Differential repair of CTG trinucleotide repeats by TALEN and CRISPR-Cas9 nucleases: roles of RAD50 and RAD52 in resection and repair.

Trinucleotide repeat expansions involving CTG or CAG triplets are responsible for several neurodegenerative disorders including myotonic dystrophy and Huntington's disease. We previously showed that a TALEN-induced double-strand break was very efficient at contracting CTG repeats in yeast, and understanding this mechanism is important for future applications in human cells. Here, we show that RAD51, POL32 and DNL4 are completely dispensable for double-strand break repair within CTG repeats, the only required genes being RAD50 and RAD52. In the absence of RAD50, there was no detectable break resection or processing. When the same experiment was performed with a CRISPR-Cas9 nuclease, CTG repeat contractions were seldom observed and large deletions around the repeat tract were frequent. Altogether, these results show that trinucleotide repeat contractions occur by Single-Strand Annealing when the double-strand break is induced by a TALEN, but an unfaithful repair mechanism leads to large chromosomal rearrangements when Cas9 is making the break.