Mutations of SURF-1 in Leigh disease associated with cytochrome c oxidase deficiency.

Leigh disease associated with cytochrome c oxidase deficiency (LD[COX-]) is one of the most common disorders of the mitochondrial respiratory chain, in infancy and childhood. No mutations in any of the genes encoding the COX-protein subunits have been identified in LD(COX-) patients. Using complementation assays based on the fusion of LD(COX-) cell lines with several rodent/human rho0 hybrids, we demonstrated that the COX phenotype was rescued by the presence of a normal human chromosome 9. Linkage analysis restricted the disease locus to the subtelomeric region of chromosome 9q, within the 7-cM interval between markers D9S1847 and D9S1826. Candidate genes within this region include SURF-1, the yeast homologue (SHY-1) of which encodes a mitochondrial protein necessary for the maintenance of COX activity and respiration. Sequence analysis of SURF-1 revealed mutations in numerous DNA samples from LD(COX-) patients, indicating that this gene is responsible for the major complementation group in this important mitochondrial disorder.

A single cell complementation class is common to several cases of cytochrome c oxidase-defective Leigh's syndrome.

A generalized defect of complex IV (cytochrome C oxidase, COX) is frequently found in subacute necrotizing encephalomyelopathy (Leigh's syndrome), the most common mitochondrial disorder in infancy. We previously demonstrated the nuclear origin of the COX defect in one case, by fusing nuclear DNA-less cytoplasts derived from normal fibroblasts with mitochondrial DNA (mtDNA)-less transformant fibroblasts derived from a patient with COX-defective [COX(-)] Leigh's syndrome. The resulting cybrid line showed a specific and serve COX(-) phenotype. Conversely, in the present study, we demonstrated that a COX(+) phenotype could be restored in hybrids obtained by fusing COX(-) transformant fibroblasts of seven additional Leigh's syndrome patients with mtDNA-less, COX(-) tumor-derived rho degree cells. Both these results are explained by the presence of a mutation in a nuclear gene. In a second set of experiments, in order to demonstrate whether COX(-) Leigh's syndrome is due to a defect in the same gene, or in different genes, we tested several hybrids derived by fusing our original COX(-) cell line with each of the remaining seven cell lines. COX activity was evaluated in situ by histochemical techniques and in cell extracts by a spectrophotometric assay. No COX complementers were found among the resulting hybrid lines. This result demonstrates that all our cases were genetically homogeneous, and suggests that a major nuclear disease locus is associated with several, perhaps most, of the cases of infantile COX(-) Leigh's syndrome. This information should make it easier to identify the gene responsible.

Nuclear DNA origin of cytochrome c oxidase deficiency in Leigh's syndrome: genetic evidence based on patient's-derived rho degrees transformants.

Defects of the respiratory chain carrying out oxidative phosphorylation (OXPHOS) are the biochemical hallmark of human mitochondrial disorders. Faulty OXPHOS can be due to mutations in either nuclear or mitochondrial genes, that are involved in the synthesis of individual respiratory subunits or in their post-translational control. The most common mitochondrial disorder of infancy and childhood is Leigh's syndrome, a severe encephalopathy, often associated with a defect of cytochrome c oxidase (COX). In order to demonstrate which genome is primarily involved in COX-deficient (COX(-))-Leigh's syndrome, we generated two lines of transmitochondrial cybrids. The first was obtained by fusing nuclear DNA-less cytoplasts derived from normal fibroblasts, with mitochondrial DNA-less (rho degree) transformant fibroblasts derived from a patient with COX(-))-Leigh's syndrome. The second cybrid line was obtained by fusing rho degree cells derived from 143B.TK- human osteosarcoma cells, with cytoplasts derived from the same patient. The first cybrid line showed a specific and severe COX(-) phenotype, while in the second all the respiratory chain complexes, including COX, were normal. These results indicate that the COX defect in our patient is due to a mutation of a nuclear gene. The use of cybrids obtained from 'customized', patient-derived rho degree cells can have wide applications in the identification of respiratory chain defects originated by nuclear DNA-encoded mutations, and in the study of nuclear DNA-mitochondrial DNA interactions.

Isolation of a new gene in the Friedreich ataxia candidate region on human chromosome 9 by cDNA direct selection.

The Friedreich ataxia (FRDA) locus is localized on chromosome 9q13 in an interval less than 1 Mb between markers D9S202/FR1 and FR5. We cloned the FRDA candidate region in YACs, and we started a systematic search for transcripts in this region using the cDNA selection approach. Several overlapping cDNA clones mapping near the telomeric end of the FRDA minimum genetic region were isolated. Zoo blot analysis demonstrated that these cDNAs are well conserved among different species. A transcript of 4.8 kb was identified by hybridization to a Northern blot containing human brain poly(A)+ RNA. Partial sequence of these clones showed 100% homology with a previously described anonymous brain cDNA (EST01251). A search for mutations of this gene in FRDA patients and carriers is in progress. No mutations have been found to date, but we have identified a DNA polymorphism. This polymorphism was nonrecombinant with the disease in a previously described FRDA pedigree in which a recombination had occurred with more telomeric markers.

Linkage disequilibrium between FD1-D9S202 haplotypes and the Friedreich's ataxia locus in a central-southern Italian population.

We used two recently described genetic markers in the region of the Friedreich's ataxia locus to study 33 affected pedigrees from central-southern regions of Italy. These markers are predicted, by physical mapping, to be localised more closely to the Friedreich's ataxia locus than other previously described markers. No recombination was found between these markers and the disease locus. Strong linkage disequilibrium is present between the compound haplotype and the disease locus. Since this population was also previously studied by using three other more distal genetic markers, a total of five markers has been used to identify the extended haplotype. Homozygosity in consanguineous pedigrees was also studied. Extended haplotype analysis and homozygosity studies suggest the presence of few common disease causing mutations in our population.

Characterization of a subfamily of zinc finger genes expressed in human hematopoietic cell lines.

We isolated by low stringency screening of a human erythroleukemia cDNA library (K562) 45 independent clones hybridizing to a Krüppel-like (HF.10) zinc finger cDNA. The expression of 15 such cDNAs in human hematopoietic cell lines was investigated. Preliminary sequence analysis of the zinc finger motifs in these cDNAs indicate that they belong to a subclass of the Cys-Cys/His-His motif, showing the highest homology to the Wilm's tumor and EGR1, EGR2 cDNAs.

Characterization of a subfamily of zinc finger genes expressed in human hematopoietic cell lines.

We isolated by low strigency screening of a human erythroleukemia cDNA library (K562) 45 indipendent clones hybridizing to a Kruppel-like (HF.10) zinc finger cDNA. The expression of 15 such cDNAs in human hematopoietic cell lines was investigated. Preliminary sequence analysis of the zinc finger motifs in these cDNAs indicate that they belong to a subclass of the Cys-Cys/His-His motif, showing the highest homology to the Wilm's tumor and EGR1, EGR2 cDNAs.