Human cytochrome oxidase deficiency

[Pathogenetic basis of optic nerve atrophy in methanol poisoning]

Optic nerve atrophy is a pathomorphological consequence of diseases of the peripheral neuron of the visual pathway, manifested as atrophy of nerve fibers of varying severity. The toxic effect of methanol is mainly associated with formic acid and formaldehyde, which suppress the cytochrome system, inhibit oxidative phosphorylation, and thereby cause a deficiency of adenosine triphosphoric acid, to which brain and retinal tissues are especially susceptible. When formiate accumulates, tissue respiration is disrupted, leading to pronounced tissue hypoxia. As a result of such methanol metabolism, metabolic acidosis occurs. Tissue hypoxia develops in the first few hours as a result of the action of formic acid on the respiratory enzyme chain at the cytochrome oxidase level. Hypoxia and, as a consequence, a decrease in energy supply lead to a disruption of biological oxidation and the development of apoptosis in the optic nerve fibers. Understanding the process of optic nerve atrophy development at the pathogenetic level in methyl alcohol intoxication will help make a correct early diagnosis and prescribe timely treatment.

Evidence that the catalytic mechanism of heme a synthase involves the formation of a carbocation stabilized by a conserved glutamate

In eukaryotes and many aerobic prokaryotes, the final step of aerobic respiration is catalyzed by an aa3-type cytochrome c oxidase, which requires a modified heme cofactor, heme a. The conversion of heme b, the prototypical cellular heme, to heme o and ultimately to heme a requires two modifications, the latter of which is conversion of a methyl group to an aldehyde, catalyzed by heme a synthase (HAS). The N- and C-terminal halves of HAS share homology, and each half contains a heme-binding site. Previous reports indicate that the C-terminal site is occupied by a heme b cofactor. The N-terminal site may function as the substrate (heme o) binding site, although this has not been confirmed experimentally. Here, we assess the role of conserved residues from the N- and C-terminal heme-binding sites in HAS from prokaryotic (Shewanella oneidensis) and eukaryotic (Saccharomyces cerevisiae) species - SoHAS/CtaA and ScHAS/Cox15, respectively. A glutamate within the N-terminal site is found to be critical for activity in both types of HAS, consistent with the hypothesis that a carbocation forms transiently during catalysis. In contrast, the residue occupying the analogous C-terminal position is dispensable for enzyme activity. In SoHAS, the C-terminal heme ligands are critical for stability, while in ScHAS, substitutions in either heme-binding site have little effect on global structure. In both species, in vivo accumulation of heme o requires the presence of an inactive HAS variant, highlighting a potential regulatory role for HAS in heme o biosynthesis.

Mitochondrial Respiratory Chain Supercomplexes: From Structure to Function

Mitochondrial oxidative phospho rylation, the center of cellular metabolism, is pivotal for the energy production in eukaryotes. Mitochondrial oxidative phosphorylation relies on the mitochondrial respiratory chain, which consists of four main enzyme complexes and two mobile electron carriers. Mitochondrial enzyme complexes also assemble into respiratory chain supercomplexes (SCs) through specific interactions. The SCs not only have respiratory functions but also improve the efficiency of electron transfer and reduce the production of reactive oxygen species (ROS). Impaired assembly of SCs is closely related to various diseases, especially neurodegenerative diseases. Therefore, SCs play important roles in improving the efficiency of the mitochondrial respiratory chain, as well as maintaining the homeostasis of cellular metabolism. Here, we review the structure, assembly, and functions of SCs, as well as the relationship between mitochondrial SCs and diseases.

A novel variant in the COX15 gene causing a fatal infantile cardioencephalomyopathy: A case report with clinical and molecular review

The cytochrome c-oxidase (COX) enzyme, also known as mitochondrial complex IV (MT-C4D), is a transmembrane protein complex found in mitochondria. COX deficiency is one of the most frequent causes of electron transport chain defects in humans. Therefore, high energy demand organs and tissues are affected in patients with mutations in the COX15 gene, with variable phenotypic expressiveness. We describe the case of a male newborn with hypertrophic cardiomyopathy and serum and cerebrospinal fluid hyperlacticaemia, whose exome sequencing revealed two variants in a compound heterozygous state: c.232G > A; p.(Gly78Arg), classified as likely pathogenic, and c.452C > G; p.(Ser151Ter), as pathogenic; the former never previously described in the literature.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Mitochondrial oxidative phosphorylation disorders in children: Phenotypic, genotypic and biochemical correlations in 85 patients from South India

Mitochondrial oxidative phosphorylation (OXPHOS) disorders account for a variety of neuromuscular disorders in children. In this study mitochondrial respiratory chain enzymes were assayed in muscle tissue in a large cohort of children with varied neuromuscular presentations from June 2011 to December 2013. The biochemical enzyme deficiencies were correlated with the phenotypes, magnetic resonance imaging, histopathology and genetic findings to reach a final diagnosis. There were 85 children (mean age: 6.9±4.7years, M:F:2:1) with respiratory chain enzyme deficiency which included: isolated complex I (n=50, 60%), multiple complexes (n=24, 27%), complex IV (n=8, 9%) and complex III deficiencies (n=3, 4%). The most common neurological findings were ataxia (59%), hypotonia (59%) and involuntary movements (49%). A known mitochondrial syndrome was diagnosed in 27 (29%) and non-syndromic presentations in 57 (71%). Genetic analysis included complete sequencing of mitochondrial genome, SURF1, POLG1&2. It revealed variations in mitochondrial DNA (n=8), SURF1 (n=5), and POLG1 (n=3). This study, the first of its kind from India, highlights the wide range of clinical and imaging phenotypes and genetic heterogeneity in children with mitochondrial oxidative phosphorylation disorders.

