Drug hunters have been particularly interested in the repeating C-A-G letters of genetic code that lead to Huntington’s disease (HD). The number of CAG repeats gets bigger in vulnerable brain cells over time and may hold the key for slowing or stopping HD. Many scientists have been asking what happens to HD symptoms if we stop this expansion. Recent work from a group in London led by Dr. Gill Bates examined exactly this, seeking to define the threshold of CAG repeats needed to cause disease. Let’s discuss what her team found!

We’re all just alphabet soup

The genetic code of every living organism is made up of only 4 letters – C, A, G, and T. They’re combined in different ways to make every gene in our body. That’s a lot of diversity for just 4 letters!

Within the huntingtin gene that leads to HD is a stretch of repeating C-A-G letters. People with HD are born with 36 or more CAG repeats in the huntingtin gene. As a person grows older, we know the number of CAG repeats can shift and wobble in some cells, getting bigger over time.

This ongoing CAG expansion is called “somatic instability”. This specifically happens in brain cells damaged by HD. It’s important to note that the CAG repeat size is relatively stable in blood. So a blood test showing 42 CAGs at the age of 18 will very likely still show 42 CAGs at age 50. But the brain cells of that person could have more than 100 CAG repeats, and a few may even have 200 repeats or more.

Expansions may be the key

Some scientists think that preventing CAG repeats from increasing in the brain may be key to stopping HD altogether. But no one knows how many CAGs are too many in the brain, or at what age CAG increases start to happen.

Several important genetic studies in the past few years have suggested that longer CAG repeats could help explain why brain cells die in HD. For example, people who develop HD earlier or later than expected have changes in genes that impact somatic instability of the CAG repeat in huntingtin. These genes are called “modifiers” – they modify the age at which someone starts to show symptoms of HD.

What’s interesting is that modifier genes mostly participate in the same process in the body, called mismatch repair, which is known to affect somatic instability of the CAG repeat. Very suspicious! This suggests that somatic instability of the CAG repeat is pretty important in HD.

Since somatic instability in brain cells may contribute to how these cells die, and since mismatch repair genes impact somatic instability, HD researchers are now very interested in drugs that target mismatch repair genes. Perhaps by targeting the right mismatch repair gene, we can stop somatic instability of the CAG repeat in vulnerable brain cells. The hope is that a drug which does this could slow or stop HD.

A numbers game

It turns out that we can stop somatic instability in the brain! At least we can in mice, for right now. Several pharmaceutical companies are developing HD drugs targeting mismatch repair genes and somatic instability in HD (for example, LoQus23, Rgenta, and Voyager Pharmaceuticals).

But no one really knows how long a CAG repeat must be to damage brain cells, or how early you might need to stop somatic instability in people as a treatment for HD. Recent studies in HD mice have tried to help answer these questions by looking at the impact of stopping somatic instability in HD mice with different CAG repeat lengths.

What’s helpful about HD mice is that they are born with many more CAG repeats than people with HD – because HD researchers want mice to develop symptoms of HD much faster than people do. For example, a type of mouse that models HD called “Q111” has over 100 CAG repeats. Another HD mouse model called “Q175” has about 185 CAG repeats. Both the Q111 and Q175 HD mice show symptoms of HD in less than a year.

Defining the threshold

Researchers think this threshold of about 100 CAGs may be the number of repeats needed to kill brain cells in people with HD. So what happens if you stop somatic instability in these HD mice? Do the mice get better? The answer for mice born with 185 CAG repeats, surprisingly, is no. They still develop HD, even when somatic instability is halted.

In a newly published study from the lab of Dr. Gill Bates at University College London, Q175 mice having about 185 CAG repeats were altered so that they didn’t have the mismatch repair gene MSH3. MSH3 is a high priority target for HD drug hunters since somatic instability stops altogether when MSH3 is gone.

As expected, somatic instability stopped almost completely in the brains of Q175 mice when MSH3 was eliminated. But these mice still developed features of HD, even though MSH3 was eliminated and somatic instability of the CAG repeat was halted.

