Cisplatin is the first-line chemotherapeutic agent for the treatment of oral squamous cell carcinoma (OSCC). However, the intrinsic or acquired resistance against cisplatin remains a major obstacle to treatment efficacy in OSCC. Recently, mitochondrial DNA (mtDNA) alterations have been reported in a variety of cancers. However, the role of mtDNA alterations in OSCC has not been comprehensively studied. In this study, we evaluated the correlation between mtDNA alterations (mtDNA content, point mutations, large-scale deletions, and methylation status) and cisplatin sensitivity using two OSCC cell lines, namely SAS and H103, and stem cell-like tumour spheres derived from SAS. By microarray analysis, we found that the tumour spheres profited from aberrant lipid and glucose metabolism and became resistant to cisplatin. By qPCR analysis, we found that the cells with less mtDNA were less responsive to cisplatin (H103 and the tumour spheres). Based on the findings, we theorised that the metabolic changes in the tumour spheres probably resulted in mtDNA depletion, as the cells suppressed mitochondrial respiration and switched to an alternative mode of energy production, i.e. glycolysis. Then, to ascertain the origin of the variation in mtDNA content, we used MinION, a nanopore sequencer, to sequence the mitochondrial genomes of H103, SAS, and the tumour spheres. We found that the lower cisplatin sensitivity of H103 could have been caused by a constellation of genetic and epigenetic changes in its mitochondrial genome. Future work may look into how changes in mtDNA translate into an impact on cell function and therefore cisplatin response. Cis-diamminedichloroplatinum (II), or cisplatin, is one of the most commonly used chemotherapy agents in the treatment of various solid tumours such as ovarian, colorectal, prostate, lung, and head and neck tumours 1-5. To date, the intrinsic or acquired resistance of cancer cells to cisplatin remains a challenge in the chemotherapy of several cancers including oral squamous cell carcinoma (OSCC) 3,6. OSCC, which affects the epithelial layer of the oral cavity, is a common malignant tumour of the head and neck with low survival rates and high risks of recurrence 7. The well-characterized mode of action of cisplatin is via causing the formation of DNA adducts upon its binding to the nucleophilic N7 sites of purines, which further leads to DNA damage responses and apoptosis 2,6,8. Cisplatin resistance in general involves reduced DNA damage due to an increase in DNA adduct repair, reduced drug uptake, or increased drug inactivation 1,3,4,6. Activation of these mechanisms depends on multiple factors including genetic changes, epigenetic alterations at both molecular and cellular levels, and heterogeneity among cancer cells 4,9,10. The recently proposed cancer stem cells (CSCs) model highlighted tumour heterogeneity as an important basis of treatment resistance and relapse in cancer. According to the model, CSCs comprise a tumourigenic subpopulation where they exhibit stem cell-like features ...
