Andrew Paul May

Guide RNA Functional Modules Direct Cas9 Activity and Orthogonality

The RNA-guided Cas9 endonuclease specifically targets and cleaves DNA in a sequence-dependent manner and has been widely used for programmable genome editing. Cas9 activity is dependent on interactions with guide RNAs, and evolutionarily divergent Cas9 nucleases have been shown to work orthogonally. However, the molecular basis of selective Cas9:guide-RNA interactions is poorly understood. Here, we identify and characterize six conserved modules within native crRNA:tracrRNA duplexes and single guide RNAs (sgRNAs) that direct Cas9 endonuclease activity. We show the bulge and nexus are necessary for DNA cleavage and demonstrate that the nexus and hairpins are instrumental in defining orthogonality between systems. In contrast, the crRNA:tracrRNA complementary region can be modified or partially removed. Collectively, our results establish guide RNA features that drive DNA targeting by Cas9 and open new design and engineering avenues for CRISPR technologies.

Unraveling the potential of CRISPR-Cas9 for gene therapy.

The molecular machinery from the prokaryotic clustered regularly interspaced short palindromic repeats (CRISPR)-Cas immune system has broadly been repurposed for genome editing in eukaryotes. In particular, the sequence-specific Cas9 endonuclease can be flexibly harnessed for the genesis of precise double-stranded DNA breaks, using single guide RNAs that are readily programmable. The endogenous DNA repair machinery subsequently generates genome modifications, either by random insertion or deletions using non-homologous end joining (NHEJ), or designed integration of mutations or genetic material using homology-directed repair (HDR) templates. This technology has opened new avenues for the investigation of genetic diseases in general, and for gene therapy applications in particular.