Instead of Shutting Down HTT, Rewrite Its Splicing Path: Base Editing Alleviates Huntington’s Disease Pathology in Mice

The challenge of treating Huntington’s disease lies not only in reducing the disease-causing protein, but also in avoiding the careless sacrifice of its normal physiological functions. A team at the University of Illinois Urbana-Champaign has proposed another approach: rather than completely shutting down the HTT gene, it fine-tunes how cells read it, making the resulting huntingtin protein less likely to be cleaved into fragments that aggregate and damage neurons.

The study, published in *Nature Biomedical Engineering*, targeted the splice acceptor of HTT exon 13. After screening 141 CRISPR base editors, the researchers altered a single base without cutting both strands of DNA, prompting cells to skip either all or a small portion of exon 13. One resulting product lacked 39 bases and retained the protein reading frame while removing a site associated with caspase-6 cleavage.

The team then used two AAV9 vectors to deliver the split editor through direct injection into the striatum of YAC128 Huntington’s disease mice. The better-performing adenine base editor achieved an editing rate of approximately 16% across striatal tissue; cell-enrichment analysis estimated the effective neuronal editing rate at approximately 18%. Although this proportion was not high, it reduced HTT protein fragments in the striatum by approximately 60%, while the proportion of cells containing mutant HTT inclusions also declined significantly.

The pathological changes were also reflected in the animals’ performance. Treated mice showed improvements in grip strength, rotarod, and limb-clasping tests, while magnetic resonance imaging indicated that atrophy of the striatum and cerebral cortex was also suppressed. However, no differences were observed in open-field activity or body weight. Animal numbers were limited across the experiments, so these findings should be regarded as signals of mechanism and efficacy rather than predictions of effects in humans.

### Background

Existing HTT-lowering strategies mostly target RNA, and some approaches reduce both mutant and normal HTT; other allele-specific designs are limited by patients’ genotypes. This approach does not depend on a specific mutant haplotype, and the DNA modification may persist over the long term. However, “not completely shutting it down” does not mean that preservation of normal function has been demonstrated. In cell experiments, full-length HTT still declined, and editing also produced changes in neighboring bases, indicating that both precision and tolerability require broader evaluation.

The greatest practical obstacle is delivery. The study used intracranial stereotactic injection and dual AAV9 vectors, which cannot yet be directly translated into a treatment covering the extensive regions of the human brain. Once permanent editing occurs, it is also difficult to reverse. The research team is exploring less invasive, nonviral delivery methods. Future work will also need to use genome-wide methods to investigate off-target effects and confirm long-term neurological and immune safety at different doses.

In addition, YAC128 mice retain their own normal Hdh gene and do not simultaneously carry normal human HTT, potentially masking functional losses caused by non-allele-specific editing. The next steps toward preclinical development will include validating protein function in models containing both normal and mutant human HTT, assessing target engagement and tolerability in large animals, and defining a therapeutic window that produces efficacy without excessively depleting normal HTT.