Transmitochondrial embryonic stem cells containing pathogenic mtDNA mutations are compromised in neuronal differentiation

Objectives:  Defects of the mitochondrial genome (mtDNA) cause a series of rare, mainly neurological disorders. In addition, they have been implicated in more common forms of movement disorders, dementia and the ageing process. In order to try to model neuronal dysfunction associated with mitochondrial disease, we have attempted to establish a series of transmitochondrial mouse embryonic stem cells harbouring pathogenic mtDNA mutations.

Materials and methods: Transmitochondrial embryonic stem cell cybrids were generated by fusion of cytoplasts carrying a variety of mtDNA mutations, into embryonic stem cells that had been pretreated with rhodamine 6G, to prevent transmission of endogenous mtDNA. Cybrids were differentiated into neurons and assessed for efficiency of differentiation and electrophysiological function.

Results:  Neuronal differentiation could occur, as indicated by expression of neuronal markers. Differentiation was impaired in embryonic stem cells carrying mtDNA mutations that caused severe biochemical deficiency. Electrophysiological tests showed evidence of synaptic activity in differentiated neurons carrying non‐pathogenic mtDNA mutations or in those that caused a mild defect of respiratory activity. Again, however, neurons carrying mtDNA mutations that resulted in severe biochemical deficiency had marked reduction in post‐synaptic events.

Conclusions:  Differentiated neurons carrying severely pathogenic mtDNA defects can provide a useful model for understanding how such mutations can cause neuronal dysfunction.

Human CIA30 is involved in the early assembly of mitochondrial complex I and mutations in its gene cause disease

In humans, complex I of the respiratory chain is composed of seven mitochondrial DNA (mtDNA)-encoded and 38 nuclear-encoded subunits that assemble together in a process that is poorly defined. To date, only two complex I assembly factors have been identified and how each functions is not clear. Here, we show that the human complex I assembly factor CIA30 (complex I intermediate associated protein) associates with newly translated mtDNA-encoded complex I subunits at early stages in their assembly before dissociating at a later stage. Using antibodies we identified a CIA30-deficient patient who presented with cardioencephalomyopathy and reduced levels and activity of complex I. Genetic analysis revealed the patient had mutations in both alleles of the NDUFAF1 gene that encodes CIA30. Complex I assembly in patient cells was defective at early stages with subunits being degraded. Complementing the deficiency in patient fibroblasts with normal CIA30 using a novel lentiviral system restored steady-state complex I levels. Our results indicate that CIA30 is a crucial component in the early assembly of complex I and mutations in its gene can cause mitochondrial disease.

The importance of liver biopsy in the investigation of possible mitochondrial respiratory chain disease

The diagnosis of mitochondrial respiratory chain deficiency is usually made by analysis of mitochondrial respiratory chain activity in muscle biopsy. We describe 4 patients in whom the diagnosis was based on mitochondrial respiratory chain deficiency in liver alone. In 3 patients, liver complex IV activity was deficient, and the 4th patient had liver complex I deficiency (relative to citrate synthase and complex II activity). The enzyme activities in skeletal muscle biopsies from these patients were normal or equivocal. The age at presentation and the neurological symptoms differed from one patient to another. All 3 patients with complex IV deficiency had non-specific white matter changes on brain MRI. None of the patients had clinical or biochemical evidence of liver disease. These findings illustrate the wide variety of presentations associated with liver mitochondrial respiratory chain deficiency. They also demonstrate the importance of mitochondrial respiratory chain enzyme analysis in liver, in addition to muscle, even in cases where the primary clinical deficit is neurological and there is no liver disease.

