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G H Thomas

Publications and source records attributed to G H Thomas.

At least 37 records · Page 2Linked to original sources
PubMed2000

Cryptic subtelomeric translocations in the 22q13 deletion syndrome.

Cryptic subtelomeric rearrangements are suspected to underlie a substantial portion of terminal chromosomal deletions. We have previously described two children, one with an unbalanced subtelomeric rearrangement resulting in deletion of 22q13-->qter and duplication of 1qter, and a second with an apparently simple 22q13-->qter deletion. We have examined two additional patients with deletions of 22q13-->qter. In one of the new patients presented here, clinical findings were suggestive of the 22q13 deletion syndrome and FISH for 22qter was requested. Chromosome studies suggested an abnormality involving the telomere of one 22q (46,XX,?add(22)(q13. 3)). FISH using Oncor D22S39 and Vysis ARSA probes confirmed a terminal deletion. A multi-telomere FISH assay showed a signal from 19qter on the deleted chromosome 22. Results were confirmed with 19qtel and 22qtel specific probes. The patient is therefore trisomic for 19qter and monosomic for 22qter. The patient's mother was found to have a translocation (19;22)(q13.42;q13.31). We also re-examined chromosomes from two patients previously diagnosed with 22q deletions who were not known to have a rearrangement using the multi-telomere assay. One of these patients was found to have a derivative chromosome 22 (der(22)t(6;22)(p25;q13)). No evidence of rearrangement was detected in the other patient. Thus we have found the 22q13 deletion to be associated with a translocation in three of four patients. This report illustrates the usefulness of examining patients with hypotonia, severe language delay, and mild facial dysmorphism for this syndrome and suggests that most of these deletions may be unbalanced subtelomeric rearrangements.

Abnormalities, Multiple↗
PubMed1999

Clinical, cytogenetic, and fluorescence in situ hybridization findings in two cases of "complete ring" syndrome.

The term "ring syndrome" was proposed to describe a phenotype of growth failure without major malformations due to a ring autosome. The growth failure is thought to be caused by instability of the ring chromosome leading to aneusomy and cell death. Most previous studies of ring chromosomes were based on standard cytogenetic banding techniques and were limited to microscopically detectable deletions in the ring chromosomes. We report on two patients with complete ring (4) and ring (9) chromosomes, respectively. The first was a 15-month-old girl and the second was a 16-month-old boy. They both presented with severe, symmetrical growth failure and normal psychomotor development in the absence of malformations. Their parents had a normal phenotype. The first case had a whorled pattern of hyperpigmentation and hypopigmentation on part of the face and chest, and the second case had a patchy hyperpigmented rash on the trunk. Peripheral blood karyotype of the first patient was 46,XX, r(4)(p16.3q35.2) and of the second 45,XY,-9/46,XY,r(9)(p24q34.3). G-band analysis suggested no loss of material in the ring chromosomes. These findings were confirmed by fluorescence in situ hybridization (FISH) analysis using chromosome-specific subtelomeric probes. The common human telomeric sequences were intact in the first patient but absent in the second patient. The cytogenetic and FISH data in our two cases provide further evidence for the existence of a "complete ring" phenotype independent of the autosome involved. Pigmentary skin changes are a useful clinical sign of mosaicism caused by the ring instability.

Chromosomes, Human, Pair 4↗
PubMed1999

Competition between Escherichia coli strains expressing either a periplasmic or a membrane-bound nitrate reductase: does Nap confer a selective advantage during nitrate-limited growth?

The physiological role of the periplasmic nitrate reductase, Nap, one of the three nitrate reductases synthesized by Escherichia coli K-12, has been investigated. A series of double mutants that express only one nitrate reductase were grown anaerobically in batch cultures with glycerol as the non-fermentable carbon source and nitrate as the terminal electron acceptor. Only the strain expressing nitrate reductase A grew rapidly under these conditions. Introduction of a narL mutation severely decreased the growth rate of the nitrate reductase A strain, but enhanced the growth of the Nap(+) strain. The ability to use nitrate as a terminal electron acceptor for anaerobic growth is therefore regulated primarily by the NarL protein at the level of transcription. Furthermore, the strain expressing nitrate reductase A had a substantial selective advantage in competition with the strain expressing only Nap during nitrate-sufficient continuous culture. However, the strain expressing Nap was preferentially selected during nitrate-limited continuous growth. The saturation constants for nitrate for the two strains (which numerically are equal to the nitrate concentrations at half of the maximum specific growth rate and therefore reflect the relative affinities for nitrate) were estimated using the integrated Monod equation to be 15 and 50 microM for Nap and nitrate reductase A respectively. This difference is sufficient to explain the selective advantage of the Nap(+) strain during nitrate-limited growth. It is concluded that one physiological role of the periplasmic nitrate reductase of enteric bacteria is to enable bacteria to scavenge nitrate in nitrate-limited environments.

