[show abstract][hide abstract] ABSTRACT: Mitochondrial oxidative phosphorylation (OXPHOS) is responsible for generating the majority of cellular ATP. Complex III (ubiquinol-cytochrome c oxidoreductase) is the third of five OXPHOS complexes. Complex III assembly relies on the coordinated expression of the mitochondrial and nuclear genomes, with 10 subunits encoded by nuclear DNA and one by mitochondrial DNA (mtDNA). Complex III deficiency is a debilitating and often fatal disorder that can arise from mutations in complex III subunit genes or one of three known complex III assembly factors. The molecular cause for complex III deficiency in about half of cases, however, is unknown and there are likely many complex III assembly factors yet to be identified. Here, we used Massively Parallel Sequencing to identify a homozygous splicing mutation in the gene encoding Ubiquinol-Cytochrome c Reductase Complex Assembly Factor 2 (UQCC2) in a consanguineous Lebanese patient displaying complex III deficiency, severe intrauterine growth retardation, neonatal lactic acidosis and renal tubular dysfunction. We prove causality of the mutation via lentiviral correction studies in patient fibroblasts. Sequence-profile based orthology prediction shows UQCC2 is an ortholog of the Saccharomyces cerevisiae complex III assembly factor, Cbp6p, although its sequence has diverged substantially. Co-purification studies show that UQCC2 interacts with UQCC1, the predicted ortholog of the Cbp6p binding partner, Cbp3p. Fibroblasts from the patient with UQCC2 mutations have deficiency of UQCC1, while UQCC1-depleted cells have reduced levels of UQCC2 and complex III. We show that UQCC1 binds the newly synthesized mtDNA-encoded cytochrome b subunit of complex III and that UQCC2 patient fibroblasts have specific defects in the synthesis or stability of cytochrome b. This work reveals a new cause for complex III deficiency that can assist future patient diagnosis, and provides insight into human complex III assembly by establishing that UQCC1 and UQCC2 are complex III assembly factors participating in cytochrome b biogenesis.
[show abstract][hide abstract] ABSTRACT: Mutations of mitochondrial DNA are linked to many human diseases. Despite the identification of a large number of variants in the mitochondrially-encoded rRNA (mt-rRNA) genes, the evidence supporting their pathogenicity is, at best, circumstantial. Establishing the pathogenicity of these variations is of major diagnostic importance. Here, we aim to estimate the disruptive effect of mt-rRNA variations on the function of the mitochondrial ribosome. In the absence of direct biochemical methods to study the effect of mt-rRNA variations, we relied on the universal conservation of the rRNA fold to infer their disruptive potential. Our method, named Heterologous Inferential Analysis or HIA, combines conservational information with functional and structural data obtained from heterologous ribosomal sources. Thus, HIA's predictive power is superior to the traditional reliance on simple conservation indexes. By using HIA we have been able to evaluate the disruptive potential for a subset of uncharacterized 12S mt-rRNA variations. Our analysis revealed the existence of variations in the rRNA component of the human mitoribosome with different degrees of disruptive power. In cases where sufficient information regarding the genetic and pathological manifestation of the mitochondrial phenotype is available, HIA data can be used to predict the pathogenicity of mt-rRNA mutations. In other cases, HIA analysis will allow the prioritisation of variants for additional investigation. Eventually, HIA-inspired analysis of potentially pathogenic mt-rRNA variations, in the context of a scoring system specifically designed for these variants, could lead to a powerful diagnostic tool.
Human Molecular Genetics 10/2013; · 7.69 Impact Factor
[show abstract][hide abstract] ABSTRACT: The advent of massive parallel sequencing is rapidly changing the strategies employed for the genetic diagnosis and research of rare diseases that involve a large number of genes. So far, it is not clear whether these approaches perform significantly better than conventional single gene testing as requested by clinicians. The current yield of this traditional diagnostic approach depends on a complex of factors which include gene-specific phenotype traits, and the relative frequency of involvement of specific genes. In order to gauge the impact of the paradigm shift that is occurring in molecular diagnostics, we assessed traditional Sanger based sequencing (in 2011), and exome sequencing followed by targeted bioinformatics analysis (in 2012) for 5 different conditions that are highly heterogeneous, and for which our center provides molecular diagnosis. We find that exome sequencing has a much higher diagnostic yield than Sanger sequencing for deafness, blindness, mitochondrial disease, and movement disorders. For microsatellite-stable colorectal cancer this was low under both strategies. Even if all genes that could have been ordered by physicians had been tested, the larger number of genes captured by the exome would still have led to a clearly superior diagnostic yield at a fraction of the cost. This article is protected by copyright. All rights reserved.
