Pyruvate dehydrogenase deficiency: molecular basis for intrafamilial heterogeneity

Manifestations of X-linked pyruvate dehydrogenase complex deficiency in female PDHA1 carriers

BACKGROUND AND PURPOSE

Pyruvate dehydrogenase complex deficiency is in up to 90% caused by pathogenic variants in the X-linked PDHA1 gene. We aimed to investigate female relatives of index patients with PDHA1-related disease to (i) describe the prevalence of female PDHA1 carriers, (ii) determine whether they had symptoms and signs, and (iii) delineate the associated phenotype.

METHODS

In a national population-based study, we identified 37 patients with pathogenic variants in PDHA1. Sanger sequencing for the presence of the pathogenic variant was performed in their mothers and female relatives. The identified female carriers were clinically assessed, and their medical records were reviewed.

RESULTS

The proportion carrying a de novo variant was 86%. We identified seven female PDHA1 carriers from five families. Five of them exhibited clinical features of the disease and were previously undiagnosed; all had signs of peripheral axonal neuropathy, four presented with strokelike episodes including two with Leigh-like lesions, and three had facial stigmata.

CONCLUSIONS

PDHA1-related disease is underrecognized in heterozygous female carriers. Peripheral axonal neuropathy, strokelike and Leigh-like changes, and facial dysmorphism should raise suspicion of the disorder. Genetic analysis and clinical examination of potential female carriers are important for genetic counseling and have implications for treatment.

Dosage Compensation in Females with X-Linked Metabolic Disorders

Through the use of new genomic and metabolomic technologies, our comprehension of the molecular and biochemical etiologies of genetic disorders is rapidly expanding, and so are insights into their varying phenotypes. Dosage compensation (lyonization) is an epigenetic mechanism that balances the expression of genes on heteromorphic sex chromosomes. Many studies in the literature have suggested a profound influence of this phenomenon on the manifestation of X-linked disorders in females. In this review, we summarize the clinical and genetic findings in female heterozygotic carriers of a pathogenic variant in one of ten selected X-linked genes whose defects result in metabolic disorders.

Differential phenotypic expression of a novel PDHA1 mutation in a female monozygotic twin pair

Pyruvate dehydrogenase complex (PDC) deficiency caused by mutations in the X-linked PDHA1 gene has a broad clinical presentation, and the pattern of X-chromosome inactivation has been proposed as a major factor contributing to its variable expressivity in heterozygous females. Here, we report the first set of monozygotic twin females with PDC deficiency, caused by a novel, de novo heterozygous missense mutation in exon 11 of PDHA1 (NM_000284.3: c.1100A>T). Both twins presented in infancy with a similar clinical phenotype including developmental delay, episodes of hypotonia or encephalopathy, epilepsy, and slowly progressive motor impairment due to pyramidal, extrapyramidal, and cerebellar involvement. However, they exhibited clear differences in disease severity that correlated well with residual PDC activities (approximately 60% and 20% of mean control values, respectively) and levels of immunoreactive E1α subunit in cultured skin fibroblasts. To address whether the observed clinical and biochemical differences could be explained by the pattern of X-chromosome inactivation, we undertook an androgen receptor assay in peripheral blood. In the less severely affected twin, a significant bias in the relative activity of the two X chromosomes with a ratio of approximately 75:25 was detected, while the ratio was close to 50:50 in the other twin. Although it may be difficult to extrapolate these results to other tissues, our observation provides further support to the hypothesis that the pattern of X-chromosome inactivation may influence the phenotypic expression of the same mutation in heterozygous females and broadens the clinical and genetic spectrum of PDC deficiency.

Molecular characterization of 82 patients with pyruvate dehydrogenase complex deficiency. Structural implications of novel amino acid substitutions in E1 protein

BACKGROUND

Pyruvate dehydrogenase complex (PDHc) deficiencies are an important cause of primary lactic acidosis. Most cases result from mutations in the X-linked gene for the pyruvate dehydrogenase E1α subunit (PDHA1) while a few cases result from mutations in genes for E1β (PDHB), E2 (DLAT), E3 (DLD) and E3BP (PDHX) subunits or PDH-phosphatase (PDP1).

