[Classification of mitochondrial diseases]

Clinical and biochemical classifications of mitochondrial disorders have given way to an as yet incomplete genetic classification system based on alterations of the mitochondrial genome, the nuclear genome, or both. The first group includes mitochondrial disorders due to specific mutations of mitochondrial DNA such as the MELAS, MERRF or NARP encephalomyopathies, various conditions involving deafness (non-syndromic or associated with diabetes), Leber's optic neuropathy and a small group of cases of maternally transmitted Leigh's syndrome. All these diseases are transmitted through maternal line. conditions which are usually sporadic are due to deletion or duplication of mitochondrial DNA, and give rise to myopathies, with or without ophthalmoplegia, and to more complex disorders such as Kearns Sayre syndrome are also included. The second group is composed of all the mitochondrial disorders in which the nuclear genes which codify sub-units of mitochondrial DNA contain a genetic defect. This includes most cases of Leigh's syndrome, Alpers polydystrophies, the myoneurogastrointestinal syndrome, Barth's syndrome and Friedreich's disease. Amongst the disorders secondary to defects in communication between the nuclear and mitochondrial genomes is a progressive external ophthalmoplegic form with autosomal dominance which arises secondary to mutations on chromosomes 3 and 10. Further mitochondrial disorders due to faults in the relationship between the two genomes will probably be found in the near future.