[show abstract][hide abstract] ABSTRACT: Leigh syndrome is a devastating neurodegenerative disease, typically manifesting in infancy or early childhood. However, also late-onset cases have been reported. Since its first description by Denis Archibald Leigh in 1951, it has evolved from a postmortem diagnosis, strictly defined by histopathological observations, to a clinical entity with indicative laboratory and radiological findings. Hallmarks of the disease are symmetrical lesions in the basal ganglia or brain stem on MRI, and a clinical course with rapid deterioration of cognitive and motor functions. Examinations of fresh muscle tissue or cultured fibroblasts are important tools to establish a biochemical and genetic diagnosis. Numerous causative mutations in mitochondrial and nuclear genes, encoding components of the oxidative phosphorylation system have been described in the past years. Moreover, dysfunctions in pyruvate dehydrogenase complex or coenzyme Q10 metabolism may be associated with Leigh syndrome. To date, there is no cure for affected patients, and treatment options are mostly unsatisfactory. Here, we review the most important clinical aspects of Leigh syndrome, and discuss diagnostic steps as well as treatment options.
Journal of neurology, neurosurgery, and psychiatry 06/2013; · 4.87 Impact Factor
[show abstract][hide abstract] ABSTRACT: Background: Congenital disorders of glycosylation (CDG) form a group of inherited metabolic diseases. Although the clinical presentation shows extreme variability, the nervous system is frequently affected. Several parents of our patients diagnosed with CDG reported behavioral problems, including mood swings, depressive behavior, and anxiety. This raised the question whether patients with CDG have an increased risk for socio-emotional problems. Methods: We evaluated 18 children with confirmed CDG. The Child Behavior Checklist (CBCL) was used to screen for socio-emotional problems. To determine the disease progression and severity in CDG, the Nijmegen Paediatric CDG Rating Scale (NPCRS) was used. Results were compared to "norm scores" and to children with mitochondrial disorders and children with other chronic metabolic disorders with multisystem involvement. Results: Results showed a high prevalence of socio-emotional problems in children with CDG. Mean total scores, scores on withdrawn/depressed behavior, social problems, and somatic complaints were significantly increased. More than two thirds of our CDG patients have abnormal scores on CBCL. The mean score on social problems was significantly higher compared to our two control groups of patients with other chronic metabolic disorders. Conclusions: Patients with CDG have an increased risk of developing socio-emotional problems. A standard screening for psychological problems is recommended for the early detection of psychological problems in CDG patients.
[show abstract][hide abstract] ABSTRACT: PURPOSE: Pheochromocytomas and paragangliomas (PGLs) are neuroendocrine tumors of sympathetic and parasympathetic paraganglia. The present study investigated the relationships between genotype-specific differences in mitochondrial function and catecholamine content in PGL tumors. EXPERIMENTAL DESIGN: Respiratory chain enzyme assays and 1H-NMR spectroscopy at 500 MHz, were performed on homogenates of 35 sporadic PGLs and 59 PGLs from patients with hereditary mutations in SDHB, SDHD, SDHAF-2, VHL, RET, NF1 and MAX. RESULTS: In SDHx related PGLs, a significant decrease in complex II activity (p<0.0001) and a significant increase in complex I, III and IV enzyme activities were observed when compared to sporadic, RET and NF1 tumors. Also, a significant increase in citrate synthase (p<0.0001) enzyme activity was observed in SDHx related PGLs when compared to sporadic, VHL, RET and NF1 related ones. An increase in succinate accumulation (p<0.001) and decrease in ATP/ADP/AMP accumulation (p<0.001) was observed when compared to sporadic PGLs and PGLs of other genotypes. Positive correlations (p<0.01) were observed between respiratory chain complex II activity and total catecholamine content and ATP/ADP/AMP and total catecholamine contents in tumor tissues. CONCLUSIONS: The present study for the first time establishes relationship between determinants of energy metabolism like activity of respiratory chain enzyme complex II, ATP/ADP/AMP content and catecholamine content in PGL tumors. Also, the present study for the first time successfully uses NMR spectroscopy to detect catecholamines in PGL tumors and provides ex vivo evidence for the accumulation of succinate in PGL tumors with a SDHx mutation.
