Abstract
Radiotherapy is an effective treatment for lung cancer but lacks a reliable prediction method. Cell-free nucleic acids in plasma have been reported as a novel tumor marker. Here, we evaluate circulating succinate dehydrogenase 5 (SDH5) mRNA in plasma and SDH5 protein in tumors, assess their predictive value in lung cancer patients undergoing radiotherapy, and explore the underlying mechanisms. Methods: SDH5 expression was measured in peripheral blood samples and fresh tumor specimens from 208 non-small cell lung cancer (NSCLC) patients and correlated with clinical outcomes. SDH5 knockout mice and human xenograft mice were used to evaluate radiosensitivity. Cell growth, apoptosis, and the DNA damage response were assessed. Relevant RNA and protein levels were analyzed by qRT-PCR and Western blotting. Immunoprecipitation and GST pulldown assays were performed to detect protein-protein interactions. Polyubiquitination of p53 was examined by an in vitro ubiquitination assay. Results: Plasma and tumor SDH5 mRNA levels were positively correlated (rho=0.894, P<0.001). Patients with relatively low SDH5 levels in plasma (0.47, 0.12-0.89) and tumors (3.85, 0.96-7.23) had a better prognosis after radiotherapy (median PFS: 30.0 versus 15.0 months, hazard ratio: 0.276, 95% CI: 0.201-0.379, P<0.001). In SDH5 knockout mice, the lung epithelial cells exhibited increased DNA damage after radiation. In human lung xenograft mice, SDH5-deficient tumors had a smaller volume after radiotherapy. Furthermore, SDH5 depletion inhibits p53 degradation via the ubiquitin/proteasome pathway, which promotes apoptosis and enhances radiosensitivity in NSCLC. Conclusion: Our findings provide a novel noninvasive method for prediction of response to radiotherapy and may have significant implications for cancer radiotherapy.