Differential cellular responses to prolonged LDR-IR in MLH1-proficient and MLH1-deficient colorectal cancer HCT116 cells

Purpose: MLH1 is a key DNA mismatch repair (MMR) protein involved in maintaining genomic stability by participating in the repair of endogenous and exogenous mispairs in the daughter strands during S phase. Exogenous mispairs can result following treatment with several classes of chemotherapeutic drugs, as well as with ionizing radiation. In this study, we investigated the role of the MLH1 protein in determining the cellular and molecular responses to prolonged low–dose rate ionizing radiation (LDR-IR), which is similar to the clinical use of cancer brachytherapy. Experimental Design: An isogenic pair of MMR+ (MLH1+) and MMR− (MLH1−) human colorectal cancer HCT116 cells was exposed to prolonged LDR-IR (1.3-17 cGy/h × 24-96 h). The clonogenic survival and gene mutation rates were examined. Cell cycle distribution was analyzed with flow cytometry. Changes in selected DNA damage repair proteins, DNA damage response proteins, and cell death marker proteins were examined with Western blotting. Results: MLH1+ HCT116 cells showed greater radiosensitivity with enhanced expression of apoptotic and autophagic markers, a reduced HPRT gene mutation rate, and more pronounced cell cycle alterations (increased late-S population and a G2/M arrest) following LDR-IR compared with MLH1− HCT116 cells. Importantly, a progressive increase in MLH1 protein levels was found in MLH1+ cells during prolonged LDR-IR, which was temporally correlated with a progressive decrease in Rad51 protein (involved in homologous recombination) levels. Conclusions: MLH1 status significantly affects cellular responses to prolonged LDR-IR. MLH1 may enhance cell radiosensitivity to prolonged LDR-IR through inhibition of homologous recombination (through inhibition of Rad51).