SURF1, encoding a factor involved in the biogenesis of cytochrome c oxidase, is mutated in Leigh syndrome

Leigh disease: clinical, neuroradiologic, and biochemical study of three new cases with cytochrome c oxidase deficiency

Three cases of Leigh disease are described. In all three, symptoms began in the first months of life, with muscle hypotonia, lactic acidosis, and psychomotor delay. The diagnosis was made on the basis of the clinical characteristics, biochemical abnormalities, and typical brain magnetic resonance imaging with symmetric lesions suggesting bilateral necrosis at the level of the basal ganglia and of the midbrain. Cytochrome c oxidase (complex IV of the mitochondrial respiratory chain) deficiency was demonstrated in muscle tissue in all patients and confirmed in skin fibroblasts in patient 3. A genetic heterogeneity was present in these patients since only one had a SURF-1 gene mutation. The clinical, biochemical, and neuroradiologic aspects are discussed. Finally, the finding of facial dysmorphisms in the cytochrome c oxidase deficiency observed in one of the described cases is of extreme interest; to our knowledge, this association has never been reported in the literature.

Immunological phenotyping of fibroblast cultures from patients with a mitochondrial respiratory chain deficit

Conventional approaches to the diagnosis of mitochondrial respiratory chain diseases, using enzyme assays and histochemistry, are laborious and give limited information concerning the genetic basis of a deficiency. We have evaluated the diagnostic value of 12 monoclonal antibodies to subunits of the four respiratory chain enzyme complexes and F(1)F(0)-ATP synthase. Antibodies were used in immunological studies with skin fibroblast cultures derived from patients with diverse mitochondrial diseases, including patients in which the disease was caused by a nuclear genetic defect and patients known to harbor a heteroplasmic mutation in a mitochondrial tRNA gene. Immunoblotting experiments permitted the identification of specific enzyme assembly deficits and immunocytochemical studies provided clues regarding the genetic origin of the disease. The immunological findings were in agreement with the biochemical and genetic data of the patients. Our study demonstrates that characterization of the fibroblast cultures with the monoclonal antibodies provides a convenient technique to complement biochemical assays and histochemistry in the diagnosis of mitochondrial respiratory chain disorders.

A novel mutation in SURF1 causes skipping of exon 8 in a patient with cytochrome c oxidase-deficient leigh syndrome and hypertrichosis

Leigh syndrome is a rare pediatric neurodegenerative disorder attributed to impaired mitochondrial energy metabolism. Mutations in SURF1 have been described in several patients with Leigh syndrome associated with cytochrome c oxidase deficiency. We report a new 18-bp deletion (821del18), spanning the splice donor junction of exon 8 of SURF1, in an infant presenting with cytochrome c oxidase-deficient Leigh syndrome and hypertrichosis. cDNA sequencing demonstrated that this deletion results in a messenger lacking exon 8. RT-PCR experiments suggested a rapid degradation of the aberrant mRNA species from the 5'-end.

Shy1p occurs in a high molecular weight complex and is required for efficient assembly of cytochrome c oxidase in yeast

Surf1p is a protein involved in the assembly of mitochondrial respiratory chain complexes. However its exact role in this process remains to be elucidated. We studied SHY1, the yeast homologue of SURF1, with an aim to obtain a better understanding of the molecular pathogenesis of cytochrome c oxidase (COX) deficiency in SURF1 mutant cells from Leigh syndrome patients. Assembly of COX was analysed in a shy1 null mutant strain by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). Steady-state levels of the enzyme were found to be strongly reduced, the total amount of assembled complex being approximately 30% of control. The presence of a significant amount of holo-COX in the SHY1-disruptant strain suggests that Shy1p may either facilitate assembly of the enzyme, or increase its stability. However, our observations, based on 2D-PAGE analysis of mitochondria labelled in vitro, now provide the first direct evidence that COX assembly is impaired in a Deltashy1 strain. COX enzyme assembled in the absence of Shy1p appears to be structurally and enzymically normal. The in vitro labelling studies additionally indicate that mitochondrial translation is significantly increased in the shy1 null mutant strain, possibly reflecting a compensatory mechanism for reduced respiratory capacity. Protein interactions of both Shy1p and Surf1p are implied by their appearance in a high molecular weight complex of about 250 kDa, as shown by 2D-PAGE.

A SURF1 gene mutation presenting as isolated leukodystrophy

Mitochondrial respiratory chain defects are increasingly recognized in patients with leukodystrophy. We report the first case of leukodystrophy with systemic cytochrome oxidase deficiency caused by a loss of function mutation in the SURF1 gene in a 2-year-old girl presenting with failure to thrive, global neurodevelopmental regression, and lactic acidosis. Although all previously reported mutations in the SURF1 gene have been found in patients with cytochrome oxidase (COX)-deficient Leigh syndrome, the phenotype associated with SURF1 protein deficiency should be extended to include leukodystrophy.

The oxidative phosphorylation (OXPHOS) system: nuclear genes and human genetic diseases

The ubiquitous nature of mitochondria, the dual genetic foundation of the respiratory chain in mitochondrial and nuclear genome, and the peculiar rules of mitochondrial genetics all contribute to the extraordinary heterogeneity of clinical disorders associated with defects of oxidative phosphorylation (mitochondrial encephalomyopathies). Here, we review recent findings about nuclear gene defects in isolated OXPHOS enzyme complex deficiency. This information should help in identifying patients with mitochondrial disease and defining a biochemical and molecular basis of the disorder found in each patient. This knowledge is indispensable for accurate genetic counseling and prenatal diagnosis, and is a prerequisite for the development of rational therapies, which are still, at present, woefully inadequate.

