Identification and characterization of hydroxymethylbilane synthase mutations causing acute intermittent porphyria: evidence for an ancestral founder of the common G111R mutation.

Acute intermittent porphyria (AIP), the most common hepatic porphyria, results from the half-normal activity of hydroxymethylbilane synthase (HMB-synthase; EC 4.3.1.8), the third enzyme in the heme biosynthetic pathway. Because life-threatening acute neurologic attacks of this autosomal dominant disease are triggered by various ecogenic factors (e.g., certain drugs, hormones, alcohol, and starvation), efforts have been directed to identify and counsel presymptomatic heterozygotes in affected families to avoid the precipitating factors. Thus, to determine the nature of the mutations causing AIP in 26 unrelated enzyme-confirmed patients from Argentina, a long-range polymerase chain reaction method was developed to amplify the entire 10-kb gene in two fragments for efficient cycle sequencing and mutation detection. Eight new mutations were identified including two missense mutations (Q34P and 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, R201W, 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 G335S) in f1 E. coli resulted in 2.5% or less of the normal expressed enzyme, confirming their defective function. Thus, eight new and five previously reported HMB-synthase mutations, including a common lesion, were detected, permitting accurate identification and counseling of presymptomatic carriers in these 26 unrelated Argentinean AIP families with this dominant porphyria.

Familial porphyria cutanea tarda: characterization of seven novel uroporphyrinogen decarboxylase mutations and frequency of common hemochromatosis alleles.

Familial porphyria cutanea tarda (f-PCT) results from the half-normal activity of uroporphyrinogen decarboxylase (URO-D). Heterozygotes for this autosomal dominant trait are predisposed to photosensitive cutaneous lesions by various ecogenic factors, including iron overload and alcohol abuse. The 3.6-kb URO-D gene was completely sequenced, and a long-range PCR method was developed to amplify the entire gene for mutation analysis. Four missense mutations (M165R, L195F, N304K, and R332H), a microinsertion (g10insA), a deletion (g645Delta1053), and a novel exonic splicing defect (E314E) were identified. Expression of the L195F, N304K, and R332H polypeptides revealed significant residual activity, whereas reverse transcription-PCR and sequencing demonstrated that the E314E lesion caused abnormal splicing and exon 9 skipping. Haplotyping indicated that three of the four families with the g10insA mutation were unrelated, indicating that these microinsertions resulted from independent mutational events. Screening of nine f-PCT probands revealed that 44% were heterozygous or homozygous for the common hemochromatosis mutations, which suggests that iron overload may predispose to clinical expression. However, there was no clear correlation between f-PCT disease severity and the URO-D and/or hemochromatosis genotypes. These studies doubled the number of known f-PCT mutations, demonstrated that marked genetic heterogeneity underlies f-PCT, and permitted presymptomatic molecular diagnosis and counseling in these families to enable family members to avoid disease-precipitating factors.

The acro-osteolysis syndrome: Morphologic and biochemical studies.

The acro-osteolysis syndrome consists of dissolution of terminal phalanges of the hands and feet, dolichocephaly with multiple wormian bones, delayed closure of cranial sutures, absence of frontal sinuses, a prominent occipital ridge, skeletal demineralization, vertebral and extremity fractures, joint laxity, and coarse hair. Studies of bone morphology reveal diminished bone density and bone formation. Osteoblasts have widely dilated smooth endoplasmic reticulum. It is postulated that an abnormality of a structural protein is the pathogenic basis of this disease.