Non-invasive screening of cytochrome c oxidase deficiency in children using a dipstick immunocapture assay

Cytochrome c oxidase (CIV) deficiency is among the most common childhood mitochondrial disorders. The diagnosis of this deficiency is complex, and muscle biopsy is used as the gold standard of diagnosis. Our aim was to minimize the patient burden and to test the use of a dipstick immunocapture assay (DIA) to determine the amount of CIV in non-invasively obtained buccal epithelial cells. Buccal smears were obtained from five children with Leigh syndrome including three children exhibiting a previously confirmed CIV deficiency in muscle and fibroblasts and two children who were clinical suspects for CIV deficiency; the smear samples were analysed using CI and CIV human protein quantity dipstick assay kits. Samples from five children of similar age and five adults were used as controls. Analysis of the controls demonstrated that only samples of buccal cells that were frozen for a maximum of 4 h after collection provide accurate results. All three patients with confirmed CIV deficiency due to mutations in the SURF1 gene exhibited significantly lower amounts of CIV than the similarly aged controls; significantly lower amounts were also observed in two new patients, for whom later molecular analysis also confirmed pathologic mutations in the SURF1 gene. We conclude that DIA is a simple, fast and sensitive method for the determination of CIV in buccal cells and is suitable for the screening of CIV deficiency in non-invasively obtained material from children who are suspected of having mitochondrial disease.

A stabilizing factor for mitochondrial respiratory supercomplex assembly regulates energy metabolism in muscle

The mitochondrial respiratory chain is essential for oxidative phosphorylation and comprises multiple complexes, including cytochrome c oxidase, assembled in macromolecular supercomplexes. Little is known about factors that contribute to supercomplex organization. Here we identify COX7RP as a factor that promotes supercomplex assembly. Cox7rp-knockout mice exhibit decreased muscular activity and heat production failure in the cold due to reduced COX activity. In contrast, COX7RP-transgenic mice exhibit increased exercise performance with increased cytochrome c oxidase activity. Two-dimensional blue native electrophoresis reveals that COX7RP is a key molecule that promotes assembly of the III2/IVn supercomplex with complex I. Our study identified COX7RP as a protein that functions in I/III2/IVn supercomplex assembly and is required for full activity of mitochondrial respiration.

Additive effects of mitochondrion-targeted cytochrome CYP2E1 and alcohol toxicity on cytochrome c oxidase function and stability of respirosome complexes

Alcohol treatment induces oxidative stress by a combination of increased production of partially reduced oxygen species and decreased cellular antioxidant pool, including GSH. Recently, we showed that mitochondrion-targeted CYP2E1 augments alcohol-mediated toxicity, causing an increase in reactive oxygen species production and oxidative stress. Here, we show that cytochrome c oxidase (CcO), the terminal oxidase of the mitochondrial respiratory chain, is a critical target of CYP2E1-mediated alcohol toxicity. COS-7 and Hep G2 cell lines expressing predominantly mitochondrion-targeted (Mt(++)) CYP2E1 and livers from alcohol-treated rats showed loss of CcO activity and increased protein carbonylation, which was accompanied by a decline in the steady state levels of subunits I, IVI1, and Vb of the CcO complex. This was also accompanied by reduced mitochondrial DNA content and reduced mitochondrial mRNA. These changes were more prominent in Mt(++) cells in comparison with wild type (WT) CYP2E1-expressing or ER(+) (mostly microsome-targeted) cells. In addition, mitochondrion-specific antioxidants, ubiquinol conjugated to triphenyl phosphonium, triphenylphosphonium conjugated carboxyl proxyl, and the CYP2E1 inhibitor diallyl sulfide prevented the loss of CcO activity and the CcO subunits, most likely through reduced oxidative damage to the enzyme complex. Our results suggest that damage to CcO and dissociation of respirosome complexes are critical factors in alcohol-induced toxicity, which is augmented by mitochondrion-targeted CYP2E1. We propose that CcO is one of the direct and immediate targets of alcohol-induced toxicity causing respiratory dysfunction.

LRPPRC mutation suppresses cytochrome oxidase activity by altering mitochondrial RNA transcript stability in a mouse model

LRPPRC (leucine-rich pentatricopeptide repeat-containing) has been shown to be essential for the maturation of COX (cytochrome c oxidase), possibly by stabilizing RNA transcripts of COXI, COXII and COXIII genes encoded in mtDNA (mitochondrial DNA). We established a mouse 'gene-trap' model using ES cells (embryonic stem cells) in which the C-terminus of LRPPRC has been replaced with a β-geo construct. Mice homozygous for this modification were found to be subject to embryonic lethality, with death before 12.5 dpc (days post-coitum). Biochemical analysis of MEFs (mouse embryonic fibroblasts) isolated from homozygous mutants showed a major decrease in COX activity, with slight reductions in other respiratory chain complexes with mtDNA encoded components. Constructs of LRPPRC containing different numbers of PPRs (pentatricopeptide repeats) were expressed as recombinant proteins and tested for their ability to bind to the COXI mRNA transcript. Full binding required the first 19 PPR motifs. A specific segment of COXI mRNA was identified as the binding target for LRPPRC, encoded by mouse mtDNA nucleotides 5961-6020. These data strongly suggest that LRPPRC is involved in the maturation of COX, and is involved in stabilizing of mitochondrial mRNAs encoding COX transcripts.

