HDBuzz

Recruitment of participants into the US arm of the PTC Therapeutics PIVOT-HD trial has been paused. Since this announcement, there have been a lot of different (and confusing!) headlines about the pause in recruitment. In this article, we will lay out what is going on and what this announcement means.

What is the aim of the PIVOT-HD trial?

The PIVOT-HD trial, run by PTC Therapeutics, aims to test how the drug PTC518 might be beneficial in HD patients, by lowering levels of huntingtin protein. PTC518 can be taken in pill form, and is a type of drug called a splice modulator. This type of drug can change how genetic messages are processed which can affect the levels of the protein molecules they encode.

In the case of PTC518, the drug affects how our bodies process the genetic message made by the Huntington’s gene, resulting in lowered huntingtin protein levels. PTC518 does not discriminate between the unexpanded or the expanded forms of the huntingtin genetic message, so both the regular and toxic forms of the huntingtin protein are lowered. You can find more about this drug from a piece we wrote earlier this year about how PTC518 works.

PIVOT-HD is a Phase 2 study which will test two different doses of PTC518, with the option for a third dose depending on the results, in addition to a placebo control. The study will run for a total of 12 months, with a 3-month dose-finding period at the beginning, followed by a 9-month period in which blood, spinal fluid and other measurements will be taken of the participants, to see how they are responding to the drug.

What did the update say?

On the 18th of October, PTC released an announcement which confirmed that enrollment for the trial is active and ongoing. The announcement also confirmed that the study has approval from both European and Australian agencies to proceed as planned.

However, although enrollment for the trial had already started in the US, this has now paused. PTC reiterated in their statement that this is not due to any bad side effects seen with the drug. The reason for this pause is because the main drug regulation agency in the US, the FDA, has requested that PTC provide additional data in order for the study to continue as planned in the US.

More details on the US recruitment pause

In a November 2nd webinar hosted by the Huntington’s Disease Society of America, PTC’s Chief Operating Officer Dr. Matthew Klein elaborated on the reasons for the yellow light from the FDA. First he explained that the length of time that a drug can be tested in people usually needs to match what was tested in animals. Typically, if a company wants to test a drug for 9 months or more in humans, they must have tested the drug for at least 9 months in animals, and at doses that will match the human study.

When PTC launched the PIVOT-HD trial, they had 3 months of promising data in animals, allowing them to begin a 3 month study in humans. In the meantime, they knew they would be getting the results of their 9-month study in animals, so they could apply to extend the study up to 1-year in people. When they got the results of those longer animal studies, the safety and dosing data remained encouraging, so they applied to regulatory agencies, like the FDA in the US, and the EMA in Europe, to lengthen the study in humans.

Whereas agencies in several countries (for example, Australia, the UK, Germany, the Netherlands) approved the longer study, the FDA in the US said that they would like to see more data in animals before extending the study in people. Exactly what data has been requested is not public information at this stage. At this time, PTC is working with the FDA to move things forward in the US, while focusing their efforts on enrolling the study at active sites in other countries.

What does this mean for the PIVOT-HD clinical trial?

For the study sites outside of the US, everything will continue as planned for PIVOT-HD. As far as we know, once PTC meets the new criteria set out by the FDA, the study will also continue as planned in the US.

You may have seen that some blogs and pharma news sites have written different ideas about what this means for the future of PTC518 as a treatment for HD. Unfortunately, some of these articles are not based on facts but are instead, very speculative.

It’s important to remember that the job of the FDA is to ensure that clinical trials are safe, ethical and scientifically sound. It is not uncommon for additional data to be requested by the FDA prior to trials proceeding, their remit is to work in the best interests of the participants in the trial.

When will we know more?

The next planned update from PTC Therapeutics about this trial will be in the first half of 2023, when we will hear what they have found in the first 12 week portion of the trial. We also anticipate PTC Therapeutics will probably update the community once recruitment in the US begins again.

Whenever there is an update, HDBuzz will write again to keep the HD community informed.

In the past week or so, during and following a big HD research conference, two companies developing medicines for Huntington’s disease announced news about their huntingtin-lowering drugs. First, the pharmaceutical company Roche announced plans for a new clinical trial of tominersen. Then, the genetic medicines company Wave Life Sciences shared early data showing that its drug WVE-003 seems to be hitting the right target in the first human trial participants. Both of these drugs are delivered via spinal injection and are known as ASOs. Let’s talk about the history and the caveats surrounding these hopeful pieces of news.

