Base editing strategies to convert CAG to CAA diminish the disease-causing mutation in Huntington's disease

An expanded CAG repeat in the huntingtin gene (HTT) causes
Huntington's disease (HD). Since the length of uninterrupted CAG
repeat, not polyglutamine, determines the age-at-onset in HD, base editing
strategies to convert CAG to CAA are anticipated to delay onset by
shortening the uninterrupted CAG repeat. Here, we developed base editing
strategies to convert CAG in the repeat to CAA and determined their
molecular outcomes and effects on relevant disease phenotypes. Base
editing strategies employing combinations of cytosine base editors and
gRNAs efficiently converted CAG to CAA at various sites in the CAG repeat
without generating significant indels, off-target edits, or transcriptome
alterations, demonstrating their feasibility and specificity. Candidate BE
strategies converted CAG to CAA on both expanded and non-expanded CAG
repeats without altering HTT mRNA and protein levels. In addition, somatic
CAG repeat expansion, which is the major disease driver in HD, was
significantly decreased in the liver by a candidate BE strategy treatment
in HD knock-in mice carrying canonical CAG repeats. Notably, CAG repeat
expansion was abolished entirely in HD knock-in mice carrying
CAA-interrupted repeats, supporting the therapeutic potential of
CAG-to-CAA conversion strategies in HD and potentially other repeat
expansion disorders.