Involvement of mitochondrial DNA sequence variations and respiratory activity in late complications following radiotherapy.

Purpose: Mitochondria and ionizing radiation overlap in a number of features; for instance, both generate harmful reactive oxygen species, and that radiation can induce cell death through the intermediary of mitochondria. Because a number of genetic variations in nuclear genes are frequently associated with response to cancer treatment, the aim of this case-control study was to test the hypothesis that mitochondrial DNA (mtDNA) genetic variations can contribute to patient-to-patient variability in normal tissue response to radiotherapy.

Experimental Design: Thirty-two nasopharyngeal carcinomas patients treated with definitive radiotherapy were included. The grade (G) of s.c. and deep tissue fibrosis was scored according to the Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer grading system. Coding and RNA mtDNA (between 611 and 15,978 bp) were sequenced, and genetic variations were scored. Mitochondrial respiratory activity was measured by resazurin reduction assay.

Results: Data showed a significantly (P = 0.003) higher number of nonsynonymous genetic variations in the radiosensitive (G(2)-G(3); 16 patients) as compared with the control (G(0)-G(1); 16 patients) groups. The nonsynonymous A10398G variation in the ND3 gene was significantly associated with fibrotic reaction (P = 0.01). The radiosensitive patients had a 7-fold (95% confidence interval, 1.16-51.65) higher risk of developing moderate to severe fibrosis (G(2)-G(3)) following radiotherapy. This was significantly correlated with lower mitochondrial respiratory activity (P = 0.001).

Conclusion: Mitochondria contribute to radiation sensitivity, and genetic variations can be associated with late reactions to radiotherapy. Predictive markers of radiosensitivity should take into account mtDNA genetic variations in addition to variations in nuclear genes.