HDBuzz

Hello from Dubrovnik, Croatia, where the 2023 CHDI Therapeutics Conference will be taking place from Monday, April 24th, through Thursday, April 27th!

This conference is a big one for HD researchers worldwide, from industry, academia, and nonprofit. Dozens of scientists will give talks on all things HD, from genetics, to therapeutics, to clinical trial news.

The HDBuzz Editorial team will be on the scene starting on the morning of Tuesday, April 24th, live-tweeting scientific talks and updates on the progress of clinical trials. Our Twitter updates are compiled below. Continue to follow live updates for the rest of the conference with the hashtag #HDTC2023.

For a summary of last year’s conference, start here: https://en.hdbuzz.net/320 We’ll post summaries in article format for each day of the conference.

Knowing what we need to know

The first talk of the morning comes from Dr. Vahri Beaumont from CHDI, who will give an overview of what we still don’t understand about HD and what we need to know to better develop therapeutics. She is first discussing the history of our understanding of HD genetics and brain changes, from CAG repeats, to loss of brain cells and circuits, which scientists have come to understand through the study of human brain tissue donations and brain imaging.

We have known for a while now that people with HD who have the same CAG number can start to get symptoms at different ages. One reason for this is other genetic differences in a person’s DNA code. Scientists are studying these DNA letter changes to better understand how they might alter HD onset and might also be exploited for making new medicines for HD. Many of these other genetic differences affect “somatic instability,” in which the CAG mutation that causes HD mutates even further in some brain cells, getting even longer. Long CAG repeats in the huntingtin gene leads to an expanded huntingtin protein, which over time can be toxic to different parts of brain cells. Vahri reminds us that there is still a lot we don’t understand about the precise sequence of events which link the HTT gene expansion to the symptoms people with HD experience.

For example, we are still unclear on exactly which HTT gene product is the key player in disease – is it the message molecule? The protein? Protein clumps? Perhaps they all play a role in HD. Another question that remains unanswered is whether the “bad” copy of huntingtin is messing things up, or whether the loss of one “good” copy leaves brain cells without some function. Regardless of these questions, several therapeutic approaches that target HD genetics are already being tested in the clinic. Some focus on total huntingtin, both normal and expanded; others target only the expanded form.

The good news is that there are many different companies testing out all kinds of approaches in the clinic, testing many different hypotheses. Perhaps a combination of these therapies may be the best way to treat HD. Thanks to the generosity of brain donations from people with HD after they pass, scientists are continuing to make breakthroughs to understand the disease in people using these very precious tissue samples.

Scientists are using the most cutting-edge technology to understand what is happening with huntingtin messages and protein in different types of cells and why certain types might be more vulnerable. Using many different kinds of animal models, researchers are building a better picture of what happens inside the brain during HD and how we might intervene.
Animal models also allow us to test interventions like drugs at very early stages of HD.

Vahri points out that there are some limitations to mouse models which don’t show all the symptoms of HD in people. Scientists are continuing to develop and use multiple models so they might best test drugs before they are used in people in the clinic.

One of the biggest goals in HD research is to be able to start treatment before symptom onset. This is not easy, but a very strategic staging system, the HD-ISS https://en.hdbuzz.net/325, will help scientists achieve this.

Data Sharing

Dr. David Howland from CHDI is introducing the first official data sharing session of the conference. It will focus on huntingtin DNA and how our understanding of its structure can inform the development of therapies.

DNA interruptions

First up is Dr. Galen Wright from the University of Manitoba, who will be discussing how small variations in the huntingtin gene affect the course of HD. 2023 marks the 30th anniversary since the mapping of the HD gene, huntingtin. This gene is VERY big. Much bigger than most other genes in our bodies, which can make it challenging for scientists to study.

Galen is recapping what we’ve learned about the HD gene, the tendency of CAG repeats to expand in some brain cells over time (somatic instability), and the other genes that influence this expansion.

Three DNA letters code for a single amino acid, the building blocks of proteins. CAG codes for glutamine. Interestingly, CAA also codes for glutamine and it turns out that most folks with HD have a CAA “interruption” in their CAGs. People who don’t have this CAA interruption in their huntingtin gene get disease a lot earlier on in life, even though the protein coded for by the gene is exactly the same. This happens very rarely, but it suggests there is something about the DNA code which is important in HD. Scientists thought that these CAA interruptions would alter how the huntingtin gene might change through somatic instability, but it turns out that isn’t the case. This means there is still more work to do to understand what is going on.

When people undergo predictive testing, the overall length of the CAG repeats is measured. Although small DNA letter changes can make a big difference in HD symptoms, we are not at a point of measuring this in individuals to understand their likelihood of early or late onset.
Interestingly, there are other diseases which are caused by DNA letter changes in the huntingtin gene, including Rett syndrome and another disease called LOMARS. These diseases also affect the central nervous system like HD.

Galen’s team mined large open datasets which bring together gene association data from many different studies, not necessarily focussed on HD. They found that the huntingtin gene is linked to traits like aging and psychological symptoms. Together, this means that the huntingtin gene is probably important in lots of different roles in our nerve cells and that the biology of huntingtin is complex. Galen rightly points out that the more we learn, the more questions we have about huntingtin.

