Genetic defects in human pyruvate dehydrogenase

Deletion at chromosomal band Xp22.12-Xp22.13 involving PDHA1 in a patient with congenital lactic acidosis

We present a patient with congenital lactic acidosis, agenesis of the corpus callosum, and profound developmental delay. Assays of pyruvate dehydrogenase complex function were normal in lymphocytes, but decreased in fibroblasts. Sequencing of the PDHA1 gene did not reveal deleterious mutations, and BAC based microarray analysis did not reveal any chromosomal abnormality. However, gene dosage analysis with oligonucleotide-based chromosomal microarray revealed a deletion of Xp22.12-Xp22.13 involving complete deletion of PDHA1. This is the first report of a whole gene deletion of PDHA1 detected by oligonucleotide-based microarray.

Pyruvate dehydrogenase deficiency as a result of splice-site mutations in the PDX1 gene

Mutations in the E3-binding protein component of pyruvate dehydrogenase complex have been demonstrated in a few cases of Leigh syndrome. We report that two mutations previously detected in the E3-binding protein cDNA are the consequence of splice-site mutations. Both involved a single base substitution in the conserved dinucleotides of splice junctions, one leading to skipping of an exon and the other, to activation of a cryptic site. Our findings add to the understanding of molecular basis of E3-binding protein deficiency and indicate yet again the high frequency of splicing mutations in this gene.

First prenatal diagnosis of defects in the HsPDX1 gene encoding protein X, an additional lipoyl-containing subunit of the human pyruvate dehydrogenase complex

We have previously reported a genetic study of a neonatal lactic acidosis linked to a pyruvate dehydrogenase complex deficiency due to the absence of the protein X subunit. This rare autosomal recessive disorder is associated with specific deletions in this polypeptide which is encoded by the HsPDX1 gene, located on chromosome 11p1.3. The pathology of the patient was considered to arise from a large homozygous deletion (78del85) found at the 5' end of the HsPDX1 coding sequence. Her heterozygous mother underwent prenatal diagnosis during a subsequent pregnancy. Chorionic villus samples were used for three independent studies: (1) normal levels of the protein X component of the PDH complex were detected by immunoblotting; (2) RT-PCR analysis showed no deletion at the 5' end of the cDNA but the presence of a distinct heterozygous deletion (965del59) at its 3' end inherited from the father; (3) haplotype analysis revealed the presence of the father's mutated allele and the mother's normal allele. It was concluded that the fetus was heterozygous for this separate 3' deletion, so, it was likely to be not affected. This study permitted us to characterize more precisely the genetic abnormalities of the HsPDX1 cDNA occurring in each family's member.

Reduced Ca2+ uptake by mitochondria in pyruvate dehydrogenase-deficient human diploid fibroblasts

Physiological and pathological Ca2+ loads are thought to be taken up by mitochondria via a process dependent on aerobic metabolism. We sought to determine whether human diploid fibroblasts from a patient with an inherited defect in pyruvate dehydrogenase (PDH) exhibit a decreased ability to sequester cytosolic Ca2+ into mitochondria. Mobilization of Ca2+ stores with bradykinin (BK) increased the cytosolic Ca2+ concentration ([Ca2+]c) to comparable levels in control and PDH-deficient fibroblasts. In normal fibroblasts transfected with plasmid DNA encoding mitochondrion-targeted apoaequorin, BK elicited an increase in Ca2(+)-dependent aequorin luminescence corresponding to an increase in the mitochondrial Ca2+ concentration ([Ca2+]mt) of 2.0 +/- 0.2 microM. The mitochondrial uncoupling agent carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone blocked the BK-induced [Ca2+]mt increase, although it did not affect the [Ca2+]c transient. Basal [Ca2+]c and [Ca2+]mt in control and PDH-deficient cells were similar. However, confocal imaging of the potential-sensitive dye JC-1 indicated that the percentage of highly polarized mitochondria was reduced from 30 +/- 1% in normal cells to 19 +/- 2% in the PDH-deficient fibroblasts. BK-elicited [Ca2+]mt transients in PDH-deficient cells were reduced to 4% of control, indicating that PDH-deficient mitochondria have a decreased ability to take up cytosolic Ca2+. Thus cells with compromised aerobic metabolism have a reduced capacity to sequester Ca2+.