Updates from Roche

First, let’s recap Roche’s work in HD research in recent years. Roche had been running a global clinical trial, GENERATION-HD1, starting in 2019, to test a drug called tominersen in people with HD. This was a Phase 3 trial, a big study designed to determine whether a drug can slow or stop the progression of HD symptoms. An earlier, shorter trial, run by Ionis Pharmaceuticals had already shown that tominersen appeared safe and that it could lower the amount of huntingtin protein in the spinal fluid of people with HD.

The 800 participants with HD in GENERATION-HD1 hailed from all over the globe, and were split into three groups, known as cohorts. One group received tominersen via spinal injection every 8 weeks, one every 16 weeks, and one received placebo (spinal injections but no drug). Their participation was planned to last for two years, with the option to continue in what’s known as an “open label” trial, in which every participant could choose to receive tominersen regularly after they completed their two year period in GENERATION-HD1.

In March of 2021, Roche stopped dosing people in the trial, because the data were starting to reveal potential safety issues. Tominersen not only had no overall benefit for people with HD, but the people in the 8 week group seemed to be doing worse, symptomatically, than those in other groups. It’s still not completely clear why this is the case, but it could have been due to the high dose of tominersen (120 mg) and frequent dosing leading to an immune reaction that was problematic over long periods of time. This would override the potential benefit of lowering huntingtin.

Wait, so why another trial?

Over the course of the past 18 months since the tough news came about GENERATION-HD1, Roche has been busy analyzing (and sharing) data from the trial, to try and understand what went wrong, and whether there might be a future for tominersen in HD. A glimmer of hope came in late 2021 when a new analysis showed that some participants in the trial might have benefitted from tominersen. Specifically, younger people who began the trial in earlier stages of HD did not seem to experience a worsening of symptoms over the course of their participation.

This finding comes with many caveats because the trial wasn’t designed to split people into smaller analysis groups, so it’s not clear how significant this finding is or if it will hold true for a larger group of people. However, Roche believed the data was hopeful enough to test tominersen again, this time in younger people with earlier signs and symptoms of HD. Taking all the data along with community and expert input into account, they have now announced what the new trial will look like. This happened in a presentation and community statement released during the recent conference of the European Huntington’s Disease Network (EHDN) in Bologna, Italy.

The new tominersen trial

This new tominersen trial will be called GENERATION-HD2, and if all goes as planned it will begin enrolling people with HD in early 2023. This is a Phase 2 study, looking at both safety and how huntingtin levels are lowered with different doses of tominersen. It will also involve three cohorts of participants, all receiving spinal injections every 4 months for 16 months. One group will get a lower dose (60 mg) of tominersen, one will get a higher dose (100 mg), and one will get placebo (injection with no drug). They aim to recruit about 360 people in 15 countries.

We’re waiting for more information on locations and exact guidelines for eligibility, but right now what we know is that the trial will recruit people aged 25 to 50, with very early subtle signs of HD or early movement symptoms. GENERATION-HD1 and the open label extension are now over, but people who previously participated in these trials of tominersen will not be eligible for GENERATION-HD2.

Just like with GENERATION-HD1, an independent data monitoring committee (iDMC) will review the data as the trial progresses. Roche scientists hope that lower, less frequent doses, and earlier treatment could help to overcome the safety issues and might reveal some of the potential benefits of huntingtin lowering with tominersen.

Updates from Wave

At nearly the same time that we got news from Roche that GENERATION-HD2 would be launching, another piece of news regarding huntingtin lowering emerged. This came from Wave Life Sciences, who are also using ASO drugs to try and improve HD symptoms.

Before we can talk about Wave’s news, we have to remind ourselves of some basic HD Biology. Every human has two copies of the HD gene – the one from mom and one from dad. In the vast majority of cases, HD patients have only one mutant (expanded) copy of the HD gene – the one they inherited from their parent with HD. Their other copy doesn’t have the genetic change required to drive the development of HD.

Wave’s approach to huntingtin lowering is a little different from that taken by Roche and Ionis. Instead of trying to use an ASO to silence both copies of the HD gene in treated patients, Wave is only targeting the expanded copy of the HD gene.

We’ve covered the ideas behind Wave’s approach before on HDbuzz, but basically the concept is that preserving the levels of the normal huntingtin protein could be important. Although researchers still don’t understand everything the HD gene does, we know it’s a very important one, and so keeping some healthy protein around might prove beneficial.

A selective approach to treat HD

What’s difficult about this selective approach is that, so far, we don’t have effective ASOs that target the actual HD mutation. So Wave has come up with a clever way to target only the expanded copy of the HD gene, by taking advantage of other tiny spelling changes in the HD gene.