The use of information theory in evolutionary biology

Information is a key concept in evolutionary biology. Information stored in a biological organism's genome is used to generate the organism and to maintain and control it. Information is also that which evolves. When a population adapts to a local environment, information about this environment is fixed in a representative genome. However, when an environment changes, information can be lost. At the same time, information is processed by animal brains to survive in complex environments, and the capacity for information processing also evolves. Here, I review applications of information theory to the evolution of proteins and to the evolution of information processing in simulated agents that adapt to perform a complex task.

Mutations in C12orf62, a factor that couples COX I synthesis with cytochrome c oxidase assembly, cause fatal neonatal lactic acidosis

We investigated a family in which the index subject presented with severe congenital lactic acidosis and dysmorphic features associated with a cytochrome c oxidase (COX)-assembly defect and a specific decrease in the synthesis of COX I, the subunit that nucleates COX assembly. Using a combination of microcell-mediated chromosome transfer, homozygosity mapping, and transcript profiling, we mapped the gene defect to chromosome 12 and identified a homozygous missense mutation (c.88G>A) in C12orf62. C12orf62 was not detectable by immunoblot analysis in subject fibroblasts, and retroviral expression of the wild-type C12orf62 cDNA rescued the biochemical phenotype. Furthermore, siRNA-mediated knockdown of C12orf 62 recapitulated the biochemical defect in control cells and exacerbated it in subject cells. C12orf62 is apparently restricted to the vertebrate lineage. It codes for a very small (6 kDa), uncharacterized, single-transmembrane protein that localizes to mitochondria and elutes in a complex of ∼110 kDa by gel filtration. COX I, II, and IV coimmunoprecipated with an epitope-tagged version of C12orf62, and 2D blue-native-polyacrylamide-gel-electrophoresis analysis of newly synthesized mitochondrial COX subunits in subject fibroblasts showed that COX assembly was impaired and that the nascent enzyme complex was unstable. We conclude that C12orf62 is required for coordination of the early steps of COX assembly with the synthesis of COX I.

Role of Surf1 in heme recruitment for bacterial COX biogenesis

Biogenesis of the mitochondrial cytochrome c oxidase (COX) is a highly complex process involving subunits encoded both in the nuclear and the organellar genome; in addition, a large number of assembly factors participate in this process. The soil bacterium Paracoccus denitrificans is an interesting alternative model for the study of COX biogenesis events because the number of chaperones involved is restricted to an essential set acting in the metal centre formation of oxidase, and the high degree of sequence homology suggests the same basic mechanisms during early COX assembly. Over the last years, studies on the P. denitrificans Surf1 protein shed some light on this important assembly factor as a heme a binding protein associated with Leigh syndrome in humans. Here, we summarise our current knowledge about Surf1 and its role in heme a incorporation events during bacterial COX biogenesis. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Reversible infantile respiratory chain deficiency: a clinical and molecular study

OBJECTIVE

To characterize the clinical features and clarify the pathogenicity of "benign cytochrome c oxidase deficiency myopathy."

METHODS

The study included 8 patients with the phenotype of this disease. Six patients underwent muscle biopsies and all the 8 underwent mitochondrial DNA analyses. To confirm the pathogenicity of the detected mitochondrial DNA mutation, we performed northern blot analysis, using muscle specimens, and blue native polyacrylamide gel electrophoresis and respiratory chain enzyme activity assay of transmitochondrial cell lines (cybrids).

RESULTS

Clinical symptoms were limited to skeletal muscle and improved spontaneously in all cases; however, 2 siblings had basal ganglia lesions. In all patients, we identified a homoplasmic m.14674T>C or m.14674T>G mitochondrial transfer RNA-glutamate mutation. Northern blot analysis revealed decreased levels of mitochondrial transfer RNA-glutamate molecules. Muscle specimens and cybrids derived from patients showed decreased activity of respiratory complexes IV, and/or I, III; however, this was normal in naive myoblasts.

INTERPRETATION

Identification of a novel m.14674T>G mutation in addition to m.14674T>C indicated the importance of this site for disease causation. Analyses of cybrids revealed the pathogenicity of m.14674T>C mutation, which resulted in defects of cytochrome c oxidase and multiple respiratory chain enzymes. Furthermore, patients with basal ganglia lesions provided new insights into this disease, in which only skeletal muscle was thought to be affected. Normal respiratory chain enzyme activities in naive myoblasts suggested the compensatory influence of nuclear factors, which may be a clue to understanding the mechanisms of spontaneous recovery and low penetrance in families carrying the mutation.

Assessment of the perimortem protocol in neonates for the diagnosis of inborn errors of metabolism

AIM

To assess the efficacy of the perimortem protocol in neonates with suspected inborn errors of metabolism (IEM).

METHODS

Retrospective analysis of medical records from January 2000 through December 2007 was performed. Only neonates (</=1 month of life) in whom the perimorterm protocol was applied were included in the study. The samples were collected following the instructions of our exitus kit, which contains the perimortem protocol and the material for the extraction of biological specimens.