Respiratory chain enzyme analysis in muscle and liver

Respiratory Chain (RC) enzyme analysis remains the mainstay for diagnosis of children suspected of having a RC disorder. A previous international workshop suggested a set of criteria for the ideal approach to diagnosis but concluded that probably no single centre fulfilled all these criteria. Major practical issues relate to the range of tissues tested, whether frozen tissue biopsies can be used reliably, assay methods, difficulty in defining realistic reference ranges, and the lack of an external quality assurance scheme. We discuss these issues and describe our experience over the last decade with assaying RC enzymes in over 600 skeletal muscle and 300 liver biopsies from patients, a range of different controls (other known inborn errors, end-stage liver disease, post-mortem samples) and single donated normal muscle and liver samples assayed on more than 100 occasions over 5- to 10-year periods. Our experience is that 'sick' tissues have wider 'normal' ranges than 'healthy' tissues. Caution is therefore needed to ensure that secondary RC defects are not misdiagnosed as primary RC defects. We describe diagnostic criteria that integrate the results of RC enzyme assays with clinical, histological, metabolic and molecular investigations to determine whether the overall diagnostic certainty is possible, probable or definite.

Leigh disease caused by the mitochondrial DNA G14459A mutation in unrelated families

Leigh disease can be caused by defects of both nuclear and mitochondrially encoded genes. One mitochondrial DNA mutation, G14459A, has been associated with both respiratory chain complex I deficiency and Leber's hereditary optic neuropathy, with or without dystonia. Here, we report the occurrence of this mutation in 3 complex I-deficient patients from 2 separate pedigrees who presented with Leigh disease, with no evidence or family history of Leber's hereditary optic neuropathy or dystonia.

The mitochondrial DNA C3303T mutation can cause cardiomyopathy and/or skeletal myopathy

OBJECTIVE

Several mutations in mitochondrial DNA have been associated with infantile cardiomyopathy, including a C3303T mutation in the mitochondrial transfer RNA(Leu(UUR)) gene. Although this mutation satisfied generally accepted criteria for pathogenicity, its causative role remained to be confirmed in more families. Our objective was to establish the frequency of the C3303T mutation and to define its clinical presentation.

STUDY DESIGN

Families with cardiomyopathy and maternal inheritance were studied by polymerase chain reaction/restriction fragment length polymorphism analysis looking for the C3303T mutation.

RESULTS

We found the C3303T mutation in 8 patients from 4 unrelated families. In one, the clinical presentation was infantile cardiomyopathy; in the second family, proximal limb and neck weakness dominated the clinical picture for the first 10 years of life, when cardiac dysfunction became apparent; in the third family, 2 individuals presented with isolated skeletal myopathy and 2 others with skeletal myopathy and cardiomyopathy; in the fourth family, one patient had fatal infantile cardiomyopathy and the other had a combination of skeletal myopathy and cardiomyopathy.

CONCLUSIONS

Our findings confirm the pathogenicity of the C3303T mutation and suggest that this mutation may not be rare. The C3303T mutation should be considered in the differential diagnosis of skeletal myopathies and cardiomyopathy, especially when onset is in infancy.

Respiratory chain complex I deficiency: an underdiagnosed energy generation disorder

OBJECTIVE

To define the spectrum of clinical and biochemical features in 51 children with isolated complex I deficiency.

BACKGROUND

Mitochondrial respiratory chain defects are one of the most commonly diagnosed inborn errors of metabolism. Until recently there have been technical problems with the diagnosis of respiratory chain complex I defects, and there is a lack of information about this underreported cause of respiratory chain dysfunction.

METHODS

A retrospective review of clinical features and laboratory findings was undertaken in all diagnosed patients who had samples referred over a 22-year period.

RESULTS

Presentations were heterogeneous, ranging from severe multisystem disease with neonatal death to isolated myopathy. Classic indicators of respiratory chain disease were not present in 16 of 42 patients in whom blood lactate levels were normal on at least one occasion, and in 23 of 37 patients in whom muscle morphology was normal or nonspecific. Ragged red fibers were present in only five patients. Tissue specificity was observed in 19 of 41 patients in whom multiple tissues were examined, thus the diagnosis may be missed if the affected tissue is not analyzed. Nine patients had only skin fibroblasts available, the diagnosis being based on enzyme assay and functional tests. Modes of inheritance include autosomal recessive (suggested in five consanguineous families), maternal (mitochondrial DNA point mutations in eight patients), and possibly X-linked (slight male predominance of 30:21). Recurrence risk was estimated as 20 to 25%.