What could this mean? Shouldn’t stopping somatic instability prevent the mice from developing HD? Gill’s group reasons that mice born with 185 CAG repeats already have too many repeats in the brain, so stopping expansions below 185 CAG will probably be necessary to treat HD in people.

This parallels the conclusions of a previous study which eliminated MSH3 in Q111 mice that have 100 CAG repeats, fewer than the 185 CAG repeats studied by Gill. In this other study, Dr. Vanessa Wheeler showed that Q111 mice without MSH3 have no somatic instability and have improved cellular markers of HD. So stopping somatic instability in brain cells before they reach 100 CAG repeats may be necessary for this strategy to work in people.

When should we treat HD?

This begs the question many people are asking lately: when should we treat HD? How early would a person with HD need to be treated to stop their brain cells from expanding across the threshold of 100 CAG repeats? Some brain cells appear to have 100 CAG repeats before people start to show measurable symptoms of HD. So it may be necessary to treat people even before they start to develop symptoms.

Treating people before they develop symptoms of HD poses lots of difficult questions that no one quite has the answers to yet. However, many brilliant scientists are now looking at CAG repeats directly in brains of people with HD to find answers. These insights detailing the threshold of CAG toxicity will help scientists to design better drugs and upcoming clinical trials to target somatic instability as a potential HD therapy.

A recently published collaboration between academic researchers and pharmaceutical companies was successful at detecting huntingtin in tears. The scientists were looking for a new, easy way to track Huntington’s disease (HD). If you don’t mind shedding a tear or two, they found it!

Biomarkers – biological metrics in tune with disease progression

Tracking disease progression is not only medically important to ensure patients are living a healthy life, but it’s also important for developing medicines for diseases like HD. Biological metrics that are in tune with disease progression are called biomarkers. There are different kinds of biomarkers, from images of organs, to tests of metabolism, to measurements made in body fluids.

Biomarkers are tools that researchers can use to assess how well a potential medicine is working. If a drug slows or stops the progression of a disease according to one or more biomarkers, it could mean that drug is working!

HD researchers have been working to identify biomarkers that not only track with disease progression, but also change before someone ever starts to show symptoms. Having very early HD biomarkers would allow researchers to know if a medicine is helping someone before they ever start to show disease onset. Since lots of studies are starting to indicate that the earlier we treat HD, the better off someone will be, good biomarkers will be critical for future trials.

How do we currently track HD progression?

We’ve known for a long time that HD causes brain cells to die. So imaging, like MRIs, has been used to track brain cell loss as HD progresses. However, it’s not always easy and convenient (or cheap!) to jump in an MRI machine. There are big advantages to finding easier, more accessible ways to track HD progression.

The HD research field has been moving toward identifying biomarkers in biofluids, like blood and the cerebrospinal fluid (CSF) that bathes the brain and spine. The two most notable biofluid biomarkers for HD have been neurofilament light (NfL) and the huntingtin protein (HTT) itself.

NfL has been detected in both blood and CSF. NfL is released from brain cells when they die. So as HD progresses and more brain cells are lost, amounts of NfL rise. Researchers have shown that NfL is increased in people with HD up to 24 years before they even start to show clinical symptoms! This currently makes NfL our most sensitive biomarker to track HD progression.

Getting more specific

However, NfL isn’t specific for HD. It’s released from brain cells that are dying for any reason. This could make it tricky to precisely follow HD progression if there are other reasons someone might have lost brain cells, like an illness or a hard hit to the head. To specifically track HD, researchers have turned to HTT itself.

Detecting expanded HTT in blood and CSF has been difficult. Overall, expanded HTT isn’t produced by the body in large amounts, so there isn’t much there to begin with. This means ultra-sensitive techniques must be used. HTT is also inside the cell, making it hard to get to in blood. It can be accessed more easily in CSF, but that requires a lumbar puncture. Because of this, researchers are now turning to other biofluids, like tears!

It’s just something in (both my) eyes

No one prefers to get a jab in their vein or back, if other options are available. To see if biomarkers of HD progression can be obtained more easily, researchers from the Netherlands and Germany teamed up and looked at tear fluid.

To get the tears, a small strip of special paper is placed on the lower eyelid, just touching the eye. The tears are wicked onto the paper and the strip is removed after 5 minutes.

