PubMed Health⌕ Search

Biomedical subjects

G A Thomas

Publications and source records attributed to G A Thomas.

At least 19 recordsLinked to original sources
PubMed1992

Oestrogen receptor protein and mRNA in adenocarcinoma of the uterine cervix.

We have investigated the oestrogen receptor (ER) status of 20 cervical adenocarcinomas by immunocytochemistry for ER protein and non-isotopic in situ hybridisation for ER mRNA. Both methods, which are applicable to paraffin sections, were developed and validated in breast carcinomas with known ER content. Six cervical adenocarcinomas contained immunocytochemically demonstrable ER protein; all contained ER mRNA, but staining was less intense in poorly differentiated areas of four tumours. This disparity between protein and mRNA detection needs further investigation as does the possibility that oestrogens may play a role in the pathogenesis of cervical adenocarcinoma.

Adenocarcinoma↗
PubMed1992

Production of thyroid tumours in mice by demethylating agents.

Previous studies on thyroid tumorigenesis in rodents have explored the role of oncogene mutation in tumour development. However, many mutagens are also known to decrease DNA methylation, another factor known to be important in the regulation of gene expression. We report the results of a small study to examine whether the demethylating agents 5-azacytidine and 5-aza-2-deoxycytidine could promote radiation- or goitrogen-induced tumorigenesis in the mouse, and whether they could potentiate the effect of a combined radiation and goitrogen regime. Three single doses of either 5-azacytidine or 5-aza-2-deoxycytidine increased the frequency of lesions in the goitrogen-treated animals; one of these lesions was a metastasising carcinoma. Thyroid carcinomas are very rare in mice in the absence of mutagen treatment, and metastasis is particularly unusual. There was no significant difference, at the dosages used, between the number of tumours induced by demethylating agents and those induced by a mutagen (radiation) in goitrogen-treated animals. Unlike goitrogens, demethylating agents did not promote radiation-induced tumorigenesis, and they did not produce any significant potentiation of the conventional regime of radiation and goitrogen at the dosages used. This study suggests that the role of non-genotoxic factors, such as agents affecting patterns of DNA methylation, warrants consideration in thyroid tumorigenesis.

Animals↗
PubMed1991

The hydrogen-bonding structure in parallel-stranded duplex DNA is reverse Watson-Crick.

Raman spectra of the parallel-stranded duplex formed from the deoxyoligonucleotides 5'-d-[(A)10TAATTTTAAATATTT]-3' (D1) and 5'-d[(T)10ATTAAAATTTATAAA]-3' (D2) in H2O and D2O have been acquired. The spectra of the parallel-stranded DNA are then compared to the spectra of the antiparallel double helix formed from the deoxyoligonucleotides D1 and 5'-d(AAATATTTAAAATTA-(T)10]-3' (D3). The Raman spectra of the antiparallel-stranded (aps) duplex are reminiscent of the spectra of poly[d(A)].poly[d(T)] and a B-form structure similar to that adopted by the homopolymer duplex is assigned to the antiparallel double helix. The spectra of the parallel-stranded (ps) and antiparallel-stranded duplexes differ significantly due to changes in helical organization, i.e., base pairing, base stacking, and backbone conformation. Large changes observed in the carbonyl stretching region (1600-1700 cm-1) implicate the involvement of the C(2) carbonyl of thymine in base pairing. The interaction of adenine with the C(2) carbonyl of thymine is consistent wtih formation of reverse Watson-Crick base pairing in parallel-stranded DNA. Phosphate-furanose vibrations similar to those observed for B-form DNA of heterogenous sequence and high A,T content are observed at 843 and 1092 cm-1 in the spectra of the parallel-stranded duplex. The 843-cm-1 band is due to the presence of a sizable population of furanose rings in the C2'-endo conformation. Significant changes observed in the regions from 1150 to 1250 cm-1 and from 1340 to 1400 cm-1 in the spectra of the parallel-stranded duplex are attributed to variations in backbone torsional and glycosidic angles and base stacking.

Base Composition↗
PubMed1991

Raman spectroscopic studies of the DNA cro binding site conformation, free and bound to cro protein.

Raman spectra of the DNA binding site for cro repressor protein were obtained in the presence and absence of bound cro protein. The 17 base pair fragment is a consensus sequence of the six cro binding sites in phage lambda, except that the second base to the right of the center of pseudosymmetry is altered. Analysis of the spectrum of the free DNA indicates that the molecule exists in a B-like conformation with deviations from the usual B form occurring mainly in the bands assigned to A-T vibrations. The spectrum of the bound DNA was obtained by subtracting the spectrum of free cro from the spectrum of the complex which was estimated to be 90% bound. The DNA undergoes significant structural changes upon binding to the protein; most notable of these changes is a destacking of the G-C bases reflected by increases in the 1240, 1262, and 1320 cm-1 bands. A decrease in the 1361 cm-1 band that occurs has also been assigned to a destacking in guanine bases. The appearance of a 705 cm-1 band and the decrease and downshift of the 670 cm-1 band are consistent with the appearance of A-like character in the A-T region of the binding site when the protein binds; however, the spectra indicate that the entire binding site remains in a distorted B-like conformation. We use the 705 cm-1 band to estimate A-like character because the 800-850 cm-1 region is obscured by interference from strong protein bands. Other shifts in both intensity and position cannot be assigned to characteristic changes in conformation and therefore must be attributed to the protein influencing the structure in a novel way.

