Introduction The DNA damage response (DDR) is an innate cellular response allowing cells to halt the cell cycle and repair DNA damage sustained by activating various mechanisms. The efficacy of conventional cancer treatment modalities is related in their ability to induce DNA damage. Constitutive activation of the ataxia telangectasia mutated (ATM) dependent DDR and repair pathways have been reported in (pre) malignant human tissues and may undermine the efficacy of current cancer therapies. Inhibition of proteins involved in the DDR cascade is an attractive therapeutic concept that may overcome resistance to current cytotoxics and potentiate the effects of radiotherapy.
Methods A tumour microarray was created using 179 sporadic colorectal cancers; 152 were of the microsatellite stable phenotype. The microarray was interrogated using antibodies against proteins of the DDR signalling cascade. A colorectal cancer cell line model was utilised to asses the functionality of the constitutively activated DNA damage pathway. ATM inhibition in combination with ionising irradiation was analysed in the cell line model using radioactive quantification of DNA synthesis, flow cytometric cell separation, clonogenic survival and immunoblotting.
Results Phosphorylated Chk2 threonine-68, a surrogate marker of the DDR, was present in 22% of microsatellite-stable colorectal tumours and 33% of tumours with the microsatellite instability phenotype. High p53 staining was present in 53% of microsatellite stable cancers and 26% microsatellite instable cancers.
P21-null HCT116 cells display constitutive activation of the ATM DDR 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. P21-null HCT116 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.
Conclusion In a colorectal cancer cell line model constitutive activation of the ATM DDR pathway reflected a non-functional pathway and inhibition of ATM in these circumstances was unable to potentiate the efficacy of ionising irradiation. Basal Chk2 threonine-68 phosphorylation in colorectal cancer may reflect a deregulated ATM DDR pathway and/or checkpoint adaption.
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.
Disclosure of Interest None Declared
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