Common cancer-associated imbalances in the DNA damage response confer sensitivity to single agent ATR inhibition

// Fiona K. Middleton 1 , Miranda J. Patterson 1 , Claire J. Elstob 1 , Sarah Fordham 1 , Ashleigh Herriott 1 , Mark A. Wade 1 , Aiste McCormick 1 , Richard Edmondson 1 , Felicity E.B. May 1 , James M. Allan 1 , John R. Pollard 2 and Nicola J. Curtin 1 1 Newcastle University, Northern Institute for Cancer Research, Newcastle upon Tyne, UK 2 Vertex Pharmaceuticals (Europe) Limited, Milton Park, Abingdon, Oxfordshire, UK Correspondence to: Nicola J. Curtin, email: // Keywords : ATR, p53, DNA damage response, DNA-PKcs, synthetic lethality Received : April 07, 2015 Accepted : September 22, 2015 Published : October 15, 2015 Abstract ATR is an attractive target in cancer therapy because it signals replication stress and DNA lesions for repair and to S/G2 checkpoints. Cancer-specific defects in the DNA damage response (DDR) may render cancer cells vulnerable to ATR inhibition alone. We determined the cytotoxicity of the ATR inhibitor VE-821 in isogenically matched cells with DDR imbalance. Cell cycle arrest, DNA damage accumulation and repair were determined following VE-821 exposure. Defects in homologous recombination repair (HRR: ATM, BRCA2 and XRCC3) and base excision repair (BER: XRCC1) conferred sensitivity to VE-821. Surprisingly, the loss of different components of the trimeric non-homologous end-joining (NHEJ) protein DNA-PK had opposing effects. Loss of the DNA-binding component, Ku80, caused hypersensitivity to VE-821, but loss of its partner catalytic subunit, DNA-PKcs, did not. Unexpectedly, VE-821 was particularly cytotoxic to human and hamster cells expressing high levels of DNA-PKcs. High DNA-PKcs was associated with replicative stress and activation of the DDR. VE-821 suppressed HRR, determined by RAD51 focus formation, to a greater extent in cells with high DNA-PKcs. Defects in HRR and BER and high DNA-PKcs expression, that are common in cancer, confer sensitivity to ATR inhibitor monotherapy and may be developed as predictive biomarkers for personalised medicine.