Association of cerebral dysgenesis and lactic acidemia with X-linked PDH E1 alpha subunit mutations in females

We describe an infant girl who presented at age 4 1/2 months with developmental delay, infantile spasms, hypotonia, and elevated lactate levels in the blood and cerebrospinal fluid. She had minor dysmorphic features. Muscle phosphorus magnetic resonance spectroscopy demonstrated reduced phosphocreatine and increased inorganic phosphate, suggesting a defect in oxidative energy metabolism. Pyruvate dehydrogenase activity in cultured fibroblasts was reduced (0.35 nmol/mg mitochondrial protein/min; controls 0.7-1.1 nmol/mg mitochondrial protein/min). Immunoblotting demonstrated a reduced amount of pyruvate dehydrogenase (PDH) E1 alpha immunoreactive protein with normal amounts of E2 protein. Single-strand conformational polymorphism analysis of E1 alpha cDNA prepared from fibroblasts disclosed an abnormal migration pattern, suggesting heterozygosity for a mutant allele. Dideoxy-fingerprinting of PCR-amplified genomic DNA was used to localize the mutation to exon 10. Direct sequencing demonstrated a novel 13-bp insertion mutation that would lead to premature termination of the protein product. This study further extends the allelic heterogeneity underlying PDH deficiency. The demonstration of bioenergetic abnormalities in muscle emphasizes that hypotonia in PDH deficiency may have combined peripheral and central etiologies. The results further suggest that the association of cerebral dysgenesis with lactic acidemia in females may be a useful clue to PDH deficiency.

Pyruvate dehydrogenase deficiency

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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.

Clinical diversity of pyruvate dehydrogenase deficiency

Clinical features, magnetic resonance, and biochemical studies are reported in 7 children with pyruvate dehydrogenase (PDH) deficiency. These findings confirm the diverse clinical presentation of this condition, although neurological abnormalities are consistent features. Imaging results are also varied. Six of the children were investigated with proton magnetic resonance spectroscopy and lactate was demonstrated in brain in all patients. Regional variation in the lactate signal was observed in those patients in whom 2 regions were examined. Advances in molecular genetics have provided some explanations for the clinical variation in pyruvate dehydrogenase deficiency.

Prenatal diagnosis of pyruvate dehydrogenase E1 alpha subunit deficiency

Pyruvate dehydrogenase (PDH) E1 alpha subunit deficiency is an X-linked inborn error of metabolism affecting males and females with equal frequency. The diagnosis is usually based on determination of enzyme activity, although this may present difficulties in some females because of X-inactivation patterns favouring expression of the normal X chromosome. This is a particular problem for prenatal diagnosis using chorionic villus cells where normal enzyme assay results do not necessarily exclude the diagnosis and confirmatory X-inactivation analysis may be complicated by variable methylation of active and inactive X chromosomes. We describe prenatal diagnosis in two pregnancies in a family following diagnosis of a PDH E1 alpha deficient male. The first prenatal diagnosis was performed by enzyme assay, but by the time of the subsequent pregnancy, the underlying mutation in the affected male had been identified and direct gene analysis was possible. This study highlights the limitations of diagnosis of PDH E1 alpha deficiency based on measurement of the gene product and illustrates the need for mutation analysis in affected individuals.

X chromosome inactivation and the diagnosis of X linked disease in females

In studies of female patients with suspected deficiency of the E1 alpha subunit of the pyruvate dehydrogenase complex, we have found that X inactivation ratios of 80:20 or greater occur at sufficient frequency in cultured fibroblasts to make exclusion of the diagnosis impossible in about 25% of cases. Pyruvate dehydrogenase E1 alpha subunit deficiency is an X linked inborn error of metabolism which is well defined biochemically and is unusual in that most heterozygous females manifest the condition. The diagnosis is usually established by measurement of enzyme activity and the level of immunoreactive protein and these analyses are most commonly performed on cultured fibroblasts from the patients. Skewed patterns of X chromosome inactivation make it impossible to exclude the diagnosis if the normal X chromosome is expressed in the majority of cells. While most of the observed variation appears to be the expected consequence of random X inactivation, it may be further exaggerated by sampling and subsequent expansion of the cells for analysis.

Enzyme activities of the mitochondrial energy generating system in skeletal muscle tissue of preterm and fullterm neonates

Quadriceps muscle specimens from autopsy of 28 neonates (gestational age 25-42 weeks) were investigated to determine pyruvate and malate oxidation rates and several enzymes of the mitochondrial oxidative process. In general, the levels of all mitochondrial parameters measured, including carnitine levels, were lower in the neonates who died within the first week of life than those in the control group (age > 5 years). Pyruvate and malate oxidation rates (P < 0.05), activities of pyruvate dehydrogenase complex (P < 0.10) and succinate: cytochrome c oxidoreductase (P < 0.05) increased significantly with gestational age. Pyruvate oxidation rates (P < 0.05) as well as activities of citrate synthase (P < 0.05) and NADH:Q1 oxidoreductase (P < 0.05) were significantly lower in the group of very preterm infants at an age of 1-7 days compared with very preterm infants at an age between 3-8 weeks. We conclude from our study that special reference values are necessary for a correct biochemical diagnosis of mitochondrial encephalomyopathies in the neonatal period. Differences between preterm and fullterm children of the same age (1 week) indicate a maturational process in human muscle tissue during gestation. Comparison of two different age groups within the very preterm neonates point to a postnatal maturation of the mitochondrial energy metabolism, at least in preterm neonates.

