Pantethine treatment is effective in recovering the disease phenotype induced by ketogenic diet in a pantothenate kinase-associated neurodegeneration mouse model

Pantothenate kinase-associated neurodegeneration, caused by mutations in the PANK2 gene, is an autosomal recessive disorder characterized by dystonia, dysarthria, rigidity, pigmentary retinal degeneration and brain iron accumulation. PANK2 encodes the mitochondrial enzyme pantothenate kinase type 2, responsible for the phosphorylation of pantothenate or vitamin B5 in the biosynthesis of co-enzyme A. A Pank2 knockout (Pank2^−/−) mouse model did not recapitulate the human disease but showed azoospermia and mitochondrial dysfunctions. We challenged this mouse model with a low glucose and high lipid content diet (ketogenic diet) to stimulate lipid use by mitochondrial beta-oxidation. In the presence of a shortage of co-enzyme A, this diet could evoke a general impairment of bioenergetic metabolism. Only Pank2^−/− mice fed with a ketogenic diet developed a pantothenate kinase-associated neurodegeneration-like syndrome characterized by severe motor dysfunction, neurodegeneration and severely altered mitochondria in the central and peripheral nervous systems. These mice also showed structural alteration of muscle morphology, which was comparable with that observed in a patient with pantothenate kinase-associated neurodegeneration. We here demonstrate that pantethine administration can prevent the onset of the neuromuscular phenotype in mice suggesting the possibility of experimental treatment in patients with pantothenate kinase-associated neurodegeneration.

Altered sulfide (H(2)S) metabolism in ethylmalonic encephalopathy

Hydrogen sulfide (sulfide, H(2)S) is a colorless, water-soluble gas with a typical smell of rotten eggs. In the past, it has been investigated for its role as a potent toxic gas emanating from sewers and swamps or as a by-product of industrial processes. At high concentrations, H(2)S is a powerful inhibitor of cytochrome c oxidase; in trace amounts, it is an important signaling molecule, like nitric oxide (NO) and carbon monoxide (CO), together termed "gasotransmitters." This review will cover the physiological role and the pathogenic effects of H(2)S, focusing on ethylmalonic encephalopathy, a human mitochondrial disorder caused by genetic abnormalities of sulfide metabolism. We will also discuss the options that are now conceivable for preventing genetically driven chronic H(2)S toxicity, taking into account that a complete understanding of the physiopathology of H(2)S has still to be achieved.

A multi-center comparison of diagnostic methods for the biochemical evaluation of suspected mitochondrial disorders

A multicenter 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 be 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.

Mutations in COX7B cause microphthalmia with linear skin lesions, an unconventional mitochondrial disease

Microphthalmia with linear skin lesions (MLS) is an X-linked dominant male-lethal disorder associated with mutations in holocytochrome c-type synthase (HCCS), which encodes a crucial player of the mitochondrial respiratory chain (MRC). Unlike other mitochondrial diseases, MLS is characterized by a well-recognizable neurodevelopmental phenotype. Interestingly, not all clinically diagnosed MLS cases have mutations in HCCS, thus suggesting genetic heterogeneity for this disorder. Among the possible candidates, we analyzed the X-linked COX7B and found deleterious de novo mutations in two simplex cases and a nonsense mutation, which segregates with the disease, in a familial case. COX7B encodes a poorly characterized structural subunit of cytochrome c oxidase (COX), the MRC complex IV. We demonstrated that COX7B is indispensable for COX assembly, COX activity, and mitochondrial respiration. Downregulation of the COX7B ortholog (cox7B) in medaka (Oryzias latipes) resulted in microcephaly and microphthalmia that recapitulated the MLS phenotype and demonstrated an essential function of complex IV activity in vertebrate CNS development. Our results indicate an evolutionary conserved role of the MRC complexes III and IV for the proper development of the CNS in vertebrates and uncover a group of mitochondrial diseases hallmarked by a developmental phenotype.

