SURFEIT-1 gene analysis and two-dimensional blue native gel electrophoresis in cytochrome c oxidase deficiency

Causes of Death in Adults with Mitochondrial Disease

INTRODUCTION

Mitochondrial diseases are a clinically, biochemically and genetically heterogeneous group of disorders with a variable age of onset and rate of disease progression. It might therefore be expected that this variation be reflected in the age and cause of death. However, to date, little has been reported regarding the 'end-of-life' period and causes of death in mitochondrial disease patients. For some specific syndromes, the associated clinical problems might predict the cause of death, but for many patients, it remains difficult to provide an accurate prognosis.

AIMS

To describe a retrospective cohort of adult mitochondrial disease patients who had attended the NHS Highly Specialised Services for Rare Mitochondrial Diseases in Newcastle upon Tyne (UK), evaluate life expectancy and causes of death and assess the consequences for daily patient care.

METHODS

All deceased adult patients cared for at this centre over a period of 10 years were included in the study. Patient history, data on laboratory findings, biochemical investigations and genetic studies were analysed retrospectively.

RESULTS

A total of 30 adult mitochondrial patients died within the time period of the study. The main mitochondrial disease-related causes of death in this patient cohort were respiratory failure, cardiac failure and acute cerebral incidents such as seizures and strokes. In almost half of the patients, the cause of death remained unknown. Based on our study, we present recommendations regarding the care of patients with mitochondrial disease.

The heme a synthase Cox15 associates with cytochrome c oxidase assembly intermediates during Cox1 maturation

Cox1, the core subunit of the cytochrome c oxidase, receives two heme a cofactors during assembly of the 13-subunit enzyme complex. However, at which step of the assembly process and how heme is inserted into Cox1 have remained an enigma. Shy1, the yeast SURF1 homolog, has been implicated in heme transfer to Cox1, whereas the heme a synthase, Cox15, catalyzes the final step of heme a synthesis. Here we performed a comprehensive analysis of cytochrome c oxidase assembly intermediates containing Shy1. Our analyses suggest that Cox15 displays a role in cytochrome c oxidase assembly, which is independent of its functions as the heme a synthase. Cox15 forms protein complexes with Shy1 and also associates with Cox1-containing complexes independently of Shy1 function. These findings indicate that Shy1 does not serve as a mobile heme carrier between the heme a synthase and maturing Cox1 but rather cooperates with Cox15 for heme transfer and insertion in early assembly intermediates of cytochrome c oxidase.

High-resolution melting analysis of 15 genes in 60 patients with cytochrome-c oxidase deficiency

Cytochrome-c oxidase (COX) deficiency is one of the common childhood mitochondrial disorders. Mutations in genes for the assembly factors SURF1 and SCO2 are prevalent in children with COX deficiency in the Slavonic population. Molecular diagnosis is difficult because of the number of genes involved in COX biogenesis and assembly. The aim of this study was to screen for mutations in 15 nuclear genes that encode the 10 structural subunits, their isoforms and two assembly factors of COX in 60 unrelated Czech children with COX deficiency. Nine novel variants were identified in exons and adjacent intronic regions of COX4I2, COX6A1, COX6A2, COX7A1, COX7A2 and COX10 using high-resolution melting (HRM) analysis. Online bioinformatics servers were used to predict the importance of the newly identified amino-acid substitutions. The newly characterized variants updated the contemporary spectrum of known genetic sequence variations that are present in the Czech population, which will be important for further targeted mutation screening in Czech COX-deficient children. HRM and predictive bioinformatics methodologies are advantageous because they are low-cost screening tools that complement large-scale genomic studies and reduce the required time and effort.

A mutation in C2orf64 causes impaired cytochrome c oxidase assembly and mitochondrial cardiomyopathy

The assembly of mitochondrial respiratory chain complex IV (cytochrome c oxidase) involves the coordinated action of several assembly chaperones. In Saccharomyces cerevisiae, at least 30 different assembly chaperones have been identified. To date, pathogenic mutations leading to a mitochondrial disorder have been identified in only seven of the corresponding human genes. One of the genes for which the relevance to human pathology is unknown is C2orf64, an ortholog of the S. cerevisiae gene PET191. This gene has previously been shown to be a complex IV assembly factor in yeast, although its exact role is still unknown. Previous research in a large cohort of complex IV deficient patients did not support an etiological role of C2orf64 in complex IV deficiency. In this report, a homozygous mutation in C2orf64 is described in two siblings affected by fatal neonatal cardiomyopathy. Pathogenicity of the mutation is supported by the results of a complementation experiment, showing that complex IV activity can be fully restored by retroviral transduction of wild-type C2orf64 in patient-derived fibroblasts. Detailed analysis of complex IV assembly intermediates in patient fibroblasts by 2D-BN PAGE revealed the accumulation of a small assembly intermediate containing subunit COX1 but not the COX2, COX4, or COX5b subunits, indicating that C2orf64 is involved in an early step of the complex IV assembly process. The results of this study demonstrate that C2orf64 is essential for human complex IV assembly and that C2orf64 mutational analysis should be considered for complex IV deficient patients, in particular those with hypertrophic cardiomyopathy.

