Genetic heterogeneity in propionic acidemia patients with alpha-subunit defects. Identification of five novel mutations, one of them causing instability of the protein

The inherited metabolic disease propionic acidemia (PA) can result from mutations in either of the genes PCCA or PCCB, which encode the alpha and beta subunits, respectively, of the mitochondrial enzyme propionyl CoA-carboxylase. In this work we have analyzed the molecular basis of PCCA gene defects, studying mRNA levels and identifying putative disease causing mutations. A total of 10 different mutations, none predominant, are present in a sample of 24 mutant alleles studied. Five novel mutations are reported here for the first time. A neutral polymorphism and a variant allele present in the general population were also detected. To examine the effect of a point mutation (M348K) involving a highly conserved residue, we have carried out in vitro expression of normal and mutant PCCA cDNA and analyzed the mitochondrial import and stability of the resulting proteins. Both wild-type and mutant proteins were imported into mitochondria and processed into the mature form with similar efficiency, but the mature mutant M348K protein decayed more rapidly than did the wild-type, indicating a reduced stability, which is probably the disease-causing mechanism.     

Functional characterization of PCCA mutations causing propionic acidemia

Propionic acidemia (PA, MIM 232000 and 232050) is caused by a deficiency of mitochondrial biotin-dependent propionyl-CoA carboxylase (PCC, EC 6.4.1.3), a heteropolymeric enzyme composed of alpha and beta subunits, which are encoded by the PCCA and PCCB genes, respectively. The PCCA protein (alpha subunit) is responsible for the formation of carboxybiotin upon hydrolysis of ATP and contains a C-terminal biotin-binding domain and a biotin carboxylase domain, defined by homology with other biotin-dependent carboxylases, some of them characterized structurally. More than 24 mutations have been found in the PCCA gene in patients with PA, among them 14 missense mutations and one in-frame deletion, for which the precise molecular effect is unknown. In this study, we have established the pathogenicity of 11 PCCA mutations (10 missense and an in-frame deletion) by expression studies in deficient fibroblasts and in a cell-free in vitro system, and analyzed the effect of each mutation on PCC activity, protein stability and domain structure. The results show that most mutant proteins show an increased turnover and are functionally deficient, suggesting that the structural alterations they cause are incompatible with normal assembly to produce a stable, functional PCC oligomer. These results are discussed in the context of the genotype-phenotype correlations in PCCA-deficient PA patients.     

Genetics of propionic acidemia in a Mennonite-Amish kindred.

A large Mennonite kindred was found to have propionic acidemia (complementation group pcc C) in at least four different sibships. Even within this kindred and this complementation group (where etiology may be assumed to be identical), there is a wide range of symptoms exhibited by homozygous pcc C-deficient individuals. The inbreeding coefficients (f) for the affected sibships ranged from 4.776 X 10(3) to 2.003 X 10(-2). Data from this study strongly support the single-locus autosomal recessive mode of inheritance. Three couples were found to be common in the ancestry (9--11 generations ago) of all eight parents of the four affected sibships. Relative likelihoods for a member of each of those couples to have been the early carrier of the defective allele were calculated at 1539, 278, and 1. Thus, one couple was designated the most likely earliest-known transmitter of the pcc-deficient allele.     

Changes in plasma amino acid concentrations with increasing age in patients with propionic acidemia

The objective of the study is to analyze plasma amino acid concentrations in propionic acidemia (PA) for the purpose of elucidating possible correlations between propionyl-CoA carboxylase deficiency and distinct amino acid behavior. Plasma concentrations of 19 amino acids were measured in 240 random samples from 11 patients (6 families) with enzymatically and/or genetically proven propionic acidemia (sampling period, January 2001–December 2007). They were compared with reference values from the literature and correlated with age using the Pearson correlation coefficient test. Decreased plasma concentrations were observed for glutamine, histidine, threonine, valine, isoleucine, leucine, phenylalanine and arginine. Levels of glycine, alanine and aspartate were elevated, while values of serine, asparagine, ornithine and glutamate were normal. For lysine, proline and methionine a clear association was not possible. Significant correlations with age were observed for 13 amino acids (positive correlation: asparagine, glutamine, proline, alanine, histidine, threonine, methionine, arginine; negative correlation: leucine, phenylalanine, ornithine, glutamate and aspartate). This study gives new insight over long-term changes in plasma amino acid concentrations and may provide options for future therapies (e.g., substitution of anaplerotic substances) in PA patients.     