RESULTS

Among the 42 neonates studied, in 28 an IEM was suspected during hospitalization and 15 (36%) were diagnosed with IEM. Mitochondrial disorders were the most frequent diagnosis (8 patients), followed by urea cycle disorders (3 patients), organic acidemias (2 patients), one patient with congenital disorder of glycosylation (CDG type Ia), and one patient with molybdenum cofactor deficiency. Sepsis and other life-threatening conditions appeared to have a biochemical profile very similar to IEM.

CONCLUSION

This protocol was especially useful for collecting all biological samples in patients with rapidly fatal evolution with a non-specific diagnostic suspicion, and to collect special tissues in previously diagnosed patients. However, only the combination of clinical and biochemical data could lead to a diagnosis which would be confirmed by enzymatic/genetic studies.

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.

SURF-1 gene mutation associated with leukoencephalopathy in a 2-year-old

Mutations in the nuclear SURF-1 gene lead directly to cytochrome-c oxidase deficiency, the most common respiratory chain defect in Leigh syndrome, a neurodegenerative mitochondrial disease involving the deep gray matter and brain stem. We describe the second documented case in the literature to have a SURF-1 mutation presenting with diffuse leukodystrophy, adding to the growing number of cases of mitochondrial syndromes presenting with white matter disease. We examine magnetic resonance imaging (MRI) findings, which suggest that high-grade cytotoxic edema on diffusion-weighted imaging may be a helpful diagnostic feature in differentiating mitochondrial leukodystrophy from other, more common leukodystrophies. We show how MRI white matter findings may progress to include the brain stem, suggesting that a leukodystrophy due to respiratory chain defects can precede more classic Leigh syndrome deep gray matter radiographic findings.

Cytochrome c oxidase deficiency: patients and animal models

Cytochrome c oxidase (COX) deficiencies are one of the most common defects of the respiratory chain found in mitochondrial diseases. COX is a multimeric inner mitochondrial membrane enzyme formed by subunits encoded by both the nuclear and the mitochondrial genome. COX biosynthesis requires numerous assembly factors that do not form part of the final complex but participate in prosthetic group synthesis and metal delivery in addition to membrane insertion and maturation of COX subunits. Human diseases associated with COX deficiency including encephalomyopathies, Leigh syndrome, hypertrophic cardiomyopathies, and fatal lactic acidosis are caused by mutations in COX subunits or assembly factors. In the last decade, numerous animal models have been created to understand the pathophysiology of COX deficiencies and the function of assembly factors. These animal models, ranging from invertebrates to mammals, in most cases mimic the pathological features of the human diseases.

Role of nuclear-encoded subunit Vb in the assembly and stability of cytochrome c oxidase complex: implications in mitochondrial dysfunction and ROS production

CcO (cytochrome c oxidase) is a multisubunit bigenomic protein complex which catalyses the last step of the mitochondrial electron transport chain. The nuclear-encoded subunits are thought to have roles either in regulation or in the structural stability of the enzyme. Subunit Vb is a peripheral nuclear-encoded subunit of mammalian CcO that is dramatically reduced under hypoxia. Although it has been shown to contain different ligand-binding sites and undergo modifications, its precise function is not known. In the present study we generated a cell line from RAW 264.7 murine macrophages that has a more than 80% reduced level of Vb. Functional analysis of these cells showed a loss of CcO activity, membrane potential and less ability to generate ATP. Resolution of complexes on blue native gel and two-dimensional electrophoretic analysis showed an accumulation of subcomplexes of CcO and also reduced association with supercomplexes of the electron transfer chain. Furthermore, the mitochondria from CcO Vb knock-down cells generated increased ROS (reactive oxygen species), and the cells were unable to grow on galactose-containing medium. Pulse-chase experiments suggest the role of the CcO Vb subunit in the assembly of the complex. We show for the first time the role of a peripheral, non-transmembrane subunit in the formation as well as function of the terminal CcO complex.

Lack of cytochrome c in mouse fibroblasts disrupts assembly/stability of respiratory complexes I and IV

Cytochrome c (cyt c) is a heme-containing protein that participates in electron transport in the respiratory chain and as a signaling molecule in the apoptotic cascade. Here we addressed the effect of removing mammalian cyt c on the integrity of the respiratory complexes in mammalian cells. Mitochondria from cyt c knockout mouse cells lacked fully assembled complexes I and IV and had reduced levels of complex III. A redox-deficient mutant of cyt c was unable to rescue the levels of complexes I and IV. We found that cyt c is associated with both complex IV and respiratory supercomplexes, providing a potential mechanism for the requirement for cyt c in the assembly/stability of complex IV.

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.

Two variants of the assembly factor Surf1 target specific terminal oxidases in Paracoccus denitrificans

Biogenesis of cytochrome c oxidase (COX) relies on a large number of assembly proteins, one of them being Surf1. In humans, the loss of Surf1 function is associated with Leigh syndrome, a fatal neurodegenerative disorder. In the soil bacterium Paracoccus denitrificans, homologous genes specifying Surf1 have been identified and located in two operons of terminal oxidases: surf1q is the last gene of the qox operon (coding for a ba(3)-type ubiquinol oxidase), and surf1c is found at the end of the cta operon (encoding subunits of the aa(3)-type cytochrome c oxidase). We introduced chromosomal single and double deletions for both surf1 genes, leading to significantly reduced oxidase activities in membrane. Our experiments on P. denitrificans surf1 single deletion strains show that both Surf1c and Surf1q are functional and act independently for the aa(3)-type cytochrome c oxidase and the ba(3)-type quinol oxidase, respectively. This is the first direct experimental evidence for the involvement of a Surf1 protein in the assembly of a quinol oxidase. Analyzing the heme content of purified cytochrome c oxidase, we conclude that Surf1, though not indispensable for oxidase assembly, is involved in an early step of cofactor insertion into subunit I.