Update on inborn errors of metabolism: primary lactic acidemia

Initially thought to be rare, primary lactic acidemia is diagnosed with increasing frequency. Elevations in lactate and pyruvate are markers for a variety of metabolic blocks. Although there have been great strides made in the diagnosis and treatment of lactic acidemia, much remains to be learned. As laboratory techniques improve, clinicians will be able to make an exact enzymatic diagnosis on an increasing percentage of patients. Specific enzymatic diagnosis also will help clinicians determine inheritance patterns, recurrence risks, and methods of prenatal diagnosis.

X-linked pyruvate dehydrogenase E1 alpha subunit deficiency in heterozygous females: variable manifestation of the same mutation

Three female patients are described with pyruvate dehydrogenase (PDH) deficiency as a result of mutation in the X-linked gene for the E1 alpha subunit of the complex. Two of these patients illustrate typical presentations of PDH E1 alpha deficiency, with severe neurological dysfunction, degenerative changes and developmental anomalies in the brain, together with variable lactic acidosis. The third patient extends the known spectrum of the condition to include mild to moderate mental retardation and seizures in an adult. All three patients have the same mutation in the PDH E1 alpha gene. This mutation, a C-to-T substitution in a CpG dinucleotide in amino acid codon 302 (designated R302C), results in the replacement of arginine by cysteine at this position. The mildly affected adult was the mother of one of the other patient, making this the first described instance of mother-to-daughter transmission of a mutation causing PDH E1 alpha deficiency. The genetic basis of the variable expression of X-linked PDH E1 alpha deficiency in heterozygous females is discussed.

Investigation of enzyme defects in children with lactic acidosis

Screening for enzyme deficiencies was carried out in cultured skin fibroblasts and leukocytes of 19 patients with lactic acidosis and neurological problems. Pyruvate carboxylase deficiency was demonstrated in three cases. Reduced pyruvate oxidation was found in seven cultures; six showed no significant stimulation of the oxidation rate by methylene blue and in three a decreased pyruvate dehydrogenase complex activity was confirmed. Methylene blue restored a near normal oxidation rate in the seventh culture which had decreased cytochrome c oxidase activity.

Pyruvate dehydrogenase E1 alpha deficiency

Pyruvate dehydrogenase (PDH) deficiency has long been recognized as the most common defined cause of primary lactic acidosis in infancy and early childhood. More recently, it has also been described in patients with subacute/chronic neurodegenerative disease without significant metabolic acidosis. The great majority of cases of PDH deficiency result from a genetic defect in the E1 alpha subunit of the complex. PDH E1 alpha deficiency is an X-linked inborn error of metabolism in which a high proportion of heterozygous females manifest the condition. In this review of 29 patients with PDH E1 alpha deficiency, particular emphasis is given to those aspects of the disorder which are specifically related to the X chromosome location of the PDH E1 alpha gene. These include the broad spectrum of clinical presentations and problems of diagnosis, especially antenatal diagnosis, in females.

Mutations and polymorphisms in the pyruvate dehydrogenase E1 alpha gene

We present an update on mutations and polymorphisms in the human X chromosome located pyruvate dehydrogenase E1 alpha gene. A total of 20 different mutations are tabulated. The mutations include deletions, insertions, and point mutations. Certain sequences seem particularly prone to mutation. Most of the mutations are found in exons 10 and 11. Furthermore, four of the mutations are seen in unrelated patients. Little is known about how the mutations affect the structure or function of the pyruvate dehydrogenase complex.

Lactic acidosis and mitochondrial disorders

Characterization of the biochemical basis of various inherited disorders associated with lactic acidosis has increased dramatically in recent years. These include defects of enzymes of gluconeogenesis, pyruvate oxidation, and electron transport. Clinical manifestations of these disorders show great variation and overlap, frequently involving the central nervous system as well as skeletal and cardiac muscle. Several of these enzymes are large complexes of subunits encoded by multiple genes; the electron transport chain complexes include subunits encoded by both nuclear and mitochondrial genes. This great complexity complicates analysis of specific mutations, despite considerable progress in defining the primary structure of component proteins and their genes. With few exceptions, treatment of disorders associated with congenital lactic acidosis remains unsatisfactory.

Sequence conservation in the alpha and beta subunits of pyruvate dehydrogenase and its similarity to branched-chain alpha-keto acid dehydrogenase

Amino acid sequence comparison of 8 alpha and 6 beta subunits of the alpha-keto acid dehydrogenase (E1) component of the pyruvate dehydrogenase complex and branched-chain alpha-keto acid dehydrogenase complex form multiple species was performed by computer analysis. In addition to 2 previously recognized regions of homology in the alpha subunit, a 3rd region of extensive homology was identified in E1 alpha, and may be one of the sites involved in subunit interaction. E1 beta contains 4 regions of extensive homology. Region 1 contains 10 amino acids that are homologous to a 10-amino acid stretch in Escherichia coli E1. Regions 2 and 3 have sequence homologies with other dehydrogenases suggesting that these regions may be involved in catalysis.

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.