Genetic heterogeneity in Leigh syndrome

Development and Functions of Mitochondria in Early Life

Mitochondria are highly dynamic organelles of bacterial origin in eukaryotic cells. These play a central role in metabolism and adenosine triphosphate (ATP) synthesis and in the production and regulation of reactive oxygen species (ROS). In addition to the generation of energy, mitochondria perform numerous other functions to support key developmental events such as fertilization during reproduction, oocyte maturation, and the development of the embryo. During embryonic and neonatal development, mitochondria may have important effects on metabolic, energetic, and epigenetic regulation, which may have significant short- and long-term effects on embryonic and offspring health. Hence, the environment, epigenome, and early-life regulation are all linked by mitochondrial integrity, communication, and metabolism.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

A Quantitative Proteome Map of the Human Body

Determining protein levels in each tissue and how they compare with RNA levels is important for understanding human biology and disease as well as regulatory processes that control protein levels. We quantified the relative protein levels from over 12,000 genes across 32 normal human tissues. Tissue-specific or tissue-enriched proteins were identified and compared to transcriptome data. Many ubiquitous transcripts are found to encode tissue-specific proteins. Discordance of RNA and protein enrichment revealed potential sites of synthesis and action of secreted proteins. The tissue-specific distribution of proteins also provides an in-depth view of complex biological events that require the interplay of multiple tissues. Most importantly, our study demonstrated that protein tissue-enrichment information can explain phenotypes of genetic diseases, which cannot be obtained by transcript information alone. Overall, our results demonstrate how understanding protein levels can provide insights into regulation, secretome, metabolism, and human diseases.

The genetic nomenclature of recessive cerebellar ataxias

The recessive cerebellar ataxias are a large group of degenerative and metabolic disorders, the diagnostic management of which is difficult because of the enormous clinical and genetic heterogeneity. Because of several limitations, the current classification systems provide insufficient guidance for clinicians and researchers. Here, we propose a new nomenclature for the genetically confirmed recessive cerebellar ataxias according to the principles and criteria laid down by the International Parkinson and Movement Disorder Society Task Force on Classification and Nomenclature of Genetic Movement Disorders. We apply stringent criteria for considering an association between gene and phenotype to be established. The newly proposed list of recessively inherited cerebellar ataxias includes 62 disorders that were assigned an ATX prefix, followed by the gene name, because these typically present with ataxia as a predominant and/or consistent feature. An additional 30 disorders that often combine ataxia with a predominant or consistent other movement disorder received a double prefix (e.g., ATX/HSP). We also identified a group of 89 entities that usually present with complex nonataxia phenotypes, but may occasionally present with cerebellar ataxia. These are listed separately without the ATX prefix. This new, transparent and adaptable nomenclature of the recessive cerebellar ataxias will facilitate the clinical recognition of recessive ataxias, guide diagnostic testing in ataxia patients, and help in interpreting genetic findings. © 2018 International Parkinson and Movement Disorder Society.

Leigh Syndrome: An MR Study of Three Cases

Leigh syndrome (LS), or subacute necrotizing encephalomyelopathy, is the most common childhood mitochondrial encephalopathy, accounting for more than 50% of cases in this age group. Its estimated incidence is 1:40,000 – 1:77,000 liveborn infants a year. LS is a rare progressive multisystem fatal disorder inherited by autosomal recessive, X-linked and maternal transmission. Clinical onset is predominantly in the first two years of life (average: six months); 50% of patients die within a year, even though there are later- and even adult-onset forms with a more protracted evolution. LS is due to a deficit of various respiratory chain and Krebs cycle enzymes resulting in insufficient production of adenosine triphosphate (ATP), in particular cytochrome-c-oxidase (COX), pyruvate carboxylase, pyruvate dehydrogenase complex and complex I of the respiratory chain, which share an autosomal recessive and X-linked mode of transmission. Cases with maternal inheritance (MILS) are due to a mitochondrial DNA (mtDNA) point mutation. LS is clinically heterogeneous in relation to the severity of the metabolic dysfunction and is characterized by muscle involvement and especially CNS disorders, particularly psychomotor retardation, ocular symptoms, hypotonia and pyramidal signs. Death is most commonly due to respiratory failure, status epilepticus and sudden coma. The major neuropathological findings, first described by Leigh in 1951, are symmetrical foci of spongy necrosis associated with vessel proliferation and reactive gliosis in basal nuclei, brainstem and thalamus grey matter. The neuronal metabolic alteration can also affect the white matter, resulting in delayed myelination or hypomyelination. The diagnosis rests on clinical signs, elevated CSF lactate, pyruvate and alanine, and biochemical and neuroradiological data. We describe two patients with LS studied with morphological MR associated with diffusion and spectroscopy techniques to assess the diagnostic potential of standard MR imaging and establish whether the association of functional MR methods can improve its diagnostic accuracy. A case of LS with a post-mortem MR study is also described. Three patients with a diagnosis of LS based on clinical, CSF and laboratory data were studied on a GE SIGNA EXCITE 1.5 T unit using an eight-channel phased-array head coil to acquire standard sequences (SE T1; TSE DP T2; FLAIR) and echo-planar diffusion-weighted sequences (DWI; b= 1000 s/mm2) with calculation of ADC maps. The spectroscopic study used single-voxel (TE/TR ms ? 144/1500) and multi-voxel techniques (TE/TR ms = 144/1000) at the level of the basal nuclei. Bilateral and symmetrical involvement of basal nuclei grey matter with T2 hyperintensity was a consistent finding in the morphological MR study. In one patient, associated white matter involvement with T2 hyperintensity in periventricular and retrotrigonal areas reflected delayed myelination or hypomyelination. The deep grey matter changes, sometimes associated with white matter lesions, suggested a diagnosis of subacute necrotizing encephalomyelopathy, in line with the literature. Acute-phase ADC values in affected areas were lower than those of normal grey and white matter and displayed signal hyperintensity on DWI. Reduced ADC values are associated with restricted water diffusivity typical of cytotoxic edema. Spectroscopy showed a high lactate peak, reflecting altered anaerobic glycolysis, and a reduced NAA peak in affected areas, which are however non-specific findings. The most informative study in these patients is standard MR associated with functional techniques, which can confirm the diagnosis obtained with morphological imaging.

