A human CAGinSTEM platform for decoding HTT repeats' somatic instability links CAG interruption to HD pathology in neurons.

Somatic CAG instability in the mutant Huntingtin (HTT) gene is increasingly recognized as a key hallmark of Huntington's disease (HD). Using our novel human CAGinSTEM platform, we manipulated cis genetic elements influencing instability in human HD neurons, monitoring repeat length. Quality-controlled CRISPR-engineered stem cells with increasing CAG lengths and clinical haplotypes were analyzed using third-generation sequencing. Our findings link interruptions in the CAG repeat, especially the loss or duplication of the penultimate CAA of canonical alleles, to significant instability modulation. Notably, four internal CAA interruptions completely abolish CAG instability, reversing HD phenotypes such as altered striatal fate acquisition and nuclear disorganization. This platform highlights the role of cis modifiers, emphasizing the direct influence of HTT DNA repeat composition on CAG instability and providing a robust framework for modeling HTT repeat instability in vitro.