Acute intermittent porphyria: characterization of a novel mutation in the structural gene for porphobilinogen deaminase. Demonstration of noncatalytic enzyme intermediates stabilized by bound substrate

A proteomics approach to detect tissue-wide adaptive changes in the pancreas associated with increased pancreatic α-amylase activity in domestic cattle (Bos taurus)

We used a proteomics-based approach to investigate potential regulatory proteins in the pancreas of domestic cattle (Bos taurus) that were associated with differences in pancreatic α-amylase activity. Two groups of 48 and 45 crossbred steers in years 1 and 2, respectively, were fed a high moisture corn-based diet and were ranked according to their pancreatic α-amylase activity. Steers (n=18) with high, medium, and low α-amylase activity were selected, with 3 for each activity range and 9 for each experimental year, and their proteomic profiles were compared. Pancreatic samples from each animal were fractionated using 2D-HPLC and fractions detected using UV spectrophotometry. Software analysis revealed 119 common protein fractions among the 18 animals, and statistical analysis revealed 10 of these fractions differing (P<0.10) in abundance between animals from the high and low pancreatic α-amylase activity groups. Five protein fractions identified after tandem mass spectrometry analysis and database searches were found to match proteins with protein-binding, nucleotide/DNA-binding or enzymatic capabilities. Bioinformatics analysis of these fractions revealed porphobilinogen deaminase, a DNA-binding protein, and a putative S1 peptidase that increased in abundance with increasing α-amylase activity; with a putative ATP/GTP binding protein decreasing in abundance with increasing pancreatic α-amylase activity. Changes in these fractions may represent adaptations of the pancreas in domestic cattle that are associated with differences in α-amylase activity.

Clinical and biochemical characteristics and genotype-phenotype correlation in 143 Finnish and Russian patients with acute intermittent porphyria

Acute intermittent porphyria (AIP), resulting from a deficiency of porphobilinogen deaminase (PBGD) in heme biosynthesis, is genetically heterogeneous and manifests with variable penetrance. The clinical outcome, prognosis, and correlation between PBGD genotype and phenotype were investigated in 143 Finnish and Russian AIP patients with 10 mutations (33G-->T, 97delA, InsAlu333, R149X, R167W, R173W, R173Q, R225G, R225X, 1073delA). Thirty-eight percent of the patients had experienced 1 or more acute attacks during their lives. The proportion of symptomatic patients has decreased dramatically from 49% to 17% among patients diagnosed before and after 1980, respectively. Patients with the R167W and R225G mutations showed lower penetrance (19% and 11%, respectively) and recurrence rate (33% and 0%, respectively) than patients with other mutations (range, 36%-67% and 0%-66%, respectively). Moreover, urinary excretions of porphyrins and their precursors were significantly lower in these patients (porphobilinogen [PBG], 47 +/- 10 vs. 163 +/- 21 micromol/L, p < 0.001; uroporphyrin, 130 +/- 40 vs. 942 +/- 183 nmol/d, p < 0.001). Erythrocyte PBGD activity did not correlate with PBG excretion in remission or with the clinical severity of the disease. Mutations R167W and R225G resulted in milder biochemical abnormalities and clinical symptoms indicating a milder form of AIP in these patients. In all AIP patients, normal PBG excretion predicted freedom from acute attacks. The risk of symptoms was highest for female patients with markedly increased PBG excretion (>100 micromol/L). Proper counseling contributed to the prevention of subsequent attacks in 60% of previously symptomatic and in 95% of previously symptom-free patients.

Heterogeneity of acute intermittent porphyria: a subtype with normal erythrocyte porphobilinogen deaminase activity in Germany

Patients with acute intermittent porphyria can be subdivided into three groups, according to the porphobilinogen deaminase activity in their erythrocytes. The first group has lowered, the second overlapping and the third normal porphobilinogen deaminase activity. Of 385 acute intermittent porphyria patients 5% had normal porphobilinogen deaminase activity. Gene carriers of acute intermittent porphyria, which have normal porphobilinogen deaminase activity but display slight, moderate or high aberrations of excretion, are recognized by analysis of urinary haem precursors and faecal porphyrins. Six individuals suffering from acute intermittent porphyria were detected in three families with normal porphobilinogen deaminase. There were no differences in the latent and clinical phases of acute intermittent porphyria between patients with lowered and those with normal porphobilinogen deaminase. One female with normal activity in erythrocytes, in which the porphyria disease process is triggered by barbiturates and carbamazepine, is presented. After therapy with high doses of glucose and omission of inducing agents, this woman was free of symptoms, and the excretion of different urinary porphyrin precursors and porphyrins decreased by between 65 and 93%.

Epidemiology and clinical characteristics of seizures in patients with acute intermittent porphyria

The objectives of this study were to investigate the lifetime prevalence of epileptic seizures in a population with acute intermittent porphyria (AIP) and to characterize the seizures and the seizure-triggering factors. A letter was sent to all patients with known AIP in Sweden registered at the National Porphyria Center (n = 294). The medical records of patients who had had epileptic seizures were reviewed in detail. The letter was answered by 268 patients (91.2%). Ten patients (3.7%) reported epileptic seizures. Eight were women (mean age 54.1 years, range 30-81 years), and 2 were men (mean age 19 years, range 9-29 years). Six patients had tonic-clonic seizures and 4 had partial seizures becoming secondarily generalized. Serum sodium levels were low in 3 patients (mean 110, range 103-120 mM), and normal in 5. Excretion of 5-aminolevulinic acid (ALA) in the urine was increased in 4 patients at the time of the seizures. In 6 patients, the seizures were associated with an acute attack of AIP (all patients with hyponatremia included). The lifetime prevalence of AIP-associated seizures was 2.2% of all those with known AIP and 5.1% of all those with manifest AIP. Epileptic seizures among persons with AIP are less common than has been previously described.