The neuroimaging of Leigh syndrome: case series and review of the literature

Leigh syndrome by definition is (1) a neurodegenerative disease with variable symptoms, (2) caused by mitochondrial dysfunction from a hereditary genetic defect and (3) accompanied by bilateral central nervous system lesions. A genetic etiology is confirmed in approximately 50% of patients, with more than 60 identified mutations in the nuclear and mitochondrial genomes. Here we review the clinical features and imaging studies of Leigh syndrome and describe the neuroimaging findings in a cohort of 17 children with genetically confirmed Leigh syndrome. MR findings include lesions in the brainstem in 9 children (53%), basal ganglia in 13 (76%), thalami in 4 (24%) and dentate nuclei in 2 (12%), and global atrophy in 2 (12%). The brainstem lesions were most frequent in the midbrain and medulla oblongata. With follow-up an increased number of lesions from baseline was observed in 7 of 13 children, evolution of the initial lesion was seen in 6, and complete regression of the lesions was seen in 3. No cerebral white matter lesions were found in any of the 17 children. In concordance with the literature, we found that Leigh syndrome follows a similar pattern of bilateral, symmetrical basal ganglia or brainstem changes. Lesions in Leigh syndrome evolve over time and a lack of visible lesions does not exclude the diagnosis. Reversibility of lesions is seen in some patients, making the continued search for treatment and prevention a priority for clinicians and researchers.

Metabolic disturbances in diseases with neurological involvement

Degeneration of specific neuronal populations and progressive nervous system dysfunction characterize neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. These findings are also reported in inherited diseases such as phenylketonuria and glutaric aciduria type I. The involvement of mitochondrial dysfunction in these diseases was reported, elicited by genetic alterations, exogenous toxins or buildup of toxic metabolites. In this review we shall discuss some metabolic alterations related to the pathophysiology of diseases with neurological involvement and aging process. These findings may help identifying early disease biomarkers and lead to more effective therapies to improve the quality of life of the patients affected by these devastating illnesses.

Metabolic causes of epileptic encephalopathy

Epileptic encephalopathy can be induced by inborn metabolic defects that may be rare individually but in aggregate represent a substantial clinical portion of child neurology. These may present with various epilepsy phenotypes including refractory neonatal seizures, early myoclonic encephalopathy, early infantile epileptic encephalopathy, infantile spasms, and generalized epilepsies which in particular include myoclonic seizures. There are varying degrees of treatability, but the outcome if untreated can often be catastrophic. The importance of early recognition cannot be overemphasized. This paper provides an overview of inborn metabolic errors associated with persistent brain disturbances due to highly active clinical or electrographic ictal activity. Selected diseases are organized by the defective molecule or mechanism and categorized as small molecule disorders (involving amino and organic acids, fatty acids, neurotransmitters, urea cycle, vitamers and cofactors, and mitochondria) and large molecule disorders (including lysosomal storage disorders, peroxisomal disorders, glycosylation disorders, and leukodystrophies). Details including key clinical features, salient electrophysiological and neuroradiological findings, biochemical findings, and treatment options are summarized for prominent disorders in each category.

Sudden death in Leigh syndrome: an autopsy case

The present report describes the sudden death of a 3-year-old female child who had been clinically diagnosed with Leigh syndrome.Leigh syndrome is a heterogeneous progressive neurodegenerative disorder, which is characterized by focal or bilateral lesions in the thalamus, basal ganglia, brainstem, cerebellum, and spinal cord. Affected patients exhibit a variable clinical picture that frequently includes psychomotor retardation or regression, recurrent episodes of vomiting, failure to thrive, and signs of brainstem and basal ganglia dysfunction.The child was found dead in bed. Autopsy described the presence of symmetrical, necrotizing lesions scattered within the basal ganglia, thalamus, diencephalon, brainstem, and spinal-cord gray matter and revealed the presence of gastric contents in the upper and lower airways. We report the results of genetic investigations and describe the histological and immunohistochemical features that confirmed the diagnosis. These findings suggest that Leigh syndrome should be regarded as predisposing children to sudden death, especially by asphyxia secondary to the neurological disorder.

