Clinical, biochemical and molecular analyses of six patients with isolated cytochrome c oxidase deficiency due to mutations in the SCO2 gene

Genetic aetiologies of acute liver failure

Acute liver failure (ALF) is a rare, rapidly evolving, clinical syndrome with devastating consequences where definitive treatment is by emergency liver transplantation. Establishing a diagnosis can be challenging and, historically, the cause of ALF was unidentified in up to half of children. However, recent technological and clinical advances in genomic medicine have led to an increasing proportion being diagnosed with monogenic aetiologies of ALF. The conditions encountered include a diverse group of inherited metabolic disorders each with prognostic and treatment implications. Often these disorders are clinically indistinguishable and may even mimic disorders of immune regulation or red cell disorders. Rapid genomic sequencing for children with ALF is, therefore, a key component in the diagnostic work up today. This review focuses on the monogenic aetiologies of ALF.

Protein Transduction Domain-Mediated Delivery of Recombinant Proteins and In Vitro Transcribed mRNAs for Protein Replacement Therapy of Human Severe Genetic Mitochondrial Disorders: The Case of Sco2 Deficiency

Mitochondrial disorders represent a heterogeneous group of genetic disorders with variations in severity and clinical outcomes, mostly characterized by respiratory chain dysfunction and abnormal mitochondrial function. More specifically, mutations in the human SCO2 gene, encoding the mitochondrial inner membrane Sco2 cytochrome c oxidase (COX) assembly protein, have been implicated in the mitochondrial disorder fatal infantile cardioencephalomyopathy with COX deficiency. Since an effective treatment is still missing, a protein replacement therapy (PRT) was explored using protein transduction domain (PTD) technology. Therefore, the human recombinant full-length mitochondrial protein Sco2, fused to TAT peptide (a common PTD), was produced (fusion Sco2 protein) and successfully transduced into fibroblasts derived from a SCO2/COX-deficient patient. This PRT contributed to effective COX assembly and partial recovery of COX activity. In mice, radiolabeled fusion Sco2 protein was biodistributed in the peripheral tissues of mice and successfully delivered into their mitochondria. Complementary to that, an mRNA-based therapeutic approach has been more recently considered as an innovative treatment option. In particular, a patented, novel PTD-mediated IVT-mRNA delivery platform was developed and applied in recent research efforts. PTD-IVT-mRNA of full-length SCO2 was successfully transduced into the fibroblasts derived from a SCO2/COX-deficient patient, translated in host ribosomes into a nascent chain of human Sco2, imported into mitochondria, and processed to the mature protein. Consequently, the recovery of reduced COX activity was achieved, thus suggesting the potential of this mRNA-based technology for clinical translation as a PRT for metabolic/genetic disorders. In this review, such research efforts will be comprehensibly presented and discussed to elaborate their potential in clinical application and therapeutic usefulness.

The mitochondrion: a central architect of copper homeostasis

All known eukaryotes require copper for their development and survival. The essentiality of copper reflects its widespread use as a co-factor in conserved enzymes that catalyze biochemical reactions critical to energy production, free radical detoxification, collagen deposition, neurotransmitter biosynthesis and iron homeostasis. However, the prioritized use of copper poses an organism with a considerable challenge because, in its unbound form, copper can potentiate free radical production and displace iron-sulphur clusters to disrupt protein function. Protective mechanisms therefore evolved to mitigate this challenge and tightly regulate the acquisition, trafficking and storage of copper such that the metal ion is rarely found in its free form in the cell. Findings by a number of groups over the last ten years emphasize that this regulatory framework forms the foundation of a system that is capable of monitoring copper status and reprioritizing copper usage at both the cellular and systemic levels of organization. While the identification of relevant molecular mechanisms and signaling pathways has proven to be difficult and remains a barrier to our full understanding of the regulation of copper homeostasis, mounting evidence points to the mitochondrion as a pivotal hub in this regard in both healthy and diseased states. Here, we review our current understanding of copper handling pathways contained within the organelle and consider plausible mechanisms that may serve to functionally couple their activity to that of other cellular copper handling machinery to maintain copper homeostasis.

Noninvasive diagnostics of mitochondrial disorders in isolated lymphocytes with high resolution respirometry

Background

Mitochondrial diseases belong to the most severe inherited metabolic disorders affecting pediatric population. Despite detailed knowledge of mtDNA mutations and progress in identification of affected nuclear genes, diagnostics of a substantial part of mitochondrial diseases relies on clinical symptoms and biochemical data from muscle biopsies and cultured fibroblasts.

