Editor—Isolated cytochrome c oxidase (COX) deficiency is one of the most frequent causes of respiratory chain defects in humans1 and results in a variety of clinical manifestations including Leigh syndrome (LS), hepatic failure, and encephalomyopathy.2-4 COX, the terminal complex of the mitochondrial respiratory chain, is composed of 13 subunits, three of them being encoded by mitochondrial DNA genes. Nuclear genes encode the 10 other subunits. However, a much larger number of proteins of nuclear origin are required for the correct assembly and function of COX. More than 30 different genetic complementation groups for COX assembly have been isolated in yeast.5 Somatic cell genetic studies have shown that most cases of LS associated with COX deficiency belong to one complementation group.6 7 The gene defect in this group was mapped to chromosome 9q34 and analysis of a candidate gene (SURF1) showed several mutations, all of which predict a truncated protein.2 8

While mutations in the three mitochondrial COX genes,COX I-III, have been reported in a few patients,9-14 no mutations in any of the nuclear genes encoding the COX protein subunits have been identified so far.15 To date, mutations in three nuclear genes have been identified in patients with isolated COX deficiencies:SURF1 mutations in patients presenting with LS,2 8 SCO2 mutations in patients with fatal hypertrophic cardiomyopathy and encephalopathy,16 17 and aCOX10 mutation in one family with mitochondrial encephalopathy presenting with ataxia, severe muscle weakness, ptosis, pyramidal syndrome, and status epilepticus.18 While all three genes are involved in COX assembly, they appear to be associated with three distinct clinical entities. In particular, SURF1 mutations have so far been reported only in LS.19 Similarly, the few patients with mutations in the nuclear gene encoding the flavoprotein …