Huntington’s disease research news. In plain language. Written by scientists. For the global HD community.
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COVID-19, short for coronavirus disease 2019, has taken the world by storm in almost every sense – many people have been infected with the SARS-CoV-2 virus, it’s created shopping pandemonium in stores, and many people are isolated at home. But behind that frenzied storm, scientists around the world have been working tirelessly to move research forward at an unprecedented speed so that we can understand the virus and develop a treatment or vaccine. How does this situation affect the HD community? And what does COVID-19 mean for HD research? What does COVID-19 mean for HD patients and families? A key question for many in the HD community right now is: Am I, or is my loved one, at greater risk for COVID-19 because of HD? The answer to that is – it depends. On its own, having the genetic mutation that causes HD doesn’t make anyone more or less susceptible to COVID-19 than someone without HD. What would make an HD individual more susceptible to COVID-19 is if they had any underlying conditions that put them in the “high-risk” category. Those can be as obvious as having asthma or being a smoker. But this can also include HD individuals who are symptomatic since we know that swallowing, clearing secretions from the lungs, and self-understanding of limitations can be impaired by HD. Advice from various global HD organizations can be found here: •https://www.hda.org.uk/getting-help/covid-19-information-advice •https://hdsa.org/wp-content/uploads/2020/03/COVID-Statement-3-18-20-final.pdf •https://www.huntingtonsociety.ca/novel-coronavirus-covid-19-and-huntington-disease-what-you-should-know/ To stay safe and healthy we should all continue doing what the WHO recommends – wash our hands regularly for 20 seconds with hot water, clean surfaces with a disinfectant, and practice social distancing. Social distancing means only coming in contact with members of your household and only going out for essential things, like an essential job, grocery store run, or to get medication from the pharmacy. Everyone should also remain vigilant for the symptoms of COVID-19, which include fever, a dry cough, shortness of breath, and fatigue. Some HD patients at particularly high risk may need to isolate themselves even more strictly. You should seek advice from the above sources and your health provider if you are concerned. What does COVID-19 mean for HD research? Many scientists who usually spend all day in the lab studying HD have been asked to stay home so that they can practice social distancing and remain safe. This means that HD-related research will slow for the short time during this pandemic. A big concern is ensuring that precious samples are kept safe, and experiments that had to be shut down were paused in a way that preserves them to be restarted once it’s safe to hang out in the lab again. While HD researchers may not be going into lab every day, they’re still hard at work to combat this disease. They may not be doing experiments at the bench, but they’re reading papers to develop their next idea, compiling data to better understand HD, and writing papers to disseminate what they’ve learned to the world. The labs may be quieter, but HD researchers are still hard at work in their fight against HD. What about clinical trials? With many countries’ entire healthcare systems turned over to providing care for people with COVID illness, and many doctors and nurses diverted from research into frontline care, an impact on Huntington’s disease clinical trials is inevitable. However, all those involved are doing everything they can to minimize the impact and carry on with whatever trial activity they can. In practice, the impact will vary quite a bit from one site to another, and from one trial to another. Some sites may still be enrolling new patients, while many will be forced to pause recruitment of new participants and focus on continued care and dosing of patients already involved. Many sites will likely convert onsite trial visits into telephone calls, or postpone visits until it is safer to carry them out in person. Decisions about what activity can carry on are largely determined locally, by the hospitals and local and national governing bodies that direct healthcare resources. Trial sponsors (companies like Wave, Roche and UniQure) fund, support and organize the trials. So far, all the trial sponsors we’ve heard from have indicated that they continue to be committed to running and completing the trials despite the interruption the viral pandemic may cause. It may be that some modifications need to be made later, to compensate for trials that were unexpectedly interrupted. For instance, they might need to treat existing patients for longer, or recruit additional patients to make up for lost time. And later, the regulatory agencies like the FDA might need to be more flexible when considering data from trials with higher than normal levels of missing data. With so much unknown about how long COVID will impact things, it’s difficult to be more specific, but the smart people who invented this cool generation of HD drugs and brought them to trials, are now working full time to keep those trials running as well as humanly possible. Could there be a silver lining? Science and research, and public policy informed by science not superstition, are the key to getting humanity through this crisis. The challenge has already changed scientific research for the better, in some quite fundamental ways, that could provide benefits long after COVID-19 is an unhappy memory. In a very short time, scientists from around the world have united to study the virus and share their findings to benefit everyone. The number of scientific publications about COVID-19 is rising dramatically week after week. In an effort to increase the pace of research about COVID-19, nearly all relevant scientific literature has been made open access, meaning it’s currently available for free to everyone – for now at least. You can see just how much work is being done to understand and combat COVID-19 at LitCovid (and read as much as you want!): https://www.ncbi.nlm.nih.gov/research/coronavirus/ Research has already told us a lot about the virus. We know it can be spread from person-to person, either through direct contact with someone else who has the virus or by coming in contact with droplets produced by someone who has the virus, such as a sneeze or a cough – similar to how the flu is transmitted. However, COVID-19 is unlike the flu in many ways – it’s much more fatal, we currently have no vaccine against it, and it’s a new virus so we still have a lot to learn. It can take up to 14 days after SARS-CoV-2 exposure to bring on COVID-19 symptoms, which is why many doctors are recommending a 14 day isolation period. However, we are now learning that a portion of the population may remain asymptomatic. This means they show no symptoms, but do have the virus and can pass it to other people. In fact, the asymptomatic portion of the population may be as high as 20 to 30%! This is why social distancing and staying at home when possible are critical for not spreading the virus – without widespread testing, we don’t truly know who does or doesn’t have the virus, so isolation is the key to staying healthy. Dramatic rollout of drug trials Many members of the HD community already have a head start on understanding how important clinical trials are for determining the safety and function of drugs before they’re distributed widely. It’s something HD patients and families are learning right now first hand with the Roche Phase III Tominersen trials (formerly Ionis-HTTRx and RG6042), and it’s something that will also have to be done, in an accelerated way, for any drug used to combat COVID-19. For COVID-19, researchers are trying to start on second base by repurposing drugs that are already approved by the FDA for something else, but may have an alternative use for helping COVID-19 patients. Because they’re already approved and on the market, they’ve already passed safety trials, making them faster to use. The WHO (World Health Organization) prioritized 4 such drugs or drug combinations that they think have the best chance of working against COVID-19 and have established a global trial to determine how well these drugs work, called SOLIDARITY – a fitting name for the global effort that has come together to work against this virus. Remdesivir is a drug that prevents viral replication, which means it stops the virus from increasing in number. It was initially designed to combat the Ebola virus, and has shown promise for COVID-19. The drug that has gotten the most attention, at least in the United States, is chloroquine, a derivative of which is called hydroxychloroquine. While some people remain eagerly optimistic about this drug, it has limitations and still needs to be tested. The third drug is a combination of ritonavir and lopinavir, which has been approved to treat HIV infections. The last drug is the same combination of ritonavir and lopinavir with the addition of interferon-beta. Interferon-beta helps regulate inflammation and has shown promise in treating a different viral disease, MERS (Middle East Respiratory Syndrome). This too shall pass This virus has undoubtedly brought a stressful and scary time for the entire world, but there have been a few bright spots. And while the pandemic will eventually fade away, we will be left with its silver lining. Many have been able to spend additional time at home with loved ones, even if that means having a computer on their lap. Scientific discoveries are being made at break-neck speed as the global research community comes together to fight a common goal. And last but not least, dogs around the world are rejoicing that their 2 legged friends are spending every night staying in. So stay safe and stay healthy, for this too shall pass.
