Researchers at the University of Illinois Urbana-Champaign used a near-PAMless SpRY adenine base editor in YAC128 mice to target the splice acceptor of HTT exon 13. Published in Nature Biomedical Engineering on July 29, 2026, the intervention produced 63 per cent fewer cells containing mutant huntingtin inclusions and improved motor function.
Targeting Exon 13 Instead of the CAG Repeat
Huntington’s disease is traditionally understood through the lens of the expanded polyglutamine repeat that drives the condition. Yet an investigative team took a different route in a study published in Nature Biomedical Engineering. Rather than attempting to directly correct the expanded CAG repeat within the genetic code, the researchers chose to edit the splice acceptor of HTT exon 13.
This genetic adjustment triggered full or partial exon skipping. Specifically, the partial event excised 39 bases while preserving the reading frame of the transcript. That precise structural modification eliminated a vulnerable site where proteolysis typically generates aggregation-prone N-terminal huntingtin fragments.
Vector Delivery and Editing Efficiency in Mouse Models
To test the approach in a living system, the team divided a near-PAMless SpRY adenine base editor between two AAV9 vectors. They injected these vectors bilaterally into the striatum of one-month-old YAC128 mice, observing the subjects as they matured over a twelve-month period.
By the time the mice reached 12 months of age, sequencing and tissue analysis revealed that editing reached approximately 16 per cent in bulk striatal tissue, accompanied by nine per cent exon skipping. While these editing percentages left the majority of the tissue unmodified, the downstream biological impact proved substantial.
Measurable Reductions in Pathological Aggregates
The physical and behavioral outcomes in the treated animal models tracked closely with the molecular modifications. Treated mice displayed 60 per cent fewer N-terminal huntingtin fragments and 63 per cent fewer cells containing mutant huntingtin inclusions when compared to untreated controls.
Beyond molecular counts, the mice showed tangible physiological improvements. Grip strength, rotarod performance, and limb clasping all improved following the administration of the base editor vectors. Furthermore, measurements of brain architecture indicated that striatal and cortical volume loss was successfully reduced.
Mitochondrial Dysfunction and Cellular Mechanics
The cellular degradation characteristic of the condition extends deep into subcellular structures, particularly energy-producing organelles. Scientific literature points out that mutant huntingtin interacts with the mitochondrial protein Drp1, elevating GTPase Drp1 enzymatic activity and driving abnormal mitochondrial dynamics.
This pathological interaction increases mitochondrial fragmentation, impairs anterograde mitochondrial movement along axons, and triggers synaptic degeneration in neurons. Therapeutic strategies aim to block this toxic protein interaction, protect synapses, and restore normal mitochondrial balance across vulnerable brain regions like the caudate, putamen, cortex, and hippocampus.
Technical Hurdles and What Lies Ahead
Despite the promising motor and cellular outcomes, the study authors noted important technical limitations in their preclinical evaluation. The base editor generated bystander substitutions alongside the intended edit. Additionally, the YAC128 mouse model retains endogenous mouse Hdh and lacks a healthy human HTT allele, which constrained researchers’ ability to fully assess potential consequences for normal huntingtin protein function.
Specificity testing for the intervention was restricted to predicted off-target sites.
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