Molecular basis of acute intermittent porphyria

Acute intermittent porphyria is an inherited disease of haem biosynthesis that results from mutation of the gene for the enzyme porphobilinogen deaminase. Many different mutations have been located throughout the gene. The three-dimensional structure of the enzyme helps in understanding how these mutations lead to inactivation even when, in some cases, the mutated product is abundant and folded correctly.

Porphobilinogen deaminase gene structure and molecular defects

Porphobilinogen deaminase (PBGD) is the third enzyme of the heme biosynthetic pathway. The half-normal activity of human PBGD causes acute intermittent porphyria (AIP), an autosomal dominant inherited disease. Two PBGD isoforms, one ubiquitous and one erythroid specific, are encoded by a single gene localized to chromosomal region 11q24.1-11q24.2. The 10-kb PBGD gene comprises 15 exons and two distinct promoters initiate the ubiquitous and the erythroid transcripts by alternative splicing. In AIP, diagnosis of asymptomatic heterozygotes is crucial to prevent occurrence of life-threatening acute attacks by avoiding known precipitating factors. Difficulties with the biochemical diagnosis could be overcome by the ability to identify the PBGD gene defects in AIP patients. Mutational analysis of the PBGD gene or the use of intragenic polymorphisms offer accurate identification of the gene carriers. To date, 58 mutations and 10 polymorphisms have been reported at the PBGD locus. The great heterogeneity of the mutations in AIP patients requires appropriate screening and diagnostic strategies to identify gene defects in AIP families.

Acute intermittent porphyria: diagnostic conundrums

Acute intermittent prophyria is a genetic disorder of haem biosynthesis caused by defects in the gene encoding hydroxymethylbilane synthase on the long arm of chromosome 11. Every effort should be made to identify gene carriers amongst the relatives of patients known to have acute intermittent porphyria as they are at risk of developing potentially fatal neurogenic attacks if exposed to precipitating factors. Erythrocyte hydroxymethylbilane synthase activity was determined in 46 members of two large well characterised families by assaying enzyme activity by both high performance liquid chromatography (HPLC) and fluorimetric assays. Additionally, hydroxymethylbilane synthase immunoreactivity was determined by a sandwich-type ELISA. Statistically significant correlations were observed between erythrocyte hydroxymethylbilane synthase activity assayed by HPLC and by the fluorimetric assay, and enzyme protein concentration (r = 0.85, p < 0.001 and r = 0.80, p < 0.001, respectively). The assay of hydroxymethylbilane synthase immunoreactive concentration in erythrocytes was useful in excluding acute intermittent porphyria in one patient in whom unequivocal assignment of porphyric status was not possible by assaying enzyme activity alone. Erythrocyte hydroxymethylbilane synthase activity assayed by HPLC and fluorimetry showed approximately equal diagnostic performances, both giving rise to a dichotomic distribution of values, with overlap zones of 6% (1/16) and 22% (2/9), respectively, at the "cut off" applied.

Acute intermittent porphyria: identification and expression of exonic mutations in the hydroxymethylbilane synthase gene. An initiation codon missense mutation in the housekeeping transcript causes "variant acute intermittent porphyria" with normal expression of the erythroid-specific enzyme

Acute intermittent porphyria (AIP), an autosomal dominant inborn error, results from the half-normal activity of the heme biosynthetic enzyme, hydroxymethylbilane synthase (EC 4.3.1.8). Diagnosis of AIP heterozygotes is essential to prevent acute, life-threatening neurologic attacks by avoiding various precipitating factors. Since biochemical diagnosis is problematic, the identification of hydroxymethylbilane synthase mutations has facilitated the detection of AIP heterozygotes. Molecular analyses of unrelated AIP patients revealed six exonic mutations: an initiating methionine to isoleucine substitution (M1I) in a patient with variant AIP, which precluded translation of the housekeeping, but not the erythroid-specific isozyme; four missense mutations in classical AIP patients, V93F, R116W, R201W, C247F; and a nonsense mutation W283X in a classical AIP patient, which truncated the housekeeping and erythroid-specific isozymes. Each mutation was confirmed in genomic DNA from family members. The W283X lesion was found in another unrelated AIP family. Expression of each mutation in Escherichia coli revealed that R201W, C247F, and W283X had residual activity. In vitro transcription/translation studies indicated that the M1I allele produced only the erythroid-specific enzyme, while the other mutant alleles encoded both isozymes. These mutations provide insight into the molecular pathology of classic and variant AIP and facilitate molecular diagnosis in AIP families.

The three-dimensional structures of mutants of porphobilinogen deaminase: toward an understanding of the structural basis of acute intermittent porphyria

Mutations in the human gene for the enzyme porphobilinogen deaminase give rise to an inherited disease of heme biosynthesis, acute intermittent porphyria. Knowledge of the 3-dimensional structure of human porphobilinogen deaminase, based on the structure of the bacterial enzyme, allows correlation of structure with gene organization and leads to an understanding of the relationship between mutations in the gene, structural and functional changes of the enzyme, and the symptoms of the disease. Most mutations occur in exons 10 and 12, often changing amino acids in the active site. Several of these are shown to be involved in binding the primer or substrate; none modifies Asp 84, which is essential for catalytic activity.