A collaborative team of scientists from Canada and Japan have identified a small molecule which can change the CAG-repeat length in different lab models of Huntington's disease. CAG repeats are unstable Huntington’s disease is caused by a stretch of C, A and G chemical letters in the Huntingtin gene, which are repeated over and over again until the number of repeats passes a critical limit; at least 36 CAG-repeats are needed to result in HD. In fact, these repeats can be unstable, and carry on getting bigger throughout HD patients’ lives, but the rate of change of the repeat varies in different tissues of the body. In the blood, the CAG repeat is quite stable, so an HD genetic blood test result remains reliable. But the CAG repeat can expand particularly fast in some deep structures of the brain that are involved in movement, where they can grow to over 1000 CAG repeats. Scientists think that there could be a correlation between repeat expansion and brain cell degeneration, which might explain why certain brain structures are more vulnerable in HD. But why? This raises the question, what is it that’s causing the CAG repeat to get bigger? It seems to be something to do with DNA repair. We’re all exposed continually to an onslaught of DNA damage every day, from sunlight and passive smoking, to ageing and what we eat. Over millions of years, we’ve evolved a complex web of DNA repair systems to rapidly repair damage done to our genomes before it can kill our cells or cause cancer. Like all cellular machines, that DNA repair machinery is made by following instructions in certain genes. In effect, our DNA contains the instructions for repairing itself, which is quite trippy but also fairly cool. We’ve known for several years that certain mouse models of HD have less efficient systems to repair their DNA, and those mice have more stable CAG repeats. What's more, deleting certain DNA repair genes altogether can prevent repeat expansion entirely. But hang on, isn’t our DNA repair system meant to protect against mutations like these?? Well normally, yes. However, it appears a specific DNA repair system, called mismatch repair, sees the CAG repeat in the huntingtin gene as an error, and tries to repair it, but does a shoddy job and introduces extra repeats. Why does this matter? There’s been an explosion of interest in this field recently, largely because huge genetic studies in HD patients have found that several DNA repair genes can affect the age HD symptoms start and the speed at which they progress. One hypothesis to explain these findings is that slowing down repeat expansion slows down the disease. What if we could make a drug that stops, or even reverses repeat expansion? Maybe we could slow down or even prevent HD. So what's new? Chris Pearson’s group in Toronto have developed a compound called naphthyridine-azaquinolone, which we’ll just refer to more easily as ‘NA’, which binds CAG repeats and could prevent repeat expansion. Using cells from HD patients in a tissue dish, NA was shown to successfully slow, and possibly even lead to a small reduction in CAG repeat length. Pearson showed that blocking transcription, the process in which genes are used as templates to make proteins, prevents repeat expansion. This suggests that during transcription, the huntingtin repeat might be bent into an abnormal shape, which mismatch repair machinery in the cell recognises and then tries to repair. However, precisely how NA works in this process remains unclear. Pearson’s team injected NA into one side of the brain of an HD mouse model. They targeted the striatum, a region known to show lots of CAG expansion. Compared to the untreated side, NA prevented expansion and even caused some shrinkage of the repeat number. Next, they showed NA reduced the build-up of clumps of toxic huntingtin protein in the mice’s cells. It is not clear yet whether the treated mice have improved symptoms or increased lifespan. This will be important for scientists to work out before deciding whether preventing repeat expansion has potential as a therapy for people. What's the catch? A huge obstacle to making new drugs is getting them into the cells that most need them; in the case of HD, that means throughout deep regions of the brain. NA is able to freely enter different cells once in the brain, but this current version of the molecule has not yet been shown to cross the blood-brain-barrier. Scientists might need to modify and improve the NA molecule to avoid needing to be directly injected into the brain. Fiddling around with DNA repair, one of our body’s major defence systems, could be dangerous, and there’s the potential for major side effects like cancer. Pearson showed that NA didn’t affect the core function of mismatch repair, which is to remove DNA bases when they get put in the wrong place. The researchers carefully analyzed the rate of mutations across the whole genome, and there was no detectable increase in the rate at which they were found, compared to controls when they were treated with NA. It is possible to imagine treating HD patients at an early age, before they develop any symptoms; this might stabilise the CAG repeat and could prevent or at least delay the onset. CAG repeat shrinkage in their sperm or eggs could even mean they wouldn’t pass the disease on to their children. However, for NA there is still a lot of work to do. For starters, we would need to show that preventing CAG expansion slows down the disease, we would then need to come up with a way to get NA into the deep regions of the brain, and finally we would need to be sure it is safe with limited side-effects. Early treatment could also mean being exposed to risks like cancer for even longer, so there's clearly a lot to be worked out. In summary, NA is an exciting research compound, but there is still a long road ahead before something like it might be a drug that could be taken by people to prevent or treat Huntington's disease.
Our new writers Rachel Harding and Sarah Hernandez report from the Huntington’s Disease Therapeutics Conference - the biggest annual gathering of HD researchers. Tuesday morning - HD genotype and phenotype Good morning from sunny Palm Springs! We’re excited to be here for the 15th Annual HD Therapeutics Conference. This year in addition to Ed and Jeff, we’re joined by HD Buzz’s newest writers - Dr. Rachel Harding and Dr. Sarah Hernandez. Joel Stanton is compiling our live twitter output into our daily posts. Session 1 is called “Genotype and Phenotype”, talks focused on how the HD mutation (genotype) changes HD symptoms (phenotype). First up is Dr Seth Ament, describing his lab's work on trying to map the first changes that happen in the brain cells of mice carrying the HD mutation. The cells in our body, including our brain, have DNA that encodes more than 20,000 genes. Which genes are turned on or off in a given cell determines how that cell works. Ament’s lab has been studying this shift in what genes are expressed, and strives to understand the specific factors that make HD cells express different sets of genes, in hopes that it might be fixable. First Ament describes his work mapping the locations where the huntingtin protein, the product of the HD gene, sticks to DNA. The most obvious way that huntingtin could change what genes are turned on or off is by sticking directly to DNA. In fact, the huntingtin protein sticks to different parts of the genome in HD mice compared to regular mice. This suggests huntingtin may be doing something directly to DNA that is important to understanding HD. In mutant HD mice, huntingtin sticks to DNA in places where there’s lots of action - genes being read out and used. This suggests mutant huntingtin might be doing something unique in areas where genes are being actively used. Could this change in huntingtin sticking to DNA in different areas help explain how cells with mutant huntingtin get their sets of cells slightly scrambled? In fact, the regions that huntingtin sticks to in mutant mice contain the very genes that are changed in HD cells, suggesting this might be true. Ament’s team found a surprising relationship: they can predict how active or inactive a given region of DNA is by how well the huntingtin protein sticks to it. Next up, Ament describes his lab's efforts in mapping which genes are changed in HD brains. Amazing new technologies allow researchers to map genes in individual cells. Ament’s lab works with these techniques in NIH’s BRAIN Initiative. Ament’s lab used these new techniques to examine changes in more then 13,000 individual cells from HD mouse brains. There are a number of different types of cells in the brain, and Ament’s approach allows him to map changes in each of those cell types separately. This gives a much clearer picture than just mushing them up and analysing together. These results paint the way to a much more refined understanding of what exactly is happening in each type of cell - which might help us understand how to treat each cell type individually. Up next is Hemali Phatnani of the New York Genome Center, who describes her work understanding the changes in the brain and spinal cord of patients with ALS (Lou Gehrig’s disease), or another related disease called FTD. Phatnani works with a large team of ALS clinics to get access to rare samples donated by ALS patients. These are analysed and the data made available immediately to researchers around the world, a great model of open science! Like HD, ALS is a complex disease; different cell types in the brain undergo different changes during the course of the disease. Phatnani’s team has helped develop new methods to map changes in cells from ALS brains. Their techniques allow them to study cell specific changes, and their data are available for any researcher (or curious non-researcher) to explore at als-st.nygenome.org. Next is Sumanjit Jayadev from the University of Washington Medical Center. Jayadev is interested in studying a particular type of brain cell called microglia. Microglia play a role in the progression of HD, and getting rid of microglia in mouse models can help with HD symptoms. Scientists have known for a while that HD causes an inflammatory response, and Jayadev is interested in which brain cells (other than neurones) are playing a role in this inflammation. Genes involved in inflammation are risk factors for Alzheimer’s disease (AD). Jayadev is looking at which which cells in the brain these genes are switched on, to identify risk for disease using a cool technique which provides data at the level of single cells. Examining these changes in single cells has allowed researchers to identify subtypes of microglia, and looking at these subtypes identified one particular type of microglia only present in AD. Jayadev is working on this project with the folks at Sage Bionetworks who are experts in open science and open data in biomedical research. With all the data generated, then can monitor disease progression of AD by looking at which genes are switched on where and when. This can help define different populations of patients which helps clinicians and researchers understand how the disease process works. If we can apply this to HD, then perhaps in the future by knowing the age and CAG length of a patient, doctors could make informed decisions about how to treat that specific patient. The next talk is by William Yang, a researcher from UCLA who studies HD. The Yang Lab generates lots of really large datasets from different HD mouse models, looking at how different genes are switched on, which proteins are present in different cell types, and bring all this data together to compare HD to controls. In these big data sets, scientists can search for patterns and correlations which might indicate how some genes work together in HD mouse models. These patterns can be mapped using computational methods to understand how certain cell types in the brain contribute to changes in the gene expressions they observe. In this talk, Yang is focusing on the analysis that was done using control mice without HD, where his team showed that this technique identified important functions and which cell types contribute to these functions. When data from the HD mice model is overlaid on the map, they find that genes that regulate sleep/wake cycles and DNA repair and changed - confirming findings researchers have reported before. This map can also be used to test lots of new theories researchers have about HD - a welcome tool! Tuesday afternoon - Somatic instability The afternoon session focuses on the process of somatic instability. Simply put, somatic instability occurs