Defects in cytochrome oxidase assembly in humans: lessons from yeast

The biogenesis of the inner mitochondrial membrane enzyme cytochrome c oxidase (COX) is a complex process that requires the actions of ancillary proteins, collectively called assembly factors. Studies with the yeast Saccharomyces cerevisiae have provided considerable insight into the COX assembly pathway and have proven to be a fruitful model for understanding the molecular bases for inherited COX deficiencies in humans. In this review, we focus on critical steps in the COX assembly pathway. These processes are conserved from yeast to humans and are known to be involved in the etiology of human COX deficiencies. The contributions from our studies in yeast suggest that this organism remains an excellent model system for delineating the molecular mechanisms underlying COX assembly defects in humans. Current progress suggests that a complete picture of COX assembly will be achieved in the near future.

Defects in energy homeostasis in Leigh syndrome French Canadian variant through PGC-1alpha/LRP130 complex

Leigh syndrome French Canadian variant (LSFC) is an autosomal recessive neurodegenerative disorder due to mutation in the LRP130 (leucine-rich protein 130 kDa) gene. Unlike classic Leigh syndrome, the French Canadian variant spares the heart, skeletal muscle, and kidneys, but severely affects the liver. The precise role of LRP130 in cytochrome c oxidase deficiency and hepatic lactic acidosis that accompanies this disorder is unknown. We show here that LRP130 is a component of the PGC-1alpha (peroxisome proliferator-activated receptor coactivator 1-alpha) transcriptional coactivator holocomplex and regulates expression of PEPCK (phosphoenolpyruvate carboxykinase), G6P (glucose-6-phosphatase), and certain mitochondrial genes through PGC-1alpha. Reduction of LRP130 in fasted mice via adenoviral RNA interference (RNAi) vector blocks the induction of PEPCK and G6P, and blunts hepatic glucose output. LRP130 is also necessary for PGC-1alpha-dependent transcription of several mitochondrial genes in vivo. These data link LRP130 and PGC-1alpha to defective hepatic energy homeostasis in LSFC, and reveal a novel regulatory mechanism of glucose homeostasis.

Are primary supratentorial intracerebral hemorrhages surrounded by a biochemical penumbra? A microdialysis study

OBJECTIVE

Opinions vary regarding the indications for surgical evacuation of spontaneous intracerebral hemorrhages (ICH) and whether or not penumbra zones surround them.

METHODS

We performed intracerebral microdialysis (mean duration, 3.5 d) after surgical evacuation of ICH in 22 patients. Probes were placed in the parenchyma within 1 to 2 cm of the evacuated hematoma; a postoperative computed tomographic scanning verified their positions. The catheters were perfused with an artificial cerebrospinal fluid solution at 0.3 microl/min. Biochemical variables (glucose, pyruvate, lactate, glutamate, and glycerol) were analyzed and displayed at the bedside. The levels obtained were compared with previous data from normal human brains and the pericontusional penumbra zones of patients with severe traumatic brain lesions.

RESULTS

During 1 to 12 hours after surgery, interstitial levels of glucose (median level, 1.3 mmol/L; interquartile range, 0.6-2.2 mmol/L) were within normal variations, whereas the levels of lactate (median level, 6.4 mmol/L; interquartile range, 3.9-9.0 mmol/L), glutamate (median level, 14 micromol/L; interquartile range, 5-370 micromol/L), and glycerol (median level, 190 micromol/L; interquartile range, 74-380 micromol/L), as well as the lactate/pyruvate ratio (median ratio, 35; interquartile range, 23-50) were increased. A gradual normalization of the lactate/pyruvate ratio and glycerol level was observed within 48 hours.

CONCLUSION

The area close to an evacuated ICH exhibits a biochemical pattern similar to that of the biochemical penumbra zone surrounding focal traumatic brain contusions. The presence of a penumbra zone around large ICH may be of importance for making surgical decisions.

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.

[Cytochrome c oxydase-deficient Leigh syndrome with homozygous mutation in SURF1 gene]

Leigh syndrome is a heterogeneous disorder, usually due to a defect in oxidative metabolism. Mutations in SURF1 gene have been identified in patients with cytochrome c oxidase deficiency. We report a homozygous splice site deletion [516-2_516-1delAG] in a young girl presenting with cytochrome c oxidase-deficient Leigh syndrome. Identification of molecular defect is indispensable for genetic counselling and prenatal diagnosis.

