A missense mutation of cytochrome oxidase subunit II causes defective assembly and myopathy

We report the first missense mutation in the mtDNA gene for subunit II of c ytochrome c oxidase (COX). The mutation was identified in a 14-year-old boy with a proximal myopathy and lactic acidosis. Muscle histochemistry and mi tochondrial respiratory-chain enzymology demonstrated a marked reduction in COX activity. Immunohistochemistry and immunoblot analyses with COX subuni t-specific monoclonal antibodies showed a pattern suggestive of a primary m tDNA defect, most likely involving CO II; for COX subunit II (COX II). mtDN A-sequence analysis demonstrated a novel heteroplasmic T-->A transversion a t nucleotide position 7,671 in CO II. This mutation changes a methionine to a lysine residue in the middle of the first N-terminal membrane-spanning r egion of COX II. The immunoblot studies demonstrated a severe reduction in cross-reactivity, not only for COX II but also for the mtDNA-encoded subuni t COX III and for nuclear-encoded subunits Vb, Via, VIb, and VIc. Steady-st ate levels of the mtDNA-encoded subunit COX I showed a mild reduction, but spectrophotometric analysis revealed a dramatic decrease in COX I-associate d heme a(3) levels. These observations suggest that, in the COX protein, a structural association of COX II with COX I is necessary to stabilize the b inding of heme a(3) to COX I.