While this technology theoretically lets you only reduce the mutant copy of HD, it has a limitation that a given person must have the right little spelling changes for Wave’s targeted drug to work. So not every HD patient could use any of the ASOs that Wave is currently testing in clinical trials.

An earlier version of their ASO failed in clinical trials; it wasn’t harmful, but it just didn’t lower huntingtin. Wave went back to the drawing board and made a new ASO with different chemistry which they hope will work better. Wave is now running a trial called SELECT-HD to test whether this new selective ASO drug can lower the mutant Huntingtin protein, and if that has benefit for HD patients. This is a relatively small trial – 18 participants have been enrolled to date and they expect around 36 total. Each participant is getting either placebo, or one of several doses of Wave’s selective ASO.

The goal of Wave’s study is to see if the drug is safe, and to try and figure out ahead of time what doses of the drug are effective at reducing levels of the huntingtin protein in the spinal fluid. This trial intentionally doesn’t include enough people to determine whether the drug has an impact on HD symptoms – as we saw with the GENERATION-HD1 trial, that takes many hundreds of participants. Exposing that many people to a drug at this early stage would be unethical, so Wave will dose a smaller group of volunteers with the drug to establish its safety.

Lowering of the HD protein

Last week, Wave provided us with an update from their trial. In a planned monitoring of the efficiency of the drug, Wave was able to see that treatment with 30 or 60 mg of drug led to a reduction of levels of the mutant Huntingtin protein. This is the first time that Wave’s HD ASOs have been shown to lower levels of the target protein.

The data are a little more complicated than we’re used to hearing about from Roche, because Wave has the goal of lowering the mutant HD gene, but preserving the regular one. Scientists at Wave have developed a test that they believe allows them to measure both of these things from the same sample. So, for the first time, we’re able to see what impact a treatment has on the levels of both normal and mutant huntingtin.

Wave shared data in their update which indicates that their drug was able to lower the levels of mutant huntingtin by 20-30%. This did not seem to be matched by a reduction in levels of regular Huntingtin, which stayed about the same. This is, as far as we know, the first time anyone has ever selectively lowered only one copy of a protein inside of a human body.

Increased neurofilament levels?

There was some news in the press release that gave us pause, however. While there weren’t any adverse events associated with treatment with Wave’s ASOs, some participants showed an increase in the levels of Neurofilament in their spinal fluid. We’ve discussed neurofilament before at Buzz – basically it’s a marker of brain cell damage.

In HD and other brain diseases, levels of neurofilament are increased, due to the harm caused to brain cells in these conditions. An attractive idea is to use neurofilament levels to try and see if an experimental drug lowers the levels of neurofilament as the damage to brain cells is halted or reduced.

The Wave study hasn’t run long enough to expect to see a big rescue of the neurofilament levels in HD patients. In the spinal fluid samples from some of the patients in the study, Wave saw an unexpected increase in the levels of neurofilament. No information has been provided by Wave as to why this might be happening but they plan to monitor this carefully.

Long-time readers might remember that in the early days of Roche’s development of tominersen, something similar happened. The people in the first human study of tominersen saw some increases in neurofilament as well. In the case of tominersen, those increases went away over time, for reasons that no one yet understands.

It could be that the increase in neurofilament that Wave is seeing in some of their SELECT-HD study could get back to normal, as happened in the early studies with tominersen, but we just don’t know at this stage. We need more data from longer treatments in more patients to understand what these short-term increases in neurofilament might mean.

Key takeaways

The past few years have been a tough time for people in the HD community. The pandemic years have brought a pandemic of difficult news, most recently with the halting of the VIBRANT-HD trial being run by Novartis with yet another huntingtin lowering approach.

It’s a welcome change to have some more positive news on this front, and good to see advances being made by multiple companies, with distinct approaches to huntingtin-lowering. Until we have effective drugs for HD, we won’t know which approach will be most effective, so it’s critical that we continue testing different approaches to this really tricky problem.

That said, it’s important to remember that huntingtin-lowering remains an unproven approach to treat HD in people. Even if we can safely lower the levels of total huntingtin or just the mutant huntingtin protein in people, we don’t yet know if this will help slow or halt the progression of symptoms. But that’s why we need to run these clinical trials to test out these approaches and hopefully find something which works.