AIM

To report molecular characterization of 82 PDHc-deficient patients and analyze structural effects of novel missense mutations in PDHA1.

METHODS

PDHA1 variations were investigated first, by exon sequencing using a long range PCR product, gene dosage assay and cDNA analysis. Mutation scanning in PDHX, PDHB, DLAT and DLD cDNAs was further performed in unsolved cases. Novel missense mutations in PDHA1 were located on the tridimensional model of human E1 protein to predict their possible functional consequences.

RESULTS

PDHA1 mutations were found in 30 girls and 35 boys. Three large rearrangements, including two contiguous gene deletion syndrome were identified. Novel missense, frameshift and splicing mutations were also delineated and a nonsense mutation in a mosaic male. Mutations p.Glu75Ala, p.Arg88Ser, p.Arg119Trp, p.Gly144Asp, p.Pro217Arg, p.Arg235Gly, p.Tyr243Cys, p.Tyr243Ser, p.Arg245Gly, p.Pro250Leu, p.Gly278Arg, p.Met282Val, p.Gly298Glu in PDHA1 were predicted to impair active site channel conformation or subunit interactions. Six out of the seven patients with PDHB mutations displayed the recurrent p.Met101Val mutation; 9 patients harbored PDHX mutations and one patient DLD mutations.

CONCLUSION

We provide an efficient stepwise strategy for mutation screening in PDHc genes and expand the growing list of PDHA1 mutations analyzed at the structural level.

Pyruvate dehydrogenase deficiency: identification of a novel mutation in the PDHA1 gene which responds to amino acid supplementation

The pyruvate dehydrogenase complex (PDHc) is an intramitochondrial multienzyme system, which plays a key role in aerobic glucose metabolism by catalysing the oxidative decarboxylation of pyruvate to acetyl-CoA. Genetic defects in the PDHc lead to lactic acidemia and neurological abnormalities. In the majority of the cases, the defect appears to reside in the E(1)alpha subunit, the first catalytic component of the complex. The report is on a 6-year-old Portuguese boy with mild neurological involvement and low PDHc activity with absence of E1alpha on immunoblotting analysis. Molecular studies showed a novel and "de novo" mutation in the PDHA1 gene, R253G. Treatment with arginine aspartate showed complete clinical and biochemical recovery. We hypothesise that arginine aspartate acts as a chemical or pharmacological chaperone, and suggest amino acid supplementation as a possible therapy in PDHA1 mutations with mild phenotypes.

CONCLUSION

our results encourage the use of amino acid supplementation to overcome the metabolic/biochemical changes induced by PDHA1 gene specific mutations associated with mild PDHc phenotypes.

Females with PDHA1 gene mutations: a diagnostic challenge

Biochemical analysis was performed in muscle tissue and in fibroblasts of four unrelated females consecutively diagnosed with a 'de novo' point mutation in the PDHA1 gene. Pyruvate dehydrogenase E1 subunit deficiency was confirmed in the muscle sample of all patients, however, in three out of four cases the activity of the pyruvate dehydrogenase complex in fibroblasts showed a normal activity. A skewed inactivation was confirmed of the maternal X chromosome in fibroblasts in all children. Due to the possibility of a skewed X inactivation pattern enzyme measurements in fibroblasts are not always reliable for the diagnosis of a PDHc defect in females. Based on the overlapping features of PDHc deficiency with those of the disorders of the oxidative phosphorylation we suggest performing a fresh muscle biopsy for detailed biochemical analysis in females with a suspected pyruvate dehydrogenase deficiency, followed by molecular genetic analysis of the PDHA1 gene.