Bacteriophage lambda↗
PubMed1991

Loss of heterozygosity on 6q, 16q, and 17p in human central nervous system primitive neuroectodermal tumors.

The loss of genetic material from specific chromosomal locations in a given tumor type has been taken for evidence of the importance of tumor suppressor genes at these loci in the genesis of the tumor. The primitive neuroectodermal tumor of the central nervous system has such a loss on 17p in one-third of tumors. In this report, a detailed analysis of 17p loss in 23 tumors has been performed using 10 probes mapping to this region. In addition, an analysis for allelic deletion on chromosomes 6q, 16q, and 22q has been performed. Six of the 23 tumors showed loss of markers on 17p, and the area of common loss spanned 17p11.2 to 17pter. Five of 23 tumors showed loss of markers on 6q, and 3 showed loss on 16q. No tumor lost markers on 22q. Only one tumor showed loss at more than one location. These data suggest that primitive neuroectodermal tumors either are a heterogeneous group of tumors with more than one mechanism leading to a tumor or that more than one recessive oncogene may play a role in the genesis of these tumors.

Adult↗
PubMed1991

Evidence for and possible mechanisms of non-genotoxic carcinogenesis in the rodent thyroid.

Thyroid tumours are a common finding in toxicity tests in rodents. It is known that prolonged administration of antithyroid drugs leads to the development of multiple thyroid tumours, and the role of genotoxic and non-genotoxic mechanisms in this needs definition. The role of drugs with an antithyroid action in thyroid carcinogenesis requires a knowledge of thyroid physiology. This review briefly discusses the anatomy and physiology of the thyroid before concentrating on the cellular pathology of the changes that take place in the transition from a normal to a neoplastic thyroid cell. The malignant cell is characterised by excess growth and invasiveness. The normal thyroid cell does not possess an unlimited growth potential because of a growth-desensitising mechanism (GDM) of the antioncogene type. Spontaneous thyroid carcinogenesis requires three key steps which are presumed to arise by mutation and clonal selection: the loss of the GDM, the acquisition of TSH-independent growth, and the acquisition of invasiveness. The sequence of the cell biological changes involved is not fully understood, but it has been shown that IGF-1 is a necessary co-factor for the growth-stimulating effect of TSH in the normal cell, and that autocrine production of IGF-1 is a feature of spontaneous thyroid adenomas. Another early change that has been shown in both experimental and human thyroid tumours is mutation of one of the ras oncogenes. In carcinogenesis due to the prolonged administration of an agent known to interfere with thyroid hormone metabolism and to induce a high TSH, two rather than three key steps will be required for carcinogenesis, as the development of TSH independent growth will not confer any selective advantage. We have shown that monoclonal lesions induced in this way regress when the goitrogen is withdrawn and therefore retain TSH dependence. The development of the other two key changes--the loss of the GDM and the acquisition of invasiveness--may be due to genotoxic or non-genotoxic mechanisms. They can occur in man in the absence of any known mutagenic agent. In patients with dyshormonogenesis a congenital defect in one of the steps of thyroid hormone synthesis is associated with multiple tumour production. It is reassuring that in these patients, exposed to decades of high TSH levels, benign lesions are common, but malignant thyroid tumours are very rare. The occurrence of thyroid tumours following the use of substances known to interfere with thyroid hormone metabolism does not itself exclude a genotoxic component to the carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Reversibility of the malignant phenotype in monoclonal tumours in the mouse.

