Modulation of Genome Editing Outcomes by Cell Cycle Control of Cas9 Expression

Targeting specific chromosomal sequences for genome modification or regulation during particular phases of the cell cycle may prove useful in creating more precise, predictable genetic changes. Here, we present a system using a fusion protein comprised of a programmable DNA modification protein, Cas9, linked to a cell cycle regulated protein, geminin, as well as green fluorescent protein (GFP) for visualization. Despite the large size of Cas9 relative to geminin, cells were observed to express Cas9-GFP-geminin at levels which oscillate with the cell cycle. These fusion proteins are also shown to retain double-strand break (DSB) activity at specific chromosomal sequences to produce both indels and targeted integration of donor ssDNA. Most importantly, the ratio of ssDNA donor integration to non-homologous end joining (NHEJ) was observed to increase, suggesting that cell cycle control Cas9 expression may be an effective strategy to bias DNA repair outcomes.