Mimicking a SURF1 allele reveals uncoupling of cytochrome c oxidase assembly from translational regulation in yeast

Defects in mitochondrial energy metabolism lead to severe human disorders, mainly affecting tissues especially dependent on oxidative phosphorylation, such as muscle and brain. Leigh Syndrome describes a severe encephalomyopathy in infancy, frequently caused by mutations in SURF1. SURF1, termed Shy1 in Saccharomyces cerevisiae, is a conserved assembly factor for the terminal enzyme of the respiratory chain, cytochrome c oxidase. Although the molecular function of SURF1/Shy1 is still enigmatic, loss of function leads to cytochrome c oxidase deficiency and reduced expression of the central subunit Cox1 in yeast. Here, we provide insights into the molecular mechanisms leading to disease through missense mutations in codons of the most conserved amino acids in SURF1. Mutations affecting G(124) do not compromise import of the SURF1 precursor protein but lead to fast turnover of the mature protein within the mitochondria. Interestingly, an Y(274)D exchange neither affects stability nor localization of the protein. Instead, SURF1(Y274D) accumulates in a 200 kDa cytochrome c oxidase assembly intermediate. Using yeast as a model, we demonstrate that the corresponding Shy1(Y344D) is able to overcome the stage where cytochrome c oxidase assembly links to the feedback regulation of mitochondrial Cox1 expression. However, Shy1(Y344D) impairs the assembly at later steps, most apparent at low temperature and exhibits a dominant-negative phenotype upon overexpression. Thus, exchanging the conserved tyrosine (Y(344)) with aspartate in yeast uncouples translational regulation of Cox1 from cytochrome c oxidase assembly and provides evidence for the dual functionality of Shy1.

Coa3 and Cox14 are essential for negative feedback regulation of COX1 translation in mitochondria

Coa3 and Cox14 form assembly intermediates with newly synthesized Cox1 and are required for association of the Mss51 translational activator with these complexes.

Regulation of eukaryotic cytochrome oxidase assembly occurs at the level of Cox1 translation, its central mitochondria-encoded subunit. Translation of COX1 messenger RNA is coupled to complex assembly in a negative feedback loop: the translational activator Mss51 is thought to be sequestered to assembly intermediates, rendering it incompetent to promote translation. In this study, we identify Coa3 (cytochrome oxidase assembly factor 3; Yjl062w-A), a novel regulator of mitochondrial COX1 translation and cytochrome oxidase assembly. We show that Coa3 and Cox14 form assembly intermediates with newly synthesized Cox1 and are required for Mss51 association with these complexes. Mss51 exists in equilibrium between a latent, translational resting, and a committed, translation-effective, state that are represented as distinct complexes. Coa3 and Cox14 promote formation of the latent state and thus down-regulate COX1 expression. Consequently, lack of Coa3 or Cox14 function traps Mss51 in the committed state and promotes Cox1 synthesis. Our data indicate that Coa1 binding to sequestered Mss51 in complex with Cox14, Coa3, and Cox1 is essential for full inactivation.

SURF1 missense mutations promote a mild Leigh phenotype

SURF1 gene mutations are the most common cause of Leigh syndrome (LS), a rare progressive neurodegenerative disorder of infancy, characterized by symmetric necrotizing lesions and hypervascularity in the brainstem and basal ganglia, leading to death before the age of 4 years. Most of the reported mutations create premature termination codons, whereas missense mutations are rare. The aim of the study was to characterize the natural history of LS patients carrying at least one missense mutation in the SURF1 gene. Nineteen such patients (8 own cases and 11 reported in the literature) were compared with a reference group of 20 own c.845_846delCT homozygous patients, and with other LS(SURF-) cases described in the literature. Disease onset in the studied group was delayed. Acute failure to thrive and hyperventilation episodes were rare, respiratory failure did not appear before the age of 4 years. Dystonia, motor regression and eye movement dissociation developed slowly. The number of patients who survived 7 years of life totaled 9 out of 15 (60%) in the 'missense group' and 1 out of 26 (4%) patients with mutations leading to truncated proteins.

