MTHFD2 is required for DNA repair and implicated in LUAD radiotherapy resistance

DNA damage-based radiotherapy (RT) is a conventional and effective local treatment for lung adenocarcinoma (LUAD). However, residual or recurrent tumors often occur due to radio-resistance. The underlying mechanisms by which radio-resistance occurs are still not fully understood. Novel predictive markers and potential therapeutic targets for LUAD radio-resistance need to be investigated. Bioinformatic tools were used to evaluate the biological functions of MTHFD2 in LUAD patients. The association of MTHFD2 with radiotherapy efficacy in LUAD patients was confirmed by immunohistochemistry. The baseline radioresponsiveness of different LUAD cells was identified and cells with acquired radio-resistance were generated. MTHFD2 in malignant phenotype and DNA damage after irradiation (IR) in LUAD cells were investigated by Western blotting, colony formation and neutral comet assays, etc. LC-MS and co-immunoprecipitation analyses were used to determine the interaction between MTHFD2 and XRCC6. Mouse models were used to verify these effects in vivo. In this study, we showed that methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) plays an important role in intrinsic and acquired radio-resistance in LUAD. High MTHFD2 expression correlates with poor RT response and reduced radiation-induced DNA damage. Mechanistically, MTHFD2 directly interacts with XRCC6 to facilitate XRCC6/XRCC5 complex formation, promoting non-homologous end joining (NHEJ)-mediated repair of DNA double-strand breaks, as evidenced by LC-MS and co-immunoprecipitation (Co-IP). This study establishes MTHFD2 as a master regulator of NHEJ-mediated radio-resistance through direct interaction with XRCC6. Our findings propose MTHFD2 expression as both a predictive biomarker for radiotherapy stratification and a therapeutic target for radiosensitization in LUAD.