Three novel splice mutations in the PCCA gene causing identical exon skipping in propionic acidemia patients

Propionyl-CoA carboxylase (PCC) is a mitochondrial, biotin-dependent enzyme involved in the catabolism of branched chain amino acids, odd chain fatty acids, and other metabolites. PCC consists of non-identical subunits, α and β, encoded by the PCCA and PCCB genes, respectively. Inherited deficiency of PCC due to mutations in either the PCCA or the PCCB gene results in propionic acidemia (PA), a clinically heterogeneous disorder with a severe, often lethal, neonatal form, and a mild, later onset form. To characterize PCCA gene mutations responsible for PCC deficiency, we analyzed RT-PCR products obtained from cultured fibroblasts from Spanish PCC-α deficient patients. In three patients, smaller than normal PCR products were observed, and sequence analysis revealed the deletion of a 54-bp exon in the cDNA. Sequencing of genomic DNA from these three patients led to the identification of three novel mutations in the PCCA gene, two short deletions and one small insertion, adjacent to short direct repeats, and all of them affecting the consensus splice sites of the skipped exon. These mutations, 1771IVS-2del9, 1824IVS+3del4, and 1824IVS+3insCT, are the cause of the aberrant splicing of the PCCA pre-mRNA and result in an in-frame deletion of 54 nucleotides in the cDNA, probably leading to an unstable protein structure which is responsible for the lack of activity leading to PCC deficiency in these patients. Received: 6 June 1997 / Accepted: 14 July 1997     

Evaluation of biotin responsiveness in cultured fibroblasts from patients with propionic acidemia: Absence of response by structurally altered carboxylases

We have demonstrated that, although propionyl CoA carboxylase (PCC) activity is deficient in fibroblast extracts from PCC-deficient patients belonging to the two major and two minor genetic complementation groups, the activity of another biotin-dependent carboxylase, -methylcrotonyl CoA carboxylase (MCC), is normal. Moreover, MCC activity is stimulated when the fibroblasts are cultured in high concentrations of biotin, in the same way that it is in normal fibroblasts, whereas the depressed PCC activity remains essentially unchanged. Because these results are parallel with the in vivo failure of high-dose biotin to stimulate PCC activity in peripheral blood leukocytes, we conclude that the biotin responsiveness of PCC in cultured fibroblasts from patients with PCC deficiency may be used to predict or confirm biotin responsiveness in vivo.     

Interorgan amino acid interchange in propionic acidemia: the missing key to understanding its physiopathology

Amino Acids - Propionic acidemia is an inborn error of metabolism caused by a deficiency in the mitochondrial enzyme propionyl-CoA carboxylase that converts the propionyl CoA to methyl malonyl CoA....     

Neonatal onset of organic acidemia (propionic) diagnosed by tandem mass spectrometry

Propionic acidemia is an autosomal recessive disorder as a result of a deficient activity of propionyl-CoA carboxylase. Propionyl CoA is metabolized by propionyl-CoA carboxylase to methylmalonyl CoA. Propionic acidemia is a major cause of ketotic hyperglycinemia. This disorder is characterized by episodic vomiting, dehydratation, feeding intolerante, lethargy, hypotonia, metabolic acidosis, ketosis and hyperammonemia. The patient presented herein was a full-term female newborn with encephalopathy in the first days of life. She presented hypoglycemia, metabolic acidosis with increased anion gap, ketosis, hyperammonemia, anemia, leukopenia and thrombocytopenia. The brain ultrasonography was normal. The tandem mass expectrometry done by Pediatrix was abnormal, with the acylcarnitine results consistent with an organic acidemia. The parents are consanguineus and have a history of abortus, miscarriage and neonatal death, characteristics suggestive of the presence of genetic defects.     

Genetic heterogeneity in patients having ataxia telangiectasia

UV-irradiated Chinese hamster cells on post-irradiation treatment with caffeine in growth medium for 24 h gave rise to biphasic UV-survival curves. At caffeine concentrations between 0.001 and 0.1 mM, control and caffeine-grown cells had similar survival curves initially from 0 to 30 J/m². At fluences greater than 30 J/m², there was effectively only little further killing of caffeine-grown cells beyond that observed at 30 J/m². At concentrations of caffeine greater than 0.5 mM, there was a gradual sensitization in the early part of the survival curve with increasing caffeine concentrations; but at fluences greater than 3 J/m², the slopes in the survival curves decreased very much.It has been proposed that the initial sensitization observed at low UV fluences is due to the suppression of post-replication repair by caffeine. After high fluences of UV exposures in these excision-deficient cells, in the presence of caffeine, the possibility of an induced ‘SOS’-like repair process has been suggested. This suggestion was supported by the observation that caffeine increased the yield of the UV-induced 8-azaguanine-resistant mutants only for the cell population exposed to UV fluences greater than 30 J/m².     

Human propionyl-CoA carboxylase beta subunit gene: exon-intron definition and mutation spectrum in Spanish and Latin American propionic acidemia patients.