Mice lacking COX10 in skeletal muscle recapitulate the phenotype of progressive mitochondrial myopathies associated with cytochrome c oxidase deficiency

We have created a mouse model with an isolated cytochrome c oxidase (COX) deficiency by disrupting the COX10 gene in skeletal muscle. Missense mutations in COX10 have been previously associated with mitochondrial disorders. Cox10p is a protoheme:heme-O-farnesyl transferase required for the synthesis of heme a, the prosthetic group of the catalytic center of COX. COX10 conditional knockout mice were generated by crossing a LoxP-tagged COX10 mouse with a transgenic mouse expressing cre recombinase under the myosin light chain 1f promoter. The COX10 knockout mice were healthy until approximately 3 months of age when they started developing a slowly progressive myopathy. Surprisingly, even though COX activity in COX10 KO muscles was <5% of control muscle at 2.5 months, these muscles were still able to contract at 80-100% of control maximal forces and showed only a 10% increase in fatigability, and no signs of oxidative damage or apoptosis were detected. However, the myopathy worsened with time, particularly in female animals. This COX10 KO mouse allowed us to correlate the muscle function with residual COX activity, an estimate that can help predict the progression pattern of human mitochondrial myopathies.

Intracerebral microdialysis in severe brain trauma: the importance of catheter location

OBJECT

Intracerebral microdialysis has attracted increasing interest as a monitoring technique during neurological/neurosurgical intensive care. The purpose of this study was to compare cerebral energy metabolism, an indicator of secondary excitotoxic injury and cell membrane degradation close to focal traumatic lesions ("penumbra zones") and in remote and apparently intact brain regions of the ipsilateral and contralateral hemispheres.

METHODS

The study included 22 consecutive patients with a mean age 44 +/- 17 years and an estimated postresuscitation Glasgow Coma Scale motor score less than 5. Altogether 40 microdialysis catheters with radiopaque tips were inserted. Two catheters could not be localized on postoperative computerized tomography (CT) scans and were excluded from the analysis. The perfusates were analyzed at the patient's bedside for levels of glucose, pyruvate, lactate, glutamate, and glycerol with the aid of a CMA 600 Analyzer. The positions of eight (22%) of the 36 catheters were reclassified after a review of findings on CT scans. Except for pyruvate the values of all biochemical variables and the lactate/pyruvate (L/P) ratio were significantly different in the penumbra zone when compared with mean values found in "normal" tissue ipsilateral to the parenchymal damage and in contralateral normal tissue (p < 0.001). In the penumbra zone a slow normalization of the L/P ratio and levels of glutamate and glycerol were observed. In normal tissue these parameters remained within normal limits.

CONCLUSIONS

Data obtained from intracerebral microdialysis can be correctly interpreted only if the locations of the catheters as they relate to focal brain lesions are visualized. A "biochemical penumbra zone" surrounds focal traumatic brain lesions. It remains to be proven whether therapeutic interventions can protect the penumbra zone from permanent damage.

White matter involvement in mitochondrial diseases

White matter involvement is recently being realized as a common finding in mitochondrial disorders. It is considered an inherent part of the classical mitochondrial syndromes which are usually associated with alterations in the mitochondrial DNA such as: Leigh disease, Kearns-Sayre syndrome, mitochondrial encephalomyopathy lactic acidosis, and stroke like episodes, mitochondrial neuro-gastro-intestinal encephalomyopathy and Leber's hereditary optic neuropathy. White matter involvement is also described in mitochondrial disorders due to mutations in the nuclear DNA which are transmitted in an autosomal pattern. MRI findings suggestive of a mitochondrial disease are: small cyst-like lesions in abnormal white matter, involvement of both cerebral and cerebellar white matter, and a combination of a leukoencephalopathy with bilateral basal ganglia lesions. The clinical manifestations may be disproportionate to the extent of white matter involvement. Other organs may frequently be involved. The onset is often in infancy with a neurodegenerative course. The finding of a leukoencephalopathy in a patient with a complex neurologic picture and multisystem involvement should prompt a thorough mitochondrial evaluation.

A report of three patients with an interstitial deletion of chromosome 15q24

Partial monosomy of the q2 region of chromosome 15 has been infrequently reported. Moreover, interstitial deletions involving 15q22-q24 have been described in only nine patients to date. The phenotype of these reported individuals is subject to the extent of the deletion but typically includes altered muscle tone and significant developmental delays. In addition, eye abnormalities, such as strabismus, microphthalmia, or colobomas, ear abnormalities including cleft earlobe and preauricular tags, and urogenital defects are common features. Congenital heart defects, diaphragmatic hernia, abnormalities of the central nervous system, and skeletal anomalies have been reported but appear to be less frequent clinical manifestations. In this report, we describe three new patients with interstitial deletions involving 15q24, two with cryptic deletions identified by fluorescence in situ hybridization (FISH) with a probe for the PML gene and one with a cytogenetically visible deletion of 15q22.3-q24. The clinical presentation of these individuals is similar to those previously described and includes global developmental delays, hypotonia, and genital abnormalities in the males. The identification of these three cases demonstrates that the above clinical features are associated with a new cytogenetic deletion syndrome. Furthermore, we suggest that FISH analysis with a probe for the PML gene be performed in patients with these physical findings.