Dissecting DNA repair

Next up is Dr. Anna Pluciennik from Thomas Jefferson University. Anna’s lab studies how mutations in our DNA letter code occur and how these might lead to disease. The mutations are caused by damage to DNA, which happens an estimated 50,000 times daily! We have evolved many ways to repair DNA to avoid a build up of mutations.

Anna’s team studies a specific type of DNA repair called mis-match repair, which corrects a situation where the 2 strands of the DNA helix aren’t properly matched so the helix structure is a bit wonky. These wonky structures are recognised by special machinery which can then try to fix these problems to correct the DNA letter code. Ironically, in some cases (like with CAG repeats) this machinery actually makes things worse.

Anna’s lab studies biochemistry and she likened it to disassembling a car into its thousands of parts to understand how they all work together. This allows her team to work out details that can’t necessarily be observed in complex cell cultures or in animal models. In her lab, Anna’s team makes a proxy for the HD mutation to understand how repair machinery might recognise and try to fix it. She studies the expansion of CAG repeats, which can cause them to stick out of the DNA helix, a structure known as an “extrusion.”

Using this proxy, Anna’s lab is dissecting which DNA repair proteins do what. This type of dissection is important for future studies that might target such proteins with drugs which could help to treat people with HD. Anna’s work is helping to understand how different amounts of each protein might tip the balance to decide whether the machinery corrects DNA damage as it should, or inadvertently makes things worse.

DNA structure influences function

The final talk of this first morning session is from Dr. Natalia Gromak, from the University of Oxford. Natalia’s team studies special structures called R-loops which may be important in HD. R-loops are formed when the messenger copy of the DNA code, called RNA, is being made. If the message RNA copy interacts with the DNA like a zipper, it forms a sort of bubble in the DNA.

These structures have important roles in certain functions in cells, but can also interfere with things causing disease, so they must be balanced carefully. Very early on, a link was made between R-loops and neurodegenerative disease including ALS. Natalia’s group has generated a list of proteins that interact with R-loop structures in hopes of understanding their roles in biology and how these might go wrong, causing disease. 50+ diseases have repetitive DNA sequences which are expanded – just like in HD.

The Gromak lab found that R-loops are formed in regions with repetitive DNA, and have studied R-loops in Friedreich’s ataxia. The question for this conference of course is whether R-loops play a role in HD. Natalia’s group found that there are more R-loops in blood cells derived from HD patients, and found the same result in HD mutation-bearing neurons grown in a dish. There is also more DNA damage in both of these cell types. Next questions for the team are whether R-loops form on the repetitive sequence in the HD gene, whether they can affect the further expansion of this region (somatic instability), and whether huntingtin lowering has any effect on the R-loops seen in HD cells.

CRISPR and HD

Up next following a caffeine break is Dr. Michael Brodsky, from UMass Chan Medical School. Michael’s lab uses CRISPR technologies which can be deployed in the lab to make very precise edits of genomic DNA sequences. Targeting the root cause of HD, the CAG expansion in the huntingtin gene, is the most sensical way to treat the disease, but this is easier said than done. Gene editing would be one way to do this, but we’ve had to wait for the technology to catch up.

10 years ago, this was all a pipedream but technologies have improved so rapidly that we are now very seriously studying gene editing as a possible therapy for HD which is very exciting! Michael points out that gene editing is permanent, so much care must go into making sure there are no unintended changes. Another challenge for using gene editing for HD is that the drug must be delivered into neurons, which is no mean feat. The gene editing must also be very precise. This means that ideally only the expanded huntingtin gene is targeted, so there are limited or no changes to the normal huntingtin gene – also a tall order.

Michael’s group is taking two approaches to specifically gene edit the expanded huntingtin gene. The first is to target small letter changes (called SNPs) in the rest of the huntingtin gene DNA which tend to be associated with the expanded version. The Brodsky lab is first trying out these experiments in all sorts of different HD mouse models, results of which suggest that they are able to specifically edit just the expanded huntingtin gene – great news!

An alternative approach to specifically gene edit the expanded huntingtin gene is to actually reduce the size of the CAG expansion back to the normal range. Michael’s group has been successful doing this in HD mice and cells in a dish. There are still some kinks to work out before this can be developed as a potential treatment for HD but they are cautiously optimistic that further research will help to define a path forward.

More CRISPR and HD!

The next talk, from Dr. Ben Kleinstiver of Harvard/MGH, will also focus on DNA editing. He runs a genomic technology development group that is working on how to alter the expansion of CAG repeats and eventually create therapeutics. Ben’s lab focuses on the many ways that CRISPR can be used to make many different types of changes to DNA. They are engineering the CRISPR machinery to tailor these changes even further.

His main research question is, “what genome editing tools can be used to alter or shorten CAG repeats?” The lab takes different approaches to cutting repeats, interrupting them, or replacing single DNA letters or sequences. Because CRISPR evolved as a way for bacteria to combat virus attacks, there are still some limitations to using the CRISPR machinery to treat diseases. Ben’s group is working on overcoming these limitations to allow better access to different parts of the HD gene. Techniques include using different types of DNA-cutting or letter-replacing enzymes, and applying different methods to direct them at DNA sequences. Then they measure whether CAG repeats get shorter. The goal is to fine-tune the editing and customize it for the huntingtin gene.

This is Ben’s first HD conference! It’s exciting to see how CRISPR experts are directing their efforts towards HD. As technologies continue to advance we hope they can be applied to future human therapeutics.