Frameshift mutations in exons 9 and 10 of the porphobilinogen deaminase gene produce a crossreacting immunological material (CRIM)-negative form of acute intermittent porphyria

Single-strand conformation polymorphism analysis was used to screen all 15 exons of the porphobilinogen deaminase gene from 13 patients with acute intermittent porphyria. Unique banding patterns in two amplified gene fragments, one containing exon 9 and another containing exon 10, were further investigated. Sequence analysis of cloned genomic DNA revealed a single base pair insertion in the middle of exon 9 in one patient and a single base pair deletion near the 3' end of exon 10 in two related patients. Both mutations change the reading frame of the mRNA transcript and predict proteins that are normal at their NH2-terminal ends but contain novel, unrelated sequences at their COOH-terminal ends and are prematurely terminated. Frameshift mutations in the porphobilinogen deaminase gene are uncommon; this is the first report of an insertion mutation causing acute intermittent porphyria.

Molecular basis of acute intermittent porphyria: mutations and polymorphisms in the human hydroxymethylbilane synthase gene

Acute intermittent porphyria (AIP) is an autosomal dominant inborn error of metabolism that results from the half-normal activity of the third enzyme in the heme biosynthetic pathway, hydroxymethylbilane synthase (HMB-synthase). AIP is an ecogenetic condition, with life-threatening acute attacks precipitated by various factors including drugs, alcohol, fasting, and certain hormones. Biochemical diagnosis is problematic and the identification of mutations in the HMB-synthase gene provides accurate detection of presymptomatic heterozygotes, permitting avoidance of the acute precipitating factors. Two HMB-synthase isozymes are encoded by the HMB-synthase gene: one unique to erythroid cells and the other a housekeeping isozyme present in all cells. These two isozymes arise from a single gene by alternative splicing. The recent isolation of the cDNAs and entire genomic sequence encoding the HMB-synthase isozymes has facilitated the detection of diagnostically useful intragenic polymorphisms and disease-causing mutations. Of the 36 mutations identified to date, most caused the classic form of AIP. These mutations included small deletions and insertions, point mutations and RNA splice junction alterations and resulted in the half-normal activity of both the erythroid-specific and housekeeping isozymes. Most AIP mutations were private; however, certain mutations were frequently found in Dutch (R116W) and Swedish (W198X) AIP families. A variant form of AIP, in which patients have normal erythroid activity, but half-normal activity of the housekeeping isozyme, resulted from two mutations at the exon 1/intron 1 boundary, each altering splicing of the hepatic-specific transcript. In addition, 10 polymorphisms in the HMB-synthase gene have been identified that are useful for the diagnosis of presymptomatic AIP heterozygotes in families whose specific mutations have not been determined.

Detection of eleven mutations causing acute intermittent porphyria using denaturing gradient gel electrophoresis

Acute intermittent porphyria (AIP) is an autosomal dominant disease characterized by mutations of the gene coding for porphobilinogen deaminase (PBGD). Until now, sixteen different mutations have been described. In an effort to investigate further the molecular epidemiology of AIP, we have undertaken a systematic study of different exons of the PBGD gene from a large number of unrelated patients. Here, we have examined seven of the fifteen exons of the gene from 43 unrelated Dutch and French AIP patients using denaturing gradient gel electrophoresis after polymerase chain reaction amplification. Eleven new mutations were found, accounting for the enzymatic defect in about half of the patients. This study further documents the molecular heterogeneity of the mutations responsible for AIP and describes an efficient strategy to detect the mutations in patients with previously unknown abnormalities.

Detection of a high mutation frequency in exon 12 of the porphobilinogen deaminase gene in patients with acute intermittent porphyria

Direct cDNA sequencing was performed on asymmetrically amplified transcripts from the porphobilinogen deaminase (PBG-D) gene of thirteen unrelated individuals with acute intermittent porphyria. Four different mutations and a polymorphic site were detected in exon 12 of the gene, four being the result of single base substitutions and one being caused by dinucleotide deletion. All of these mutations are located in domain 3 of the PBG-D molecule, with the single base substitutions affecting the hydrophobic interfaces between domains 1 and 3. The dinucleotide deletion results in a frame-shift producing a premature stop codon.

Genetic carrier detection in Norwegian families with acute intermittent porphyria

Early detection of carriers of acute intermittent porphyria (AIP) is of great value as an assistance for correct diagnosis and prevention of attacks. In order to complement traditional biochemical methods, restriction fragment length polymorphism (RFLP) studies as well as analysis for a previously identified point mutation were included in a study of three Norwegian AIP families. Several asymptomatic carriers could be identified, and the study thus demonstrates the usefulness of the combination of biochemical and genetic analysis.

