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

Abstract

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 (Gly124-->Glu) changes a Gly that is strictly conserved in Surfl homologs of 12 species. The substitution G618 C (Asp202-->His), changing an Asp that is conserved only in mammals, appears to be a polymorphism. The mutation T751 C changes Ile246 to Thr, a position at which a hydrophobic amino acid is conserved in all eukaryotic and some bacterial species. Replacing Ile246 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.

New Splicing-site Mutations in the SURF1Gene in Leigh Syndrome Patients

“…Further studies are necessary to characterize the COX deficiency affecting patients 4, 6, and 7. However, we have confirmed that SURF1 mutations are responsible for Leigh syndrome associated with severe COX deficiency (12) and found a new phenotype of late Leigh syndrome associated with SURF1 mutations never described previously (2,3,8,13). …”

“…Three of these mutations have already been described. The most frequent mutation reported in SURF1 gene, the [312_321del311_312insAT], was found in patients 1 and 2 (2, 3); the [737TϾC] mutation was detected in patient 2 (8), and a mutation in the splicing donor site of intron 3 [240ϩ1GϾT] was found in patient 3 (9).…”

“…Each RNA sample (5 g) was reverse-transcribed using d(T)18 primers and a first-strand cDNA synthesis kit (Amersham Pharmacia Biotech) according to the manufacturer's instructions. The coding sequence from exon 4 to exon 9 was amplified by PCR as described by Poyau et al (8). We also used three new primer pairs; the first pair was chosen to study patient 3.…”

“…We found no mutations in DNAs from patients 6 or 7, but we could detect a homozygous polymorphism in intron 8,[835ϩ25CϾT] in the DNA of patient 4.…”

Mutations of theSCO1Gene in Mitochondrial CytochromecOxidase Deficiency with Neonatal‐Onset Hepatic Failure and Encephalopathy

“…So far, COX deficiency has been ascribed to mutations of either mtDNA-encoded COX subunits (Keightley et al 1996;Comi et al 1998;Hanna et al 1998;Bruno et al 1999;Clark et al 1999;Rahman et al 1999;Hoffbuhr et al 2000) or nuclear genes involved in assembly of functional complexes-namely, SURF1, SCO2, and COX10. Indeed, mutations in the nuclear SURF1 gene (MIM 185620 and MIM 256000) usually cause Leigh subacute necrotizing encephalomyopathy (Zhu et al 1998;Tiranti et al 1999;Poyau et al 2000) and have occasionally been associated with other clinical presentations (von Kleist-Retzow et al, in press). On the other hand, patients harboring SCO2 mutations (MIM 604272 and MIM 604377) have presented with encephalocardiomyopathy (Papadopoulou et al 1999;Jaksch et al 2000), whereas COX10 mutations (MIM 602125) have accounted for tubulopathy and leukodystrophy in one family (Valnot et al 2000).…”