Even more CRISPR!

Next up is Kathryn Woodburn from Life Edit Therapeutics who will give the last talk before we break for lunch. Kathryn works on ways to target the expanded copy of the huntingtin gene with editing technologies. Life Edit Therapeutics is looking at how different versions of the CRISPR machinery, especially those found in plants, can be used to customize editing of the expanded huntingtin gene.

Their approach to treating HD will be to use viruses to deliver their editing machinery to the brain. So far they have tried this in different kinds of HD mice with different versions and different doses of their gene editing drugs. They are able to decrease levels of harmful huntingtin protein by 40% while leaving the healthy protein intact! To get expanded huntingtin gene specificity, their approach is to target specific DNA signatures which are only found in the expanded version of the gene. Life Edit Therapeutics are looking at a few different signatures to do this and so far the data looks promising.

Making sure that there are no unwanted off-target effects is a challenging task and the scientists at Life Edit are working to get this figured out as quickly as possible. That’s all for the morning’s session!

HD Genetic Modifiers

Day One’s afternoon’s session will focus on progress being made in the study of HD genetic modifiers.

Understanding MSH3 in HD

Large scale human genetic studies, known as GWAS, have allowed researchers to identify these genetic modifiers, other genes that influence when HD symptoms begin. The first talk is from CHDI scientists, Dan Felsenfeld and Tasir Haque, who will be telling us all about their big team effort studying a gene identified in the GWAS called MSH3, and how they might be able to make drugs targeting this protein.

MSH3, as you may remember from the earlier talks, recognises wonky bits of mismatched DNA which need to be fixed. The CAG expansion in the huntingtin gene is prone to create these wonky bits, and it’s thought that MSH3 activity at the huntingtin gene may inadvertently increase the number of CAG repeats in brain cells (somatic instability). Scientists think that MSH3 could be a good target for medicines as switching off this gene seems to be beneficial in animal models of HD, as it reduces somatic instability – the expansion of the CAG number – in the huntingtin gene.

Completely switching off a gene is quite challenging to do in people, so instead scientists are making so-called “small molecule” drugs, which could potentially be taken by mouth, that aim to stop MSH3 working so well in cells. Dan’s team has considered different ways to inhibit MSH3 and created a toolbox of materials and protocols to study their small molecules. This will help other researchers hoping to make drugs targeting MSH3. To make better drugs, it helps to be able to “see” the MSH3 protein. Using clever techniques, it is possible to create 3D models of the protein, and then the scientists can see where and how their molecule binds.

Tasir Haque is now showing some animations that zoom in on different parts of the MSH3 protein and where the drugs fit in. Lots of squiggles that have great meaning for structural biologists! Using these models, they can work out how to better improve these early stage drug molecules to better fit into all the nooks and crannies of the MSH3 protein surface, which should improve their properties.

Drugs targeting MSH3

Next up is Caroline Benn from LoQus23 Therapeutics, a company that is also working to develop drugs targeting MSH3 – it’s a hot area! LoQus23 is taking a slightly different approach to the CHDI program on MSH3 – they are making molecules which target a different region of the protein. This is good news for the HD field as it’s great to be able to test out multiple approaches! Although their approach of targeting different regions of MSH3 is more difficult, they have pulled it off and found two such series of molecules which are very potent and selective, meaning they bind the MSH3 protein very tightly without affecting other proteins.

LoQus23 has also established a way to measure somatic instability in cells in a dish to test how well their molecules work. These are complex experiments which take weeks from start to finish.They will also be able to use this platform to find new targets, besides MSH3, which play a similar role in this part of DNA damage repair which is so important in HD.

Next up is James Fleming from Pfizer. This company is also developing drugs to target the pathway involving MSH3. Pfizer are taking a similar approach to the CHDI team, and like the other folks, have developed a suite of tools and methods to test their molecules for the ability to stop expansion of CAG repeats.

Like other pharmaceutical companies, Pfizer takes a series of steps to screen potential drug compounds, understand how they interact with the proteins they target, and then test them in cells and in animals. They too are using 3D models and chemical tests to show that their drugs can stick to the protein complex that MSH3 is a part of, which has helped them make these molecules better and better over time. The next step is to test these drugs in cells grown in a dish. A lot of this work is focused on the minute details of the protein chemistry, structure, and energetics. Suffice it to say that mathematics figures into the drug development process!

To bring these studies into animals and then later into humans will require a drug with the right properties: the ability to target MSH3, the ability for the body to break it down, and the ability for the drug to get into the brain. Not a small task! After better understanding the properties of a new drug in cells and animals, it may then be tested for safety in people. Right now this is a bit far in the future for all of the compounds presented today, but it’s exciting to see that companies are moving the work forward.

Massive datasets to identify genetic modifiers

Kicking off the last session of the day is Jim Gusella from Harvard, who will be telling us about genetic modifiers of HD on behalf of a large consortium of scientists who study HD genetics.

Jim begins his talk by acknowledging all of the HD families who have so generously shared their data and samples with HD researchers over the years, without whom, these large-scale analyses would not be possible. An interesting finding we have known about for a while now, is that folks with the same CAG number may start to have symptoms at very different ages. Genetic modifiers are markers in the DNA which can explain this early or late onset of symptoms.

