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...

No abstract

We have determined the primary structure of a mutant insulin receptor of a leprechaun patient born from a consanguineous marriage. A characteristic feature of leprechaunism is an extreme resistance to insulin. In this patient the insulin resistance seems to result from an observed lack of insulin binding to intact cells. Solubilizanion of cells in non-ionic detergents leads to the appearance of insulin receptors which can bind insulin. However, the insulin-stimulated autophosphorylation of the receptor's : subunit is markedly reduced. Cloning and sequencing of cDNA derived from insulin receptor mRNA of this patient revealed a leucine-to-proline mutation at position 233 in the a subunit. By means of DNA amplification we found that the patient is homozygous for this mutation and that the parents and two grandparents from the consanguineous line are heterozygous. The heterozygous individuals all show decreased insulin binding to cultured fibroblasts. In addition, they are mildly insulin resistant in vivo. These observations show a linkage between the leucine-to-proline mutation and the observed insulin resistance in this family. We therefore conclude that the mutation in the homozygous form is responsible for the extreme insulin resistance in the leprechaun patient. The mutation for the first time characterizes a region in the insulin receptor which seems to be involved in transmitting the insulin binding signal to the tyrosine kinase domain.

No abstract

X-linked agammaglobulinemia (XLA) is a severe humoral immunodeficiency disease of man. The inheritance of the disease is X-linked recessive. Female carriers can not be distinguished by immunologic assays. We investigated the localization of the disease gene on the X chromosome, utilizing nine polymorphic X chromosomal markers. In a single eight generation pedigree we found close linkage of the disease gene to the restriction fragment length polymorphism (RFLP) recognized by the DNA probe p19-2; the maximum lod score was 3.30 at a recombination fraction of 0.06. Addition of the lod scores for p19-2 obtained from seven other XLA pedigrees did not show the expected increase of the total score. This suggested genetic heterogeneity. We used the p19-2 marker as a reference point to search for pedigrees which had the disease gene at a different location. One pedigree provided a lod score of -3.14 at a recombination fraction of 0.06 with the p19-2 marker. We postulate that XLA is not a single genetic entity.