Research news

“Somatic expansion” is a hot topic in Huntington’s disease research. Somatic expansion is a process in which CAG repeats lengthen in some cells during aging. It’s thought to control how early HD symptoms appear. A group of researchers from Toronto, Canada recently identified proteins that may play an important role in regulating this process. Understanding how these proteins regulate somatic expansion in Huntington’s disease may hold the key for unlocking therapeutics for CAG repeat diseases.

Repetition is (the) key

Huntington’s disease (HD) is referred to as a “CAG repeat expansion disease” – it’s caused by an increase in the number of CAG repeats in the huntingtin gene. Everyone has the huntingtin gene – in fact, everyone even has a repetitive CAG sequence within their huntingtin gene. It’s just that people who will go on to develop HD have more CAGs within the huntingtin gene compared to people without HD.

But HD isn’t the only disease caused by CAG repeats. There are over 70 different diseases associated with nerve cell breakdown that are caused by repetitive DNA tracts! In a way, this is good, because we can look to the research in these other diseases and find similarities to learn more about HD.

One thing in common with many of these diseases caused by repetitive DNA tracts is something called “somatic instability”, also called “somatic expansion”. This is a biological phenomenon where a repetitive DNA track gets bigger in some cells as the person ages. This ongoing expansion of the disease-causing CAG tract in HD is thought to contribute to accelerated disease progression. HDBuzz recently wrote about somatic expansion, which you can read about here.

For HD, somatic expansion of the CAG repeat tract in the huntingtin gene preferentially happens in brain cells. Specifically in brain cells that are vulnerable to dying as someone with HD ages. Emerging scientific research seems to suggest that if we can get a handle on the perpetual expansion of CAGs in the huntingtin gene, we may be able to keep brain cells healthy and delay when symptoms appear. In a perfect world, even pushing that into the realm of never. But to do that, we first have to understand the intricate biological details behind somatic expansion in HD.

How exactly do CAGs get added?

DNA is made up of 2 complementary strands of genetic material, creating a double helix. This may conjure up images of a gently turning, intertwined ribbon from 8th grade biology. Each strand contains letters of the genetic code – C, A, G, or T – that interlock with the genetic code on the complementary strand like Lego pieces.

When cells need to make a protein coded by a certain gene, the DNA strands are unwound, and the Lego pieces are unlocked. After the protein is made, the DNA strands snap back together, with complementary strands finding their original alphabetic partners.

However, when the DNA contains a repetitive sequence, like a long strand of CAGs repeated over and over, it can be difficult to discern exactly which Lego piece went where. This can cause some of the genetic code to misalign and match with the complementary strand ahead of where it should. This creates a loop-out structure – one strand is nice and straight, and the other has a looped-out piece of DNA with no mate. This is a big no-no in cell biology…

There’s a reason your mind conjures the smooth-sided, intertwined ribbon when “double helix” is mentioned. DNA strands always bind to their complementary mate. DNA is never single stranded. When it is, proteins immediately intervene, chopping out or adding DNA to the looped-out structure that threatens the elegant, softly twisted natural form of DNA.

Often, to ensure that DNA strands once again perfectly match with their alphabetic mates, additional letters are added – like adding additional Legos to make sure each aligns with the matching pieces on the other side. This ensures that both DNA strands have matching mates on each side. For the huntingtin gene, this can mean that additional CAG repeats are added, and the CAG repeat expansion gets longer. The result is often earlier onset of HD symptoms. Understanding how the cell decides whether to chop or add DNA letters to a loop out structure could be the key to understanding somatic expansion, and to controlling it.

Cellular editing decisions defined

Researchers at the Hospital for Sick Children (SickKids) in Toronto, Canada recently identified proteins that play a key role in the cellular decision process of chopping or adding DNA to loop outs. This work, spearheaded by Dr. Terence Gall-Duncan and led by Dr. Christopher Pearson, was recently published in the prestigious scientific journal Cell. The work from the team at SickKids adds to our understanding of somatic instability in HD while identifying proteins that could be targeted for therapeutic gain.

