Garry K. Brown

SURF1, encoding a factor involved in the biogenesis of cytochrome c oxidase, is mutated in Leigh syndrome

Leigh Syndrome (LS) is a severe neurological disorder characterized by bilaterally symmetrical necrotic lesions in subcortical brain regions that is commonly associated with systemic cytochrome c oxidase (COX) deficiency. COX deficiency is an autosomal recessive trait and most patients belong to a single genetic complementation group. DNA sequence analysis of the genes encoding the structural subunits of the COX complex has failed to identify a pathogenic mutation. Using microcell-mediated chromosome transfer, we mapped the gene defect in this disorder to chromosome 9q34 by complementation of the respiratory chain deficiency in patient fibroblasts. Analysis of a candidate gene (SURF1) of unknown function revealed several mutations, all of which predict a truncated protein. These data suggest a role for SURF1 in the biogenesis of the COX complex and define a new class of gene defects causing human neurodegenerative disease.

Mutations in the X-linked pyruvate dehydrogenase (E1) α subunit gene (PDHA1) in patients with a pyruvate dehydrogenase complex deficiency

Defects in the pyruvate dehydrogenase (PDH) complex are an important cause of primary lactic acidosis, a frequent manifestation of metabolic disease in children. Clinical symptoms can vary considerably in patients with PDH complex deficiencies, and almost equal numbers of affected males and females have been identified, suggesting an autosomal recessive mode of inheritance of the disease. However, the great majority of PDH complex deficiencies result from mutations in the X‐linked pyruvate dehydrogenase (E1) α subunit gene (PDHA1). The major factors that contribute to the clinical variation in E1α deficiency and its resemblance to a recessive disease are developmental lethality in some males with severe mutations and the pattern of X‐inactivation in females.

Comparison of the relative levels of the 3243 (A-->G) mtDNA mutation in heteroplasmic adult and fetal tissues.

In this report, levels of the 3243 A to G mtDNA mutation associated with the mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome were measured in different heteroplasmic tissues of subjects in a kindred including adults with variable clinical phenotypes and a fetus. The relative proportions of mutant mtDNA varied widely (0.03 to 0.67) between identical tissues of the six different subjects and between different tissues of the same subjects. In the one adult for whom sufficient data were available there was an apparent correlation between the distribution of mutant mtDNA and clinical presentation. A woman without neurological symptoms who died prematurely with a cardiomyopathy and lactic acidosis had higher proportions of mutant in heart (0.49, SD 0.02), skeletal muscle (0.56, SD 0.01), and liver (0.55, SD 0.12) than in other tissues studied (for example, kidney, 0.03, SD 0.01). A strikingly different result was found in a 24 week old fetus in whom there was little variation in heteroplasmy in different tissues (average proportion of mutant mtDNA in six tissues, 0.53, SD 0.02). These observations add cardiomyopathy to the growing list of presenting features of the 3243 mtDNA mutation. The unique results from the fetus suggest also that selection pressures acting on either wild type or 3243 mutant mtDNA (rather than variation from replicative segregation of the heteroplasmic mtDNA) may be responsible primarily for the variable levels of 3243 mutant mtDNA in different heteroplasmic tissues of adults.

Variation in mitochondrial DNA levels in muscle from normal controls. Is depletion of mtDNA in patients with mitochondrial myopathy a distinct clinical syndrome

SummaryRecent studies have identified a group of patients with cytochrome oxidase (COX) deficiency presenting in infancy associated with a deficiency of mtDNA in muscle or other affected tissue (Moraes et al 1991). We used a novel approach to compare the level of mitochondrial (mtDNA) compared to nuclear DNA in skeletal muscle from a group of patients and controls, based on dot blots that were hybridized with a mtDNA probe labelled with35S[dCTP] and a reference nuclear DNA probe labelled with [32P]dCTP.The ratio of mtDNA to nuclear DNA varied in samples from different muscles of the same individual. Secondly, fetal muscle had very low levels of mtDNA compared to nuclear DNA, and data from older controls (cross-sectional rather than sequential) suggest that this increases rapidly over the first 3 months after birth and thereafter more slowly. Four patients with COX deficiency had levels of mtDNA that were below the age-specific range defined by ‘normal’ quadriceps muscle. The clinical features of two of these patients were similar to earlier case reports of mtDNA depletion. In three patients the clinical course was relatively benign compared to cases that have previously been described.Levels of mtDNA in skeletal muscle from some patients with other forms of muscle disease were also found to be low, suggesting that mtDNA depletion, possibly related to depletion of mitochondria, may be a relatively non-specific response of muscle to various pathological processes. However, there does appear to be a distinctive group of young patients with reduced cytochrome oxidase activity in muscle, in whom marked mtDNA depletion reflects the primary defect.