when long repeated DNA sequences are unstable in certain cell types. First up this afternoon is Darren Monckton from the University of Glasgow. He is interested in studying how somatic instability can drive Huntington’s and how targeting instability could be a good strategy for making new drugs for HD. We have known for a while now that the CAG-length can vary in tissues with some cells having much longer lengths than others. This is similar to what we see in other expansion diseases such a myotonic dystrophy, The variation in the CAG-repeat length gets larger as patients get older, suggesting there is more instability. Similarly the longer the CAG-repeat, the greater the variability and therefore more I fired instability. It is worth noting that this doesn’t mean your CAG number gets longer overall as you age, this will stay the same. it just means that in a few cells, the CAG-repeat length can sometimes increase. CAG encodes for the amino acid glutamine, which is why you will sometimes hear HD called a polyglutamine disease. But glutamines can also be encoded by CAA, so although the DNA is different, the protein which is made will be the same with a CAG substituted for a CAA. This happens, for example in the HD gene, where a long “C-A-G” repeat grows longer in come cell types. There is a more detailed post on HDBuzz about this here. A recent interesting finding showed that the presence of CAA in some places instead of CAG is better at maintaining repeat length stability. The next interesting question becomes, what is driving these changes that cause somatic instability? Evidence is no pointing to DNA repair, a recent hot topic in HD research. Identifying specific “drivers” (or genes) of this instability could have therapeutic implications for HD. Researchers are now working to fogure out which of these drivers are most important in patients, as well as working out which are suitable for targeting with a new therapy or medicine. Now the researchers are looking to see how the instability or changes in the CAG-length, varies for a specific patient over time, all thanks to those patients who contributed samples and data to EnrolHD. If we can monitor and track HD progression by measuring instability with a simple blood test, this could be a less intrusive way for doctors to monitor patients disease progression and suggest the best way for how they might be treated. Next up is Karen Usdin from the NIH who has been looking at somatic instability in mouse models of a different disease which affects the nervous system called fragile X. Like HD, fragile X is a repeat expansion disease, but instead of CAG-expansion, this disease has a CGG expansion. We can learn a lot from other expansion diseases as scientists believe that there are lots of similarities in the drivers of the diseases. Also like HD, the CGG repeat that causes Fragile X is affected by somatic instability, and genes involved in DNA repair influence this process. Uddin is finding, at least in Fragile X mice, that altering levels of DNA repair genes prevents the expansion of the CGG repeat and even removes some of the repeats! It’s super to hear from scientists at this meeting from outside the field of HD, who share a lot of interesting ideas and knowledge which might help push HD research forward faster! The last talk of the session is from Ravi Iyer of CHDI, discussing one of the drug discovery programs scientists are interested in. One of the goals of this drug discovery program is to identify small molecules that will stabilise CAG expansion in HD. CHDI is working with lots of different companies to discover small molecules using lots of different techniques - collaboration on these tricky projects helps push things forward faster. One of the ways in which they identify small molecules that might work, is by looking at detailed models of the structures of molecules which they want to target, such as proteins involved in DNA repair. The best part about programs that seek to identify small molecules in that they could be taken as a pill if they were shown to be effective treatments for HD. While the prospect of small molecule treatment is very exciting, researchers have to be extra cautious that the small molecules they want to use aren’t having unexpected side effects. Making small molecules which might be medicines for HD is an exciting project, but we are still way off learning whether this will be successful. A big team of scientists lead by CHDI are working hard towards which and we’re excited to see updates in the coming meetings. Now we’re hearing from Brian Bettencourt from Triplet Therapeutics, one of the many companies working in this area, who will be telling us more about therapeutically targeting somatic instability. One of their goals is to stop somatic instability of the CAG expansion and delay the onset of HD, hopefully to an age that's so old it's not a realistic lifespan! This has been such a promising area of research that scientists have to prioritize which molecules to work on first. This allows them to efficiently work toward their goal of developing promising therapeutics for HD as fast as possible. After prioritizing candidates for safety and low risk, Bettencourt's group developed molecules that target 8 different genes. While this is a large number of targets, it's a small enough number to get done relatively quickly. Again, this research is part of a larger collaboration, as Bettencourt is working with HDBuzz's Jeff Carroll to study these different potential therapeutic targets. Bettencourt's group should be reporting back on some of these studies next year, so stay tuned to see if targeting somatic expansion can be used to treat HD. The last speaker of the day is HDBuzz’s very own Jeff Carroll, who will be wrapping up the session on somatic instability. One of the things Carroll is interested in is understanding the effect of lowering huntingtin on non-brain tissue, such as the liver. Interestingly, lowering huntingtin reduces somatic instability specifically in some tissues, but not others. The Carroll lab wanted to understand these findings in more detail so are collaborating with Sarah Tabrizi to look in human neuron cells. Huntingtin lowering in a different mouse model that doesn't have HD (but has ataxia, a different CAG repeat disease) shows that somatic instability is also reduced there, perhaps suggesting that the huntingtin protein in general has a role in somatic instability. If huntingtin is playing a role in genome stability, and how well our DNA is maintained, the Carroll lab is keen to work out how that might be happening and how this would affect HD patients. This is a very new observation so folks are excited to explore this finding! And that's all for Day 1! Stay tuned for our write ups of day 2 and 3 and keep up with the conversation on Twitter.
Rachel and Sarah report from the Huntington’s Disease Therapeutics Conference - the biggest annual gathering of HD researchers. Be sure to catch up on Day 1 and Day 2. Thursday morning - Huntingtin lowering Good morning everyone! We are back for Day 3 at the CHDI meeting in Palm Springs which is all about huntingtin lowering. There are a LOT of researchers and companies interested in huntingtin lowering! We know lowering huntingtin protein (HTT) in mice and rats does help with HD symptoms and progression, and that it is possible to lower HTT in other animals. However, the million dollar question is whether we can lower HTT in humans and whether this helps treat patients. Other big questions are when we should lower HTT - do we need to do this before patients show symptoms, or later? And do we need to lower HTT in specific brain areas? Or should this be done in the whole brain? All things that current clinical trials are trying to answer. We can be optimistic that clinical trials will help us answer these questions. Scientists have developed lots of ways to try and lower HTT as well as all sorts of tools to measure HTT levels in the brain. There are also a lot of alternative strategies in the pipeline. Our first speaker is Ignacio Munoz-Sanjuan from CHDI, discussing the timing of HTT lower strategies. Munoz-Sanjuan is also very actively involved in patient outreach in Latin America. Munoz-Sanjuan started a non-profit centered around his efforts in patient outreach called Factor H. Scientists want to make sure any therapies they develop are safe for patients. HD affects the whole body so whilst we focus on HTT lowering in the brain, its important to understand the effects of treatment on the whole patient. Neurodegeneration is a difficult problem to target - scientists in other fields such as Alzheimer's and Parkinson's are also concerned about how best to treat patients, it's not an easy task to make these new medicines, so we should be cautious moving forward. One attractive area for HTT lowering are small molecules. These are medicines that would be taken as a pill, so lots of people are quite interested in developing this strategy further. But this likely wouldn't specifically target expanded HTT, but rather total HTT. One key question in HTT lowering is timing - when should we be treating to stop the disease? Can we reverse any damage that has been done or can we treat at later disease stages? Researchers don't want to give unnecessary medicines to patients if they don't have to. The brain is a really complex organ and it is important to remember that while mouse models are useful for investigating some aspects of HD progression, a mouse brain is not a human brain. Using lots of different models of HD is key for studying how well drugs might work. However, mice are critical for advancing HD research. Since timing of treatment is a hot topic, researchers have been following what happens to reversal of cellular effects in mice. Very excitingly, researchers are finding that HTT lowering in mice can prevent and reverse deficits in striatal neurons - the most affected cell type in the brain of HD patients - amazing news! Now, we move onto the promise of HD biomarkers. A quick recap that these are the measures scientists can make in patients to track how HD is progressing. Good biomarkers can help inform clinicians on how and when to best treat patients. CHDI has spent a long time developing new imaging techniques that will let us visualise how much HTT is in the brain, and whereabouts it is. To do this, they have developed a new molecule that specifically binds to the HTT protein molecule when it forms into certain clump structures. This is called a PET tracer or PET ligand, because the scanners it shows up in are called PET scanners - short for positron emission tomography. The molecule is able to cross into the brain so could be used to help track HTT lowering. Already, CHDI and team are measuring lowering of these HTT clumps in different HD mouse models. They are trying HTT lowering in different areas of the mouse brain and also treating mice which are different ages. Another question scientists are trying answer is by how much should we lower HTT protein in cells? What level might help treat patients? What levels are safe? We can use the HTT PET ligand and other experiments to measure the lowering levels after treatment and can then see which mouse models recover at which levels of lowering. This might help translate discoveries from mice to humans. The next speaker is Mark Bevan from Northwestern University, discussing his work lower HTT in a specific brain region. The research Bevan is sharing with us today is focused on how HTT lowering in certain brain areas might change the way the brain works and how this could affect symptoms in patients. Like many others, Bevan is interested in cell type-specific differences caused by HD. His group is finding that specific kinds of neurons are less active, while others seem to be unaffected in mouse models of HD. By looking in HD models of mice, Bevan has found that there are differences in the way neurons talk to one another and is using his experiments looking at neuron-to-neuron communication to study the effect of HTT lowering in these mice. By lowering HTT in HD mice, Bevan is seeing that motor deficits are improved - the mice are able to move longer distances at a faster speed. Great news since current human clinical trials haven't yet disclosed data regarding changes they might be seeing in symptoms. Bevan and colleagues are continuing to look at HTT lowering on other motor deficits and so forth in their mouse models.They hope their findings might inform how HTT lowering therapies could translate to the clinic. Next up is Marcy MacDonald from Massachusetts