Detection of pyruvate dehydrogenase E1 alpha-subunit deficiencies in females by immunohistochemical demonstration of mosaicism in cultured fibroblasts

Deficiency of the E1 alpha-subunit of the pyruvate dehydrogenase (PDH) complex is an X-linked inborn error of metabolism and one of the major causes of lactic acidosis in children. Although most heterozygous females manifest symptoms of the disease, it is often difficult to establish the diagnosis as results based on measurement of total PDH activity, and E1 alpha-immunoreactive protein in patient fibroblasts may be ambiguous because of the variability in the pattern of X chromosome inactivation. We report the development of a set of monoclonal antibodies (MAbs) specific to four subunits of the PDH complex that can be used for detection of PDH E1 alpha deficiency. We also show that anti-E1 alpha and anti-E2 MAbs, when used in immunocytochemical analysis, can detect mosaicism in cell cultures from female patients in which as few as 2-5% of cells express the deficiency. This immunocytochemical approach, which is fast, reliable, and quantitative, will be particularly useful in identifying females with PDH E1 alpha-subunit deficiency as a precursor to mutation analysis.

Differential effects of two mutations at arginine-234 in the alpha subunit of human pyruvate dehydrogenase

The most common mutation in the alpha subunit of the pyruvate dehydrogenase (E1) component of the human pyruvate dehydrogenase complex (PDC) is arginine-234 to glycine and glutamine in 12 and 3 patients, respectively. Interestingly, these two mutations at the same amino acid position cause E1 (and hence PDC) deficiency by apparently different mechanisms. Recombinant human R234Q E1 had similar V(max) (25.7 +/- 4.4 units/mg E1) and apparent K(m) (101 +/- 4 nM) values for TPP as recombinant wild-type human E1, while R234G E1 had no significant change in V(max) (33.6 +/- 4.7 units/mg E1) but had a 7-fold increase in its apparent K(m) value for TPP (497 +/- 25 nM). Both of the R234 mutant proteins had similar apparent K(m) values for pyruvate. Both R234Q and R234G mutant proteins displayed similar phosphorylation rates of sites 1 and 2 by pyruvate dehydrogenase kinase 2 (PDK2) and site 3 by PDK1 compared to wild-type E1. Phosphorylated R234Q E1, R234G E1, and wild-type E1 also had similar dephosphorylation rates of sites 1 and 2 by phosphopyruvate dehydrogenase phosphatase 1. The rate of dephosphorylation of site 3 was about 50% for R234Q E1 and without a significant change for R234G E1 compared to the wild type. The data indicate that the patients with the R234G E1 mutation are symptomatic due to a decreased ability of this mutant protein to bind TPP, whereas the patients with the R234Q E1 mutation are symptomatic due to a decreased rate of dephosphorylation of site 3, hence keeping the enzyme in a phosphorylated/inactivated form.

Mutations in the X-linked pyruvate dehydrogenase (E1) alpha 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) alpha subunit gene (PDHA1). The major factors that contribute to the clinical variation in E1alpha 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. To date, 37 different missense/nonsense and 39 different insertion/deletion mutations have been identified in the E1alpha subunit gene of 130 patients (61 females and 69 males) from 123 unrelated families. Insertion/deletion mutations occur preferentially in exons 10 and 11, while missense/nonsense mutations are found in all exons. In males, the majority of missense/nonsense mutations are found in exons 3, 7, 8 and 11, and three recurrent mutations at codons R72, R263 and R378 account for half of these patients with missense/nonsense mutations (25 of 50). A significantly lower number of females is found with missense/nonsense mutations (25). However, 36 females out of 55 affected patients have insertion/deletion mutations. The total number of female and male patients is thus almost the same, although a difference in the distribution of the type of mutations is evident between both sexes. In many families, the parents of the affected patients were studied for the presence of the PDHA1 mutation. The mutation was never present in the somatic cells of the father; in 63 mothers studied, 16 were carriers (25%). In four families, the origin of the new mutation was determined to be twice paternal and twice maternal.

Neonatal presentations of mitochondrial metabolic disorders

Because of the high energy requirements of the growing neonate, disorders of mitochondrial metabolism caused by defects in fatty acid oxidation, pyruvate metabolism, and the respiratory chain may often present in the neonatal period. Common neonatal presentations are hypotonia, lethargy, feeding and respiratory difficulties, failure to thrive, psychomotor delay, seizures, and vomiting. Laboratory clues include alterations in the levels of lactate, pyruvate (and the lactate/pyruvate ratio), glucose, and ketone bodies. Diagnosis usually depends on specific enzyme assays or on molecular genetic analysis. Without treatment, most infants die in the first few days or months of life. In the last decade, there have been significant advances in the understanding of the molecular basis of these disorders. This review discusses the major subgroups of mitochondrial disorders, focusing on defects of pyruvate oxidation, the Krebs cycle, and the respiratory chain. Disorders caused by respiratory chain defects may involve nuclear DNA, mitochondrial DNA, or intergenomic signaling. Recognition and early diagnosis of these conditions are important in the genetic counseling of these families.

