Cas goes rogue

Sundaresan, R. et al. Cell Rep. 21, 3728–3739 (2017).

Conventional wisdom has it that Cas nucleases need guide RNAs to find and cleave their targets, but this may not always be true. Sundaresan et al. report that, given high enough manganese concentrations, certain Cas species cleave DNA randomly without any sequence specificity and without guide RNA. They tested Cas9 from Streptococcus pyogenes (SpCas9), the most widely used Cas protein for genome editing, and Cas9 and Cpf1 from Francisella tularensis novicida (Fno). Purified SpCas9 and FnoCpf1 degrade linear and circular single-strand DNA at 250 μm Mn²⁺, and FnoCas9 nicks double-stranded DNA (dsDNA) at 500 mm Mn²⁺—levels these proteins could encounter in a cell. Sundaresan et al. did not observe nonspecific activity of SpCas9 on dsDNA, so it is unlikely that this finding will impact the nuclease's usefulness, but it is something to keep in mind.