Methods

To investigate manifestation of oxidative phosphorylation defects in isolated lymphocytes, digitonin-permeabilized cells from 48 children were analyzed by high resolution respirometry, cytofluorometric detection of mitochondrial membrane potential and immunodetection of respiratory chain proteins with SDS and Blue Native electrophoreses.

Results

Evaluation of individual respiratory complex activities, ATP synthesis, kinetic parameters of mitochondrial respiratory chain and the content and subunit composition of respiratory chain complexes enabled detection of inborn defects of respiratory complexes I, IV and V within 2 days. Low respiration with NADH-dependent substrates and increased respiration with glycerol-3-phosphate revealed complex I defects; changes in p₅₀ for oxygen and elevated uncoupling control ratio pointed to complex IV deficiency due to SURF1 or SCO2 mutation; high oligomycin sensitivity of state 3-ADP respiration, upregulated mitochondrial membrane potential and low content of complex V were found in lymphocytes with ATP synthase deficiency due to TMEM70 mutations.

Conclusion

Based on our results, we propose the best biochemical parameters predictive for defects of respiratory complexes I, IV and V manifesting in peripheral blood lymphocytes.

General significance

The noninvasiveness, reliability and speed of an approach utilizing novel biochemical criteria demonstrate the high potential of isolated lymphocytes for diagnostics of oxidative phosphorylation disorders in pediatric patients.

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Analysis of inborn mitochondrial disorders in peripheral blood lymphocytes

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Detection of specific defects of respiratory chain complexes I, IV and V

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Manifestation of cytochrome c oxidase deficiency due to SCO2 mutations

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Rapid and noninvasive diagnostics/screening appropriate for pediatric patients

Neonatal onset of mitochondrial disorders in 129 patients: clinical and laboratory characteristics and a new approach to diagnosis

INTRODUCTION

Mitochondrial disorders (MD) may manifest in neonates, but early diagnosis is difficult. In this study, clinical and laboratory data were analyzed in 129 patients with neonatal onset of MD to identify any association between specific mitochondrial diseases and their symptoms with the aim of optimizing diagnosis.

MATERIALS AND METHODS

Retrospective clinical and laboratory data were evaluated in 461 patients (331 families) with confirmed MD.

RESULTS

The neonatal onset of MD was reported in 28% of the patients. Prematurity, intrauterine growth retardation and hypotonia necessitating ventilatory support were present in one-third, cardiomyopathy in 40%, neonatal seizures in 16%, Leigh syndrome in 15%, and elevated lactate level in 87%. Hyperammonemia was observed in 22 out of 52 neonates. Complex I deficiency was identified in 15, complex III in one, complex IV in 23, complex V in 31, combined deficiency of several complexes in 53, and PDH complex deficiency was identified in six patients. Molecular diagnosis was confirmed in 49 cases, including a newborn with a 9134A>G mutation in the MTATP6 gene, which has not been described previously.

CONCLUSION

The most significant finding is the high incidence of neonatal cardiomyopathy and hyperammonemia. Based on our experience, we propose a diagnostic flowchart applicable to critically ill neonates suspicious for MD. This tool will allow for the use of direct molecular genetic analyses without the need for muscle biopsies in neonates with Alpers, Barth, MILS and Pearson syndromes, SCO1, SCO2, TMEM70, ATP5E, SUCLG1 gene mutations and PDH complex deficiency.

Biochemical diagnosis of mitochondrial disorders

Establishing a diagnosis in patients with a suspected mitochondrial disorder is often a challenge. Both knowledge of the clinical spectrum of mitochondrial disorders and the number of identified disease-causing molecular genetic defects are continuously expanding. The diagnostic examination of patients requires a multi-disciplinary clinical and laboratory evaluation in which the biochemical examination of the mitochondrial functional state often plays a central role. In most cases, a muscle biopsy provides the best opportunity to examine mitochondrial function. In addition to activity measurements of individual oxidative phosphorylation enzymes, analysis of mitochondrial respiration, substrate oxidation, and ATP production rates is performed to obtain a detailed picture of the mitochondrial energy-generating system. On the basis of the compilation of clinical, biochemical, and other laboratory test results, candidate genes are selected for molecular genetic testing. In patients in whom an unknown genetic variant is identified, a compatible biochemical phenotype is often required to firmly establish the diagnosis. In addition to the current role of the biochemical analysis in the diagnostic examination of patients with a suspected mitochondria disorder, this report gives a future perspective on the biochemical diagnosis in view of both the expanding genotypes of mitochondrial disorders and the possibilities for high throughput molecular genetic diagnosis.

