Oral cancer ranks as the 10th most common type of cancer worldwide (1). Tongue squamous cell carcinoma (TSCC) is the most prevalent and aggressive form of oral cancer, accounting for 25–40% of all oral cancer cases (2). Rapid local invasion, high rates of lymph nodal metastasis and high recurrence rates are striking features of TSCC, causing malfunction of mastication, speech and deglutition, and leading to poor survival and quality of life (3). In spite of the advances in chemotherapy, radiotherapy and surgical therapy, the prognosis and overall survival rates of patients with TSCC have not been significantly improved over the last decade, with a current overall five-year survival rate of <50% (4). Epidemiological study has revealed that tongue cancer is associated with a variety of factors, including age, geographic environment, family history, malnutrition, infectious agents, and chronic alcohol, tobacco and betel nut consumption (5). However, the pathogenesis of tongue cancer is yet to be fully understood.

Mitochondria are organelles wherein aerobic respiration is conducted to supply energy for a diverse range of cellular events including fatty acid oxidation (6), calcium signaling (7), apoptosis (8), biosynthesis and biogenesis (9,10). Each mitochondrion possesses its own 16,569 bp circular DNA (mtDNA) (11), characteristically having 37 encoding genes for 13 subunits of respiratory chain complexes (I, III, IV and V), 22 transfer (t)RNAs and 2 ribosomal (r)RNAs (12), and a non-coding region (D-loop) involved in regulation of mtDNA replication and transcription (13–15). Unlike nuclear DNA, mtDNA lacks the protection of histones and efficient repair mechanisms. Thus, mtDNA is more vulnerable to oxidative damage by excessive reactive oxygen species (ROS), a byproduct of oxidative phosphorylation in the mitochondria (16). This may initiate a vicious cycle: Damaged mtDNA can lead to an inefficient oxidative phosphorylation system, which in turn gives rise to more ROS, causing further damage to mtDNA and the genome. Importantly, the resulting genomic instability directly contributes to carcinogenesis.

Early on, it was proposed that injured mitochondria, for instance when there is alteration of oxidative phosphorylation or changes in the number, shape and function of mitochondria, may be an important contributing factor in many cancer types (17). Recently, an increasing number of mtDNA mutations, in the form of point mutations, deletions, insertions and mitochondrial microsatellite instability (mtMSI), have been reported in various cancers, including head and neck (18), pancreatic (19), colorectal (20,21) and breast cancers (22–24). However, to date, the mtDNA alterations in TSCC remain poorly defined. To profile the mtDNA alterations in tongue cancer, the present study sequenced and analyzed the mitochondrial genomes of tongue carcinoma, adjacent normal tissue and matched peripheral blood from 8 patients with TSCC.

Somatic mutations in the mtDNA have been increasingly observed in human cancers over recent years (21,23,24). To contrast the many studies focusing on the control region (24,26), the present study examined the entire mitochondrial genomes in cancer tissues, adjacent normal tissues and matched peripheral blood samples obtained from 8 patients with TSCC. As a result, synonymous mutations, polymorphisms and MSI were identified in TSCC.

In previous studies, the T16519C substitution in the control region has been demonstrated to be correlated with increased risk of breast cancer (22) and with worse prognosis in pancreatic cancer (19), and to exhibit a higher frequency in patients with malignant melanoma and metastasis compared with healthy controls (26). These findings indicate a potential link between the T16519C polymorphism and cancer progression. In addition to variation within the control region, encoding sequence variation in the mitochondrial genome has also been reported as relevant to cancer. The ND3 A10398G non-synonymous polymorphism, which results in the substitution of alanine by threonine within the NADH dehydrogenase subunit of Complex I, has been shown to be associated with an increased risk of breast cancer in European-American women (22). However, in another study, the 10398A variant was reported to increase the risk of breast cancer in African-American women but not in Caucasian women (27). A possible explanation for the conflicting results may be that the A10398G variation was in linkage disequilibrium with other causative polymorphisms in the aforementioned study populations. In the present study, one synonymous mutation and 21 polymorphisms were observed in 8 patients with TSCC. Of note, almost all the polymorphisms were concentrated in the control region and in genes encoding Complexes I, III and V, and rRNA. Theoretically, these variations may have potential impact on mitochondrial function via affecting the replication, transcription or translation events in mitochondria.

In the current study, the MSI of the mitochondrial genome in TSCC was analyzed. It appeared that mtMSI was most frequently observed at the D310 locus (2/8 for the D310 locus vs. undetected for the D514 or D16184 locus). This finding is in line with previous reports (21,28). The D310 locus is considered to be the most variable region in mtDNA, and alterations in this region, including the 303-C insertion, T310C mutation or T deletion, 315-C (CC) insertion and G316C mutation located within a poly-C stretch of HVS II, may lead to MSI. The C-stretch structure is of particular interest, since it is involved in the formation of the persistent RNA-DNA hybrid that leads to initiation of mtDNA heavy-strand replication (29). Therefore, alterations of this region may have an impact on replication and transcription of the mitochondrial genome. Studies have also suggested that MSI at the non-coding D-loop region could alter the rate of mtDNA replication, thereby disrupting mitochondrion-induced apoptosis (30,31). Regarding the biological significance of mtMSI in cancer, there are studies having associated mtMSI with less differentiated hepatocellular carcinoma, and late-stage progression and poor prognosis of non-small cell lung cancer (32), as well as poor prognosis in colorectal (20) and breast (24) cancer.

In summary, the current preliminary results suggest a relatively high frequency of mtDNA polymorphisms and mtMSI in TSCC. However, the generalization of this phenomenon requires caution, since the present study used a relatively small sample size and lacked healthy controls. Further large cohort studies are needed to validate the findings and identify possible correlations between the mtDNA variations and clinical parameters of patients with TSCC. As increasing evidence supports that mitochondrial genomic variations may serve a role in tumor development, further investigations are expected to uncover the molecular mechanisms by which mtDNA variations affect tumor progression, and their value in cancer risk assessment and management.