RFLP analysis of three different types of acute intermittent porphyria

Hydroxymethylbilane synthase gene mutations and polymorphisms in Brazilian families with acute intermittent porphyria

Acute intermittent porphyria (AIP), an autosomal dominant disorder, is caused by a deficiency of hydroxymethylbilane synthase (HMBS). In the present study, we sought to establish a correlation between HMBS activity with the presence of mutations and polymorphisms. Enzyme activity was measured in red blood cells of four Brazilian unrelated AIP families (n = 124) and in blood donors (n = 80). The HMBS mutations in AIP family members were studied by PCR-SSCP followed by direct sequencing. Six intragenic SNPs (1345 G>A, 1500 T>C, 2377 C>A, 2478 A>G, 3581 A>G, and 7064 C>A) were determined by PCR-RFLP. Abnormal SSCP patterns in exons 7, 9, 12, and 15 were observed. DNA sequencing analysis revealed one nonsense mutation, R149X, two missense mutations, G111R and L338P, and one deletion, CT 730-731. All mutation carriers had lower enzyme activity. All polymorphisms, except 2377 C>A and 7064 C>A, showed no significant differences compared with previous reports. Mutation screening allowed the detection of the missense mutation, L338P, and the 730_731delCT deletion, two as yet unreported mutations in Brazilian AIP patients. Our findings also showed a high frequency of 2478 A>G and 3581 A>G polymorphism combinations suggesting that these polymorphisms contributed to enzymatic activity reduction in our study population.

Diagnostic strategy, genetic diagnosis and identification of new mutations in intermittent porphyria by denaturing gradient gel electrophoresis

Acute intermittent porphyria (AIP) is an autosomal dominant inherited disease of heme metabolism caused by mutations in the hydroxymethylbilane synthase gene. Diagnosing AIP during an acute attack using traditional biochemical markers is unproblematic, but it can be difficult to obtain a definite diagnosis in asymptomatic carriers. These limitations may, however, be solved through a genetic approach for diagnosing AIP carrier status. A mutation screening assay based on the denaturing gradient gel electrophoresis (DGGE) principle was established in a setup that allows within 24 hr to pinpoint which of the 15 exons of the hydroxymethylbilane synthase gene carries the underlying mutation, and thereby reduces subsequent sequencing, needed to determine the specific mutation, to this particular gene region. To evaluate sensitivity and specificity of the DGGE assay, samples from 22 AIP patients with known mutations and six healthy controls were examined in a blinded design. Following unblinding, it was revealed that in all 22 AIP samples the correct mutation carrying region had been pointed out. In two samples containing a previously undescribed polymorphism, this additional region was also pointed out. All controls were correctly characterized as normal in the DGGE assay. Subsequently, to evaluate the assay in the clinical setting, samples from six previously uncharacterized Danish AIP probands were examined and the underlying mutation detected in all six. In conclusion, a simple and sensitive mutation screening assay based on the DGGE principle allows genetic diagnosis of AIP in a routine setting and may be used as an additional tool in genetic counseling of AIP families.

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: 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.

Developmental change in activity of red cell porphobilinogen deaminase and its electrophoretic variant in the Japanese population

The activity of porphobilinogen deaminase (PBGD), an enzyme whose partial deficiency is associated with acute intermittent porphyria (AIP), changes during development. Little is known about the postnatal change of PBGD activity and the prevalence of its electrophoretic variant in the Japanese population. The activity of PBGD was measured fluorometrically in 194 infants aged 0-12 months, while isoelectric focusing of PBGD was performed in 400 healthy Japanese adults aged 20-45 years and 30 children with various hematological disorders aged 1-15 years. The PBGD level was 1.9 times higher in the neonates than in the adults, decreased abruptly during the first month of life, and reached the adult level at the age of 9 months. None of the 400 healthy Japanese adults and the 30 children with hematological disorders showed any electrophoretic variant. These results suggest that there is no need to consider any polymorphism in the gene dose study of PBGD and that the biochemical screening of AIP is applicable to since the late infancy.

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 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.

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.

Single-strand conformation polymorphism (SSCP) analysis applied to the diagnosis of acute intermittent porphyria

The single-strand conformation polymorphism (SSCP) technique was used to detect carriers of the known point mutation in the first exon of the porphobilinogen deaminase gene in Finnish and Swedish families. The SSCP technique was a reliable and convenient way of distinguishing patients from healthy members in a family. This point mutation is thought to result from a splicing defect of the mRNA. The PCR-based analyses of a patient's cDNA did not reveal the presence of an abnormal mRNA population, suggesting that no abnormal mRNA is synthesized or that it is too unstable to be detected.