General Hospital whose talk is intriguingly titled “the outer limits”! From the Enroll-HD study, researchers are finding there is lots of variability that comes from factors other than CAG length. While this has been a big finding in the HD field, it really indicates how complex this disease is, even though we know the genetic cause. Another huge thanks to all the Enroll-HD participants. Hopefully you've noticed a theme that many researchers are making great use of the data you're providing - thank you! While we all have the HTT gene and HD patients have a longer CAG than others, we also have other small variations in the HTT gene sequence that researchers like MacDonald are interested in studying and targeting therapeutically. MacDonald's team has generated stacks of data on what can modify disease progression in HD. They are sharing all their data so that scientists all over the world can work together on it - many pairs of eyes are always better than one. Serendipitously, MacDonald identified people that have reduced expression of HTT. These people are just fine, so researchers know that reducing HTT to at least that level shouldn't have detrimental effects. Since HTT expression levels vary within the population and people seem to be just fine, this is really good news for HTT lowering strategies. It suggests that altering HTT levels might not have negative side effects. One thing to note though, is that when HTT is lowered (specifically in liver), the cells are less able to sustain stress in mouse models. So again, it's very important for researchers to test safety thoroughly for all HTT lower strategies. Because HTT expression levels vary within the population, researchers also need to be aware that the starting levels of HTT for patients in HTT lowering trials may differ. So one could imagine in the future a more personalized approach may be adopted for each patient. These are all points that companies running HTT lowering trials are considering in their trial design. We all look forward to hearing more updates from these trials in the afternoon session. Thursday afternoon - taking huntingtin lowering into the clinic Good afternoon! This session focuses on translating HTT lowering to the clinic. Our first talk is by Charlotte Sumner from Johns Hopkins. She'll be discussing some of the challenges associated with targeting genes therapeutically. While Sumner primarily focuses on a different neurodegenerative disease called spinal muscular atrophy (SMA), there's currently an ASO treatment for SMA so the HD field can learn a lot from watching what they're doing. Similarly to HD, we know the precise genetic causes for SMA. However, our understanding of what's happening with the protein molecules in the cell is much more hazy so the drivers of disease are not completely clear. Because the genetics are clear, there are a number of different gene therapies which have been developed for SMA. In addition to an approved ASO therapy, there are also small molecules (taken in a pill) which lower the target gene currently under review at the FDA. A single dose or "one-shot" gene replacement therapy which fixes the DNA sequence directly has also been shown to work well in young children with SMA and work is ongoing to see if this treatment would work well for older patients. Some patients in the SMA trials improved dramatically across various metrics that were measured. These findings are very encouraging for the field of HD research, where we hope to apply some of the successful strategies they've found in the field of SMA. Earlier in the conference we tweeted about researchers who were interested in determining when is the best time to treat HD. In the field of SMA they've found that timing really matters, so it's great to see that HD research is on the right track. Because some SMA patients have responded differently to treatments, SMA researchers are keen to get more patient data to understand potential reasons for this variation. This reinforces why studies such as Enroll-HD will be helpful for HD research. Now SMA researchers are trying to work out the best way to treat and monitor their patients i.e. how much of the medicine to give? When and how often should it be given? This should hopefully further improve patient outcomes. Again, similarly to HD, SMA researchers are interested in neurofilament as a biomarker for disease progression. They are monitoring neurofilament levels in patients who are treated with the different SMA therapies and neurofilament levels seem to drop over time with treatment. The next talk is from Anastasia Khvorova who works at the University of Massachusetts Medical School and will be telling us about HTT lowering using a technique called RNAi. Similar to ASOs, RNAi-based therapeutics target the message of HTT rather than the DNA or the protein, acting to destroy the middle step so that protein is never produced. You can read more about RNAi and how it differs from ASOs here. To test how the HTT-targeting RNAi affects disease, Khvorova and her team first analyzed the effects in mice. The first step was to measure how widely their treatment spread in the brain - very promising results! After treatment, they found HTT was significantly reduced in many areas of the brain. This work targeted both expanded and unexpanded HTT, but they're also working on approaches that will just target expanded HTT. Making some clever tweaks to the RNAi molecules, Khvorova and team were able to make their treatment selective for just the expanded HTT message so that only this protein is lowered, not the unexpanded. However, it should be noted that this will only work in ~35 % patients who have a slight difference in their huntingtin gene sequence called a SNP (pronounced "snip"). This allows the RNAi treatment to select for the expanded over the unexpanded HTT. Next they wanted to see how their RNAi treatment worked in larger animal models, so they moved from mice to sheep. Using sheep they tested various delivery methods for the treatment finding they could inject into the brain or CSF and it works the same. After sheep, Khvorova and colleagues moved into monkeys and again saw that the RNAi treatment spread fairly nicely across the brain and through the spinal cord. It stays in these regions for quite a long time so they don't expect to have to treat very frequently.In a very early safety study in these monkeys, the therapy seems safe at the dose tested. Similarly, early data from a sheep safety study showed the therapy was safe under the conditions that the scientists tested. The exciting news is that HTT is significantly lowered in these early studies in monkeys. The levels of other genes seem to be unchanged which means the off-target or side effects appear to be low in the way the scientists measured in their experiment. Nonetheless, looking for even small changes in other genes is really important so work is ongoing by Khvorova and colleagues to make sure that there are no differences and confirm the safety of this therapy. This technology is thought to be very promising by Khovrova and colleagues, as well as CHDI. It could be used to change the levels of other proteins in the brain such as those identified as modifiers for symptom onset in HD patients or other targets. One of the things they're paying attention to is cost. They're trying to keep the cost down so it can be widely available to all HD patients. We're really looking forward to more updates about this promising research as they move toward the clinic! Our next speaker is Astrid Valles-Sanchez from uniQure. She'll be telling us about uniQure's approach to lower HTT. UniQure has a treatment called AMT-130 to lower HTT and is currently performing a clinical trial to determine the safety of this treatment. AMT-130 is designed to be a one-time injection of HTT lowering treatment into the brain. Valles-Sanchez is focusing her talk on biomarkers they're assessing to measure how effective this treatment will be to modify HD disease progression. When they look in a pig model, they find that their treatment is detected in the CSF out to 2 years. In monkeys, they detected their treatment out to 6 months, when the animals were sacrificed. Similarly to other studies, uniQure want to check that their therapy is spreading out across the brain to see where it might be working. In the pig model 12 months after treatment, they analyzed tissue from different areas across the brain to see how effectively their treatment lowered HTT. They find significant lowering of expanded HTT with the strongest lowering in brain regions most affected by HD. uniQure also looked at expanded HTT levels in the CSF after treating HD pig models with AMT-130, but in this particular experiment, the levels in the CSF do not correlate to levels of HTT found in the brain. Scientists at uniQure are interested in using magnetic resonance spectroscopy (MRS), a non-invasive way to look at the brain, to see whether there are any changes in chemicals called metabolites that are found in different areas of the brain after treatment. The final talk of the conference is from Scott Schobel who will be telling us about some very preliminary new results from the Roche trial. These results come from a 15 month open label extension in manifest (symptomatic) HD patients, we should be a bit cautious in interpreting this data as its still very early days, but its exciting nonetheless to see what they found. And thats is for this years HD Theraputics Conference! Be sure to catch up on Day 1 here, and Day 2 here and keep up with the conversation on Twitter. As many projects in the field of HD research, this has been a large collaborative effort of many researchers working together to get this out. RG6042 has a new name! It's called tominersen which is what the therapy will be called from now on. Patients in the original safety study were kept on an open label extension which means that after the safety trial ended, they continued to receive the therapy and the data we will see today is what scientists have found since the end of the safety study. Recap - this therapy is NOT selective for the expanded HTT gene, it is designed to lower both expanded and unexpanded huntingtin protein levels. Folks at Roche and their collaborators have been working for many years now with different animal models to work out how much they would need to reduce levels of huntingtin protein and how they might measure that lowering of protein by looking at the CSF. It was two years ago at the CHDI meeting that Sarah Tabrizi from UCL delivered the first results of the safety study. In just 2 short years we've made huge advances in HTT lowering with many strategies now available - very exciting! From the safety study, it looked as though HTT protein levels could be reduced with tominersen in a dose-dependent manner. This means that the protein was lowered more in patients who received more of the treatment in the small number of patients tested. The open label extension tested 2 dosing strategies - participants either were dosed either every month or every other month. This type of design was critical for determining how often patients would need to take tominersen. They found that good huntingtin lowering was observed even when tominersen was taken every other month. The two patient groups had very few folks not complete this extension study - we are all grateful to these patients for their commitment to the trial! Given the metrics that Roche examined, the data indicates that tominersen should be taken every other month rather than every month. Because they found the less frequent dosing was effective in lowering HTT, they modified the strategy for the next arm of the study (Phase III) to reduce the number of doses and test dosing every 16 weeks - much less demanding for patients! Tracking levels of neurofilament, a proposed HD biomarker to monitor disease progression, could be helpful to see how effective the treatment is and Roche are continuing to investigate neurofilament levels in the clinical trial. They found that NFL levels have an initial rise, but amounts seem to decrease and even out by the 15 month mark. Roche are interested in understanding the NfL biomarker further and are looking into the underlying biology which might link HTT and NfL. There is more clinical work in the pipeline at Roche which will hopefully continue to inform HD researchers about how effective this therapy might be. Schobel has kindly shared his slides online! And that’s all folks! We hope these live tweets have been helpful for you all to follow along, we’re excited for next year already!