Large-scale deletion and point mutations of the nuclear NDUFV1 and NDUFS1 genes in mitochondrial complex I deficiency

Reduced nicotinamide adenine dinucleotide (NADH):ubiquinone oxidoreductase (complex I) is the largest complex of the mitochondrial respiratory chain and complex I deficiency accounts for approximately 30% cases of respiratory-chain deficiency in humans. Only seven mitochondrial DNA genes, but >35 nuclear genes encode complex I subunits. In an attempt to elucidate the molecular bases of complex I deficiency, we studied the six most-conserved complex I nuclear genes (NDUFV1, NDUFS8, NDUFS7, NDUFS1, NDUFA8, and NDUFB6) in a series of 36 patients with isolated complex I deficiency by denaturing high-performance liquid chromatography and by direct sequencing of the corresponding cDNA from cultured skin fibroblasts. In 3/36 patients, we identified, for the first time, five point mutations (del222, D252G, M707V, R241W, and R557X) and one large-scale deletion in the NDUFS1 gene. In addition, we found six novel NDUFV1 mutations (Y204C, C206G, E214K, IVS 8+41, A432P, and del nt 989-990) in three other patients. The six unrelated patients presented with hypotonia, ataxia, psychomotor retardation, or Leigh syndrome. These results suggest that screening for complex I nuclear gene mutations is of particular interest in patients with complex I deficiency, even when normal respiratory-chain-enzyme activities in cultured fibroblasts are observed.

Abnormal calcium homeostasis in fibroblasts from patients with Leigh disease

Recently, we reported that in various cell lines under conditions of deenergization of the mitochondrial membrane, the release of Ca(2+) from the endoplasmic reticulum (ER) does not produce the expected activation of store-operated calcium channels (SOCs) in the plasma membrane. In the present work, we examined the activation of SOCs in fibroblasts derived from three patients with Leigh disease (LS). We identified mutations in the SURF-1 gene in all these cells. Consequently, cytochrome oxidase (COX) deficiency was found in all these (LS(COX)) cell lines and, thus, the main mitochondrial mechanism of generation of the electrochemical proton gradient on the mitochondrial membrane was naturally depressed. We demonstrated that, in untreated LS(COX) fibroblasts, the rate of Ca(2+)-inflow through SOCs was low compared to the fibroblasts from healthy individuals even after thapsigargin-induced maximal release of Ca(2+) from the ER. Moreover, the pretreatment of LS(COX) fibroblasts with a protonophore did not modify this rate. Thus, in LS(COX) fibroblasts, the activation of SOCs was naturally impaired. Our findings suggest that altered calcium metabolism, apart from severe energy production failure, may also contribute to developing pathological conditions in patients with COX-deficient Leigh disease related to SURF-1 gene mutation.

Cytochrome c oxidase-deficient patients have distinct subunit assembly profiles

Cytochrome c oxidase (COX) deficiency is the most common respiratory chain defect in childhood and is clinically heterogeneous. We report a study of six patients with COX deficiencies. Two of the patients had as yet undefined defects, three patients had Surf-1 mutations, and one patient had a 15-base pair deletion in the COX III subunit. We show that quantitative measurements of steady-state levels of subunits by monoclonal antibody reactivity, when used in combination with a discontinuous sucrose gradient methods, provide an improved diagnosis of COX deficiencies by distinguishing between kinetic, stability, and assembly defects. The two mutants of undefined etiology had a full complement of subunits with one stable and the other partially unstable to detergent solubilization. Both are likely to carry mutations in nuclear-encoded subunits of the complex. The three Surf-1 mutants and the COX III mutant each had reduced steady-state levels of subunits but variable associations of the residual subunits. This information, as well as aiding in diagnosis, helps in understanding the genotype-phenotype relationships of COX deficiencies and provides insight into the mechanism of assembly of the enzyme complex.

New splicing-site mutations in the SURF1 gene in Leigh syndrome patients

The gene SURF1 encodes a factor involved in the biogenesis of cytochrome c oxidase, the last complex in the respiratory chain. Mutations of the SURF1 gene result in Leigh syndrome and severe cytochrome c oxidase deficiency. Analysis of seven unrelated patients with cytochrome c oxidase deficiency and typical Leigh syndrome revealed different SURF1 mutations in four of them. Only these four cases had associated demyelinating neuropathy. Three mutations were novel splicing-site mutations that lead to the excision of exon 6. Two different novel heterozygous mutations were found at the same guanine residue at the donor splice site of intron 6; one was a deletion, whereas the other was a transition [588+1G>A]. The third novel splicing-site mutation was a homozygous [516-2_516-1delAG] in intron 5. One patient only had a homozygous polymorphism in the middle of the intron 8 [835+25C>T]. Western blot analysis showed that Surf1 protein was absent in all four patients harboring mutations. Our studies confirm that the SURF1 gene is an important nuclear gene involved in the cytochrome c oxidase deficiency. We also show that Surf1 protein is not implicated in the assembly of other respiratory chain complexes or the pyruvate dehydrogenase complex.

Mutations in the SURF1 gene associated with Leigh syndrome and cytochrome C oxidase deficiency

Cytochrome c oxidase (COX) deficiency is one of the major causes of Leigh Syndrome (LS), a fatal encephalopathy of infancy or childhood, characterized by symmetrical lesions in the basal ganglia and brainstem. Mutations in the nuclear genes encoding COX subunits have not been found in patients with LS and COX deficiency, but mutations have been identified in SURF1. SURF1 encodes a factor involved in COX biogenesis. To date, 30 different mutations have been reported in 40 unrelated patients. We aim to provide an overview of all known mutations in SURF1, and to propose a common nomenclature. Twelve of the mutations were insertion/deletion mutations in exons 1, 4, 6, 8, and 9; 10 were missense/nonsense mutations in exons 2, 4, 5, 7, and 8; and eight were detected at splicing sites in introns 3 to 7. The most frequent mutation was 312_321del 311_312insAT which was found in 12 patients out of 40. Twenty mutations have been described only once. We also list all polymorphisms discovered to date.