The team broke down the science behind a protein called RPA – replication protein A. The job of RPA in the cell is to bind to DNA when the helix is unwound and it’s single stranded. There’s a different version of RPA that is unique to humans and monkeys, creating an alternative version of RPA called Alt-RPA. Both versions, RPA and Alt-RPA, bind to DNA loop-outs, like the ones that are created when CAGs in the huntingtin gene can’t find their mate when DNA strands are separated.

The experiments in this new paper show that when cells have more Alt-RPA, CAG expansions get bigger. But when the standard version of RPA is present, fewer CAG expansions are added. So it seems that Alt-RPA controls the cellular decision to add DNA to loop-outs while RPA decides to chop!

Something else interesting about this finding is that Alt-RPA is only found in monkeys and humans, with very strong levels found in humans – the only species to have HD. This may be a start to understanding why HD specifically and only affects humans.

The team did a large-scale interaction study to identify other proteins with which RPA and Alt-RPA were interacting. They found that Alt-RPA specifically interacted with proteins that regulate CAG repeat instability! One of the most striking proteins identified that specifically interacts with Alt-RPA was MSH3.

MSH3 is a major regulator of age of symptom onset in HD and was originally identified from samples given by thousands of HD families for a study called Gem-HD. Having lots of samples from HD families, from studies like GeM-HD and Enroll-HD, has rapidly advanced the identification genes that modify age of symptom onset, like MSH3. This new work from the group at SickKids may be the link for understanding how MSH3 helps to control somatic expansion in the huntingtin gene.

The team tested the effect of changing levels of RPA in mice that model a disease similar to HD – spinocerebellar ataxia (SCA1), which is also caused by a CAG repeat. When they increased levels of the standard version of RPA, the SCA1 mice’s symptoms improved, including the instability of its CAG repeats.

What does this all mean for HD?

There are several companies currently working on drugs as a treatment option for HD that target MSH3 as a modifier associated with somatic instability. Voyager Therapeutics is working to develop a harmless virus that targets MSH3 that can be injected into the blood to reach the brain. LoQus23 Therapeutics is working to target MSH3 using small molecules that could be taken as a pill. Pfizer has also jumped on the MSH3 bandwagon and is testing drugs to move toward clinical trials.

These new results from the team at SickKids don’t mean that we’re ready to add RPA or Alt-RPA to the drug lineup just yet though. This work still needs to be tested in mice that model HD to see if changing these proteins can improve behavior and molecular effects associated with HD. However, they do get the research world closer to understanding the precise mechanism that controls somatic instability. Knowing exactly how the cell makes the decision to add or chop DNA when a loop out structure is formed opens the door for designing more drugs to test in trials, not just those that target MSH3.

In recent years, HD research headlines have trended toward huntingtin lowering: experimental therapies that target the root genetic cause of HD. But there are also several drugs in development to treat HD that do not aim to lower huntingtin. Some of these are aimed at managing individual symptoms of HD, like managing involuntary movements, or improving cognition. Others take more preventative approaches, like preserving the health of brain cells or slowing down the expansion of CAG repeats in the huntingtin gene.

Beyond huntingtin lowering

HD is caused by an expansion in one section of the huntingtin gene, so cells produce an extra-long form of the huntingtin protein. Expanded huntingtin proteins are believed to be toxic, especially to the brain cells responsible for control of mood, movement, and memory. Naturally, this has made lowering the amount of expanded huntingtin in the brain and body a major priority of efforts to treat HD.

Despite trial failures that have rocked the HD community in recent years, huntingtin-lowering remains a viable approach to treat HD, and dozens of companies and academic labs are working to make this an accessible reality. However, all of our metaphorical HD research eggs are not just in this one basket.

As novel science weaves a broader understanding of HD biology, potential new avenues of treating the disease are coming into focus, and there are dozens of companies and academic labs working to develop these strategies into accessible realities, too. In this article, we’ll explore some of these approaches to treating HD.