Rachel and Sarah report from the Huntington’s Disease Therapeutics Conference - the biggest annual gathering of HD researchers. Be sure to catch up on Day 1! Good morning everyone! We are back for day 2 of the CHDI therapeutics conference in Palm Springs. Lots of exciting talks coming up! Wednesday morning - The path to prevention Our first speaker is Ariana Mullin from Critical Path Institute, a non-profit formed in response to a governmental call to foster new medical products. Folks at the CPath are interested in bringing together research from lots of different groups. For this, they have built a research framework so that everyone is on the same page and language/ definitions are consistent for all of these researchers. The overall aim is speeding up drug development and licensing. HDBuzz wrote about the Critical Path Institute a little while ago. The next speaker is Swati Sathe from the CHDI. Sathe is going to be telling us about work towards defining different stages and symptoms of HD using lots of data from Enroll-HD. Enroll-HD has been collecting lots of data about HD patients for many years now. With lots of data, researchers can look for patterns which could be interesting, think about how best to plan clinical trials and craft policy. Importantly, folks who participate in Enroll-HD don't have to know their genetic status, so scientists can still get data to advance HD research while patient privacy is protected. Enroll-HD collects data from HD patients whether they have symptoms yet or not. All of this data is important for understanding how HD progresses as patients get older. Many researchers can use the data gathered from Enroll-HD to help guide future studies. This large type of dataset will help advance HD research forward more quickly. Enroll-HD is not the only big data project in HD. TRACK-HD was another study which looked at changes in the brain structure over time in patients. Patient participation in these studies, helping scientists collect big data sets, is very valuable for HD research. A new and exciting initiative is Self Enroll. This would be a digital version of Enroll-HD where patients could provide data and updates without having to travel to an Enroll-HD site. Removing this barrier would hopefully encourage even more patients to participate. Check out more about Enroll-HD. Next up is Sarah Tabrizi from UCL who will be telling us about her team's research studying young HD adults. By studying HD in young adults, researchers can try to find the best time to treat HD. We have known for a long time now that HD patient brains begin to change even long before they have symptoms which might be detected in the clinic. Again, we have confidence in this finding due to the huge datasets from TRACK-HD, PREDICT-HD, Enroll-HD and other studies. In these studies examining very early changes that occur, researchers are also focused on identifying biomarkers - molecules that can be used to judge disease progression - that will also help to determine if treatments are working. For all of the participants in the study lead by Tabrizi, brain structure and function was extensively mapped, generating tons of data. Along with detailed brain maps, Tabrizi is also collecting lots of data from CSF and blood to track the difference in these biomarkers over time and between patients with and without HD comparing between patients before and after symptoms become obvious. What they found in the extensive thinking tests was that overall there were no differences between the participants with and without the HD gene in the way that they think. One of the biomarkers that was examined was neurofilament light (NfL) - a marker recently shown to increase as disease progresses. They found that NfL levels are raised in HD patients very early, before they show any symptoms or clear brain shrinkage. We have written about NfL before. Measuring changes in the CSF levels of NfL could be a good biomarker for measuring how HD progresses in patients, even many years before they become symptomatic. This could help guide monitoring and treatment of patients in very early stages of the disease. The next speaker is Jianying Hu from IBM T. J. Watson Research Center. Hu will also be talking about studying HD progression. Hu and IBM are working with CHDI to understand HD disease progression using lots of data collected from various studies. You're probably noticing 2 themes: lots of people working together, and lots of data used from these trials. Massive thanks to all the participants! Using these large datasets, Hu is gathering information not only on the HD population as a whole, but also for each individual patient to improve outcomes. This data can then be used by doctors to better treat HD patients. Because all of these studies collect different types of information there's lots of different data for HD researchers to use. Hu and colleagues can use lots of cool cutting-edge computational methods to help build a model of HD progression with all of this data. Hu and her team are using data from all these HD clinical trials that has been collected over 4 decades - wow! One of the ways this computational model could be used is to predict how symptoms might progress for HD patients, which could be helpful for clinicians to work out how best to treat and monitor patients. Using the computational model that they developed, Hu defined 9 different "disease states", or stages of HD. To track progression, this model can be used to predict where the patient falls within these states. This data is being used to drive discovery of therapies for HD specifically aimed at slowing progression. Their next steps focus on defining very early stages of the disease in more detail. The next talk is being given by Steven McCarroll from Harvard who will be telling us about his research into HD by doing single cell analysis experiments. McCarroll's work is focused on understanding what HD is doing at the cellular level - so identifying targets for therapeutics and biomarkers for progression. While people typically think of neurons when they think of cells in the brain, there are actually a variety of different cell types. For a disease like HD that affects every cell, it's critical to understand how HD is affecting cells of the brain other than neurons. Using more clever computational methods called machine learning, McCarroll's group is able to sort out the different types of cells from the brain — AND they made all of this cool software free for the community, there are over 25,000 downloads so far! In fact, this software was able to find new types of cells in the brain. This is important as these new cell types were not consistently found in different species such as mice but they are in primates like monkeys. Next, the McCarroll lab wanted to apply this technology to see if they could monitor HD progression and look for biomarkers of different stages of disease, by seeing which types of cells were found in patients early on and then later in the disease. We've known for a while that one of the most vulnerable cell type in HD are spiny projection neurons, or SPNs. With McCarroll's single cell technology, he was able to show there were less and less SPNs as HD progresses. Now McCarroll is switching gears to discuss biomarkers. Identifying new biomarkers for disease progression is critical not only to track HD over time, but also to definitively assess if therapeutics are having a beneficial effect. One of the biomarkers he's exploring is mostly found in the SPNs. This means if this biomarker is tracked over time, it could correlate to loss of SPNs as HD progresses. This would be a great way for researchers to measure SPN loss without needing brain samples. That’s it for today! Be sure to have caught up with Day 1 here, stay tuned for our write up of Day 3 which is focussed on huntingtin protein lowering (now available here and keep up with the conversation on Twitter!
A recently published study in the journal ‘Neuron’ has identified new potential therapeutic targets for the treatment of Huntington’s disease (HD). The work conducted by Professor Myriam Heiman and colleagues used cutting-edge genetic technologies and discovered several genes could modify HD progression in their models in the lab. Many of these genes have not been linked to HD before and could be exciting new targets for researchers to pursue when developing drugs and treatments for the HD patient community. Ambitious screening of the whole genome Cells in our bodies contain DNA encoding thousands of genes, each a recipe giving instructions to our cells on how to make a different protein molecule. These recipes are transcribed from our DNA into a message called the messenger RNA. The RNA is then translated by cellular machinery into protein molecules. Scientists can manipulate these processes in the lab to understand the role of different genes in our bodies. Genetic screens look to understand the role of a single gene in different contexts, in this case, the researchers were interested as to the role of all the different genes in our cells in protecting against the damaging effects of the HD mutation. So the idea is to mess with every single gene in turn, to try and discover whether that gene has any impact on HD symptoms. Genetic screen technologies can work in lots of different ways, but they all aim to stop or lower the expression of proteins from specific genes. Genes may be targeted directly by editing the genome itself. Other technologies interfere with the message RNA which is transcribed from the gene and is essential for the cells to make the protein which the gene encodes. This might sound familiar to HDBuzz readers as these are similar technologies to those being used in huntingtin-lowering therapies which are currently being assessed in various clinical trials. Whereas these huntingtin-lowering therapies target just the huntingtin gene, in the case of this genetic screen, the researchers are targeting every gene in the genome, one-by-one, to work out the role they play in HD. The Broad Institute, where Prof. Heiman is based, is a world leader in developing libraries which can be used for genetic screens. In this study the researchers used two different technologies in their screen, both of which are delivered into brain cells by special types of viruses. Firstly, short-hairpin RNAs which target the messenger RNA and turn down expression of the gene by interrupting the message from being translated into the functional protein molecule. Secondly, CRISPR was used to directly edit the gene sequence in the genome, disrupting its ability to be switched on to make the protein it encodes. Systematic genetic screens in different animal models have been around for decades for more simple systems such as worms and flies. However, these types of experiments have been much more technically challenging to perform for the brains of mammals which has been a hindrance to scientists interested in conducting these screens to understand different neurodegenerative diseases. Prof. Heiman’s team were able to overcome these difficulties by finding a way to pool and concentrate the reagents which need to be injected into the brains of the mice in the genetic screen, and were able to directly target the striatum which is the area of the brain they were interested in studying. The striatum is the most heavily impacted brain region in HD patients, which is why this area was chosen. 20,000+ genes were investigated in this one study Rather than look at familiar genes associated with neurodegeneration in their mouse models, the scientists in this study took an unbiased approach and completed a genome-wide screen to look at the role played by almost every gene. In fact, they screened nearly all of the approximately 22,000 genes found in mice! This was an incredibly ambitious approach and provides a wealth of data to researchers in the HD field and beyond. As this was the first systematic screen of all genes in the mammalian central nervous system, the researchers used normal mice with no known mutations to work out which genes are important in brain cell survival in normal conditions. Genes which had been previously identified in systematic screens in more simple models such as flies and worms were shown to be important in mice too. In this study however, many new genes were identified including several which play a role in metabolism in cells. These were not previously identified in other systematic screens in flies or worms, which is probably because the mammalian central nervous system requires more energy and is more dependent on genes which help the cells make energy. These findings are a good reminder of how important it is for scientists to consider their research findings in the context of the animal models being investigated. In addition to the control mouse model, two different HD mouse models were used in this experiment, R6/2 and zQ175, both of which are extensively described in the HD research literature. By comparing the genes identified in the screen in the HD mouse models to those identified in the control mice, scientists could work out which genes were specifically important for HD, rather than genes which affect brain cell function more generally. For the genetic screens conducted on the two HD mouse models used in the study, approximately 500 genes were identified as being important in HD progression. Many of these genes play roles in pathways scientists have previously identified in other studies such as the genome-wide association studies (GWAS) which looked for genes which can alter the age-of-onset of HD symptoms in human patients. These include genes involved with DNA damage repair pathways which maintain the integrity of our genetic material as well as genes in transcription pathways which regulate how the messenger RNA is processed in cells and therefore which protein molecules are made. New gene targets were identified in the screen too, including genes belonging to the Nme family. Nme genes have been previously reported to be linked to spread in some cancers but this is the first time they have been connected to HD. Heiman and colleagues think that targeting the Nme pathway may be important in helping the brain cells get rid of mutant huntingtin protein in HD brains. If we can design therapeutics which modulate this pathway, this could be a potential way to help treat HD. New leads for making new HD medicines Even with lots of ground-breaking clinical trials underway testing different therapies for HD patients, it is important that researchers continue to look for alternative ways to potentially make new medicines for HD. This research provides a wealth of data on HD as it works in mouse model brains and also gives us ideas of new targets to pursue as potential drug targets, which may one day end up in the drug discovery pipeline. It will be exciting to see how these new leads are followed up by researchers around the world and also how this technology might be applied to other neurodegenerative diseases.