We remain optimistic that at least one of the approaches currently being tested in the clinic or the lab may prove beneficial for people with HD. It’s thanks to the brave and selfless volunteers who participate in this research that we may find a treatment. Stay tuned to HDBuzz for more news on these trials as they develop.

A group of scientists from the EPFL in Lausanne, Switzerland have published a paper in the Journal of the American Chemical Society, describing clumps made up of a fragment of the huntingtin protein. A word that’s commonly used to describe these is “aggregates.” Using very powerful microscopes, the team was able to zoom in and look closely at the details of the 3D structures of these samples. The build-up of huntingtin protein aggregates is thought to be an important feature of Huntington’s disease (HD), contributing to the progression of the disease. But until recently we knew very little about what they looked like. With these exciting new glimpses of aggregates under the microscope, scientists hope to build tools to visualize them in the brains of people with HD, or even send harmful aggregates to the trash can in brain cells.

Aggregates, amyloids and fibres – what does this all mean?

Many diseases affecting the brain, including neurodegenerative diseases like Parkinson’s, Alzheimer’s and Huntington’s, are characterised by the build up of clumps of protein molecules in brain cells. In HD, it is a small and sticky fragment of the huntingtin protein itself that forms these clumps, which scientists often refer to as huntingtin aggregates.

“Aggregate” is a fancy word for when lots of copies of the same protein molecule stick together to make much larger three-dimensional structures. Sometimes these aggregates are a jumbled mess of lots of protein molecules all higgledy piggledy. But other times, the molecules are much more organised and form repetitive structures. Some of these more organised structures look like fibres and are called amyloids or fibrils.

You can think of these different organisations of protein molecules like a tower of Jenga bricks. Each brick represents a single protein molecule. When the bricks are all stacked neatly together into a tower, this looks a bit like protein amyloids or fibrils. But when the bricks eventually fall down into a messy pile, this is more similar to what we think a disorganised protein aggregate might look like.

Scientists are generally (and annoyingly) lovers of jargon so you will see that they often use all these words interchangeably. But for the purposes of this article, we are going to be focussed on huntingtin fibrils; organised three-dimensional fibres made up of lots and lots of copies of a small and sticky fragment of the huntingtin protein.

Of mice and men… and bacteria

Aggregation of the huntingtin protein is a long-documented feature of Huntington’s disease. In brains from people who have passed from HD, we can use dyes and other nifty chemical labels to see these aggregates under the microscope in different types of nerve cells. In animal models of HD, which are genetically engineered to make the small sticky fragment of the huntingtin protein, scientists have shown that these aggregates accumulate over time. In many HD model animals, the level of aggregates in different parts of the brain are associated with the severity of HD-like symptoms.

One of the problems with looking at the aggregates in the brain is that there are lots of other molecules in the cells where we find aggregates, so we generally have to use special stains which stick to the aggregates to see them. However, this approach doesn’t give us very detailed insight into the types of aggregates present or their 3D structures.

To overcome this problem, scientists look at highly pure samples of aggregates which they make synthetically in the lab. Harmless bacteria are engineered by the scientists to be huntingtin protein factories, making lots and lots of copies of this molecule. The scientists can then fish out huntingtin from the bacteria and use these samples to make fibrils in a test tube which look similar to those we see in people. The fibrils can be made with unexpanded huntingtin protein or expanded huntingtin, corresponding to the huntingtin protein with and without the HD mutation. This means that scientists can investigate the effects of the HD mutation on the fibrils.

What can mighty microscopes reveal about these aggregates?

After making these synthetic huntingtin fibril samples, the team of researchers from Switzerland looked at them using a fancy piece of equipment called a cryogenic electron microscope. This type of microscope allows you to really zoom in and see the fibrils in lots of detail. The fibrils are extremely small – only 3-10 nanometers across, about 100,000 times smaller than the thickness of your fingernails – but are easily visible under this type of microscope.

In this study, the scientists took lots of pictures of the fibrils using the microscope and then used special software to average together similar looking images. This averaging process improves the quality of the image, which makes the features of the fibrils easier to see – a bit like changing the contrast or brightness on your phone screen to see the display more clearly.

From these images of the fibrils, the scientists were able to measure their dimensions and work out how all the huntingtin protein molecules were organised. They could see that they were stacked together and lined up into flat ribbons, looking a bit like if you took lots of Jenga bricks and lined them all up next to each other to make a thin, single layer of bricks. Many ribbons of huntingtin are layered on top of each other in the fibrils, which would be as though you added more and more layers of lined up Jenga bricks on top of the first.