More and more evidence is pointing to a particular driver of the onset and speed at which HD gets worse over time: the expansion of CAG repeats in some cells. This process, known as somatic instability, seems to be linked directly to genetic modifiers. The power of these modifier studies comes from the number of patient samples which are analysed – more data means higher confidence conclusions. In the most recent study, 11698 participants’ data were analysed which is amazing!

A problem with HD research, and science in general, is that many of the samples analysed are from Europeans or folks of European heritage. In this later dataset, the team is working to include a more diverse group of patients in the data. With such a wealth and diversity of data, it is possible to zoom out on a large scale and make general predictions of how genetic modifiers – tiny changes in other genes – affect when people with HD might reach certain stages of HD.

It is important to emphasize that this is a way to have more confidence about what other genes most affect HD in people. This is different from being able to predict onset or disease course in an individual person with HD. Jim’s team and the genetic consortium are also looking at how subtle differences and “interruptions” in the sequence of CAGs in the huntingtin gene affect the DNA structure and the tendency of the repeats to become unstable and grow longer. The good news from this most recent dataset is that MSH3, the topic of the previous session, is still a very significant modifier by all of the analyses Jim and colleagues used. That lends a lot of credibility to all of these approaches targeting somatic instability and trying to stop the expansion of the CAG repeat or shrink it.

CAG expansions in specific brain cells

Next up is Nathaniel Heintz, based at Rockefeller University, who will be talking to us about his work on understanding which genes are switched on or off in HD. The Heintz lab developed a series of techniques that allow scientists to “sort” the nuclei of many different cells and look at genetic messages in many cell types. This has become an important way to study why certain cells are most vulnerable in HD and other diseases. These analyses use post-mortem brain tissue samples, enabled by the amazing generosity of the HD community.

The striatum, an area in the center of the brain, is most deeply affected by HD. Heintz’s team is able to sort through different kinds of cells in the striatum and have discovered that CAG repeat expansion occurs most often in one type of cell, medium spiny neurons.

We have known for a long time that medium spiny neurons (MSNs) are lost in great numbers in HD. There are different types of MSNs and oddly it turns out that both those which are vulnerable in HD and those which survive are subject to CAG repeat expansion. The reason these medium spiny neurons seem to have higher levels of expansion could be due to the higher levels of MSH3 which are found in these cells but that link is not yet proved. Medium spiny neurons also have huge numbers of genes that are switched on or off in HD – 1000 turned on and 500 turned off! Many of the genes affected are involved in DNA damage repair – again reinforcing the important role this likely plays in HD.

Ongoing work is addressing the questions of when medium spiny neurons are affected in HD and how best to intervene. They are also looking at other brain areas and going layer by layer to understand exactly which types of cells are becoming damaged or lost.

Speed of CAG expansions

The last speaker for the day is Steve McCarroll, from Harvard. Steve’s lab looks at which genes are switched on or off at the levels of single cells, instead of a big mixture of loads of different types of cells – an incredibly detailed approach. He uses a fruit analogy to talk about the power of this technique – you can compare cell types like different kinds of berries, the same cell type in different people like apples to apples, the differences between different cells of the same type, like looking at two blueberries.

From these single cell analyses, they can work out which cells disappear over the course of HD, confirming previous findings that medium spiny neurons and spiny projection neurons are the most vulnerable kinds of nerve cells. They can also work out exactly which cells have CAG expansions – this seems to suggest that the vulnerable medium spiny neurons are the most likely to have the most expansion. The CAG expansion during a person’s lifetime in these cells seems to be very specific JUST to the huntingtin gene, and only the HD gene, not other genes which have similar kinds of DNA code. The majority of these vulnerable brain cells have moderate expansion in the CAG repeat, but a small subset have huge expansions which scientists haven’t quite figured out the reason for just yet.

According to the McCarroll lab’s data, the moderate expansions seem to happen very slowly over time, but the more exaggerated expansions happen much more rapidly. The key question is, at what threshold of CAG repeat number does the expansion speed up, causing damage and death of these vulnerable nerve cells? To try and figure this out, the McCarroll lab can compare individual neurons with different CAG numbers, and split them into groups to better understand what lengths are most problematic. They grouped cells by CAG number and strangely didn’t see too many differences in genes turning on and off at lower repeat lengths. The most profound changes occur in cells with very, very long CAG repeats, more than 180.

McCarroll is proposing a very different way of thinking about HD pathology and how the disease works over time. There is some interesting chatter in the audience! But this is why it’s so good that all these scientists can get together at this meeting to debate all these ideas.

Tune in tomorrow!

That’s all for today, folks. We’re breaking for the night, but will be back tomorrow morning! Remember to continue to follow live updates for the rest of the conference with the hashtag #HDTC2023.

Top line results of the PROOF-HD study, run by Prilenia Therapeutics and testing pridopidine, have been announced at the American Academy of Neurology convention. Sadly the trial outcome was negative. We recap the history of pridopidine in Huntington’s disease, review the trial results, and figure out where this disappointing result leaves us.

The drug: pridopidine

Pridopidine has been under investigation as a possible treatment for Huntington’s disease since the early 2000s and has a long and colourful history. It was initially developed by the Swedish company Neurosearch who called it Huntexil.