A new publication used tiny 3D brain models created from human cells to show that the mutation that causes HD could lead to early changes in brain development. However, it’s clear that HD patients can, and do, develop fully mature brain cells that maintain normal function, in most cases, for decades. So let’s put these findings into context and dig into what these developmental changes that have been discovered using human cells in a dish might mean for HD patients. Getting human brain cells without collecting brain samples Even though HD is unique to humans, most organisms have a version of the gene that is mutated to cause HD – huntingtin, or Htt for short. A variety of organisms can be used for studying HD and each model can inform different aspects of how the disease works. For example, if a scientist wants to know if an experimental treatment could benefit HD, they could use fruit flies or even worms to get those answers. While flies and worms are quite different than humans, they have very short lifespans (about 14 days for fruit flies) so scientists can get their answers quickly. If they want to know what will happen in a more complex brain, scientists often choose mice. But to understand the effects that their work will have in humans, scientists need to test their ideas in humans - or at least human cells. In 2006 two separate scientists showed that you can reverse the biological timeline of a skin cell, priming it to turn into any other cell type in the body. More recently, blood cells have even been used. These primed cells are called “induced pluripotent stem cells”, or iPSCs. If scientists are interested in studying a brain disease like HD, they can then turn those iPSCs into the cell types of interest, like neurons. And even better, if the skin or blood cells are from an HD patient, scientists then have everything they need to study the neurons of that patient without having to take a brain sample. Not only super cool science, but also great news for HD patients, who would like to hang onto their brains! Usually, cells are grown on the flat surface of a Petri dish, but recently researchers have devised a way to coax iPSCs to grow into 3 dimensional balls of cells - which resemble a little brain at an early stage of development. These 3D structures are called brain organoids and are akin to a tiny model of a brain. Growing these cells in 3D allows researchers to study the way that they organize as the organoid grows, informing very early events in development within the brain. But while these tiny brain-like structures seem to have similar early developmental patterns to a human brain, it’s not a working replica and they don’t possess the capacity for cognitive function. You are a beautiful and unique snowflake In a recent study, these brain organoids were used to investigate the impact that the mutation that causes HD has on their development. They did this using 4 different cell lines that are identical in every way, except one: the HD gene. But, wait. How can 4 different cell lines be identical and different? You can think of people as snowflakes – we’re all unique in our own way, not just with obvious physical differences, like different hair color or eye shape, but also at the genetic level. Everyone has a slightly unique makeup in the code of their DNA that makes them different. So while 2 people may have the genetic code necessary for hands, one may have very long fingers and another may have short fingers. If researchers take cells from 2 people, one with HD and one without, their cells will not only contain the different CAG lengths of that person’s HTT gene, but will also contain all the other genetic differences that make them uniquely them! This can confuse results a bit though because researchers won’t know if any changes they measure are because of differences in their HD gene or if they’re because of another unique alteration in that person’s DNA. So back to those identically different cells – to prevent any confusion in their study about whether the results are from different CAG lengths in the HD gene or some other unique DNA code a person has, researchers used a series of cell lines originating from a single cell line that has been genetically altered only within the HTT gene so that it contains CAG repeats of different sizes. In this case, the CAG repeat tract was increased from 30 (to represent someone without a risk for HD) to 45, 65, or 81 (representing adult-, adolescent-, or juvenile-onset HD, respectively) while all other genes in these cells remained identical. So now the researchers can be sure that any differences they observe between these cell lines are explicitly due to the changes they induced in the HD gene. Pretty clever! Early-onset juvenile HD may not be a purely degenerative disorder When using all 4 cell lines to create organoids, the first thing the researchers noticed was that even though organoids from all 4 lines were the same size, the HD organoids developed smaller internal structures that developmentally lead to the formation of important brain cells called neurons, suggesting that brain development is blunted. However, this was only observed in organoids that correspond to adolescent- (CAG of 65) and juvenile-onset HD (CAG of 81), while the organoids that represents adult-onset HD (CAG of 45) were similar to the organoids representing someone without HD. So what does this mean? The authors interpreted their findings to mean that the mutation that causes HD, particularly in cases of juvenile-onset, stunts brain development. However, an alternative idea is that the mutation that causes HD may just delay development. To test this, the authors examined older organoids – they measured the difference between the organoids that have 30 and 81 CAGs and found that they still had smaller internal structures, even at this later time point. So it appears that, at least for juvenile-onset HD cases, brain development in these organoids is not just delayed, but rather stalled. However, the adolescent- and adult-onset organoids weren’t included in this specific experiment. Another key finding from this study suggests that the juvenile-onset organoids develop into neurons more quickly than the organoids without the HD mutation. But if you’ve been staying up-to-date on your HD organoid literature, you may find this a bit confusing because a paper that came out about a year ago found the exact opposite – that HD organoids derived from iPSCs develop into neurons more slowly than organoids without HD. So does this mean that one study is right and the other is wrong? No. Even though the 2 studies found opposite effects in the speed of HD organoid neurodevelopment, each study was performed slightly differently, using different cell lines and measuring effects at different time points. What both studies agree on is that the mutation that causes HD leads to early changes in neurodevelopment. But, just because results suggest early changes in development doesn’t mean that these changes can’t be compensated for. In fact, the authors of the more recent study identified a drug with the ability to partially restore the lower measurements they observed in the juvenile-onset HD organoids! But what about the organoids that represent adolescent- and adult-onset HD? If you’re a stickler for details, you may have noticed that most of the findings of this study just focus on organoids that represent juvenile-onset HD, which represent about 5-10% of the HD patient population. This means these experiments are assessing a rare form of an already rare disease. However, the authors of this study are diligent about interpreting their findings in the context of what their data represents, saying, “Overall, these findings suggest that HD, at least in its early-onset juvenile forms, may not be a purely neurodegenerative disorder and that abnormal neurodevelopment may be a component of its pathophysiology”. Hot off the presses One thing to note about this study is that it’s currently published in a repository called BioRxiv (pronounced “bio archive”). BioRxiv is a phenomenal resource because it publishes data ahead of print and is available to everyone. While this gets data out to the masses sooner, it also means that it hasn’t undergone the scientific process of “peer review”, which is an unbiased evaluation of the work by other scientists in the field who are unconnected to the project. Peer review is critical for maintaining the rigor of scientific studies and provides the authors of the work a thoughtful outside perspective from other experts in their field. Because this study hasn’t yet undergone peer review, reviewers might request additional work prior to publication to clarify some of the results or even request further examination of the organoids that represent adolescent- and adult-onset HD. So you can think of this study like an unfinished book at the moment – we’ll have to tune back in after its final publication to get the full story. Do these developmental changes ever normalize? While the organoids are very cool because they can tell us about HD-related changes at the cellular level that occur early in development using human cells, we really need data from patients to interpret the effect that any changes may or may not have on a fully developed human. Another study did just that and examined the sizes of different brain structures of children and adolescents (age 6 to 18) with and without the adult-onset form of the HD mutation using MRI. These are kids with no symptoms of HD, whose parents have agreed to allow them to participate in research to better understand the very earliest changes caused by the HD mutation. This study reported a larger striatum (one of the primary brain regions affected by the mutation that causes HD) in HD mutation-carrying kids early on, from age 6 to 11, while HD gene-negative kids have a larger striatum later, from age 11 to 18. So it seems that the gene-positive kids have more rapid neurodevelopment, at least of the striatum, but that gene-negative kids eventually catch up and end up having a larger striatum at the ages examined in this study. However, this difference appears to be quite modest, with only about a 1mL swing – about ¼ of one gummy bear for perspective. Studies like these that use non-invasive methods capable of detecting very small changes are exactly what’s needed to assess the contribution that HD has on brain development. They will help interpret findings from studies that represent very early development, such as the organoid study in a dish, in the context of human patients. Ultimately, research demonstrating brain developmental changes resulting from HD is new, and while biologically interesting, researchers don’t yet know what it all means in the context of the disease. However, it’s important to remember that researchers are also working discovering mechanisms that can compensate for any brain developmental changes they report.
