Constitutive activation of the ATM DNA damage response pathway in cancer represents a deregulated pathway
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Constitutive activation of the ATM dependent DNA damage response and repair pathways have been reported in pre-malignant and malignant human tissues and may undermine the efficacy of genotoxic cancer therapies. Therefore, ATM inhibitors may overcome resistance to current cytotoxics and potentiate the effects of radiotherapy. A colorectal cancer model was investigated to develop a framework for the rational use of ATM inhibitors. HCT116 p21-/- cells display constitutive activation of the ATM DNA damage response but display a defect in the ionising radiation induced S-phase checkpoint, termed radioresistant DNA synthesis. This radioresistant phenotype is associated with increased basal levels of Cdc25A protein, deficient DNA damage-induced degradation of Cdc25A and Chk2 mis-localisation. HCT116 p21-/- and SW620 cells, which exhibit basal Chk2 threonine-68 phosphorylation, were unable to abrogate the S-phase checkpoint when treated with an ATM inhibitor, suggesting that the ATM– Chk2 arm is non-functional in these cells: inhibition of ATM did not potentiate the efficacy of ionising irradiation. To assess activation of the pathway a tumour microarray was created using 179 treatment naïve sporadic colorectal cancers; 152 were of the microsatellite stable phenotype. Phosphorylated Chk2 threonine-68 was present in 22 % of microsatellite-stable colorectal tumours and 33 % of tumours with the microsatellite instability phenotype. In a colorectal cancer cell line model constitutive activation of the ATM DDR pathway reflected an attenuated ATM-Chk2 axis and inhibition of ATM in these circumstances was unable to potentiate the efficacy of ionising irradiation. Basal Chk2 threonine-68 phosphorylation may reflect a deregulated ATM DNA damage response pathway and/or checkpoint adaption and therefore use of an ATM inhibitor in this background may have limited efficacy. A predictive model is proposed that integrates functionality of the ATM-Chk2 axis, p53 mutation status and defects in DNA repair pathways when considering ATM inhibitor therapy. Ultimately, molecular phenotyping and functional analysis of processes deregulated in cancer will permit individualisation of current treatment modalities, improving their efficacy and limiting patient toxicity.