The Effect of Isocitrate Dehydrogenase on the Epigenetics of Human Mitochondrial DNA

摘要

Aberrant metabolism has become an increasingly interesting area of cancer biology. In many cancers including lower grade glioma, glioblastomas and some leukemias, a mutation in the metabolic enzyme Isocitrate Dehydrogenase (IDH), has been found in more than 70% of cases and has been shown to lead to a distinct hypermethylator phenotype. IDH commonly converts isocitrate to alpha-ketoglutarate in normal cell metabolism. Three isoforms of this enzyme are found in humans: IDH1, IDH2 and IDH3. Studies on IDH1, the cytosolic isoform, have revealed that mutations in the enzyme’s binding site lead to a novel gain of function: the synthesis of an oncogenic metabolite, 2-hydroxyglutarate (2HG). 2HG competitively inhibits alpha-ketoglutarate dependent enzymes such as the TET 5-methylcytosine (5mC) oxygenases and histone demethylases. These oxygenases are responsible for the hydroxymethylation (5hmC) of cytosine residues, ultimately leading to demethylation and gene re-expression. Thus, mutant IDH may lead to an elevation in 5mC levels producing the hypermethylator phenotype described. A similar gain-of-function mutation in IDH2, the mitochondrial isoform of IDH1, has been associated with leukemias and gliomas lacking IDH1 mutations. Mutations in IDH1, IDH2 and TET2 are mutually exclusive, and each yields a similar hypermethylator phenotype. IDH2, along with IDH3, is primarily involved in the TCA cycle and energy production for the cell. Recently, the Taylor lab has uncovered evidence of 5mC and 5hmC residues in mitochondrial DNA, established and maintained by mtDNMT1 and TET2. Changing levels of mtDNMT1 appears to alter the patterns and levels of mtDNA transcription from the mitochondrial genome. We hypothesized that mutant IDH would produce a similar effect on the mitochondrial genome as that found in the nuclear genome and result in a decrease in the level of 5-hydroxymethylcytosine, as well as a subsequent increase in the level of 5-methylcytosine caused by the competitive inhibition of the TET enzymes by 2-hydroxyglutarate accumulation. Using molecular biology techniques such as Western blots and MeDIP (methylated DNA immunoprecipitation) I aim to uncover the role of IDH mutation on mitochondrial DNA methylation and its effect on energy production in mammalian cells.