We have determined the mutation in a patient with acute intermittent porphyria. The mRNA coding for porphobilinogen deaminase was reverse transcribed then the cDNA was enzymatically amplified in vitro. Upon sequencing of a polymerase chain reaction product of abnormal size we found that this fragment lacked exon 12 of the gene. We analysed a genomic fragment containing exon 12 and determined that the patient was heterozygous for a point mutation G A at the last position of exon 12. We propose that this base change is responsible for an abnormal processing of the mutant allele such that exon 12 is missing in the mature mRNA. The resulting aberrant mRNA encodes a truncated protein which is inactive but stable and can be detected using antibodies directed against the normal enzyme.

An inherited deficiency of porphobilinogen deaminase [porphobilinogen ammonia-lyase (polymerizing), EC 4.3.1.8] in humans is responsible for the autosomal dominant disease acute intermittent porphyria. Different classes of mutations have been described at the protein level suggesting that this is a heterogeneous disease. It was previously demonstrated that porphobilinogen deaminase is encoded by two distinct mRNA species expressed in a tissue-specific manner. Analysis of the genomic sequences indicated that these two mRNAs are transcribed from two promoters and only differ in their first exon. The first mutation identified in the human porphobilinogen deaminase gene is a single-base substitution (G --A) in the canonical 5' splice donor site of intron 1. This mutation leads to a particular subtype of acute intermittent porphyria characterized by the restriction of the enzymatic defect to nonerythropoietic tissues. Hybridization analysis using oligonucleotide probes after in vitro amplification of genomic DNA offers another possibility of detecting asymptomatic carriers of the mutation in affected families.Acute intermittent porphyria (AIP) is a metabolic disorder characterized by attacks of neurological dysfunction with abdominal pain, hypertension, tachycardia, and peripheral neuropathy. Most attacks are precipitated by drugs, alcohol, caloric deprivation, infections, or endocrine factors (1). AIP results from a partial deficiency of the third enzyme of heme biosynthesis, porphobilinogen deaminase [PBGD; porphobilinogen ammonia-lyase (polymerizing), EC 4.3.1.8] that is inherited as an autosomal dominant trait (1). Early detection of gene carriers is important in the prevention of attacks, as they can be advised to avoid precipitating factors. Since asymptomatic carriers do not consistently excrete abnormal amounts of porphyrins and the porphyrin precursors, 5-aminolevulinic acid and porphobilinogen, the best method for detecting carriers in most AIP families is the determination of erythrocyte PBGD activity (1). There are, however, limitations to this approach as a screening method. Erythrocyte PBGD levels are affected by erythrocyte age and the presence of other diseases (2), and there is some overlap between values for normal individuals and AIP patients (1). In addition, some families have been described with clinical and biochemical criteria indicating AIP, but without the PBGD deficiency in the erythrocytes of the patients (3-5); in fact the distribution of PBGD activity in these AIP patients and their relatives is identical to that in healthy controls (3, 5), in contrast to most AIP families in which erythrocyte PBGD levels show a biphasic distribution. These features point to the existence of a subset of AIP families in whom the mutation is not expressed in erythrocytes.We demonstrated (6) that two isoforms of PBGD, one found in nonerythroid cells and the other found only in erythroid cells, are translated from two mRNAs that differ solely in their 5' termini. These mRNAs are produced through alte...

Two mutations of the porphobilinogen (PBG) deaminase gene resulting in cross-reacting immunological material (CRIM) positive forms of acute intermittent porphyria (AIP) have been identified by in vitro amplification of cDNA and cloning of the amplified products in a bacterial expression vector. Both mutations resulted from G to A transitions in exon 10 of the gene and produced arginine to glutamine substitutions in the abnormal protein. Expression of mutant cDNA in Escherichia coli reveals that one but not the other of these amino acid changes results in a striking decrease of the optimal pH of the mutated enzyme. One or the other of these two mutations accounted for the defect causing AIP in six unrelated patients among the eight patients evaluated with the CRIM positive subtype of this disorder. (J. Clin. Invest. 1990Invest. . 86:1511Invest. -1516

Uroporphyrinogen decarboxylase deficiency in man is responsible for familial porphyria cutanea tarda and hepatoerythropoietic porphyria. A recent study of a family with hepatoerythropoietic porphyria showed that the enzyme defect resulted from rapid degradation of the protein in vivo. Cloning and sequencing of a complementary DNA for the mutated gene revealed that the mutation was due to the replacement of a glycine residue by a glutamic acid residue at position 281. This base change leads to a protein that is very rapidly degraded in the presence of cell lysate. Characterization of the mutation will allow comparison of this defect in a homozygous patient with defects in other patients with familial porphyria cutanea tarda.