Copper and homocysteine in cardiovascular diseases

High blood copper (Cu) and homocysteine (Hcy) concentrations have been independently reported as risk factors for cardiovascular diseases. When they are simultaneously measured, a concomitant increase in both parameters in association with vascular dysfunction has been observed. Cu chelator penicillamine can significantly diminish the inhibitory effect of Hcy on endothelial function, which has led to the interpretation that Cu mediates the deleterious effect of Hcy. However, Cu itself has been shown to be beneficial to the cardiovascular system. In particular, Cu promotion of angiogenesis has been well documented. Cu stimulates endothelial cell proliferation and differentiation and promotes microtubule formation in cultured saphenous veins. High levels of Hcy do not affect the process of microtubule formation, but the combination of Cu and Hcy leads to a significant inhibitory effect. Under other conditions, Cu does not affect, but Hcy inhibits, the endothelium-dependent relaxation of blood vessels and the combination of both augments the inhibition. Why does Cu produce adverse effects when it co-exists with Hcy? Cu forms complexes with Hcy and the Cu-Hcy complexes possess a deleterious potential due to their redox properties. Cu chelation can remove Cu from the Cu-Hcy complexes, but leaves behind high levels of Hcy and produces Cu deficiency. An alternative approach should focus on the reduction of Hcy, but maintenance of Cu, making detrimental Cu beneficial. A comprehensive understanding of Cu speciation and a development of selective modulation of Cu coordination to Cu-binding molecules to avoid Cu-Hcy complex formation would effectively improve the condition of cardiovascular disease.

A homozygous mutation in the SCO2 gene causes a spinal muscular atrophy like presentation with stridor and respiratory insufficiency

Infants with deficiency of cytochrome c oxidase (COX) due to SCO2 mutations observed so far usually demonstrated early cardiomyopathy, encephalopathy and lactic acidosis. Milder spinal muscular atrophy-like (SMA-like) phenotype was also rarely reported. The aim is to present 18 Polish patients with SCO2 mutations. Molecular study revealed p.E140K mutation in all cases (on 32 alleles); p.Q53X mutation and novel p.M177T change were identified in single patients. In three families no second mutation was found. Thirteen p.E140K homozygotes presented in infancy with floppiness and remarkable stridor. Survival motor neuron (SMN) gene deletion was excluded. Mild to moderate lactic academia was found. Neurological involvement manifested as spasticity and psychomotor retardation. In some patients strabismus, ptosis and episodes of seizures were seen. During second half of the year chronic respiratory failure with artificial respiration dependency appeared in all homozygotes. Heart involvement was never present at the beginning. Rapidly progressive hypertrophic cardiomyopathy developed in several patients at the terminal stage. The stridor was constant and striking feature. Skeletal muscle biopsy was performed in 16 patients including 11 homozygotes. Four pathological patterns were discerned - from neurogenic muscle changes, including spinal muscular atrophy (SMA) to unspecific findings. Histochemical cytochrome c oxidase (COX) deficit was not a constant feature. Significant decrease in respiratory chain complex IV activity was detected in muscle homogenate by spectrophotometric method only in 7 out of 12 examined cases.

CONCLUSIONS

1/Mutations of SCO2 gene should be considered as a possible cause of neurogenic skeletal muscle features (including SMA-like) in infants with encephalomyopathy even in the absence of heart involvement and COX deficit; 2/Inspiratory stridor may be symptomatic of SCO2 gene mutation(s).

