Identification and expression of mutations in the hydroxymethylbilane synthase gene causing acute intermittent porphyria (AIP)

Background: Acute intermittent porphyria (AIP), an autosomal dominant inbor n error, results from the half-normal activity of the heme biosynthetic enz yme hydroxymethylbilane synthase (EC 4.3.1.8; HMB-synthase). This disease i s characterized by acute, life-threatening neurologic attacks that are prec ipitated by various drugs, hormones, and other factors. The enzymatic and/o r biochemical diagnosis of AIP heterozygotes is problematic; therefore, eff orts have focused on the identification of HMB-synthase mutations so that h eterozygotes can be identified and educated to avoid the precipitating fact ors. In Spain, the occurrence of AIP has been reported, but the nature of t he HMB-synthase mutations causing AIP in Spanish families has not been inve stigated. Molecular analysis was therefore undertaken in nine unrelated Spa nish ATP patients. Materials and Methods: Genomic DNA was isolated from affected probands and family members of nine unrelated Spanish families with AIP. The HMB-synthas e gene was amplified by long-range PCR and the nucleotide sequence of each exon was determined by cycle sequencing. Results: Three new mutations, a missense, M212V; a single base insertion, g 4715insT; and a deletion/insertion, g7902ACT-->G, as well as five previousl y reported mutations (G111R, R116W, R149X R167W, and R173W) were detected i n the Spanish probands. Expression of the novel missense mutation M212V in E. coli revealed that the mutation was causative, having <2% residual activ ity. Conclusions: These studies identified the first mutations in the HMB-syntha se gene causing AIP in Spanish patients. Three of the mutations were novel, while five previously reported lesions were found in six Spanish families. These findings enable accurate identification and counseling of presymptom atic carriers in these nine unrelated Spanish AIP families and further demo nstrate the genetic heterogeneity of mutations causing AIP.