Anaerobiosis↗
PubMed1999

Microbial reduction of technetium by Escherichia coli and Desulfovibrio desulfuricans: enhancement via the use of high-activity strains and effect of process parameters.

Escherichia coli and Desulfovibrio desulfuricans reduce Tc(VII) (TcO(4)(-)) with formate or hydrogen as electron donors. The reaction is catalyzed by the hydrogenase component of the formate hydrogenlyase complex (FHL) of E. coli and is associated with a periplasmic hydrogenase activity in D. desulfuricans. Tc(VII) reduction in E. coli by H(2) and formate was either inhibited or repressed by 10 mM nitrate. By contrast, Tc(VII) reduction catalyzed by D. desulfuricans was less sensitive to nitrate when formate was the electron donor, and unaffected by 10 mM or 100 mM nitrate when H(2) was the electron donor. The optimum pH for Tc(VII) reduction by both organisms was 5.5 and the optimum temperature was 40 degrees C and 20 degrees C for E. coli and D. desulfuricans, respectively. Both strains had an apparent K(m) for Tc(VII) of 0.5 mM, but Tc(VII) was removed from a solution of 300 nM TcO(4)(-) within 30 h by D. desulfuricans at the expense of H(2). The greater bioprocess potential of D. desulfuricans was shown also by the K(s) for formate (>25 mM and 0.5 mM for E. coli and D. desulfuricans, respectively), attributable to the more accessible, periplasmic localization of the enzyme in the latter. The relative rates of Tc(VII) reduction for E. coli and D. desulfuricans (with H(2)) were 12.5 and 800 micromol Tc(VII) reduced/g biomass/h, but the use of an E. coli HycA mutant (which upregulates FHL activities by approx. 50%) had a similarly enhancing effect on the rate of Tc reduction. The more rapid reduction of Tc(VII) by D. desulfuricans compared with the E. coli strains was also shown using cells immobilized in a hollow-fiber reactor, in which the flow residence times sustaining steady-state removal of 80% of the radionuclide were 24.3 h for the wild-type E. coli, 4.25 h for the upregulated mutant, and 1.5 h for D. desulfuricans.

Biomass↗
PubMed1998

A novel and ubiquitous system for membrane targeting and secretion of cofactor-containing proteins.

We report the identification of the proteins encoded by the mttABC operon (formerly yigTUW), which mediate a novel Sec-independent membrane targeting and translocation system in Escherichia coli that interacts with cofactor-containing redox proteins having a S/TRRXFLK "twin arginine" leader motif. A pleiotropic-negative mutant in mttA prevents the periplasmic localization of twin arginine redox enzymes, including nitrate reductase (NapA) and trimethylamine N-oxide reductase (TorA). The mutation also prevents the correct localization of the integral membrane molybdoenzyme dimethylsulfoxide reductase (DmsABC). The DmsA subunit has a twin arginine leader. Proteins with a Sec-dependent leader or which assemble spontaneously in the membrane are not affected by this mutation. MttA, B, and C are members of a large family of related sequences extending from archaebacteria to higher eukaryotes.

2S Albumins, Plant↗
PubMed1998

Drosophila betaHeavy-spectrin is essential for development and contributes to specific cell fates in the eye.

The spectrin membrane skeleton is a ubiquitous cytoskeletal structure with several cellular roles, including the maintenance of cell integrity, determination of cell shape and as a contributor to cell polarity. We have isolated mutations in the gene encoding &bgr ;Heavy-spectrin in Drosophila, and have named this essential locus karst. karst mutant individuals have a pleiotropic phenotype characterized by extensive larval lethality and, in adult escapers, rough eyes, bent wings, tracheal defects and infertility. Within karst mutant eyes, a significant number of ommatidia specifically lack photoreceptor R7 alongside more complex morphological defects. Immunolocalization of betaHeavy-spectrin in wild-type eye-antennal and wing imaginal discs reveals that betaHeavy-spectrin is present in a restricted subdomain of the membrane skeleton that colocalizes with DE-cadherin. We propose a model where normal levels of Sevenless signaling are dependent on tight cell-cell adhesion facilitated by the betaHeavy-spectrin membrane skeleton. Immunolocalization of betaHeavy-spectrin in the adult and larval midgut indicates that it is a terminal web protein, but we see no gross morphological defects in the adult apical brush border in karst mutant flies. Rhodamine phalloidin staining of karst mutant ovaries similarly reveals no conspicuous defect in the actin cytoskeleton or cellular morphology in egg chambers. This is in contrast to mutations in alpha-spectrin, the molecular partner of betaHeavy-spectrin, which affect cellular structure in both the larval gut and adult ovaries. Our results emphasize the fundamental contribution of the spectrin membrane skeleton to normal development and reveals a critical interplay between the integrity of a cell's membrane skeleton, the structure of cell-cell contacts and cell signaling.