Clinical Cancer Research 05/2013; · 7.84 Impact Factor
[show abstract][hide abstract] ABSTRACT: Whole exome sequencing is a powerful tool to detect novel pathogenic mutations in patients with suspected mitochondrial disease. However, the interpretation of novel genetic variants is not always straightforward. Here, we present two siblings with a severe neonatal encephalopathy caused by complex V deficiency. The aim of this study was to uncover the underlying genetic defect using the combination of enzymatic testing and whole exome sequence analysis, and to provide evidence for causality by functional follow-up. Measurement of the oxygen consumption rate and enzyme analysis in fibroblasts were performed. Immunoblotting techniques were applied to study complex V assembly. The coding regions of the genome were analysed. Three-dimensional modelling was applied. Exome sequencing of the two siblings with complex V deficiency revealed a heterozygous mutation in the ATP5A1 gene, coding for complex V subunit α. The father carried the variant heterozygously. At the messenger RNA level, only the mutated allele was expressed in the patients, whereas the father expressed both the wild-type and the mutant allele. Gene expression data indicate that the maternal allele is not expressed, which is supported by the observation that the ATP5A1 expression levels in the patients and their mother are reduced to ∼50%. Complementation with wild-type ATP5A1 restored complex V in the patient fibroblasts, confirming pathogenicity of the defect. At the protein level, the mutation results in a disturbed interaction of the α-subunit with the β-subunit of complex V, which interferes with the stability of the complex. This study demonstrates the important value of functional studies in the diagnostic work-up of mitochondrial patients, in order to guide genetic variant prioritization, and to validate gene defects.
[show abstract][hide abstract] ABSTRACT: It is estimated that the human mitochondrial proteome consists of 1,000-1,500 distinct proteins. The majority of these support the various biochemical pathways that are active in these organelles. Individuals with an oxidative phosphorylation disorder of unknown cause provide a unique opportunity to identify novel genes implicated in mitochondrial biology. We identified a homozygous deletion of CEP89 in a patient with isolated complex IV deficiency, intellectual disability and multisystemic problems. CEP89 is an ubiquitously expressed and highly conserved gene of unknown function. Immunocytochemistry and cellular fractionation experiments showed that CEP89 is present both in the cytosol and in the mitochondrial intermembrane space. Furthermore, we ascertained in vitro that downregulation of CEP89 resulted in a severe decrease in complex IV in-gel activity and altered mobility, suggesting that the complex is aberrantly formed. Two-dimensional BN-SDS gel analysis revealed that CEP89 associates with a high molecular weight complex. Together, these data confirm a role for CEP89 in mitochondrial metabolism. In addition, we modeled CEP89 loss of function in Drosophila. Ubiquitous knockdown of fly Cep89 decreased complex IV activity and resulted in complete lethality. Furthermore, Cep89 is required for mitochondrial integrity, membrane depolarization and synaptic transmission of photoreceptor neurons, and for (sub)synaptic organization of the larval neuromuscular junction. Finally, we tested neuronal Cep89 knockdown flies in the light-off jump reflex habituation assay, which revealed its role in learning. We conclude that CEP89 proteins play an important role in mitochondrial metabolism, especially complex IV activity, and are required for neuronal and cognitive function across evolution.