Variable phenotypic expression of genotypic abnormalities in the porphyrias

The inherited porphyrias are the consequence of inherited deficiencies of enzymes in the heme synthesis pathway; they exhibit classical Mendelian inheritance patterns. The acute porphyrias (acute intermittent, porphyria variegata, hereditary coproporphyria) result from 50% (approx.) deficiencies of specific enzymes, which demonstrate autosomal dominant inheritance. However, only approx. 10% of subjects who inherit a porphyrin enzyme deficiency develop the corresponding acute porphyria and in most instances there is no obvious reason why one patient with an enzyme deficiency is symptomatic whereas another is not. Control of heme synthesis is achieved by the repressor effect of heme on the enzyme ALA synthase. Acute attacks of porphyria can be precipitated in susceptible persons by drugs, ethanol, starvation, hormones, stress and infection. The mechanism is usually by induction of ALA synthase activity. The molecular biology of porphyria variegata and hereditary coproporphyria is large unexplored. Acute intermittent porphyria is due to a partial deficiency of the enzyme porphobilinogen deaminase in the liver. The location of the gene for this enzyme has been identified on the long arm of chromosome 11. Acute intermittent porphyria is a genetically heterogenous disease with the abnormality frequently being a point mutation affecting synthesis of the enzyme.

High prevalence of a point mutation in the porphobilinogen deaminase gene in Dutch patients with acute intermittent porphyria

Acute intermittent porphyria (AIP) is an autosomal dominant disease characterized by a deficiency of porphobilinogen deaminase (PBGD). Up to now 14 different mutations have been described. In an effort to investigate the molecular epidemiology of AIP we have undertaken a systematic study of different exons of the PBGD gene from a large number of unrelated patients. Here, exon 8 from 82 unrelated Dutch and French AIP patients was examined using single strand confirmation polymorphism analysis (SSCP) after polymerase chain reaction (PCR) amplification. A single base mutation, C to T, at position 346 of the sequence coding for PBGD was observed in 15 Dutch families but in only 1 French family. A simple PCR assay is described to facilitate the diagnosis of this common mutation at the DNA level.

Detection of seven point mutations in the porphobilinogen deaminase gene in patients with acute intermittent porphyria, by direct sequencing of in vitro amplified cDNA

Direct cDNA sequencing has been performed on asymmetrically amplified transcripts from the human porphobilinogen deaminase gene. Lymphocytes from 30 patients with acute intermittent porphyria were the source of mRNA; of the seven separate point mutations detected, three were silent, whereas four resulted in amino acid changes. Three of these changes involved highly conserved amino acids, and the remaining one a conserved charge. One of these mutations was predicted to cause structural alterations in the protein product. The application of this method to affected families allows the direct identification of these heterogeneous mutations, thus permitting the unequivocal detection of carriers.

High frequency of mutations in exon 10 of the porphobilinogen deaminase gene in patients with a CRIM-positive subtype of acute intermittent porphyria

Acute intermittent porphyria (AIP) is an autosomal dominant disease characterized by a partial deficiency of porphobilinogen (PBG) deaminase. Different subtypes of the disease have been defined, and more than 10 different mutations have been described. We focused our study on exon 10, since we previously found that three different mutations were located in this exon and that two of them seemed to be relatively common. We used denaturing gradient gel electrophoresis (DGGE) after in vitro amplification to detect all possible mutations in exon 10 in 41 unrelated AIP patients. In about one-fourth of these patients we could distinguish three abnormal migration patterns, indicating the presence of various mutations. Additional sequencing demonstrated the presence of three different single-base substitutions. Two of these mutations had already been described. A third one consisted of a C-to-T transition located at position 499 of the PBG deaminase mRNA and resulted in an Arg-to-Trp substitution. All three mutations were found in patients with cross-reacting immunological material (CRIM)-positive forms of AIP. The high frequency of these mutations make DGGE analysis of exon 10 a useful approach allowing the direct direction of the DNA abnormality in most of the families with the CRIM-positive subtype of AIP.

CRIM-positive mutations of acute intermittent porphyria in Finland

Acute intermittent porphyria (AIP) is a dominantly inherited metabolic disease caused by a partial deficiency of the third enzyme, porphobilinogen deaminase (PBGD), in the heme biosynthetic pathway. AIP has been divided into two subtypes according to the ratio of enzyme polypeptide concentration and enzyme activity measured in erythrocytes: cross-reacting immunologic material (CRIM) positive or negative. In this study six out of the seven known CRIM-positive AIP families in Finland were analyzed and two also previously identified mutations in the PBGD gene were found to be responsible for AIP in this genetically isolated population. The search for mutations was focused on exon 10 based on previously found mutations. SSCP analysis revealed a known polymorphism but the two mutations in that region were found only by direct sequencing of the PCR products. A G518-->A substitution changing Arg173 to Gln was found in three families and a C499-->T substitution changing Arg167 to Trp was detected in three families. DNA analyses of the family members revealed that conventional assays of erythrocyte PBGD activity identified correctly only 72% of the carriers for the AIP mutation.

Identification of the most common mutation within the porphobilinogen deaminase gene in Swedish patients with acute intermittent porphyria

Acute intermittent porphyria (AIP) is a metabolic disorder characterized by a partial deficiency of the porphobilinogen deaminase (PBGD, EC 4.3.1.8) activity. Previous haplotype analysis combined with genealogical data suggested a common origin of the PBGD gene mutation in the AIP families originating from northern Sweden (Lappland), where the highest prevalence of the disease (1 in 1500) is observed. An AIP family from Lappland consisting of two patients and two unaffected subjects was investigated. The genomic DNA fragments of the PBGD gene were amplified by polymerase chain reaction (PCR) and directly sequenced, and the sequence of the coding region was compared with the normal sequence to identify the mutation. A base substitution, G to A, in exon 10 of the PBGD gene was identified. The mutation changes the codon for Trp198 to a stop codon (nonsense mutation) and creates a recognition site for the restriction enzyme Nhe I. Screening of 33 Swedish AIP families showed that 15 had this mutation. Genealogical data revealed that 12 of the 15 families were related to the northern family. This finding supports the hypothesis of a "founder effect" of the mutation in the families originating from Lappland. In addition, a method is described for detection of specific sequences in the genome by one-sided PCR using Taq polymerase. This method is simple, fast, and economical and can be substituted for hybridization analysis using allele-specific oligonucleotides.

