Abstract
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-associated (Cas) systems function as an adaptive immune system in bacteria and archaea for defense against invading viruses and plasmids (Barrangou and Marraffini, 2014). The effector nucleases from some class 2 CRISPR-Cas systems have been repurposed for heterologous targeting in eukaryotic cells ( Jinek et al., 2012 ; Cong et al., 2013 ; Mali et al., 2013 ; Zetsche et al., 2015 ). However, the genomic environments of eukaryotes are distinctively different from that of prokaryotes in which CRISPR-Cas systems have evolved. Mammalian heterochromatin was found to be a barrier to target DNA access by Streptococcus pyogenes Cas9 (SpCas9), and nucleosomes, the basic units of the chromatin, were also found to impede target DNA access and cleavage by SpCas9 in vitro ( Knight et al., 2015 ; Hinz et al., 2015 ; Horlbeck et al., 2016 ; Isaac et al., 2016 ). Moreover, many CRISPR-Cas systems characterized to date often exhibit inactivity in mammalian cells and are thus precluded from gene editing applications even though they are active in bacteria or on purified DNA substrates. Thus, there is a need to devise a means to alleviate chromatin inhibition to increase gene editing efficiency, especially on difficult-to-access genomic sites, and to enable use of otherwise inactive CRISPR-Cas nucleases for gene editing need. Here we describe a proxy-CRISPR protocol for restoring nuclease activity of various class 2 CRISPR-Cas nucleases on otherwise inaccessible genomic sites in human cells via proximal targeting of a catalytically dead Cas9 ( Chen et al., 2017 ). This protocol is exemplified here by using Campylobacter jejuni Cas9 (CjCas9) as nuclease and catalytically dead SpCas9 (SpdCas9) as proximal DNA binding protein to enable CjCas9 to cleave the target for gene editing using single stranded DNA oligo templates.