Human Molecular Genetics 04/2013; · 7.69 Impact Factor
[show abstract][hide abstract] ABSTRACT: OBJECTIVE: To identify the mutated gene in a group of patients with an unclassified heritable white matter disorder sharing the same, distinct MRI pattern. METHODS: We used MRI pattern recognition analysis to select a group of patients with a similar, characteristic MRI pattern. We performed whole-exome sequencing to identify the mutated gene. We examined patients' fibroblasts for biochemical consequences of the mutant protein. RESULTS: We identified 6 patients from 5 unrelated families with a similar MRI pattern showing predominant abnormalities of the cerebellar cortex, deep cerebral white matter, and corpus callosum. The 4 tested patients had a respiratory chain complex I deficiency. Exome sequencing revealed mutations in NUBPL, encoding an iron-sulfur cluster assembly factor for complex I, in all patients. Upon identification of the mutated gene, we analyzed the MRI of a previously published case with NUBPL mutations and found exactly the same pattern. A strongly decreased amount of NUBPL protein and fully assembled complex I was found in patients' fibroblasts. Analysis of the effect of mutated NUBPL on the assembly of the peripheral arm of complex I indicated that NUBPL is involved in assembly of iron-sulfur clusters early in the complex I assembly pathway. CONCLUSION: Our data show that NUBPL mutations are associated with a unique, consistent, and recognizable MRI pattern, which facilitates fast diagnosis and obviates the need for other tests, including assessment of mitochondrial complex activities in muscle or fibroblasts.
[show abstract][hide abstract] ABSTRACT: Elevated urinary excretion of 3-methylglutaconic acid is considered rare in patients suspected of a metabolic disorder. In 3-methylglutaconyl-CoA hydratase deficiency (mutations in AUH), it derives from leucine degradation. In all other disorders with 3-methylglutaconic aciduria the origin is unknown, yet mitochondrial dysfunction is thought to be the common denominator. We investigate the biochemical, clinical and genetic data of 388 patients referred to our centre under suspicion of a metabolic disorder showing 3-methylglutaconic aciduria in routine metabolic screening. Furthermore, we investigate 591 patients with 50 different, genetically proven, mitochondrial disorders for the presence of 3-methylglutaconic aciduria. Three percent of all urine samples of the patients referred showed 3-methylglutaconic aciduria, often in correlation with disorders not reported earlier in association with 3-methylglutaconic aciduria (e.g. organic acidurias, urea cycle disorders, haematological and neuromuscular disorders). In the patient cohort with genetically proven mitochondrial disorders 11 % presented 3-methylglutaconic aciduria. It was more frequently seen in ATPase related disorders, with mitochondrial DNA depletion or deletion, but not in patients with single respiratory chain complex deficiencies. Besides, it was a consistent feature of patients with mutations in TAZ, SERAC1, OPA3, DNAJC19 and TMEM70 accounting for mitochondrial membrane related pathology. 3-methylglutaconic aciduria is found quite frequently in patients suspected of a metabolic disorder, and mitochondrial dysfunction is indeed a common denominator. It is only a discriminative feature of patients with mutations in AUH, TAZ, SERAC1, OPA3, DNAJC19 TMEM70. These conditions should therefore be referred to as inborn errors of metabolism with 3-methylglutaconic aciduria as discriminative feature.
Journal of Inherited Metabolic Disease 01/2013; · 4.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: Coenzyme Q(10) (ubiquinone or CoQ(10)) serves as a redox carrier in the mitochondrial oxidative phosphorylation system. The reduced form of this lipid-soluble antioxidant (ubiquinol) is involved in other metabolic processes as well, such as preventing reactive oxygen species (ROS) induced damage from the mitochondrial membrane. Primary coenzyme Q(10) deficiency is a rare, autosomal recessive disorder, often presenting with neurological and/or muscle involvement. Until now, five patients from four families have been described with primary coenzyme Q(10) deficiency due to mutations in COQ2 encoding para-hydroxybenzoate polyprenyl transferase. Interestingly, four of these patients showed a distinctive renal involvement (focal segmental glomerular sclerosis, crescentic glomerulonephritis, nephrotic syndrome), which is only very rarely seen in correlation with mitochondrial disorders. The fifth patient deceases due to infantile multi organ failure, also with renal involvement. Here we report a novel homozygous mutation in COQ2 (c.905C>T, p.Ala302Val) in a dizygotic twin from consanguineous Turkish parents. The children were born prematurely and died at the age of five and six months, respectively, after an undulating disease course involving apneas, seizures, feeding problems and generalized edema, alternating with relative stable periods without the need of artificial ventilation. There was no evidence for renal involvement. We would like to raise awareness for this potentially treatable disorder which could be under diagnosed in patients with fatal neonatal or infantile multi-organ disease.