DNA-based drugs called antisense oligonucleotides, or ASOs, are now in multiple clinical trials in Huntington's disease, aiming to lower production of the harmful mutant huntingtin protein in the brain. Wave Life Sciences has been running parallel trials of two new ASO drugs, administered by injection into the spine. Just before the new year, Wave announced that the drug in the PRECISION-HD2 trial had successfully lowered the concentration of mutant huntingtin in the spinal fluid. The reduction was quite modest, at 12%, so the company will be adding a higher-dose cohort to both its trials. While the investment community seems disappointed that another trial arm is needed, and we need to see the results in full, to us it's good news that there are now multiple huntingtin-lowering drugs in the world. Lowering huntingtin The genetic mutation that causes Huntington's disease does damage to the brain by telling cells to make a harmful protein, mutant huntingtin. Reducing production of this protein - or Huntingtin Lowering - is the biggest focus of drug development in HD. A drug called HTTRx made a big splash a couple of years ago when it was reported that it had successfully lowered the production of mutant huntingtin in the spinal fluid of HD patients. That drug has been renamed RG6042 and is now being tested by Roche/Genentech in the GENERATION-HD1 trial which will hopefully tell us whether lowering huntingtin production slows the progression of the disease. RG6042 is a drug made from DNA that interrupts the protein production chain. DNA drugs like that are called antisense oligonucleotides or ASOs. Wave Life Sciences was the second company to start testing ASO drugs for Huntington's disease. Wave wants to achieve the same aim – lowering mutant huntingtin – but with a twist. Every person has two copies of the huntingtin gene - one inherited from mom, and one from dad. One abnormal copy is enough to cause HD by causing cells to make the mutant protein. But those cells also produce the normal or healthy version of the protein. Scientists call this healthy version of a gene or protein "wild-type" because it's the one most commonly seen "in the wild". Roche's RG6042 has equal effects on the mutant and healthy version of huntingtin - it cannot distinguish between the two production lines and is expected to lower mutant and wild-type huntingtin equally. Wave's ASO drugs aim to target just the mutant version of the huntingtin protein, leaving wild-type production relatively unaltered. This is much harder to do, which is why Wave had to design two different drugs, each targeting a little single-letter genetic spelling differences that are sometimes passed down along with the mutation that causes HD. These spelling differences don't do anything in themselves, but they can be used to steer the drug to the mutant side of the protein production line, in people who have the right genetic markers in the right place. Wave estimates that about two-thirds of the HD population will have one or other of the necessary genetic markers to make them suitable for treatment with one of their two drugs. Wave's two trials launched in 2017. They were called PRECISION-HD1 and PRECISION-HD2, testing drugs called WVE-120101 and WVE-120102 respectively. Within each trial, patients were allocated randomly to treatment with the drug or placebo (injection without any drug). Four different doses of the drug were tried as the trial proceeded, which is important to remember as we look at the results of this study. The trials were short - about five months' treatment per patient. The headlines Wave's latest press release sets out the first results from the PRECISION-HD2 trial. The release announces that WVE-120102 successfully lowered mutant huntingtin in the spinal fluid, when all of the active treatment arms were looked at together and compared against the placebo-treated group. Wave's announcement gives a figure of about 12% for the degree of mutant huntingtin lowering. If a drug is working, we expect higher doses to produce a bigger effect. This is called a dose-dependent response, and if you can show it in a clinical trial, it strengthens the case that your drug is doing what you intended. Without giving much detail, Wave's announcement states that the huntingtin-lowering did show a dose-dependent response at the highest doses tested when looking across all of the treatment groups together. To be clear - Wave has not yet released enough information for us to understand exactly how the amount of mutant Huntingtin in the spinal fluid is related to the dose of the drug given in the PRECISION-HD2 study. We expect that, as commonly happens with these small early trials, more data will become available soon and we'll be able to evaluate this relationship. Important but easily glossed-over is the primary reason behind the trial: safety. From the information given, the short-term safety looks good. 'Adverse events' were no more common in drug-treated patients than in those receiving the placebo. In itself, that is a very solid result from this first-in-human trial. Apples and oranges? The first person to climb a mountain has a tough job, but gets lots of cheers. The second person to the summit may have an easier time, thanks to the first person mapping out a route - but is likely to be asked questions like "how did your time compare?" when they get there. It's similar with drugs. Roche's RG6042 was the first ASO drug to lower huntingtin, and two years down the line, we have much more detail about how they did it and the full results of the trial have been published. It's inevitable that Wave's results will be scrutinised to see how they compare. Such comparisons may not be terribly helpful, because of the important differences between Wave's drugs and Roche's – but let's do it anyway and see what we can learn. How does Wave's 12% reduction in mutant huntingtin compare? Well, RG6042 reduced mutant huntingtin by roughly 40 to 60% in patients on higher doses. 12% is less than 40%, so that means the Wave drug is less good, right? Not so fast... Fundamentally, no drug has yet been shown to slow progression of HD, so we don't know how much mutant huntingtin reduction is ideal. Furthermore, we don't yet know whether reducing only mutant Huntingtin, as Wave is trying to do, is going to be more beneficial and safer than RG6042, which targets both forms of Huntingtin. That's why we do these studies - so we can figure out what approach is safest and has the biggest impact on HD symptoms. Another important wrinkle to keep in mind is that the doses of drug used in the two trials were very different - the highest dose in the RG6042 study was 120 milligrams and the highest dose tested in the PRECISION-HD2 study was 16 milligrams - that's a big difference! Based on these results showing their drug was safe at lower doses, Wave has already announced it will now add an extra dosing arm to the PRECISION-HD2 trial, to test higher doses – 32 milligrams per injection. That's twice the amount tested at the highest dose in this trial. So the 12% mutant huntingtin reduction they're reporting may well be a stepping stone to a bigger reduction from a higher dose. Adding extra dosing arms like this is a fairly common strategy in drug development, where it can be very difficult to predict what dose will be ideal, even if very detailed work is done in animals before going into humans. Sometimes it is necessary to keep increasing the dose, guided by some measure of success, until some hint of a problem is seen, then step back to the previous dose and test that in a bigger trial. Testing a higher dose will help Wave find whether greater reductions in mutant huntingtin can be achieved, and whether doing so is safe. It may be necessary to go even higher, depending on what the results of the new 32 milligram dose show. Wave has also added a 32 milligram dose to its other trial, PRECISION-HD1. Because of this, the final results of both trials will now arrive later than initially planned, in late 2020. Mutant, wild-type and total There's another complication to understanding these results: remember that the Wave drug is trying to lower the mutant form of the protein without reducing the wild-type form, whereas the Roche drug is expected to lower them both equally. So even if both drugs achieved the same degree of mutant huntingtin reduction, there is more happening behind the scenes that the headline 'mutant huntingtin' percentage doesn't tell us. We don't yet have any clear idea whether lowering wild-type huntingtin alongside the mutant form makes any difference, and until Roche's big trial completes, we are unlikely to find out. To us, this is another reason to be cautiously pleased that a reduction in mutant huntingtin has been reported, and wait as patiently as we can for more information. Talking of wild-type huntingtin – what can we say about whether Wave's drug succeeded in leaving it unaltered while lowering the mutant version? So far, not a lot! For reasons to do with how awkward the protein is, we can measure the level of mutant huntingtin quite accurately, but there is no direct way of measuring how much wild-type huntingtin the spinal fluid contains. We can measure the total amount of huntingtin in spinal fluid – that's the combined pool of mutant and wild-type. When Wave did that, they found that the drug hadn't altered it. That might seem weird - if they reduced mutant huntingtin by 12%, and didn't change the level of wild-type huntingtin, then surely the total level of protein should fall by 6%? Possibly - but every measurement has error in it, and simple assumptions like that might be built on shaky foundations. What's certainly true is that with a small reduction in mutant huntingtin, it is very hard to say anything for sure about the drug's effects on wild-type protein. At this point, we don't think any conclusions can be drawn on that front. We need more information, from more people, before we can start to understand the relationship between changes in mutant and total Huntingtin in the spinal fluid of HD patients in these studies. Life's complicated One thing we've noticed in the wake of this announcement is a fair amount of speculation on social media and in the news. There seems to be a 'received wisdom' among investment folks that these results should disappointing for Wave. We don't really agree with that position, which seems to have come from an over-simplistic comparison of the headline percentages in mutant huntingtin reduction, and the potentially expensive addition of a new higher dose arm. In fact, RG6042 went through exactly the same process when it was first tested in patients by Ionis Pharmaceuticals. Initially, four dosing levels were planned, but then a fifth, higher dose was added when the trial was already well underway. The main difference here is that Wave has announced their initial results at the same time as the decision to add another dosing arm. Our advice here is – as ever – to take speculation in the news and especially on social media with a large pinch of salt. Try to get your information from many sources, and if things are confusing, it may well be because nobody knows the full answer. As scientists driven by progress towards effective treatments for HD, we are interested above all in facts and data. Assuming Wave's announcement is an accurate reflection of the trial data, it represents an important milestone: for the first time, there are multiple drugs in the world that can lower mutant huntingtin in the spinal fluid of patients. Critically, we have drugs that target total Huntingtin, and others that target only mutant Huntingtin, allowing us to compare the risks and benefits of both approaches, in the only place that matters, which is HD patients. Many questions remain unanswered, and for now we have to be OK with that. What's the best dose of Wave's drugs? Will Wave's drugs slow progression of Huntington's disease? How will they compare with other huntingtin-lowering drugs? These questions will take much longer to answer, and we must be patient and determined to get the trials done and hope that clear answers will emerge. For now, we're cautiously pleased that 2020 has begun with a little ray of light.
Several approaches are being taken to lower the amount of the toxic huntingtin protein as a way to treat Huntington's disease. Last week, a study reported a new strategy that helps target huntingtin for disposal by the cell. This approach is in its earliest stages and requires more testing, but the concept is sure to be investigated further. Nipping the problem in the bud The one good thing we can say about Huntington’s disease is that, unlike many neurodegenerative diseases, we know the exact culprit: the huntingtin gene. Everyone has the huntingtin gene, but people with HD have an expansion in theirs. The huntingtin gene acts as a recipe for the huntingtin protein, a molecular machine with many jobs in the cell. When a person inherits an expanded huntingtin gene, there’s a corresponding expansion in their huntingtin protein. For reasons that are not entirely understood, this expanded, or ‘mutant’ protein is toxic to brain cells. Since we know the exact cause of HD, many treatment approaches aim to nip the problem in the bud: lower the amount of toxic mutant huntingtin protein. The most advanced of these approaches are already in clinical trials (read about them here). But researchers aren’t stopping there – many other approaches are being investigated (read about them here). A new approach A study published in the journal Nature last week reported a possible new way to lower mutant huntingtin levels. Researchers turned to one of the cell’s waste management systems, called ‘autophagy’. Autophagy is an orderly way for cells to recycle unnecessary or damaged parts. The unwanted parts are swallowed up by big bags of digestive juices, and broken down, just like garbage bags left on the curb are thrown into a city garbage truck and hauled away. Imagine if we had a set of molecular 'handcuffs' that could tether the mutant huntingtin protein to the garbage truck? Then it would always be cleaned up, with no chance to accumulate and cause problems in the cell. That’s exactly what a research team from Shanghai set out to find. Molecular handcuffs with the right fit The team started their search with a list of existing drug candidates – things like FDA-approved drugs and natural remedies. This is called a drug library. They stamped each of the small molecules into tiny, clean-cut dots arranged in a grid on a glass dish. Then they turned to a protein called LC3, which is in charge of capturing cargo destined for disposal in the cell. LC3 is like the garbage collector who hangs off the back of the truck, methodically picking up garbage bags around the neighbourhood and dumping them into the compactor. In the study, the LC3 protein was passed over the plate of small molecules in hopes that some of the small molecules would fit the shape of LC3, latch on, and stick LC3 onto the dish. The same process was then done with mutant huntingtin, with some of the small molecules fitting its shape, latching on, and binding mutant huntingtin to the plate. A fancy light-bouncing technique was then used to detect any dots on the plate that had captured both proteins – mutant huntingtin and LC3. The molecules in these dots were the first candidate molecular handcuffs that could link mutant huntingtin to the LC3 garbage collector. To further refine the search, normal (non-expanded) huntingtin protein was also passed over the plate, with the purpose of ruling out any molecules that bound to normal huntingtin. The reason for this is that normal huntingtin has many important functions in the cell, so it makes sense to search for drugs that selectively lower the toxic mutant huntingtin, leaving normal huntingtin alone. Although the research team started with a relatively short list of small molecules for a study of this type, they were apparently lucky enough to find not one, but two that stuck to both mutant huntingtin and LC3. Based on the chemical structures of these ‘hits’, they then came up with two more potential mutant huntingtin-LC3 handcuffs, for a total of four. Do the handcuffs help get rid of of huntingtin? The candidate molecules were first tested in cells grown in a dish. From HD mouse model brain cells, to skin cells from HD patients, to HD patient cells that have been converted to neurons, the molecules appeared to lower the amount of mutant huntingtin while leaving normal huntingtin alone. The same was true in a fruit fly model of HD, and three of the four candidates even reduced mutant huntingtin when injected into the brains of HD model mice. HD patient cells, grown in a dish and converted to neurons, usually die more easily than those from a person without HD. The candidate small molecules improved this somewhat, and also increased the lifespan and climbing ability of HD model fruit flies. In model mice, some HD-like symptoms were also improved. Does this mean we have a treatment for HD? As we have reiterated many times here at HDBuzz, mice are not people and so far, every potential drug that worked in mice has failed in humans. The excitement behind this study lies in the idea of tethering mutant huntingtin to the cellular waste disposal system, an idea that is sure to be followed up and refined as the research moves forward. It is also an approach that could work really well in combination with others that are already being tested. Just as a bath will empty faster if you turn off the faucet and pull out the plug, so reducing the manufacture of huntingtin protein and speeding up its removal from cells could be a powerful combo. One thing raising eyebrows among HD drug-hunters about this study is just how lucky the team was to find two molecules that did what they wanted, even though the library they started with wasn't enormous. That doesn't mean the results are untrue, but it could mean that the test of handcuff stickiness was easier to pass than they thought. If so, there might be unanticipated "off-target" effects if the molecules they found are generally sticky, and just happened to stick more to mutant huntingtin and LC3 than to healthy huntingtin. All this calls for what one of our particularly smart friends called "orthogonal validation". That means that the handcuff molecules need to be tested by an independent team of researchers, to double check that they are as good as they sound, and to check for any potential downsides that the first team might have missed. Since the molecular handcuffs identified in this study are already available to HD researchers everywhere, you can bet they will be added to laboratory tool sets around the world.