Mitochondrial DNA 11777C>A mutation associated Leigh syndrome: case report with a review of the previously described pedigrees

Here, we present a male infant with clinical signs of typical Leigh syndrome. The first symptom, myoclonus was presented already on the 5th day of life; at 7 months of age limb convulsions and cerebral paresis with hypotonia were developed. At the age of 11 months, MRI verified increased signal intensity in the entire mesencephalon and medulla oblongata; while gray matter proton spectroscopy revealed presence of lactate increase in the brain. At age of 17 months, the child died in cardiorespiratory arrest. After autopsy, the diagnosis of Leigh syndrome was established; using DNA isolated from skeletal muscle and liver, heteroplasmic (>50%) mitochondrial 11777C>A was detected in the fourth subunit of NADH dehydrogenase enzyme (MTND4) encoding gene, which causes Arg --> Ser replacement. The mutation was also detected in low copy number in blood of mother. Albeit this mutation type is well recognized as a typical mtDNA mutation, according the reports available on the PubMed, this mutation was described only in four patients with wide phenotypic variations; here, we reviewed the characteristic clinical features of them. Taken together, the earliest onset of symptoms, the nature of the first presentation signs, the most rapid progression, the character of minor additional symptoms, and the early fatal outcome differentiate the phenotypic variant of the proband presented here from cases reported so far by others.

Unusual findings in Leigh syndrome caused by T8993C mutation

The pathological nature of Leigh syndrome is highly variable and depends on the underlying mitochondrial or nuclear genome defect. Mitochondrial m.8993T>G and m.8993T>C mutations are responsible for both NARP (neurogenic weakness, ataxia and retinitis pigmentosa) and Leigh syndrome depending on the amount of mutant mtDNA. The clinical findings of Leigh syndrome caused by the m.8993T>C mutation are less severe than those associated with the m.8993T>G mutation, and ragged red fibers, oligoclonal bands in cerebrospinal fluid, and additional deficiencies of respiratory enzyme complexes are usually not found. This report presents a two year old girl with Leigh syndrome caused by a m.8993T>C mutation. Interestingly she had ragged red fibers in muscle tissue, oligoclonal bands in CSF and focal deficient histochemical staining for complexes I and IV.

Leigh-like subacute necrotising encephalopathy in Yorkshire Terriers: neuropathological characterisation, respiratory chain activities and mitochondrial DNA

Our knowledge of molecular mechanisms underlying mitochondrial disorders in humans has increased considerably during the past two decades. Mitochondrial encephalomyopathies have sporadically been reported in dogs. However, molecular and biochemical data that would lend credence to the suspected mitochondrial origin are largely missing. This study was aimed to characterise a Leigh-like subacute necrotising encephalopathy (SNE) in Yorkshire Terriers and to shed light on its enzymatic and genetic background. The possible resemblance to SNE in Alaskan Huskies and to human Leigh syndrome (LS) was another focus of interest. Eleven terriers with imaging and/or gross evidence of V-shaped, non-contiguous, cyst-like cavitations in the striatum, thalamus and brain stem were included. Neuropathological examinations focussed on muscle, brain pathology and mitochondrial ultrastructure. Further investigations encompassed respiratory-chain activities and the mitochondrial DNA. In contrast to mild non-specific muscle findings, brain pathology featured the stereotypic triad of necrotising grey matter lesions with relative preservation of neurons in the aforementioned regions, multiple cerebral infarcts, and severe patchy Purkinje-cell degeneration in the cerebellar vermis. Two dogs revealed a reduced activity of respiratory-chain-complexes I and IV. Genetic analyses obtained a neutral tRNA-Leu(UUR) A-G-transition only. Neuropathologically, SNE in Yorkshire Terriers is nearly identical to the Alaskan Husky form and very similar to human LS. This study, for the first time, demonstrated that canine SNE can be associated with a combined respiratory chain defect. Mitochondrial tRNA mutations and large genetic rearrangements were excluded as underlying aetiology. Further studies, amongst relevant candidates, should focus on nuclear encoded transcription and translation factors.

Leigh's disease associated with a dorsal midbrain syndrome

Leigh's disease is a rare progressive neurodegenerative condition caused by a mitochondrial cytopathy. The authors present the case of a 9-year-old girl with dorsal midbrain syndrome causing convergence retraction nystagmus. Magnetic resonance imaging, skin biopsy, and genetic testing confirmed the cause to be Leigh's disease due to two SURF1 mutations. To the authors' knowledge, this is the first reported case of dorsal midbrain syndrome caused by a mitochondrial cytopathy.

Assembly of the oxidative phosphorylation system in humans: what we have learned by studying its defects

Assembly of the oxidative phosphorylation (OXPHOS) system in the mitochondrial inner membrane is an intricate process in which many factors must interact. The OXPHOS system is composed of four respiratory chain complexes, which are responsible for electron transport and generation of the proton gradient in the mitochondrial intermembrane space, and of the ATP synthase that uses this proton gradient to produce ATP. Mitochondrial human disorders are caused by dysfunction of the OXPHOS system, and many of them are associated with altered assembly of one or more components of the OXPHOS system. The study of assembly defects in patients has been useful in unraveling and/or gaining a complete understanding of the processes by which these large multimeric complexes are formed. We review here current knowledge of the biogenesis of OXPHOS complexes based on investigation of the corresponding disorders.