Genetic heterogeneity of the porphobilinogen deaminase gene in Swedish families with acute intermittent porphyria

Acute intermittent porphyria (AIP) is an autosomal dominant metabolic disorder affecting the enzyme porphobilinogen (PBG) deaminase in the heme biosynthetic pathway. The highest prevalence of the disorder has been observed in Scandinavia, especially in northern Sweden (Lappland) where it occurs with a prevalence of 1 in 1500. Biochemical assays of the activity and concentration of PBG deaminase in red blood cells, haplotyping with 4 intragenic restriction fragment length polymorphisms (RFLPs) (MspI, PstI, BstNI, ApaLI) using the polymerase chain reaction (PCR) and screening for known base substitutions by oligonucleotide probes was performed in 28 Swedish AIP families. There was no close relationship between haplotype, biochemical findings (PBG deaminase activity, enzyme-linked immunosorbent assay [ELISA], and excess urinary excretion of delta-aminolevulinic acid or PBG), and a specific mutation. Three different haplotypes were identified. The haplotype 2/1/1/2 (MspI/PstI/BstNI/ApaLI; +/-/-/+) was found to be the most frequent among gene carriers (P less than 0.001). The disease segregated with the haplotype 2/1/1/2 in the 10 families originating from northern Sweden. All 28 families were screened for three known point mutations. Only one was found to carry one of these mutations. Thus, the genetic background of AIP is heterogeneous in Sweden.

Molecular heterogeneity of acute intermittent porphyria: identification of four additional mutations resulting in the CRIM-negative subtype of the disease

Four mutations of the porphobilinogen (PBG) deaminase gene that result in cross-reacting immunological material (CRIM)-negative forms of acute intermittent porphyria (AIP) have been identified by in vitro amplification of cDNA from patients and by cloning of the amplified products in a bacterial expression vector. One mutation is a single base deletion which causes a frameshift and which is expected to result in the synthesis of a truncated protein. Two other mutations consist of single base substitutions and lead to amino acid changes. The fourth mutation is a single base substitution producing an aberrant splicing and resulting in an mRNA which would encode a protein missing three amino acids. DNAs from 16 unrelated CRIM-negative AIP patients were screened for the presence of these four mutations, by hybridization with oligonucleotides specific for each of the mutations, but none of the four mutations was identified in additional patients. The results indicate that mutations responsible for CRIM-negative AIP are highly heterogenous.

Hereditary hepatic porphyria due to homozygous delta-aminolevulinic acid dehydratase deficiency: studies in lymphocytes and erythrocytes

Activities of delta-aminolevulinic acid (ALA) dehydratase and porphobilinogen (PBG) deaminase, and haem content were determined in EB-virus transformed lymphocytes from two patients with homozygous ALA dehydratase deficiency, and their family members to determine the expression of the specific gene defect in this cell type. ALA dehydratase activity, but not PBG deaminase activity or haem content, was markedly decreased in lymphocyte preparations from both patients with homozygous enzyme deficiency, and moderately decreased in subjects heterozygous for enzyme deficiency. Immunochemical quantitation of erythrocyte ALA dehydratase suggested the presence of a cross-reactive material in a patient with a late-onset of acute hepatic porphyria due to the homozygous enzyme deficiency.

Acute intermittent porphyria caused by a C----T mutation that produces a stop codon in the porphobilinogen deaminase gene

A mutation of the porphobilinogen (PBG) deaminase gene that produces the cross-reacting immunological material (CRIM)-negative type of acute intermittent porphyria (AIP) has been identified in one of 43 unrelated patients with this form of the disorder. The mutation is a C----T transition that abolishes a PstI recognition site in exon 9 of the gene and converts a codon for glutamine to a stop codon.

Enzyme heterogeneity in the porphyrias

The use of immunological methods for measuring enzyme mass has identified several varieties of porphyria in which the reduction of porphyrin enzyme activity is not accompanied by a corresponding change in the enzyme mass. Currently, acute intermittent porphyria and hepatoerythropoietic porphyria have exhibited this phenomenon. In porphyria cutanea tarda, it has recently been shown that the pattern of enzyme deficiency in erythrocytic and nonerythrocytic tissues does not strictly follow the inheritance pattern (familial and sporadic) previously described. Also, contrary to previous dogma, some cases of type 1 porphyria cutanea tarda appear to be positive for cross reactive immunological material (CRIM).

Linkage disequilibrium between DNA polymorphisms within the porphobilinogen deaminase gene

Three restriction fragment length polymorphisms (RFLPs) (MspI, PstI, ScrFI/BstNI) within the human porphobilinogen deaminase (PBG-D) gene have been studied in 47 unrelated patients with the autosomal dominant disorder, acute intermittent porphyria (AIP), and in 92 control subjects. Each enzyme identified a two-allele polymorphism with allele frequencies close to 0.50: however, marked linkage disequilibrium limited the number of observed haplotypes to four, of which one is uncommon. No association was detected between any haplotype and AIP.

