Mitochondrial mutations in hepatocellular carcinomas and fibrolamellar carcinomas
Mitochondrial mutations are well documented in hepatocellular carcinoma, but their role in carcinogenesis remains unclear. To clarify their significance, a comprehensive analysis was performed of hepatocellular carcinomas (N=24), including quantifying the total mitochondrial DNA levels, quantifying the levels of mitochondrial DNA with the common deletion, and complete sequencing of the mitochondrial control region. In addition, these studies were expanded and reinforced by analysis of fibrolamellar carcinomas (N=15), a unique type of liver carcinoma that has increased numbers of mitochondria on electron microscopy. Overall, approximately 50% of hepatocellular carcinomas had lower levels of total mitochondrial DNA than paired non-neoplastic tissues. Interestingly, despite their increased numbers of mitochondria, primary fibrolamellar carcinomas had lower levels of total mitochondrial DNA. In contrast, metastatic fibrolamellar carcinomas had greatly increased mitochondrial DNA levels. Overall, deletions in the control region were associated with lower total DNA levels in typical hepatocellular carcinoma, but somatic single base pair mutations were not. In fact, almost all single base pair mutations were either reversions to the wild-type sequence or known population polymorphisms, strongly suggesting they are not directly oncogenic. Complete sequencing of the entire mitochondrial genome in fibrolamellar carcinomas identified several somatic mutations, but no consistent pattern of mutations was found. Overall, the levels of the common deletion were highest in tissues with lower total mitochondrial DNA. In conclusion, control region deletions, but not somatic mutations, may influence total DNA copy numbers. Somatic control region mutations in hepatocellular carcinoma are not directly oncogenic but instead may be adaptive.