Neurosearch thought pridopidine was able to stabilise levels of dopamine, which is important for movement control. They hoped it might therefore suppress involuntary movements and improve voluntary movements. They ran two trials called MermaiHD and HART, but the drug did not show conclusive benefits for movement control.

In 2012, Teva Pharmaceuticals purchased the right to develop pridopidine, and ran a third study called PRIDE-HD which tested several different doses of pridopidine, again with the aim of improving movement function.

The PRIDE-HD study ended in 2016 with a negative result for movement improvement, but a curious finding when the data were scrutinised after the event. For one of the dose levels tested, there was an apparent stabilisation in a clinical score called total functional capacity or TFC.

TFC is a score out of 13 that estimates someone’s ability to work, undertake household tasks, care for themselves and so on. TFC tends to decline steadily as Huntington’s disease progresses, and a drug to slow or halt the decline in TFC would be very appealing.

One mystery at the time was how pridopidine might have a beneficial effect on function without actually impacting movement control, which is what it was supposed to do.

The twist: a change of mechanism

While Teva was studying pridopidine in the PRIDE-HD study, its scientists were making new discoveries about how the drug actually worked.

Unexpectedly they found that its main action was nothing to do with dopamine, but was instead targeting a protein called sigma-1 receptor or S1R, which is involved in helping neurons survive under conditions of stress. You can read about this in detail in this HDBuzz article.

These findings about pridopidine caused a rethink about what the drug might be able to do in the brain. Improving movement control would be a symptomatic benefit, while extending the survival of neurons would be a disease-modifying result that could actually slow progression of HD.

Prilenia and PROOF-HD

The rights to pridopidine were then moved to a new company called Prilenia Therapeutics. Buoyed by the new findings about S1R, Prilenia launched the PROOF-HD trial in 2020.

PROOF-HD would be pridopidine’s fourth attempt to meaningfully impact Huntington’s disease. The trial enrolled 499 participants with HD and tested one dose of pridopidine (45mg per day) against placebo.

The primary outcome measure was TFC, so the trial was asking whether pridopidine could slow the progression of HD over 15 months, by comparing TFC changes for participants receiving the drug or placebo.

PROOF-HD was classified as a phase 3 trial, meaning a positive result would let Prilenia get approval for pridopidine to be prescribed to HD patients.

A negative result

PROOF-HD finished in March of this year, and the top-line results were announced today at the American Academy of Neurology meeting in Boston, USA, by the trial’s Principal Investigator, Dr Andy Feigin.

We won’t sugar-coat this: the trial results were unfortunately negative. The drug did not slow progression of HD as measured by the TFC.

Failing to meet its primary endpoint means that pridopidine will not get licensed by the FDA and other regulatory agencies.

All trials have secondary endpoints, which are measurements of special interest that might suggest the drug is doing something useful even if it doesn’t meet its primary endpoint. Unfortunately Feigin reported that PROOF-HD failed to meet its secondary endpoints, too.

Where now?

The news of a negative result for PROOF-HD will of course be a big disappointment for everyone who took part in the trial and the whole HD community.

The HD community is becoming all too used to hearing news from clinical trials which do not pan out as we would have hoped. But it’s worth reminding ourselves of a few fundamental truths.

First: a negative trial is not a failed trial, as long as we learn everything we can from it. The data from PROOF-HD will help us learn more about the effect of pridopidine and how to design better drugs and trials. Our daily updates from the currently-running HD Therapeutics Conference will give you a very thorough rundown of what’s being developed and what trials are underway.

Second: Every trial will be negative until one is positive. Many other trials are running or coming soon that test drugs targeting solid, known features of HD.

And finally: this community is robust, smart, and determined. We know how to turn sadness and disappointment into a positive, unstoppable drive to succeed. We know how to get right back on the horse and test the next promising drug, to ensure that not a single day is lost in bringing about the future we are all working for: a future we create together where we have safe, effective treatments to slow or prevent HD.

The most obvious changes related to Huntington’s disease (HD) happen to neurons, the nervous system’s messenger cells that send and receive information throughout the brain and spinal cord. However, many different cell types are affected by HD. A recently published article reviewed research findings from various labs, describing how a specific type of brain cell, called an astrocyte, contributes to HD. This review article details why researchers need to pay attention to more than just neurons to develop effective treatments for HD. Let’s find out why!

The brain – more than just neurons

The huntingtin (HTT) gene is found in every cell in our bodies. That means that the expanded CAG repeat within the HTT gene that causes HD is also found in every cell. But there are certain organs, like the brain, that are more sensitive to being harmed by expanded HTT. Within the brain, there are certain regions that have proven to be particularly vulnerable in HD, such as the striatum – a portion of the brain that sits almost exactly in the center and helps control things like decision-making and voluntary movement.

The striatum is made up of various cell types, including neurons, which we hear a lot about in HD. Neurons are the tree-shaped cells that transmit electrical signals allowing us to think, feel, and move. They also happen to be the cell type that’s most affected in HD, losing their ability to function as the disease progresses. But the most abundant cell type in the striatum isn’t neurons, it’s actually a cell type called glia.

Glia are support cells that act to keep the brain healthy. There are several different types of glia, and newer evidence in the HD field has taught us that these different cell types also play a role in HD. Understanding how they contribute to HD and how they’re affected throughout the disease could help develop new therapeutics.

