Exon 1 donor splice site mutations in the porphobilinogen deaminase gene in the non-erythroid variant form of acute intermittent porphyria

Abstract

Acute intermittent porphyria (AIP) is an autosomal dominant disorder caused by a partial defect of the heme biosynthesis enzyme, porphobilinogen deaminase (PBGD). PBGD is encoded by two distinct mRNA species expressed in a tissue-specific manner from a single gene. One transcript is expressed in erythroid tissues, while the housekeeping transcript is expressed in all tissues. In classical AIP (95% of cases) the housekeeping and the erythroid-specific enzymes both have half-normal activity in erythroid and non-erythroid tissues, whereas in the variant non-erythroid form of the disease the enzymatic defect is present only in non-erythroid cells. A large allelic heterogeneity of mutations (n>135) has been demonstrated in classical AIP, but to date only three different mutations have been characterized in the non-erythroid variant form of AIP. We describe the molecular abnormalities responsible for the non-erythroid variant form of AIP in two French and two German unrelated AIP patients with normal PBGD activity in the erythrocytes. Three different splicing defects located in the intron 1 donor splice site were identified: a 33+1 g-->a mutation, previously described in a Dutch family, was found in two patients; two novel mutations (33+2 t-->a, 33+5 c-->g) affected the two remaining patients. All the mutations resulted in the activation of a cryptic splice site 67 bp downstream in intron 1, leading to a frameshift and a premature stop codon in exon 4. Mutations in the exon 1 donor splice site are involved in eight of the nine non-erythroid variant AIP families reported in the literature. These data show that most mutations causing the non-erythroid variant AIP are exon 1 splice defects, in contrast with classical AIP, where missense mutations are chiefly involved. Moreover, the allelic heterogeneity of PBGD gene defects causing the non-erythroid variant AIP is demonstrated, with five different mutations identified. These mutations could be easily detected by a single denaturing gradient gel electrophoresis which also allows the presymptomatic detection of gene carriers in the affected families.

A novel mutation in a family with non-erythroid variant form of acute intermittent porphyria

“…These include four point mutations at the exon 1/intron 1 junction of the PBGD gene and one in the initiation of the translation codon of the non-erythroid transcript (Grandchamp et al 1989a,b;Kauppinen et al 1995;Puy et al 1997Puy et al , 1998Chen et al 1994). Three point mutations in the exon 1/intron 1 junction were demonstrated to cause a splicing defect, leading to a 67-bp intronic sequence aberrantly transcribed and a premature stop codon (Puy et al 1998). Because these mutations are located upstream from the promoter for the erythroid transcript, they affect only the non-erythroid transcript, resulting in half-normal PBGD activity in non-erythroid tissues.…”

“…Mutation detection methods used in investigating this variant form of AIP included single-strain conformation polymorphism (SSCP) analysis or denaturing gradient gel electrophoresis (DGGE) followed by DNA sequencing (Chen et al 1994;Kauppinen et al 1995;Puy et al 1997Puy et al , 1998. In view of the fact that all five known mutations causing this variant form of AIP were clustered in a small region of DNA (Fig.…”

Porphobilinogen Deaminase Gene Mutations in Polish Patients with Non-Erythroid Acute Intermittent Porphyria

“…Reports were published on 10 different mutations in exon and intron 1 leading to non-erythroid AIP. They included: 1 mutation in the promoter region evoking transcription weakness [20], 1 missense mutation in the translational initiating codon of a systemic isoform [21], 8 mutations yielding abnormal exon 1 splicing (leading to nucleotides retention and frameshift). The nonerythroid AIP was also caused by a frameshift mutation in exon 3, which resulted in the formation of the stop codon of the housekeeping isoform.…”

Dolor abdominal y síndrome de secreción inadecuada de hormona antidiurética (SIADH) en un paciente diagnosticado de porfiria aguda intermitente

“…Las manifestaciones clínicas aparecen durante la edad adulta y son más frecuentes en mujeres. Las principales manifestaciones de PAI son dolor abdominal neuropático, neuropatía periférica y alteraciones mentales (1,2). El dolor abdominal suele ser intenso, es el síntoma más frecuente y generalmente es el síntoma inicial de un ataque agudo.…”