Tears contain a surprising number of proteins – close to 1,500! Biomarkers from tears are also being explored to track other diseases, like Alzheimer’s, Parkinson’s, and multiple sclerosis. Because of this, the researchers thought tears might be a good source for HD biomarkers.

They found that amounts of expanded HTT were higher in tears from people that carry the gene for HD, whether they currently had symptoms or not. While their data were quite accurate in determining if someone carried the gene for HD, this test doesn’t appear to be sensitive enough to determine years from symptom onset or distinguish those who are experiencing symptoms from those who aren’t.

A new tool for the box

Finding new and novel ways to identify biomarkers expands our toolbox and offers easier ways for people with HD to track disease progression. Using tears to look at expanded HTT means researchers now have a new tool to examine HD in a fluid that can be collected in a non-invasive way.

Researchers will continue to advance biomarkers that are easy to collect and track with HD progression very early. Having sensitive biomarkers that can be used to measure HD before someone ever shows symptoms will set us up for success when we start testing preventative treatments. When that day arrives, we’ll be ready with tears of joy.

Long repetitive sequences of C-A-G letters in the DNA code are associated with at least 12 genetic diseases, including Huntington’s disease (HD). A group of scientists in Massachusetts, USA, have recently developed a new genetic strategy to study how CAG repeats can lead to harmful proteins being made in cells, causing cells to become unhealthy. Their findings showed that expanded CAG repeats can interfere with a process called ‘splicing’, which chops up and organises genetic message molecules before they are turned into proteins.

CAG repetition

Our DNA is a genetic code that holds instructions for making thousands of different proteins, the molecular machines that run our cells. This code is made of four building blocks or ‘bases’: C, A, G, and T. DNA is arranged like a twisted ladder with two DNA strands bound together in a helix, each made of a string of bases. The bases on one DNA strand pair with bases on the opposite DNA strand to form the ‘rungs’ of the ladder.

HD is known as a ‘CAG repeat expansion disease’. Everyone has a repetitive sequence of C-A-G DNA letters in their huntingtin gene, but people who go on to develop HD have over 36 C-A-G repeats. The number of CAG repeats can increase over time, called repeat expansion, and this seems to happen mainly in cells that get the most unhealthy in HD such as brain cells.

If we can understand exactly how a longer CAG repeat itself makes cells sick, we may be able to keep brain cells healthy and delay when HD symptoms appear. There are also other diseases caused by expansions in CAG repeats, including spinocerebellar ataxias and myotonic dystrophies. Trying to find similarities between what happens in cells affected by these other diseases may help us learn more about what goes on in HD.

Cutting scenes in the genetic script

When a cell wants to make a protein coded by a certain gene, the two DNA strands unwind and separate from each other. Cellular machinery then reads the opened-up DNA base code and makes a copy of it, called an RNA message molecule, a bit like making a photocopy of a recipe from a cookery book.

However, before any RNA message molecules are read by the next set of cellular machinery to make the corresponding protein, an essential process needs to take place. Much like editing out unnecessary scenes from a film to make a final polished version, this process involves editing the RNA message to remove all of the waffly bits of genetic code copied from DNA which aren’t actually needed to make a protein. The process of going from the unedited RNA message molecule to a shorter more succinct message is called ‘splicing’. During splicing, non-essential sections of the unedited message are cut out and the important sections that remain are pasted together to produce what is known as ‘mature’ RNA. This final mature RNA product has only the necessary instructions that the cell needs to make proteins.

Expanded CAG repeats can cause genetic plot twists

In diseases caused by expanding CAGs, the CAG repeat in the DNA is copied into the RNA message, which can cause abnormal proteins to be made. In the case of HD, an extra-long version of the huntingtin protein is made. A group of scientists led by Dr Jain in Cambridge, Massachusetts, previously found that repeat-containing RNA messages, along with the proteins made from them, combine to form toxic clumps in cells which can cause serious damage.