Pantothenate kinase-associated neurodegeneration: altered mitochondria membrane potential and defective respiration in Pank2 knock-out mouse model

Neurodegeneration with brain iron accumulation (NBIA) comprises a group of neurodegenerative disorders characterized by high brain content of iron and presence of axonal spheroids. Mutations in the PANK2 gene, which encodes pantothenate kinase 2, underlie an autosomal recessive inborn error of coenzyme A metabolism, called pantothenate kinase-associated neurodegeneration (PKAN). PKAN is characterized by dystonia, dysarthria, rigidity and pigmentary retinal degeneration. The pathogenesis of this disorder is poorly understood and, although PANK2 is a mitochondrial protein, perturbations in mitochondrial bioenergetics have not been reported. A knock-out (KO) mouse model of PKAN exhibits retinal degeneration and azoospermia, but lacks any neurological phenotype. The absence of a clinical phenotype has partially been explained by the different cellular localization of the human and murine PANK2 proteins. Here we demonstrate that the mouse Pank2 protein localizes to mitochondria, similar to its human orthologue. Moreover, we show that Pank2-defective neurons derived from KO mice have an altered mitochondrial membrane potential, a defect further corroborated by the observations of swollen mitochondria at the ultra-structural level and by the presence of defective respiration.

C19orf12 and FA2H mutations are rare in Italian patients with neurodegeneration with brain iron accumulation

Neurodegeneration with brain iron accumulation (NBIA) defines a wide spectrum of clinical entities characterized by iron accumulation in specific regions of the brain, predominantly in the basal ganglia. We evaluated the presence of FA2H and C19orf12 mutations in a cohort of 46 Italian patients with early onset NBIA, which were negative for mutations in the PANK2 and PLA2G6 genes. Follow-up molecular genetic and in vitro analyses were then performed. We did not find any mutations in the FA2H gene, although we identified 3 patients carrying novel mutations in the C19orf12 gene. The recent discovery of new genes responsible for NBIA extends the spectrum of the genetic investigation now available for these disorders and makes it possible to delineate a clearer clinical-genetic classification of different forms of this syndrome. A large fraction of patients still remain without a molecular genetics diagnosis, suggesting that additional NBIA genes are still to be discovered.

Morphologic evidence of diffuse vascular damage in human and in the experimental model of ethylmalonic encephalopathy

Ethylmalonic encephalopathy (EE) is a rare autosomal recessive disorder characterized by early onset encephalopathy, chronic diarrhoea, petechiae, orthostatic acrocyanosis and defective cytochrome c oxidase (COX) in muscle and brain. High levels of lactic, ethylmalonic and methylsuccinic acids are detected in body fluids. EE is caused by mutations in ETHE1, a mitochondrial sulphur dioxygenase. By studying a suitable mouse model, we found that loss of ETHE1 leads to accumulation of sulphide, which is a poison for COX and other enzymatic activities thus accounting for the main features of EE. We report here the first autopsy case of a child with a genetically confirmed diagnosis of EE, and compare the histological, histochemical and immunohistochemical findings with those of the constitutive Ethe1 (-/-) mice. In addition to COX depleted cells, widespread endothelial lesions of arterioles and capillaries of the brain and gastrointestinal tract were the pathologic hallmarks in both organisms. Our findings of diffuse vascular damage of target critical organs are in keeping with the hypothesis that the pathologic effects of ETHE1 deficiency may stem from high levels of circulating hydrogen sulphide rather than the inability of specific organs to detoxify its endogenous production.

Metabolic consequences of mitochondrial coenzyme A deficiency in patients with PANK2 mutations

Pantothenate kinase-associated neurodegeneration (PKAN) is a rare, inborn error of metabolism characterized by iron accumulation in the basal ganglia and by the presence of dystonia, dysarthria, and retinal degeneration. Mutations in pantothenate kinase 2 (PANK2), the rate-limiting enzyme in mitochondrial coenzyme A biosynthesis, represent the most common genetic cause of this disorder. How mutations in this core metabolic enzyme give rise to such a broad clinical spectrum of pathology remains a mystery. To systematically explore its pathogenesis, we performed global metabolic profiling on plasma from a cohort of 14 genetically defined patients and 18 controls. Notably, lactate is elevated in PKAN patients, suggesting dysfunctional mitochondrial metabolism. As predicted, but never previously reported, pantothenate levels are higher in patients with premature stop mutations in PANK2. Global metabolic profiling and follow-up studies in patient-derived fibroblasts also reveal defects in bile acid conjugation and lipid metabolism, pathways that require coenzyme A. These findings raise a novel therapeutic hypothesis, namely, that dietary fats and bile acid supplements may hold potential as disease-modifying interventions. Our study illustrates the value of metabolic profiling as a tool for systematically exploring the biochemical basis of inherited metabolic diseases.