CONCLUSION

Heterogeneous clinical features, tissue specificity, and absence of lactic acidosis or abnormal mitochondrial morphology in many patients have resulted in underdiagnosis of respiratory chain complex I deficiency.

Respiratory chain complex III [correction of complex] in deficiency with pruritus: a novel vitamin responsive clinical feature

We report a child with an isolated complex III respiratory chain deficiency and global developmental delay who had severe pruritus with elevated plasma bile acid levels. A liver biopsy showed micronodular cirrhosis, and enzymologic evaluation demonstrated an isolated complex III deficiency in both liver and muscle. His pruritus improved and serum bile acid levels decreased after treatment with menadione and vitamin C.

HIV-1 protein Vpr causes gross mitochondrial dysfunction in the yeast Saccharomyces cerevisiae

The biological effects of the HIV-1 accessory protein, Vpr, have been studied in yeast expression systems. In our previous study [1], employing the pCUP1-vpr copper-inducible expression cassette, Vpr was shown to cause growth arrest and structural defects. In this study yeast constitutively expressing vpr, through elevated copy number and/or elevated transcription levels, displayed no growth arrest in fermentative growth conditions while Vpr was produced at much lower levels than in the inducible expression system. However, such cells were respiratory deficient and unable to utilise ethanol or glycerol as the sole carbon source. They exhibited gross mitochondrial dysfunction displayed in the loss of respiratory chain complex I, II, III, IV and citrate synthase activities. The effects on mitochondria required a C-terminal domain of Vpr that contains a conserved amino acid sequence motif HFRIGCRHSRIG. These results suggest that the widely observed phenomenon of 'Vpr-induced growth arrest' in human cells could be due to mitochondrial dysfunction.

Mitochondrial electron transport chain defect presenting as hypoglycemia

A profoundly deaf female infant was found to have hypoglycemia and lactic acidemia after an episode of decreased oral intake and vomiting. Electron transport chain (ETC) enzyme studies revealed a combination defect of complexes I, III, and IV in liver but not in skeletal muscle. This case highlights the fact that defects of the ETC are clinically highly heterogeneous and should be considered with hypoglycemia and lactic acidosis in the absence of a glycogen storage disorder. Moreover, ETC defects can occur with a biochemical profile suggestive of a fatty acid oxidation disorder.

Mitochondrial myopathy with tRNA(Leu(UUR)) mutation and complex I deficiency responsive to riboflavin

Deficiency of complex I (reduced nicotinamide adenine dinucleotide dehydrogenase-ubiquinone oxidoreductase) of the mitochondrial respiratory chain may be seen as a pure myopathy or as a neuromuscular disorder at presentation. Efficacy of long- term therapy for these disorders is yet to be established. We report the case of a female patient with complex I deficiency and skeletal myopathy, who has had a sustained clinical response to riboflavin during 3 years of therapy. Molecular studies found no mutations in the putative flavin mononucleotide binding site in the 51 kd subunit of complex I, but a T-to-C transition at nucleotide 3250 in the mitochondrial DNA tRNA(Leu(UUR)) gene was identified. This mutation has been reported in one other family in that five members had fatigue with or without muscle weakness. There were also five cases of unexplained infant deaths in that family and two cases in the family reported here. Riboflavin therapy should be attempted in all patients with complex I deficiency when the clinical presentation is one of isolated skeletal myopathy.