RFLP analysis of three different types of acute intermittent porphyria

Restriction fragment length polymorphism (RFLP) analysis was performed in three Finnish families with different subtypes of acute intermittent porphyria (AIP): 1) cross-reacting immunological material (CRIM)-negative with low erythrocyte porphobilinogen (PBG)-deaminase activity, 2) CRIM-positive with low PBG-deaminase activity and 3) CRIM-negative with normal PBG-deaminase activity. The disease-associated RFLP haplotype (A2B1C2) of the PBG-deaminase gene was the same in each family. In all three families, RFLP linkage analysis resulted in highly positive lod scores. The maximal lod score (4.3) was obtained at the recombinant fraction of zero, thus confirming a tight linkage of AIP to the PBG-deaminase locus. Of the 62 family members tested, 30 had the disease-associated haplotype; in 5 of them, conventional tests for AIP were normal and in one, uncertain. RFLP analysis can thus reveal new gene carriers and help in the diagnosis of individuals with uncertain results in other laboratory tests.

Regulation of the genes for heme pathway enzymes in erythroid and in non-erythroid cells

There are eight enzymes in the heme biosynthetic pathway and three enzymes in the heme catabolic pathway. Enzymatic defects in heme biosynthesis lead to clinical conditions termed porphyrias. cDNAs for five of the eight enzymes in the heme biosynthetic pathway and two of the three enzymes in the heme catabolic pathway have been cloned and characterized in mammalian cells. At least two enzymes exist as isozymes between erythroid and non-erythroid tissues. One is delta-aminolevulinic acid synthase (ALAS), and the erythroid and hepatic isozymes are coded by two separate genes. The other is porphobilinogen deaminase (PBGD), and both the erythroid and the non-erythroid PBGD mRNA are transcribed from a single PBGD gene by alternate transcription and splicing. There is also a significant tissue-specific control of expression of the uroporphyrinogen decarboxylase gene which is expressed as a unique mRNA in all tissues.

Preparative free solution isoelectric focusing of human erythrocyte uroporphyrinogen I synthase in an ampholyte pH gradient

A free solution electrofocusing method for uroporphyrinogen I synthase (EC 4.3.1.8) in an Ampholine pH gradient on a preparative scale is described. Partial purification of the enzyme was achieved in a 4-h focusing run. Enzyme activity was found in the pH range of pH 5.1 to pH 7.0. Complete separation of the most basic and most acidic isozyme from the control and the acute intermittent porphyria (AIP) patient was obtained in this single-step procedure. The level of enzyme activity has been shown to be reduced to about half the normal value in erythrocytes of two patients from a family with AIP. A shift of maximal activity toward the acidic side of the pH gradient was observed with the abnormal enzyme. In contrast to the normal isozyme set with seven isozyme bands, the fluorescence of the three basic bands and the second acidic band was greatly reduced, whereas the intermediate forms showed increased fluorescence intensity.

Mutations in acute intermittent porphyria detected by ELISA measurement of porphobilinogen deaminase

To study the existence of different mutations in acute intermittent porphyria, erythrocyte porphobilinogen deaminase activity and enzyme protein concentration were investigated in 125 porphyria gene carriers from 31 families, and in 121 apparently healthy controls. Porphobilinogen deaminase concentration (micrograms/gHb) was quantified using a recently developed double-sandwich ELISA. The ratio of enzyme catalytic activity to the concentration of enzyme protein was expressed as the porphobilinogen specific activity (nkat/g). The controls had a mean porphobilinogen deaminase concentration of 160 +/- 35 micrograms/gHb and a specific activity of 762 +/- 127 nkat/g. Two different types of mutation causing acute intermittent porphyria were detected. The majority (91%) of gene carriers, from 25 families, had a diminished porphobilinogen deaminase concentration of 102 +/- 18 micrograms/gHb, with a slightly lowered specific activity of 634 +/- 105 nkat/g. In 9% of the gene carriers, representing six different families, an increase in porphobilinogen deaminase concentration to 269 +/- 46 micrograms/gHb, and a highly significant reduction in specific activity to 234 +/- 48 nkat/g, were found, which indicates the presence of a different mutation.

Porphobilinogen deaminase in human erythrocytes: purification of two forms with apparent molecular weights of 40 kDa and 42 kDa

Porphobilinogen deaminase was purified from human erythrocytes by ion-exchange chromatography, gel filtration and hydrophobic interaction chromatography. Two forms of the enzyme were isolated, with apparent molecular weights of 40 kDa and 42 kDa, and in relative amounts of 85% and 15%, respectively. Both forms were found to have an N-terminal amino acid sequence identical to that published for the erythropoietic form of porphobilinogen deaminase, as deduced from a cDNA clone. The two forms present could each be separated into three differently charged subforms by Mono Q chromatography.