Astrocytes are stealing the limelight

A particular type of glia that has gotten a lot of attention in HD research lately is a cell type called astrocytes. These are star-shaped cells that support neurons by making sure they have a nice environment in which they can thrive – they balance the chemicals around the neurons, provide nutrients, and protect them. An astrocytes motto is, “happy neuron, healthy brain”!

It takes a lot of astrocytes to make sure neurons in the brain remain happy and healthy. About 20-40% of the brain is made up of astrocytes! Unlike neurons, astrocytes continue to multiply throughout their lifetime. Also unlike neurons, astrocytes don’t die in large numbers over the course of HD, but they do appear to change. These changes are thought to alter their ability to support neurons and keep them healthy. Ultimately, those changes might contribute to the unique vulnerability of neurons in HD.

To summarize what the HD field has learned about astrocytes, Dr. Baljit Khakh from the University of California, Los Angeles and Dr. Steve Goldman from the University of Rochester teamed up to write a comprehensive review of scientific findings from the last 10 years on this topic. Drs. Khakh and Goldman are both experts in neurodegenerative diseases and have largely focused their careers on studying glia and astrocytes. Their review covered what we know about astrocytes from human brains and mouse models while suggesting how we can use this information to develop therapeutics.

Chicken or the egg

Tissue samples from people who have generously donated their brains to research have been instrumental in our understanding of astrocytes. These precious samples have taught us that in the HD brain, astrocytes change shape and lose the molecular “tags” that contribute to their unique identity. These changes in astrocytes progress with HD severity and are thought to reduce their ability to function properly. However, it’s not clear from human brains if changes in astrocytes are a cause or consequence of the vulnerability of neurons in HD.

To understand cause and consequence between astrocytes and neurons in HD, scientists turn to animal models. Animal models allow researchers to ask and answer intricate biological questions that can’t be done with human tissue samples.

Astrocytes – both cause and consequence

When they looked at astrocyte shape and function in HD mice, researchers found similar changes to what they saw in human brains. Researchers also discovered that changes in astrocyte shape were observed before neurons lose the ability to communicate. Additionally, researchers noted changes in potassium and calcium levels produced by astrocytes. Neurons use these elements to communicate throughout the brain and body. These findings may suggest that HD leads to changes in astrocytes that cause breakdown in neurons.

Using genetic manipulation techniques in mice, researchers lowered only the expanded copy of HTT exclusively in astrocytes or exclusively in neurons. This technique allowed them to tease out which cell type causes specific consequences of HD. Quite a clever tactic! They discovered that symptoms of HD in mice, like changes in behavior, primarily come from neurons and those changes in neurons disturb the function of astrocytes.

However these results seem to be a head scratcher – which comes first and which effects the other? It’s a bit of a chicken and egg scenario. While it’s not entirely clear if astrocytes or neurons are the cause or effect, it is clear that both cell types contribute to certain symptoms of HD and are affected by HD.

Another group used cell replacement to examine the role of astrocytes in HD. They transplanted non-HD astrocytes into a mouse with HD and found that the mice had fewer symptoms and lived longer. They also did the reverse experiment by transplanting HD astrocytes into a non-HD mouse and found that those mice developed HD symptoms. These findings suggest that at least some HD symptoms are caused by astrocytes and that replacing sick astrocytes could be an approach to consider to reduce HD symptoms.

Working together for effective treatments

The studies highlighted in this review paper suggest that animal models accurately mimic HD changes in astrocytes that we observe in humans. From these animals we’ve learned that neurons appear to be the primary drivers of HD symptoms. However, astrocytes themselves can also cause HD changes and their reduced function in HD further disrupts neurons.

The authors suggest that the most effective therapeutic strategies will likely require a two-pronged approach: lowering expanded HTT in neurons while restoring the ability of astrocytes to create a supportive environment in the brain. So while we might hear about certain therapeutic strategies more often, like HTT lowering, scientists all over the world are approaching therapeutics from different angles.

Work in the field is ongoing to fully understand the contribution that astrocytes have in HD. However, so far researchers have shown that both neurons and astrocytes are affected by HD. The cell types work together and understanding how each is influenced by the other can lead to the development of effective therapeutic strategies.

A group led by Dr. Sandrine Humbert from the French National Institute for Health and Medical Research published new work in the prestigious journal Science. Dr. Humbert’s team did some really cool science in mice to look at how both the expanded and unexpanded copies of huntingtin (HTT) affect mouse “symptoms” of Huntington’s disease (HD). Let’s break down the experiments they did and what their findings suggest.

Different forms of HTT contribute to HD

The genetic cause of HD is an extra stretch of DNA in a gene called Huntingtin (HTT) that leads to an expanded form of the HTT protein. The vast majority of people with HD inherit one copy of the gene from their parent without HD, and an expanded copy from their parent with HD. This means that for a person with HD, half their HTT protein is completely regular and unexpanded, while the other half is expanded.

The expanded HTT protein is thought to be the cause of HD-related symptoms. However, there are questions about what problems are caused by the presence of expanded protein, versus what problems are caused by the absence of enough unexpanded protein.

To address questions about the balance of expanded and unexpanded HTT protein levels in HD, researchers can use a variety of genetic tricks in mice, to manipulate where, when, and how much HTT protein is made, or “expressed,” in the brain and body.