Interestingly, the researchers found that the HD mutation led to changes in the dimensions of the huntingtin protein fibrils, as well as changes in the number of ribbons of huntingtin stacked on top of each other. The scientists also made fibrils from an even smaller fragment of the huntingtin protein which is missing a region right at the beginning of the molecule. They showed that these fibrils were much more disorganised and were made up of a mixture of different organisations of the huntingtin protein molecules.

These findings are important because they show that the Huntington’s Disease mutation and other regions of the huntingtin gene affect the 3D structure and organization of huntingtin protein fibrils. Fibrils which are uniform of more disorganised, might gum up the works in different ways so this is important to understand.

How will this work help people affected by Huntington’s disease?

Our in-depth understanding of the structure of aggregates in the Huntington’s disease brain is still somewhat in its infancy but we can look to work in other disease areas to see what promise this type of study can hold (beyond generating really cool images of the fibrils of course).

In the field of Alzheimer’s disease research, this type of approach is now being used to look at fibrils from the brains of patients who have passed. This work has revealed an astonishing level of detail of the fibril structures, showing precisely where each atom is located. Comparing fibrils from people with different forms of Alzheimer’s disease, scientists could see subtle differences in their organisation and showed that there are differences among patients, animal models of Alzhiemer’s disease, and the synthetic fibrils generated in the lab. For other types of fibrils scientists have examined, the variation from patient to patient is significant, although it is not yet clear how this relates to symptoms or disease severity.

Other studies show how brain imaging molecules called PET ligands bind to the fibrils. The Huntington’s field has a PET ligand which binds to fibrils (we wrote about this recently on HDBuzz) but we don’t yet know exactly where it binds on these structures, so maybe one day scientists will be able to use this approach to better understand the PET ligand.

Overall, the work by the researchers at the EPFL is an exciting step forward as we begin to understand more about huntingtin fibrils and has laid a foundation for future studies where we might glean more information about this important feature of HD.

DNA repair and CAG repeat instability

The effect of HTT lowering on CAG repeat expansions

Welcome to last day of the @hdfcures conference! We’ll only be sharing a few talks from today’s sessions, which focus on DNA repair. The first is from HDBuzz’s very own Jeff Carroll!

Jeff will be sharing his work on HTT lowering and how this might influence the stability of the CAG number in mouse models of HD. This is part of a process called somatic instability which we previously wrote about on HDBuzz.

Scientists have found that buildup of HTT within a cellular compartment called the nucleus, where our genetic material is stored, might be driving aspects of HD. This might be because of interactions HTT has with that genetic material – the DNA

It seems that the huntingtin protein molecule is binding to genes which we know are very important in HD. Interestingly, it looks like huntingtin is binding on to the end of genes, where expression of the gene ends. Very spooky!

When they looked to see which groups of genes huntingtin seems to be hanging out near, it looks like these are mainly genes with lower expression in HD and HD animal models. While cool, it’s not clear what this all means just yet.

Now Jeff is switching gears to look at somatic expansion in HD mouse models. His team found that when HTT levels are lowered, the amount of expansion is reduced when they looked in the liver, but in the brain, they don’t see the same effect.

It turns out that the HTT-targeting ASO causes the machinery involved in gene expression, a process called transcription, to be thrown off the DNA at the HTT gene. Scientists have found that transcription is important for somatic expansion so Jeff thinks this might be why the ASO reduces expansion.

Lowering HTT using a different tool, Jeff’s data shows that lowering only the expanded form of HTT prevents expansion of the CAG repeat – somatic instability. This is great news since there’s been a lot of talk at this meeting about the contribution of somatic instability to HD and what it could mean for therapeutic development

It turns out that the HTT lowering ASO also reduces somatic instability at other genes which have lots of CAG repeats. It’s not quite clear what’s going on just yet but Jeff and his team are on the case to follow up on this interesting data.

The role of modifiers in CAG repeat expansion

Our next talk is from Anna Pluciennik, who will be sharing her work on DNA repair and CAG expansions. Anna’s work is focused on understanding how mistakes in reading DNA can lead to additions of CAG repeats.

When the gene has lots of CAGs, like HTT, DNA slips out forming a little loop. This little DNA loop is recognized by molecular machines in the cell that can increase those repeats.

Normally, cells can repair this, but it seems in diseases like HD there are also problems with the proteins that repair these slip outs. Understanding more about these DNA slip outs at the CAGs and proteins that repair DNA could tell us something about the cause of HD.

Interestingly, many genes that modify the age of onset of HD – “modifiers” – also happen to be these proteins that repair DNA. It’s all connected!

