Although much of the current funding is aligned to continuing to further understand the functional details of the nuclear genome, the mitochondrion and its modest complement of DNA and protein is emerging as a crucial component of the biological networking of nuclear pathways . Mitochondria are eukaryotic organelles involved in many important physiological processes, including metabolism, signaling, apoptosis, cell cycle, differentiation and responsible for energy production . It has been well documented that the enhanced production of mitochondrial reactive oxygen species (ROS), most notably superoxide, hydroxyl radicals, and hydrogen peroxide is a prominent byproduct of cancer cell metabolism . Within various cells, tissues and organs, mtDNA copy number is different, and this difference can also occur in a given type of cell under different conditions or internal or external microenvironments [34, 35]. Unlike nuclear DNA, mtDNA is present at a consistently high level in each cell , and mtDNA mutation rate is much higher than that of nuclear DNA [18, 21]. Mitochondrial aberrants, including mtDNA mutations and copy number variations, have been frequently identified in different types of human cancers, including gastric cancer [28–30, 36, 37], suggesting that mtDNA aberrations play a critical role in gastric tumorigenesis. However, the prognostic values of mtDNA aberrants, particularly copy number variations, in gastric cancer patients ramain largely unclear.
In this study, we investigated relative mtDNA copy number in a cohort of gastric cancers and normal gastric tissues (control subjects) using real-time quantitative PCR approach. Our data showed that the majority of the cancer patients had low levels of mtDNA copy number as compared to control subjects, although mean mtDNA content was a little bit higher in gastric cancer patients than control subjects. In line with this study, the previous studies have demonstrated that mtDNA depletion is frequently found in gastric cancers as compared with normal gastric tissues [28, 29], implicating that low mtDNA content is involved in the formation and progression of gastric cancer. Moreover, we did not find the association of mtDNA content with most of clinicopathological features, such as gender, age, tumor localization, tumor size, differentiation, tumor invasion, TNM stage and survival status. However, we found that the patients with lymph node metastasis had a lower mtDNA copy number than the patients without lymph node metastasis, although the difference between two groups was not statistically significant.
To further explore the association of mtDNA content with clinicopathological characteristics and poor survival of gastric cancer patients, we categorized the patients into three groups based on two cutoff points (the lower and upper limit of 95% confidence interval for all control subjects), such as decreased mtDNA content (<3.61 copies), normal mtDNA content or reference (3.61-5.35 copies) and increased mtDNA content (>5.35 copies). Our findings showed that variable mtDNA content (whatever decreased or increased mtDNA content) was closely associated with an increased risk of lymph node metastasis for gastric cancer patients as compared to reference. Strikingly, when gastric cancer patients were further categorized into early-stage and late-stage groups based on TNM stage, variable mtDNA content was not asscoiated with lymph node metastasis for the patients with early-stage tumors. However, both decreased and increased mtDNA content significantly increased the risk of of lymph node metastasis for the patients with late-stage tumors. These observations suggest that copy number variations of mtDNA may be invloved in gastric cancer progression. Similar to our findings in the present study, a previous study showed that mtDNA content was increased gradually from the non-cancerous esophageal mucosa to esophageal squamous cell carcinoma (ESCC) and then the metastatic lymph nodes . Moreover, our data showed that variable mtDNA content was associated with cancer-related death of the patients with late-stage tumors. Collectively, our findings suggest that variable mtDNA content may contribute to poor clinical outcomes of gastric cancer patients, particularly the patients with advanced tumors. Next, we evaluated the effect of variable mtDNA content on poor survival of gastric cancer patients. Our data showed that both decreased and increased mtDNA content were not associated with overall survival of gastric cancer patients. However, when the patients were categorized into early-stage and late-stage tumor groups, increased mtDNA content was strongly associated with poor survival in the latter, but not in the former, as supported by a previous study that high mtDNA copy number may contribute to the high bioenergetic function of mitochondria and further confer an advantage for malignant behaviors of cancer cells, such as tumor invasion .