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

Abstract

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.

Mitochondrial Epigenetics and Environmental Exposure

“…More recently, it has been shown that methylation of the Dloop in the mitochondrial genome is decreased in colorectal cancer tissues compared with non-cancerous tissues [97].…”

Experimental mitochondria-targeted DNA methylation identifies GpC methylation, not CpG methylation, as potential regulator of mitochondrial gene expression

“…Such supporting evidence, as reviewed by us elsewhere 11 , includes the discovery of a) a mitochondria-targeted human DNA methyltransferase 1 transcript variant (mtDNMT1) 12 , b) the presence of both CpG and CpH (where H is A, T or C) methylation 8, 12–15 and, importantly, c) correlations with diseases such as cancer 16 , Down syndrome 17 and diabetes 18 . Although several of these papers hint toward an effect of mtDNA methylation on mitochondrial gene expression 12, 16, 18–20 , a direct causal link has yet to be demonstrated.…”

Can Mitochondria DNA Provide a Novel Biomarker for Evaluating the Risk and Prognosis of Colorectal Cancer?

“…Feng et al investigated 44 colorectal cancer tissues and found the ratio of the methylation in D-loop region in colorectal cancer tissues was less than that in the noncancerous tissues [32]. Gao et al found the similar results in 65 colorectal cancer patients [33]. Govatati et al investigated the D-loop region of CRC patients in south Indian origin and put forward that D-loop sequence alterations were inherent risk factor for CRCs [34].…”