To find out exactly how longer CAG repeats cause the production of harmful RNA and proteins, Rachel Anderson and colleagues within the Jain team recently developed a clever new method to look in detail at the precise genetic message in RNA molecules containing large CAG repeats. Interestingly, they found that CAG repeats in RNA cause mistakes to be made during splicing of that RNA message molecule. Expanded CAG repeats in RNA cause other sections of the message molecule, sometimes far away from the CAG repeat itself, to be cut and pasted into or next to the repeat during splicing.

Here, the expanded CAG repeat can act like the opening credits of a film, into which the final scenes of the film get mistakenly inserted out of order. When this happens, the plot of the film no longer makes sense. Similarly, the final RNA message doesn’t make much sense when other sections of genetic information are inserted into the CAG repeat during splicing. This leads to the creation of many different repeat-containing mature RNAs with unexpected sequences.

The researchers found that the longer the CAG repeat in the RNA message, the more faulty splicing events that occurred. This is interesting as the CAG number in HD tracks with the age at which symptoms start and the rate at which they progress. The researchers showed that when they stopped all splicing events in cells using a chemical, repeat-containing RNA messages did not form clumps in cells and so did not cause cell toxicity.

Protein production glitches

So far, these results explain how expanded CAG repeats lead to abnormal and incorrectly spliced mature RNA messages, but what happens when these messages are read to make proteins?
Any mature RNAs that are ready to be read by cellular machinery to make a protein contain a ‘start’ signal, like a green traffic light. The researchers found that sometimes when repeat-containing RNAs are incorrectly spliced, more of these start signals are found before the repeat, causing many different proteins to be made from a single RNA message than normal. The researchers altered these start signals in the CAG repeat-containing RNAs to turn them off and found that this stopped abnormal proteins from being made.

The researchers also studied the RNA messages containing CAGs that were copied from genes associated with CAG repeat expansion diseases, including spinocerebellar ataxia and myotonic dystrophy. The researchers showed that expanded CAGs copied from these genes also caused abnormal splicing into the repeat, which again contained more protein reading start signals which may cause more abnormal proteins to be made.

What does this mean for CAG repeat expansion diseases?

Understanding how important processes in cells are impacted by long CAG repeats can help researchers piece together exactly how cells become unhealthy in CAG repeat expansion diseases and point to which processes can be targeted with therapeutics. The findings from this study add another piece to the puzzle of what happens in cells, suggesting expanded CAG repeats in RNA interfere with splicing, which can lead to damaging proteins being made.

Importantly, these experiments were performed in cell types, such as kidney cells, which are easy to grow and manage in the lab but are not most affected by HD. Therefore, these cells may not accurately reflect what causes cells to become sick in HD. A lot more work is needed looking at how expanded repeats alter RNA splicing and protein production in cell and animal models of HD. Nonetheless, targeting splicing may be a potentially exciting avenue that researchers can pursue to develop medicines for HD and other repeat expansion diseases.

In two recent studies, researchers looked at how different parts of the brain are affected by CAG expansions in Huntington’s disease (HD) at the level of individual brain cells. The scientists looked at post-mortem brains from people with and without HD to track molecular changes in different brain regions called the cortex and striatum. These studies have provided new insights into what contributes to HD. Let’s get into it!

Specific brain areas are prone to damage in HD

For a long time now, we have known that some areas of the brain are affected more than others in people with HD. Specific types of brain cells in these vulnerable parts of the brain tend to die off more quickly than others, in a process known as degeneration.

However, the underlying reasons for why some cells are affected more than others are not very clear. Lots of researchers from around the world have been trying to figure this out, as it might shine a light on exactly how HD progresses and give us clues as to how we might treat it.

In two recent papers coming from the same laboratory at Rockefeller University in New York, scientists have looked very closely at molecular changes which happen in different types of brain cells in HD. Using generously donated post-mortem brain samples from people with or without HD, the team carefully separated the brain tissue into individual cells.

The two studies focused on different parts of the brain; the first concentrating on a region called the cortex, and the second looking into cells which make up the striatum and cerebellum. Each brain region is made up of lots of different types of cells so they used special markers to sort all the cells and work out which cells were which type. They were then able to measure all sorts of molecular changes from the different types of cells using cutting-edge genetics technologies.