Previous research in the HD field has shown that just expressing the expanded copy of the HTT protein for short periods during development in mice is enough to cause symptoms associated with HD when the mouse is older. But what’s also interesting is that the same thing happens when the normal, unexpanded copy of HTT is removed from the mouse during development!

Dr. Humbert’s team recently set out to explore in more depth the effects of losing or gaining different forms of HTT using specialized mouse models of HD.

Communication breakdown

Brain cells communicate, in part, by sending electrical signals throughout the brain. To study brain diseases, researchers will often measure these electrical signals to see how well brain cells are talking to one another.

Dr Humbert’s group ran experiments to compare electrical signals in the developing brains of mice with or without expanded HTT. The researchers found differences in these electrical signals when mice were very young. However, the electrical signals leveled out when the mice were older and eventually matched the mice with unexpanded HTT.

This seems to suggest the brain is able to compensate for changes in brain cell communication that it experiences early on in HD. The big question is – does this compensation last long enough and do these changes contribute to HD-related symptoms experienced later in life?

Cause and effect conundrum

HD researchers have long explored this cause-and-effect conundrum, questioning whether certain symptoms associated with HD are caused by the presence of the expanded HTT protein or loss of the regular HTT copy.

Dr. Humbert’s group asked if gaining expanded HTT or losing unexpanded HTT causes the changes they see in brain cell communication in their HD mice. They used creative molecular tools to turn off expression of only the regular HTT copy in nerve cells in the brain.

Interestingly, nerve cells with lower amounts of the regular HTT protein responded in a similar way as those that express the expanded HTT protein, except their electrical signals didn’t level out over time. So it seems the brain cells without regular HTT did worse than those that have expanded HTT!

The researchers think this means that the HTT protein is necessary for communication between brain cells when mice are very young, and the presence of either form (expanded or regular) helps compensate as mice get older.

HTT affects nerve cell size and shape

Next the researchers looked at how the different forms of the HTT protein influence the shape of the nerve cells. Nerve cells are shaped like trees – with a cell body that contains many branches at the top, a long trunk, and a branched “root” system at the bottom of the cell.

They found that nerve cells from mice expressing the expanded HTT copy were less complex and had fewer branches when the mice were very young. As the mice got older though, they caught up in size and shape to match nerve cells from mice without the expanded HTT copy.

Interestingly, when they repeated the experiment that decreased the amount of regular HTT in nerve cells in mice, they had similar findings as before – they matched what was observed in the HD mice, except there was no compensation as the mice got older. This once again suggests that loss of regular HTT causes larger negative effects than expression of expanded HTT!

For this experiment they also added a drug called CX516 that increases the ability of nerve cells to send electrical signals. Using cool science, they added this drug to mouse embryos without HTT in their nerve cells before they were born. Excitingly, this drug improved the shape and size of the nerve cells. This suggests loss of the HTT protein affects the way cells communicate through electrical signals, but when that is restored, the brain can compensate!

Balance is key

The next question the researchers asked was if improving electrical communication in HD mouse brains with CX516 would affect HD “symptoms” in mice. They looked at different tests that examine mouse behavior, like how well mice can cross an open gap or how they travel through mazes. They found that CX516 improved how the HD mice performed on these tasks.

Interestingly, on all of the tests that looked at the effects of CX516 in mice without the regular HTT protein, the mice did worse. So even though CX516 increases electrical communication between brain cells, it seems that in mice without HD this does harm. These results highlight how delicate the communication circuits between cells are and show that tipping the scales too far in the other direction can also be bad.

It’s worth noting that the researchers won’t explore CX516 as a potential medicine for HD. It was previously studied as a possible treatment for Alzheimer’s disease, but it didn’t work very well. It’s more likely that they’ll look at ways to target brain cell communication in the same way CX516 does to further understand how the brain can compensate for HD-related changes.

Compensating for communication deficits

There are a few caveats to this work. The first is that CX516 was given to the HD mice before they were born. This undoubtedly leads to questions about how early we have to treat to see some of these effects.

While it might seem like this work suggests that we need to treat people that have expanded HTT almost as soon as they are born, that might not be the case. The brain is really good at compensation! There are lots of redundant pathways that ensure if something goes wrong, other mechanisms make up for it.

This is why the experiments in this paper saw a leveling off of electrical signals and changes in size and shape of nerve cells as mice got older – the brain was compensating. So treating HD when people are adults might still allow the brain time to compensate for changes related to HTT expression when people are young.

The second caveat to this work is that mice aren’t people! Seeing effects in mice is no guarantee that we’ll see the same thing in humans.

What we learned

Rather than studying HTT lowering for therapeutic gain, this new work by Dr. Humbert’s group lowered different forms of HTT as a tool – it allowed them to gain a deeper understanding of the effects caused by different forms of the HTT protein in mice that model HD. Studies like these could be very informative for therapeutics aimed at lowering HTT, giving us a more complete picture of what’s happening in HD.

This work also details some of the effects caused by the disease and the effects caused by the brain trying to compensate. Understanding that last bit – how the brain compensates – could help find balance in the brain and develop treatments for HD.

Roche released a community letter last month, detailing how their Phase II clinical trial to study the huntingtin-lowering drug, tominersen, is now underway. Learn more about what this means in this article and at the recent HDSA Research Webinar, with representatives from the company.

