Novel mutations in 3-phosphoglycerate dehydrogenase (PHGDH) are distributed throughout the protein and result in altered enzyme kinetics.

Three-phosphoglycerate dehydrogenase (3-PGDH) deficiency is a rare recessive inborn error in the biosynthesis of the amino acid L-serine characterized clinically by congenital microcephaly, psychomotor retardation, and intractable seizures. The biochemical abnormalities associated with this disorder are low concentrations of L-serine, D-serine, and glycine in cerebrospinal fluid (CSF). Only two missense mutations (p.V425M and p.V490M) have been identified in PHGDH, the gene encoding 3-PGDH, but it is currently unclear how these mutations in the carboxy-terminal regulatory domain of the protein affect enzyme function. We now describe five novel mutations in five patients with 3-PGDH deficiency; one frameshift mutation (p.G238fsX), and four missense mutations (p.R135W, p.V261M, p.A373T, and p.G377S). The missense mutations were located in the nucleotide binding and regulatory domains of 3-PGDH and did not affect steady-state expression, protein stability, and protein degradation rates. Patients' fibroblasts displayed a significant, but incomplete, reduction in maximal enzyme activities associated with all missense mutations. In transient overexpression studies in HEK293T cells, the p.A373T, p.V425M, and p.V490M mutations resulted in almost undetectable enzyme activities. Molecular modeling of the p.R135W and p.V261M mutations onto the partial crystal structure of 3-PGDH predicted that these mutations affect substrate and cofactor binding. This prediction was confirmed by the results of kinetic measurements in fibroblasts and transiently transfected HEK293T cells, which revealed a markedly decreased V(max) and an increase in K(m) values, respectively. Taken together, these data suggest that missense mutations associated with 3-PGDH deficiency either primarily affect substrate binding or result in very low residual enzymatic activity.

Hyperinsulinism in short-chain L-3-hydroxyacyl-CoA dehydrogenase deficiency reveals the importance of beta-oxidation in insulin secretion.

A female infant of nonconsanguineous Indian parents presented at 4 months with a hypoglycemic convulsion. Further episodes of hypoketotic hypoglycemia were associated with inappropriately elevated plasma insulin concentrations. However, unlike other children with hyperinsulinism, this patient had a persistently elevated blood spot hydroxybutyrylcarnitine concentration when fed, as well as when fasted. Measurement of the activity of L-3-hydroxyacyl-CoA dehydrogenase in cultured skin fibroblasts with acetoacetyl-CoA substrate showed reduced activity. In fibroblast mitochondria, the activity was less than 5% that of controls. Sequencing of the short-chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD) genomic DNA from the fibroblasts showed a homozygous mutation (C773T) changing proline to leucine at amino acid 258. Analysis of blood from the parents showed they were heterozygous for this mutation. Western blot studies showed undetectable levels of immunoreactive SCHAD protein in the child's fibroblasts. Expression studies showed that the P258L enzyme had no catalytic activity. We conclude that C773T is a disease-causing SCHAD mutation. This is the first defect in fatty acid beta-oxidation that has been associated with hyperinsulinism and raises interesting questions about the ways in which changes in fatty acid and ketone body metabolism modulate insulin secretion by the beta cell. The patient's hyperinsulinism was easily controlled with diazoxide and chlorothiazide.

Tyrosinaemia type I--de novo mutation in liver tissue suppressing an inborn splicing defect.