Journal of the neurological sciences 01/2013; · 2.32 Impact Factor
[show abstract][hide abstract] ABSTRACT: The m.3243A>G is the most prevalent pathogenic mtDNA mutation but little is known about its inheritance. We studied 34 families containing 56 mother-child relations and 82 intersibling relations to investigate its transmission. We found a significant correlation between mother and child heteroplasmy levels (r = 0.679, p < 0.001). In mothers with a heteroplasmy level of below 25% we found 30% offspring without detectable mutation, while in mothers with a heteroplasmy level of above 25%, 100% of the offspring showed the m.3243A>G mutation. Heteroplasmy levels between siblings also correlated (r = 0.512, p < ;0.001), but had limited extra predictive value because of outliers. These new data on inheritance of the m.3243A>G mutation might be of value in counseling patients and preventing transmission of the mutation.
[show abstract][hide abstract] ABSTRACT: Mutations in PLA2G6 gene have variable phenotypic outcome including infantile neuroaxonal dystrophy, atypical neuroaxonal dystrophy, idiopathic neurodegeneration with brain iron accumulation and Karak syndrome. The cause of this phenotypic variation is so far unknown which impairs both genetic diagnosis and appropriate family counseling. We report detailed clinical, electrophysiological, neuroimaging, histologic, biochemical and genetic characterization of 11 patients, from 6 consanguineous families, who were followed for a period of up to 17 years. Cerebellar atrophy was constant and the earliest feature of the disease preceding brain iron accumulation, leading to the provisional diagnosis of a recessive progressive ataxia in these patients. Ultrastructural characterization of patients' muscle biopsies revealed focal accumulation of granular and membranous material possibly resulting from defective membrane homeostasis caused by disrupted PLA2G6 function. Enzyme studies in one of these muscle biopsies provided evidence for a relatively low mitochondrial content, which is compatible with the structural mitochondrial alterations seen by electron microscopy. Genetic characterization of 11 patients led to the identification of six underlying PLA2G6 gene mutations, five of which are novel. Importantly, by combining clinical and genetic data we have observed that while the phenotype of neurodegeneration associated with PLA2G6 mutations is variable in this cohort of patients belonging to the same ethnic background, it is partially influenced by the genotype, considering the age at onset and the functional disability criteria. Molecular testing for PLA2G6 mutations is, therefore, indicated in childhood-onset ataxia syndromes, if neuroimaging shows cerebellar atrophy with or without evidence of iron accumulation.
PLoS ONE 01/2013; 8(10):e76831. · 3.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Defects of mitochondrial oxidative phosphorylation (OXPHOS) are associated with a wide range of clinical phenotypes and time courses. Combined OXPHOS deficiencies are mainly caused by mutations of nuclear genes that are involved in mitochondrial protein translation. Due to their genetic heterogeneity it is almost impossible to diagnose OXPHOS patients on clinical grounds alone. Hence next generation sequencing (NGS) provides a distinct advantage over candidate gene sequencing to discover the underlying genetic defect in a timely manner. One recent example is the identification of mutations in MTFMT that impair mitochondrial protein translation through decreased formylation of Met-tRNAMet.
Here we report the results of a combined exome sequencing and candidate gene screening study. We identified nine additional MTFMT patients from eight families who were affected with Leigh encephalopathy or white matter disease, microcephaly, mental retardation, ataxia, and muscular hypotonia. In four patients, the causal mutations were identified by exome sequencing followed by stringent bioinformatic filtering. In one index case, exome sequencing identified a single heterozygous mutation leading to Sanger sequencing which identified a second mutation in the non-covered first exon. High-resolution melting curve-based MTFMT screening in 350 OXPHPOS patients identified pathogenic mutations in another three index cases. Mutations in one of them were not covered by previous exome sequencing.
