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Background: Colorectal cancer as a mortal disease affected both sexes of all ethnic and racial human groups. Former studies have indicated some mutations in the mitochondrial DNA (mtDNA) in different human cancers. Complex I NADH has the most subunits encoded by mtDNA. For a better understanding of the mtDNA abnormality in colorectal cancer some genes of this complex is screened for existence of mutations. Methods: One of the main regions of the mtDNA encoding protein was screened by PCR-RFLP followed by DNA sequencing. The obtained sequences were aligned with the revised Cambridge Reference Sequence (rCRS). Each altera-tion recorded as single nucleotide polymorphisms (SNPs), deletions or insertions. Results: Eight mutations were found in 15 samples out of 30 studied populations and no mutation detected in other 15 samples. Among these 15 mutated samples, 7 different mutations were found in 7 patients, that means one mutation per patient and the 8th mutation (T4216C) was common in the rest of 8 samples; in other words T4216C mutation in 27% of tested samples was identified (8 patients out of 30 patients). The existence of T4216C mutation was found to be sig-nificantly different (p ≤ 0.05) between tumoral patient′s tissue and adjacent normal tissue. Conclusions: Results showed that a high frequency of somatic alterations of mtDNA occurs during the carcinogenesis and/or the progression of colorectal cancer. Based on the mtDNA mutation pattern observed in this study and other pre-viously studies it is believed that looking for somatic mutations in mtDNA would be one of the diagnostic values in early detection of cancer.