Identification and characterization of synthetic viability with ERCC1 deficiency in response to interstrand crosslinks in lung cancer

Purpose: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 (D) cell lines where we identified a synthetic viable phenotype in response to ICLs with ERCC1 deficiency. Experimental Design:We utilized the CRISPR-Cas9 system to create a panel of ERCC1D lung cancer cell lines which we characterized. Results:We observe that loss of ERCC1 hypersensitizes cells to cisplatin when wildtype (WT) p53 is retained, while there is only modest sensitivity in cell lines that are p53^mutant/null. Additionally, when p53 is disrupted by CRISPR-Cas9 (p53*) in ERCC1D/p53^WT cells, there is reduced apoptosis and increased viability after platinum treatment. These results were recapitulated in two 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 ERCC1D/p53^WT and ERCC1D/p53* cells. Finally, we provide evidence that cisplatin tolerance in the context of ERCC1 deficiency relies on DNA-PKcs and BRCA1 function. Conclusions: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- repair may be able to partially compensate for loss of ERCC1.

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