Assembly factors and ATP-dependent proteases in cytochrome c oxidase biogenesis

Eukaryotic cytochrome c oxidase (CcO), the terminal enzyme of the energy-transducing mitochondrial electron transport chain is a hetero-oligomeric, heme-copper oxidase complex composed of both mitochondrially and nuclear-encoded subunits. It is embedded in the inner mitochondrial membrane where it couples the transfer of electrons from reduced cytochrome c to molecular oxygen with vectorial proton translocation across the membrane. The biogenesis of CcO is a complicated sequential process that requires numerous specific accessory proteins, so-called assembly factors, which include translational activators, translocases, molecular chaperones, copper metallochaperones and heme a biosynthetic enzymes. Besides these CcO-specific protein factors, the correct biogenesis of CcO requires an even greater number of proteins with much broader substrate specificities. Indeed, growing evidence indicates that mitochondrial ATP-dependent proteases might play an important role in CcO biogenesis. Out of the four identified energy-dependent mitochondrial proteases, three were shown to be directly involved in proteolysis of CcO subunits. In addition to their well-established protein-quality control function these oligomeric proteolytic complexes with chaperone-like activities may function as molecular chaperones promoting productive folding and assembly of subunit proteins. In this review, we summarize the current knowledge of the functional involvement of eukaryotic CcO-specific assembly factors and highlight the possible significance for CcO biogenesis of mitochondrial ATP-dependent proteases.

Analysis of mouse models of cytochrome c oxidase deficiency owing to mutations in Sco2

Mutations in SCO2, a protein required for the proper assembly and functioning of cytochrome c oxidase (COX; complex IV of the mitochondrial respiratory chain), cause a fatal infantile cardioencephalomyopathy with COX deficiency. We have generated mice harboring a Sco2 knock-out (KO) allele and a Sco2 knock-in (KI) allele expressing an E-->K mutation at position 129 (E129K), corresponding to the E140K mutation found in almost all human SCO2-mutated patients. Whereas homozygous KO mice were embryonic lethals, homozygous KI and compound heterozygous KI/KO mice were viable, but had muscle weakness; biochemically, they had respiratory chain deficiencies as well as complex IV assembly defects in multiple tissues. There was a concomitant reduction in mitochondrial copper content, but the total amount of copper in examined tissues was not reduced. These mouse models should be of use in further studies of Sco2 function, as well as in testing therapeutic approaches to treat the human disorder.

A novel mutation in the SCO2 gene in a neonate with early-onset cardioencephalomyopathy

Mutations in the SCO2 gene [SCO cytochrome oxidase deficient homolog 2 (yeast)] causing cytochrome c oxidase deficiency have been reported in at least in 26 patients with fatal infantile cardioencephalomyopathy. Mutation 1541G > A affecting protein stability is associated with the majority of cases, and the other 11 described mutations have more serious deleterious structural consequences for the protein product. Reported here is a novel case caused by compound heterozygosity of SCO2. The child presented at the age of 3 weeks with failure-to-thrive, muscular hypotonia, hypertrophic cardiomyopathy, and lactic acidemia. Leigh syndrome was diagnosed based on magnetic resonance imaging findings. Immunohistochemical and enzymatic investigations on muscle indicated totally absent cytochrome c oxidase activity. Both parents had mild mental retardation. Sequence analysis in the patient and in his parents revealed heterozygous mutation c.418G > A in exon 2 inherited from the father and maternally inherited heterozygous insertion of 19bp at position 17 in the coding region of the SCO2 gene. Respiratory chain enzyme activity measurements indicated normal activity in both parents, although the mother's cytochrome c oxidase activity was lower. This gene may be involved in the etiology of the mother's mental retardation.

Loss of function of Sco1 and its interaction with cytochrome c oxidase

Sco1 and Sco2 are mitochondrial copper-binding proteins involved in the biogenesis of the Cu(A) site in the cytochrome c oxidase (CcO) subunit Cox2 and in the maintenance of cellular copper homeostasis. Human Surf1 is a CcO assembly factor with an important but poorly characterized role in CcO biogenesis. Here, we analyzed the impact on CcO assembly and tissue copper levels of a G132S mutation in the juxtamembrane region of SCO1 metallochaperone associated with early onset hypertrophic cardiomyopathy, encephalopathy, hypotonia, and hepatopathy, assessed the total copper content of various SURF1 and SCO2-deficient tissues, and investigated the possible physical association between CcO and Sco1. The steady-state level of mutant Sco1 was severely decreased in the muscle mitochondria of the SCO1 patient, indicating compromised stability and thus loss of function of the protein. Unlike the wild-type variant, residual mutant Sco1 appeared to migrate exclusively in the monomeric form on blue native gels. Both the activity and content of CcO were reduced in the patient's muscle to approximately 10-20% of control values. SCO1-deficient mitochondria showed accumulation of two Cox2 subcomplexes, suggesting that Sco1 is very likely responsible for a different posttranslational aspect of Cox2 maturation than Sco2. Intriguingly, the various SURF1-deficient samples analyzed showed a tissue-specific copper deficiency similar to that of SCO-deficient samples, suggesting a role for Surf1 in copper homeostasis regulation. Finally, both blue native immunoblot analysis and coimmunoprecipitation revealed that a fraction of Sco1 physically associates with the CcO complex in human muscle mitochondria, suggesting a possible direct relationship between CcO and the regulation of cellular copper homeostasis.