Towards the professional status of dental hygiene in Alberta

Presently, despite formal advances toward professional status and the acquisition of some professional attributes, dental hygiene may or may not be recognized as a profession. While dental hygienists in Alberta have a professional association that regulates the practice of dental hygiene, other professional attributes have yet to be attained. As the province's dental hygiene leaders begin to prepare recommendations to the Alberta government for the practice and direction of dental hygiene in Alberta, it is valuable for them to understand the different perceptions held by those affected. The study reported in this paper investigated the perceptions of Alberta dental hygienists and dentists regarding the professional status of dental hygiene in the province. Dental hygienists were selected to participate according to employment setting, while dentists were randomly selected. The questionnaire consisted of both open and closed ended questions, including Likert Scale items. Data were collected from questionnaires returned by 111 dental hygienists and 109 dentists. Two main points emerged from this study. The first is that although all dental hygienists recognize dental hygiene as an emerging profession, perceptions held by dental hygienists employed in private practice settings vary from those held by dental hygienists employed in traditional community care or alternative practice settings. The second recurrent theme is that dentists, while recognizing the expertise of dental hygienists, wish to retain economic control of dental hygiene. The article concludes by offering recommendations designed to increase the development of dental hygiene as a profession.

Leigh syndrome: clinical features and biochemical and DNA abnormalities

We investigated the etiology of Leigh syndrome in 67 Australian cases from 56 pedigrees, 35 with a firm diagnosis and 32 with some atypical features. Biochemical or DNA defects were determined in both groups, ie, 80% in the tightly defined group and 41% in the "Leigh-like" group. Eleven patients had mitochondrial DNA point mutations (nucleotide [nt] 8993 T to G, nt 8993 T to C, or nt 8344 A to G) and 1 Leigh-like patient had a heteroplasmic deletion. Twenty-nine patients had enzyme defects, ie, 13 respiratory chain complex I, 9 complex IV, and 7 pyruvate dehydrogenase complex (PDHC). Complex I deficiency is more common than recognized previously. Six PDHC-deficient patients had mutations in the X-chromosomal gene encoding the E1alpha subunit of PDHC. Parental consanguinity suggested autosomal recessive inheritance in two complex IV-deficient sibships. We found no strong correlation between the clinical features and basic defects. An assumption of autosomal recessive inheritance (frequently made in the past) would have been wrong in nearly one-half (11 of 28 tightly defined and 18 of 41 total patients) of those in whom a cause was found. A specific defect must be identified if reliable genetic counseling is to be provided.

Maternal tyrosinaemia II: management and successful outcome

A 25-year-old woman with tyrosinaemia type II was treated from the 5th week of pregnancy with a protein-restricted diet supplemented with a tyrosine/phenylalanine-free amino acid mixture. Tyrosine concentrations were maintained in the range 100-200 mumol/l by restricting natural protein intake to 0.16 g/kg per 24 h in early pregnancy, with increases up to 0.38 g/kg per 24 h in the last trimester. This treatment maintained plasma phenylalanine concentrations in the range 20-40 mumol/l. Maternal weight gain and fetal growth were normal, and the mother remained asymptomatic throughout the pregnancy. A normal infant was born at term with length, weight and head circumference between the 25-50th per centiles.

Pregnancy in phenylketonuria: dietary treatment aimed at normalising maternal plasma phenylalanine concentration

The transport characteristics of the placenta, which favour higher phenylalanine concentrations in the fetus than in the mother, and regression data of head circumference at birth against phenylalanine concentration at conception in maternal phenylketonuria (PKU), suggest that treatment of maternal PKU should ideally aim to maintain plasma phenylalanine concentration within the normal range throughout pregnancy. A patient with classical PKU was treated from before conception by aiming to maintain plasma phenylalanine concentration within the range 50-150 mumol/l and tyrosine within the range 60-90 mumol/l. The diet was supplemented with phenylalanine-free amino acids (100-180 g/day) and tyrosine (0-5 g/day). Plasma amino acid concentrations were monitored weekly by amino acid analyser. Dietary phenylalanine intake ranged from 6 mg/kg/day at conception to 30 mg/kg/day at delivery. Normal weight gain and fetal growth were maintained throughout the pregnancy. A normal baby was born at term with a head circumference of 35.5 cm; at 1 year of age no abnormality is detectable. These results show that with careful monitoring and compliance it is possible, and may be advisable, to maintain plasma phenylalanine concentration within the normal range in the management of PKU pregnancy.