Severe infantile encephalomyopathy caused by a mutation in COX6B1, a nucleus-encoded subunit of cytochrome c oxidase

Cytochrome c oxidase (COX) deficiency, one of the most common respiratory-chain defects in humans, has been associated with mutations in either mitochondrial DNA genes or nucleus-encoded proteins that are not part in but promote the biogenesis of COX. Mutations of nucleus-encoded structural subunits were sought for but never found in COX-defective patients, leading to the conjecture that they may be incompatible with extra-uterine survival. We report a disease-associated mutation in one such subunit, COX6B1. Nuclear-encoded COX genes should be reconsidered and included in the diagnostic mutational screening of human disorders related to COX deficiency.

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.

Novel mitochondrial mutation in the ND4 gene associated with Leigh syndrome

We analyzed the complete mitochondrial genome of a 3-month-old female child with basal ganglionic lesions and other clinical features suggestive of Leigh syndrome, which is caused by variations in mitochondrial and nuclear genes. Our study revealed a novel, homoplasmic T11984C missense mutation in ND4 gene, which replaces a highly conserved amino acid tyrosine with histidine. Computational analysis showed that this mutation alters the secondary structure of ND4 subunit. As the mutation observed in this study was novel and homoplasmic, we speculate that there could be interplay of this mitochondrial mutation along with nuclear gene(s) in the pathogenesis.

Supercomplexes and subcomplexes of mitochondrial oxidative phosphorylation

Dimerization or oligomerization of ATP synthase has been proposed to play an important role for mitochondrial cristae formation and to be involved in regulating ATP synthase activity. We found comparable oligomycin-sensitive ATPase activity for monomeric and oligomeric ATP synthase suggesting that oligomerization/monomerization dynamics are not directly involved in regulating ATP synthase activity. Binding of the natural IF1 inhibitor protein has been shown to induce dimerization of F1-subcomplexes. This suggested that binding of IF1 might also dimerize holo ATP synthase, and possibly link dimerization and inhibition. Analyzing mitochondria of human rho zero cells that contain mitochondria but lack mitochondrial DNA, we identified three subcomplexes of ATP synthase: (i) F1 catalytic domain, (ii) F1-domain with bound IF1, and (iii) F1-c subcomplex with bound IF1 and a ring of subunits c. Since both IF1 containing subcomplexes were present in monomeric state and exhibited considerably reduced ATPase activity as compared to the third subcomplex lacking IF1, we postulate that inhibition and induction of dimerization of F1-subcomplexes by IF1 are independent events. F1-subcomplexes were also found in mitochondria of patients with specific mitochondrial disorders, and turned out to be useful for the clinical differentiation between various types of mitochondrial biosynthesis disorders. Supramolecular associations of respiratory complexes, the "respirasomes", seem not to be the largest assemblies in the structural organization of the respiratory chain, as suggested by differential solubilization of mitochondria and electron microscopic analyses of whole mitochondria. We present a model for a higher supramolecular association of respirasomes into a "respiratory string".

Magnetic resonance imaging and pathologic findings associated with necrotizing encephalitis in two Yorkshire terriers

Two young adult Yorkshire terriers had neurologic signs consistent with forebrain and brainstem involvement or forebrain involvement alone. On magnetic resonance imaging studies there were asymmetric bilateral lesions mainly in the cerebral cortex, and in the diencephalon. These areas were hyperintense on T2-weighted and FLAIR images, but hypointense or isointense on T1-weighted images. Lesions had a varying degree of contrast enhancement. Areas which were isointense on T1-weighted images had no contrast enhancement or only foci of contrast enhancement. Lesions with hypointensity in T1-weighted images had no enhancement or more frequently ring-like enhancement around the lesion. Necrotizing encephalitis was confirmed pathohistologically in both dogs. The degree of contrast enhancement appeared to be related to the degree of lymphohistiocytic inflammation on histologic examination.

Neuropathologic aspects of cytochrome C oxidase deficiency

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

The role of mitochondria in the pathogenesis of neurodegenerative diseases

A growing body of evidence indicates that mitochondrial dysfunction may play an important role in the pathogenesis of many neurodegenerative disorders. Because mitochondrial metabolism is not only the principal source of high energy intermediates, but also of free radicals, it has been suggested that inherited or acquired mitochondrial defects could be the cause of neuronal degeneration as a consequence of energy defects and oxidative damage. Mitochondrial respiratory chain dysfunction has been reported in association with primary mitochondrial DNA abnormalities, and also as a consequence of mutations in nuclear genes directly involved in mitochondrial functions, such as SURF1, frataxin, and paraplegin. Defects of oxidative phosphorylation and increased free radical production have also been observed in diseases that are not due to primary mitochondrial abnormalities. In these cases, the mitochondrial dysfunction is likely to be an epiphenomenon, which, nevertheless, could be of importance in precipitating a cascade of events leading to cell death. In either case, understanding the role of mitochondria in the pathogenesis of neurodegenerative diseases could be important for the development of therapeutic strategies in these disorders.

The significance of reduced respiratory chain enzyme activities: clinical, biochemical and radiological associations

BACKGROUND

Mitochondrial diseases are an important group of neurometabolic disorders in children with varied clinical presentations and diagnosis that can be difficult to confirm.