Mitochondria and the heart

Since the identification of the first pathogenic mutations of mitochondrial DNA in 1988, a plethora of information about human mitochondrial diseases has been brought to light. Not surprisingly, many of these disorders affect the myocardium, because this tissue relies heavily upon oxidative metabolism. This review focuses on disorders of the respiratory chain, the only area of mammalian cellular metabolism under the control of two genomes, nuclear and mitochondrial. Consequently, defects of aerobic synthesis of adenosine triphosphate (ATP) can be due to mutations of either genome. We describe genetic mitochondrial cardiomyopathies and briefly review mouse models and the mitochondrial theory of presbycardia.

The genetics and pathology of oxidative phosphorylation

The mitochondrial oxidative phosphorylation (OXPHOS) system is the final biochemical pathway in the production of ATP. The OXPHOS system consists of five multiprotein complexes, the individual subunits of which are encoded either by the mitochondrial or by the nuclear genome. Defects in the OXPHOS system result in devastating, mainly multisystem, diseases, and recent years have seen the description of the underlying genetic mutations in mitochondrial and nuclear genes. Advances in this arena have profited from progress in various genome projects, as well as improvements in our ability to create relevant animal models.

The role of chaperone-assisted folding and quality control in inborn errors of metabolism: protein folding disorders

Molecular chaperones are present in the various compartments of the cell and assist the folding of newly synthesized proteins. Compared to wild-type proteins, missense mutant proteins are generally synthesized in a normal fashion, but may be impaired in their folding. A broad array of diseases that are due to misfolding of mutant proteins may be labelled conformational diseases: aggregation diseases, such as Alzheimer disease; diseases caused by negative dominance from misfolded structural proteins, such as hypertrophic cardiomyopathy; and disorders where the misfolded protein is degraded by intracellular proteases. Many metabolic disorders belong to this last category, where the so-called protein quality control systems, comprising chaperones and proteases, attempt to eliminate folding intermediates or misfolded proteins. On the basis of in vitro experiments with a limited number of missense mutations identified in patients with phenylalanine hydroxylase and fatty acid oxidation deficiencies, we discuss the cellular fate of missense mutant proteins. We find that the balance between folding to functional conformers, retention (holding) and degradation of folding intermediates or misfolded proteins is dependent on the nature of the mutation and on the efficiency of the quality control. For example, low temperature may promote formation of functional conformers, while elevated temperature usually promotes retention and degradation. We conclude that disorders caused by many missense mutations are complex diseases in which the mutation itself is a necessary major primary component, but that its effect may be modified by cellular conditions and possibly by genetic variations in the quality control systems. We suggest that this new knowledge about cell handling may open new avenues of understanding of the cell pathology and treatment of patients with metabolic disorders.

Clinical and molecular studies of mitochondrial disease

Our knowledge of mitochondrial disorders has expanded enormously through advances in mitochondrial DNA (mtDNA) research during the last decade. This has also provided information about the complicated genotype-phenotype relationship based on distinct characteristics of mtDNA itself. These include multicopy, vulnerability to the mutation, and cytoplasmic inheritance. There are complications of heteroplasmy, tissue/cell specificity, accumulation of mutations and material inheritance.

A genomewide linkage-disequilibrium scan localizes the Saguenay-Lac-Saint-Jean cytochrome oxidase deficiency to 2p16

Leigh syndrome (LS) affects 1/40,000 newborn infants in the worldwide population and is characterized by the presence of developmental delay and lactic acidosis and by a mean life expectancy variously estimated at 3-5 years. Saguenay-Lac-Saint-Jean (SLSJ) cytochrome oxidase (COX) deficiency (LS French-Canadian type [LSFC] [MIM 220111]), an autosomal recessive form of congenital lactic acidosis, presents with developmental delay and hypotonia. It is an LS variant that is found in a geographically isolated region of Quebec and that occurs in 1/2,178 live births. Patients with LSFC show a phenotype similar to that of patients with LS, but the two groups differ in clinical presentation. We studied DNA samples from 14 patients with LSFC and from their parents, representing a total of 13 families. Because of founder effects in the SLSJ region, considerable linkage disequilibrium (LD) was expected to surround the LSFC mutation. We therefore performed a genomewide screen for LD, using 290 autosomal microsatellite markers. A single marker, D2S1356, located on 2p16, showed significant (P < 10(-5)) genomewide LD. Using high-resolution genetic mapping with additional markers and four additional families with LSFC, we were able to identify a common ancestral haplotype and to limit the critical region to approximately 2 cM between D2S119 and D2S2174. COX7AR, a gene encoding a COX7a-related protein, had previously been mapped to this region. We determined the genomic structure and resequenced this gene in patients with LSFC and in controls but found no functional mutations. Although the LSFC gene remains to be elucidated, the present study demonstrates the feasibility of using a genomewide LD strategy to localize the critical region for a rare genetic disease in a founder population.

Sequence variations in the NDUFA1 gene encoding a subunit of complex I of the respiratory chain

NDUFA1 is one of the 36 nuclear genes encoding subunits of the mitochondrial complex I involved in the respiratory chain. The human NDUFA1 has been cloned, completely sequenced and mapped to Xq24. In the present study, we searched for sequence variations in NDUFA1 as causative defects in complex I deficiency using genomic DNA of 152 patients with various clinical phenotypes. The patient sample consisted of 54 patients (46 male and 8 female) with Leber heriditary optic neuropathy (LHON) from 48 unrelated families from Germany and 98 patients (72 male and 26 female) with biochemically proven complex I deficiency including Leigh syndrome. Patient DNA was used to amplify all three exons, including the exon/intron boundaries and the promoter region of NDUFA1 for heteroduplex analysis and direct sequencing. In the 152 patients tested, no mutation was found that could be related to any of the disease phenotypes included. However, three single-nucleotide polymorphisms (SNPs) located in the promoter region (SNP G/C at nt -71 and SNP T/C at nt -189) and in intron 1 (SNP T/G nt 1454) were discovered. Allele frequencies of the SNPs were estimated in a German and Estonian control population and compared to complex I-deficient patients. There was no significant difference between the control population, the LHON patients, or the severely affected patients with complex I deficiency, excluding an association of the polymorphisms with the diseases. Our results suggest that mutations in NDUFA1 do not cause the gender difference observed in clinically severe and complex phenotypes with complex I deficiency.