On the mechanism and biology of cytochrome oxidase inhibition by nitric oxide

The detailed molecular mechanism for the reversible inhibition of mitochondrial respiration by NO has puzzled investigators: The rate constants for the binding of NO and O2 to the reduced binuclear center CuB/a3 of cytochrome oxidase (COX) are similar, and NO is able to dissociate slowly from this center whereas O2 is kinetically trapped, which altogether seems to favor the complex of COX with O2 over the complex of COX with NO. Paradoxically, the inhibition of COX by NO is observed at high ratios of O2 to NO (in the 40-500 range) and is very fast (seconds or faster). In this work, we used simple mathematical models to investigate this paradox and other important biological questions concerning the inhibition of COX by NO. The results showed that all known features of the inhibition of COX by NO can be accounted for by a direct competition between NO and O2 for the reduced binuclear center CuB/a3 of COX. Besides conciliating apparently contradictory data, this work provided an explanation for the so-called excess capacity of COX by showing that the COX activity found in tissues actually is optimized to avoid an excessive inhibition of mitochondrial respiration by NO, allowing a moderate, but not excessive, overlap between the roles of NO in COX inhibition and in cellular signaling. In pathological situations such as COX-deficiency diseases and chronic inflammation, an excessive inhibition of the mitochondrial respiration is predicted.

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.

The role of the LRPPRC (leucine-rich pentatricopeptide repeat cassette) gene in cytochrome oxidase assembly: mutation causes lowered levels of COX (cytochrome c oxidase) I and COX III mRNA

Leigh syndrome French Canadian (LSFC) is a variant of cytochrome oxidase deficiency found in Québec and caused by mutations in the LRPPRC (leucine-rich pentatricopeptide repeat cassette) gene. Northern blots showed that the LRPPRC mRNA levels seen in skeletal muscle>heart>placenta>kidney>liver>lung=brain were proportionally almost opposite in strength to the severity of the enzymic cytochrome oxidase defect. The levels of COX (cytochrome c oxidase) I and COX III mRNA visible on Northern blots were reduced in LSFC patients due to the common (A354V, Ala354-->Val) founder mutation. The amount of LRPPRC protein found in both fibroblast and liver mitochondria from LSFC patients was consistently reduced to <30% of control levels. Import of [(35)S]methionine LRPPRC into rat liver mitochondria was slower for the mutant (A354V) protein. A titre of LRPPRC protein was also found in nuclear fractions that could not be easily accounted for by mitochondrial contamination. [35S]Methionine labelling of mitochondrial translation products showed that the translation of COX I, and perhaps COX III, was specifically reduced in the presence of the mutation. These results suggest that the gene product of LRPPRC, like PET 309p, has a role in the translation or stability of the mRNA for mitochondrially encoded COX subunits. A more diffuse distribution of LRPPRC in LSFC cells compared with controls was evident when viewed by immunofluorescence microscopy, with less LRPPRC present in peripheral mitochondria.

Self-inflicted orodental injury in a child with Leigh disease

Leigh disease is an inherited progressive mitochondrial neurodegenerative disease that affects the neurological, respiratory and cardiovascular systems and is associated with retardation of the intellectual and physical development. This report describes the case of a 4-year-old boy with Leigh disease who presented with self-inflicted traumatic injury to the teeth, alveolar bone, lips and tongue during repeated episodes of intense orofacial spasms. Conservative management of the injury included repositioning the fractured alveolar bone, splinting the traumatized teeth and planning for a mouthguard. However, after a second incident of severe self-induced injury to the teeth and alveolar bone, extraction of the anterior teeth became inevitable to protect the child from further self-mutilation and to allow healing of the injured tissues.

Neonatal seizures and limb malformations associated with liver-specific complex IV respiratory chain deficiency

An eight-week-old infant, the fourth child of consanguineous parents presented with intractable neonatal seizures. The mother had two previous miscarriages. The infant initially presented on day one with multifocal myoclonus, complex partial and generalised tonic-clonic seizures. On examination, there were dysmorphic hands and feet, with absent nails and terminal phalanges of the fingers and toes, hepatomegaly, marked axial and peripheral hypotonia and severe global developmental delay. Ophthalmological assessment showed 'salt and pepper' pigmentary retinopathy. The urinary organic acid profile revealed a marked increase in tricarboxylic acid metabolites. Urinary phosphate reabsorption was reduced at 84%. Type I fibre atrophy was seen on muscle histology, and a cytochrome c oxidase deficiency was found only on enzymology of liver tissue. Limb malformations associated with respiratory chain defects have rarely been reported. To our knowledge, this child has the most severe limb anomaly associated with a tissue-specific complex IV respiratory chain defect.

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.

Clinical and diagnostic characteristics of complex III deficiency due to mutations in the BCS1L gene

We investigated two siblings of a Spanish family presenting with congenital lactic acidosis. They had severe failure to thrive, liver dysfunction, and renal tubulopathy. An isolated biochemical complex III deficiency was detected in liver. A search for mutations in the human bc1 synthesis like (BCS1L) gene was undertaken. Direct sequencing revealed a missense mutation R45C and a nonsense mutation R56X, both located in exon 1 of BCS1L. The missense mutation in combination with a loss of function of the second allele is responsible for the isolated complex III deficiency in this family.

A mutation in mitochondrial DNA-encoded cytochrome c oxidase II gene in a child with Alpers-Huttenlocher-like disease

OBJECTIVE

Cytochrome c oxidase (COX) deficiency has been demonstrated in some patients with Alpers-Huttenlocher disease, but no genetic background has been identified. Our objective was to determine the molecular defect underlying the mitochondrial respiratory chain deficiency in a child with Alpers-Huttenlocher-like progressive cerebrohepatic disease.

