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Biomedical subjects

G A Mitchell

Publications and source records attributed to G A Mitchell.

At least 19 recordsLinked to original sources
PubMed1992

Ornithine delta-aminotransferase mutations in gyrate atrophy. Allelic heterogeneity and functional consequences.

Ornithine delta-aminotransferase is a nuclear-encoded mitochondrial matrix enzyme which catalyzes the reversible interconversion of ornithine and alpha-ketoglutarate to glutamate semialdehyde and glutamate. Inherited deficiency of ornithine delta-aminotransferase results in ornithine accumulation and a characteristic chorioretinal degeneration, gyrate atrophy of the choroid and retina. We have surveyed the ornithine delta-aminotransferase genes of gyrate atrophy patients for mutations. Using a variety of techniques, we discovered and molecularly characterized 21 newly recognized ornithine delta-aminotransferase alleles. We determined the consequences of these and three previously described mutations on ornithine delta-aminotransferase mRNA, antigen, and enzyme activity in cultured fibroblasts. The majority (20/24) of these alleles produce normal amounts of normally sized ornithine delta-aminotransferase mRNA. By contrast, only 2/24 had normal amounts of ornithine delta-aminotransferase antigen. Reproducing these mutations by site-directed mutagenesis and expressing the mutant ornithine delta-aminotransferase in Chinese hamster ovary cells confirms that several of these mutations inactivate ornithine delta-aminotransferase and cause gyrate atrophy in these patients.

Alleles↗
PubMed1992

Hyperammonemia-hyperornithinemia-homocitrullinuria syndrome: neurologic, ophthalmologic, and neuropsychologic examination of six patients.

We report the clinical, electrophysiologic, ophthalmologic, and neuropsychologic features of six patients with hyperammonemia-hyperornithinemia-homocitrullinuria syndrome, an inborn error of ornithine metabolism. Pyramidal signs, decreased vibration sense, bucco-facio-lingual dyspraxia, and learning difficulties or subnormal intelligence were found in the majority. Anomalies of peripheral nerve conduction velocity and of evoked potentials were common, and one patient had markedly abnormal white matter images on cranial magnetic resonance imaging. One patient had retinal depigmentation and chorioretinal thinning. The clinical severity varied greatly among patients; in general, the three younger patients had less neurologic and intellectual impairment than did the three older patients. Only two of our patients have had episodes of symptomatic hyperammonemia. We conclude that hyperammonemia-hyperornithinemia-homocitrullinuria syndrome can be associated with widespread manifestations in the central and peripheral nervous systems. Although the control of hyperammonemia is an essential element in the treatment of these patients, the relationship of hyperammonemia to the chronic neuropsychologic problems of these patients is unclear.

Adolescent↗
PubMed1991

Splice-mediated insertion of an Alu sequence inactivates ornithine delta-aminotransferase: a role for Alu elements in human mutation.

In studies of mutations causing deficiency of ornithine delta-aminotransferase (EC 2.6.1.13), we found an allele whose mature mRNA has a 142-nucleotide insertion at the junction of sequences from exons 3 and 4. The insert derives from an Alu element in ornithine delta-aminotransferase intron 3 oriented in the direction opposite to transcription (an "antisense Alu"). A guanine----cytosine transversion creates a donor splice site in this Alu, activating a cryptic acceptor splice site at its 5' end and causing splice-mediated insertion of an Alu fragment into the mature ornithine-delta-aminotransferase mRNA. We note that the complement of the Alu consensus sequence has at least two cryptic acceptor sites and several potential donor sequences and predict that similar mutations will be found in other genes.

Amino Acid Sequence↗
PubMed1989

At least two mutant alleles of ornithine delta-aminotransferase cause gyrate atrophy of the choroid and retina in Finns.