Mutations in the complex I NDUFS2 gene of patients with cardiomyopathy and encephalomyopathy

Human complex I is built up and regulated by genes encoded by the mitochondrial DNA (mtDNA) as well as the nuclear DNA (nDNA). In recent years, attention mainly focused on the relation between complex I deficiency and mtDNA mutations. However, a high percentage of consanguinity and an autosomal-recessive mode of inheritance observed within our patient group as well as the absence of common mtDNA mutations make a nuclear genetic cause likely. The NDUFS2 protein is part of complex I of many pro- and eukaryotes. The nuclear gene coding for this protein is therefore an important candidate for mutational detection studies in enzymatic complex I deficient patients. Screening of patient NDUFS2 cDNA by reverse transcriptase-polymerase chain reaction (RT-PCR) in combination with direct DNA sequencing revealed three missense mutations resulting in the substitution of conserved amino acids in three families.

The P(174)L mutation in the human hSCO1 gene affects the assembly of cytochrome c oxidase

Mutations of the yeast SCO1 gene result in impaired COX assembly. Recently, heterozygous mutations in the human homologue hSCO1 have been reported in infants suffering from neonatal ketoacidotic coma and isolated COX deficiency (Valnot et al., 2000). One of the hSCO1 alleles harboured a frame shift mutation resulting in a premature stop codon, the other a missense mutation leading to a substitution of proline(174) by leucine. This position is next to the essential CXXXC motif, which is conserved in all Sco1p homologues. We used chimeric proteins with the amino-terminal portion derived from yeast Sco1p and carboxy-terminal portion including the CXXXC motif from the human hSco1p to provide experimental evidence for the pathogenic nature of the P(174)L mutation. These chimeras are able to complement yeast sco1 null mutants. Introduction of the P(174)L mutation affects the function of these chimeric proteins severely, as shown by impaired COX assembly and loss of COX activity.

Human cytochrome oxidase deficiency

The human cytochrome oxidase complex is a multisubunit assembly in the inner mitochondrial membrane responsible for the terminal event in electron transport in which molecular oxygen is reduced. Various phenotypic forms of cytochrome oxidase deficiency have been recognized, the major varieties involving degeneration of the brain stem and basal ganglia (Leigh syndrome) and lactic acidemia. Others include a fatal infantile form, a benign reversible form, and forms with cardiomyopathy. Early recognition of complementation groups within, for instance, the Leigh syndrome group has recently been followed up with a description of the gene defect for three of the nuclear-encoded forms of cytochrome c oxidase (COX) deficiency. The three genes indicted, SURF1 for Leigh syndrome, COX 10 for leukodystrophy and tubulopathy, and SCO2 for the cardiomyopathic form, all have a role in the assembly of the mature cytochrome oxidase complex. The description of these gene defects and the role these genes play are discussed in terms of what can be learned about COX assembly and about the etiology of the different phenotypic forms of the disease.

Mutations of the SCO1 gene in mitochondrial cytochrome c oxidase deficiency with neonatal-onset hepatic failure and encephalopathy

Cytochrome c oxidase (COX) catalyzes both electron transfer from cytochrome c to molecular oxygen and the concomitant vectorial proton pumping across the inner mitochondrial membrane. Studying a large family with multiple cases of neonatal ketoacidotic comas and isolated COX deficiency, we have mapped the disease locus to chromosome 17p13.1, in a region encompassing two candidate genes involved in COX assembly-namely, SCO1 and COX10. Mutation screening revealed compound heterozygosity for SCO1 gene mutations in the patients. The mutated allele, inherited from the father, harbored a 2-bp frameshift deletion (DeltaGA; nt 363-364) resulting in both a premature stop codon and a highly unstable mRNA. The maternally inherited mutation (C520T) changed a highly conserved proline into a leucine in the protein (P174L). This proline, adjacent to the CxxxC copper-binding domain of SCO1, is likely to play a crucial role in the tridimentional structure of the domain. Interestingly, the clinical presentation of SCO1-deficient patients markedly differs from that of patients harboring mutations in other COX assembly and/or maturation genes.

The pathophysiology of mitochondrial biogenesis: towards four decades of mitochondrial DNA research

Mitochondria are with very few exceptions ubiquitous organelles in eukaryotic cells where they are essential for cell life and death. Mitochondria play a central role not only in a variety of metabolic pathways including the supply of the bulk of cellular ATP through oxidative phosphorylation (OXPHOS), but also in complex processes such as development, apoptosis, and aging. Mitochondria contain their own genome that is replicated and expressed within the organelle. It encodes 13 polypeptides all of them components of the OXPHOS system, and thus, the integrity of the mitochondrial DNA (mtDNA) is critical for cellular energy supply. In the past 12 years more than 50 point mutations and around 100 rearrangements in the mtDNA have been associated with human diseases. Also in recent years, several mutations in nuclear genes that encode structural or regulatory factors of the OXPHOS system or the mtDNA metabolism have been described. The development of increasingly powerful techniques and the use of cellular and animal models are opening new avenues in the study of mitochondrial medicine. The detailed molecular characterization of the effects produced by different mutations that cause mitochondrial cytopathies will be critical for designing rational therapeutic strategies for this group of devastating diseases.

