Therapeutic validation of MMR-associated genetic modifiers in a human ex vivo model of Huntington’s disease

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Abstract
The pathological huntingtin ( HTT ) trinucleotide repeat underlying Huntington’s disease (HD) continues to expand throughout life. Repeat length correlates both with earlier age at onset (AaO) and faster progression, making slowing its expansion an attractive therapeutic approach. Genome-wide association studies have identified candidate variants associated with altered AaO and progression, with many found in DNA mismatch repair (MMR) associated genes. We examine whether lowering expression of these genes affects the rate of somatic expansion in human ex vivo models using HD iPSCs and HD iPSC-derived striatal neurons. We have generated a stable CRISPR interference HD iPSC line in which we can specifically and efficiently lower gene expression from a donor carrying over 125 CAG repeats. Lowering expression of each member of the MMR complexes MutS (MSH2, MSH3 & MSH6), MutL (MLH1, PMS1, PMS2 & MLH3) and LIG1 resulted in characteristic MMR deficiencies. Reduced MSH2, MSH3 and MLH1 slowed repeat expansion to the largest degree, while lowering either PMS1, PMS2 and MLH3 slowed it to a lesser degree. These effects were recapitulated in iPSC derived striatal cultures where MutL factor expression was lowered. Here, reducing the expression of MMR factors by CRISPRi to levels typically reached by current therapeutics effectively slows the pathogenic expansion of the HTT CAG repeat tract. We highlight members of the MutL family as potential therapeutic targets to slow repeat expansion with the aim to delay onset and progression of HD, and potentially other repeat expansion disorders exhibiting somatic instability. GRAPHICAL

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