One of those modifier proteins that Anna is interested in is called FAN1. Anna and her team can make FAN1 protein in the lab and look to see what other molecules it might be working with. They found that FAN1 interacts with DNA only when CAG slip outs are present. Her lab is doing lots of experiments to find other proteins that are required for this process.

Understanding exactly what’s going on and what proteins are involved will help the team understand if they can disrupt this process to reduce the slip outs. Ultimately, they hope this could help them reduce CAG expansions in HD.

Different forms and fragments of the HTT protein

The last talk of the conference is by Gill Bates, who will put HTT splicing into perspective for HTT-lowering therapeutics. HTT splicing is something we’ve heard a lot about lately with recent trials around PTC-518 and branaplam.

Splicing is the fancy science name for the process by which genetic messages are processed and chopped up before they get turned into protein molecules. If the huntingtin genetic message is spliced differently, then different forms or fragments of the huntingtin protein molecule will be made.

Dr. Bates’ team looked at lots of these different forms and fragments so that they could systematically ask what each is doing. Interestingly, they found that there is one particular fragment – called “exon 1” – which may be super important.

This exon 1 fragment contains the CAG repeats, but is missing much of the rest of the HTT gene. So it seems that this particular fragment may be causing much of the trouble in HD.

Since scientists like to give molecules specific names once they know they’re important, this exon 1 fragment of the huntingtin protein has been named HTT1a.

Using various tools in lab, they have shown that HTT1a is also made into a little protein fragment and can be found in different mouse models of HD. When they looked in brains generously donated from people with HD, they also found this little HTT1a fragment there.

It seems that the HTT1a protein fragment is important for beginning the formation of toxic protein clumps, called aggregates. Aggregates are a common feature in HD in both people and our animal and cell models of HD.

Dr. Bates has focused on developing tools to specifically look at the small HTT1a protein. This has been tricky because HTT protein fragments, like HTT1a, are hard to handle and make in the lab as they are rather sticky.

Interestingly, when they look in certain mouse models of HD over time, they find full length HTT levels go down as the mice age but levels of the HTT1a clumps go up. This suggests the HTT1a fragment becomes more prevalent as the HD mice get more sick.

Gill’s team is also looking at measuring the really enormous full-length HTT protein molecule. There are lots of different ways to do this but nearly all of these experiments get confused by a mixture of expanded and unexpanded HTT.

All of this work is very important because all of the HTT lowering clinical trials rely on these tests to work out if their drug is working or not by measuring changes in the HTT levels in different samples.

One important thing Gill’s work points out is that it’s really critical to measure various forms of the HTT protein – both full length and fragments that seem to be very toxic and contribute to disease.

An interesting question Gill asked was, what happens if we could make a mouse that doesn’t produce the toxic HTT1a fragment given how important it appears to be in HD?

Gill’s team have used some clever genetics tricks to make a mouse which only makes the full-length HTT protein but not the HTT1a fragment.

When they compare these mice to the same strain that DOES express HTT1a and look at protein clump formation in the brain, they find they do eventually form, just much later than expected and to a lesser degree.

While this might seem to suggest that even without HTT1a, mice can form toxic protein clumps, the caveat with this interpretation is that these mice did have a very small amount of HTT1a still present. So that small amount may be driving this pathology.

No experiment is perfect, but these results strongly suggest that a significant amount of the toxicity associated with the HTT protein is because of the HTT1a fragment.

That’s all for our reporting from the @hdfcures conference! HDBuzz looks forward to tweeting future HDF symposia. We hope you all enjoyed following along and we look forward to sharing more HD research with you soon!

To learn more about the Hereditary Disease Foundation, visit their website. To learn more about the science discussed at #HDF2022, tune into a live webinar on September 15th at noon EST! Register here You can also follow HDF on Facebook, Instagram, and Twitter to ensure you don’t miss future webinar updates.

Pre-clinical work moving toward trials

New tools to lower HTT showing promise in animal models

Welcome back! The first talk we will be tweeting about today is from Anastasia Khvorova, who will be telling us about her teams work on lowering of Huntingtin using technology called RNAi.

One of the problems in studying drug delivery to the human brain is that animal models, even large ones, all have much smaller brains than us! Mouse and even monkey brains are tiny by comparison, so Anastasia’s lab use sheep as they have fairly large brains. In these sheep brains, the Khvorova lab can measure how drugs are able to spread and work across the different regions of the brain.