Cytochrome c oxidase assembly in primates is sensitive to small evolutionary variations in amino acid sequence

Respiring mitochondria require many interactions between nuclear and mitochondrial genomes. Although mitochondrial DNA (mtDNA) from the gorilla and the chimpanzee are able to restore oxidative phosphorylation in a human cell, mtDNAs from more distant primate species are functionally incompatible with human nuclear genes. Using microcell-mediated chromosome and mitochondria transfer, we introduced and maintained a functional orangutan mtDNA in a human nuclear background. However, partial oxidative phosphorylation function was restored only in the presence of most orangutan chromosomes, suggesting that human oxidative phosphorylation-related nuclear-coded genes are not able to replace many orangutan ones. The respiratory capacity of these hybrids was decreased by 65%-80%, and cytochrome c oxidase (COX) activity was decreased by 85%-95%. The function of other respiratory complexes was not significantly altered. The translation of mtDNA-coded COX subunits was normal, but their steady-state levels were approximately 10% of normal ones. Nuclear-coded COX subunits were loosely associated with mitochondrial membranes, a characteristic of COX assembly-defective mutants. Our results suggest that many human nuclear-coded genes not only cannot replace the orangutan counterparts, but also exert a specific interference at the level of COX assembly. This cellular model underscores the precision of COX assembly in mammals and sheds light on the nature of nuclear-mtDNA coevolutionary constraints.

Mitochondrial disorders

The rate of advance of our understanding of mitochondrial pathology continues to accelerate. Trends in genotype-phenotype correlations in mitochondrial DNA mutations continue to be developed; the latest of these is the association of exercise intolerance with cytochrome b mutations and onset in infancy of multisystem disorders associated with cytochrome oxidase assembly defects. New models for mitochondrial disease are being developed. Drugs, toxins and deficiency of nuclear encoded proteins that are targeted at mitochondria are now recognized as important causes of secondary mitochondrial respiratory chain deficiency.

Characterization of human SCO1 and COX17 genes in mitochondrial cytochrome-c-oxidase deficiency

At least three proteins, COX17p, SCO1p, and its homologue SCO2p are thought to be involved in mitochondrial copper transport to cytochrome-c-oxidase (COX), the terminal enzyme of the respiratory chain. Recently, we and others have shown that mutations in SCO2 are associated with a lethal infantile hypertrophic cardiomyopathy (HCMP) with COX-deficiency. The majority of patients with a similar phenotype were, however, negative for SCO2 mutations, suggesting the other genes as candidates for this disorder. Here we report on the genomic organization of SCO1 and COX17 on human chromosomes 17 and 3 respectively, and the complete sequence analysis of COX17 and SCO1 in 30 patients with COX deficiency. Using a panel of human:mouse-monochromosomal hybrids, the expression of COX17 was specifically restricted to chromosome 3, indicating that the previously reported sequence on chromosome 13 represents a pseudogene. DNA sequence analysis of SCO1 and COX17 in nine patients with severe COX deficiency and fatal HCMP, and in 21 patients with other COX deficiency disorders, did not reveal any pathogenic mutations or polymorphisms. We conclude that neither SCO1 nor COX17 are common causes of COX deficiency disorders.

Mitochondrial encephalomyopathy

Mitochondrial encephalomyopathy is a disease based on multisystemic mitochondrial dysfunction. Pathologic, biochemical and molecular genetic approaches to the disease have revealed the complex features of the phenotype and its relationship to the genotype. Last decade's great success of the mtDNA study must move research of the disease into the next phase.

Determination of enzyme activities for prenatal diagnosis of respiratory chain deficiency

Genetic counselling and prenatal diagnosis are major issues of mitochondrial respiratory chain deficiency, especially as these conditions are largely untreatable. In the absence of known mitochondrial or nuclear gene mutations, measurement of respiratory chain enzyme activities represents the only possibility to prevent recurrence of the disease in affected families. We carried out enzymatic prenatal diagnosis in 21 pregnancies from 10 unrelated couples using uncultured choriocytes and/or amniocytes. Twelve babies were born and are healthy, seven pregnancies were discontinued early on because of an enzyme deficiency detected prenatally. In two cases, a fetus which appeared normal after early and/or late prenatal diagnosis, turned out to be affected. We conclude that a deficient enzyme activity is indicative of recurrence, but a normal result at 10 weeks of gestation does not give conclusive evidence as to the outcome of the pregnancy. We therefore suggest the following procedure: (1) a choriocentesis or an amniocentesis in early pregnancy when the proband expresses the disease in cultured skin fibroblasts; (2) a second amniocentesis at 28 weeks' gestation should be offered to avoid false negative results due to a possible late expression of the disease, in combination with: (3) a careful and repeated ultrasound survey for detection of growth failure in the third trimester; (4) prenatal diagnosis should not be performed in case of late onset clinical symptoms in the proband; and (5) parents should be aware of the possibility of false negative results. Prenatal diagnosis should not be proposed for a complex I deficiency as this enzyme activity cannot be accurately measured in fetal cells.

Clinical differences in patients with mitochondriocytopathies due to nuclear versus mitochondrial DNA mutations

Defects in oxidative phosphorylation (OXPHOS) are genetically unique because the different components involved in this process, respiratory chain enzyme complexes (I, III, and IV) and complex V, are encoded by nuclear and mitochondrial genome. The objective of the study was to assess whether there are clinical differences in patients suffering from OXPHOS defects caused by nuclear or mitochondrial DNA (mtDNA) mutations. We studied 16 families with > or = two siblings with a genetically established OXPHOS deficiency, four due to a nuclear gene mutation and 12 due to a mtDNA mutation. Siblings with a nuclear gene mutation showed very similar clinical pictures that became manifest in the first years (ranging from first months to early childhood). There was a severe progressive course. Seven of the eight children died in their first decade. Conversely, siblings with a mtDNA mutation had clinical pictures that varied from almost alike to very distinct. They became symptomatic at an older age (ranging from childhood to adulthood), with the exception of defects associated with Leigh or Leigh-like phenotype. The clinical course was more gradual and relatively less severe; four of the 26 patients died, one in his second year, another in her second decade and two in their sixth decade. There are differences in age at onset, severity of clinical course, outcome, and intrafamilial variability in patients affected of an OXPHOS defect due to nuclear or mtDNA mutations. Patients with nuclear mutations become symptomatic at a young age, and have a severe clinical course. Patients with mtDNA mutations show a wider clinical spectrum of age at onset and severity. These differences may be of importance regarding the choice of which genome to study in affected patients as well as with respect to genetic counseling.