All novel mutations predict a loss-of-function or result in a severe decrease in MTFMT protein in patients’ fibroblasts accompanied by reduced steady-state levels of complex I and IV subunits. Being present in 11 out of 13 index cases the c.626C > T mutation is one of the most frequent disease alleles underlying OXPHOS disorders. We provide detailed clinical descriptions on eleven MTFMT patients and review five previously reported cases.
Molecular Genetics and Metabolism 01/2013; · 2.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Complexome profiling is a novel technique which uses shotgun proteomics to establish protein migration profiles from fractionated blue native electrophoresis gels. Here we present a dataset of blue native electrophoresis migration profiles for 953 proteins by complexome profiling. By analysis of mitochondrial ribosomal complexes we demonstrate its potential to verify putative protein-protein interactions identified by affinity purification - mass spectrometry studies. Protein complexes were extracted in their native state from a HEK293 mitochondrial fraction and separated by blue native gel electrophoresis. Gel lanes were cut into gel slices of even size and analyzed by shotgun proteomics. Subsequently, the acquired protein migration profiles were analyzed for co-migration via hierarchical cluster analysis. This dataset holds great promise as a comprehensive resource for de novo identification of protein-protein interactions or to underpin and prioritize candidate protein interactions from other studies. To demonstrate the potential use of our dataset we focussed on the mitochondrial translation machinery. Our results show that mitoribosomal complexes can be analyzed by blue native gel electrophoresis, as at least four distinct complexes. Analysis of these complexes confirmed that 24 proteins that had previously been reported to co-purify with mitoribosomes indeed co-migrated with subunits of the mitochondrial ribosome. Co-migration of several proteins involved in biogenesis of inner mitochondrial membrane complexes together with mitoribosomal complexes suggested the possibility of co-translational assembly in human cells. Our data also highlighted a putative ribonucleotide complex that potentially contains MRPL10, MRPL12 and MRPL53 together with LRPPRC and SLIRP.
PLoS ONE 01/2013; 8(7):e68340. · 3.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Leigh syndrome (LS) is a progressive neurodegenerative disease caused by either mitochondrial or nuclear DNA mutations resulting in dysfunctional mitochondrial energy metabolism. Mutations in genes encoding for subunits of the respiratory chain or assembly factors of respiratory chain complexes are often documented in LS cases. Nicotinamide adenine dinucleotide (NADH):ubiquinone oxidoreductase (complex I) enzyme deficiencies account for a significant proportion of mitochondrial disorders, including LS. In an attempt to expand the repertoire of known mutations accounting for LS, we describe the clinical, radiological, biochemical and molecular data of six patients with LS found to have novel mutations in two complex I subunits (NDUFV1 and NDUFS2). Two siblings were homozygous for the previously undescribed R386C mutation in NDUFV1, one patient was a compound heterozygote for the R386C mutation in NDUFV1 and a frameshift mutation in the same gene, one patient was a compound heterozygote for the R88G and R199P mutations in NDUFV1, and two siblings were compound heterozygotes for an undescribed E104A mutation in NDUFS2. After the novel mutations were identified, we employed prediction models using protein conservation analysis (SIFT, PolyPhen and UCSC genome browser) to determine pathogenicity. The R386C, R88G, R199P, and E104A mutations were found to be likely pathogenic, and thus presumably account for the LS phenotype. This case series broadens our understanding of the etiology of LS by identifying new molecular defects that can result in complex I deficiency and may assist in targeted diagnostics and/or prenatal diagnosis of LS in the future.
[show abstract][hide abstract] ABSTRACT: A multicentre comparison of mitochondrial respiratory chain and complex V enzyme activity tests was performed. The average reproducibility of the enzyme assays is 16% in human muscle samples. In a blinded diagnostic accuracy test in patient fibroblasts and SURF1 knock-out mouse muscle, each lab made the correct diagnosis except for two complex I results. We recommend that enzyme activities are evaluated based on ratios, e.g. with complex IV or citrate synthase activity. In spite of large variations in observed enzyme activities, we show that inter-laboratory comparison of patient sample test results is possible by using normalization against a control sample.