Many patients with tyrosinaemia type 1 have a mosaic pattern of fumarylacetoacetase (FAH) immunopositive or immunonegative nodules in liver tissue. This phenomenon has been explained by a spontaneous reversion of the mutation in one allele to a normal genotype, but only a few nodules have been examined. We now report on a Norwegian patient, compound heterozygous for the mutations IVS12g(+5)-->a and G(1009-->)A, with liver mosaicism, but with an immunopositive nodule in which both primary mutations were intact. In the immunopositive hepatocytes of this nodule, genetic analyses showed a new mutation, C(1061-->)A, 6 bp upstream of the primary mutation IVS12g(+5)-->a in the FAH gene. The splicing defect caused by the primary mutation is most likely suppressed by the new mutation due to improvement of the splicing site. In the same liver we demonstrate another nodule of regenerating immunopositive tissue due to reversion of one of the primary mutations to a normal genotype. Together with the original cells this makes a triple mosaicism of hepatocytes with one, two or three point mutations in the FAH gene.

Purification of human liver fumarylacetoacetase using immunoaffinity chromatography.

A method is described to purify fumarylacetoacetase from crude human liver extracts using immunoaffinity chromatography. Immobilized rabbit antibodies specific for beef liver fumarylacetoacetase were used as an immunoadsorbent. With this rapid and specific procedure human liver fumarylacetoacetase could be purified to apparent homogeneity. The molecular weight of native human liver fumarylacetoacetase is approximately 83000 as estimated by gel filtration. The two subunits have a molecular weight of approximately 41000, as determined by sodium dodecyl sulphate polyacrylamide gel electrophoresis. Purified human liver fumarylacetoacetase has a broad pH optimum with a maximum at pH 7.2 and a Km = 2.1 microM towards fumarylacetoacetate.

Considering Fabry, but Diagnosing MPS I: Difficulties in the Diagnostic Process.

INTRODUCTION: Recent studies have indicated that a proportion of patients with renal failure, left ventricular hypertrophy, or cryptogenic stroke have sequence variants in their aGal A gene (Fabry disease), which has resulted in an increase in diagnostic activities for this disorder. The diagnostic process for lysosomal storage disorders may result in findings of unknown clinical significance. Here we report such an unexpected outcome. CASE: A 32-year-old male presented at the emergency department because of a transient ischemic attack. Extensive investigations revealed no cause and an initial diagnosis of cryptogenic stroke was made. Subsequently, aGal A activity was measured in a bloodspot and was shown to be normal, but the activity of alpha-L-iduronidase (IDUA), used as reference enzyme, was unexpectedly low: 0.5 umol/L (ref = 1.7-14.3). A diagnosis of IDUA deficiency, mucopolysaccharidosis type 1S or Scheie disease was considered. IDUA gene analysis revealed two homozygous sequence alterations: a silent sequence change (979C > T) in exon 7 (N297N) and an unknown missense mutation 875A > T (R263W). Physical examination was completely normal, without clinical signs of mucopolysaccharidosis type I (MPS I). Leukocyte IDUA activity was also low: 2.1 nmol/mg prot/h (ref = 14-40 nmol prot/h), but higher than the patient range of <0.1 nmol/mg prot/h. Urinary glycosaminoglycan levels were normal both quantitatively and qualitatively. It was concluded that there was low IDUA activity without clinical symptoms and the diagnosis of mucopolysaccharidosis I was discarded. CONCLUSION: The diagnostic process for lysosomal storage disorders may result in biochemical abnormalities of unknown clinical significance. Early evaluation by a specialist in inborn errors of metabolism may help to avoid anxiety in patients and unnecessary additional analyses.

Phosphorylase b kinase deficiency in man: a review.

Phosphorylase b kinase is involved in the activation of glycogen phosphorylase and is thus involved in the breakdown of glycogen. The enzyme exists as several tissue specific isoenzymes of which the muscle enzyme (rabbit) has been most characterized. It is a multimeric protein composed of four subunits, alpha, beta, gamma and delta. The four subunits are coded on different chromosomes, the alpha, beta and gamma subunit genes being on the X, 16 and 17 chromosomes respectively. The delta subunit is a calmodulin and confers calcium sensitivity on phosphorylase b kinase. Tissue specificity of the enzyme is conferred, at least in some cases, by variation in the gamma subunit. Seven different clinical types of phosphorylase b kinase deficiency have been described. The most common type is X-linked and affects the liver only; other types affect liver, muscle and liver, muscle or heart and have an autosomal recessive mode of inheritance, while in some types the mode of inheritance is not clear. Diagnosis based on the study of erythrocytes or leukocytes can be misleading due to the tissue specific nature of the enzyme, and liver or muscle biopsies may be required.

