D-loop somatic mutations and ∼5 kb "common" deletion in mitochondrial DNA: important molecular markers to distinguish oral precancer and cancer.

Apart from genomic DNA, mutations at mitochondrial DNA (mtDNA) have been hypothesized to play vital roles in cancer development. In this study, ∼5 kb deletion and D-loop mutations in mtDNA and alteration in mtDNA content were investigated in buccal smears from 104 healthy controls and 74 leukoplakia and 117 cancer tissue samples using Taqman-based quantitative assay and re-sequencing. The ∼5 kb deletion in mtDNA was significantly less (9.8 and 10.5 folds, P < 0.0001) in cancer tissues compared to control and leukoplakia tissues, respectively. On the other hand, somatic mutations in D-loop, investigated in 54 controls, 50 leukoplakias and 56 cancer patients, were found to be significantly more in cancer tissues, but not in leukoplakia tissues, compared to control (Z-score = 5.4). MtDNA contents were observed to be significantly more in leukoplakia (2.1 folds, P = 0.004) and cancer (1.6 folds, P = 0.03) tissues compared to control tissues. So, D-loop somatic mutations and ∼5 kb deletion patterns could be used as distinguishing markers between precancer and cancer tissues. This observation further suggests that somatic mutations in D-loop may facilitate carcinogenesis and cancer cells with less ∼5 kb deletion, i.e., intact mtDNA, may become resistant to apoptosis.