Novel heteroplasmic frameshift and missense somatic mitochondrial DNA mutations in oral cancer of betel quid chewers.

Mitochondrial DNA (mtDNA) has been proposed to be involved in carcinogenesis because of its high susceptibility to oxidative DNA damage and limited repair mechanisms. For investigation of the potential role of somatic mtDNA mutations in the tumorigenesis of oral cancer, we screened the occurrence of mtDNA mutations by the temporal temperature gradient gel electrophoresis method. We amplified the entire mitochondrial genome by use of 32 pairs of overlapping primers, and to identify the mutations, we sequenced DNA fragments showing different banding patterns between normal and tumor mtDNA. Fourteen of eighteen (77.8%) oral carcinomas displayed somatic mtDNA mutations, with a total of 26 mutations. Among them, six were in the mRNA coding region. Three were missense mutations (C14F, H186R, T173P) in NADH dehydrogenase subunit 2, and one was a frameshift mutation, 9485delC, in cytochrome c oxidase subunit III. Eight (44%) tumors had insertion or deletion mutations in the nucleotide position 303-309 poly C region of the D-loop. Multiple large deletions were also observed. Our results demonstrate that somatic mtDNA mutations occur in oral cancer. Some missense and frameshift mutations may play an important role in the tumorigenesis of this carcinoma. More extensive biochemical and molecular studies will be necessary for determining the pathologic effect of these somatic mutations.