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Biomedical subjects

G A Mitchell

Publications and source records attributed to G A Mitchell.

At least 73 records · Page 4Linked to original sources
PubMed1993

3-Hydroxy-3-methylglutaryl coenzyme A lyase (HL). Cloning of human and chicken liver HL cDNAs and characterization of a mutation causing human HL deficiency.

3-Hydroxy-3-methylglutaryl coenzyme A lyase (HL) catalyzes the final step of ketogenesis, an important pathway of mammalian energy metabolism. HL deficiency is an autosomal recessive inborn error in man leading to episodes of hypoglycemia and coma. Using the N-terminal peptide sequence of purified chicken liver HL, we designed degenerate sequence primers and amplified an 89-base pair (bp) chicken liver HL cDNA fragment. Longer cDNA clones for chicken (1384 bp) and human (1575 bp) HL were obtained by library screening. The peptide sequence predicted from the chicken clone contains two peptides from purified chicken HL. Mature human and chicken HL are 298-residue peptides. The sequence of the human clone predicts a 27-residue mitochondrial leader and a 31.6-kDa mature HL peptide. Human fibroblast and liver RNA contain a single 1.7-kilobase HL message. Two Acadian French-Canadian siblings with HL deficiency were homozygous for a 2-base pair deletion within the Ser-69 codon (S69fs(-2)), predicted to result in a truncated nonfunctional HL peptide lacking a complete active site. S69fs(-2) was not present in 12 other HL-deficient patients of 10 other ethnic origins, showing that HL deficiency is genetically heterogeneous.

Amino Acid Sequence↗
PubMed1993

Angiotensin II, sodium, and cardiovascular hypertrophy in spontaneously hypertensive rats.

Angiotensin II (Ang II) may cause cardiovascular hypertrophy as a consequence of increased blood pressure or possibly by direct trophic actions. To dissociate Ang II and blood pressure in young spontaneously hypertensive rats (SHR), we used sodium loading during angiotensin converting enzyme inhibitor treatment. Animals were treated between 6 and 10 weeks of age with perindopril to lower Ang II and blood pressure, or with perindopril and 1% saline drinking fluid or perindopril and aldosterone infusion to lower Ang II but maintain high blood pressure. Blood pressure, heart weight, and media/lumen ratio of mesenteric resistance arteries were studied while rats were on treatment at 10 weeks of age and 15 weeks after treatment at 25 weeks of age. Perindopril lowered blood pressure and inhibited the development of cardiovascular hypertrophy. Saline or aldosterone restored high blood pressure during perindopril treatment and resulted in increased heart weight/body weight and resistance artery media/lumen ratios in direct proportion to the elevation of blood pressure. Because increased structure occurred despite perindopril treatment, we conclude that direct trophic actions of Ang II are not essential for the development of cardiovascular hypertrophy in young SHR and that the antitrophic actions of angiotensin converting enzyme inhibitors depend more on changes in blood pressure than on Ang II. However, restoration of blood pressure and structure by sodium during perindopril treatment raises the possibility that the design of the cardiovascular system and blood pressure may depend indirectly on Ang II through effects on sodium metabolism.

Angiotensin II↗
PubMed1993

Future trends of welfare benefits in the '90s.

A number of economic and demographic factors have had a marked effect on welfare plans in Canada and will continue to affect them for the rest of the decade. This situation presents major challenges for plan trustees.

Canada↗
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↗
PubMed1987

UV-B radiation and photosynthetic irradiance acclimate eggplant for outdoor exposure.

Treatment of greenhouse-grown eggplant (Solanum melongena L. var. esculentum Nees. 'Burpee's Black Beauty') seedlings with supplemental photosynthetically active radiation from cool-white fluorescent lamps increased growth of plants subsequently transferred outdoors relative to growth of plants that received no supplemental radiation or were shaded to 45% of solar irradiation in the greenhouse before transfer outdoors. Eggplant seedlings transferred outdoors were placed under plastic tarps either to provide relative protection from solar ultraviolet-B (UV-B) radiation (280-315 nm) using Mylar film or to allow exposure to UV-B using cellulose acetate. Protection of seedlings from UV-B radiation resulted in greater leaf expansion than for UV-B-exposed seedlings, but no change in leaf or shoot dry weight occurred after 9 days of treatment. Specific leaf weight increased in response to UV-B exposure outdoors. Exposure of eggplant to UV-B radiation from fluorescent sunlamps in the greenhouse also decreased leaf expansion and leaf and shoot dry weight gain after 5 days of treatment. However, there were no differences in leaf or shoot dry weight relative to control plants after 12 days of UV-B treatment, indicating that UV-B treated plants had acclimated to the treatment and actually had caught up with non-UV-B-irradiated plants in terms of growth.

Acclimatization↗