Linking somatic expansion to when symptom start

One of the changes the scientists looked at in each cell was the CAG number in the huntingtin gene. HD is defined at the genetics level as people who have more than 36 repeating C-A-G DNA letters in their huntingtin gene, with most folks with HD having 40-50 CAGs compared to people without HD who have somewhere around 18 CAGs.

For some time now we have known that in certain types of cells this CAG number is not stable and will change over the course of someone’s lifetime, often getting much longer. This process of CAG increases in some cells is known as somatic expansion. It’s important to note that blood cells happen to be a cell type with stable CAG repeat numbers compared to other cell types. So if you received a genetic test when you were 18, that number will almost surely be the same if you were to get tested again at age 50.

Somatic expansion became a hot topic in HD research when studies of genetic modifiers, traits which change the age of symptom onset, pointed to the exact genes which we think control somatic expansion. Together, this suggests there is a link between how big a CAG number gets during the lifetime of someone with HD and how early they will experience symptoms of the disease.

Thanks to tremendous advances in DNA sequencing technology, we can now look at how long the CAG number is in each individual cell. In fact, this is exactly what the team from Rockefeller did. So what did they find?

Cool conclusions from the cortex

In the first study published at the start of this year, the scientists zoomed into a part of the brain called the cortex – the outer part of the brain with all the wrinkles. Studies which have done detailed brain scans of people with HD have shown a thinning of this part of the brain. They’ve also found that connections between brain cells are lost in this part of the brain over the course of the disease and that the cells tend to die early on. Changes in the cortex cause cognitive decline and psychiatric symptoms that many people with HD experience.

The scientists found that a specific type of brain cell, called Layer 5a corticostriatal projection neurons (phew, what a mouthful!), is lost in people with HD. These cells die early during the disease, in both humans and monkeys. While these cells are found in the wrinkly cortex, they connect all the way to the centre of the brain, to the striatum, the region that’s most vulnerable in HD.

Interestingly, the team found that increases in the CAG number happen in many different types of nerve cells in the cortex, including those that remain relatively healthy. CAG increases were seen in the vulnerable Layer 5a brain cells but also in other cells called Betz cells, which aren’t so badly affected by HD. This pointed the researchers to the conclusion that having an increase in CAG number is not enough on its own to cause cells to get sick.

Study surprises from the striatum

In the second study, the researchers focused on the striatum, a brain region in the very center of our heads and the part of the brain most affected by HD. A type of brain cell, called medium spiny neurons or MSNs, are found in this part of the brain and are known by researchers to be the most vulnerable to death in folks with HD.

When the team looked at the CAG number in individual cell types from this part of the brain, they found that the MSNs had the biggest increase in their CAG number. However, other cells in the striatum which are not so affected in HD, such as a type of nerve cell called ChAT+, also had big changes in their CAG number.

The researchers looked at cells from the brain of someone with a similar brain disease to HD called SCA3 (spinocerebellar ataxia type 3), which is caused by an increase in CAGs in a gene called Ataxin3. Folks with SCA3 suffer loss of brain cells but this is not specific to the MSN cell type like it is in HD.

In this disease, they also found that the CAG number increased in the MSN cells, but not other types of brain cells, even though the MSN cells in these folks’ brains weren’t so affected. This means that MSNs may just be particularly prone to expanding CAG repeats, regardless of what gene has the long CAG repeats. However the ever expanding CAG repeat may not be the direct reason that those cells die.

So what does this all mean?

What both of these studies point to is the idea that increasing CAG number in HD could be just one of the necessary steps towards cells getting sick. On its own, somatic expansion might be insufficient to cause that cell to die, as the researchers report CAG expansions in cells not vulnerable to death in HD, like the Betz cells.

Both studies also looked at other features of these cells. They did a deep dive into the genes that are turned on or off in every cell in brains of people with and without HD. What they found is that HD causes global changes in the types of genes that are on or off. This has also been shown by many other researcher groups before. Researchers think that these changes may cause toxicity, affecting the health of the cells, eventually contributing to their death.

The researchers think that connection issues could also be contributing to cell death. In the cortex, they found alterations in vulnerable cells that change the way they can connect and communicate with cells in other areas of the brain. This disconnection not only reduces the ability of one brain area to communicate with another, but it also weakens the cells themselves over time.

