Taking A Short Cut: New Routes to Huntingtin-Lowering Therapies

A new study from researchers at the University of Washington and University College London explores different ways to lower the huntingtin proteinhuntingtin protein The protein produced by the HD gene. in Huntington disease (HD). Their findings suggest that not all forms of harmful huntingtin are equally damaging in models of HD – and that targeting the worst offender could lead to new approaches to therapies.

A Hybrid Gene with a Built-In Target

HD is caused by an extra-long stretch of the DNA letters CAG within the huntingtin (HTTHTT one abbreviation for the gene that causes Huntington’s disease. The same gene is also called HD and IT-15) gene, causing cells to produce an expanded form of the HTTHTT one abbreviation for the gene that causes Huntington’s disease. The same gene is also called HD and IT-15 protein. Over time, this extended protein can misfold, interfere with the cell’s normal activities, and form sticky clumps inside brain cells. 

To combat this, many experimental HD therapies aim to reduce levels of the HTTHTT one abbreviation for the gene that causes Huntington’s disease. The same gene is also called HD and IT-15 protein. But researchers are increasingly asking whether some forms of the protein are more harmful than others, and whether targeting those specific forms could be more beneficial. 

To tackle this question, a team of UK and US scientists led by Drs. Gillian Bates and Jeff Carroll worked with a specially engineered mouse model of HD. In this mouse, a small stretch of the normal mouse huntingtin gene was replaced with a human version carrying the a very long CAG repeatCAG repeat The stretch of DNA at the beginning of the HD gene, which contains the sequence CAG repeated many times, and is abnormally long in people who will develop HD expansion that causes HD-like symptoms in this mouse. 

You can think of it like swapping a few key pages in an instruction manual with pages from a different edition. The result is a hybrid – or “chimeric” – gene that behaves more like the human version.

This clever design gave researchers a unique advantage: it created a genetic “handle” that allowed them to target only the expanded huntingtin, while leaving the regular version untouched.

When Messages Get Cut Short: The Rise of HTT1a

Genes don’t produce proteins directly – they first create RNARNA the chemical, similar to DNA, that makes up the ‘message’ molecules that cells use as working copies of genes, when manufacturing proteins. messages that are carefully edited before being used. These messages contain exonsExons The small fraction of our DNA that is directly used to instruct cells how to make proteins, which represent the actual instructions, and introns, sections that are normally removed before making proteins. But in HD, this process can go wrong.

Think of it like sending a long email but accidentally hitting “send” halfway through. The incomplete message can be confusing or worse.

The expanded CAG repeatCAG repeat The stretch of DNA at the beginning of the HD gene, which contains the sequence CAG repeated many times, and is abnormally long in people who will develop HD that causes HD sits within the very beginning of the huntingtin gene. Sometimes the huntingtin message gets cut short, producing smaller fragments. One shortened product that has attracted increasing attention is called HTT1a.

Some studies suggest that this shortened protein fragment may be especially harmful: more prone to slipping into the nucleusnucleus A part of the cell containing genes (DNA) of cells, clumping together, and disrupting the normal activity of genes.

Two Ways to Lower Huntingtin

The team compared two different treatment strategies in the HD mice using antisense oligonucleotidesASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene (ASOsASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene) – small DNA-like molecules that bind to RNARNA the chemical, similar to DNA, that makes up the ‘message’ molecules that cells use as working copies of genes, when manufacturing proteins. messages and trigger their destruction.

One approach lowered all huntingtin, both healthy and expanded. The other was more precise, using an ASOASOs A type of gene silencing treatment in which specially designed DNA molecules are used to switch off a gene designed to recognize the human-specific stretch that flanks the HTT1a fragment that had been inserted into the HD gene in this mouse model. By homing in on this “humanized” sequence, the treatment selectively targeted the harmful huntingtin message while sparing the normal mouse version—and at the same time blocked production of the HTT1a fragment.

Same Goal, Different Outcome

Both approaches reduced levels of expanded huntingtin – but that’s where the similarities ended.

Surprisingly, in this mouse model, lowering all huntingtin did not have a big impact. Toxic protein clumps remained, and gene activity in brain cells stayed disrupted.

In contrast, the selective approach had a more dramatic effect. It reduced HTT1a, cleared away protein aggregatesaggregate Lumps of protein that form inside cells in Huntington’s disease and some other degenerative diseases, and restored gene activity toward normal levels. Even when full-length huntingtin was reduced, disease-related problems persisted unless HTT1a was also lowered.

This suggests that simply lowering huntingtin may not be enough – it may also matter which forms you target. The findings add to growing evidence that the HTT1a fragment could play an outsized role in driving HD. 

A Shift in Strategy

Many current HD therapies aim to lower huntingtin broadly. But this study suggests a more refined approach may be possible – one that targets the expanded gene and blocks production of its most toxic forms.

There’s still work to do before this can be applied in people. But by zeroing in on what may be key drivers of damage, this research offers a potential new path toward treatments.

If this message sounds similar to another recent HDBuzz article, that’s because the two studies are pieces of the same puzzle. That study showed that targeting HTT1a may be especially beneficial in some models; this one shows that leaving HTT1a behind might limit the benefits of huntingtin-lowering therapies. Together, they strengthen the case that HTT1a deserves serious attention. 

Important Questions Remain

As promising as these findings are, there are still some important caveats. The mice in this study are born with more than 100 CAGs in every cell carrying the engineered HD gene, while most people with adult-onset HD inherit around 40–50. In people, CAG repeats can grow over decades through somatic expansionsomatic expansion A process in which the CAG repeat in the Huntingtin gene can change over a person’s lifetime in some cells of the body, particularly in the brain., particularly in vulnerable brain cells, creating a patchwork of different repeat lengths. These mice essentially start life with very long repeats that some human cells may only reach much later. This makes them useful for studying long CAG repeats, but may accelerate or exaggerate processes like HTT1a production compared with HD in people.

Another question surrounds the treatments. The most effective treatment both selectively targeted the expanded gene and reduced the toxic HTT1a fragment – making it difficult to tease apart which factor really drove the benefit. Was it the precision of targeting only expanded huntingtin, or the ability to block HTT1a? Future experiments will need to separate these effects more cleanly, for example by comparing allele-selective lowering of full-length huntingtin with approaches that also reduce HTT1a.

There’s also a practical challenge for translating this strategy into people. The approach used here relies on a genetic difference engineered into the mouse, meaning that similar drugs targeting exonExons The small fraction of our DNA that is directly used to instruct cells how to make proteins 1 or intron 1 in humans would likely lower both healthy and harmful huntingtin. That raises an important question for the field: is it better to precisely target only the expanded gene but leave HTT1a untouched, or to broadly lower huntingtin if it means hitting this particularly toxic fragment? Answering that question will be key for designing the next generation of HD therapies.

Summary

• Researchers compared two huntingtin-lowering strategies in an HD mouse model to ask whether which forms of huntingtin are lowered matters as much as how much huntingtin is reduced.
• Broad lowering of both regular and expanded huntingtin had surprisingly little effect on protein clumps or disrupted gene activity when the shortened HTT1a form remained in mice that model HD.
• A more selective approach that targeted expanded huntingtin and blocked HTT1a production reduced protein aggregatesaggregate Lumps of protein that form inside cells in Huntington’s disease and some other degenerative diseases and shifted gene activity back toward normal in these mice.
• The results strengthen the case for HTT1a as an important therapeutic target, but leave a key question unresolved: are the benefits due to selectively lowering expanded huntingtin, eliminating HTT1a, or both?