AIM

To report the significance of reduced respiratory chain enzyme (RCE) activity in muscle biopsy samples from children.

METHODS

Retrospective odds ratio was used to compare clinical and biochemical features, DNA studies, neuroimaging, and muscle biopsies in 18 children with and 48 without reduced RCE activity.

RESULTS

Children with reduced RCE activity were significantly more likely to have consanguineous parents, to present with acute encephalopathy and lactic acidaemia and/or within the first year of life; to have an axonal neuropathy, CSF lactate >4 mmol/l; and/or to have signal change in the basal ganglia. There were positive associations with a maternal family history of possible mitochondrial cytopathy; a presentation with failure to thrive and lactic acidaemia, ragged red fibres, reduced fibroblast fatty acid oxidation and with an abnormal allopurinol loading test. There was no association with ophthalmic abnormalities, deafness, epilepsy or myopathy.

CONCLUSION

The association of these clinical, biochemical and radiological features with reduced RCE activity suggests a possible causative link.

Mechanisms of unexpected death and autopsy findings in Leigh syndrome (subacute necrotising encephalomyelopathy)

A 21-year-old previously-well woman who was undergoing medical investigations for problems with balance and suspected multiple sclerosis, developed a headache and breathing difficulties, and died suddenly and unexpected at home. The autopsy was unremarkable except for pulmonary and cerebral oedema. However, subsequent microscopy of the brain revealed characteristic features of Leigh syndrome with multifocal areas of astrogliosis, capillary proliferation, and parenchymal vacuolation. While Leigh syndrome is more commonly diagnosed in infancy, manifestations may occur throughout early life into adulthood. Sudden and unexpected death is a rare presentation that may be associated with cerebral necrosis and oedema. An awareness of the variable manifestations of Leigh syndrome is necessary in forensic practice as not all cases will present in a typical manner and sudden death may occur before a diagnosis has been established. The heritable nature of this condition makes accuracy of diagnosis essential.

Subcomplexes of human ATP synthase mark mitochondrial biosynthesis disorders

OBJECTIVE

METHODS

We describe biochemically and clinically relevant aspects of mitochondrial ATP synthase, the enzyme that supplies most ATP for the cells energy demand.

RESULTS

Analyzing human Rho zero cells we could identify three subcomplexes of ATP synthase: F1 catalytic domain, F1 domain with bound natural IF1 inhibitor protein, and F1-c subcomplex, an assembly of F1 domain and a ring of F(O)-subunits c. Large amounts of F1 subcomplexes accumulated also in mitochondria of patients with specific mitochondrial disorders. By quantifying the F1 subcomplexes and other oxidative phosphorylation complexes in parallel, we were able to discriminate three classes of defects in mitochondrial biosynthesis, namely, mitochondrial DNA depletion, mitochondrial transfer RNA (tRNA) mutations, and mutations in the mitochondrial ATP6 gene.

INTERPRETATION

The relatively simple electrophoretic assay used here is a straightforward approach to differentiate between various types of genetic alterations affecting the biosynthesis of oxidative phosphorylation complexes and will be useful to guide molecular genetic diagnostics in the field of mitochondrial neuromuscular disorders.

[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.

Mss51p and Cox14p jointly regulate mitochondrial Cox1p expression in Saccharomyces cerevisiae

Mutations in SURF1, the human homologue of yeast SHY1, are responsible for Leigh's syndrome, a neuropathy associated with cytochrome oxidase (COX) deficiency. Previous studies of the yeast model of this disease showed that mutant forms of Mss51p, a translational activator of COX1 mRNA, partially rescue the COX deficiency of shy1 mutants by restoring normal synthesis of the mitochondrially encoded Cox1p subunit of COX. Here we present evidence showing that Cox1p synthesis is reduced in most COX mutants but is restored to that of wild type by the same mss51 mutation that suppresses shy1 mutants. An important exception is a null mutation in COX14, which by itself or in combination with other COX mutations does not affect Cox1p synthesis. Cox14p and Mss51p are shown to interact with newly synthesized Cox1p and with each other. We propose that the interaction of Mss51p and Cox14p with Cox1p to form a transient Cox14p-Cox1p-Mss51p complex functions to downregulate Cox1p synthesis. The release of Mss51p from the complex occurs at a downstream step in the assembly pathway, probably catalyzed by Shy1p.

Variable penetrance of a familial progressive necrotising encephalopathy due to a novel tRNA(Ile) homoplasmic mutation in the mitochondrial genome

INTRODUCTION

We present a family comprising a clinically normal mother and two daughters, each with severe encephalopathy with onset in late childhood. A third daughter had died previously of an earlier onset but neuropathologically similar disease.

METHODS

Sequence analysis of the entire mtDNA was carried out in muscle, fibroblasts, and lymphocytes of the affected daughters and unaffected mother. Biochemical analysis of individual respiratory chain enzymes was performed on the same tissues, and on several transmitochondrial cybrid clones containing the nucleus of a 143B.206 osteosarcoma cell line and the mutant mtDNA.

