Peeking at huntingtin and learning from a PET study

A recent publication discusses a non-invasive way of measuring levels of expanded HTT protein in the brain, using an imaging tool called a PET tracer. The results were variable, but there’s still a lot to learn from the study as development of HTT tracers continues!

Measuring Levels Of Huntingtin In The Brain

One of the hallmarks of Huntington’s disease (HD) is the buildup of sticky protein clumps in brain cells. Known as “aggregates,” they contain bits of expanded HTT protein and other debris. Huntingtin aggregates tend to increase over time and there’s a lot of evidence to suggest that they contribute to damage in the HD brain. 

We have known for many years that these structures exist because scientists have studied brain tissue from animals and from people after they have passed. More recently, sensitive tests have been developed to measure levels of expanded HTT in living people using samples of CSF, the fluid that surrounds the brain and spinal cord. 

These methods have transformed our ability to test genetic therapies aimed at lowering HTT, but they are imperfect. It’s not clear exactly how well levels of expanded HTT from CSF reflect levels inside of brain cells. Wouldn’t it be great if there were a way to visualize in real time, without needles or the need to store and test samples, the amount of protein in there? 

That way, researchers could monitor how well a HTT-lowering drug was working, understand better how protein buildup corresponds with symptoms, and figure out who might be a good candidate for a clinical trial in the earliest stages of HD.  

Brain Imaging As A Way To Measure Expanded HTT?

Having a way to see features of the brain non-invasively is the idea behind PET tracers to image expanded HTT. A big HD research foundation, known as CHDI, has been working on this for several years, and they recently published a human study in collaboration with a clinical team in Leuven, Belgium. 

Positron emission tomography, also known as a PET scan, involves a radioactive molecule called a PET ligand. In this HD study, the ligand was designed to stick to clumps of the expanded HTT protein. The radioactive part of the ligand emits tiny particles that lead to a reaction producing bursts of energy (photons). A PET scanner detects these photons and calculates where they came from, and then a computer image shows all the places where the ligand is stuck.

Participants in a PET study received the tracer, then gave it time to reach the brain. Then they laid in a scanner that took images showing where expanded HTT is located and how much of it is present. Afterwards, the body cleared the tracer away. 

Over the past several years, CHDI had designed and selected an expanded HTT tracer, tested it in tissue, in living mice and monkeys, and most recently in a few healthy people to make sure it could be used safely. The recent study, called iMagemHTT, tested the tracer in 12 people with HD (who have expanded HTT in their brains) and 12 people without HD (who do not). They broke these groups down into younger and older individuals as well. 

Unfortunately, the tracer stuck to a lot of stuff that wasn’t expanded HTT, in some people more than others. This led to a lot of variability between individuals, both with and without HD.

The Challenges Of PET Imaging

The main goal of this study was to test the expanded HTT PET ligand in a larger group of people to look at its “dynamics” in the brain. This means making observations about how well it sticks to expanded HTT, how fast the body clears it away, how consistent it is between different people, and whether it can actually be used to measure levels of the expanded HTT protein in people with HD, compared to control participants who should have no expanded HTT. 

One challenge with PET imaging is that often the ligand will stick to things it’s not supposed to. When this one sticks to something that is not HTT, it lights up the brain in unexpected places and creates a “noisy” picture. Even in people without HD, who have no expanded HTT in their brains, there will be some tracer that grabs onto other proteins and shows up on the screen. Researchers who specialize in brain scans can apply all sorts of complex math afterwards to analyze the images, balance out individual and group differences, and help to see the real “signal” through the “noise.”  

There are many, many factors that can influence variability in measurements – things like age and individual brain differences, but even things like the time of day, or whether a person has eaten beforehand! For this reason, the team did multiple scans on the same person, sometimes in the same day, sometimes a week apart.   

What The Scanners Saw

In this HD study, the expanded HTT tracer entered the brain and was cleared away at rates typical for PET scans, and importantly, it did this similarly in people with and without HD. That tells us that HD doesn’t affect the ability of a person’s cells to take up the tracer or to break it down, which could skew the results. 

