Genetic defects causing mitochondrial respiratory chain disorders and disease.

Genetic mitochondrial defects of the respiratory chain show marked phenotypic variability. Laboratory diagnosis is complicated and includes biochemical screening tests, tissue histopathology, functional enzyme studies, and molecular tests where available. Normal respiratory chain function necessitates the co-ordinated expression of over 100 different gene loci, and the interaction of two genetic systems, the nuclear and mitochondrial genomes. Thus genetic counselling for the mitochondrial disorders is extremely challenging. In this review, the classes of mitochondrial and nuclear defects that give rise to functional abnormalities of the mitochondrial respiratory chain are discussed, with specific instructive examples described in some detail.