METHODS

The entire coding region of mitochondrial DNA was analyzed by conformation-sensitive gel electrophoresis and sequencing. Biochemical and morphologic investigations were performed on tissue biopsy material, including oximetric and spectrophotometric analyses of oxidative phosphorylation, histochemistry, and electron microscopy.

RESULTS

Postmortem histologic examination revealed a marked loss of neurons in the olivary nuclei and a spongy change in the calcarine cortex, fatty infiltration and micronodular cirrhosis of the liver, and atrophic ovaries. A novel heteroplasmic 7706G>A mutation was found in the COX II gene. The median degree of the mutant heteroplasmy was 90% in 5 tissues examined but was lower in the blood of asymptomatic maternal relatives. The distribution of the mutant heteroplasmy was skewed to the left in single muscle fibers of the proband and her mother. The 7706G>A mutation converts a hydrophobic alanine in a conserved transmembrane segment to hydrophilic threonine.

CONCLUSIONS

The 7706G>A mutation is pathogenic and may lead to impaired dioxygen transfer to the active site of COX. The clinical phenotype of this patient resembled that in Alpers-Huttenlocher disease, suggesting that analysis of mitochondrial DNA is worthwhile in patients with a progressive cerebrohepatic disease.

Identification of a gene causing human cytochrome c oxidase deficiency by integrative genomics

Identifying the genes responsible for human diseases requires combining information about gene position with clues about biological function. The recent availability of whole-genome data sets of RNA and protein expression provides powerful new sources of functional insight. Here we illustrate how such data sets can expedite disease-gene discovery, by using them to identify the gene causing Leigh syndrome, French-Canadian type (LSFC, Online Mendelian Inheritance in Man no. 220111), a human cytochrome c oxidase deficiency that maps to chromosome 2p16-21. Using four public RNA expression data sets, we assigned to all human genes a "score" reflecting their similarity in RNA-expression profiles to known mitochondrial genes. Using a large survey of organellar proteomics, we similarly classified human genes according to the likelihood of their protein product being associated with the mitochondrion. By intersecting this information with the relevant genomic region, we identified a single clear candidate gene, LRPPRC. Resequencing identified two mutations on two independent haplotypes, providing definitive genetic proof that LRPPRC indeed causes LSFC. LRPPRC encodes an mRNA-binding protein likely involved with mtDNA transcript processing, suggesting an additional mechanism of mitochondrial pathophysiology. Similar strategies to integrate diverse genomic information can be applied likewise to other disease pathways and will become increasingly powerful with the growing wealth of diverse, functional genomics data.

The effect of short-term dimethylglycine treatment on oxygen consumption in cytochrome oxidase deficiency: a double-blind randomized crossover clinical trial

OBJECTIVE

To study the effectiveness of dimethylglycine (DMG) on oxygen consumption (VO(2)) in children with Saguenay-Lac-Saint-Jean cytochrome-c oxidase (SLSJ-COX) deficiency (OMIM 220111).

STUDY DESIGN

In a crossover randomized double-blind clinical trial, 5 children with SLSJ-COX deficiency, who were stable and old enough to comply with VO(2) measurement, were treated with placebo or DMG for 3 days, and with the alternate treatment after a 2-week washout period. VO(2) was measured by indirect calorimetry before and after treatment. Dietary caloric intake was calculated for 3 days before each measurement. Mean caloric intakes per day were 1562 and 1342 kcal x m(-2) before and during placebo, 1,336 and 1,380 before and during DMG, respectively.

RESULTS

DMG was well tolerated and, in all cases, resulted in markedly increased blood DMG levels (617 + 203 mmol x L(-1)), versus 0 to 2 mmol x L(-1) without treatment. Mean VO(2) was lower after administration of either DMG (-1 +/- 3 mL x min(-1) x m(-2)) or placebo (-6 +/- 4), but neither difference was statistically significant. There was no detectable effect of DMG treatment on blood levels of lactate, pyruvate, bicarbonate, or pH. VO(2) values of patients (range, 101-135 mL x min(-1) x m(-2)) were lower than published norms (150-160 mL x min(-1) x m(-2)).

CONCLUSION

This study suggests that treatment with DMG does not substantially change VO(2) in children with SLSJ-COX deficiency.

Prerequisites and strategies for prenatal diagnosis of respiratory chain deficiency in chorionic villi

Prenatal diagnosis for respiratory chain deficiencies is a complex procedure that requires a thorough diagnostic work-up of the index patient. This includes confirmation of the clinical and metabolic evaluations through histological and enzymatic examinations of tissue biopsies. Prenatal diagnosis currently relies on biochemical assays of respiratory chain complexes in chorionic villi or amniocytes and is possible by mutation analysis of nuclear genes in a limited but increasing proportion of cases. Based on a recent survey of prenatal diagnosis in families with complex I and complex IV deficiencies, performed at Nijmegen Centre for Mitochondrial Disorders (NCMD), prerequisites and strategies for performing prenatal diagnosis have been developed to increase reliability. Biochemical investigations in chorionic villi can be done reliably if the respiratory chain enzyme deficiency is expressed in both skeletal muscle and skin fibroblasts to rule out tissue specificity. No mitochondrial DNA defects must be suspected or established. The NCMD does not offer prenatal diagnosis until all the prerequisites have been confirmed. We expect prenatal diagnosis at the molecular level to become more feasible in time as the mutational spectrum broadens with advances in medical research.

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.