A systematic CRISPR screen defines mutational mechanisms underpinning signatures caused by replication errors and endogenous DNA damage

Published: 19 April 2022| Version 3 | DOI: 10.17632/ymn3ykkmyx.3
Serena Nik-Zainal


Mutational signatures are imprints of pathophysiological processes arising through tumorigenesis. We generated isogenic CRISPR–Cas9 knockouts (∆) of 43 genes in human induced pluripotent stem cells, cultured them in the absence of added DNA damage and performed whole-genome sequencing of 173 subclones. ∆OGG1, ∆UNG, ∆EXO1, ∆RNF168, ∆MLH1, ∆MSH2, ∆MSH6, ∆PMS1 and ∆PMS2 produced marked mutational signatures indicative of them being critical mitigators of endogenous DNA modifications. Detailed analyses revealed mutational mechanistic insights, including how 8-oxo-2′-deoxyguanosine elimination is sequence context specific while uracil clearance is sequence context independent. Mismatch repair (MMR) deficiency signatures are engendered by oxidative damage (C > A transversions) and differential misincorporation by replicative polymerases (T > C and C > T transitions), and we propose a reverse template slippage model for T > A transversions. ∆MLH1, ∆MSH6 and ∆MSH2 signatures were similar to each other but distinct from ∆PMS2. Finally, we developed a classifier, MMRDetect, where application to 7,695 whole-genome-sequenced cancers showed enhanced detection of MMR-deficient tumors, with implications for responsiveness to immunotherapies.



University of Cambridge


Genomics, Cancer Genetics, CRISPR/Cas9