RESULTS

Genetic analyses revealed in both daughters and mother the presence of a novel mutation in the tRNA(Ile) gene of mtDNA, which was homoplasmic in fibroblasts, lymphocytes, and skeletal muscle of the two patients. It was also homoplasmic in fibroblast and skeletal muscle samples of the mother, and approximately 97% heteroplasmic in her lymphocytes. Combined defects of complexes I and IV of the mitochondrial respiratory chain were found not only in fibroblasts of the two probands, but surprisingly also in those of their clinically unaffected mother. The respiratory chain defect segregated in transmitochondrial cybrids containing the nucleus of a 143B.206 osteosarcoma cell line and the mutant mtDNA, indicating that the latter was responsible for the biochemical phenotype.

DISCUSSION

Our results support the concept that homoplasmic mutations in tRNA genes can be responsible for mitochondrial disorders characterised by extremely variable penetrance. Albeit still unexplained, this phenomenon has important consequences in the nosological characterisation, clinical management, and genetic counselling of mitochondrial disorders.

Mitochondrial encephalomyopathies: diagnostic approach

Mitochondrial diseases have extremely heterogeneous clinical presentations due to the ubiquitous nature of mitochondria and the dual genetic control of the respiratory chain. Thus, mitochondrial disorders can be multisystemic (mitochondrial encephalomyopathies) or confined to a single tissue, and they can be sporadic or transmitted by mendelian or maternal inheritance. Mendelian disorders are usually inherited as autosomal recessive traits, tend to present earlier in life, and usually "breed true" in each family. By contrast, mitochondrial DNA-related diseases usually start later and vary in their presentation within members of the same family. Precise diagnosis is often a challenge; we go through the traditional steps of the diagnostic process, trying to highlight clues to mitochondrial dysfunction in the family history, physical and neurological examinations, routine and special laboratory tests, and histo-chemical and biochemical results of the muscle biopsy. The ultimate goal is to reach, whenever possible, a definitive molecular diagnosis, which permits rational genetic counseling and a prenatal diagnosis.

A mitochondrial ATPase 6 mutation is associated with Leigh syndrome in a family and affects proton flow and adenosine triphosphate output when modeled in Escherichia coli

A multidisciplinary strategy was used to identify the molecular defect in a family with Leigh syndrome (LS). The propositus presented severe developmental delay, an ataxic-spastic gait and seizures. She died at 3.5 y of age from cardiorespiratory arrest. Postmortem examination disclosed pathological features typical of LS. A 12-y-old sister is affected with the same disease. Respiratory chain enzyme complex activities in skeletal muscle biopsy were normal. Adenosine triphosphate (ATP) synthesis during oxidative phosphorylation in skin fibroblasts mitochondria showed a severely hampered ATP production. Mitochondrial DNA sequencing revealed a new mutation in the ATPase 6 gene (T9176G). Site-directed mutagenesis in Escherichia coli strains was used to measure H+ pumping and ATP synthesis. Results were comparable to findings obtained in human cells. These data corroborate the use of E. coli strains as a feasible "animal" model for functional studies in pathogenic mutations of the ATPase 6 gene.

A zebrafish model for pyruvate dehydrogenase deficiency: rescue of neurological dysfunction and embryonic lethality using a ketogenic diet

Defects in the pyruvate dehydrogenase (PDH) complex result in severe neurological dysfunction, congenital lactic acidosis, growth retardation, and early death. Current treatments for PDH deficiency are administered postnatally and are generally unsuccessful. Because many patients with this disease are born with irreversible defects, a model system for the development of effective pre- and postnatal therapies would be of great value. In a behavioral genetic screen aimed to identify zebrafish with visual function defects, we previously isolated two alleles of the recessive lethal mutant no optokinetic response a (noa). Here we report that noa is deficient for dihydrolipoamide S-acetyltransferase (Dlat), the PDH E2 subunit, and exhibits phenotypes similar to human patients with PDH deficiency. To rescue the deficiency, we added ketogenic substrates to the water in which the embryos develop. This treatment successfully restored vision, promoted feeding behavior, reduced lactic acidosis, and increased survival. Our study demonstrates an approach for establishing effective therapies for PDH deficiency and other congenital diseases that affect early embryonic development.

Patterned Purkinje cell death in the cerebellum

The object of this review is to assemble much of the literature concerning Purkinje cell death in cerebellar pathology and to relate this to what is now known about the complex topography of the cerebellar cortex. A brief introduction to Purkinje cells, and their regionalization is provided, and then the data on Purkinje cell death in mouse models and, where appropriate, their human counterparts, have been arranged according to several broad categories--naturally-occurring and targeted mutations leading to Purkinje cell death, Purkinje cell death due to toxins, Purkinje cell death in ischemia, Purkinje cell death in infection and in inherited disorders, etc. The data reveal that cerebellar Purkinje cell death is much more topographically complex than is usually appreciated.

