Cas9 nickase-mediated contractions of CAG/CTG repeats are transcription-dependent and replication-independent

There is currently no disease-modifying treatment for expanded CAG/CTG repeat disorders. Given that longer repeat tracts lead to an earlier age of disease onset and faster progression, contracting them is expected to improve symptoms and/or delay onset. We have previously demonstrated that the Cas9 D10A nickase can effectively contract CAG/CTG repeats when targeted to the repeat tract itself. However, the mechanism remains unclear. Here, we tested whether nickase-mediated contractions depend on transcription or on replication using human cell models. We find that transcription promotes contractions and that they occur independently of the rate of cell division. These results support the therapeutic potential of this approach in non-dividing cells.