Autosomal recessive liver phosphorylase kinase deficiency caused by a novel splice-site mutation in the gene encoding the liver gamma subunit (PHKG2).

To facilitate mutation analysis of patients with an autosomal recessive form of liver phosphorylase kinase deficiency, the genomic structure of the gene encoding the testis/liver gamma subunit (PHKG2) was established. The gene consist of 10 exons. The translation start site is located in exon 2. Analysis of DNA from two female siblings, affected with liver phosphorylase kinase deficiency, by exon specific amplification followed by direct sequencing, revealed a single donor splice site mutation in the PHKG2 gene, IVS4 + 1(g --> a). The mutation leads to the skipping of exon 4, which results in a frameshift, starting at nucleotide 272, a premature stop codon after 32 additional amino acids, and subsequent loss of the catalytic site. It is concluded that deficiency of phosphorylase kinase in liver of the patients is caused by the IVS4 + 1(g --> a) mutation. In the patients described here, this genotype is associated with development of liver fibrosis.

Autosomal recessive phosphorylase kinase deficiency in liver, caused by mutations in the gene encoding the beta subunit (PHKB).

The association of autosomal recessive phosphorylase kinase deficiency in liver of a 3 1/2-year-old female child with mutations in the gene encoding the common part of the beta subunit of phosphorylase kinase is reported. The proband had a severe deficiency of phosphorylase kinase in liver, while the phosphorylase kinase activity in erythrocytes was only slightly diminished. She had no symptoms of muscle involvement. The complete coding sequences of the liver gamma subunit and of the beta subunit of phosphorylase kinase of the proband were analyzed for the presence of mutations, by either reverse-transcribed PCR or SSCP analysis. Three deviations from the normal sequence were found in the region encoding the common part of the beta subunit of phosphorylase kinase-namely, a 1827G-->A (W609X) transition, a 2309A-->G (Y770C) transition, and a deletion of nucleotides 2896-2911-whereas no mutations were detected in the sequence encoding the liver gamma subunit of phosphorylase kinase. The 1827G-->A mutation and the deletion both result in the formation of early stop codons. Investigation of DNA showed that the deletion is caused by a splice-acceptor site mutation (IVS30(-1),g-->t). Family analysis revealed that the 1827G-->A and IVS30(-1),g-->t substitutions are located on different parental chromosomes and that compound heterozygosity for these mutations segregates with the disease. The 2309A-->G mutation was detected in 2%-3% of the normal population. Thus, it is concluded that the deficiency of phosphorylase kinase in this proband is caused by compound heterozygosity for the 1827G-->A and the IVS30(-1),g-->t mutations and that the 2309A-->G mutation is a polymorphism. This implies that a defect in the sequence encoding the common part of the beta subunit of phosphorylase kinase may present as liver phosphorylase kinase deficiency.

Spectrum of mutations in the fumarylacetoacetate hydrolase gene of tyrosinemia type 1 patients in northwestern Europe and Mediterranean countries.

Hereditary tyrosinemia type 1 (HT1) is a rare metabolic disease caused by a deficient activity of the enzyme fumarylacetoacetase (FAH). To investigate the molecular heterogeneity of tyrosinemia, the geographic distribution and the genotype-phenotype relationship, we have analyzed the FAH genotype of 25 HT1 patients. Mutation screening was performed by PCR amplification of exons 1-14 of the FAH gene, followed by SSCP analysis and direct sequencing of the amplified exons. Fourteen different mutations were found, of which seven were novel, viz. three missense mutations (G158D, P261L, F405H), a deletion of three nucleotides causing a deletion of serine (DEL366S) and three splice site mutations: IVS2+1(g-t), IVS6-1(g-c), IVS8-1(g-c). The splice site mutations IVS6-1(g-t) and IVS12+5(g-a) were frequently found in countries around the Mediterranean and northwestern Europe, respectively. No clear correlation between the genotype and the three major HT1 subtypes could be established.