IN CONCLUSION

(i) The presence of a missense mutation in the SURF1 gene may correlate with a milder course and longer survival of Leigh patients, (ii) normal magnetic resonance imaging (MRI) findings, normal blood lactate value, and only mild decrease of cytochrome c oxidase (COX) activity are not sufficient reasons to forego SURF1 mutation analysis in differential diagnosis.

High prevalence of SURF1 c.845_846delCT mutation in Polish Leigh patients

Leigh syndrome is a neuropathological disorder with typical morphological changes in brain, appearing regardless of diverse molecular background. One of the most common enzymatic defects in Leigh patients is cytochrome c oxidase deficiency associated with recessive mutations in the SURF1 gene. To assess the SURF1 mutation profile among Polish patients we studied 41 affected children from 34 unrelated families by PCR-SSCP and sequencing. Four novel mutations, c.39delG, c.752-1G>C, c.800_801insT, c.821A>G, and five described pathogenic changes, c.311_312insAT312_321del10, c.688C>T, c.704T>C, c.756_757delCA, c.845_846delCT, were identified in 85.3% of analysed probands. One mutation, c.845_846delCT, was identified in 77.6% of SURF1 alleles. Up to now, it has been reported only in 9% of alleles in other parts of the world. The deletion was used as LS(SURF1-) marker in population studies. Eight heterozygous carriers of the mutation were found in a cohort of 2890 samples. The estimated c.845_846delCT allele frequency is 1:357 (0.28+/-0.2%), and the lowest predicted LS(SURF1-) frequency in Poland 1:126,736.births. Relatively high frequency of LS(SURF1-) in Poland with remarkable c.845_846delCT mutation dominance allows one to start the differential diagnosis of LS in each patient of Polish (and probably Slavonic) origin from the direct search for c.845_846delCT SURF1 mutation.

Successful application of preimplantation genetic diagnosis for Leigh syndrome

OBJECTIVE

To perform preimplantation genetic diagnosis (PGD) for a SURF1 gene mutation of the Leigh syndrome to transfer unaffected or carrier embryo/embryos.

DESIGN

Case report.

SETTING

Clinical IVF laboratory.

PATIENT(S)

A couple carrying an nt769 G/A mutation that is associated with Leigh syndrome.

INTERVENTION(S)

Oocytes were fertilized by means of intracytoplasmic sperm injection. The resulting embryos were biopsied 3 days after fertilization. One blastomere was taken and whole-genome amplification was performed. Amplification of the mutation site was achieved by polymerase chain reaction (PCR) and restriction digestion was completed. Gel Imager was used to measure the digests of normal and mutant load.

MAIN OUTCOME MEASURE(S)

Embryo testing by means of PGD-PCR and pregnancy. Successful preimplantation genetic diagnosis for a SURF1 gene mutation and transfer of healthy or carrier embryos.

RESULT(S)

Successful singleton pregnancy resulting in the delivery of healthy baby girl.

CONCLUSION(S)

We report the first case of successful PGD for Leigh syndrome resulting in delivery of a healthy newborn.

Suppression mechanisms of COX assembly defects in yeast and human: insights into the COX assembly process

Eukaryotic cytochrome c oxidase (COX) is the terminal enzyme of the mitochondrial respiratory chain. COX is a multimeric enzyme formed by subunits of dual genetic origin whose assembly is intricate and highly regulated. In addition to the structural subunits, a large number of accessory factors are required to build the holoenzyme. The function of these factors is required in all stages of the assembly process. They are relevant to human health because devastating human disorders have been associated with mutations in nuclear genes encoding conserved COX assembly factors. The study of yeast strains and human cell lines from patients carrying mutations in structural subunits and COX assembly factors has been invaluable to attain the current state of knowledge, even if still fragmentary, of the COX assembly process. After the identification of the genes involved, the isolation and characterization of genetic and metabolic suppressors of COX assembly defects, reviewed here, have become a profitable strategy to gain insight into their functions and the pathways in which they operate. Additionally, they have the potential to provide useful information for devising therapeutic approaches to combat human disorders associated with COX deficiency.

Light and electron microscopy characteristics of the muscle of patients with SURF1 gene mutations associated with Leigh disease

AIMS

Leigh syndrome (LS) is characterised by almost identical brain changes despite considerable causal heterogeneity. SURF1 gene mutations are among the most frequent causes of LS. Although deficiency of cytochrome c oxidase (COX) is a typical feature of the muscle in SURF1-deficient LS, other abnormalities have been rarely described. The aim of the present work is to assess the skeletal muscle morphology coexisting with SURF1 mutations from our own research and in the literature.

METHODS

Muscle samples from 21 patients who fulfilled the criteria of LS and SURF1 mutations (14 homozygotes and 7 heterozygotes of c.841delCT) were examined by light and electron microscopy.

