HDBuzz

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.

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.

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.

Huntingtin-lowering therapies are being pursued in the clinic by many companies

Many companies are exploring huntingtin-lowering as a strategy for treating HD. HD is caused by a mutation in the huntingtin gene, which leads to the production of a faulty version of the huntingtin protein. The faulty protein causes all sorts of problems in the brain and including the death of nerve cells, which results in the symptoms of HD. Huntingtin-lowering drugs aim to reduce the levels of the faulty huntingtin protein in the brain , with a goal of slowing or stopping HD’s progression.

Huntingtin-lowering treatments are being developed using a variety of different approaches, such as anti-sense oligonucleotides (Wave Life Science and Roche) or viral gene therapies (uniQure). One problem is that the drugs these companies have developed cannot easily spread through the whole body, so they are given to patients through an infusion into the spinal fluid, or by direct injection into the brain. Giving drugs this way is expensive and demanding for patients so this type of therapy could not be trivially rolled out to the global HD community.

To overcome these problems, researchers are keen to develop “small molecule therapies” which would be cheaper to manufacture and administer. Small molecule drugs can be formulated to be taken orally as a pill or syrup, like most common medicines you may already be taking, such as pain killers or an allergy medication. Because they can hitch a ride in the bloodstream, small molecule drugs are also better at spreading to nearly all the organs of the body. Some small molecule drugs, although not all, can even make the leap from the blood into the brain – enabling treatment of the body and brain with a single drug.

Branaplam lowers huntingtin but was originally designed to treat another disease, SMA

Two different companies, Novartis and PTC Therapeutics, are both testing small molecule drugs which can lower huntingtin in HD patients. The drugs from both companies are called splice modulators because they target how our cells which edits genetic messages, a process call splicing. Each genetic message can be thought of like a story book, and when the story is over, the final part of the message reads the genetic equivalent of “the End” to tell the cell that the sequence for that message is complete. Splice modulator drugs rejig the pages of the story book so “The End” is read the ending, so the cell destroys the message and doesn’t make the associated protein at all. Just like you would toss a book that made no sense with a premature ending and read, “Once upon a time, The End”.

The splice modulator developed by Novartis is called branaplam, a drug originally developed for a completely different disease called spinal muscular atrophy (SMA), because it also changes the levels of a protein called SMN2, which underlies that disease. Very unexpectedly, scientists at Novartis discovered branaplam also changes the levels of the huntingtin protein in different models so wanted to explore if this drug might be a good treatment for people with HD in a trial called VIBRANT-HD.

Branaplam has bad side effects for some people treated with this drug

VIBRANT-HD aimed to work out if branaplam was safe and effective at lowering huntingtin levels but, before recruitment was completed, dosing for the trial was paused due to safety concerns. The decision to pause the trial was made by an independent Data Monitoring Committee, who assess data generated by the trial before the doctors, patients, or study sponsor (Novartis) know the outcomes to ensure participants are safe in case issues arise.

We have since learned in this most recent announcement that Novartis has decided to end all development of branaplam for HD due to safety concerns associated with the drug. When dosing was paused back in August, information was released indicating that there were issues in some study participants with a condition called peripheral neuropathy – damage to nerve cells outside of the brain and spinal cord. In this most recent announcement, Novartis have provided further information about safety issues seen in many, although not all, participants.

As we expected to learn, symptoms and changes in neurological examinations consistent with peripheral neuropathy were confirmed as being observed in some participants. Some participants also had increased levels of neurofilament light chain (NfL), a lab test used to assess injury or damage to nerve cells. This means that there may be damage to the nervous system after branaplam treatment. Also of concern is the observation that there was an increase in the size of a region of the brain called the ventricles. The ventricles are a fluid filled space deep in the brain and an increase in the size of this region can mean several different things, which we don’t yet have enough information to fully understand. In their letter, Novartis state that no clinical symptoms have been associated with these brain scan findings to date.

What does this mean for HD patients who received branaplam?

Novartis have stated that all study participants who received branaplam will continue to be monitored. We don’t yet know if the side effects experienced by participants in the trial are permanent or whether they will get better now that dosing with the drug is stopped, so monitoring of symptoms is important.

What can we learn from trials that end this way?

Trial failures like this can be very hard-hitting and it is very normal to feel upset about this type of news, especially for the brave and dedicated members of the HD community who participated in this trial.
Despite this sad development, there is still a lot we can learn from trials which don’t turn out as we had hoped. Tons of data is collected throughout the course of trials and more will continue to be collected in the coming months as things formally conclude. This data can give us important insights into what might have happened so that the community can learn and move on from this trial. Novartis has stated that they are committed to sharing what they learn with HD families, researchers, and other professionals in the HD community.

Do we know why branaplam didn’t work as we had hoped?

This announcement is the latest in a series of disappointing news regarding HD trials so what’s going on? It’s important to note that branaplam was not developed to treat HD. We knew unexpected side effects were possible, because as well as lowering huntingtin, branaplam also changes the levels of the SMN2 protein, as well as potentially others. Changing the levels of lots of different proteins can disrupt the intricate processes performed by nerve cells, which could explain some of the symptoms observed.

In fact, in some animal studies, Novartis note in their announcement that toxicity of the nerves was seen as a side effect of branaplam treatment, which is why they included robust safety monitoring procedures in the VIBRANT-HD trial. Interestingly, children with SMA treated with branaplam do not seem to have these symptoms, which is why there was still optimism that this would not prove to be a problem in HD patients. We will likely learn more about why this happened as more data from the trial is compiled and analysed.

What does this mean for the other splice-modulator drugs to treat HD?

Other companies are working to develop a splice modulator to treat HD, including Roche who are doing pre-clinical research in this area. Another trial, called PIVOT-HD, will be testing the splice modulator PTC-518 developed by PTC therapeutics which is very similar to branaplam. This trial is underway in Europe and Australia although recruitment is paused in the US as PTC work to provide some extra data to the US regulatory agency, the FDA. It’s important to note that PTC-518 was specifically designed for HD, and data from PTC indicates this drug spreads more efficiently into the brain than branaplam, so the hope is that the side effects observed for branaplam won’t be an issue for PTC-518; we will learn more as the trial proceeds.

When will we learn more?

Novartis have vowed to keep the community updated as their analysis of the data from the trial proceeds. HDBuzz will write another article as soon as we learn any more information about branaplam or the VIBRANT-HD trial.

It’s important to remember clinical trials are some of the biggest and most complicated experiments which we can run, with no guarantees of good outcomes, but each trial adds to our knowledge and brings us closer to finding drugs to treat HD. We are extremely grateful to the brave and selfless HD community members who participated in this trial.