Other research groups are still testing the hypothesis that somatic expansion is the major driver of HD. Different scientists are using different technologies to measure the CAG numbers and early previews of these datasets at the recent therapeutics meeting suggest that this can lead to different results. We expect to see a lot more work in this space in the future.

Science only made possible by patient families

It is really important to note that nearly all of the findings of these two very important studies were made possible by the researchers having access to extremely precious post-mortem brain samples. In both studies, the scientists compared brain material from people either with or without HD, who had passed, who generously donated their brains to science to further research. This is an amazing selfless act that has tremendous impacts for research to better understand HD, with the ultimate aim of one day finding a drug to slow, stop, or reverse the disease.

Whilst brain donation is not something that everyone might be comfortable or able to do, if this is something you are interested in doing, the HDSA, HSC, the Brain Donor Project, and other patient organisations, have information and resources about what this decision involves and next steps.

HDBuzz is back for Day 2 of the CHDI HD Therapeutics Conference: Wednesday February 28th in Palm Springs, California. This article summarizes our real-time updates of the conference in community-friendly language.

It’s a brain disease

This morning’s session is titled “It’s a brain disease” and will feature talks about BRAINSSSS! HD scientists are a bit like zombies – they love brains! The session chairs posed big questions to the audience, such as: why does HD affect the brain? Why does it affect certain cells in the brain? And why does HD affect people at the time in their life that it does. This season’s talks tried to address these questions from a variety of different angles.

Christopher Walsh: DNA “signatures” of brain cells in aging and disease

The first speaker of the day was Dr. Christopher Walsh from Boston Children’s Hospital & Harvard Medical School who spoke to us about somatic expansion in the brain. We mentioned that somatic expansion was a hot topic in HD as of late, and many of the talks at CHDI proved that true! Somatic expansion is the increase in the CAG number in some types of cells over the course of someone’s lifetime. There seems to be a link between the amount of somatic expansion and disease progression. Lots of scientists are pursuing this to understand how and why this occurs and also how we might treat it.

Chris’s team look at human brains, generously donated by folks to science. They examine individual cells in those brains, then try to identify single letter changes in the DNA code that are linked to somatic instability.

Over their lifetime, cells accumulate changes in their DNA code, forming “signatures” that scientists can use to classify those cells. Different diseases and cell types have different DNA “signatures,” which researchers can track and categorise. In nerve cells, these changes in DNA signatures happen with age. Chris says this is a bit like a molecular “clock” which gives insight to that person’s degree of aging.

It turns out that the majority of these DNA signature changes are happening in genes that are most important for nerve cells. It also looks like these changes are fairly unique to neurons over other types of brain cells. What’s rather cool about these signatures is that it gives scientists clues about the birth of brain cells, and the common stem cells that gave rise to nerve cells with different DNA signatures. This helps us understand how the brain is formed and how it changes over someone’s lifetime.

Folks in Chris’s team are also using this tracking system to look at how the brain changes in diseases like Alzheimer’s and other neurological disorders, and how this trajectory differs from healthy brains. For example, they were able to use their molecular clock to figure out that brain cells from some people with Alzheimer’s had DNA changes equivalent to around 10 additional years of age compared to people without Alzheimer’s.

Although most of this talk was focused on findings in other diseases, the methods this lab has developed could be an asset to the HD community as scientists continue to pursue DNA repair for HD therapeutics and understand how HD changes the brain.

Mark Bevan: electrical activity in the HD brain

Next on the roster was Mark Bevan from Northwestern University. Mark’s team study HD mice to understand how their brains are affected by the HD mutation, especially an area called the STN, which is thought to suppress movements. Mark recapped some of what we know about CAG repeat expansion: that it may play a role in brain cells getting sick, and that long repeats trigger even more expansion. HD mice with really long CAG repeats have measurable signs of HD – they move and behave differently.

Mark’s lab measures the electrical activity between neurons to understand the activity of different types of cells as mice move around. These techniques involve implanting an electrode into the brain. With this technique, they can get into the nitty gritty of which cells are firing in HD mice and in non-HD mice to unpick exactly what is working and what is going wrong. This is helping them to understand why the HD mice have movement symptoms.

