Deficiency of argininosuccinate synthetase causes arginine auxotrophy in lower organisms and causes citrullinemia in humans and cattle. Previously, seven missense mutations, four mutations associated with an absence of an exon in mRNA, and one splicing mutation have been identified in human neonatal citrullinemia. Reverse transcription of mRNA, amplification of cDNA and sequencing of cDNA clones were used to identify two additional missense mutations causing citrullinemia. One mutation involves substitution of leucine for serine at position 18 (S18L) and the other a substitution of cysteine for arginine at position 86 (R86C). Both of these mutations represent C----T transitions in CpG dinucleotides, and eight of nine missense mutations causing human citrullinemia involve similar transitions in CpG dinucleotides. The nucleotide coding sequence and deduced amino acid analysis are available for four mammalian species, yeast and three bacterial species. Six of nine missense mutations in humans occur in amino acid positions that are completely conserved in these organisms. Mutations causing human citrullinemia are extremely heterogeneous, and all non-consanguineous individuals studied to date are compound heterozygotes.
Human prolidase (PEPD, iminodipeptidase, EC 3.4.13.9) and related deficiencies were analyzed in terms of the nature and molecular biology of the enzyme and the molecular events seen in patients with this deficiency. The analyses were based on findings concerning isolation of the enzyme, development of specific antibodies and molecular cloning of cDNA and genome DNA of human prolidase. The studies revealed that human prolidase is a homo-dimer of an identical subunit 492 amino acid residues. The gene for prolidase (PEPD gene) was localized on chromosome 19, spanned more than 130 x 10(3) base-pairs and split into 15 exons. Molecular defects in prolidase deficiency were then analyzed. Two patients with the polypeptide-positive phenotype of the disease carried a mis-sense mutation of exon 12. Two siblings with a polypeptide-negative phenotype carried a gene deletion that encompassed exon 14. These mutations were not found in ten other patients with the disease, hence the molecular defects in prolidase deficiency are apparently highly heterogeneous.
Histidase (histidine ammonia-lyase, EC 4.3.1.3) catalyzes the deamination of L-histidine to trans-urocanic acid in the liver and skin of mammals. Histidase deficiency results in increased histidine and histamine in blood, and decreased urocanic acid in blood and skin. In this review we discuss current research on: (1) the mechanism of formation of an unusual residue, dehydroalanine, at the active site of histidase; and (2) the role of urocanic acid as an ultraviolet light-induced immunoregulator in the skin, and the implications of urocanic acid deficiency for human histidinemia. Genetic mechanisms that may account for the 1% of histidinemic patients with neurological impairments are considered briefly.
Maple syrup urine disease (MSUD) results from an inborn metabolic error caused by a deficiency of the branched-chain alpha-ketoacid dehydrogenase complex (BCKDC). cDNA clones encoding the E1 alpha subunit of BCKDC from rat and human liver have been isolated and characterized. The chromosomal location of E1 alpha on chromosome 19q13.1-13.2 has been determined using complementary methods. The etiology of MSUD has been studied by determining the enzyme activity, protein mass and mRNA level of BCKDC in fibroblasts from a human family and Polled Hereford calves, both with classic MSUD. A TACTyr to AACAsn substitution at residue 394 of the E1 alpha subunit was identified in the human patient by using enzymatic amplification of mRNA followed by DNA sequencing. Amplification of both mRNA and genomic DNA, in combination with allele-specific oligonucleotide hybridization, demonstrated that the patient was a compound heterozygote, inheriting an allele with a structural mutation from the father, and an allele from the mother containing a presumably cis-acting defect in regulation that abolished the expression of one of the E1 alpha alleles. The results revealed for the first time that a case of MSUD was caused by structural and regulatory mutations involving the E1 alpha subunit. Recent studies by others have demonstrated that the same structural mutation as is found in this patient is responsible for the high incidence of MSUD in the Philadelphia Mennonite population.(ABSTRACT TRUNCATED AT 250 WORDS)
Gyrate atrophy (GA) is an autosomal recessive eye disease characterized by progressive loss of vision due to chorioretinal degeneration. It is associated with a deficiency of the mitochondrial enzyme ornithine aminotransferase (OATase) with consequent hyperornithinemia. Although the clinical phenotype is largely confined to the eye, OATase deficiency is a systemic disorder. A step toward delineation of the enzyme defect in GA at the molecular level has been made by cloning and characterizing the cDNA and structural gene for OATase. The structural gene for OATase maps to chromosome 10 (10q26) and OATase-related sequences map to the X chromosome (Xp11.2). A diverse number of mutations at the OATase locus in GA patients of varied ethnic origins have been defined employing polymerase chain reaction and other molecular biological techniques. The majority of these mutations are of the missense type although a splicing mutation in one patient has recently been identified. The functional consequences of some of these mutations have been tested and confirmed in a eukaryotic expression system. These mutations demonstrate the allelic heterogeneity, which extends to both pyridoxine responsive and non-responsive forms of GA, reflecting the clinical and biochemical heterogeneity observed in this disease. The molecular studies in addition to providing information on the structure/function of the enzyme will facilitate understanding of the retinal pathophysiology in this disorder.