A novel SURF1 mutation results in Leigh syndrome with peripheral neuropathy caused by cytochrome c oxidase deficiency

We report on a 5-year-old boy with clinical and neuroradiological evidence of Leigh syndrome and peripheral neuropathy. Skeletal muscle biopsy showed decreased cytochrome c oxidase stain. Ultrastructurally, the nerve biopsy showed a defect of myelination. Biochemical analyses of muscle homogenate showed cytochrome c oxidase deficiency (15% residual activity). SURF1 gene analysis identified a novel homozygous nonsense mutation which predicts a truncated surf1 protein.

Activities of mitochondrial oxidative phosphorylation enzymes in cultured amniocytes

Amniocytes represent a population of foetal cells that can be used for prenatal diagnosis in families with suspected mitochondrial oxidative phosphorylation (OXPHOS) defects. In this paper, we present a complex protocol for evaluation of the function of mitochondrial OXPHOS enzymes in cultured amniocytes using three independent and complementary methods: (a) spectrophotometry as a tool for determination of the capacities of mitochondrial respiratory-chain enzymes (NADH ubiquinone oxidoreductase, succinate- and glycerophosphate cytochrome c reductase, cytochrome c oxidase and citrate synthase); (b) polarography as a tool for the evaluation of mitochondrial OXPHOS enzyme functions in situ using digitonin-permeabilised amniocytes (rotenone-sensitive oxidation of pyruvate+malate, antimycin A-sensitive oxidation of succinate, KCN-sensitive oxidation of cytochrome c, ADP-activated substrate oxidation) and (c) cytofluorometric determination of tetramethyl rhodamine methyl ester (TMRM) fluorescence in digitonin-permeabilised amniocytes as a sensitive way to determine the mitochondrial membrane potential under steady-state conditions (state 4 with succinate). These protocols are presented together with reference control values using 9-22 independent cultures of amniocytes.

Energetics in the pathogenesis of neurodegenerative diseases

Mitochondria have been linked to both necrotic and apoptotic cell death, which are thought to have a major role in the pathogenesis of neurodegenerative diseases. Recent evidence shows that nuclear gene defects affecting mitochondrial function have a role in the pathogenesis of Friedreich's ataxia, Wilson's disease and hereditary spastic paraplegia. There is also accumulating evidence that mitochondrial dysfunction might have a role in the pathogenesis of amyotrophic lateral sclerosis, Huntington's disease, Parkinson's disease and Alzheimer's disease. If this is so, a number of therapeutic targets are implicated that might result in novel treatments for neurodegenerative diseases.

Isolation and sequence of the human cytochrome c oxidase subunit VIIaL gene

The gene for human cytochrome c oxidase subunit VIIa liver isoform (COX7AL) was isolated and its sequence determined and analyzed. The three introns of the gene are considerably larger than those of the heart isoform of subunit VIIa (COX7AH), but the position of the introns relative to the cDNA sequences is homologous between the two genes. Comparison with other isolated COX7AL genes suggests that the promoter region binding motifs for transcription factors have evolved along with the coding region. In fibroblasts cultured originally from a Leigh's disease patient, a shortened COX7AL cDNA was identified by RT-PCR, consisting of exon I joined to exon IV, omitting exons II and III. No mutation could be identified in COX7AL of the patient, suggesting that the shortened cDNA is due to an alteration of the genome during cell culture. A surprising transcription of COX7AH was observed in cultured fibroblasts, suggesting a potential utility of these cells for study of its gene expression.

Myc and YY1 mediate activation of the Surf-1 promoter in response to serum growth factors

The human Surf-1 and Surf-2 genes are divergently transcribed and share a single bi-directional promoter. The addition of serum growth factors to serum-starved cells activates transcription in the Surf-1 direction, but has no effect on transcription in the Surf-2 direction. Mutations that block the binding of YY1 to a site immediately downstream of the major Surf-1 transcription start point abolish this response to serum factors. Here we show that over-expression of mitogen-activated protein (MAP) kinase phosphatase MKP-1, an inhibitor of the MAP kinase cascade, also blocks the response the Surf-1 promoter to serum factors. YY1 has previously been shown to interact with several transcription factors including Myc. We show that although the Surf-1/Surf-2 promoter does not contain Myc binding sites (E-boxes), Myc over-expression, or the activation of a Myc-oestrogen receptor fusion protein, activates transcription in the Surf-1 direction and that this response to Myc requires a functional YY1 binding site. Our data suggest that the MAP kinase cascade is required for the stimulation of Surf-1 promoter activity and that the Myc-YY1 interaction mediates this response.