RESULTS

Diffuse decreased activity or total deficit of COX was revealed histochemically in all examined muscles. No ragged red fibres (RRFs) were seen. Lipid accumulation and fibre size variability were found in 14 and 9 specimens, respectively. Ultrastructural assessment showed several mitochondrial abnormalities, lipid deposits, myofibrillar disorganisation and other minor changes. In five cases no ultrastructural changes were found. Apart from slight correlation between lipid accumulation shown by histochemical and ultrastructural techniques, no other correlations were revealed between parameters investigated, especially between severity of morphological changes and the patient's age at the biopsy.

CONCLUSION

Histological and histochemical features of muscle of genetically homogenous SURF1-deficient LS were reproducible in detection of COX deficit. Minor muscle changes were not commonly present. Also, ultrastructural abnormalities were not a consistent feature. It should be emphasised that SURF1-deficient muscle assessed in the light and electron microscopy panel may be interpreted as normal if COX staining is not employed.

Two-dimensional blue native/SDS gel electrophoresis of multiprotein complexes from Helicobacter pylori

The study of protein interactions constitutes an important domain to understand the physiology and pathogenesis of microorganisms. The two-dimensional blue native/SDS-PAGE was initially reported to analyze membrane protein complexes. In this study, both cytoplasmic and membrane complexes of a bacterium, the strain J99 of the gastric pathogen Helicobacter pylori, were analyzed by this method. It was possible to identify 34 different proteins grouped in 13 multiprotein complexes, 11 from the cytoplasm and two from the membrane, either previously reported partially or totally in the literature. Besides complexes involved in H. pylori physiology, this method allowed the description of interactions involving known pathogenic factors such as (i) urease with the heat shock protein GroEL or with the putative ketol-acid reductoisomerase IlvC and (ii) the cag pathogenicity island CagA protein with the DNA gyrase GyrA as well as insight on the partners of TsaA, a peroxide reductase/stress-dependent molecular chaperone. The two-dimensional blue native/SDS-PAGE combined with mass spectrometry is a potential tool to study the differences in complexes isolated in various situations and also to study the interactions between bacterial and eucaryotic cell proteins.

Detection and analysis of protein–protein interactions in organellar and prokaryotic proteomes by native gel electrophoresis: (Membrane) protein complexes and supercomplexes

It is an essential and challenging task to unravel protein-protein interactions in their actual in vivo context. Native gel systems provide a separation platform allowing the analysis of protein complexes on a rather proteome-wide scale in a single experiment. This review focus on blue-native (BN)-PAGE as the most versatile and successful gel-based approach to separate soluble and membrane protein complexes of intricate protein mixtures derived from all biological sources. BN-PAGE is a charge-shift method with a running pH of 7.5 relying on the gentle binding of anionic CBB dye to all membrane and many soluble protein complexes, leading to separation of protein species essentially according to their size and superior resolution than other fractionation techniques can offer. The closely related colorless-native (CN)-PAGE, whose applicability is restricted to protein species with intrinsic negative net charge, proved to provide an especially mild separation capable of preserving weak protein-protein interactions better than BN-PAGE. The essential conditions determining the success of detecting protein-protein interactions are the sample preparations, e.g. the efficiency/mildness of the detergent solubilization of membrane protein complexes. A broad overview about the achievements of BN- and CN-PAGE studies to elucidate protein-protein interactions in organelles and prokaryotes is presented, e.g. the mitochondrial protein import machinery and oxidative phosphorylation supercomplexes. In many cases, solubilization with digitonin was demonstrated to facilitate an efficient and particularly gentle extraction of membrane protein complexes prone to dissociation by treatment with other detergents. In general, analyses of protein interactomes should be carried out by both BN- and CN-PAGE.

Neuropathologic aspects of cytochrome C oxidase deficiency

Cytochrome c oxidase (COX) deficiency is an important cause of myopathy or encephalomyopathy. Considering the structural complexity of COX, its dual genetic control, and the several nuclear genes needed for its proper assembly, the phenotypic heterogeneity is not surprising. From a morphologic view point, the application of histochemistry and immunohistochemistry to the study of COX deficiency in muscle has revealed specific patterns that -we believe- are helpful both for diagnosis and for directing sequencing studies of either mitochondrial DNA (mtDNA) or nuclear DNA (nDNA) genes. Similar studies in brain have shown that patients with mutations in mtDNA appear to have different patterns of COX deficiency from patients with mutations in nDNA genes. The recent discovery of mutations in COX assembly genes coupled with the potential to generate knock-out mice with these mutations holds the promise of providing the neuropathologist with the animal models needed to study the pathogenesis of COX deficiency in brain and muscle.

