ClinVar

Dey et al. (2000) reported a novel mitochondrial DNA alteration in a 12-year-old boy with myopathy. They demonstrated a single nucleotide insertion (an adenine) in a polyadenine stretch at mitochondrial DNA positions 4366-4369 in the MTTQ gene. This altered the length of the evolutionarily conserved anticodon loop from 7 to 8 bases. The insertion was heteroplasmic and was abundant in the patient's muscle. Lower proportions of mutated mitochondrial DNA were observed in skin fibroblasts, but were below detectable levels in white blood cells. A muscle biopsy of the patient showed ragged-red fibers and an unusually high percentage of cytochrome c oxidase-deficient fibers (89%). The pathogenicity of the mutation was also evidenced by the fact that fibers harboring lower levels of the mutation showed normal cytochrome c oxidase activity.