De-methylation of displacement loop of mitochondrial DNA is associated with increased mitochondrial copy number and nicotinamide adenine dinucleotide subunit 2 expression in colorectal cancer

DNA methylation occurs in the displacement loop (D-loop) region of mammals; however, D-loop regions of certain tumor tissue types were found to be de-methylated. Whether hypomethylation of the D-loop region is involved in the regulation of the mitochondrial DNA (mtDNA) copy number and nicotinamide adenine dinucleotide subunit 2 (ND-2) expressions in colorectal cancer has remained elusive. In the present study, the association between methylation status of the D-loop region, mtDNA copy number and ND-2 expression was investigated in 65 colorectal cancer specimens and their corresponding non-cancerous tissues. In addition, a de-methylation experiment was performed on the Caco-2 colorectal cancer cell line by using 5-aza-2'-deoxycytidine (5-Aza). The methylation rate of the D-loop region in all 65 colorectal cancer tissues was markedly reduced when compared with that of their corresponding non-cancerous tissues (13.8 vs. 81.5%; P<0.05). Furthermore, the methylation rate of the D-loop region in colorectal cancer tissues was markedly decreased in clinicopathological stages III and IV compared with that in clinicopathological stages I and II (7.1 and 0% vs. 25 and 16%; P<0.05). In addition, the mean relative mtDNA copy number and ND-2 expression in colorectal cancer tissues were increased compared with those in the corresponding non-cancerous tissues. De-methylation of the D-loop region was associated with an elevated mtDNA copy number and an increased ND-2 expression. Furthermore, the mtDNA copy number and ND-2 expression in Caco-2 cells were significantly increased after 5-Aza treatment. In conclusion, de-methylation of the D-loop region is likely to be involved in the regulation of the mtDNA copy number and ND-2 expression.