Tissue-specific cytochrome c oxidase assembly defects due to mutations in SCO2 and SURF1

The biogenesis of eukaryotic COX (cytochrome c oxidase) requires several accessory proteins in addition to structural subunits and prosthetic groups. We have analysed the assembly state of COX and SCO2 protein levels in various tissues of six patients with mutations in SCO2 and SURF1. SCO2 is a copper-binding protein presumably involved in formation of the Cu(A) centre of the COX2 subunit. The function of SURF1 is unknown. Immunoblot analysis of native gels demonstrated that COX holoenzyme is reduced to 10-20% in skeletal muscle and brain of SCO2 and SURF1 patients and to 10-30% in heart of SCO2 patients, whereas liver of SCO2 patients' contained normal holoenzyme levels. The steady-state levels of mutant SCO2 protein ranged from 0 to 20% in different SCO2 patient tissues. In addition, eight distinct COX subcomplexes and unassembled subunits were found, some of them identical with known assembly intermediates of the human enzyme. Heart, brain and skeletal muscle of SCO2 patients contained accumulated levels of the COX1.COX4.COX5A subcomplex, three COX1-containing subcomplexes, a COX4.COX5A subcomplex and two subcomplexes composed of only COX4 or COX5A. The accumulation of COX1.COX4.COX5A subcomplex, along with the virtual absence of free COX2, suggests that the lack of the Cu(A) centre may result in decreased stability of COX2. The appearance of COX4.COX5A subcomplex indicates that association of these nucleus-encoded subunits probably precedes their addition to COX1 during the assembly process. Finally, the consequences of SCO2 and SURF1 mutations suggest the existence of tissue-specific functional differences of these proteins that may serve different tissue-specific requirements for the regulation of COX biogenesis.

Post-transcriptional silencing and functional characterization of the Drosophila melanogaster homolog of human Surf1

Mutations in Surf1, a human gene involved in the assembly of cytochrome c oxidase (COX), cause Leigh syndrome, the most common infantile mitochondrial encephalopathy, characterized by a specific COX deficiency. We report the generation and characterization of functional knockdown (KD) lines for Surf1 in Drosophila. KD was produced by post-transcriptional silencing employing a transgene encoding a dsRNA fragment of the Drosophila homolog of human Surf1, activated by the UAS transcriptional activator. Two alternative drivers, Actin5C-GAL4 or elav-GAL4, were used to induce silencing ubiquitously or in the CNS, respectively. Actin5C-GAL4 KD produced 100% egg-to-adult lethality. Most individuals died as larvae, which were sluggish and small. The few larvae reaching the pupal stage died as early imagos. Electron microscopy of larval muscles showed severely altered mitochondria. elav-GAL4-driven KD individuals developed to adulthood, although cephalic sections revealed low COX-specific activity. Behavioral and electrophysiological abnormalities were detected, including reduced photoresponsiveness in KD larvae using either driver, reduced locomotor speed in Actin5C-GAL4 KD larvae, and impaired optomotor response as well as abnormal electroretinograms in elav-GAL4 KD flies. These results indicate important functions for SURF1 specifically related to COX activity and suggest a crucial role of mitochondrial energy pathways in organogenesis and CNS development and function.

Unusual clinical presentations in four cases of Leigh disease, cytochrome C oxidase deficiency, and SURF1 gene mutations

Mutations in the SURF1 gene are the most frequent causes of Leigh disease with cytochrome c oxidase deficiency. We describe four children with novel SURF1 mutations and unusual features: three had prominent renal symptoms and one had ragged red fibers in the muscle biopsy. We identified five pathogenic mutations in SURF1: two mutations were novel, an in-frame nonsense mutation (834G-->A) and an out-of-frame duplication (820-824dupTACAT). Although renal manifestations have not been described in association with SURF1 mutations, they can be part of the clinical presentation. Likewise, mitochondrial proliferation in muscle (with ragged red fibers) is most unusual in Leigh disease but might be part of an emerging phenotype.

Decreased affinity for oxygen of cytochrome-coxidase in Leigh syndrome caused bySURF1mutations

Mutations in the gene SURF1 prevent synthesis of cytochrome- c oxidase (COX)-specific assembly protein and result in a fatal neurological disorder, Leigh syndrome. Because this severe COX deficiency presents with barely detectable changes of cellular respiratory rates under normoxic conditions, we analyzed the respiratory response to low oxygen in cultured fibroblasts harboring SURF1 mutations with high-resolution respirometry. The oxygen kinetics was quantified by the partial pressure of oxygen (Po2) at half-maximal respiration rate (P50) in intact coupled cells and in digitonin-permeabilized uncoupled cells. In both cases, the P50in patients was elevated 2.1- and 3.3-fold, respectively, indicating decreased affinity of COX for oxygen. These results suggest that at physiologically low intracellular Po2, the depressed oxygen affinity may lead in vivo to limitations of respiration, resulting in impaired energy provision in Leigh syndrome patients.