Targeting cholesterol

Cholesterol is a type of fat molecule found throughout the body. You’re probably familiar with its roles in heart health or hormone production, but you may not know it is especially important for the health of connections between cells in the brain. Maintaining optimal levels of cholesterol in the brain is tricky; cholesterol molecules are large, and it is difficult for them to pass freely between the brain and the blood that circulates through the rest of the body. A special enzyme, CYP46A1, helps eliminate excess amounts of cholesterol in the brain, but it can stop working properly in Huntington’s disease.

Asklepios BioPharmaceutical (AskBio) is developing an experimental gene therapy that targets this enzyme. AskBio’s drug, AB-1001, is delivered directly to brain tissues in a single dose on each side of the brain with an MRI-guided brain surgery. AB-1001 tells the body to produce more CYP46A1 to help restore a better balance of cholesterol in the brain.

AskBio hopes that repairing this cholesterol pathway will support the overall health of neurons, and could also help the brain lower its own levels of mutant huntingtin protein without affecting levels of healthy huntingtin. Currently, this drug is being studied in a small group of people with HD in a Phase I/II trial in France that began at the end of 2022. While there is no news yet as to the drug’s safety or efficacy, the results of this safety study will determine whether a larger trial will take place.

Preserving connections between brain cells

Synapses are the connections between brain cells that allow them to communicate. Sometimes these connections stop working as well as they should, and a part of the immune system, called complement, gets rid of them. This process, called synaptic pruning, is especially important in early phases of brain development, but occurs throughout a person’s life.

It’s a bit like trimming back an overgrowing shrub in a garden that might block sunlight or monopolize nutrients from surrounding plants. A complement protein called C1q attaches itself to declining synapses, causing them to be cleared, to make sure healthy synapses can continue to do their job effectively.

In HD, C1q proteins become overactive and can tell the rest of the complement system to begin breaking down healthy brain cells instead of damaged ones. If C1q protein levels could be managed, it might help preserve healthy synapses for longer to support the brain’s resiliency against the onset of HD. The company Annexon has been developing an experimental therapy to block C1q and calm over-activity in the complement system.

ANX-005 is an antibody therapy that is delivered with an IV; in 2022, a Phase II trial was completed to check its safety and efficacy in people with HD. The study didn’t have a placebo group to compare the effects of ANX-005 to the natural progression of HD, so the results are a bit difficult to interpret. However, the findings indicate that HD symptoms were stabilized in some participants, particularly those who started out with a more active complement system. Annexon is planning for a larger, placebo-controlled Phase II/III study to begin in 2024.

Slowing somatic expansion

DNA is constantly being pulled apart and put back together again to be used as a blueprint to make message molecules, called RNA, which in turn encode proteins. Our cells perform these tasks nearly 2 trillion times per day—they literally have it down to a science. This also means that there are plenty of opportunities for mistakes. Our bodies plan for this, and have machinery to detect and fix errors: DNA mismatch repair proteins.

Certain stretches of DNA pose an extra challenge for these auto-correct proteins. In people with HD, DNA mismatch repair proteins are more prone to slip on the extra CAG repeats in the huntingtin gene, like a needle might get caught on a scratch in a record. Sometimes this results in even more CAG repeats– especially in the cells of the striatum, the part of the brain that controls movement and mood.

This tendency for the expanded stretch of the huntingtin gene to grow over time is called somatic instability. While some cells are more prone to CAG repeat expansion over time, like in the brain or liver, this phenomenon is less likely to occur in other types of cells, like those in our blood. This means that the results of a person’s genetic blood test wouldn’t be changed over time by somatic instability.

Some scientists think that as CAG repeats in huntingtin gene grow longer, the resulting huntingtin proteins become even more dysfunctional and toxic. Scientists are still understanding what this means, but it is believed that somatic expansion contributes to the death of brain cells in HD, making it a key therapeutic target to treat the disease.

LoQus23 and Pfizer are researching drugs to slow or stop somatic expansion in the mutated stretch of the huntingtin gene with the goal of slowing or stopping the progression of HD. While still in early stages of development, they are targeting some of the proteins involved in DNA mismatch repair to accomplish these aims, and many more companies and academic researchers have an interest in pursuing HD treatments related to somatic instability.

