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Researchers at the University of Washington School of Medicine have identified a specific fragment of the mutant protein responsible for Huntington’s disease that may play a central role in driving disease pathology. The study, published Science Translational Medicine, examined how different strategies for lowering the huntingtin protein affect disease-related changes in mouse models. The team found that reducing a short, toxic fragment known as huntingtin 1a (HTT1a), rather than targeting the full-length protein alone, led to stronger improvements in molecular markers associated with the disease.
“I hope we’re wrong, but the science behind our findings is solid,” said senior author Jeffrey Carroll, PhD, an associate professor of neurology at the University of Washington School of Medicine in Seattle. “To succeed, we may need to design new treatments that also target this specific region of the protein.”
Huntington’s disease is inherited and is caused by a mutation in the huntingtin gene, in which an expanded CAG repeat produces an abnormal protein that accumulates in brain cells. This accumulation disrupts cellular processes and leads to neuronal death. Symptoms typically emerge in midlife, often in a person’s 40s, beginning with involuntary movements, coordination problems, and cognitive changes, and progressing to loss of motor and cognitive function over the next 10 to 15 years.
For this study, the University of Washington team began by focusing on how the mutant huntingtin protein is processed inside cells. Prior work had shown that the gene can produce not only a full-length protein but also shorter fragments. One of these fragments, HTT1a, is generated when the pre-mRNA is cut early, producing a truncated but highly toxic version of the protein.
To see how this fragment may affect disease pathology, the team used antisense oligonucleotides (ASOs), short DNA sequences designed to bind to specific regions of messenger RNA and block protein production. They developed two approaches: one that reduced all forms of huntingtin protein (PanASO) and another that selectively targeted the mutant gene and its early RNA sequence (MutASO), thereby reducing both the full-length mutant proteins and the HTT1a fragments.
The team then treated mouse models of Huntington’s with these ASOs to see their effects. Outcomes were assessed by measuring gene expression changes and the presence of protein aggregates, a hallmark of Huntington’s disease. “We set out to compare the efficacy and safety of allele-selective lowering of mHTT with those of non–allele-selective lowering using antisense oligonucleotides (ASOs) in heterozygous HttQ111 (Q111) mice,” the researchers wrote.
The data showed a clear difference between the two methods. Mice treated with the allele-selective ASO targeting intron 1 exhibited reduced levels of HTT1a, near elimination of protein aggregates, and improved gene expression profiles. In comparison, reducing only the full-length protein without affecting HTT1a had little effect.
The findings build on earlier studies that have suggested Huntington’s disease manifests from a toxic gain of function linked to the expanded CAG repeat. Previous research had also indicated that HTT1a is prone to aggregation and may enter the nucleus, where it disrupts gene regulation. “Undoubtedly, HTT (or N-terminal fragments of it) must enter the nucleus to form aggregates, and one proposal for HTT1a’s high toxicity is the known ability of this HTT isoform to translocate into the nucleus, preferentially seed aggregate formation, and drive transcriptional dysregulation,” the researchers wrote.
Based on their findings, the University of Washington researchers believe that HTT1a may be a primary driver of pathology. “Our findings suggest that HTT1a may have a disproportionate impact on aggregate formation and transcriptional dysregulation and that lowering the levels of HTT1a could provide benefit when designing HTT-lowering–based therapeutic strategies for HD,” the researchers wrote.
This would require a significant change in drug development strategies. Many current efforts to develop therapies for Huntington’s focus on reducing levels of the entire huntingtin protein, not just the HTT1a fragment. This new finding may help explain why some of the current approaches to treating the disease show limited benefits.
The researchers acknowledged that a limitation of their study was it could not isolate the HTT1a reduction without also lowering levels of the full-length mutant protein. They also noted that genetic differences between mouse models and human could hamper the allele-selective approach taken.
Next steps in this line of research will seek to address these issues including efforts to more precisely target the HTT1a fragment and potentially targeting the RNA that produces it.