NovelSURF1 mutation in a child with subacute encephalopathy and without the radiological features of Leigh Syndrome

Mutations in SURF1, a gene involved in cytochrome-c oxidase (COX) assembly, cause COX deficiency and Leigh Syndrome (LS). Typical presentation is in the first year of life, with failure to thrive, psychomotor regression, ataxia, signs of brainstem dysfunction, and peripheral neuropathy. Progression is rapid and patients usually die of respiratory failure before 2 years of age. LS is characterized by symmetrical bilateral lesions in the brainstem and basal ganglia, revealed premortem as signal hyperintensities in T2-weighted MRI imaging. Here, we describe a 10-year-old boy with a novel mutation in SURF1 associated with an unusually mild clinical course. At 39 months, there were no MRI lesions, and a follow-up MRI at 8 years of age showed only brainstem and cerebellar involvement without lesions in the basal ganglia or subthalamic nuclei. These data confirm that the spectrum of MRI findings in LS is variable and that SURF1 mutations should be considered in patients with encephalomyopathy and COX deficiency even when early MRI findings are negative.

Leigh disease with mitochondrial DNA A8344G mutation: case report and brief review

Leigh disease, subacute necrotizing encephalomyelopathy, is a neurodegenerative disorder often seen in infancy or childhood but rarely reported in adults. Genetic heterogeneity is well recognized, and the associated etiologies include both mitochondrial and nuclear DNA defects. We describe an infant presenting with developmental delay and then progressive multisystem disorder and neuroradiologic features of Leigh disease. He and his maternal relatives all have the A8344G mitochondrial DNA mutation. However, only minor clinical features are seen in his maternal relatives, with migraine being the most common problem. Additionally the A8344G mitochondrial DNA mutation is associated with spinocerebellar degeneration, other nonspecific mitochondrial encephalomyopathies, atypical Charcot-Marie-Tooth disease, and progressive external ophthalmoplegia. The A8344G mitochondrial DNA mutation may present with Leigh disease or other different atypical clinical features without myoclonic epilepsy and ragged red fibers.

Coenzyme Q-responsive Leigh's encephalopathy in two sisters

A 31-year-old woman had encephalopathy, growth retardation, infantilism, ataxia, deafness, lactic acidosis, and increased signals of caudate and putamen on brain magnetic resonance imaging. Muscle biochemistry showed succinate:cytochrome c oxidoreductase (complex II-III) deficiency. Both clinical and biochemical abnormalities improved remarkably with coenzyme Q10 supplementation. Clinically, when taking 300mg coenzyme Q10 per day, she resumed walking, gained weight, underwent puberty, and grew 20cm between 24 and 29 years of age. Coenzyme Q10 was markedly decreased in cerebrospinal fluid, muscle, lymphoblasts, and fibroblasts, suggesting the diagnosis of primary coenzyme Q10 deficiency. An older sister has similar clinical course and biochemical abnormalities. These findings suggest that coenzyme Q10 deficiency can present as adult Leigh's syndrome.

Mitochondrial diseases

Since the first reports of disorders associated with mitochondrial DNA (mtDNA) defects more than a decade ago, the small mtDNA circle has been a Pandora's box of pathogenic mutations associated with human diseases. The "morbidity map" of mtDNA has gone from one point mutation and a few deletions in 1988 to more than 110 point mutations as of September, 2001. Nuclear DNA defects affecting mitochondrial function and mtDNA replication and integrity have also been identified in the past few years and more are expected. As a result, human "mitochondrial" diseases have evolved beyond the novelty diagnoses of a decade ago into an important area of medicine, and thus, the diagnostic principles of these disorders ought to be familiar to the clinician. In this article, the authors, we summarize the principles of mitochondrial genetics and discuss the common phenotypes, general diagnostic approach, and possible therapeutic venues for these fascinating disorders.

Dichloroacetate therapy in Leigh syndrome with a mitochondrial T8993C mutation

A 6-year-old female with Leigh syndrome associated with a T-to-C mutation at nucleotide 8993 of mitochondrial deoxyribonucleic acid (T8993C) was treated with dichloroacetate, once during the first acute deterioration after a febrile illness and another time when she demonstrated subacute regression without precipitating events. Dichloroacetate reversed the clinical course on both occasions, and diffuse lesions in the midbrain revealed on magnetic resonance imaging during the second episode disappeared completely. However, dichloroacetate could not prevent the second acute deterioration associated with a febrile illness that occurred during the second treatment. Thus dichloroacetate treatment, although limited, was effective for T8993C-associated Leigh syndrome.

The genetics of primary dystonias and related disorders

Dystonias are a heterogeneous group of disorders which are known to have a strong inherited basis. This review details recent advances in our understanding of the genetic basis of dystonias, including the primary dystonias, the 'dystonia-plus' syndromes and heredodegenerative disorders. The review focuses particularly on clinical and genetic features and molecular mechanisms. Conditions discussed in detail include idiopathic torsion dystonia (DYT1), focal dystonias (DYT7) and mixed dystonias (DYT6 and DYT13), dopa-responsive dystonia, myoclonus dystonia, rapid-onset dystonia parkinsonism, Fahr disease, Aicardi-Goutieres syndrome, Hallervorden-Spatz syndrome, X-linked dystonia parkinsonism, deafness-dystonia syndrome, mitochondrial dystonias, neuroacanthocytosis and the paroxysmal dystonias/dyskinesias.