Human short-chain L-3-hydroxyacyl-CoA dehydrogenase: cloning and characterization of the coding sequence.

The cDNA encompassing the complete coding sequence of human liver short-chain L-3-hydroxyacyl-CoA dehydrogenase (SCHAD) was isolated and characterized. Screening of a cDNA library combined with rapid amplification of 5' cDNA ends resulted in a SCHAD cDNA sequence of 1877 bp. It encodes a protein of 314 amino acids with a calculated molecular weight of 34.3 kDA containing a mitochondrial import signal peptide of 12 amino acids and 302 amino acids of mature SCHAD protein. The deduced amino acid sequence of the mature protein shows a 92 percent identity with SCHAD from pig heart. Northern blot analysis reveals SCHAD mRNA to be expressed in liver, kidney, pancreas, heart and skeletal muscle. The human SCHAD gene was mapped by fluorescence in situ hybridization to chromosome 4q22-26.

Structural organization of the human short-chain L-3-hydroxyacyl-CoA dehydrogenase gene.

The third step in the mitochondrial beta-oxidation spiral of short-chain fatty acids is catalyzed by short-chain L-3-hydroxyacyl-CoA dehydrogenase (HADHSC; EC 1.1.1.35). We have determined the structural organization of the human HADHSC gene by sequencing of cloned genomic amplification products, obtained using HADHSC-specific cDNA-based primers, as well as by direct sequencing of an isolated PAC clone containing the HADHSC gene. Upon comparison with the HADHSC cDNA sequence, HADHSC was shown to encompass at least eight exons, ranging in size from 73 to 158 bp, and 7 introns. The total HADHSC gene spans approximately 49 kb. The HADHSC 5'-flanking region was characterized with an AluI plasmid library constructed from a partially AluI-digested PAC clone containing the human HADHSC gene. Several typical promoter elements such as a CAAT-box, Sp1, AP1, and AP2 sites were found, while a TATA-box was apparently absent. Among other putative regulatory elements, a NRRE-1 site was identified. By radiation hybrid panel, assisted fine-mapping HADHSC was linked to marker AFM070TH5, corresponding to Chromosome (Chr) 4q22-26, and a putative HADHSC pseudogene was linked to marker D15S1324, located at Chr 15q17-21. Knowledge of the genomic organization and 5'-flanking region of HADHSC will enable genomic mutation analysis of patients suspected of HADHSC deficiency, as well as facilitate the investigation into the transcriptional regulation of short-chain fatty acid oxidizing gene products in general and HADHSC expression in particular.

Hereditary tyrosinemia type 1: novel missense, nonsense and splice consensus mutations in the human fumarylacetoacetate hydrolase gene; variability of the genotype-phenotype relationship.

The complete fumarylacetoacetate hydrolase (FAH) genotype of probands of thirteen unrelated families with hereditary tyrosinemia type 1 (HT 1) was established. The screening was performed by analysis of exons 2-14 of the FAH gene by using the polymerase chain reaction (PCR) and of the mRNA by reverse transcription/PCR. Nine different mutations were identified, of which six are novel. Three mutations involve consensus sequences for correct splicing, viz. IVS 6-1 (g-t), IVS 7-1 (g-a) and IVS 12 + 5 (g-a). Two missense mutations (C193R and G369V) and three nonsense mutations (R237X, E357X and E364X) were found. One silent mutation N232N was associated with the skipping of exon 8 from the FAH mRNA. Analysis of the effect of the respective mutations on the FAH mRNA showed a strong reduction of FAH mRNA levels in association with the nonsense mutations, and normal levels with the missense mutations. The splice consensus mutations give deletions of complete or small parts of exon sequences from the FAH mRNA. Data suggest a founder effect for several of the mutations, with a frequency for both the IVS 6-1 (g-t) and IVS 12 + 5 (g-a) mutations of approximately 30% in the HT 1 probands. No strict correlation between genotype and phenotype, i.e. the acute, subacute or chronic form of HT 1, was evident.