Pyruvate dehydrogenase complex deficiency and altered respiratory chain function in a patient with Kearns-Sayre/MELAS overlap syndrome and A3243G mtDNA mutation

Combined alteration of the pyruvate dehydrogenase complex and respiratory chain function is described in a 21 year-old male patient with overlapping MELAS (mitochondrial encephalomyopathy, lactic acidosis, and 'stroke-like' episodes) and Kearns-Sayre syndrome. Progressive external ophthalmoplegia, pigmentary retinopathy and right bundle branch block were present when he experienced the first 'stroke-like' episode at 18 years old. The A>G tRNALeu(UUR) point mutation at nucleotide 3243 of the mitochondrial DNA was predominant in muscle tissue (79%) and present, but at lower levels in fibroblasts (49%) and blood cells (37%). Biochemical analysis revealed diminished activities of pyruvate dehydrogenase (23%) and respiratory chain complexes I and IV (57%, respectively) in muscle, but normal activities in the fibroblasts. Immunochemical studies of the muscular pyruvate dehydrogenase components showed normal content of E1alpha, E1beta and E2 protein. Molecular screening of the E1alpha gene did not indicate a nuclear mutation. These observations suggest that mitochondrial DNA defects may be associated with altered nuclear encoded enzymes which are actively imported into mitochondria and constitute components of the mitochondrial matrix. Biochemical workup of mitochondrial disorders should not be restricted to the respiratory chain even if mitochondrial DNA mutations are present.

Genetics and developmental delay

The discovery of new cytogenetic and molecular genetic techniques and principles has been explosive in recent years. A secure diagnosis based on molecular evidence has become possible for many syndromes previously only clinically defined, which has helped enormously in predicting children's developmental progress, in allowing knowledgeable surveillance for potential associated health problems, in genetic counseling, and in prenatal diagnosis. This article reviews several of the most significant recently described cytogenetic and molecular genetic principles and techniques in relation to the child who presents with developmental delay.

Pyruvate dehydrogenase deficiency: the relation of the E1 alpha mutation to the E1 beta subunit deficiency

We report 7 patients with pyruvate dehydrogenase (PDH) deficiency caused by mutations of the PDH-E1 alpha subunit. Each patient had a different mutation; 4 with duplicate insertions, 1 with a deletion of tandem repeat, and 2 with point mutations. Five of the mutations were novel, thus confirming allelic heterogeneity. Immunoblot analysis revealed decreased immunoreactivity for the E1 alpha and E1 beta subunits in every patient. Pulse-labeling and chase study for the E1 alpha and E1 beta subunits revealed that initial synthesis of the mutant E1 alpha subunit was normal and posttranslational degradation was complete by 48 hours. However, the post-translational degradation rate of the E1 beta subunit varied from one patient to another. Factors other than instability of the E1 beta monomer must contribute to the degradation rate of this subunit in the presence of an E1 alpha subunit mutation. Including this series, 3 patients with the S312 deletion and 5 with the R302C point mutation have been reported, and all of these patients are female. These findings suggest that these two loci are hot spots for gene mutations, and may be lethal in the male fetus.

The treatment of congenital lactic acidoses

Congenital lactic acidoses form a heterogeneous group of disorders: this paper considers primarily defects of the pyruvate dehydrogenase complex and the respiratory chain. Attempts to treat these disorders are hampered by uncertainty concerning the pathophysiology and by the central role of the enzymes in cellular metabolism. Few strategies are of proven efficacy, though many have been tried, including dietary manipulation, enhancement of residual enzyme activity, artificial electron acceptors and free-radical scavengers. Evaluation of treatment is complicated by the rarity, heterogeneity and unpredictable course of the diseases. Double-blind placebo-controlled trials are needed.

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.