Identification and characterization of a common set of complex I assembly intermediates in mitochondria from patients with complex I deficiency

Deficiencies in the activity of complex I (NADH: ubiquinone oxidoreductase) are an important cause of human mitochondrial disease. Complex I is composed of at least 46 structural subunits that are encoded in both nuclear and mitochondrial DNA. Enzyme deficiency can result from either impaired catalytic efficiency or an inability to assemble the holoenzyme complex; however, the assembly process remains poorly understood. We have used two-dimensional Blue-Native/SDS gel electrophoresis and a panel of 11 antibodies directed against structural subunits of the enzyme to investigate complex I assembly in the muscle mitochondria from four patients with complex I deficiency caused by either mitochondrial or nuclear gene defects. Immunoblot analyses of second dimension denaturing gels identified seven distinct complex I subcomplexes in the patients studied, five of which could also be detected in nondenaturing gels in the first dimension. Although the abundance of these intermediates varied among the different patients, a common constellation of subcomplexes was observed in all cases. A similar profile of subcomplexes was present in a human/mouse hybrid fibroblast cell line with a severe complex I deficiency due to an almost complete lack of assembly of the holoenzyme complex. The finding that diverse causes of complex I deficiency produce a similar pattern of complex I subcomplexes suggests that these are intermediates in the assembly of the holoenzyme complex. We propose a possible assembly pathway for the complex, which differs significantly from that proposed for Neurospora, the current model for complex I assembly.

Functional alteration of cytochrome c oxidase by SURF1 mutations in Leigh syndrome

Subacute necrotising encephalomyopathy (Leigh syndrome) due to cytochrome c oxidase (COX) deficiency is often caused by mutations in the SURF1 gene, encoding the Surf1 protein essential for COX assembly. We have investigated five patients with different SURF1 mutations resulting in the absence of Surf1 protein. All of them presented with severe and generalised COX defect. Immunoelectrophoretic analysis of cultured fibroblasts revealed 85% decrease of the normal-size COX complexes and significant accumulation of incomplete COX assemblies of 90-120 kDa. Spectrophotometric assay of COX activity showed a 70-90% decrease in lauryl maltoside (LM)-solubilised fibroblasts. In contrast, oxygen consumption analysis in whole cells revealed only a 13-31% decrease of COX activity, which was completely inhibited by detergent in patient cells but not in controls. In patient fibroblasts ADP-stimulated respiration was 50% decreased and cytofluorometry showed a significant decrease of mitochondrial membrane potential DeltaPsi(m) in state 4, as well as a 2.4-fold higher sensitivity of DeltaPsi(m) to uncoupler. We conclude that the absence of the Surf1 protein leads to the formation of incomplete COX complexes, which in situ maintain rather high electron-transport activity, while their H(+)-pumping is impaired. Enzyme inactivation by the detergent in patient cells indicates instability of incomplete COX assemblies.

Mutations in COX15 produce a defect in the mitochondrial heme biosynthetic pathway, causing early-onset fatal hypertrophic cardiomyopathy

Deficiencies in the activity of cytochrome c oxidase (COX), the terminal enzyme in the respiratory chain, are a frequent cause of autosomal recessive mitochondrial disease in infants. These patients are clinically and genetically heterogeneous, and all defects so far identified in this group have been found in genes coding for accessory proteins that play important roles in the assembly of the COX holoenzyme complex. Many patients, however, remain without a molecular diagnosis. We have used a panel of retroviral vectors expressing human COX assembly factors in these patients to identify the molecular basis for the COX deficiency by functional complementation. Here we show that overexpression of COX15, a protein involved in the synthesis of heme A, the heme prosthetic group for COX, can functionally complement the isolated COX deficiency in fibroblasts from a patient with fatal, infantile hypertrophic cardiomyopathy. Mutation analysis of COX15 in the patient identified a missense mutation (C700T) on one allele, changing a conserved arginine to tryptophan (R217W), and a splice-site mutation in intron 3 on the other allele (C447-3G), resulting in a deletion of exon 4. This splicing error introduces a frameshift and a premature stop codon, resulting in an unstable mRNA and, likely, a null allele. Mitochondrial heme A content was reduced in the patient's heart and fibroblast mitochondria, and levels of heme O were increased in the patient's heart. COX activity and the total amount of fully assembled enzyme were reduced by 50%-70% in patient fibroblasts. Expression of COX15 increased heme A content and rescued COX activity. These results suggest that reduced availability of heme A stalls the assembly of COX. This study establishes COX15 as an additional cause, along with SCO2, of fatal infantile, hypertrophic cardiomyopathy associated with isolated COX deficiency.

