[Mitochondrial DNA methylation: Controversies, issues and perspectives]

DNA methylation is an epigenetic mechanism that has been largely probed regarding eukaryotic nuclear genome and bacteria, and its role is especially crucial in the regulation of gene expression. In mammals, it is almost exclusively acting on a cytosine preceding a guanine (CpG), whereas it presents itself mainly in a non-CpG context in bacteria's DNA. Conversely to nuclear and bacterial genomes, the existence of methylation in the mitochondrial genome is still widely debated. This controversy has been attributed to structural differences between the nuclear and mitochondrial genomes, and to the techniques used to study methylation of cytosines, which were rather optimized for the study of nuclear DNA. However, novel studies suggest that cytosine methylation is truly existing in mitochondria, and that it is mostly found in a non-CpG context, just like in their evolutionary relative, the bacteria.

[The underestimated coding potential of mitochondrial DNA]

Mitochondria are ancient organelles that emerged from the endosymbiosis of free-living proto-bacteria. They still retain a semi-autonomous genetic system with a small genome. Mitochondrial DNA (mtDNA) codes for 13 essential proteins for the production of ATP, the sequences of which are relatively conserved across Metazoans. The discovery of additional mitochondria-derived peptides (MDPs) indicates an underestimated coding potential. Humanin, an anti-apoptotic peptide, is likely independently transcribed from within the 16S rRNA gene, as are recently described SHLPs. MOTS-c, discovered in silico, has been demonstrated to be involved in metabolism and insulin sensitivity. Gau, is a positionally conserved open reading frame (ORF) sequence found in the antisense strand of the COX1 gene and its corresponding peptide is strictly colocalized with mitochondrial markers. In bivalves with doubly uniparental inheritance of mtDNA, male and female mtDNAs each carry a separate additional gene possibly involved in sex determination. Other MDPs likely exist and their investigation will shed light on the underestimated functional repertoire of mitochondria.