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Rather Than Deleting Huntingtin, Rewriting Its “Splicing Route”: Base Editing Mitigates Brain Pathology in Mice
Researchers used in vivo CRISPR base editing to skip HTT exon 13, preventing the mutant protein from being cleaved into toxic fragments prone to aggregation; brain atrophy was subsequently reduced in mice, but delivery, off-target effects, long-term safety, and other hurdles remain before the approach can become a human treatment.
The challenge of Huntington’s disease lies not only in patients producing abnormal huntingtin protein, but also in the protein being cleaved into shorter, toxic fragments that aggregate more readily. A study published in *Nature Biomedical Engineering* proposes another therapeutic approach: rather than directly deleting the HTT gene or repairing the disease-causing CAG repeat sequence, it alters RNA splicing so that the protein bypasses a critical cleavage step.
The research team screened 141 CRISPR base-editing combinations targeting HTT splicing elements and ultimately selected the splice acceptor sequence of exon 13. After this site was rewritten, cells skipped exon 13 when processing HTT precursor messenger RNA, producing a huntingtin protein isoform that was less susceptible to cleavage by caspase-6.
The central feature of this design is that it intercepts the pathological process at an intermediate stage. Huntington’s disease is caused by abnormal expansion of the CAG sequence in HTT exon 1. After the mutant protein is cleaved by caspase-6, it forms N-terminal fragments that readily accumulate into aggregates and damage neurons in the striatum and cerebral cortex. The study did not remove the disease-causing repeat sequence, but instead sought to make the mutant protein less prone to fragmentation.
The researchers delivered the base-editing system to the striatum of mouse models of Huntington’s disease. The results showed increased exon 13 skipping and reductions in toxic N-terminal fragments and huntingtin protein aggregation; compared with the control group, atrophy of the striatum and cerebral cortex was also alleviated. These findings extend the main conclusions of a 2024 preprint, which have now been formally published following peer review.
Base editing can alter specific sites without cutting both strands of DNA and could theoretically avoid some of the risks of large deletions or complex repair products caused by conventional CRISPR nucleases. However, the DNA changes it leaves are likewise long-lasting; if editing occurs at unintended sites, or if exon 13 skipping affects the function of normal huntingtin protein, the consequences could be difficult to reverse.
The evidence currently remains limited to mice. The study’s core findings primarily concern pathological indicators such as protein fragments, aggregation, and brain atrophy, and are not yet sufficient to demonstrate that the approach can improve patients’ symptoms, slow disease progression, or extend survival. Before it can advance to humans, the extent of delivery within the brain, effective dose, immune responses, DNA and RNA off-target effects, and the safety of long-term editor expression must still be clarified; efficacy and tolerability in large animals will also be critical tests.