Identification and characterization of hydroxymethylbilane synthase mutations causing acute intermittent porphyria: Evidence for an ancestral founderof the common G111R mutation

Acute intermittent porphyria (AIP), the most common hepatic porphyria, resu lts from the half-normal activity of hydroxymethylbilane synthase (HMB-synt hase; EC 4.3.1.8), the third enzyme in the heme biosynthetic pathway. Becau se life-threatening acute neurologic attacks of this autosomal dominant dis ease are triggered by various ecogenic factors (e.g., certain drugs, hormon es, alcohol, and starvation), efforts have been directed to identify and co unsel presymptomatic heterozygotes in affected families to avoid the precip itating factors. Thus, to determine the nature of the mutations causing AIP in 26 unrelated enzyme-confirmed patients from Argentina, a long-range pol ymerase chain reaction method was developed to amplify the entire 10-kb gen e in two fragments for efficient cycle sequencing and mutation detection. E ight new mutations were identified including two missense mutations (Q34P a nd G335S) four small deletions (728delCT, 815delAGGA, 948delA, and 985del12 ), a single base insertion (666insA), and a splice site mutation (IVS12(+1) ). In addition, five previously reported mutations (G111R, R173W, Q204X, R2 01W, and 913insC) were detected. Notably, G111R was identified in 12 of the 26 (46%) presumably unrelated propositi; however, haplotype analysis with intragenic and flanking markers indicated an ancestral founder. Expression of the two new missense mutations (Q34P and Q335S) in E. coli resulted in 2 .5% or less of the normal expressed enzyme, confirming their defective func tion. Thus, eight new and five previously reported HMB-synthase mutations, including a common lesion, were detected, permitting accurate identificatio n and counseling of presymptomatic carriers in these 26 unrelated Argentine an AIP families with this dominant porphyria. Am. J. Med. Genet. 86:366-375 , 1999. (C) 1999 Wiley-Liss, Inc.