Gyrate atrophy of the choroid and retina (GA) is an inherited chorioretinal degeneration caused by deficiency of ornithine delta-aminotransferase (OAT; L-ornithine: 2-oxo-acid aminotransferase; EC 2.6.1.13). GA is one of the "Finnish genetic diseases," a group of several rare monogenic disorders that occur with increased frequency in the Finnish population. Using a combination of RNase A protection, genomic cloning, and polymerase chain reaction amplification of genomic DNA, we found one of two missense mutant OAT alleles to be present in each of 16 Finnish GA pedigrees. The first mutation R180T, in which arginine-180 is replaced by threonine, was present in homozygous form in patients from two pedigrees. The second mutation L402P, in which leucine-402 is replaced by proline, was present in homozygous form in patients from 14 pedigrees. Neither mutation was present in 19 Finnish controls. L402P was not present in 18 non-Finnish GA patients but R180T was found in an American GA patient. We constructed full-length mutant cDNAs by amplifying patient cDNA with the polymerase chain reaction and cloning a restriction fragment containing the mutation into an otherwise normal human OAT cDNA. These mutant cDNAs were then expressed in CHO-K1 cells, which lack endogenous OAT. Both R180T and L402P inactivate OAT. These results show molecular heterogeneity in GA alleles even in the Finnish population.

Alleles↗
PubMed1989

Selection against lethal alleles in females heterozygous for incontinentia pigmenti.

Studies of five heterozygous females from three kindreds segregating incontinentia pigmenti indicate that cells expressing the mutation have been eliminated from skin fibroblast cultures and in varying degrees from hematopoietic tissues. Clonal analysis was carried out using G6PD variants and methylation patterns at the HPRT locus. Our results confirm X linkage in these families and suggest that selection against cells expressing mutations that are lethal to males in utero may help ameliorate the deleterious phenotype in carrier females.

Adolescent↗
PubMed1988

Human ornithine-delta-aminotransferase. cDNA cloning and analysis of the structural gene.

Ornithine-delta-aminotransferase (OAT) is a nuclear-encoded, mitochondrial matrix enzyme which, in rat, is expressed as basal levels in most tissues but is induced in liver by high dietary protein and in kidney by estrogen and thyroxine administration. In man, the hereditary deficiency of OAT results in ornithine accumulation and the blinding disease gyrate atrophy of the choroid and retina. We cloned near full length rat and human liver OAT cDNAs and demonstrated OAT expression in a variety of tissues from each species. We mapped the human OAT structural gene to chromosome 10, cloned 40 kilobase pairs of genomic DNA containing the complete OAT structural gene, and determined its organization. It is 21 kilobase pairs in length, and contains 11 exons. Exon 2 has been absent from all cDNAs studied and was detected by homology to X-linked processed OAT pseudogenes. The 5'-flanking region of the OAT gene has features of housekeeping genes (GC enrichment and three Sp1 binding consensus sequences) and tissue-specific, inducible genes (TATA box-like element and two CCAAT boxes). A 22-base pair region of partial dyad symmetry containing homology to estrogen responsive elements overlaps the OAT transcription site. Another 5' sequence, GTATCCTGCCCTC, is homologous to sequences in the promoter regions of the genes of three urea cycle enzymes.

Amino Acid Sequence↗
PubMed1988

An initiator codon mutation in ornithine-delta-aminotransferase causing gyrate atrophy of the choroid and retina.

Gyrate atrophy of the choroid and retina (GA) is an autosomal recessive chorioretinal degeneration caused by deficiency of the mitochondrial matrix enzyme, ornithine-delta-aminotransferase (OAT). To study the molecular basis of the mutations causing GA, we cloned and sequenced the human OAT cDNA and determined the intron-exon arrangement of the structural gene. Using the cDNA template, we synthesized antisense RNA probes and performed RNase A protection experiments with RNA from four Lebanese GA patients. We found a probe-target mismatch at the 5' end of the first coding exon and amplified this region of the patients' genomic DNA using the polymerase chain reaction. Sequence analysis showed a G----A transition, changing the initiator ATG (methionine) codon to ATA. This mutation segregates with the GA allele in both pedigrees. Initiation of translation at the closest in-frame methionine codon would truncate OAT by 138 amino acids, eliminating the entire mitochondrial leader sequence and 113 amino acids of the mature peptide.

Choroid↗
PubMed1988

Glutaric acidemia type II. Comparison of pathologic features in two infants.

Glutaric acidemia type II (GA II) is a metabolic disorder caused by deficiency of electron transport flavoprotein or its oxyreductase. It is characterized by acidosis, hypoglycemia, hyperammonemia, organic aciduria, and "sweat-sock" odor. Neonatal GA II differs from most inborn metabolic errors in that there are prominent congenital malformations. We recently observed two infants at autopsy with GA II whose malformations included: subcortical renal glomerular cysts, renal medullary dysplasia, cerebral pachygyria, pulmonary hypoplasia, and facial dysmorphism. In addition, there was lipid accumulation in liver, heart, and renal tubular epithelium, tissues that use fatty acids as a primary source of energy. Review of previous reports of 12 patients showed that these lesions are typical of neonatal GA II. The pattern of lesions, in particular the striking localization of renal dysplasia to the medulla, suggests that the malformations may be the consequence of an accumulation of toxic metabolites that is not corrected by placental transfer.