Coronary thrombolysis is the treatment of choice for patients with acute Q wave myocardial infarcts who have no contraindication to such therapy. However, the time required for thrombolysis to occur and the possibility of reocclusion of the infarct-related artery following thrombolytic therapy are problems. The time required for thrombolysis to occur with currently available agents ranges from 40 to 60 minutes and the frequency of reocclusion of the infarct-related artery after tissue-type plasminogen activator is 10 to 20%. We review experimental studies and clinical evaluations in which attempts have been made to develop adjunctive therapies that when coupled with available thrombolytic interventions might shorten the time to thrombolysis and delay or prevent reocclusion. From the studies done to date, it appears that a combination of thromboxane synthesis inhibitor and receptor antagonist with a serotonin receptor antagonist and heparin shortens the time to thrombolysis and delays or prevents coronary artery reocclusion in experimental canine models with copper coil-induced coronary artery thrombi. A monoclonal antibody to the platelet glycoprotein IIb/IIIa receptor given with tissue plasminogen activator and heparin also shortens the time to thrombolysis and delays or prevents reocclusion in experimental canine models. A mutant tissue plasminogen activator with a glycosylation defect and prolonged systemic clearance delays coronary artery reocclusion following lysis of three-hours coronary thrombi, induced by a copper coil. Thrombin inhibitors, including heparin, and synthetic inhibitors, given with tissue plasminogen activator and aspirin, appear to shorten the time to thrombolysis and delay or prevent coronary artery reocclusion in experimental canine models.(ABSTRACT TRUNCATED AT 250 WORDS)
The rare hereditary metabolic disorder alcaptonuria is characterized by the inability to metabolize homogentisic acid, an intermediary compound in the catabolism of the aromatic amino acids phenylalanine and tyrosine. The essentially complete deficiency of homogentisic acid oxidase causes a striking accumulation of homogentisic acid and a derived melanin-like pigment in the connective tissues; the latter is termed ochronosis. Urinary homogentisic acid is oxidized rapidly and becomes a brown or black pigment if alkali is added. Older alcaptonurics have intensely pigmented (ochronotic) connective tissues, primarily the cartilaginous joint surfaces, ribs, intervertebral disks, ear cartilage, etc. They also have an unusual type of arthritis affecting the large weight-bearing joints, i.e. hips, knees and spine, but not the small joints of the hands and feet, as in rheumatoid arthritis. A mechanistic explanation for ochronotic arthritis has not been worked out, but it is clear that accumulation of homogentisic acid in the connective tissues directly or indirectly leads to the arthritic changes. A detailed analysis of the events leading to alcaptonuric arthritis should be worthwhile since it is a model form of arthritis secondary to a well-defined metabolic disorder that must persist for many years before the arthritic complications appear. Possibly other, more common types of arthritis, develop secondarily to metabolic disturbances that involve chemical mediators less obvious, or less easily detected, than homogentisic acid.
Non-ketotic hyperglycinemia is caused by a molecular lesion involved in the glycine cleavage system and shows striking features representing the impaired central nervous system. For the study on molecular genetics of non-ketotic hyperglycinemia, we have isolated several cDNA clones, each encoding human glycine decarboxylase of H-protein, two of the four component enzymes of the glycine cleavage system. Although one of eight patients with this disease resulting from a lesion of glycine decarboxylase had the glycine decarboxylase gene deleted at a 5' region, they showed no common aberration detectable by glycine decarboxylase cDNA. Using the H-protein cDNA, we have demonstrated the rearranged structures, identified by one of the undetectable 5.0 and 5.5 kb SacI fragments, in the genomes of patients in whom there was an impaired expression of H-protein or glycine decarboxylase. The aberration of the 5.5 kb SacI fragment was associated with a defect of the 5.2 kb EcoRI fragment. Multiple genomic lesions are suggested for non-ketotic hyperglycinemia, and their implications in pathogenesis are discussed.
The application of the tools of molecular biology has led to a profound increase in our current understanding of the nature of the disease states associated with defects in the phenylalanine hydroxylase (PAH) gene. Over the past decade, the PAH cDNA has been cloned and the primary structure of the PAH protein has been determined. The PAH cDNA clone has served as an invaluable probe to define the molecular structure and chromosomal location of the PAH locus in both man and other organisms. Southern analysis using the PAH cDNA as a hybridization probe has revealed the presence of numerous restriction fragment-length polymorphisms (RFLPs) in the PAH gene, which have permitted the classification of normal and mutant PAH chromosomes. RFLP analysis has also permitted the implementation of prenatal diagnosis of phenylketonuria (PKU) and other related hyperphenylalaninemic disorders. Through the use of molecular cloning and polymerase chain reaction methodologies, many molecular lesions have now been identified in the PAH gene, and their association with different PAH haplotypes and disease phenotypes can now be addressed in a rational manner. Finally, the characterization of PAH mutations has enabled the population dynamics of phenylketonuria to be examined in several different populations.
