Identification of sequence polymorphisms in the mitochondrial cytochrome c oxidase genes as risk factors for hepatocellular carcinoma

Single nucleotide polymorphisms (SNPs) accumulated in the mitochondrial DNA (mtDNA) is susceptible to the tumor formation. We discovered previously that SNPs in the mitochondrial displacement loop (D‐loop) was associated with the risk of hepatocellular carcinoma (HCC).


| INTRODUCTION
Hepatocellular carcinoma (HCC) is the fifth most frequent cancer and is responsible for more than half a million deaths each year, which makes it the third leading cause of cancer deaths worldwide. 1 In China, the incidence of HCC is increasing and now accounts for 55% of all HCC cases in the world. 2,3 This disease is strongly associated with several risk factors, including chronic hepatitis B virus (HBV), chronic hepatitis C virus (HCV) infection, and alcohol abuse. 4 HBV infection is a challenging health issue in China, where about 93 million peoples are HBV carries and 30 million have chronic B hepatitis. 5 Alcohol abuse is also on the rise in China with about 6.6% of males and 0.1% of females diagnosed with alcohol dependence. 6 Many of these patients develop liver disease such as alcoholic hepatitis and cirrhosis thereby to subsequent carcinogenesis.
The human mitochondrial genome is a 16-kb closed circular duplex molecule that contains 37 genes, including two ribosomal RNAs and a complete set of 22 tRNAs. Because of its lack of the protection of histone protein, inefficient repaired mechanism, and generation of reactive oxygen species (ROS), mtDNA has high susceptibility to mutations and polymorphisms. [7][8][9] Oxygen species production could cause oxidative damage that lead to mutations and polymorphisms in the mitochondrial genes as well as affect the process of oxidative phosphorylation (OXPHOS), which might ultimately initiate carcinogenesis. 7,10-13 However, the mechanism of dysfunction of OXPHOS in HCC remains unclear. It is reported that abnormal level of ROS could induce HCC by affecting the process of oxidative phosphorylation (OXPHOS) and downregulation of the tumor suppressor genes in vitro. 14 We found that the highly polymorphic sequence in D-loop that might be related to the breast cancer risk, esophageal squamous cell carcinoma, non-Hodgkin lymphoma, kidney cancer, and lung cancer. [15][16][17][18][19][20] But few studies focused on the relationships between coding region of mtDNA and HCC. COX genes codes three subunits of respiratory complex IV, a key enzyme as the third and final enzyme of the electron transport chain of mitochondrial oxidative phosphorylation in aerobic metabolism. 21 Gene polymorphisms of COX genes (including COXI, COXII, and COXIII) contributed to the dysfunction of mitochondrial respiratory function and associated with susceptibility of prostate cancer. 22 In this study, we sequenced a region of approximately 3010 bp flanking the majority of the COX genes (including COXI, COXII, and COXIII) in mtDNA coding region from the blood DNA of HCC patients to identify cancer risk-associated single nucleotide polymorphisms (SNPs).

| Statistical analysis
The

| RESULTS
A total of 107 HCC patients were enrolled in this study. The clinical characteristics of the HCC patients and healthy controls are listed in

| DISCUSSION
The relationships between mitochondrial DNA variation and oncogenesis were demonstrated in many kinds of tumor. [23][24][25][26] We previously focused on the role of mitochondrial D-loop variation in tumor development. 27,28 In this study, we sequenced the whole COXI, COXII, and COXIII genes in the coding region of mitochondiral DNA and identified the association of 9545 nucleotide with HCC risk by χ 2 analysis.
At the evolution process, more somatic mutation occurred in mtDNA that was transferred to daughter cells as SNP, an increased proliferation rate of tumor cells and an decreased apoptotic ability of tumor cells might give rise to accumulation of mutations and polymorphisms, 13  In conclusion, analysis of the genetic polymorphisms in the mitochondrial coding region (COXI, COXII, and COXIII) may help identify patients at a higher risk for developing HCC. Further researches will be essential to clear and identify the role of these changes.

ACKNOWLEDGMENTS
This work was supported by Key basic research program of Hebei (15277706D).

CONFLICTS OF INTEREST
All authors disclose any financial, consulting, and personal relationships with other people or organizations that could influence their work.