Nuclear gene defects in respiratory chain disorders

Deficiencies in the activity of the components of the mitochondrial respiratory chain can result from mutations in genes encoded in the mitochondrial (mtDNA) or nuclear genomes. Mutations in mtDNA have been identified over the past decade in a wide spectrum of clinical disorders, and attention has now turned to identifying nuclear gene defects. Positional cloning, candidate gene analysis, and functional complementation in patient cell lines have all been used with success. Mutations in gene coding for structural subunits of the respiratory chain complexes appear to be less numerous than defects in genes associated with some aspect of the biogenesis of the respiratory chain. Despite the fact that many of the nuclear disease genes so far identified are ubiquitously expressed, tissue specificity of the biochemical and clinical phenotype is the rule rather than the exception. This selective vulnerability of different cell populations remains unexplained. The majority of patients with a biochemical deficiency in one or the other of the respiratory chain complexes do not yet have a molecular diagnosis.