Disorders related to mitochondrial membranes: pathology of the respiratory chain and neurodegeneration

Faulty oxidative phosphorylation (OXPHOS) is observed in a number of mitochondrial disorders, and may be associated with single or multiple defects of the five complexes of the respiratory chain. From the genetic standpoint, the respiratory chain is unique as it is formed by means of the complementation of two separate genetic systems: the nuclear genome and the mitochondrial genome. The nuclear genome encodes most of the protein subunits of the respiratory complexes and most of the mtDNA replication and expression systems, whereas the mitochondrial genome encodes only 13 OXPHOS subunits and some RNA components of the mitochondrial translation apparatus. Accordingly, mitochondrial disorders due to defects in oxidative phosphorylation include both Mendelian-inherited and cytoplasmic-inherited diseases. Although our knowledge of these diseases has grown at an impressive rate in the past few years, it is worth noting that, as a result of complementation in the respiratory chain of nuclear-encoded and mitochondrially-encoded polypeptides, it is often difficult to establish a precise relationship between genomic mutations and biochemical phenotypes, or to distinguish pathogenic mutations from polymorphic variants in gene sequences. This review deals with human diseases due to mutations of the structural components of the respiratory chain, particularly focusing on the recently identified disorders caused either by mutations in nuclear genes encoding subunits of the subcomplexes or by mutations in nuclear genes affecting the functional efficiency, homeostasis, and assembly of respiratory chain subcomplexes. To better focus on OXPHOS genotype-phenotype correlations, mutations of the mtDNA-encoded structural genes are also discussed.

Oxidative phosphorylation disease diagnosis

Although the mtDNA encodes only 13 polypeptide subunits of the OXPHOS enzymes, approximately 1,000 proteins are estimated to be necessary for proper OXPHOS function. Over the past 10 years a wide variety of adult and pediatric OXPHOS diseases were found to be caused by or associated with mitochondrial DNA (mtDNA) mutations and nuclear DNA mutations. These advances enhanced the ability to definitively diagnose patients, develop management plans, and provide genetic counseling. Recently described nuclear DNA and mtDNA mutations are enhancing our understanding of this complex group of diseases. The impact of these advances on our understanding of OXPHOS disease pathogenesis will be reviewed.

Screening human EST database for identification of candidate genes in respiratory chain deficiency

Disorders of mitochondrial oxidative phosphorylation (OXPHOS) are now recognized as major causes of human metabolic diseases and several mutations of mitochondrial and nuclear genes encoding respiratory chain components have been reported. Interestingly, mutations of nuclear genes involved in mitochondrial respiratory chain assembly, protein trafficking, and iron metabolism are also known to alter oxidative phosphorylation. While several hundred of these genes have been described in yeast, only a few nuclear genes have been hitherto identified in humans. Yeast gene databases present therefore an invaluable tool for identification of human homologues that should be regarded as candidate genes in OXPHOS diseases. In an attempt to identify the human counterparts of yeast genes, we developed a systematic comparison of yeast protein sequences to the GenBank dbEST database. Starting from 340 yeast protein sequences as templates, we searched the human dbEST counterparts using the BLAST similarity searching program and identified 102 groups of human EST likely to represent orthologues of yeast genes because of significant homology. This collection of human genes possibly related to mitochondrial OXPHOS may help identify nuclear genes responsible of mitochondrial disorders.

Isolated complex I deficiency in children: clinical, biochemical and genetic aspects

We retrospectively examined clinical and biochemical characteristics of 27 patients with isolated enzymatic complex I deficiency (established in cultured skin fibroblasts) in whom common pathogenic mtDNA point mutations and major rearrangements were absent. Clinical phenotypes present in this group are Leigh syndrome (n = 7), Leigh-like syndrome (n = 6), fatal infantile lactic acidosis (n = 3), neonatal cardiomyopathy with lactic acidosis (n = 3), macrocephaly with progressive leukodystrophy (n = 2), and a residual group of unspecified encephalomyopathy (n = 6) subdivided into progressive (n = 4) and stable (n = 2) variants. Isolated complex I deficiency is one of the most frequently observed disturbance of the OXPHOS system. Respiratory chain enzyme assays performed in cultured fibroblasts and skeletal muscle tissue in general reveal similar results, but for complete diagnostics we recommend enzyme measurements performed in at least two different tissues to minimize the possibility of overlooking the enzymatic diagnosis. Lactate levels in blood and CSF and cerebral CT/MRI studies are highly informative, although normal findings do not exclude complex I deficiency. With the discovery of mutations in nuclear encoded complex I subunits, adequate pre- and postnatal counseling becomes available. Finally, considering information currently available, isolated complex I deficiency in children seems to be caused in the majority by mutations in nuclear DNA.

Isolation and genomic analysis of the human surf-6 gene: a member of the Surfeit locus

The human Surfeit locus contains at least six tightly clustered genes (Surf-1 to Surf-6) of which five (Surf-1 to Surf-5) have been characterised and found not to share any sequence homology. The organisation and juxtaposition of the Surfeit genes are conserved between human and mouse. The Surf-6 gene that encodes a novel nucleolar-matrix protein with nucleic-acid binding properties has been characterised in mouse. In this work, we have isolated and analysed the human Surf-6 homologue and determined its genomic organisation in the Surfeit locus. The human Surf-6 gene has five exons spread over a distance of 4.3kb and has features of a housekeeping gene being ubiquitously expressed, having its 5' end located within a CpG rich island and lacking a canonical TATA box. The intragenic region between the 3' end of the Surf-5 gene and the 5' end of the Surf-6 gene is 3.2kb and contains a pseudogene of the ribosomal protein gene rpL21. The putative human Surf-6 protein is 361 amino acids long and includes motifs found in both the mouse and fish Surf-6 homologues, which may underlie the functions of Surf-6. Three amino acid polymorphisms have been detected at codons 163, 175 and 311 by SSCP analysis.

Mitochondrial defects in cardiomyopathy and neuromuscular disease

Over the past 11 years, a considerable body of evidence has accumulated implicating defects in the mitochondrial energy-generating pathway, oxidative phosphorylation, in a wide variety of degenerative diseases including myopathy and cardiomyopathy. Most classes of pathogenic mitochondrial DNA mutations affect the heart, in association with a variety of other clinical manifestations that can include skeletal muscle, the central nervous system (including eye), the endocrine system, and the renal system. To better understand the pathophysiologic basis of mitochondrial diseases and their role in myopathy and cardiomyopathy, several mouse models of mitochondrial disease have been prepared. Mitochondrial DNA mutations from cultured cells have been introduced into mice; nuclear DNA genes involved in mitochondrial energy production and reactive oxygen species detoxification have been genetically inactivated, which resulted in mice with hypertrophic and dilated cardiomyopathy, respectively. Physiologic characterization of these mice has confirmed the importance of decreased mitochondrial energy production, increased mitochondrial reactive oxygen species production, and the mitochondrial initiation of apoptosis in mitochondrial disease. With these insights, new therapeutic approaches for neuromuscular and cardiac disease have been suggested.

