Missense mutations in SURF1 associated with deficient cytochrome c oxidase assembly in Leigh syndrome patients

We have studied the fibroblasts of three patients suffering from Leigh syndrome associated with cytochrome c oxidase deficiency (LS-COX-). Their mitochondrial DNA was functional and all nuclear COX subunits had a normal sequence. The expression of transcripts encoding mitochondrial and nuclear COX subunits was normal or slightly increased. Similarly, the OXA1 transcript coding for a protein involved in COX assembly was increased. However, several COX-protein subunits were severely depressed, indicating deficient COX assembly. Surf1, a factor involved in COX biogenesis, was recently reported as mutated in LS-COX- patients, all mutations predicting a truncated protein. Sequence analysis of SURF1 gene in our three patients revealed seven heterozygous mutations, six of which were new : an insertion, a nonsense mutation, a splicing mutation of intron 7 in addition to three missense mutations. The mutation G385 A (Gly(124)-->Glu) changes a Gly that is strictly conserved in Surf1 homologs of 12 species. The substitution G618 C (Asp(202)-->His), changing an Asp that is conserved only in mammals, appears to be a polymorphism. The mutation T751 C changes Ile(246) to Thr, a position at which a hydrophobic amino acid is conserved in all eukaryotic and some bacterial species. Replacing Ile(246) by Thr disrupts a predicted beta sheet structure present in all higher eukaryotes. COX activity could be restored in fibroblasts of the three patients by complementation with a retroviral vector containing normal SURF1 cDNA. These mutations identify domains essential to Surf1 protein structure and/or function.