Strategic targeting of Cas9 nickase expands tandem gene arrays

Expanding tandem gene arrays facilitates adaptation through dosage effects and gene family formation via sequence diversification. However, experimental induction of such expansions remains challenging. Here we introduce a method termed break-induced replication (BIR)-mediated tandem repeat expansion (BITREx) to address this challenge. BITREx strategically places Cas9 nickase adjacent to a tandem gene array to break the replication fork that has replicated the array, forming a single-end double-strand break. This break is subsequently end-resected to become single-stranded. Since there is no repeat unit downstream of the break, the single-stranded DNA often invades an upstream unit to initiate ectopic BIR, resulting in array expansion. BITREx has successfully expanded gene arrays in budding yeast, with the CUP1 array reaching ~1 Mb. Furthermore, appropriate splint DNA allows BITREx to generate tandem arrays de novo from single-copy genes. We have also demonstrated BITREx in mammalian cells. Therefore, BITREx will find various unique applications in genome engineering.