A novel gly290asp mitochondrial cytochrome b mutation linked to a complex III deficiency in progressive exercise intolerance.

We have identified a new mitochondrial (mt) cytochrome b mutation in a 29-year-old man with progressive exercise muscle intolerance associated with a marked deficiency of complex III activity and a decreased amount of mitochondrial-encoded cytochrome b. This G to A transition at mtDNA position 15615 leads to the substitution (G290D) of a very highly conserved amino acid of cytochrome b during evolution. The mutant mtDNA was heteroplasmic (80% mutant) in patient muscle but was undetectable in blood from the patient and his healthy mother and sisters. A maternally inherited cytochrome b polymorphism was also identified in this patient. Molecular screening of 150 individuals showed that the G290D mutation associated with the described phenotype. We suggest that this molecular defect is the primary cause of the muscle disease in this patient.