As for the meat of this experiment, visualizing expanded HTT itself: unfortunately, the tracer stuck to a lot of stuff that wasn’t expanded HTT, in some people more than others. This led to a lot of variability between individuals, both with and without HD. Applying one common type of analysis, this meant it was actually pretty hard to identify brains with expanded HTT and those without. 

However, another statistical method compares a person’s own cerebellum (the back, bottom part of the brain controlling posture and coordination) with the parts most affected by HD, and then compares between groups. When they analyzed the image data in this way, the expanded HTT ligand did bind more in people with HD compared to people without. This is what we would expect, given that people without HD don’t even have expanded HTT in their brains. 

Another observation they made was that if a person had 2 scans in one day, those two scans might look quite different from one another! When they had the scans a week apart, the results were less variable. This was surprising, but it’s important information that could be used later to determine the frequency and timing of PET scans for a future study. 

There are newer PET ligands actively being tested in tissue and animals that may stick better to the expanded HTT protein and less to other structures and debris. 

Why So Variable? 

After years of testing in tissues and animals, it’s a bit disappointing that this tracer didn’t produce a blazingly strong signal that would allow us to visualize expanded HTT in people with high accuracy. In fact, the authors conclude that this PET ligand isn’t the right one to move forward with for a clinical study or diagnostics in humans. But they can learn from the process, and there are new, better compounds in development that are likely to produce a much clearer picture.  

The authors speculate about the potential reasons for the discrepancy between promising data in animals and variable data in humans. Man-made models of HD don’t capture all of the features of disease in humans, and animal and human brain cells don’t always take up and break down substances in the same way. 

Additionally, the mouse models they tested have super-long CAGs and more clumps than people, and the monkeys were injected with genetic material that leads to extra expanded HTT protein – that could be why the tracer worked more clearly in animals. The tracer could also be sticking, not just to HTT, but to other types of protein clumps that build up during normal aging in people.       

Next Steps

Even though this particular ligand didn’t provide a clear picture of expanded HTT aggregates in the brain, the researchers can quickly apply these learnings to future studies. There are newer PET ligands actively being tested in tissue and animals that may stick better to the expanded HTT protein and less to other structures and debris. 

Future studies might include multiple scans spaced farther apart at the same time of the morning or afternoon, since that seemed to decrease the variability. They could also be designed to include more people, to increase the chance of drawing solid conclusions.

Ultimately, HD scientists are not giving up on the potential to use PET imaging to measure levels of HTT protein in a safe and non-invasive way. It would represent an important addition to the toolbox of methods for tracking HD and measuring the success of clinical trials. The approach simply needs a bit more tweaking, and meanwhile there are numerous research groups working to overcome these barriers with additional techniques. We’ll be sure to provide updates as they come! 

TL;DR — What You Really Need to Know

• A new PET tracer was tested to non-invasively measure expanded huntingtin (HTT) protein in the brain, which could help track disease progression and treatment effects in Huntington’s disease (HD).
• The tracer was safe and behaved similarly in people with and without HD, meaning HD doesn’t interfere with how the body takes up or clears the tracer.
  However, the tracer showed a lot of “noise”, meaning it stuck to things other than HTT, making it hard to clearly distinguish HD brains from non-HD brains in the scans.
• Some analysis methods did reveal more tracer binding in HD-affected brain regions, but overall, the results were too inconsistent to use this tracer in clinical trials or diagnosis.
• Surprisingly, scans taken just hours apart looked quite different, while those spaced a week apart were more consistent, highlighting the need to fine-tune scan timing in future studies.
• Despite the challenges, researchers gained valuable insights, and new, more specific tracers are already in development, keeping hope alive for PET imaging as a future HD biomarker tool.

Learn more

Original research article, “PET imaging with [¹¹C]CHDI-00485180-R, designed as radioligand for aggregated mutant huntingtin, in people with Huntington’s disease” (open access).