Phenotypic consequences of a novel SCO2 gene mutation

SCO2 is a cytochrome c oxidase (COX) assembly gene. Mutations in the SCO2 gene have been associated with fatal infantile cardioencephalomyopathy. We report on the phenotype of a novel SCO2 mutation in two siblings with fatal infantile cardioencephalomyopathy. The index patient died of heart failure at 25 days of age. Muscle biopsy was performed for histology and biochemical study of the oxidative phosphorylation system complexes. The entire coding region of the SCO2 gene was sequenced. Autopsy was performed on the index patient and on a female sibling delivered at 23 weeks of gestation following termination of pregnancy during which amniocentesis and genetic testing had been performed. Muscle biopsy and biochemical analysis of heart and skeletal muscle detected a severe isolated COX-IV deficiency. Pathologic findings in both patients confirmed hypertrophic cardiomyopathy. Sequencing of the SCO2 gene showed compound heterozygous mutation; the common E140K mutation and a novel W36X nonsense mutation. Newborns with a combination of hypotonia and cardiomyopathy should be evaluated for multiple congenital anomaly syndromes, inborn errors of metabolism and mitochondrial derangements, and may require extensive diagnostic testing. Mutations in the SCO2 gene are a cause of prenatal-onset hypertrophic cardiomyopathy.

Isolated cytochrome c oxidase deficiency in G93A SOD1 mice overexpressing CCS protein

G93A SOD1 transgenic mice overexpressing CCS protein develop an accelerated disease course that is associated with enhanced mitochondrial pathology and increased mitochondrial localization of mutant SOD1. Because these results suggest an effect of mutant SOD1 on mitochondrial function, we assessed the enzymatic activities of mitochondrial respiratory chain complexes in the spinal cords of CCS/G93A SOD1 and control mice. CCS/G93A SOD1 mouse spinal cord demonstrates a 55% loss of complex IV (cytochrome c oxidase) activity compared with spinal cord from age-matched non-transgenic or G93A SOD1 mice. In contrast, CCS/G93A SOD1 spinal cord shows no reduction in the activities of complex I, II, or III. Blue native gel analysis further demonstrates a marked reduction in the levels of complex IV but not of complex I, II, III, or V in spinal cords of CCS/G93A SOD1 mice compared with non-transgenic, G93A SOD1, or CCS/WT SOD1 controls. With SDS-PAGE analysis, spinal cords from CCS/G93A SOD1 mice showed significant decreases in the levels of two structural subunits of cytochrome c oxidase, COX1 and COX5b, relative to controls. In contrast, CCS/G93A SOD1 mouse spinal cord showed no reduction in levels of selected subunits from complexes I, II, III, or V. Heme A analyses of spinal cord further support the existence of cytochrome c oxidase deficiency in CCS/G93A SOD1 mice. Collectively, these results establish that CCS/G93A SOD1 mice manifest an isolated complex IV deficiency which may underlie a substantial part of mutant SOD1-induced mitochondrial cytopathy.

Structural analysis of tissues affected by cytochrome C oxidase deficiency due to mutations in the SCO2 gene

Structural and histochemical studies carried out in a series of seven cases (from five families) with isolated cytochrome c oxidase (COX) deficiency caused by mutations in the SCO2 gene (1, 2) disclosed changes concentrated in the nervous system, skeletal muscle and myocardium. In five patients homozygous for the E140K mutation, the phenotype was predominantly neuromuscular and the average life span ranged between 9 and 15 months. In two cases, the course was more rapid (death at 7 and 11 weeks of life) and featured marked cardiac hypertrophy (3- and 4-fold increase in heart weight). This predominantly cardiomyopathic phenotype was associated with compound heterozygosity (E140K with another nonsense mutation) in the SCO2 gene. Polioencephalopathy with neurodegeneration and neuronal drop out was present in all cases with evidence that retinal neurons might be seriously affected too. Involvement of spinal motoneurons together with cytochrome c oxidase deficiency in muscle represents a "double hit" for the skeletal muscle. The mitochondrial population was not found to be significantly increased or structurally altered, with the exception of two compound heterozygotes in which the cardiac mitochondria were increased in number and size. Our report extends knowledge of the pathology of COX deficiency caused by mutations in the SCO2 gene.