An exciting new tool in the fight against Huntington's disease has just been described. An international group of scientists have developed a new, targeted, way to lower levels of the mutant huntingtin protein. Huntingtin genetics: from gene to protein Huntington's disease (HD) is caused by a genetic change - or mutation - in the DNA of a specific gene. Scientists call the gene huntingtin. Like every other bit of DNA in our cells, the huntingtin gene is comprised of four chemical letters, which repeat in unique patterns that give them their unique functions. Those four DNA letters are referred to by abbreviations for their chemical names, 'A', 'C', 'T', and 'G'. Every case of HD is caused by a lengthening of a long stretch of the DNA letters 'C-A-G' very near the beginning of the huntingtin gene. In most people - the ones not destined to develop HD - that 'C-A-G' code is repeated around 20 times or so, for reasons we still don't totally understand. HD arises when a person inherits a lengthened stretch of 'C-A-G', with the disease inevitably arising in people who inherit 40 or more 'C-A-G's. Note that everyone has two different copies of the huntingtin gene - one inherited from Mom and one from Dad. The vast majority of HD patients have a normal copy with a low number of 'C-A-G's, and the mutant copy in which they are longer. Most genes, including the huntingtin gene, are used by cells as instruction manuals for building proteins - tiny molecular machines that help cells do their work. So in the cells of people with the HD mutation, there's two different versions of the huntingtin gene, and those instructions tell the cell to make two different versions of the huntingtin protein. Huntingtin Lowering A major goal of the HD research world currently is to investigate whether "huntingtin lowering" strategies could be effective treatments for HD. The goal of huntingtin lowering treatments is to stop, or slow, the rate at which cells use the information in the huntingtin gene to make the huntingtin protein. Animal studies suggest that if we can lower the amount of huntingtin protein made from the mutant huntingtin gene, we may have a hope of reducing the symptoms of HD. A number of drug companies are using a wide range of approaches to lower huntingtin as potentially new treatments for HD. We've covered the general idea of huntingtin lowering here, with more recent updates about huntingtin lowering drugs called ASOs here and here, and other approaches here and here. And now, ZFPs The biotechnology company Sangamo Therapeutics has been working for a number of years on yet another way of lowering proteins: by controlling whether a gene gets turned on, or activated. Their technology relies on little molecular machines called zinc finger protein transcription factors. That's kind of a mouthful, so we'll just call them ZFPs for short. Just like the other huntingtin lowering technologies we've described before, the goal for researchers using ZFPs in HD is to reduce huntingtin levels in cells. While the basic idea is the same, ZFPs work in a quite unique way, compared to existing huntingtin lowering technologies. Existing huntingtin lowering drugs work by targeting an intermediate step between reading the huntingtin gene's information from DNA and making the huntingtin protein. The information in genes is first read from the DNA, copied into a closely related language called RNA and then translated into the language of proteins. This intermediate RNA message is the target of huntingtin lowering drugs currently in the clinic. But ZFPs, like those developed by Sangamo and their collaborators, work in a very different way. Our cells contain a number of proteins that include tiny little pincers, which are shaped just right for grasping specific DNA sequences. (Nerd alert - the pincers are held together by a zinc atom, which explains the funny name). ZFPs for HD? For many years, researchers have worked towards understanding naturally occurring ZFPs in the hopes that they could reprogram them to stick to new specific DNA sequences. Sangamo have been a leader in this field, and developed a sort of tool kit of custom ZFPs that can target almost any DNA sequence. Why do this, what's the point of making custom DNA-binding pincers? Well, it turns out that we can attach various payloads to these pincers, and some of them do very interesting things to the DNA where they attach. As an example, researchers know that they can fuse a sort of cellular stop sign to zinc fingers, to block the cell from activating the targeted gene. A recent publication describes Sangamo's work developing ZFPs for use in HD, which was a large-scale collaboration with CHDI foundation and a number of HD researchers around the world. After a laborious screening effort, they were able to develop new ZFPs that stick to the huntingtin gene - in the DNA - and block its activation. So, unlike other approaches that target the huntingtin RNA, cells treated with these ZFPs never turn on their huntingtin gene in the first place. Even better, the team was able to develop ZFPs that can shut off expression of only the mutant copy of the huntingtin gene, while leaving the normal copy entirely alone. Sangamo tested their ability to discriminate between one of the lowest CAG sizes that cause HD in humans (38 CAG repeats), while leaving the normal copy of huntingtin alone. Promising results in mice Having proved in cells that their new ZFPs could shut off mutant huntingtin specifically, the team next did a number of very well-conducted animal studies to see if their tool might be useful in the brains of animals that have HD-like mutations. To be comprehensive, they tested two different animal models of HD - one with very rapidly progressing symptoms, and another with more subtle long-term changes. In both cases, ZFP delivery to the brains of mice led to reductions of the huntingtin protein. It also helped some of the symptoms these mice experience, which look a bit like things we observe in HD patients. It's reasonably easy to test experimental drugs like this in mice. Researchers are able to collect brain tissue from animals and study it intensively, but similar studies are impossible in human HD patients, who get quite grumpy if you take pieces of their brain. Because translating mouse studies into humans is so difficult, the team did another set of experiments to determine whether ZFP treatment improved things in a way that we can also measure in people. In fact, using sophisticated brain scanning techniques, the team was able to observe benefits of ZFP treatment in HD mice. These well-established techniques also work in humans, so if we want to test ZFPs in human studies we can hope to look for improvements without the need for removing tissue. What are the risks and benefits of ZFPs? As with every other potential treatment for HD, there are benefits and disadvantages to the use of ZFPs. In theory, it's a much better approach to shut off the protein production from a mutant gene entirely, rather than trying to clean up the RNA and protein afterwards. We don't completely understand which RNA and protein species have toxic effects in cells, so shutting it off at the tap seems like the best approach. Moreover, the data presented by Sangamo and their collaborators shows a very nice ability to discriminate between the normal copy of the huntingtin gene and the mutant copy. Silencing just the mutant copy of the huntingtin gene and sparing the other copy is, in theory, preferable, since we still don't know every risk associated with reducing the normal copy. On the downside, the ZFPs developed by Sangamo and their collaborators are genes themselves, encoded in DNA, that must be delivered to every cell we want to treat. Using delivery of genes to treat a disease is generally known as gene therapy. To be an effective treatment for HD, ZFP gene therapy will require certain interventions. The DNA encoding the ZFPs needs to be packaged up into a virus and injected into the brain. Like any drug, the ZFPs developed by Sangamo and their collaborators could have unexpected consequences. In this case, the simplest concern about ZFPs might be that they accidentally target other genes - besides huntingtin - for lowering. The team conducted quite detailed investigations of this possibility in cells, but of course in the brain things could be more complicated. The best way to determine whether these ZFPs are as useful as we would hope is to run human studies. To support this, Sangamo has established a partnership with Japanese drug giant Takeda, who certainly have the expertise and resources to run such studies. Stay tuned to HDBuzz for any announcements about future studies with ZFPs in HD patients. Take home This exciting new study provides another arrow in our quiver as we tackle huntingtin lowering in the clinic. The study was very well-conducted, and leaves us well-poised to consider testing ZFPs in human clinical studies. It's very exciting to see that brilliant scientists around the world continue to develop new approaches to treating HD. These new ZFPs seem likely to provide exciting benefits compared to other Huntingtin lowering approaches that we look forward to seeing tested in HD patients. Stay tuned to HDBuzz for more coverage of huntingtin lowering therapies!