Methyltransferase 1 is required for nonhomologous end‐joining repair and renders hepatocellular carcinoma resistant to radiotherapy

Background and Aims: Radiotherapy is an increasingly essential therapeutic strategy in the management of hepatocellular carcinoma (HCC). Nevertheless, resistance to radiotherapy is one of the primary obstacles to successful treatment outcomes. Hence, we aim to elucidate the mechanisms underlying radioresistance and identify reliable biotargets that would be inhibited to enhance the efficacy of radiotherapy in HCC. Approach and Results: From a label‐free quantitative proteome screening, we identified transfer RNA (tRNA; guanine‐ N [7]‐) methyltransferase 1 (METTL1), a key enzyme for N7‐methylguanosine (m 7 G) tRNA modification, as an essential driver for HCC cells radioresistance. We reveal that METTL1 promotes DNA double‐strand break (DSB) repair and renders HCC cells resistant to ionizing radiation (IR) using loss‐of‐function and gain‐of‐function assays in vitro and in vivo. Mechanistically, METTL1‐mediated m 7 G tRNA modification selectively regulates the translation of DNA‐dependent protein kinase catalytic subunit or DNA ligase IV with higher frequencies of m 7 G‐related codons after IR treatment, thereby resulting in the enhancement of nonhomologous end‐joining (NHEJ)–mediated DNA DSB repair efficiency. Clinically, high METTL1 expression in tumor tissue is significantly correlated with poor prognosis in radiotherapy‐treated patients with HCC. Conclusions: Our findings show that METTL1 is a critical enhancer for HCC cell NHEJ‐based DNA repair following IR therapy. These findings give insight into the role of tRNA modification in messenger RNA translation control in HCC radioresistance.