Molecular analysis of acute intermittent porphyria in a Finnish family with normal erythrocyte porphobilinogen deaminase

Porphobilinogen deaminase, the third enzyme of the haem biosynthetic pathway, is encoded by two distinct mRNA species expressed in a tissue-specific manner from a single gene. These two mRNAs are transcribed from two promoters and only differ in their first exon. An inherited deficiency or porphobilinogen deaminase in man is responsible for the autosomal dominant disease acute intermittent porphyria. Different classes of mutations have been described at the protein level suggesting that this is a heterogeneous disease. In the present report, we describe the molecular abnormality responsible for a variant form of acute intermittent porphyria where the enzyme defect is restricted to non-erythroid cells. Upon cloning and sequencing the mutant allele of a patient from a large Finnish kindred, a single-base substitution within the 5'-splice donor sequence of intron 1 was found at the last position of exon 1 (CG----CT). The identification of this mutation allowed us to detect asymptomatic gene carriers among family members using in vitro amplification of DNA and hybridization of the target sequence to allele-specific oligonucleotides.

ELISA for measuring porphobilinogen deaminase in human erythrocytes

An ELISA method has been developed to quantitate human porphobilinogen deaminase in erythrocyte lysate. The antiserum used in the assay was raised against the erythropoietic form of human porphobilinogen deaminase. The IgG fraction was characterized by use of immunoblotting technique, rocket immunoelectrophoresis and immunotitration and shown to be monospecific. The measuring range of the method was from 4 ng to 50 pg. Intra- and inter-assay coefficients of variation were 6% and 7%, respectively. Erythrocyte lysates from 97 apparently healthy individuals were assayed giving a mean erythrocyte porphobilinogen deaminase protein concentration of 150 +/- 28 SD (micrograms/g Hb) and a specific enzyme activity of 750 +/- 140 SD (nkat/g). Eight patients with acute intermittent porphyria were also investigated. A decreased concentration of enzyme protein, i.e. 84 +/- 13 SD (micrograms/g Hb) with a normal specific activity, was found.

Dual porphyria in double heterozygotes with porphobilinogen deaminase and uroporphyrinogen decarboxylase deficiencies

A coexistent dual deficiency of porphobilinogen deaminase (PBG-D; EC 4.3.1.8) and uroporphyrinogen decarboxylase (EC 4.1.1.37) in erythrocytes was recognized in five individuals, four males and one female. Clinically, the female and one male were diagnosed as suffering from acute intermittent porphyria (AIP), and the other two males were diagnosed as having porphyria cutanea tarda (PCT). Biochemically, the excretion pattern of urinary and fecal heme precursors exhibited a complex constellation with signs characteristic for both AIP and PCT. A coexistent dual enzyme deficiency of PBG-D and URO-D could be confirmed by repeated studies over 10 years. Clinical courses of both disease manifestations were observed. Family investigations have shown that the two disorders do not consistently segregate together. The findings suggest that the dual porphyria reflects a double heterozygous condition of coexistent AIP and PCT genes in the same individual.

The porphyrias

The porphyrias are metabolic disorders in which there are excessive accumulation and excretion of porphyrins and porphyrin precursors. Each of the porphyrias has a specific enzyme defect in the pathway of heme biosynthesis that explains the pattern of biochemical abnormalities that occur. However, some patients have the enzyme defect but do not have clinical or biochemical manifestations, indicating that other factors (e.g., demand for increased heme biosynthesis) are also important in causing disease expression. The major clinical manifestations are neurologic dysfunction and photosensitivity. The precise cause of the neurologic dysfunction has not been defined, but the likely possibilities are overproduction of delta-aminolevulinic acid, which may act as a neurotoxin, or a deficiency of heme (or both). The cutaneous lesions in the porphyrias are due to the photo-sensitizing and other effects of porphyrins that are deposited in the skin or are circulating in dermal blood vessels. Therapy is directed to modify the biochemical abnormalities. Most importantly, intravenous administration of hematin is used in the treatment of acute attacks of neurologic dysfunction. Prevention remains a cornerstone in management of patients with porphyria, and those with gene defects should be counseled regarding factors that precipitate acute attacks.

DNA polymorphisms within the porphobilinogen deaminase gene in two Swedish families with acute intermittent porphyria

Two unrelated families with acute intermittent porphyria (AIP), an autosomal dominant disease related to a defect in porphobilinogen deaminase (PBG-D, EC 4.1.3.8.), were studied with regard to three restriction fragment length polymorphisms (RFLPs) (MspI, PstI, BstNI) within the PBG-D gene. The results indicate that linkage analysis of RFLPs within the gene can be used as a complement to PBG-D analysis for the diagnosis of gene carriers in families with AIP.

Alternative transcription and splicing of the human porphobilinogen deaminase gene result either in tissue-specific or in housekeeping expression

Porphobilinogen deaminase [PBGD; porphobilinogen ammonia-lyase (polymerizing), EC 4.3.1.8] is a cytosolic enzyme involved in the heme biosynthetic pathway. Two isoforms of PBGD, encoded by two mRNAs differing solely in their 5' end, are known: one is found in all cells and the other is present only in erythroid cells. We have previously shown that the human PBGD is encoded by a single gene and have now cloned and characterized this gene, which is split into 15 exons spread over 10 kilobases of DNA. We demonstrate that the two mRNAs arise from two overlapping transcription units. The first one (upstream) is active in all tissues and its promoter has some of the structural features of a housekeeping promoter; the second, located 3 kilobases downstream, is active only in erythroid cells and its promoter displays structural homologies with the beta-globin gene promoters.