X-linked liver phosphorylase kinase deficiency is associated with mutations in the human liver phosphorylase kinase alpha subunit.

Two Dutch patients with liver phosphorylase kinase (PhK) deficiency were studied for abnormalities in the PhK liver alpha (alpha L) subunit mRNA by reversed-transcribed-PCR (RT-PCR) and RNase protection assays. One patient, belonging to a large Dutch family that expresses X-linked liver PhK deficiency, had a C3614T mutation in the PhK alpha L coding sequence. The C3614T mutation leads to replacement of proline 1205 with leucine, which changes the composition of an amino acid region, containing amino acids 1195-1214 of the PhK alpha L subunit, that is highly conserved in different species. The patient showed normal levels of PhK alpha L mRNA. The second patient, from an unrelated family, was found to have a TCT (bp 419-421) deletion in the PhK alpha L coding sequence, resulting in a phenylalanine 141 deletion. The same deletion was found in the PhK alpha L coding sequence from lymphocytes of the patient's mother, together with a normal PhK alpha L coding sequence. The phenylalanine that is absent in the PhK alpha L coding sequence of the second patient is a highly conserved amino acid between species. Both the C3614T mutation and the TCT (bp 419-421) deletion were not found in a panel of 80 control X chromosomes. On the basis of these results, it is postulated that the mutations found are responsible for liver PhK deficiency in the two patients investigated.

Molecular characterization of 3-phosphoglycerate dehydrogenase deficiency--a neurometabolic disorder associated with reduced L-serine biosynthesis.

3-phosphoglycerate dehydrogenase (PHGDH) deficiency is a disorder of L-serine biosynthesis that is characterized by congenital microcephaly, psychomotor retardation, and seizures. To investigate the molecular basis for this disorder, the PHGDH mRNA sequence was characterized, and six patients from four families were analyzed for sequence variations. Five patients from three different families were homozygous for a single nucleotide substitution predicted to change valine at position 490 to methionine. The sixth patient was homozygous for a valine to methionine substitution at position 425; both mutations are located in the carboxyterminal part of PHGDH. In vitro expression of these mutant proteins resulted in significant reduction of PHGDH enzyme activities. RNA-blot analysis indicated abundant expression of PHGDH in adult and fetal brain tissue. Taken together with the severe neurological impairment in our patients, the data presented in this paper suggest an important role for PHGDH activity and L-serine biosynthesis in the metabolism, development, and function of the central nervous system.

A novel disorder of N-glycosylation due to phosphomannose isomerase deficiency.

Three siblings suffered from an unusual disorder of cyclic vomiting and congenital hepatic fibrosis. Serum transferrin isoelectric focusing showed increased asialo- and disialotransferrin isoforms as seen in the carbohydrate-deficient glycoprotein (CDG) syndrome type I. Phosphomannomutase, which is deficient in most patients with type I CDG syndrome, was found to be normal in all three patients. Structural analysis of serum transferrin revealed nonglycosylated, hypoglycosylated, and normoglycosylated transferrin molecules. These findings suggested a defect in the early glycosylation pathway. Phosphomannose isomerase was found to be deficient and the defect was present in leucocytes, fibroblasts, and liver tissue. Phosphomannose isomerase deficiency appears to be a novel glycosylation disorder, which is biochemically indistinguishable from CDG syndrome type I. However, the clinical presentation is entirely different.