Cerebral dysgenesis and lactic acidemia: An MRI/MRS phenotype associated with pyruvate dehydrogenase deficiency

Pyruvate dehydrogenase complex (PDHC) is an intramitochondrial multienzyme complex essential for the aerobic oxidation of glucose. The majority of patients with PDHC deficiency have abnormalities in the major catalytic and regulatory subunit, E1 alpha, which is encoded on the X chromosome. The clinical spectrum of PDHC deficiency is heterogeneous, particularly in heterozygous females, and diagnosis may be difficult. Three affected infant girls with PDHC deficiency were investigated. All had dysmorphic features, microcephaly with profound global developmental delay, and hypotonia. Systemic acidosis was absent, although serum lactate and pyruvate were abnormally elevated. Magnetic resonance imaging revealed hypoplasia of the corpus callosum in all patients. Proton magnetic resonance spectroscopy of brain revealed large increases in relative signal intensities for lactic acid and decreases in the relative signal intensities of N-acetylaspartate, a marker of neuronal damage or less. Phosphorus MRS of muscle revealed abnormally low phosphorylation potentials for all these patients, although the degree of abnormality was variable and not directly correlated with the amount of brain lactate. It is proposed that cerebral dysgenesis and cerebral lactic acidemia as shown by magnetic resonance imaging and proton magnetic resonance spectroscopy are useful diagnostic clues to PDHC deficiency, particularly in females in whom variable patterns of X-inactivation reduce sensitivity of laboratory diagnosis based on the biochemical studies of peripheral tissues. In addition, muscle bioenergetic abnormalities in conjunction with CNS dysfunction may contribute to profound hypotonia in this disorder.

A SURF1 gene mutation presenting as isolated leukodystrophy.

Mitochondrial respiratory chain defects are increasingly recognized in patients with leukodystrophy. We report the first case of leukodystrophy with systemic cytochrome oxidase deficiency caused by a loss of function mutation in the SURF1 gene in a 2‐year‐old girl presenting with failure to thrive, global neurodevelopmental regression, and lactic acidosis. Although all previously reported mutations in the SURF1 gene have been found in patients with cytochrome oxidase (COX)‐deficient Leigh syndrome, the phenotype associated with SURF1 protein deficiency should be extended to include leukodystrophy.

Neuropathology and pathogenesis of mitochondrial diseases

SummaryThe majority of patients with mitochondrial disease have significant neuropathology, with the most common features being spongiform degeneration, neuronal loss and gliosis. Although there is considerable overlap between different mitochondrial diseases, the nature and distribution of the lesions is sufficiently distinctive in some cases to suggest a specific diagnosis. On the other hand, a number of different defects in cerebral energy metabolism are associated with common patterns of neuropathology (e.g. Leigh syndrome), suggesting that there is a limited range of responses to this type of metabolic disturbance.There are many descriptions of neuropathological changes in patients with mitochondrial disease, but there has been remarkably little investigation of the underlying pathogenic mechanisms. Comparisons with other conditions of cerebral energy deprivation such as ischaemia/hypoxia and hypoglycaemia suggest a possible role for excitotoxicity initiated by excitatory amino acid neurotransmitters. An additional contributing factor may be peroxynitrite, which is formed from nitric oxide and the oxygen free radicals which accumulate with defects of the mitochondrial electron transport chain.Mitochondrial diseases are often characterized by episodes of neurological dysfunction precipitated by intercurrent illness. Depending on the severity of the metabolic abnormality, each of these episodes carries a risk of further neuronal death and the result is usually progressive accumulation of irreversible damage. The balance between reversible functional impairment and neuronal death during episodes of metabolic imbalance is determined by the effectiveness of various protective mechanisms which may act to limit the damage. These include protective metabolic shielding of neurons by astrocytes and suppression of electrical activity (and hence energy demands) by activation of ATP-gated ion channels. In addition, recent evidence suggests that lactic acid, the biochemical abnormality common to these conditions, may not be toxic at moderately high concentrations but may in fact be protective by reducing the sensitivity of neurons to excitotoxic mechanisms. Abnormal brain function is a major manifestation of the majority of disorders of mitochondrial energy metabolism. The neurological features of these diseases are extremely diverse and are associated with a correspondingly wide range of pathological changes. Defects have been defined in enzymes of the mitochondrial matrix and in components of the electron transport chain in the inner mitochondrial membrane and, as these all involve the central common pathways of energy generation in the cell, it is not surprising that there is considerable overlap in structural and functional abnormalities in the different conditions. However, there are some patterns of neuropathology which are characteristic for specific biochemical defects and these may be helpful for diagnosis, in addition to providing detailed information concerning the biological contribution of individual reactions.Although pathological lesions in the brain are a major feature of mitochondrial disease, there are many enzyme defects involving individual pathways of substrate oxidation in the mitochondria in which there may be remarkably little permanent structural damage to the brain. This is in spite of repeated episodes of metabolic decompensation during which neurological symptoms are prominent. In these cases, the neurological features are related to the intermittent accumulation of toxic intermediates (e.g. methylmalonic acid in methylmalonic aciduria) or reduced supply of metabolizable substrate (e.g. glucose in the fatty acidβ-oxidation defects). If unrecognized or untreated, these conditions may result in permanent structural damage, but this is seldom as extensive as that found in association with defects in the final common pathways of energy metabolism.