New perspectives on the assembly process of mitochondrial respiratory chain complex cytochrome c oxidase

The assembly of cytochrome c oxidase (COX) is a complicated process and requires a number of assembly factors to put all the necessary subunits in the correct position. Defects in COX assembly lead in particular to serious neuromuscular disorders. We demonstrated that COX-deficient patients can be associated with different assembly patterns. To obtain more insight in the biogenesis of COX in a living cell, we used yeast as a model organism to design a way to pulse label holo-COX with green fluorescent protein (GFP). Using blue native electrophoresis, we showed that the GFP-tagged subunit is incorporated into fully assembled COX and this GFP tagged complex still has enzymatic activity. This allows us to correlate the GFP fluorescence signal detected in vivo by microscopy with the synthesis, turnover and assembly of COX.

Defects in mitochondrial respiratory complexes III and IV, and human pathologies

Here, relationships between alterations in tissue-specific content, protein structure, activity, and/or assembly of respiratory complexes III and IV induced by mutations in corresponding genes and various human pathologies are reviewed. Cytochrome bc(1) complex and cytochrome c oxidase (COX) deficiencies have been detected in a heterogeneous group of neuromuscular and non-neuromuscular diseases in childhood and adulthood, presenting a number of clinical phenotypes of variable severity. Such disorders can be caused by mutations located either in mitochondrial genes or in nuclear genes encoding structural subunits of the complexes or corresponding assembly factors/chaperones. Of the defects in mitochondrial DNA genes, mutations in cytochrome b subunit of complex III, and in structural subunits I-III of COX have been described to date. As to defects in nuclear DNA genes, mutations in genes encoding the complexes assembly factors such as the BCS1L protein for complex III; and SURF-1, SCO1, SCO2, and COX10 for complex IV have been identified so far.

Shy1p is necessary for full expression of mitochondrial COX1 in the yeast model of Leigh's syndrome

SHY1 codes for a mitochondrial protein required for full expression of cytochrome oxidase (COX) in Saccharomyces cerevisiae. Mutations in the homologous human gene (SURF1) have been reported to cause Leigh's syndrome, a neurological disease associated with COX deficiency. The function of Shy1p/Surf1p is poorly understood. Here we have characterized revertants of shy1 null mutants carrying extragenic nuclear suppressor mutations. The steady-state levels of COX in the revertants is increased by a factor of 4-5, accounting for their ability to respire and grow on non-fermentable carbon sources at nearly wild-type rates. The suppressor mutations are in MSS51, a gene previously implicated in processing and translation of the COX1 transcript for subunit 1 (Cox1) of COX. The function of Shy1p and the mechanism of suppression of shy1 mutants were examined by comparing the rates of synthesis and turnover of the mitochondrial translation products in wild-type, mutant and revertant cells. We propose that Shy1p promotes the formation of an assembly intermediate in which Cox1 is one of the partners.

Respiratory chain complex I deficiency

Oxidative phosphorylation disorders make a contribution of 1 per 10,000 live births in man, of which isolated complex I deficiency is frequently the cause. Complex I, or NADH:ubiquinone oxidoreductase, is the largest multi-protein enzyme complex of the mitochondrial electron transfer chain. In complex I deficiency, various clinical phenotypes have been recognized, often resulting in multi-system disorders with a fatal outcome at a young age. Recent advances in complex I deficiency, regarding clinical, biochemical, and molecular aspects are described. However, the genetic causes of about 60% of complex I deficiency remain unclear. As a consequence, further research will be needed to clarify the genetic defects in the remaining cases. Novel strategies in which interesting non-structural nuclear-encoded disease-causing genes may be found, as well as the molecular genetic composition of human complex I, are presented.