The ups and downs of huntingtin-lowering

Tominersen is a type of drug called an ASO, which aims to lower levels of the huntingtin protein, and is delivered through spinal injections. People with Huntington’s disease make an expanded form of the huntingtin protein, due to an expansion in their huntingtin gene. By reducing the amount of the expanded huntingtin protein, scientists working on these drugs hope they might slow or halt the progression of symptoms of Huntington’s. Many companies are working on huntingtin-lowering using different types of drugs, including Roche, Wave, uniQure, and PTC therapeutics.

The path of tominersen from the research lab to this most recent clinical trial has certainly been a bumpy one. A study of tominersen which concluded in 2019 was the first to show that it was possible to lower levels of the huntingtin protein. It also appeared to be safe in people for the duration of the 3-month trial. In a subsequent Phase III trial, called GENERATION HD1, more than 800 participants were enrolled to test if tominersen might improve signs and symptoms of Huntington’s. Unfortunately, GENERATION HD1 was cut short due to safety issues. We still don’t fully understand the reasons for this, but participants who received the highest and most frequent dose of the drug did worse by many measures than patients who were given the placebo, the exact opposite of what we had hoped for.

Roche scientists then spent a long time poring over all the findings from GENERATION HD1 and uncovered some trends suggesting that tominersen may have benefitted some trial participants, especially those who were younger and began the trial with less prominent symptoms of HD. This type of analysis where scientists pick back through subsets of the data is called “post hoc”. The original GENERATION HD1 study was not designed to answer the question of whether the drug is better for this category of Huntington’s patients, but there does seem to be a potentially promising pattern. To address this question properly, Roche scientists need to run another clinical trial and this is how GENERATION HD2 came about.

GENERATION HD2 – a fresh approach to questions about tominersen

This new trial will try to answer a few different questions about the possibility of using tominersen as a treatment for Huntington’s, focusing on the safety of the drug and whether it’s properly hitting its target (huntingtin).

* First, scientists will hope to answer if lower doses of tominersen are safe as a long-term treatment for this younger, less progressed subgroup of Huntington’s patients. As with previous trials, lots of different measures will be taken to check for participant safety.
* Second, they will investigate if tominersen has impacts on biomarkers of Huntington’s, things that can be measured in blood or spinal fluid to get a picture of brain health. This will include a protein called NfL, levels of which go up in people suffering symptoms of neurodegenerative diseases.
* Thirdly, they will assess how well the drug is hitting its target in this more focused patient group. This will include a measure of the huntingtin protein itself, which we expect to be lowered, as we have seen in previous tominersen studies.
* Lastly, they’ll also look at how tominersen affects peoples’ thinking, movements, and behaviour.

Everyone recruited into this trial will be randomly assigned to one of three groups, where they will either receive a low 60 mg dose of tominersen, a higher 100 mg dose of tominersen, or a placebo dose. Both doses are lower than the 120 mg tested in GENERATION HD1. As per previous tominersen trials the drug will be given by spinal tap, but in this trial, everyone will receive their dose every 4 months for a total of 16 months of treatment and monitoring. Data collected will be assessed approximately every 4 months by an independent data monitoring committee (iDMC) which will monitor the trial safety and look at the clinical and biomarker data to see how things are progressing. This is confidential, unless there are serious issues, and completely independent of Roche’s own analysis of the data which will happen when the trial ends.

Who will be enrolled into this new trial?

This new trial will last 16 months, and approximately 360 participants will be enrolled. To follow up on their post hoc analysis from GENERATION HD1, this study will be enrolling participants aged 25-50 years old who have only the very early signs of Huntington’s. You may have read the terms “prodromal” or “early manifest,” which is the science-y way doctors and researchers refer to people with Huntington’s right around the time that movement symptoms appear.

The study will take place across 4 continents with sites in 15 countries spread across North America, Europe, South America and Oceania. Precise information about the sites will become available once they are each approved and will be posted on clinical trial directories such as www.clinicaltrials.gov (global) and www.hdtrialfinder.org (North America), but sites are expected in Argentina, Austria, Australia, Canada, Denmark, France, Germany, Italy, New Zealand, Poland, Portugal, Spain, Switzerland, UK and the USA. Each site may have slightly different rules about participant recruitment i.e. how close to the site you need to live to be considered for enrollment, and not all of the sites from previous tominersen trials will participate in the GENERATION HD2 trial. Keep in mind that most clinical trials recruit through strong relationships between doctors and patients.

Those individuals who were previously in a trial testing tominersen would only be eligible for enrollment in GENERATION HD2 if they had received the placebo dose. Roche stated that their decision to exclude individuals who previously received tominersen was not made lightly, and was made “following extensive consultation with HD experts and community leaders.” This news, and the narrower age range for eligibility, may be very disappointing for some. But Roche is committed to answering important safety questions about tominersen, based on previous data. Although this trial will focus on younger people with less advanced HD symptoms, Roche emphasised that they have not forgotten the complete range of patients which comprise the HD community, nor the commitment of previous participants, and there may be other opportunities for these folks in future.

How can I learn more about GENERATION HD2?

Roche participated in an HDSA Research Webinar last week where more of the specifics of the trial were discussed, including the precise criteria for participant enrollment and members of the Huntington’s community put their own questions directly to the scientists at Roche. You can rewatch this webinar here until early April, 2023. Stay tuned on HDBuzz for more news as things progress.