Molecular weight differences between human platelet and placental monoamine oxidase

Abstract The molecular weights of the active site subunits of human placental and platelet monoamine oxidase have been compared. These tissues were chosen as each appears to contain only one form of the enzyme, type A in placenta and type B in platelets. We found that the biochemical properties (substrate affinity and inhibitor sensitivity) clearly distinguish between the enzymes from these two sources, whether in the membrane-bound or detergent-solubilized state. The suggestion that such differences result from the existence of distinct molecular forms was strengthened by the observation that the active site subunits labelled specifically with [ 3 H]pargyline differ in their apparent molecular weights and in the electrophoretic pattern of their partial proteolytic digest products.

Clinical and genetic spectrum of pyruvate dehydrogenase deficiency: dihydrolipoamide acetyltransferase (E2) deficiency.

Pyruvate dehydrogenase deficiency is a major cause of primary lactic acidosis and neurological dysfunction in infancy and early childhood. Most cases are caused by mutations in the X‐linked gene for the E1α subunit of the complex. Mutations in DLAT, the gene encoding dihydrolipoamide acetyltransferase, the E2 core component of the complex, have not been described previously. We report two unrelated patients with pyruvate dehydrogenase deficiency caused by defects in the E2 subunit. Both patients are less severely affected than typical patients with E1α mutations and both have survived well into childhood. Episodic dystonia was the major neurological manifestation, with other more common features of pyruvate dehydrogenase deficiency, such as hypotonia and ataxia, being less prominent. The patients had neuroradiological evidence of discrete lesions restricted to the globus pallidus, and both are homozygous for different mutations in the DLAT gene. The clinical presentation and neuroradiological findings are not typical of pyruvate dehydrogenase deficiency and extend the clinical and mutational spectrum of this condition. Ann Neurol 2005;58:234–241

Pontocerebellar hypoplasia type 6: A British case with PEHO-like features†

Six subtypes of autosomal recessive pontocerebellar hypoplasia (PCH) have been identified and the genetic basis of four of these (PCH1, PCH2, PCH4, and PCH6) is known. PCH6 is associated with cerebral atrophy and multiple but variable respiratory chain defects in muscle and has been reported in one consanguineous Sephardic Jewish family. It is caused by mutations in the RARS2 gene which encodes mitochondrial arginine‐transfer RNA synthetase. Here we describe a female patient born to nonconsanguineous British parents. She presented in the neonatal period with increased respiratory rate, poor feeding and transiently elevated blood and CSF lactate levels. She went on to manifest profound developmental delay and severe microcephaly. Edema of the hands, feet, and face were suggestive of a PEHO‐like condition (progressive encephalopathy, edema, hypsarrhythmia and optic atrophy), although optic atrophy and hypsarrhythmia were absent. Cranial MRI at age 14 months showed generalized cerebral atrophy, thinning of the pons and gross atrophy and flattening of the cerebellar hemispheres. Muscle biopsies on two occasions were normal with normal respiratory chain studies. Despite the absence of respiratory chain defects, the phenotype was felt to be consistent with PCH6 and indeed two novel pathogenic RARS2 mutations were identified. Ours is the second report of PCH6 due to RARS2 mutations and demonstrates that respiratory chain abnormalities are not obligatory, whereas some features of PEHO might be present.