[Genetic counseling and prenatal diagnosis in mitochondrial diseases].
Since mitochondrial diseases lead frequently to severe phenotypes and are often hereditary, there is a need for genetic counselling of the affected families. The specific features of mitochondrial genetics, however, hamper straightforward definition of recurrence risks as in Mendelian diseases. Empirical risks were recently provided for MELAS and MERRF syndromes and for Leber hereditary optic neuropathy. In MELAS and MERFF, higher levels of mutant mtDNA in the mothers' blood were associated with an increased frequency of affected offspring. Chronic progressive external ophthalmoplegia and Kearns-Sayre syndrome are in general sporadic disorders without increased recurrence risks in the offspring. As Leigh syndrome is found with maternal, autosomal recessive or X chromosomal transmission, the definition of the molecular defect is crucial for genetic counselling. Prenatal diagnosis was reported only in one case of mitochondrial disease so far, and in our opinion it remains questionable because of the uncertain correlation of the proportion of mutant DNA in chorionic villi and in clinically relevant tissues such as brain.