DNA polymorphism of human porphobilinogen deaminase gene in acute intermittent porphyria

A common two-allele MspI restriction fragment length polymorphism of the human erythroid porphobilinogen (PBG)-deaminase gene was investigated in 33 unrelated patients with acute intermittent porphyria (AIP) and 20 controls. The polymorphism was tightly linked (lod score 3.14; no recombinants) to the locus for AIP as identified by measurement of erythrocyte PBG-deaminase activity. The frequency of the polymorphism in the AIP patients did not differ significantly from that in the controls. No common polymorphisms for eight other restriction endonucleases were found in either group. In 30 of the AIP patients no crossreacting immunological material (CRIM) was produced by the mutant PBG-deaminase allele. The MspI polymorphism enabled each PBG-deaminase allele to be distinguished in subjects heterozygous for the polymorphism; thus a major gene deletion was excluded as the cause of the CRIM-negative mutation in all of the 18 families that contained an affected CRIM-negative individual heterozygous for the polymorphism. In suitable families, the MspI polymorphism provides a more certain way of identifying carriers of the AIP gene than current enzymatic methods and major gene deletions are unlikely to be present in more than a small proportion of the commonest type of AIP, the CRIM-negative form.

Tissue-specific expression of porphobilinogen deaminase. Two isoenzymes from a single gene

Porphobilinogen deaminase (hydroxymethylbilane synthase; EC 4.3.1.8), the third enzyme of the heme biosynthetic pathway, catalyzes the stepwise condensation of four porphobilinogen units to yield hydroxymethylbilane, which is in turn converted to uroporphyrinogen III by cosynthetase. We compared the apparent molecular mass of porphobilinogen deaminase from erythropoietic and from non-erythropoietic cells by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and immune-blotting. The results indicate that two isoforms of porphobilinogen deaminase can be distinguished and differ by 2000 Da. Analysis of cell-free translation products directed by mRNAs from human erythropoietic spleen and from human liver demonstrates that the two isoforms of porphobilinogen deaminase are encoded by distinct messenger RNAs. We cloned and sequenced cDNAs complementary to the non-erythropoietic form of porphobilinogen deaminase encoding RNA. Comparison of these sequences to that of human erythropoietic mRNA [Raich et al. (1986) Nucleic Acids Res. 14, 5955-5968] revealed that the two mRNA species differ by their 5' extremity. From the mRNA sequences we could deduce that an additional peptide of 17 amino acid residues at the NH2 terminus of the non-erythropoietic isoform of porphobilinogen deaminase accounts for its higher molecular mass. RNase mapping experiments demonstrate that the two porphobilinogen deaminase mRNAs are distributed according to a strict tissue-specificity, the erythropoietic form being restricted to erythropoietic cells. We propose that a single porphobilinogen deaminase gene is transcribed from two different promoters, yielding the two forms of porphobilinogen deaminase mRNAs. Our present finding may have some relevance for further understanding the porphobilinogen deaminase deficiency in certain cases of acute intermittent porphyria with an enzymatic defect restricted in non-erythropoietic cells.

Molecular cloning and complete primary sequence of human erythrocyte porphobilinogen deaminase

We have cloned and sequenced a cDNA clone coding for human erythrocyte porphobilinogen deaminase. It encompasses the translated region, part of the 5' and the 3' untranslated regions. The deduced 344 amino acid sequence is consistent with the molecular weight and the partial amino-acid sequence of the NH2 terminal of the purified erythrocyte enzyme. Southern analysis of human genomic DNA shows that its gene is present as a single copy in the human genome and Northern analysis demonstrates the presence of a single size species of mRNA in erythroid and non-erythroid tissues and in several cultured cell lines. Quantitative determinations indicate that the amount of PBG-D mRNA is modulated both by the erythroid nature of the tissue and by cell proliferation, probably at the transcriptional level.

Porphobilinogen deaminase is unstable in the absence of its substrate

Porphobilinogen deaminase is induced during the dimethyl sulfoxide-mediated differentiation of Friend erythroleukemia cells. We have previously shown that when succinylacetone, a potent inhibitor of porphobilinogen formation, is present during the differentiation process, the induction of the enzyme is apparently suppressed. Here, we provide evidence that, in this condition, porphobilinogen deaminase is synthesized normally but does not accumulate as a consequence of an accelerated turnover. The normal half-life of the protein is 24 h but decreases to 10 h when the formation of its substrate is impaired by succinylacetone. We propose that when the enzyme is covalently bound to its substrate, a normal step in this enzymatic reaction, it is protected from proteolytic degradation, and we show that this new finding is relevant to the human disorder acute intermittent porphyria.

Genetic heterogeneity in acute intermittent porphyria: characterisation and frequency of porphobilinogen deaminase mutations in Finland

The occurrence of different porphobilinogen deaminase mutant types in 68 patients with acute intermittent porphyria from 33 unrelated families in Finland was studied with biochemical and immunological techniques. In this fairly homogenous population four different porphobilinogen deaminase mutant types were identified and their frequencies determined. Most (about 80%) of the mutations were cross reacting immunological material (CRIM) negative, including a large kindred with normal erythrocyte porphobilinogen deaminase activities. The remainder of the families had CRIM positive mutations, including an unusual type (type 2) that had an immunoreactive, non-catalytic porphobilinogen deaminase level considerably greater than the maximal theoretical ratio of CRIM to activity of 2.0 for a single mutant allele. Correlations of the amount of residual porphobilinogen deaminase activity and the occurrence of acute clinical manifestations in each mutant type suggested that CRIM positive type 2 patients may have fewer acute symptoms.