Mutations in human nuclear genes encoding for subunits of mitochondrial respiratory complex I: the NDUFS4 gene

Among the mitochondrial disorders, complex I deficiencies are encountered frequently. Although some complex I deficiencies have been associated with mitochondrial DNA mutations, in the majority of the complex I-deficient patients mutations of nuclear genes are expected. This review attempts to summarize genetic defects affecting nuclear encoded subunits of complex I reported to date focusing on those found in the NDUFS4 gene. NDUFS4 product is 18 kDa protein which appears to have a dual role in complex I, at least: cAMP-dependent phosphorylation activates the complex; non-sense mutation of NDUFS4 prevents normal assembly of a functional complex in the inner mitochondrial membrane.

Three novel SURF-1 mutations in Japanese patients with Leigh syndrome

Leigh syndrome, a severe neurodegenerative disorder, commonly is associated with cytochrome c oxidase deficiency. Recent studies in white patients indicate that SURF-1 gene mutations can cause Leigh syndrome associated with cytochrome c oxidase deficiency. When we measured cytochrome c oxidase activity in cultured lymphoblastoid cells from our Japanese patients with typical Leigh syndrome, three patients demonstrated cytochrome c oxidase deficiency. Three novel mutations of the SURF-1 gene were identified in two of these three patients with cytochrome c oxidase deficiency. All mutations predicted loss of function of the SURF-1 protein; in both patients' cells, cytochrome c oxidase activity was decreased to less than 20% of the control mean. These results indicate that cultured lymphoblastoid cells are useful for elucidating the etiology of Leigh syndrome, and that loss of function of the SURF-1 gene product can be responsible for Leigh syndrome associated with severe cytochrome c oxidase deficiency in Japanese patients.

A novel mutation in the SURF1 gene in a child with Leigh disease, peripheral neuropathy, and cytochrome-c oxidase deficiency

We report a 16-month-old boy with psychomotor regression, muscle hypotonia, peripheral neuropathy, and lactic acidosis. Brain magnetic resonance imaging showed a bilateral abnormal signal in the substantia nigra and in the subthalamic nucleus, suggestive of Leigh disease. Histochemical analysis of skeletal muscle showed decreased cytochrome-c oxidase activity. Biochemical analysis of respiratory chain enzymes in muscle homogenate and in cultured fibroblasts showed isolated cytochrome-c oxidase deficiency. Western blot analysis in fibroblasts showed the absence of Surf1 protein. Genetic analysis of the SURF1 gene revealed that the patient was compound heterozygous for a previously reported mutation at the splice-junction site of intron 3 (240 + 1G > T), and for a novel 4-bp deletion in exon 6 (531_534delAAAT). Our data further enlarge the spectrum of mutations in SURF1 gene in patients with Leigh disease and cytochrome-c oxidase deficiency, contributing to better characterization of the clinical and neuroradiologic features of this group of patients for genotype-phenotype correlations.

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

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

Neuropathological features of mitochondrial disorders

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

The expanding spectrum of nuclear gene mutations in mitochondrial disorders

Our understanding of the molecular basis of mitochondrial disorders has come primarily from the discovery of an expanding number of mutations of mtDNA. However, a variety of recent observations indicate that many syndromes are due to abnormalities in nuclear genes related to oxidative-phosphorylation (OXPHOS). Nuclear genes encode hundreds of proteins involved in mitochondrial OXPHOS. Nevertheless, the identification of these genes has proceeded at a much slower pace, compared with the discovery and characterization of mtDNA mutations. This scenario is rapidly changing, thanks to the discovery of several OXPHOS-related human genes, and to the identification of mutations responsible for different clinical syndromes.

Cytochrome c oxidase partial deficiency-associated Leigh disease presenting as an extrapyramidal syndrome

Leigh disease is a subacute neurodegenerative disorder characterized by symmetric necrotic lesions in the basal ganglia, cerebellum, thalamus, brain stem, and optical nerves and caused by altered oxidative phosphorylation. We describe the clinical, biochemical, neuroimaging, and molecular studies of a 19-year-old boy with early-onset Leigh disease manifesting as severe extrapyramidal disorder with generalized dystonia and choreoathetosis. He was born of healthy parents after an uneventful pregnancy and delivery. At the age of 2 1/2 years, after a minor respiratory infection, he developed unstable, broad-based gait and tremor of the hands. These symptoms persisted for the next several years, when ataxia became more prominent. Difficulty in swallowing, dysarthria, trunk dystonia, and marked dyskinesia of the arms and hands gradually developed. Nystagmus, transient ptosis, and strabismus also appeared. Abnormal laboratory findings included elevated plasma and cerebrospinal fluid lactate and pyruvate, with an abnormal lactate/pyruvate ratio. Cranial computed tomography and magnetic resonance imaging demonstrated signs of cerebellar atrophy, bilateral and symmetric hypodensities in the lentiform nucleus and thalamus, and transient hyperintensities of cerebral peduncles in T2-weighted sequences suggestive of Leigh disease. Muscle biopsy revealed isolated fiber atrophy, necrotic fibers undergoing phagocytosis, and no ragged-red fibers. The measured catalytic activity of cytochrome c oxidase in skeletal muscle homogenates demonstrated a partial cytochrome c oxidase deficiency No abnormalities in the mitochondrial genome and in the SURF-1 gene were found. The boy is currently receiving levodopa therapy, creatine monohydrate, and a high dosage of thiamine and lipoic acid, his condition is stabilized, and extrapyramidal symptoms are less pronounced.