In fact, using special genetic trickery, they were able to make mice without HD move like mice with HD, by stimulating similar patterns of brain cell firing. The challenge is to do the reverse: getting HD mice to move like mice that don’t model HD.

Another interesting finding is that huntingtin-lowering techniques help to restore normal brain activity in some cells but not in others. It also helps to ease the movement symptoms Mark’s team see in the HD mice.

Mark’s work focuses on brain regions that are not normally thought of as top targets for drugs, because they are affected later, or less, in HD. He reminds us that therapies will need to reach widespread areas of the brain. As an electrophysiologist (a scientist who studies electrical brain activity), Mark asks the audience to consider that changes in electrical activity could play a role in damage to neurons. It’s a reminder that a wide variety of techniques give us a clearer picture of changes to the HD brain.

Osama Al-Dalahmah: how HD affects astrocytes

Osama Al-Dalahmah is a neuropathologist based at Columbia University Irving Medical Center. He shared his work looking at how different cells and regions of the brain are affected in HD. There are over 100 different types of cells in the brain! Osama is interested in a type of cell called an astrocyte – a star-shaped cell that helps maintain health and function of neurons.

Osama and his team looked at brains which had been generously donated by folks with and without HD who had passed. There’s been so much data from human tissue this year! This highlights the collaboration between researchers and the HD community. It also gets us closer to understanding HD in the only species we care about – people!

They looked at which genes had been switched on and off in different parts of each brain. They also delved into what was happening at the level of individual cells. Armed with all of this data they could see which genes may track with increasing CAG number and other factors.

The star-shaped astrocytes have genes switched on which suggest they are responding to stress, which we know is happening as brain cells get sick. In fact, Osama’s team found that the more sick brain cells there are, the more the astrocytes are trying to make things better again. As astrocytes work to keep neurons healthy, understanding more about how HD affects them may help us understand how to improve the health of the whole brain.

Osama is trying to identify molecules that might improve the health of both astrocytes and neurons. Its early stages but it looks like his team might have found a molecule that appears to protect HD neurons in a dish!

Matthew Baffuto: understanding CAG expansion through decorations on DNA

Next we heard from Matthew Baffuto from Rockefeller University who spoke about his work on understanding changes to the CAG number in different types of brain cells. One part of this analysis is looking at an area of biology called epigenetics. Essentially, there are inherited decorations on the genetic code that make it easier or harder for a gene to be made into a message or protein.

Matthew’s research uses a special method of sorting out different types of brain cells so they can look at each type in lots of detail and see what molecular changes are happening with disease. As you may have gathered by now, there were a ton of talks using cutting edge technologies to really get into the nitty gritty of what each cell is doing in HD. This is a great example of how HD research benefits from technology developments in other fields.

One of the things Matthew wanted to tease out was which cell types have increasing CAG repeats due to somatic expansion. He looked at the decorative epigenetic markers on genes known to control this, like MSH3 and FAN1, which may sound familiar. The different decorative marks will let Matthew know if levels of MSH3, FAN1, and other DNA repair genes, are likely to be higher or lower in those cell types with somatic expansion. This is a way to examine the cause of lengthening CAG repeats.

Overall, the cells which get sicker in HD had a different array of decorative marks on DNA than cells which are less affected. Many of these decorative mark differences occur on genes known to be important in HD.

Matthew highlighted that changes in these decorative marks aren’t responsible for all the changes in HD in cells. However, his work shines light on how epigenetics can be used to understand how HD affects some drivers of disease, like somatic expansion. We need many different approaches to explain everything that’s going wrong in the cell!

Bob Handsaker – which cells gain CAGs and why?

The final talk of the morning was from Bob Handsaker who is based at Harvard Medical School and the Broad Institute. He studies the very long CAG repeats caused by somatic expansion in the brain. He began by recapping work from Peggy Shelbourne and Vanessa Wheeler who were some of the first researchers to detail that massive expansions can happen in some cells of the brain – up to 1000 CAGs in some cases!

