Esophageal Cancer Cells Exhibit Heterogeneity in DNA Double-Strand Break Repair and G2/M Checkpoint Arrest Associated With Cell Viability After Ionizing Radiation

Purpose: Esophageal cancer lacks characteristic mutations in DNA repair genes; therefore, DNA damage response (DDR) factors have not been widely explored as predictive biomarkers in esophageal cancer. In this study, we explored the potential heterogeneity of DDR capabilities following exposure to ionizing radiation (IR). Methods and Materials: DNA repair protein RAD51 homolog 1/Rad51 recombinase (RAD51), breast cancer susceptibility gene 1, and replication protein A foci formation were analyzed in 15 esophageal cancer cell lines after IR. DNA damage signaling, including phosphorylation of ataxia telangiectasia mutation, Chk2, and Chk1, was examined by immunoblotting. G2/M checkpoint arrest after IR was assessed by scoring mitotic cells. The mode of cell death and cell viability after IR were evaluated using immunofluorescence staining and colony formation assay. Results: Notably, we found significant variations in RAD51 foci formation among 15 esophageal cancer cell lines. Analyzing 2 cell lines, with the highest and lowest RAD51 foci formation each, revealed that cells with low RAD51 foci formation (DDR-defective cell lines) exhibited impaired double-strand break (DSB) end resection, and reduced ataxia telangiectasia mutation-Chk2 and ATR-Chk1 signaling. The DDR-defective cell lines showed increased mitosis with DSBs and enhanced radiosensitivity. Conversely, DDR-proficient cell lines that exhibited intact G2/M checkpoint arrest become significantly more radiosensitive when treated with ATR or WEE1 inhibitors, which abrogate G2/M checkpoint arrest and increase mitosis with DSBs. Conclusions: Esophageal cancer cell lines with lower capability of RAD51 foci formation exhibited defective DDR and G2/M checkpoint arrest associated with higher radiosensitivity. These findings suggest novel possibilities for predicting the efficacy of DNA damage-inducing cancer therapies, such as chemoradiotherapy, based on DDR proficiency, potentially guiding personalized treatment strategies for esophageal cancer.