Brain↗
PubMed1986

Clinical heterogeneity in cobalamin C variant of combined homocystinuria and methylmalonic aciduria.

We describe two patients with methylmalonic aciduria and homocystinuria (Cbl C). The disorder was not diagnosed in patient 1 until 4 1/2 years of age; he had a history of fatigue, anorexia, delirium, and spasticity. Moderate megaloblastic bone marrow changes were observed, and there was hyperreflexia of the lower limbs. His condition improved clinically with hydroxycobalamin therapy. Patient 2 was hospitalized at 6 weeks of age because of lethargy and poor feeding. She was found to have macrocytosis. Despite an initial good clinical response to hydroxycobalamin, she developed a striking pigmentary retinopathy. Methylmalonic aciduria persisted in both patients, and homocystinuria persisted in patient 1 despite therapy. The diagnosis of Cbl C disease has been confirmed in both patients by biochemical studies of cultured fibroblasts, including complementation studies. The differences in age of onset and clinical findings together with the similar biochemical findings in these two patients demonstrate the heterogeneity of phenotypic expression in patients with apparently identical abnormalities of vitamin B12 metabolism.

Amino Acid Metabolism, Inborn Errors↗
PubMed1985

Novel rhodamine tripeptide substrate for manual and automated colorimetric prothrombin time test.

A chromogenic prothrombin time test is described using the tripeptide substrate, (Sar-Pro-Arg)2-Rhodamine 110. The method is both precise and sensitive to the factors of the extrinsic pathway. The accuracy of the method was demonstrated by comparison of patient sample values to standard clotting test results. The manual substrate test was adapted to two different automated chemistry analyzers.

Autoanalysis↗
PubMed1983

Sensitivity of two Chinese hamster cell lines to SCE induction by a variety of chemical mutagens.

SCE induction in Chinese hamster Don (lung) cells was compared with that in CHO (ovary) cells exposed under identical conditions to 14 known mutagens. Test protocols used for comparison were selected following a study of Don and CHO cell responses to aflatoxin B1 and benzo[a]pyrene. In the absence of added metabolizing enzymes 9-aminoacridine, 4-nitroquinoline 1-oxide, N-methyl-N-nitrosourea, dimethylcarbamoyl chloride, beta-propiolactone, daunomycin, aflatoxin B1 and 2-aminoanthracene were directly active in both cell lines; every substance positive in CHO cells was also positive in Don cells. However, the latter detected cyclophosphamide, hydrazine sulphate, benz[c]acridine, 3-methylcholanthrene and benzo[a]pyrene without addition of S9. CHO cells did not respond equivalently to these mutagens, either in the presence or absence of S9. Other differences between the cell lines depended on chemical exposure time, S9 pre-incubation or co-incubation conditions. For example, the ability of CHO cells to detect SCEs due to 2-aminoanthracene was acutely dependent on exposure time. In addition, Don cells exhibited lower background SCE values which were less variable than those of CHO cells under the same culture conditions. Although incapable of detecting 4-dimethylaminoazobenzene (butter yellow) and not as sensitive to cyclophosphamide as certain cell lines of liver origin, the pseudodiploid Don cell line possesses other desirable characteristics required for in vitro SCE assays, particularly with regard to intrinsic metabolic activation of polycyclic aromatic hydrocarbons and related substances.

Aflatoxin B1↗
PubMed1982

Sister chromatid exchange induction in two cell lines.

Sister chromatid exchange (SCE) induction by the procarcinogen benzo (a)-pyrene (BP) was studied using two established cell lines. Chinese hamster ovary (CHO) and Chinese hamster lung (Don) cells were exposed to varying concentrations of BP in the presence and absence of a rat liver metabolizing system. Results showed a significant difference in the abilities of the two cell lines to induce SCE in the absence of liver homogenate. These experiments demonstrate that Don cells are capable of generating active metabolites from biologically inactive procarcinogens presumably due to an inherent metabolizing system which is deficient or absent in CHO cells.

Animals↗