Alleles↗
PubMed1997

Familial tandem duplication of bands q31.1 to q32.3 on chromosome 4 with mild phenotypic effect.

We describe a family carrying a direct duplication of an interstitial segment of the distal long arm of chromosome 4(q31.1q32.3) with a mild clinical phenotype. Affected family members include a 27-year-old woman and 2 of her 4 children, 1 of whom also has Klinefelter syndrome. Although 8 cases of simple duplications or insertions of 4q material have been described, only 3 include bands q31-q32, and these involve additional adjacent material. Affected members of the pedigree reported here have some of the manifestations of previously described cases of duplication 4q, but are less severely affected, suggesting that the genetic material in this region is well tolerated as a partial trisomy.

Abnormalities, Multiple↗
PubMed1997

Segregation of a familial balanced (12;10) insertion resulting in Dup(10)(q21.2q22.1) and Del(10)(q21.2q22.1) in first cousins.

An interchromosomal insertion in 3 generations of a family was ascertained through two developmentally delayed first cousins. Cytogenetic analysis using G-banding and chromosome painting showed an apparently balanced direct insertion of chromosome 10 material into chromosome 12, ins(12;10)(q15;q21.2q22.1), in the mothers and grandfather of these children. The proposita inherited only the derivative 10 chromosome, resulting in deletion of 10q21.2 --> 22.1 while her cousin inherited only the derivative 12, resulting in duplication of 10q21.2 --> 22.1. A comparison of the proposita with published deletion cases suggests a pattern of anomalies attributable to deletion of the 10q21 --> q22 region: developmental delay, hypotonia, a heart murmur, telecanthus, broad nasal root and ear abnormalities. This is the first report of a nontandem duplication of the 10q21 --> q22 region. The phenotype of the cousin with the duplication does not overlap greatly with published tandem 10q duplications. Finally, this report reaffirms the importance of obtaining family studies of patients with interstitial chromosomal abnormalities.

Chromosomes, Human, Pair 10↗
PubMed1997

Comparisons of the nucleotide substitution process among repetitive segments of the alpha- and beta-spectrin genes.

The actin-cross-linking protein spectrin is a prominent component of the membrane cytoskeleton. Spectrin is a tetramer of two antiparallel alphabeta-dimers which share a unique and ancient gene structure. The alpha-spectrin and beta-spectrin genes are composed primarily of tandemly repeated 106-amino-acid segments, each of which forms a triple alpha-helical coiled coil. Both the genes and the repeats themselves are homologous. The two genes are thought to be the result of a gene duplication event, and each gene is the product of duplications of the 106-amino-acid repeats. In this work we compare the process of molecular evolution across the repeated segments of the alpha- and beta-spectrin genes. We find that the alpha-spectrin segments have, for the most part, evolved in a homogeneous fashion, while considerable heterogeneity is found among beta-spectrin segments. Several segments with unique known functions are found to have evolved differently than the others. On the basis of heterogeneity of the evolutionary process, we suggest that at least one repeat has a unique function that has yet to be documented. We also present new statistical methods for comparing the evolutionary process between different regions of DNA sequences.

Animals↗
PubMed1997

Intragenic duplication and divergence in the spectrin superfamily of proteins.

Many structural, signaling, and adhesion molecules contain tandemly repeated amino acid motifs. The alpha-actinin/spectrin/dystrophin superfamily of F-actin-crosslinking proteins contains an array of triple alpha-helical motifs (spectrin repeats). We present here the complete sequence of the novel beta-spectrin isoform beta(Heavy)-spectrin (beta H). The sequence of beta H supports the origin of alpha- and beta-spectrins from a common ancestor, and we present a novel model for the origin of the spectrins from a homodimeric actin-crosslinking precursor. The pattern of similarity between the spectrin repeat units indicates that they have evolved by a series of nested, nonuniform duplications. Furthermore, the spectrins and dystrophins clearly have common ancestry, yet the repeat unit is of a different length in each family. Together, these observations suggest a dynamic period of increase in repeat number accompanied by homogenization within each array by concerted evolution. However, today, there is greater similarity of homologous repeats between species than there is across repeats within species, suggesting that concerted evolution ceased some time before the arthropod/vertebrate split. We propose a two-phase model for the evolution of the spectrin repeat arrays in which an initial phase of concerted evolution is subsequently retarded as each new protein becomes constrained to a specific length and the repeats diverge at the DNA level. This evolutionary model has general applicability to the origins of the many other proteins that have tandemly repeated motifs.

Actinin↗
PubMed1997

Cryptic terminal rearrangement of chromosome 22q13.32 detected by FISH in two unrelated patients.

