[Mitochondrial disorders: a classification for the 21st century].

Mitochondrial encephalomyopathies were described in the 60's when accumulation of mitochondria were observed in patients with exercise intolerance. The electronic transport chain is located in the mitochondria and forms the oxidative phosphorylation (OXPHOS) system that produces ATP in the cell. The electronic transport chain is coded by two different genomes, nuclear and mitochondrial, with different genetic characteristics. The main differences between nuclear and mitochondrial genetics are heteroplasmy and threshold effect, mitotic segregation and maternal inheritance. Mitochondrial diseases are due to defects in the genes encoding proteins involved in some mitochondrial pathways. Those genes may be ARNts, structural proteins of the electronic transport chain or associated proteins of the mitochondria (transporters or assembly proteins). Alterations in those genes may be point mutations, deletions or duplications in the mitochondrial DNA and alterations of the genomic signaling between nucleus and mitochondria.