Microscale oxygraphy reveals OXPHOS impairment in MRC mutant cells

Given the complexity of the respiratory chain structure, assembly and regulation, the diagnostic workout for the identification of defects of oxidative phosphorylation (OXPHOS) is a major challenge. Spectrophotometric assays, that measure the activity of individual respiratory complexes in tissue and cell homogenates or isolated mitochondria, are highly specific, but their utilization is limited by the availability of sufficient biological material and intrinsic sensitivity. A further limitation is tissue specificity, which usually determines attenuation, or disappearance, in cultured fibroblasts, of defects detected in muscle or liver. We used numerous fibroblast cell lines derived from patients with OXPHOS deficiencies to set up experimental protocols required for the direct readout of cellular respiration using the Seahorse XF96 apparatus, which measures oxygen consumption rate (OCR) and extra-cellular acidification rate (ECAR) in 96 well plates. Results demonstrate that first level screening based on microscale oxygraphy is more sensitive, cheaper and rapid than spectrophotometry for the biochemical evaluation of cells from patients with suspected mitochondrial disorders.

Absence of an orphan mitochondrial protein, c19orf12, causes a distinct clinical subtype of neurodegeneration with brain iron accumulation

The disease classification neurodegeneration with brain iron accumulation (NBIA) comprises a clinically and genetically heterogeneous group of progressive neurodegenerative disorders characterized by brain iron deposits in the basal ganglia. For about half of the cases, the molecular basis is currently unknown. We used homozygosity mapping followed by candidate gene sequencing to identify a homozygous 11 bp deletion in the orphan gene C19orf12. Mutation screening of 23 ideopathic NBIA index cases revealed two mutated alleles in 18 of them, and one loss-of-function mutation is the most prevalent. We also identified compound heterozygous missense mutations in a case initially diagnosed with Parkinson disease at age 49. Psychiatric signs, optic atrophy, and motor axonal neuropathy were common findings. Compared to the most prevalent NBIA subtype, pantothenate kinase associated neurodegeneration (PKAN), individuals with two C19orf12 mutations were older at age of onset and the disease progressed more slowly. A polyclonal antibody against the predicted membrane spanning protein showed a mitochondrial localization. A histopathological examination in a single autopsy case detected Lewy bodies, tangles, spheroids, and tau pathology. The mitochondrial localization together with the immunohistopathological findings suggests a pathomechanistic overlap with common forms of neurodegenerative disorders.

Ethylmalonic encephalopathy: application of improved biochemical and molecular diagnostic approaches

Ethylmalonic encephalopathy (EE, OMIM # 602473) is an autosomal recessive metabolic disorder of infancy affecting the brain, the gastrointestinal tract and peripheral vessels. It is caused by a defect in the ETHE1 gene product, which was recently shown to be part of a metabolic pathway devoted to sulphide detoxification. We report the application of improved biochemical and molecular approaches to the diagnosis of three cases of EE from two unrelated Cypriot families. The children presented all the typical biochemical hallmarks of the disease including elevated lactate and butyrylcarnitine in blood and elevated urinary excretion of ethylmalonic acid, 2-methylsuccinate, isobutyrylglycine and isovalerylglycine. We also detected an elevated level of thiosulphate in urine, which we propose as an additional biochemical marker of the disease. The proband of the first family was shown to be a compound heterozygote for a missense mutation in exon 5, L185R, and a deletion of exon 4. The deletion was identified using quantitative real-time polymerase chain reaction (qRT-PCR). Using the same technique, the proband of the second family was found to be homozygous for the exon 4 deletion. A prenatal diagnosis was performed for the second family using qRT-PCR, thus establishing the usefulness of RT-PCR in prenatal diagnosis.

Chronic exposure to sulfide causes accelerated degradation of cytochrome c oxidase in ethylmalonic encephalopathy

Ethylmalonic encephalopathy (EE) is an autosomal recessive, invariably fatal disorder associated with mutations in ETHE1, a gene encoding a mitochondrial sulfur dioxygenase (SDO). The main consequence of the absence of Ethe1-SDO is the accumulation of sulfide (H(2)S) in critical tissues, including colonic mucosa, liver, muscle, and brain. To make progress in the elucidation of the biochemical mechanisms leading to cytochrome c oxidase (COX) deficiency, we (i) generated tissue-specific conditional Ethe1 knockout mice to clarify the different contributions of endogenous and exogenous H(2)S production, and (ii) studied the development of H(2)S-driven COX deficiency in Ethe1(-/-) mouse tissues and human cells. Ethe1(-/-) conditional animals displayed COX deficiency limited to the specific targeted tissue. The accumulation of H(2)S over time causes progressive COX deficiency in animal tissues and human cells, which is associated with reduced amount of COX holoenzyme, and of several COX subunits, including mitochondrially encoded cytochrome c oxidase 1 (MTCO1), MTCO2, COX4, and COX5A. This reduction is not paralleled by consistent downregulation in expression of the corresponding mRNAs. Tissue-specific ablation of Ethe1 causes COX deficiency in targeted organs, suggesting that failure in neutralizing endogenous, tissue-specific production of H(2)S is sufficient to cause the biochemical defect but neither to determine a clinical impact nor to induce the biomarker profile typical of EE. The mechanism by which H(2)S causes COX deficiency consists of rapid heme a inhibition and accelerated long-term degradation of COX subunits. However, the pleiotropic devastating effects of H(2)S accumulation in EE cannot be fully explained by the sole defect of COX in critical tissues, but are likely consequent to several toxic actions on a number of enzymatic activities in different tissues, including endothelial lining of the small vessels, leading to multiorgan failure.

