Exon 1 donor splice site mutations in the porphobilinogen deaminase gene in the non-erythroid variant form of acute intermittent porphyria.
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.
Puy
H
H
Centre Français des Porphyries, INSERM U409, Hôpital Louis Mourier, Colombes, France.
Gross
U
U
Deybach
J C
JC
Robréau
A M
AM
Frank
M
M
Nordmann
Y
Y
Doss
M
M
D000328:Adult; D004252:DNA Mutational Analysis; D004591:Electrophoresis, Polyacrylamide Gel; D005091:Exons / Q000235:genetics; D005260:Female; D006801:Humans; D011163:Hydroxymethylbilane Synthase / Q000235:genetics*; D008875:Middle Aged; D009154:Mutation / Q000235:genetics*; D017118:Porphyria, Acute Intermittent / Q000235:genetics*; D012326:RNA Splicing / Q000235:genetics*; D012333:RNA, Messenger / Q000032:analysis; D017422:Sequence Analysis, DNA
D012333:RNA, Messenger; D011163:Hydroxymethylbilane Synthase