[What is new in pediatric neurology?]

Some significant advances in the field of pediatric neurology are reviewed. For many constitutional disorders, concepts and diagnostic procedures have progressed from various genetic techniques or from protein labeling in situ. Many neurodegenerative disorders, some poorly-defined metabolic diseases, and several syndromes associating mental retardation with neurologic or extraneurologic malformations have been characterized. In addition, for many disorders viewed as 'poorly specific' (mental retardation, epilepsy, migraine), familial forms have permitted us to define the first genes involved. In 'acquired' disorders, new data come from clinical trials (antiepileptic, anti-inflammatory drugs) rather than definite conceptual advances. Finally, clinics and biology are no longer the only approaches to brain functions, and clinical neurophysiology could encounter a second wind thanks to the techniques of functional imaging, especially in the fields of developmental neuropsychology.

Mitochondrial myopathy diagnosis

Oxidative phosphorylation (OXPHOS) accounts for approximately 95% of the adenosine triphosphate (ATP) produced by the cell. The central nervous system, peripheral nervous system, cardiac muscle, skeletal muscle, and smooth muscle are highly susceptible to dysfunction of this complex enzyme system. Although most OXPHOS diseases are multisystem disorders, the neuromuscular manifestations are often prominent and play an important role in patient diagnosis. To assist the neurologist in evaluating these complex patients, this article focuses on selected samples of OXPHOS diseases with identifiable neuromuscular abnormalities and presents an evaluation algorithm to facilitate patient diagnosis.

MITOP, the mitochondrial proteome database: 2000 update

MITOP (http://www.mips.biochem.mpg.de/proj/medgen/mitop/) is a comprehensive database for genetic and functional information on both nuclear- and mitochondrial-encoded proteins and their genes. The five species files--Saccharomyces cerevisiae, Mus musculus, Caenorhabditis elegans, Neurospora crassa and Homo sapiens--include annotated data derived from a variety of online resources and the literature. A wide spectrum of search facilities is given in the overlapping sections 'Gene catalogues', 'Protein catalogues', 'Homologies', 'Pathways and metabolism' and 'Human disease catalogue' including extensive references and hyperlinks to other databases. Central features are the results of various homology searches, which should facilitate the investigations into interspecies relationships. Precomputed FASTA searches using all the MITOP yeast protein entries and a list of the best human EST hits with graphical cluster alignments related to the yeast reference sequence are presented. The orthologue tables with cross-listings to all the protein entries for each species in MITOP have been expanded by adding the genomes of Rickettsia prowazeckii and Escherichia coli. To find new mitochondrial proteins the complete yeast genome has been analyzed using the MITOPROT program which identifies mitochondrial targeting sequences. The 'Human disease catalogue' contains tables with a total of 110 human diseases related to mitochondrial protein abnormalities, sorted by clinical criteria and age of onset. MITOP should contribute to the systematic genetic characterization of the mitochondrial proteome in relation to human disease.

Sequence conservation from human to prokaryotes of Surf1, a protein involved in cytochrome c oxidase assembly, deficient in Leigh syndrome

The human SURF1 gene encoding a protein involved in cytochrome c oxidase (COX) assembly, is mutated in most patients presenting Leigh syndrome associated with COX deficiency. Proteins homologous to the human Surf1 have been identified in nine eukaryotes and six prokaryotes using database alignment tools, structure prediction and/or cDNA sequencing. Their sequence comparison revealed a remarkable Surf1 conservation during evolution and put forward at least four highly conserved domains that should be essential for Surf1 function. In Paracoccus denitrificans, the Surf1 homologue is found in the quinol oxidase operon, suggesting that Surf1 is associated with a primitive quinol oxidase which belongs to the same superfamily as cytochrome oxidase.

Characterization of SURF-1 expression and Surf-1p function in normal and disease conditions

Loss-of-function mutations of the SURF-1 gene have been associated with Leigh syndrome with cytochrome c oxidase (COX) deficiency. Mature Surf-1 protein (Surf-1p) is a 30 kDa hydrophobic polypeptide whose function is still unknown. Using antibodies against a recombinant, hemagglutinin-tagged Surf-1p, we have demonstrated that this protein is imported into mitochondria as a larger precursor, which is then processed into the mature product by cleaving off an N-terminal leader polypeptide of approximately 40 amino acids. By using western blot analysis with specific antibodies, we showed that Surf-1p is localized in and tightly bound to the mitochondrial inner membrane. The same analysis revealed that no protein is present in cell lines harboring loss-of-function mutations of SURF-1, regardless of their type and position. Northern blot analysis showed the virtual absence of specific SURF-1 transcripts in different mutant cell lines. This result suggests that several mutations of SURF-1 are associated with severe mRNA instability. To understand better whether and which domains of the protein are essential for function, we generated several constructs with truncated or partially deleted SURF-1 cDNAs. None of these constructs, expressed into Surf-1p null mutant cells, were able to rescue the COX phenotype, suggesting that different regions of the protein are all essential for function. Finally, experiments based on blue native two-dimensional gel electrophoresis indicated that assembly of COX in Surf-1p null mutants is blocked at an early step, most likely before the incorporation of subunit II in the nascent intermediates composed of subunit I alone or subunit I plus subunit IV. However, detection of residual amounts of fully assembled complex suggests a certain degree of redundancy of this system.