Mitochondrial oxidative phosphorylation system assembly in man: recent achievements

The human oxidative phosphorylation system consists of five multi-subunit complexes of which the individual subunits, with the exception of complex II, are encoded either by mitochondrial or nuclear DNA. Consequently, a deficient enzyme activity of one or more of the complexes can be caused by mitochondrial or nuclear DNA mutations. In the past 5 years numerous mutations have been found in structural nuclear oxidative phosphorylation system genes. However, in a substantial number of patients with oxidative phosphorylation system complex deficiencies, despite extensive investigations, no mutations in the mitochondrial DNA or the structural nuclear genes have been found. Genetic defects in such patients are therefore suspected at the transcriptional, translational, post-translational level or in gene products involved in the assembly of the oxidative phosphorylation system. The latter is a complicated process, as the proteins encoded by the two genomes have to be brought together in a proper stoichiometric way to form five functional complexes. In the past year substantial progress in the knowledge of the human oxidative phosphorylation assembly process has been made. Several human assembly genes have been identified, and mutations in these genes responsible for human oxidative phosphorylation system complex-related diseases have been found. In this review, we summarize our current knowledge about human oxidative phosphorylation system assembly genes in health and disease.

Neuropathological features of mitochondrial disorders

Genetic defects affecting the mitochondrial respiratory chain comprise an important cause of encephalomyopathies. Considering the structural complexity of the respiratory chain, its dual genetic control, and the numerous nuclear genes required for proper assembly of the enzyme complexes, the phenotypic heterogeneity is not surprising. From a neuropathological view point, application of in situ hybridization and immunohistochemistry to study the choroid plexus and brain-blood barrier in "prototypes" of mitochondrial encephalopathies have revealed alterations that we think are important in the pathogenesis of central nervous system dysfunction in these disorders. As the role of the blood-cerebrospinal fluid (CSF) and brain-blood barriers in mitochondrial encephalopathies is better understood, manipulation of their functions offers promises for therapeutic interventions.

Blue native polyacrylamide gel electrophoresis: a powerful tool in diagnosis of oxidative phosphorylation defects

Catalytic activity of oxidative phosphorylation complexes is maintained following separation by Blue Native polyacrylamide gel electrophoresis (BN-PAGE). In BN-PAGE gels, using histochemical staining methods, we have demonstrated enzymatic activity of the complexes I, II, IV, and V in heart and skeletal muscle, liver, and cultured skin fibroblasts. The combination of BN-PAGE and catalytic staining can be successfully applied for detection of complex deficiencies. Tissues from 18 patients with deficiency in the oxidative phosphorylation as detected by spectrophotometric assay were used (10 patients complex IV, three patients complex I, one patient complex II, one patient complex I+III, three patients complex I+IV). The gene defect was located in nuclear DNA in five patients and mitochondrial DNA in one patient. In samples from patients with a severe deficiency, almost complete absence of the corresponding enzyme band is observed after catalytic staining in the gel. In patients with known partial deficiency, a milder decrease of the corresponding enzyme band is demonstrated. The amount of protein in complexes I, V, and III can easily be evaluated in samples from heart and skeletal muscle after separation by BN-PAGE using silver or Coomassie staining. The protein amount in complex IV is difficult to visualize by silver staining but easier by the Coomassie technique. In samples from liver and cultured skin fibroblasts, evaluation of protein amount is more difficult due to high background staining. In these tissues, immunoblotting can be done after BN-PAGE and subsequent transfer to a nitrocellulose membrane.

Antibody-based approaches to diagnosis and characterization of oxidative phosphorylation diseases

Mitochondrial disorders caused by defects in oxidative phosphorylation function are difficult to diagnose. Here we review the emerging use of antibody-based approaches for this diagnosis. Novel methods involving immunohistochemistry and immunocapture of defective enzymes for characterization are described that add to the arsenal of approaches available.

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.

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.

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.

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.

Increased synthesis and decreased stability of mitochondrial translation products in yeast as a result of loss of mitochondrial (NAD(+))-dependent isocitrate dehydrogenase

We have previously demonstrated that the yeast Krebs cycle enzyme NAD(+)-dependent isocitrate dehydrogenase (Idh) binds specifically and with high affinity to the 5'-untranslated leader sequences of mitochondrial mRNAs in vitro and have proposed a role for the enzyme in the regulation of mitochondrial translation [Elzinga, S.D.J. et al. (2000) Curr. Genet., in press]. Although our studies initially failed to reveal any consistent correlation between idh disruption and mitochondrial translational activity, it is now apparent that compensatory extragenic suppressor mutations readily accumulate in idh disruption strains thereby masking mutant behaviour. Now, pulse-chase protein labelling of isolated mitochondria from an Idh disruption mutant lacking suppressor mutations reveals a strong (2-3-fold) increase in the synthesis of mitochondrial translation products. Strikingly, the newly synthesised proteins are more short-lived than in mitochondria from wild-type cells, their degradation occurring with a 2-3-fold reduced half-life. Enhanced degradation of translation products is also a feature of yeast mutants in which tethering/docking of mitochondrial mRNAs is disturbed. We therefore suggest that binding of Idh to mitochondrial mRNAs may suppress inappropriate translation of mitochondrial mRNAs.