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 deami nase (PBGD). PBGD is encoded by two distinct mRNA species expressed in a ti ssue-specific manner from a single gene. One transcript is expressed in ery throid tissues, while che housekeeping transcript is expressed in all tissu es. In classical AIP (95% of cases) the housekeeping and the erythroid-spec ific enzymes both have half-normal activity in erythroid and non-erythroid tissues, whereas in the variant non-erythroid form of the disease the enzym atic defect is present only in non-erythroid cells. A large allelic heterog eneity of mutations (n>135) has been demonstrated in classical AIP, bur to date only three different mutations have been characterized in the non-eryt hroid variant form of AIP. We describe the molecular abnormalities responsi ble for the non-erythroid variant form of ATP in two French and two German unrelated ATP 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) aff ected the two remaining patients. All the mutations resulted in the activat ion of a cryptic splice site 67 bp downstream in intron 1, leading to a fra meshift 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 fa milies reported in the literature. These data show that most mutations caus ing the non-erythroid variant AIP are exon I splice defects, in contrast wi th classical AIP, where missense mutations are chiefly involved. Moreover, the allelic heterogeneity of PBGP gene defects causing the non-erythroid va riant AIP is demonstrated, with five different mutations identified. These mutations could be easily detected by a single denaturing gradient: gel ele ctrophoresis which also allows the presymptomatic detection of gene carrier s in the affected families.