Low Expression of ECT2 Confers Radiation Therapy Resistance Through Transcription Coupled Nucleolar DNA Damage Repair

Radioresistance contributes to poor clinical therapeutic efficacy in most cancers. Emerging evidence shows that aberrant DNA damage repair is involved in radioresistance. This study aimed to elucidate the mechanism for radioresistance and explore the precise treatment to sensitize the radioresistant tumors.Real-time polymerase chain reaction and Western blot were used to confirm the differential expression of epithelial cell transforming 2 (ECT2) in irradiation-resistant and sensitive cell lines. Laser microirradiation was used to examine the ribosome DNA (rDNA) damage response of ECT2. Biotin-identification, in vivo, in vitro binding assay, and dot blotting were used to confirm the interaction of ECT2 and PARP1. The xenograft mouse model and cell survival assay were used to assess the irradiation sensitivity with or without PARP1 inhibitor.We found the expression of ECT2 correlates with sensitivity to radiation therapy in both lung cancer and nasopharyngeal carcinoma. We demonstrated that low expression of ECT2 causes radioresistance, mainly by protecting rDNA in nucleoli from persistent irradiation exposure through transcriptional recovery prevention. ECT2 is recruited to the rDNA damage site in an ataxia-telangiectasia-mutated RNA polymerase I dependent manner. The recruited ECT2 interacts with PARP1 and facilitates the disassociation of PARP1 from rDNA in nucleoli. Thus, ECT2 deficiency results in sustained activation of PARP1, which subsequently inhibits nucleolar transcription and results in a low frequency of rDNA exposure under DNA damage. PARP inhibition synergized with irradiation can sensitize radioresistant tumors with low ECT2 expression.Our study provides a potential perspective for the application of PARP inhibitor to sensitize low-ECT2 expressing tumors to radiation therapy.