adPEO mutations in ANT1 impair ADP-ATP translocation in muscle mitochondria

Mutations in the heart and muscle isoform of adenine nucleotide translocator 1 (ANT1) are associated with autosomal-dominant progressive external opthalmoplegia (adPEO) clinically characterized by exercise intolerance, ptosis and muscle weakness. The pathogenic mechanisms underlying the mitochondrial myopathy caused by ANT1 mutations remain largely unknown. In yeast, expression of ANT1 carrying mutations corresponding to the human adPEO ones causes a wide range of mitochondrial abnormalities. However, functional studies of ANT1 mutations in mammalian cells are lacking, because they have been hindered by the fact that ANT1 expression leads to apoptotic cell death in commonly utilized replicating cell lines. Here, we successfully express functional ANT1 in differentiated mouse myotubes, which naturally contain high levels of ANT1, without causing cell death. We demonstrate, for the first time in these disease-relevant mammalian cells, that mutant human ANT1 causes dominant mitochondrial defects characterized by decreased ADP-ATP exchange function and abnormal translocator reversal potential. These abnormalities are not due to ANT1 loss of function, because knocking down Ant1 in myotubes causes functional changes different from ANT1 mutants. Under certain physiological conditions, mitochondria consume ATP to maintain membrane potential by reversing the ADP-ATP transport. The modified properties of mutant ANT1 can be responsible for disease pathogenesis in adPEO, because exchange reversal occurring at higher than normal membrane potential can cause excessive energy depletion and nucleotide imbalance in ANT1 mutant muscle cells.

Combined treatment with oral metronidazole and N-acetylcysteine is effective in ethylmalonic encephalopathy

Ethylmalonic encephalopathy is caused by mutations in ETHE1, a mitochondrial matrix sulfur dioxygenase, leading to failure to detoxify sulfide, a product of intestinal anaerobes and, in trace amounts, tissues. Metronidazole, a bactericide, or N-acetylcysteine, a precursor of sulfide-buffering glutathione, substantially prolonged the lifespan of Ethe1-deficient mice, with the combined treatment being additive. The same dual treatment caused marked clinical improvement in five affected children, with hardly any adverse or side effects.

Nonsense mutation in pseudouridylate synthase 1 (PUS1) in two brothers affected by myopathy, lactic acidosis and sideroblastic anaemia (MLASA)

Myopathy, lactic acidosis and sideroblastic anaemia (MLASA) is a rare condition that combines early-onset myopathy with lactic acidosis and sideroblastic anaemia. MLASA has been associated with a missense mutation in pseudouridylate synthase 1 (PUS1), an enzyme located in both nucleus and mitochondria, which converts uridine into pseudouridine in several cytosolic and mitochondrial tRNA positions and increases the efficiency of protein synthesis in both compartments. We examined two Italian brothers with MLSA and sequenced the PUS1 gene. We found combined defects in mitochondrial respiratory chain complexes in muscle and fibroblast homogenates of both patients, and low levels of mtDNA translation products in fibroblast mitochondria. A novel, homozygous stop mutation was present in PUS1 (E220X). The stop mutation in PUS1 is likely to determine the loss of function of the protein, since it predicts the synthesis of a protein missing 208/427 amino acid residues on the C terminus, and was associated with low mtDNA translation.

How do human cells react to the absence of mitochondrial DNA?

Background

Mitochondrial biogenesis is under the control of two different genetic systems: the nuclear genome (nDNA) and the mitochondrial genome (mtDNA). The mtDNA is a circular genome of 16.6 kb encoding 13 of the approximately 90 subunits that form the respiratory chain, the remaining ones being encoded by the nDNA. Eukaryotic cells are able to monitor and respond to changes in mitochondrial function through alterations in nuclear gene expression, a phenomenon first defined in yeast and known as retrograde regulation. To investigate how the cellular transcriptome is modified in response to the absence of mtDNA, we used Affymetrix HG-U133A GeneChip arrays to study the gene expression profile of two human cell lines, 143BTK⁻ and A549, which had been entirely depleted of mtDNA (ρ° cells), and compared it with that of corresponding undepleted parental cells (ρ⁺ cells).

