Cosegregation of the mitochondrial DNA A1555G and G4309A mutations results in deafness and mitochondrial myopathy.

We report a patient with progressive external ophthalmoplegia (PEO), exercise intolerance, and deafness after aminoglycoside exposure, harboring two pathogenic mutations in her mtDNA: an A1555G in the 12S rRNA gene and a G4309A in the tRNA(Ile) gene. Muscle histochemistry showed abundant ragged-red fibers, and biochemistry revealed normal respiratory chain function. The A1555G mutation was homoplasmic in blood from the proband and from all maternal relatives. The G4309A mutation was abundant in the proband's muscle, less abundant in her blood, still less abundant in the mother's blood, and absent in blood from other maternal relatives. Family members were asymptomatic. Our data suggest that the former mutation resulted in aminoglycoside-induced deafness and the latter caused PEO plus exercise intolerance. Copyright 2002 John Wiley & Sons, Inc.