Managing movements

A major goal of HD research is to find options to slow or stop the disease in its tracks. Another important objective is to help people with HD maintain independence and quality of life for longer by managing symptoms of the disease. One approach to this is reducing the involuntary movements that are common in people with HD, chorea. These movements may not be bothersome for some, but others may find chorea disruptive to day-to-day activities or safety.

There are currently three drugs available for treating HD chorea. Each is taken by mouth and employs similar drug chemistry to manage involuntary movements. Xenazine (tetrabenazine), Austedo (deutetrabenazine), and INGREZZA (valbenazine) all limit the activity of VMAT2 proteins. These proteins act as transport vehicles for the chemical messages that are passed between brain cells, especially dopamine. Dopamine plays a role in movement, and managing its levels in the brain can help minimize chorea.

While these three drugs are similar, and there are additional drugs that doctors may prescribe for movements alongside other HD symptoms, having options to manage chorea is a good thing. One drug may be preferable over others for a variety of reasons, including cost, dosage, and frequency. HDBuzz recently wrote about the FDA approval of INGREZZA in the United States in August 2023, as well as the other chorea-management medications currently available for people with HD. You can read more about that here.

Improving function

Early signs of HD often include slight disruptions in someone’s ability to perform day-to-day activities, such as handling their finances, remembering directions, and managing household chores. HD clinicians often use a rating scale to measure “Total Functional Capacity” (TFC), which encompasses many aspects of someone’s capacity to live and function independently. Maintaining TFC for longer could improve quality of life for people by preserving their independence.

Prilenia has been testing a drug to support total functional capacity in people with HD. Pridopidine, which is taken by mouth, has been studied in humans for more than a decade, but no large trial has met its goals of slowing the progression of HD. Pridopidine activates a protein called the sigma-1-receptor, which helps brain cells survive under stress.

The latest study of pridopidine, PROOF-HD, wrapped earlier this year, but the results are somewhat unclear. Pridopidine has a good safety profile, but was not found to effectively improve total functional capacity or movement symptoms in people with HD. The drug may have been helpful for the first year in some participants, those who were not taking certain medications that alter dopamine. Prilenia is continuing to analyze the data, and to conduct additional research to interpret these results.

Improving cognition

NDMA receptors are critical for tasks like combining and linking memories, multitasking, and effective decision making—functions that all fall under the umbrella of cognition. Sage Therapeutics is also hoping that their drug will improve early changes in these thinking abilities for people with HD. Their drug, SAGE-718, is designed to increase activity of NMDA receptors to preserve cognition in people with HD.

A small, early clinical trial showed some promising results in people with HD. Sage is now studying the drug’s safety, efficacy, and effects on cognitive performance in a series of Phase II trials called the PERSPECTIVE program. Two of these trials are currently recruiting participants in North America; while similar, they have different goals.

The DIMENSION study investigates the safety and efficacy of SAGE-718. The SURVEYOR study also assesses safety and efficacy, and evaluates the drug’s effects on tasks of daily living. The study protocol includes virtual reality simulations of things like cooking a meal, using transportation, shopping, or managing money, as well as an optional driving simulation.

Looking forward

Huntingtin-lowering therapies have dominated the HD-research landscape, but this is one among many approaches to treating HD. New paths to treat HD are being uncovered and explored all the time. This is one reason why observational research studies like Enroll-HD are so important; the greater our understanding of HD biology, the better our understanding of how to treat it, and the more drug targets are revealed to fight the disease and manage its symptoms.

While all cases of HD result from a single gene, this doesn’t mean that every person’s symptoms will progress in the same way. In an ideal world, there would be multiple strategies available to treat and slow HD that could be attuned to an individual’s symptoms and genetics. More tools in the toolbox is a good thing, and the treatment strategies described in this article are only a handful of the possibilities currently in the HD research pipeline from a few of many companies working to bring options to HD families.

