It has been proposed that the presence of heteroplasmy in the hypervariable (HV) regions of the mitochondrial DNA (mtDNA) may be an indicator of mitochondrial genome instability, mtDNA dysfunction, and, thus, may be associated with increased cancer risk. However, whether heteroplasmy in the HV regions of mtDNA could be a risk predictor of oxidative stress-related human cancers, such as breast cancer, remains to be determined. To explore the role of heteroplasmy in the HV regions of mtDNA in breast cancer etiology, we analyzed heteroplasmy in the HV regions of mtDNA in whole blood from 103 patients with breast cancer and 103 matched control subjects. Both cases and controls displayed heteroplasmies in both of the HV1 and HV2 regions. Closer examination of the prevalence of length heteroplasmy indicated that the prevalence of heteroplasmies in both of the HV1 and HV2 regions was much higher in the cases than in the controls (HV1: 68% vs 49%, P=0.007; HV2: 46% vs 25%, P=0.002). The presence of length heteroplasmies in both of the HV1 and HV2 regions was associated with 2.18- and 2.49-folds increased risk of breast cancer, respectively, (HV1: OR=2.18, 95% CI: 1.19 - 4.00; HV2: OR=2.49, 95% CI: 1.32 - 4.69). Interestingly, we observed that the controls with length heteroplasmies in both HV1 and HV2 had statistically significantly lower copy number of mtDNA than the ones without heteroplasmies. These results suggest that the length heteroplasmy in the HV regions of mtDNA could be associated with a risk of breast cancer, perhaps through affecting the copy number of mtDNA
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