[Mitochondrial disorders]

The main role of the respiratory chain is to synthesize the ATP required by all cell types. Mitochondrial disorders can affect apparently unrelated organs, usually before age 1 year, although isolated organ involvement is also seen (myocardiopathy, liver failure, etc.) and clinical onset can occur throughout life. The clinical variability of mitochondrial disorders is due to the ubiquitous nature of the mitochondrial respiratory chain. Mitochondrial disorders are studied at the metabolic, biochemical and genetic levels. Metabolic investigations can assist with the diagnosis, especially when they reveal lactic acidosis (although the absence of lactic acidosis does not rule out the diagnosis.) Enzymatic and molecular investigations are required to confirm the diagnosis, especially through enzyme studies of the different complexes of the respiratory chain in isolated mitochondria or affected tissues. The different proteins of the mitochondrial respiratory chain being encoded partly by the mitochondrial genome and partly by nuclear genes, abnormal oxidative phosphorylation can originate either in the mitochondria or in the nucleus. These genetic lesions can be point mutations or deletions of mtDNA, mutations of nuclear genes encoding subunits of the respiratory chain, or mutations in genes participating in the function and control of the respiratory chain and mtDNA.