Similar to the approach Wave Life Sciences are taking, the drugs Anastasia and her collaborators are testing target small genetic signatures which means they can lower just the toxic form of the huntingtin protein. However, one of the problems about being so specific in which form of huntingtin you are targeting, is that you need more drug to see the same effect.

We’ve talked a lot about huntingtin protein clumping up in cells, but Anastasia is looking at the huntingtin message – the recipe – forming clumps in the cell’s nucleus. She thinks this could contribute to the lengthening of CAG repeats that can cause cells to become sick.

The toxic huntingtin actually comes in a long and short form. The shorter form is thought to be responsible for the toxic clumps that we see in HD models. Anastasia tells us that we have to be careful when looking at these clumps, because they differ between models and people.

Anastasia and her team have identified compounds which are able to reduce the amount of the short form of the toxic huntingtin. They have added this to their toolkit of compounds which change the levels of important targets in HD including total huntingtin, toxic specific and MSH3, a genetic modifier of HD.

Anastasia thinks this toolkit is an excellent portfolio of different options for targeting HD, which may also help us unpick exactly which protein or protein form is important in the disease progression.

Cell replacement treatment options using stem cells

The next talk we’ll cover is by Anne Rosser from Cardiff University. As a part of the Stem Cells 4 HD initiative, she’ll give an overview of how stem cells are being used to study and potentially treat HD. Stem cells can be used in HD research for various purposes: either as a tool to understand more about HD or, perhaps, as a therapy. Anne’s talk focuses on the latter.

The overall goal of using stem cells as a therapy would be to 1) replace cells that have been damaged by disease and 2) release biological factors like chemicals or proteins that might have been lost during disease, to try and keep other cells in the brain healthy.

The cells that researchers have been interested in using for this cell replacement therapy come from “pluripotent stem cells.” These can be made from the cell of an adult, like a skin cell or blood cell, and they can be turned into almost any type of cell in the body.

We know from older studies using a different type of stem cell that cells transplanted to the brain do a good job of integrating into their new environment. This would be great news for HD, where they hope that cells added to damaged areas will form connections with other parts of the brain

Anne mentions that there are several HD labs moving this technology forward toward clinical trials. However there are challenges with such an invasive approach, including exactly which cell type to use for transplants and how to create a comparison group.

To deeply consider all of these challenges before moving forward, researchers have created the Stem Cells for HD (SC4HD) group, comprised of stem cell leaders from around the world.

The SC4HD group is standing on the shoulders of giants – learning lots from previous studies that transplanted fetal tissue in the HD brain, changing what didn’t work and using what did to move forward logically and safely. To ensure researchers have as much information as possible before moving forward, studies are being done to compare various ways to make striatal neurons, which are the most vulnerable cell type in HD.

There are a lot of variables to consider – controlling between different batches of cells, tracking the cells after they’re implanted, and ensuring they turn into the cell type we want once they’re in the brain. There’s still a lot to work out before we have this technology in humans for HD, but stem cells represent a very powerful source for cell replacement therapies.

It’s an action-packed morning, and we’ll be back after a break, tweeting briefly about a couple of short talks on impactful topics.

Datablitz: presentations from young investigators

Putting mouse models head-to-head: which is the best?

Sophie St-Cyr was selected to give a short talk related to the advantages and disadvantages of different types of mouse models we use to study HD.

There are dozens of models, grouped in different categories based on how they’re created and what HD-like signs and symptoms they have. Sophie compares different behavioral tests in different HD models and across sexes.

As an expert in mouse behavior, she made recommendations to the scientists in the audience about the use of different mouse models and how best to design their behavioral experiments.

At-home collection of samples for NfL detection in blood

Next up is Lauren Byrne from UCL who works on a protein called NfL (neurofilament light), which is released by sick cells and can be used as a biomarker of brain damage in HD. NfL levels go up as HD progresses.

Scientists might also be able to use NfL to track whether a treatment is working, and it is increasingly being measured in clinical trials to check firstly, that there are no safety issues, but secondly, to see if the drug is helping to keep the brain healthier.

Lauren’s work has been focused on developing more practical ways to measure NfL levels. Luckily NfL is a very stable protein so Lauren has developed an at-home finger-prick test to collect blood, and then post it back to the lab through the mail for analysis.

She will be running a study called iNfLuence-HD to study NfL levels and improve methods for measurement, and is also heading up the JOIN-HD registry which aims to study juvenile HD patients from all over the world.

That’s all from this morning’s session. We are breaking for lunch now and will be back with more updates on all this exciting HD research later on this afternoon.