Propionyl-CoA carboxylase (PCC) is a mitochondrial biotin-dependent enzyme composed of an equal number of alpha and beta subunits. Mutations in the PCCA (alpha subunit) or PCCB (beta subunit) gene can cause the inherited metabolic disease propionic acidemia (PA), which can be life threatening in the neonatal period. Lack of data on the genomic structure of PCCB has been a significant impediment to full characterization of PCCB mutant chromosomes. In this study, we describe the genomic organization of the coding sequence of the human PCCB gene and the characterization of mutations causing PA in a total of 29 unrelated patients-21 from Spain and 8 from Latin America. The implementation of long-distance PCR has allowed us to amplify the regions encompassing the exon/intron boundaries and all the exons. The gene consists of 15 exons of 57-183 bp in size. All splice sites are consistent with the gt/ag rule. The availability of the intron sequences flanking each exon has provided the basis for implementation of screening for mutations in the PCCB gene. A total of 56/58 mutant chromosomes studied have been defined, with a total of 16 different mutations detected. The mutation spectrum includes one insertion/deletion, two insertions, 10 missense mutations, one nonsense mutation, and two splicing defects. Thirteen of these mutations correspond to those not described yet in other populations. The mutation profile found in the chromosomes from the Latin American patients basically resembles that of the Spanish patients.     

Unequal synthesis and differential degradation of propionyl CoA carboxylase subunits in cells from normal and propionic acidemia patients.

We have characterized further the molecular basis of human inherited propionyl CoA carboxylase deficiency by measuring steady state levels of the mRNAs coding for the enzyme's two protein subunits (alpha and beta) and by estimating initial synthesis and steady state levels of the protein subunits in skin fibroblasts from controls and affected patients. We studied cell lines from both major complementation groups (pccA and pccBC) corresponding, respectively, to defects in the carboxylase's alpha and beta subunits. Analysis of pccA lines revealed the absence of alpha chain mRNA in three and an abnormally small alpha-mRNA in a fourth. Despite the presence of normal beta-mRNA in each of these pccA lines, there was complete absence of both alpha and beta protein subunits under steady state conditions, even though new synthesis and mitochondrial import of beta precursors was normal. Results in nine pccBC lines revealed normal alpha mRNA in each, while the amounts of beta-mRNA were distinctly reduced in every case. Correspondingly, alpha protein subunits were present in normal amounts at steady-state, but beta subunits were uniformly decreased. In addition, in six of the nine beta deficient cell lines, partially degraded beta-subunits were observed. To help interpret these results, synthesis and stability of carboxylase subunits were studied in intact HeLa cells using a pulse-chase protocol. Whereas alpha chains were stable over the four hour interval studied, beta chains--initially synthesized in large excess over alpha chains--were degraded rapidly reaching equivalence with alpha chains after two hours.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic characterization of mutations found in propionic acidemia and methylcrotonylglycinuria: evidence for cooperativity in biotin carboxylase

Acetyl-CoA carboxylase catalyzes the committed step in fatty acid synthesis in all plants, animals, and bacteria. The Escherichia coli form is a multifunctional enzyme consisting of three separate proteins: biotin carboxylase, carboxyltransferase, and the biotin carboxyl carrier protein. The biotin carboxylase component, which catalyzes the ATP-dependent carboxylation of biotin using bicarbonate as the carboxylate source, has a homologous functionally identical subunit in the mammalian biotin-dependent enzymes propionyl-CoA carboxylase and 3-methylcrotonyl-CoA carboxylase. In humans, mutations in either of these enzymes result in the metabolic deficiency propionic acidemia or methylcrotonylglycinuria. The lack of a system for structure-function studies of these two biotin-dependent carboxylases has prevented a detailed analysis of the disease-causing mutations. However, structural data are available for E. coli biotin carboxylase as is a system for its overexpression and purification. Thus, we have constructed three site-directed mutants of biotin carboxylase that are homologous to three missense mutations found in propionic acidemia or methylcrotonylglycinuria patients. The mutants M169K, R338Q, and R338S of E. coli biotin carboxylase were selected for study to mimic the disease-causing mutations M204K and R374Q of propionyl-CoA carboxylase and R385S of 3-methylcrotonyl-CoA carboxylase. These three mutants were subjected to a rigorous kinetic analysis to determine the function of the residues in the catalytic mechanism of biotin carboxylase as well as to establish a molecular basis for the two diseases. The results of the kinetic studies have revealed the first evidence for negative cooperativity with respect to bicarbonate and suggest that Arg-338 serves to orient the carboxyphosphate intermediate for optimal carboxylation of biotin.     

