Detection of four mutations in six unrelated South African patients with acute intermittent porphyria

Molecular characterisation of acute intermittent porphyria in a cohort of South African patients and kinetic analysis of two expressed mutants

AIMS

Acute intermittent porphyria (AIP) is a disorder of the haem biosynthetic pathway caused by mutations in the hydroxymethylbilane synthase (HMBS) gene. Knowledge of the spectrum of mutations present in South Africa is limited. This study presents the molecular profile of 20 South African patients with AIP, and the kinetic analysis of one novel expressed mutated HMBS enzyme and a previously identified mutation at the same position.

METHODS

Genomic DNA was isolated from affected probands and selected family members, the HMBS gene amplified and mutations characterised by direct sequencing and restriction enzyme analysis. One of the novel mutations (p.Lys98Glu), a previously characterised mutation at the same position (p.Lys98Arg), and the wild-type enzyme were expressed, purified and subjected to partial kinetic characterisation.

RESULTS

Four new mutations, p.Lys98Glu, p.Asp230Aspfs_*20, c.161-1G>A and c.422+3_6delAAGT, are described. Seven previously described mutations were found, while four patients revealed no mutations. Mutation analysis of five offspring of one of the probands carrying the p.Trp283X mutation revealed two asymptomatic carriers. Kinetic analysis showed that the p.Lys98Glu mutation results in loss of substrate affinity, whereas the previously described p.Lys98Arg mutation causes the loss of binding between the enzyme and its dipyrromethane cofactor, rendering the enzyme inactive.

CONCLUSIONS

This study comprises the most comprehensive characterisation of HMBS gene mutations in patients with AIP in South Africa. The biochemical characterisation of expressed HMBS mutants reveals insight into the mechanism of catalytic activity loss, which may inspire investigation into individualised therapy based on the molecular lesion identified.

Marked geographic aggregation of acute intermittent porphyria families carrying mutation Q180X in Venezuelan populations, with description of further mutations

Acute intermittent porphyria (AIP) caused by mutations in the hydroxymethylbilane synthase gene (HMBS), has been reported in almost all human populations, with varying frequencies. A founder effect for a few specific mutations in geographic regions where prevalence is high (Sweden, The Netherlands, Switzerland) has been established through haplotype analyses, while some other mutations (R26H, R26C) have been repeatedly reported in many populations with different genetic backgrounds. Epidemiological, biochemical and molecular data on AIP in Venezuela were gathered during the last two decades; 24 independent families with AIP were ascertained, based on a deficient HMBS activity and increased porphobilinogen (PBG) urinary excretion. Molecular analyses of coding and splicing regions were performed in 23 families, to establish disease-causing changes, and haplotype analyses were used to assess ancestral kinships between them. Changes were detected in 16 out of 23 families, 9 of them being different: R26H, R26C, c.87+5G>A, c.267-54_61delgaaggggt, R116W, Q180X, c.825+1G>A, c.913-1delG, and 3' UTR *277G>A. Seven mutations were found, each one in a single family; one mutation was present in two unrelated families, whereas mutation Q180X was shared by 7 independent kindreds, all of which had the same haplotype (-);T;A;T;G;T;A;G (3167delG; 3530T>C; 3581A>G; 3982T>C; 6479G>T; 7052T>C; 7064A>C; 7779G>A). Six out of seven different Q180X carrier families came from the same geographic focus (Santa Lucía, Miranda State). Dense geographic aggregation with one identical haplotype strongly suggests a remote founder phenomenon for these Venezuelan AIP families, carrying an unreported but most frequent mutation.