Genes and proteins that modify HD onset

Identifying modifiers by looking at the whole genome

Welcome back! The afternoon session will focus on genetic modifiers of HD, other genes that influence the age that HD symptoms begin. The first talk we’ll cover is by William Yang from UCLA, who studies modifiers in mouse models.

What very large sequencing studies have shown us is that many of the modifier genes that change the age of onset in HD have to do with DNA repair. The question is: how can we harness them for HD therapeutics?

Dr. Yang uses mice to study how we might be able to use these modifiers for treatments in HD. His team has created many of the mouse models that have become the standard in the field.

Dr. Yang’s lab has compared these different models at different timepoints to understand how HD changes within each model over time. In particular, he has done a deep dive on gene expression changes – how levels of genetic “recipes” go up and down.

Another key feature they examine are clumps of the HTT protein, also known as protein aggregates. This is a unique feature caused by expanded HTT that seems to occur mainly in brain cells called neurons.

The Yang lab has recently created a new mouse model that can be used to study different aspects of HD, like problems with sleep, CAG repeat expansion, and damage to specific brain areas. One of the questions they want to answer with this new model is whether genetic modifiers of HD can influence these features, like changes in gene expression or protein aggregation.

A relatively recently identified feature of HD is somatic instability – expansion of the CAG repeat in certain cells or tissues over time. This happens frequently in neurons and might be contributing to why certain types of cells become sick and die in HD.

Adding or removing certain modifier genes in these HD model mice can cause symptoms and features of HD to improve or worsen. This strengthens the case for targeting these genes with drugs in people.

Dr. Yang’s lab has found that altering levels of a specific modifier, FAN1, in HD mice can affect their behaviors, like sleep patterns and ability to walk on a rotating rod. There is also a change in protein aggregation

It seems that just targeting FAN1 alone might not be the answer to HD, but interestingly, when they also target another modifier in these mice, called MSH3, the mice get better in most of the metrics they looked at.

These types of controlled genetic experiments in mice can help to identify and confirm the right targets for drugs that could delay HD onset.

Alfy as a modifier of HD

The next presentation we’ll talk about is from Dr. Ai Yamamoto of Columbia University, who works on a protein called Alfy, that is also a genetic modifier of HD.

Large scale human studies found that tiny genetic changes in Alfy can cause the onset of HD symptoms to be much later. Ai’s lab created a specialized mouse model to study this rare genetic variation in more depth, and these mice also had delayed onset of symptoms.

Alfy is involved in breaking down clumps of harmful huntingtin protein. In both humans and mice, the Yamamoto lab has found that higher levels of Alfy can have positive effects on symptom onset.

They are now finding that Alfy’s role in clearing toxic proteins is highly important in conditions of stress, including in HD and other brain disorders.

The effect of HD on connections between different parts of the brain

This afternoon we’ve got a very exciting session about BRAINSSSS!! The talks focus on different aspects of brain function, measured with very cool new techniques.

First talk is from Dr. Lynn Raymond, of @UBC. She studies how brain cells called neurons communicate. These communicating cells are the ones that die in HD, so understanding how they start to dysfunction can give us clues about how HD arises.

While the HD gene is expressed in nearly every cell of the body, it’s neurons that cause most of the symptoms of HD. And in fact, not all neurons are impacted in the same way. The most impacted include a set of structures within the brain, deeply connected to each other, called the “cortex” and the “striatum”.

Dr. Raymond’s lab has long studied the details of how these two parts of the brain communicate using mouse models of HD. She sees very clear changes in how the HD mice learn to do new movements, something like the problems that happen in HD patients

Dr. Raymond’s lab is using existing new “deep learning” or “artificial intelligence” software tools to analyze the behavior of mice in detail that was not previously possible (@deeplabcut).

To link changes in behavior to changes in brain function, Dr. Raymond uses live, real-time, microscopes that allow tracking of brain activity in HD mice doing specific HD-relevant movements. This is a good example of why we need mice, and other models of HD – there’s no way we could record this level of detail of how brain cells talk to each other in humans with HD.

In HD mice, the cortex, an outer bit of the brain critical for our thinking ability, is hyper-excitable. There’s a lot more activity accompanying movements in HD mice, compared to controls. So it’s as if the brains of the HD mice have to work a bit harder to achieve the goal of the movement. This might be a hint for why some types of movements, and the learning of those movements, are hard for HD patients

That’s all from us for today! We’ll be back tomorrow afternoon to share a few other talks before the close of the
@hdfcures symposium. Tune back in then!

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