A tiny protein fragment may be the real driver of Huntington's disease damage

University of Washington School of Medicine / UW Medicine

What if the most promising treatments for Huntington's disease are aimed at the wrong target?

That is the uncomfortable question raised by a study published March 18 in Science Translational Medicine by researchers at the University of Washington School of Medicine. Their findings suggest that a small, often-overlooked fragment of the mutant huntingtin protein - called huntingtin 1a - may be doing most of the damage. And the antisense oligonucleotide therapies currently in clinical trials do not touch it.

The protein everyone targets and the fragment nobody watches

Huntington's disease is caused by a mutation in the huntingtin gene that produces an abnormal protein. This protein accumulates in brain cells, disrupts cellular functions, and forms large aggregates that kill neurons. The disease typically strikes in a person's 40s, progressing through uncontrolled movements, personality changes, and cognitive decline to death within 10 to 15 years. Roughly 41,000 Americans have Huntington's, with more than 200,000 at risk.

The leading experimental approach uses antisense oligonucleotides (ASOs) - short DNA sequences designed to intercept and destroy the messenger RNA (mRNA) that cells use to build the huntingtin protein. By cutting the mRNA, ASOs prevent the full protein from being assembled. Several ASO therapies are now in clinical trials.

But here is the complication: the mRNA strand does not just produce one protein product. It also generates a short fragment called huntingtin 1a. This fragment is known to be toxic to nerve cells, but its specific role in the disease has been poorly understood. Most ASO therapies bind to sites on the mRNA that block production of the full protein but leave huntingtin 1a intact.

The experiment that surprised its own researchers

Jeffrey Carroll, an associate professor of neurology at UW and the study's senior author, initially set out to compare two ASO strategies in mice carrying one copy of the mutant huntingtin gene. One approach suppressed production of all huntingtin proteins, both normal and mutant. The other selectively blocked only the mutant version.

The most effective ASO in their mouse model happened to bind very near the beginning of the mRNA strand. This meant it not only blocked the full protein but also suppressed production of huntingtin 1a. And the results were striking.

The researchers assessed treatment effectiveness by measuring the expression of more than 150 genes known to be disrupted in Huntington's disease, along with the protein aggregates that are a hallmark of the condition. The ASO that left huntingtin 1a intact produced minimal benefit - gene expression barely budged, and aggregates persisted. But the ASO that also blocked huntingtin 1a restored roughly 55% of affected gene expression to baseline levels. Protein aggregates were nearly eliminated.

Robert Bragg, the study's first author and a research scientist in Carroll's lab, described looking at treated mouse brain cells under the microscope and initially thinking he had made a mistake. He could barely find a single protein aggregate in the treated tissue.

If you lower the protein but not the fragment, nothing moves

The implication is sobering for the field. If these results hold, then ASO therapies designed to block the full huntingtin protein - while leaving huntingtin 1a untouched - may be targeting an incomplete piece of the problem. The fragment, not the full protein, may be the primary driver of disease pathology. Or at minimum, the fragment may need to be suppressed alongside the full protein for treatment to be effective.

Carroll himself acknowledged the weight of the finding with notable candor. He said he hopes the results are wrong, but that the underlying science is solid. If the findings translate to humans, it would mean that new treatments specifically designed to target the huntingtin 1a region need to be developed - a significant redirection for a field that has invested heavily in existing ASO designs.

Mouse caveats and human questions

These results come from mice, and that distinction matters enormously. Mouse models of Huntington's disease replicate some features of the human condition but not all. The specific ASO that worked best in this study was optimized for the mouse model and cannot be directly applied to human patients. The kinetics of protein production, the biology of mRNA processing, and the complexity of the human brain all introduce variables that mouse studies cannot capture.

The study also used mice carrying a single copy of the mutant gene, which mirrors the typical human genetic situation. But the progression of disease in mice occurs on an accelerated timeline and with different neuroanatomical patterns than in people. Whether huntingtin 1a plays an equally dominant role in human disease remains an open question.

Sample sizes in mouse genetic studies tend to be small, and this study is no exception. The dramatic effects observed - near-total elimination of aggregates, restoration of more than half of affected gene expression - are encouraging but will need replication in independent laboratories and ideally in different animal models before redirecting clinical strategy.

It is also worth noting that suppressing huntingtin 1a along with the full protein means suppressing more of the mRNA strand's products. The normal (non-mutant) huntingtin protein plays important roles in cells, and broader suppression could carry its own risks. The safety profile of this more aggressive approach has not been established.

What comes next for Huntington's treatment design

The immediate practical question is whether existing ASO therapies in clinical trials can be redesigned to also target huntingtin 1a, or whether entirely new molecules are needed. That answer will depend on the specific binding sites available on the human mRNA sequence and on whether the dramatic effects seen in mice translate to meaningful clinical benefit in people.

For the roughly 41,000 Americans living with Huntington's and the 200,000 who carry the gene, the study reframes the therapeutic landscape. The target may not have been wrong so much as incomplete. Getting the full picture - and designing treatments that address it - is the next challenge.