Evaluation of mutation screening by heteroduplex analysis in acute intermittent porphyria: comparison with denaturing gradient gel electrophoresis

Acute intermittent porphyria is the major autosomal dominant form of acute hepatic porphyrias. The disease is due to mutations in the gene encoding for porphobilinogen deaminase (PBGD). Many different strategies have been developed to screen for mutations. However the high prevalence (0.6 per thousand) of PBGD gene defect, the large allelic heterogeneity of mutations (n = 130), and the limitations of the PBGD enzymatic assay for asymptomatic patients' detection, require for diagnosis an efficient and easy to handle strategy for locating mutations within the PBGD gene. In a recent study the sensitivity of the denaturing gradient gel electrophoresis (DGGE) technique was 100%. However DGGE requires the preparation of gradient gels and the use of primers with long GC-clamps; thus alternative methods should be preferable in the clinical laboratory. We have compared the detection rate of DGGE with heteroduplex analysis (HA) using 16 characterized PBGD gene mutations. Six different HA conditions were used to determine the efficiency of the method, including: (1) MDE (mutation detection enhancement) gel concentration; (2) addition of urea and sodium dodecyl sulfate (SDS); (3) radioactive labelling. The sensitivity of each HA condition varied from 31 to 81% vs. 100% in DGGE analysis. HA using 1 x MDE with 15% urea with or without 0.55% SDS was the most sensitive condition. This first comparative study of DGGE and HA mutation screening methods suggests that DGGE is a more sensitive screening assay than optimized HA. However, because of its simplicity HA should be considered as an efficient alternative mutation screening method.

Molecular epidemiology and diagnosis of PBG deaminase gene defects in acute intermittent porphyria

Acute intermittent porphyria (AIP) is the major autosomal dominant form of acute hepatic porphyrias. The disease is due to mutations in the gene encoding for porphobilinogen (PBG) deaminase and is characterized by life-threatening neurovisceral attacks, often precipitated by drugs, fasting, cyclical hormonal changes, or infectious diseases. This report describes a prospective study on the molecular epidemiology of PBG deaminase gene defects in AIP. It uses a sensitive, reliable, and easy-to-handle method for routine AIP molecular diagnosis and family study based on an exon-by-exon denaturing gradient gel electrophoresis (DGGE) strategy followed by direct sequencing. Fifteen genomic DNA fragments, including all the coding sequence and covering 3.35 kb of the PBG deaminase gene, were investigated in 405 subjects from 121 unrelated French Caucasian AIP families who had not been screened previously at the DNA level. PBG deaminase gene mutations were identified in 109 families, but only 78 were of different type, and each of them had a prevalence rate < 5%. Among these mutations, 33 had not been published previously. Sixty percent of these 78 mutations were located in only three exons (exons 10, 12, and 14), 44% were missense, 18% were splice defect, 19% were frameshift, and 16% were nonsense. In addition, two de novo mutational events were characterized. The evaluation of the efficiency of the standard PBG deaminase enzymatic screening method for gene-carrier detection indicated 95% of concordancy with the molecular-based diagnosis.

Molecular analysis of porphobilinogen (PBG) deaminase gene mutations in acute intermittent porphyria: first study in patients of Slavic origin

Acute intermittent porphyria (AIP) is an autosomally dominant inherited metabolic disorders caused by decreased activity of porphobilinogen deaminase, the third enzyme in the human heme biosynthetic pathway. We report here the first mutations in the human porphobilinogen deaminase gene in seven unrelated patients from the Czech and Slovak Republics with acute intermittent porphyria. We used denaturing gradient gel electrophoresis to screen all 15 exons and exon/intron boundaries of the porphobilinogen deaminase gene. Polymerase chain reaction products of abnormal migration patterns were subjected to direct sequencing to identify the causative mutations. Thus we revealed four novel mutations and three which have been previously described. Of the four novel mutations, two were mis-sense (G24S, V267M), one was a single base insertion (158insA) that produced a stop codon 12 codons downstream, and one was a single base substitution in intron 12 (771 + 1) resulting in a splicing defect. The three previously detected mutations were mis-sense mutations (R26C, R26H, G111R). These results suggest a high allelic heterogeneity in Czech and Slovak patients.