Researchers studied a fragment of the Huntington’s disease (HD) protein in plants and found a new way to stop it from forming toxic clumps. A special plant protein that the team identified can prevent harmful buildup in plants as well as in some HD model systems, showing potential for this approach as a possible way to treat HD.

Why study HD in plants?

Plants are stuck in their environment, literally rooted to the ground, which means they cannot move if they start to experience challenging conditions such as too much sun, freezing cold or pesky predators. To help deal with the environmental troubles they can experience, plants have evolved all kinds of nifty ways to cope, which can make them very resilient to stress. Many plants can also live an extremely long time, so some scientists believe they might hold the key for studying and finding new medicines for human diseases of ageing.

HD is caused by an expansion of the number of CAGs within the huntingtin gene, which means that an expanded form of the huntingtin protein is made. The expanded huntingtin protein can form clumps, and scientists think these may cause all kinds of stresses in our cells, contributing to the signs and symptoms of HD. In this study, a group of researchers from Cologne, Germany wanted to investigate whether the resiliency of plants could be extended to dealing with stress caused by toxic clumps of the HD protein.

Plants making the HD protein grow normally

First, the research team made specially modified plants which artificially make a fragment of the HD protein. They grew some plants that had a very long CAG repeat which might be found in a person with juvenile HD (69). They also grew plants to compare them to, which had approximately the longest CAG repeat number that exists in any plant protein, but would not be likely to cause HD in humans (28).

Under normal conditions, they found that these modified plants making the HD protein grew almost exactly the same as plants without the HD protein, and that no protein clumps formed in the plant cells. They also checked that making the HD proteins did not trigger any of the stress response systems in the plants. However, if they subjected these plants to additional stress, such as high heat conditions, then they saw toxic clumps form for both forms of the HD protein.

HD proteins communicate with the cell's cleanup system within chloroplasts

Unlike human cells, plant cells have special compartments called chloroplasts which are responsible for capturing light so the plants can make food and grow. Contained within the chloroplasts are lots of specialist bits of cell machinery, that keep protein levels in balance and clean up damaged or toxic proteins, so that energy and growth stay on track.

The scientists found that these clean-up machinery assemblies had a lot of contact with expanded HD proteins, and they could see this contact happening both in chloroplasts as well as other parts of the plant cells.. In particular, there was contact between the HD protein and an enzyme called SPP which chops up other proteins during the clean up process.

Using microscopes, the team looked at the location of the HD protein within the plant cells. They could see lots of the HD protein surrounding the chloroplasts, suggesting that these special structures may help the plant to deal with the stress of making the HD protein.

Messing with the chloroplast's job makes HD protein clumps pile up

The research team then looked at how chloroplasts process the HD protein. In their studies, they saw that chloroplasts were able to take up the HD protein when it was floating nearby, and then remove it.

They then wanted to see what would happen if they stopped the chloroplasts from cleaning up proteins or from shuttling molecules in and out. They used different chemicals to disable the chloroplasts in these ways, and In both cases, the plants showed a buildup of HD protein and potentially harmful clumps. This provided even more evidence that the chloroplasts were very important in dealing with the HD protein.

A new way to decrease HD protein clumping

The SPP molecule can help with protein cleanup in plants, and it was found in contact with the HD protein. So could SPP help deal with HD protein clumps in other contexts - like in cells grown in dishes, or in an animal model of HD?

In the final part of the study, the scientists added the SPP gene in different models of HD to see what would happen to the HD protein. They first looked in human cells in a dish, and found that SPP stopped the build up of HD protein clumps.

Finally, they engineered microscopic worms to make the HD protein, with or without SPP as well. The worms with SPP had a lot less HD protein clumps and could move around better than those without it.

What does this all mean and what’s next?

It’s likely to be a long road before folks with HD are being dosed with SPP to treat symptoms. However, the research team behind this study believe that by researching plants, which can endure harsh conditions causing protein clumping, they might find even more valuable insights for treating human diseases.

This innovative, and slightly wacky, plant-based approach could hold promise for advancing possible new treatments for diseases like HD.