What many researchers are trying to figure out is whether somatic expansion is the main thing that drives HD progression or whether it is involved in another way. Bob’s team uses a fancy type of DNA sequencing which means they can measure CAG numbers up to 1000 CAGs long. Impressive! Bob and his fellow researchers see that somatic expansion really only happens in cell types which get sick in HD. This is one explanation for the vulnerability of certain cells, but it’s a bit different from what other researchers have seen.

Bob shows us a model for how a CAG repeat tract might change from ~42 repeats to 100 repeats and then gets bigger at a faster rate as the person ages. In cells with less than ~150 CAGs, Bob’s team sees very few changes in the genes that are switched on or off. In their analysis, widespread changes only seem to happen when repeats get very large.

They also looked at the speed at which cells gain CAG repeats – it takes decades to reach rapid expansion. By 80 CAGs, it seems cells start gaining more CAGs rapidly, on the order of years. And once they reach 150 CAGs, the cells gain CAGs within a matter of months. In the rapid phase, at 150+ CAGs, the cells become dysregulated. Genes that should be off are turned on and others that should be on are turned off. Bob thinks this leads to toxicity and eventually death of the neurons that undergo this rapid CAG expansion.

Bob and his team are using their model to try and map the trajectory of HD: how many brain cells are lost when different symptoms start to happen? This is a new way of looking at HD and how the disease progresses, enabled by brand new technologies. He shared data from another area called the cortex (the wrinkly outer part of the brain). In these cells they saw a similar two-phase change in CAG numbers – slow and then much faster. This suggests that the process happens across brain regions. It also looks like somatic expansion could be linked to toxic huntingtin protein clumps. Cells which have really long CAGs are also the ones which show clumps.

After so many fascinating talks on somatic expansion, there are some contrasting ideas emerging in the research space. Some scientists believe that CAG repeat expansion is the number one driver of cell vulnerability in the striatum. Others believe CAG expansion kicks off a process that leads to toxicity, but isn’t necessarily the primary player.

Although Bob, Matthew, and other folks have different ideas about what is happening with somatic instability and HD progression, this is not necessarily bad news for the HD research community. In fact this happens all the time in science! Folks have different findings and they battle it out with genuine camaraderie in meetings. Researchers can weigh the strengths and limitations of each piece of data to figure out who might be right. An enthusiastic Q&A followed this talk!

David Altshuler – learning from the Vertex approach to drug discovery

Wednesday afternoon involved a poster session where more than 100 scientists had the chance to present their work to one another, followed by a keynote presentation from David Altshuler, who is the Chief Scientific Officer at Vertex Pharmaceuticals. He is an experienced researcher and pharmaceutical executive who spoke about his work developing drugs for various diseases.

David shared the timeline of discoveries in HD research, mentioning how far we’ve come from the 1980s when some researchers thought we could learn everything we need to know from mice. Slow progress isn’t unique to HD, it’s just really hard to make good medicines for human diseases! Despite tremendous advances, there is still so much we don’t know about human health and disease.

Making new drugs is also super expensive, and many drugs fail during clinical trials, costing time and money to companies and governments, whilst leaving patients waiting without effective treatments. 75% of new medicines fail, and on average it takes more than 12 years to make a drug. That said, David made a key point that it’s encouraging that we have many different technologies available now for the development of medicines. We see this in the HD field with splicing modulators, ASOs, gene therapies and other types of drugs which are all being tested in the clinic – most of these are fairly recent technologies.

As an example of how drug discovery can be successful, he notes Vertex’s efforts on Cystic Fibrosis. The factors that got them to multiple effective drugs are many of the things we already have in the HD field – care centers, natural history studies, and funding agencies that focus on moving research forward. He cited that after 25 years of Vertex working on Cystic Fibrosis, they now have multiple drugs, several of which they developed in just 3 years and can be used in patients as young as 1 month old.

An important point David makes is that genetic therapies should not be the endpoint. These expensive and inaccessible therapies do not serve global patient populations. Vertex is committed to drug modalities which could treat many more people, such as a pill.

David ended with a strong message of hope by saying, “I know this group will find a cure for Huntington’s disease. I don’t know when, but I know you will.”

That’s all for Day 2! Stay tuned for research updates from the last day of the 2024 CHDI HD Therapeutics Conference.