Identification and Characterization of Synthetic Viability with ERCC1 Deficiency in Response to Interstrand Crosslinks in Lung Cancer.

ERCC1/XPF is a DNA endonuclease with variable expression in primary tumor specimens, and has been investigated as a predictive biomarker for efficacy of platinum-based chemotherapy. The failure of clinical trials utilizing ERCC1 expression to predict response to platinum-based chemotherapy suggests additional mechanisms underlying the basic biology of ERCC1 in the response to interstrand crosslinks (ICLs) remain unknown. We aimed to characterize a panel of ERCC1 knockout (Δ) cell lines, where we identified a synthetic viable phenotype in response to ICLs with ERCC1 deficiency. We utilized the CRISPR-Cas9 system to create a panel of ERCC1Δ lung cancer cell lines which we characterized. We observe that loss of ERCC1 hypersensitizes cells to cisplatin when wild-type (WT) p53 is retained, whereas there is only modest sensitivity in cell lines that are p53mutant/null. In addition, when p53 is disrupted by CRISPR-Cas9 (p53*) in ERCC1Δ/p53WT cells, there is reduced apoptosis and increased viability after platinum treatment. These results were recapitulated in 2 patient data sets utilizing p53 mutation analysis and ERCC1 expression to assess overall survival. We also show that kinetics of ICL-repair (ICL-R) differ between ERCC1Δ/p53WT and ERCC1Δ/p53* cells. Finally, we provide evidence that cisplatin tolerance in the context of ERCC1 deficiency relies on DNA-PKcs and BRCA1 function. Our findings implicate p53 as a potential confounding variable in clinical assessments of ERCC1 as a platinum biomarker via promoting an environment in which error-prone mechanisms of ICL-R may be able to partially compensate for loss of ERCC1.See related commentary by Friboulet et al., p. 2369.