PT  - JOURNAL ARTICLE
AU  - Eirini M Kallimasioti-Pazi
AU  - Keerthi Thelakkad Chathoth
AU  - Gillian C Taylor
AU  - Alison Meynert
AU  - Tracy Ballinger
AU  - Martijn Kelder
AU  - Sébastien Lalevée
AU  - Ildem Sanli
AU  - Robert Feil
AU  - Andrew J Wood
TI  - Heterochromatin delays CRISPR-Cas9 mutagenesis but does not influence repair outcome
AID  - 10.1101/267690
DP  - 2018 Jan 01
TA  - bioRxiv
PG  - 267690
4099  - http://biorxiv.org/content/early/2018/02/19/267690.short
4100  - http://biorxiv.org/content/early/2018/02/19/267690.full
AB  - CRISPR-Cas9 genome editing occurs in the context of chromatin, which is heterogeneous in structure and function across the genome. Chromatin heterogeneity is thought to affect genome editing efficiency, but this has been challenging to quantify due to the presence of confounding variables. Here, we develop a method that exploits the allele-specific chromatin status of imprinted genes in order to address this problem. Because maternal and paternal alleles of imprinted genes have identical DNA sequence and are situated in the same nucleus, allele-specific differences in the frequency and spectrum of Cas9-induced mutations can be attributed unequivocally to epigenetic mechanisms. We found that heterochromatin can impede mutagenesis, but to a degree that depends on other key experimental parameters. Mutagenesis was impeded by up to 7-fold when Cas9 exposure was brief and when intracellular Cas9 expression was low. Surprisingly, the outcome of mutagenic DNA repair was independent of chromatin state, with similar efficiencies of homology directed repair and deletion spectra on maternal and paternal chromosomes. Combined, our data show that heterochromatin imposes a permeable barrier that influences the kinetics, but not the endpoint of CRISPR-Cas9 genome editing, and suggest that therapeutic applications involving low-level Cas9 exposure will be particularly affected by chromatin status.