Abstract
Using budding yeast, we address how Cas9 protein and its guide RNA (gRNA) create double-strand chromosome breaks (DSBs), and explore whether binding of Cas9::gRNA influences subsequent DSB repair by nonhomologous end-joining. We created pairs of gRNAs that are complementary to opposite DNA strands but direct cleavage at the same chromosomal location. The resulting repair profiles (insertion/deletions) are different for the two ostensibly identical DSBs. Most notably, there are frequent +1 insertions that are templated after cleavage creates a 1-nt 5′ overhang that is filled in before ends are ligated. DNA polymerase 4 is required for most +1 insertions and for longer (+2 and +3) insertions. We found similar templating of +1 insertions in published studies of mammalian DSBs created by Cas9.
Harnessing CRISPR-Cas9 technology provides an unprecedented ability to modify genomic loci via DNA double-strand break (DSB) induction and repair. We analyzed nonhomologous end-joining (NHEJ) repair induced by Cas9 in budding yeast and found that the orientation of binding of Cas9 and its guide RNA (gRNA) profoundly influences the pattern of insertion/deletions (indels) at the site of cleavage. A common indel created by Cas9 is a 1-bp (+1) insertion that appears to result from Cas9 creating a 1-nt 5′ overhang that is filled in by a DNA polymerase and ligated. The origin of +1 insertions was investigated by using two gRNAs with PAM sequences located on opposite DNA strands but designed to cleave the same sequence. These templated +1 insertions are dependent on the X-family DNA polymerase, Pol4. Deleting Pol4 also eliminated +2 and +3 insertions, which are biased toward homonucleotide insertions. Using inverted PAM sequences, we also found significant differences in overall NHEJ efficiency and repair profiles, suggesting that the binding of the Cas9:gRNA complex influences subsequent NHEJ processing. As with events induced by the site-specific HO endonuclease, CRISPR-Cas9–mediated NHEJ repair depends on the Ku heterodimer and DNA ligase 4. Cas9 events are highly dependent on the Mre11-Rad50-Xrs2 complex, independent of Mre11’s nuclease activity. Inspection of the outcomes of a large number of Cas9 cleavage events in mammalian cells reveals a similar templated origin of +1 insertions in human cells, but also a significant frequency of similarly templated +2 insertions.