Continuous venovenous hemofiltration in the management of acute decompensation in inborn errors of metabolism

Continuous venovenous hemofiltration was used to treat two neonates, one with maple syrup urine disease and the other with an inborn error of long-chain fatty acid oxidation. The latter infant had hypoglycemia, hyperammonemia and lactic acidosis. In both cases, acceptable biochemical control was achieved within 8 hours. This therapy offers the potential to overcome acute crises rapidly in a wide range of inborn errors of intermediary metabolism.

Characterization of the mutations in three patients with pyruvate dehydrogenase E1 alpha deficiency

The human pyruvate dehydrogenase complex catalyses the oxidative decarboxylation of pyruvate to acetyl-CoA. Defects in several of the seven subunits have been reported, but the majority of mutations affect the E1 component and especially the E1 alpha subunit. However, the clinical presentation of patients with pyruvate dehydrogenase E1 alpha deficiency is extremely variable. Dependency of the brain on pyruvate dehydrogenase activity and localization of the gene for the somatic form of the pyruvate dehydrogenase E1 alpha subunit to the X chromosome provide the basis for a better understanding of the variation in the clinical manifestations. Further understanding of the function and interaction of subunits and the pathophysiology of pyruvate dehydrogenase deficiency necessitates the characterization of mutations in the pyruvate dehydrogenase complex. We report the analysis of three patients with pyruvate dehydrogenase E1 alpha deficiency. One female has a three base pair deletion which affects dephosphorylation of the subunit. Of two males analysed, one has a two base pair deletion causing a shift in the reading frame. The other has a base change, resulting in an Arg to His substitution. All three mutations are located near the carboxyl terminus of the subunit.

"Cerebral" lactic acidosis: defects in pyruvate metabolism with profound brain damage and minimal systemic acidosis

Six patients are described with a combination of early onset of neurological symptoms, gross cerebral changes and elevated concentrations of pyruvate and lactate in cerebrospinal fluid. Although at least five of the six patients appear to have a generalised defect in pyruvate metabolism, reflected in deficient pyruvate dehydrogenase activity in cultured fibroblasts, systemic acidosis was not a problem clinically and blood pyruvate and lactate concentrations were only slightly raised. The localisation of significant clinical and biochemical problems to the central nervous system, coupled with the difficulties in making the diagnosis if analysis of cerebrospinal fluid (CSF) is not performed, lead us to term this condition "cerebral" lactic acidosis.

Difficulties in assessing the effect of strychnine on the outcome of non-ketotic hyperglycinaemia. Observations on sisters with a mild T-protein defect

Sisters with a mild variant of non-ketotic hyperglycinaemia resulting from a defect in the T-protein of the glycine cleavage system had different clinical outcomes. The older sister was ascertained at 6 months of age because of mental retardation. She received only brief treatment with sodium benzoate from 11-15 months and at 15 years of age is profoundly retarded and has epilepsy. The younger sister was diagnosed 36 h after birth, was treated with strychnine, sodium benzoate and arginine from the neonatal period and at 27 months of age is only moderately retarded and free of seizures. The possible role of strychnine in the improved outcome is discussed.

Urinary-free amino acids in osteogenesis imperfecta

Free urinary amino acids were analyzed in patients with osteogenesis imperfecta to determine whether there were any abnormalities that could be used to improve the diagnosis and classification of this syndrome. The results obtained from 15 patients, who had either the type 1, 2, or 3 form of osteogenesis imperfecta, were compared to the values obtained from 115 age-matched controls. Elevated free amino acid levels were not found in any patient with osteogenesis imperfecta.