Intracellular calcium mishandling leads to cardiac dysfunction and ventricular arrhythmias in a mouse model of propionic acidemia

Propionic acidemia (PA) is a rare metabolic disease associated with mutations in genes encoding the α and β subunits of the enzyme propionyl-CoA carboxylase. The accumulation of toxic metabolites results in mitochondrial dysfunction, increased reactive oxygen species production and oxidative damage, which have been associated with the disease pathophysiology. Clinical symptoms are heterogeneous and include cardiac complications, mainly cardiac dysfunction and arrhythmias, which are recognized as one of the major life-threatening manifestations in patients. We aimed to investigate the molecular mechanisms underlying the cardiac phenotype using a hypomorphic mouse model (Pcca^−/−(A138T)) that recapitulates some biochemical and clinical characteristics of PA. We demonstrate that Pcca^−/−(A138T) mice present with depressed cardiac function along with impaired cell contractility when compared to the wild-type mice. Cardiac dysfunction in Pcca^−/−(A138T) mice was associated with lower systolic Ca²⁺ release ([Ca²⁺]_i transients), impairment in the sarcoplasmic reticulum (SR) Ca²⁺ load and decreased Ca²⁺ re-uptake by SR-Ca²⁺ ATPase (SERCA2a). These functional changes correlated well with the depressed activity of SERCA2a, the elevated ROS levels and SERCA2a oxidation rate in cardiomyocytes isolated from Pcca^−/−(A138T) mice. In addition, decreased SR-Ca²⁺ load in Pcca^−/−(A138T) cardiomyocytes was associated with increased diastolic Ca²⁺ release. The increase in Ca²⁺ sparks, Ca²⁺ waves and spontaneous [Ca²⁺]_i transients in Pcca^−/−(A138T) cardiomyocytes could be responsible for the induction of ventricular arrhythmias detected in these mice. Overall, our results uncover the role of impaired Ca²⁺ handling in arrhythmias and cardiac dysfunction in PA, and identify new targets for the development of therapeutic approaches for this devastating metabolic disease.     

Functional analysis of PCCB mutations causing propionic acidemia based on expression studies in deficient human skin fibroblasts

Propionic acidemia (PA) is a recessive disorder caused by a deficiency of propionyl-CoA carboxylase (PCC), a dodecameric enzyme composed of two different proteins alpha-PCC and beta-PCC, nuclear encoded by the PCCA and PCCB genes, respectively. Mutations in either gene cause PA and to date, up to 47 different allelic variations in the PCCB gene have been identified in different populations. In this work, we describe the expression studies of 18 PCCB sequence changes in order to elucidate their functional consequences. We have used a PCCB-deficient transformed fibroblast cell line to target the wild-type and mutant proteins to their physiological situation, analysing the effect of the mutations on PCC activity and protein stability. Of the 18 mutant proteins tested for activity, those carrying the L17M and A497V substitutions showed an activity similar to the wild-type one, which proves that these changes do not have any effect on protein activity. The other 16 mutant proteins exhibited two different functional behaviours, 3 retained substantial activity (K218R, R410W and N536D), and the remaining 13 proteins showed null or very low activity. Western blot analysis demonstrated instability only for the L519P, R512C and G112D mutant proteins. We have proved the pathogenicity of R67S, R165Q and G112D mutation in PCCB gene, expressed for the first time in this work. The information derived from the expression analysis is discussed in the phenotype and genotype context in order to improve the knowledge of this complex disease.     

Genetic heterogeneity of thalassemias in Mexican mestizo patients with hemolytic anemia

Twenty-eight Hb abnormalities (16 thalassemias, 10 Hb variants and 2 hereditary persistences of fetal hemoglobin) were detected in an etiopathogenic search in 131 patients with hemolytic anemia. The observed thalassemic genotypes [6 beta(0)/beta, 2 (delta beta) (0)/beta(0), 2 beta(0)/beta(+), 2 beta(0)/beta(s), 2 beta(+)/beta(+), 1 beta(+)/beta(s) and 1 (delta beta)(0)/beta] showed heterogeneity in the population studied. A native origin of some of these mutations is considered.     

Genetic heterogeneity of skin microvasculature

Angiogenesis, the formation of new blood vessels from existing vasculature, is a complex process that is essential for normal embryonic development. Current models for experimental evaluation of angiogenesis often use tissue from large vessels like the aorta and umbilical vein, which are phenotypically distinct from microvasculature. We demonstrate that the utilization of skin to measure microvascular angiogenesis in embryonic and adult tissues is an efficient way to quantify microvasculature angiogenesis. We validate this approach and demonstrate its added value by showing significant differences in angiogenesis in monogenic and polygenic mouse models. We discovered that the pattern of angiogenic response among inbred mouse strains in this ex vivo assay differs from the strain distributions of previous in vivo angiogenesis assays. The difference between the ex vivo and in vivo assays may be related to systemic factors present in whole animals. Expression analysis of cultured skin biopsies from strains of mice with opposing angiogenic response was performed to identify pathways that contribute to differential angiogenic response. Increased expression of negative regulators of angiogenesis in C57Bl/6J mice was associated with lower growth rates.