Results

Our data indicate that absence of mtDNA is associated with: i) a down-regulation of cell cycle control genes and a reduction of cell replication rate, ii) a down-regulation of nuclear-encoded subunits of complex III of the respiratory chain and iii) a down-regulation of a gene described as the human homolog of ELAC2 of E. coli, which encodes a protein that we show to also target to the mitochondrial compartment.

Conclusions

Our results indicate a strong correlation between mitochondrial biogenesis and cell cycle control and suggest that some proteins could have a double role: for instance in controlling both cell cycle progression and mitochondrial functions. In addition, the finding that ELAC2 and maybe other transcripts that are located into mitochondria, are down-regulated in ρ° cells, make them good candidates for human disorders associated with defective replication and expression of mtDNA.

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.

Identification of novel mutations in five patients with mitochondrial encephalomyopathy

MELAS, MERRF, LHON and NARP, are well-established mitochondrial syndromes associated with specific point mutations of mitochondrial DNA (mtDNA). However, these recurrent mtDNA mutations account for only a minority of mitochondrial disease cases. To evaluate the impact of novel mtDNA mutations, we performed mtDNA sequence analysis in muscle and other tissues of 240 patients with different mitochondrial neuromuscular syndromes. We identified a total of 33 subjects with novel, private or uncommon mutations. Among these, five novel mutations were found in both paediatric and adult cases. We here report on the clinical description of these patients, as well as the biochemical and molecular genetic characterization of the corresponding mutations. Patients 1 and 2 showed changes in ND genes, patient 3 carried a heteroplasmic deletion in the COI gene, patients 4 and 5 carried heteroplasmic mutations in tRNA(Trp) and tRNA(Phe), respectively. Altogether, these data indicate that mtDNA analysis must become part of the routine screening for mitochondrial disorders.

FASTKD2 nonsense mutation in an infantile mitochondrial encephalomyopathy associated with cytochrome c oxidase deficiency

In two siblings we found a mitochondrial encephalomyopathy, characterized by developmental delay, hemiplegia, convulsions, asymmetrical brain atrophy, and low cytochrome c oxidase (COX) activity in skeletal muscle. The disease locus was identified on chromosome 2 by homozygosity mapping; candidate genes were prioritized for their known or predicted mitochondrial localization and then sequenced in probands and controls. A homozygous nonsense mutation in the KIAA0971 gene segregated with the disease in the proband family. The corresponding protein is known as fas activated serine-threonine kinase domain 2, FASTKD2. Confocal immunofluorescence colocalized a tagged recombinant FASTKD2 protein with mitochondrial markers, and membrane-potential-dependent in vitro mitochondrial import was demonstrated in isolated mitochondria. In staurosporine-induced-apoptosis experiments, decreased nuclear fragmentation was detected in treated mutant versus control fibroblasts. In conclusion, we found a loss-of-function mutation in a gene segregating with a peculiar mitochondrial encephalomyopathy associated with COX deficiency in skeletal muscle. The corresponding protein is localized in the mitochondrial inner compartment. Preliminary data indicate that FASTKD2 plays a role in mitochondrial apoptosis.

Severe infantile hypotonia with ethylmalonic aciduria: case report

An 8-month-old girl was admitted to an outpatient clinic with significant hypotonia and weakness. Organic acid analysis in urine revealed a significant increase in ethylmalonic acid. A deoxyribonucleic analysis revealed the presence of a 625G>A (G-to-A substitution at nucleotide 625) variant short-chain acyl-coenzyme A dehydrogenase gene polymorphism. With the clinical, biochemical and molecular findings, short-chain acyl-coenzyme A dehydrogenase deficiency was suspected. Because 625G>A and 511C>T (C-to-T substitution at nucleotide 511) genetic variations are also present in 14% of the general population, these are considered to be genetic sensitivity variations rather than causing a disease themselves and to result in possible short-chain acyl-coenzyme A dehydrogenase deficiency in the presence of environmental factors such as fever and hunger as well as cellular, biochemical, and other genetic factors. It was stressed that severe infantile hypotonia could also be the only manifestation of ethylmalonic aciduria spectrum disorders.