Cas9 of bacterial pathogens induces DNA damage and apoptosis of human cells

CRISPR-Cas is an RNA-based defense system that enables prokaryotes to recognize invading foreign DNA by cognate crRNA guides and destroy it by CRISPR-associated Cas nucleases 1,2 . Elucidation of the interference mechanism of the Streptococcus pyogenes Type II CRISPR- Cas9 system has allowed for the successful repurposing of SpCas9 as a generic genome editing tool, with great promise for human gene therapy 3 . However, especially for therapeutic applications, some caution seems appropriate, because Cas9 systems from some human pathogens may induce a cytotoxic response via an unknown mechanism 4 . Here we show that when released in human cells, Cas9 nucleases from the pathogenic bacteria Campylobacter jejuni and S. pyogenes have the potential to cause severe DNA damage. In the absence of a CRISPR RNA guide, native Cas9 nucleases from both pathogens enter the host nucleus, where their presence leads to promiscuous double stranded DNA breaks (DSBs) and induction of cell death. DSB induction can be reduced to background levels either by saturation of CjCas9 and SpCas9 with crRNA guides or by inactivating their nuclease activity. Our results demonstrate that guide-free Cas9 of bacterial pathogens might play an important role in pathogenicity. Furthermore, we propose that saturating Cas9 with appropriate guide RNAs is crucial for efficient and safe therapeutic applications.