[show abstract][hide abstract] ABSTRACT: The mitochondrial respiratory chain complex IV (cytochrome c oxidase) is a multi-subunit enzyme that transfers electrons from cytochrome c to molecular oxygen, yielding water. Its biogenesis requires concerted expression of mitochondria- and nuclear-encoded subunits and assembly factors. In this report we describe a homozygous missense mutation in FAM36A from a patient that displays ataxia and muscle hypotonia. The FAM36A gene is a remote, putative ortholog of the fungal complex IV assembly factor COX20. mRNA and protein coexpression analyses support the involvement of FAM36A in complex IV function in mammals. The c.154A>C mutation in the FAM36A gene, a mutation that is absent from sequenced exomes, leads to a reduced activity and lower levels of complex IV and its protein subunits. The FAM36A protein is absent from patient's fibroblasts. Cells affected by the mutation accumulate subassemblies of complex IV that contain COX1 but are almost devoid of COX2 protein. We observe co-purification of FAM36A and COX2 proteins supporting that the FAM36A defect hampers the early step of complex IV assembly at the incorporation of the COX2 subunit. Lentiviral complementation of patient's fibroblasts with wild-type FAM36A increases the complex IV activity as well as the amount of holocomplex IV and of individual subunits. These results establish the function of the human gene FAM36A in complex IV assembly and support a causal role of the gene in complex IV deficiency.
Human Molecular Genetics 11/2012; · 7.69 Impact Factor
[show abstract][hide abstract] ABSTRACT: Mitochondrial disorders are a heterogeneous group of disorders affecting energy production of the body. Different consensus diagnostic criteria for mitochondrial disorders in childhood are available - Wolfson, Nijmegen and modified Walker criteria. Due to the extreme complexity of mitochondrial disorders in children, we decided to develop a diagnostic algorithm, applicable in clinical practice in Estonia, in order to identify patients with mitochondrial disorders among pediatric neonatology and neurology patients. Additionally, it was aimed to evaluate the live-birth prevalence of mitochondrial disorders in childhood. During the study period (2003-2009), a total of 22 children were referred to a muscle biopsy in suspicion of mitochondrial disorder based on the preliminary biochemical, metabolic and instrumental investigations. Enzymatic and/or molecular analysis confirmed mitochondrial disease in 5 of them - an SCO2 gene (synthesis of cytochrome c oxidase, subunit 2) defect, 2 cases of pyruvate dehydrogenase complex deficiency and 2 cases of combined complex I and IV deficiency. The live-birth prevalence for mitochondrial defects observed in our cohort was 1/20,764 live births. Our epidemiological data correlate well with previously published epidemiology data on mitochondrial diseases in childhood from Sweden and Australia, but are lower than in Finland.
[show abstract][hide abstract] ABSTRACT: Congenital disorders of glycosylation type I (CDG-I) form a growing group of recessive neurometabolic diseases. Identification of disease genes is compromised by the enormous heterogeneity in clinical symptoms and the large number of potential genes involved. Until now, gene identification included the sequential application of biochemical methods in blood samples and fibroblasts. In genetically unsolved cases, homozygosity mapping has been applied in consanguineous families. Altogether, this time-consuming diagnostic strategy led to the identification of defects in 17 different CDG-I genes. Here, we applied whole-exome sequencing (WES) in combination with the knowledge of the protein N-glycosylation pathway for gene identification in our remaining group of six unsolved CDG-I patients from unrelated non-consanguineous families. Exome variants were prioritized based on a list of 76 potential CDG-I candidate genes, leading to the rapid identification of one known and two novel CDG-I gene defects. These included the first X-linked CDG-I due to a de novo mutation in ALG13, and compound heterozygous mutations in DPAGT1, together the first two steps in dolichol-PP-glycan assembly, and mutations in PGM1 in two cases, involved in nucleotide sugar biosynthesis. The pathogenicity of the mutations was confirmed by showing the deficient activity of the corresponding enzymes in patient fibroblasts. Combined with these results, the gene defect has been identified in 98% of our CDG-I patients. Our results implicate the potential of WES to unravel disease genes in the CDG-I in newly diagnosed singleton families.
Human Molecular Genetics 07/2012; 21(19):4151-61. · 7.69 Impact Factor