Heavy Metal Ions in Normal Physiology, Toxic Stress, and Cytoprotection

As a group, heavy metals include both those essential for normal biological functioning (e.g., Cu and Zn), and nonessential metals (e.g., Cd, Hg, and Pb). Both essential and nonessential metals can be present at concentrations that disturb normal biological functions, and which evoke cellular stress responses. The cellular targets for metal toxicity include tissues of the kidney, liver, heart, and the immune response and nervous systems. Intriguingly, manipulations of specific metals, their reservoirs, and the cellular stress response can have therapeutic effects on certain diseases. In this minireview, we will consider both the biological responses to stressful levels of heavy metal cations, and experimental and clinical manipulations of these cations as a means to improve human health parameters.

The human cytochrome c oxidase assembly factors SCO1 and SCO2 have regulatory roles in the maintenance of cellular copper homeostasis

Human SCO1 and SCO2 are metallochaperones that are essential for the assembly of the catalytic core of cytochrome c oxidase (COX). Here we show that they have additional, unexpected roles in cellular copper homeostasis. Mutations in either SCO result in a cellular copper deficiency that is both tissue and allele specific. This phenotype can be dissociated from the defects in COX assembly and is suppressed by overexpression of SCO2, but not SCO1. Overexpression of a SCO1 mutant in control cells in which wild-type SCO1 levels were reduced by shRNA recapitulates the copper-deficiency phenotype in SCO1 patient cells. The copper-deficiency phenotype reflects not a change in high-affinity copper uptake but rather a proportional increase in copper efflux. These results suggest a mitochondrial pathway for the regulation of cellular copper content that involves signaling through SCO1 and SCO2, perhaps by their thiol redox or metal-binding state.

Differential utilization of two ATP-generating pathways is regulated by p53

A fundamental property of cancer cells is the preferential utilization of glycolysis over aerobic respiration to produce ATP. Renewed interest in understanding the mechanism underlying this metabolic shift in energy production is broadening our understanding of the relationship between cancer and cellular metabolism. In a recent article, Matoba et al. report that the p53 tumor suppressor regulates the expression of SCO2, a protein that is required for the assembly of cytochrome c oxidase (COX), a multimeric protein complex required for oxidative phosphorylation. The implication of these findings is that aerobic respiration is compromised in cells that lack functional p53.

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.

Retrospective, multicentric study of 180 children with cytochrome C oxidase deficiency

A retrospective, multicenter study of 180 children with cytochrome c oxidase (COX) deficiency analyzed the clinical features, prognosis, and molecular bases of the COX deficiency. Clinical symptoms including failure to thrive, encephalopathy, hypotony, Leigh syndrome, cardiac involvement, and hepatopathy appeared in most patients early after birth or in early childhood. Two thirds of all children died. Biochemical examination revealed an isolated COX deficiency in 101 children and COX deficiency combined with disturbances of other respiratory chain complexes in 79 children. Blood and cerebrospinal fluid lactate increased in 85% and 81% of examined cases, respectively. Pathogenic mutations in mitochondrial or nuclear DNA were established in 75 patients. Mutations in surfeit locus protein 1 gene (SURF1) were found in 47 children with Leigh syndrome; 2bp deletion 845-846delCT was found in 89% of independent alleles. Mutations in a mitochondrial copper-binding protein (SCO2) gene were found in nine children with encephalomyopathy and/or cardiomyopathy; all of them were homozygotes or heterozygotes for 1541G>A mutation. Different mitochondrial DNA (mtDNA) deletion or depletion were found in nine children, mtDNA mutation 3243A>G in six, mtDNA mutation 8363G>A in two children with Leigh syndrome and mtDNA mutations 8344A>G, and 9205-9206delTA in one child each. COX deficiency represents a heterogeneous group of diseases with unfavorable prognosis. Marked prevalence of two nuclear DNA mutations (845-846delCT in the SURF1 gene and 1541G>A in the SCO2 gene) associated with COX deficiency in a Slavonic population suggests the existence of regional differences in the genetic basis of COX deficiency.