Two unrelated patients with cryptic subtelomeric deletions of 22q13.3 were identified using FISH with the commercially available Oncor probe, D22S39. Proband 1 was found to have a derivative chromosome 22 resulting from the unbalanced segregation of a t(1;22)(q44;q13.32) in her mother. Additional FISH analysis of proband 1 and her mother placed the breakpoint on chromosome 22 in this family proximal to D22S55 and D22S39 and distal to D22S45. We have mapped D22S39 to within 170 kb of D22S21 using pulsed field gel electrophoresis. D22S21 is genetically mapped between D22S55 and D22S45. These data indicate that the deletion in proband 1 is smaller than in eight of nine reported del(22)(q13.3) patients. Probands 1 and 2 share features of hypotonia, developmental delay, and expressive language delay, also seen in previously reported del(22)(q13.3) patients, although proband 1 appears to be more mildly affected. Proband 1 is also trisomic for the region 1q44-->qter. This very small duplication has been previously reported only once and the patient had idiopathic mental retardation. This is the first report where 22q13.3 terminal deletion patients have been identified through the use of FISH, and the first report of a deletion of this region occurring because of missegregation of a parental balanced cryptic translocation. We feel that investigation of the frequency of del(22)(q13.3) in the idiopathic mentally retarded population is warranted and may be aided by the ability to use a commercially available probe (D22S39), which is already currently in use in a large number of cytogenetic laboratories.

Adult↗
PubMed1996

High male:female ratio of germ-line mutations: an alternative explanation for postulated gestational lethality in males in X-linked dominant disorders.

In this paper I suggest that a vastly higher rate of de novo mutations in males than in females would explain some, if not most, X-linked dominant disorders associated with a low incidence of affected males. It is the inclusion of the impact of a high ratio of male:female de novo germ-line mutations that makes this model new and unique. Specifically, it is concluded that, if an X-linked disorder results in a dominant phenotype with a significant reproductive disadvantage (genetic lethality), affected females will, in virtually all cases, arise from de novo germ-line mutations inherited from their fathers rather than from their mothers. Under this hypothesis, the absence of affected males is explained by the simple fact that sons do not inherit their X chromosome (normal or abnormal) from their fathers. Because females who are heterozygous for a dominant disorder will be clinically affected and will, in most cases, either be infertile or lack reproductive opportunities, the mutant gene will not be transmitted by them to the next generation (i.e., it will be a genetic lethal). This, not gestational lethality in males, may explain the absence of affected males in most, if not all, of the 13 known X-linked dominant diseases characterized by high ratios of affected female to male individuals. Evidence suggesting that this mechanism could explain the findings in the Rett syndrome is reviewed in detail.

Fathers↗
PubMed1995

Sister chromatid exchanges in cells defective in mismatch, post-replication and excision repair.

Three processes associated with DNA damage and genomic instability have been defined experimentally as operating during or soon after DNA replication: mismatch repair, post-replication repair and sister chromatid exchange. All these processes appear to operate on damage and/or errors in newly replicated DNA. Both mismatch repair and post-replication repair involve resynthesis of up to 1 kb of newly synthesized DNA: mismatch repair operates on single-base or slippage errors; post-replication repair operates on persistent gaps in newly synthesized DNA caused by damage on parental strands. Using colon cancer cells with different mismatch repair capacity, together with normal cells and excision-repair-defective and post-replication-repair-defective xeroderma pigmentosum (XP) cells, we analysed possible interactions between these processes. No evidence for overlap of mismatch repair with excision or post-replication repair was found. However, post-replication-repair-defective XP variant cells that were SV40 transformed showed higher UV-induced sister chromatid exchange frequencies than did untransformed cells. This suggests that sister chromatid exchanges in the XP variant are closely involved with UV-induced replication errors that are enhanced by transformation.

Caffeine↗
PubMed1994

Prenatal diagnosis of xeroderma pigmentosum and Cockayne syndrome.

In a study of fetal cells from a series of 12 pregnancies in ten families at risk for the ultraviolet light-sensitive, DNA repair-deficient diseases xeroderma pigmentosum (XP) and Cockayne syndrome (CS), we detected one XP and two CS homozygote fetuses. The diagnoses were confirmed by analysis of fetal skin fibroblasts or second amniotic samples after termination of the pregnancies. The measurement of ultraviolet light sensitivity and DNA repair depended on properties common to the seven excision repair-deficient XP complementation groups (A-G) and the two CS complementation groups (A, B). No XP variant families were included in the study, because the variant requires different testing techniques. Reliable and rapid diagnosis proved possible in all but one of the 12 pregnancies, supporting the use of these methods until the spectrum of mutations in the various XP and CS genes of the U.S. population is fully characterized and a DNA sequence-based diagnostic procedure becomes available.

Amniocentesis↗