Longterm goitrogen administration to rodents is well known to result in multiple proliferative lesions of the thyroid. The regression of these lesions on withdrawal of goitrogen has led to their neoplastic nature being questioned, and they have been regarded as 'nodules' rather than as true tumours. We have induced multiple thyroid lesions by the combined use of high dose radiation as a mutagen, together with goitrogen administration to induce prolonged TSH growth stimulation. G6PD histochemistry was used in heterozygous G6PD deficient female mice to show that all the thyroid lesions induced by this regime were monophenotypic, and therefore monoclonal in origin. The great majority of induced tumours were adenomas, a minority were carcinomas. The number of carcinomas observed was significantly lower in a group of animals from which goitrogen was withdrawn for 4 weeks prior to killing, when compared to animals killed while on goitrogen treatment. Both adenomas and carcinomas, including areas of intravascular tumour, showed morphological features of regression on withdrawal of the goitrogen. There are three key cellular changes which must occur in spontaneous thyroid carcinogenesis--escape from a growth limiting mechanism, acquisition of TSH independent growth and acquisition of invasiveness. In the natural selection of mutations or epimutations during carcinogenesis, prolonged high levels of TSH are likely to remove any selective advantage from mutations that lead to TSH independent growth. Tumours induced by a regime including prolonged goitrogen treatment may therefore develop following two rather than three key stages. They will occur with an increased frequency relative to lesions observed in spontaneous carcinogenesis, but will retain TSH dependency. We speculate that several mechanisms may lead to loss of the growth limiting mechanism, including translocation of an oncogene to the region of a TSH induced promoter. Other carcinogenic regimes may also increase the yield of tumours by creating conditions which reduce the number of essential steps required for carcinogenesis, and may involve translocation to a carcinogen inducible promoter.

Animals↗
PubMed1991

The immunohistochemical detection of involucrin in denture induced fibrous inflammatory hyperplasia of oral mucous membrane.

Involucrin is a major structural protein specific to the cross-linked cell envelope found in the stratum corneum of stratified squamous epithelium. This protein is considered to be an excellent immunohistochemical marker of normal squamous differentiation. Detection of variations to the patterns of immunostaining for involucrin may also be of value in the differential diagnosis between benign and malignant lesions. Previous studies of involucrin expression in oral mucosa have failed to clarify the effect of chronic inflammatory change upon the patterns of immunoreactivity. This study investigated involucrin staining patterns in fibrous inflammatory hyperplasia of oral mucous membrane (FIH). The results suggest that in FIH an altered pattern of involucrin immunostain occurs in areas of severe inflammatory change. This may reflect changes to the pattern of squamous differentiation in this tissue.

Dentures↗
PubMed1990

Allelotype of human malignant astrocytoma.

Astrocytoma, the most common brain tumor in humans, is usually malignant and virtually incurable. Two types of malignant astrocytomas can be distinguished histopathologically: anaplastic astrocytoma and glioblastoma multiforme. Studies using DNA markers that detect restriction fragment length polymorphisms have shown that loci on chromosomes 10 and 17p are lost frequently in tumor DNA from malignant astrocytoma patients, suggesting that tumor suppressor genes important in astrocytoma tumorigenesis may be present on 2 different chromosomes. To identify additional regions of chromosome loss, we carried out an allelotype analysis of 41 malignant astrocytoma patients using restriction fragment length polymorphism markers for each arm of every human autosome. Loss of heterozygosity was found for every autosome except chromosome 21, indicating an even greater complexity of genomic alterations than reported previously. Many tumors showed loss of heterozygosity for multiple chromosomes and the number of chromosomes involved correlated with tumor histopathology. A high-resolution restriction fragment length polymorphism study of chromosome 10 loci in these patients showed that loss of broad regions of chromosome 10 was a common event, particularly in glioblastoma multiforme. An allelotype analysis has been carried out on only one other tumor, human colorectal carcinoma. Different profiles of allele loss were observed in malignant astrocytoma and colorectal carcinoma, suggesting that the genetic events leading to these 2 human cancers may proceed along different pathways.

Alleles↗
PubMed1990

A Raman study of low frequency intrahelical modes in A-, B-, and C-DNA.

We have obtained low frequency (less than 200 cm-1) Raman spectra of calf-thymus DNA and poly(rI).poly(rC) as a function of water content and counterion species and of d(GGTATACC)2 and d(CGCGAATTCGCG)2 crystals. We have found that the Raman scattering from water in the first and second hydration shells does not contribute directly to the Raman spectra of DNA. We have determined the number of strong Raman active modes by comparing spectra for different sample orientations and polarizations and by obtaining fits to the spectra. We have found at least five Raman active modes in the spectra of A- and B-DNA. The frequencies of the modes above 40 cm-1 do not vary with counterion species, and there are only relatively small changes upon hydration. These modes are, therefore, almost completely internal. The mode near 34 cm-1 in A-DNA is mostly internal, whereas the mode near 25 cm-1 is dominated by interhelical interactions. The observed intensity changes upon dehydration were found to be due to the decrease in interhelical distance. Polymer length appears to play a role in the lowest frequency modes.

Base Sequence↗
PubMed1990

The infrared and Raman spectra of the duplex of d(GGTATACC) in the crystal show bands due to both the A-form and the B-form of DNA.

The deoxyoligonucleotide, d(GGTATACC), forms a duplex structure that crystallizes in the DNA A form. This has been shown by both X-ray diffraction studies and Raman spectroscopy (1,2). The presence of the DNA B form has been reported using diffuse X-ray scattering from a crystal of the closely related sequence d(GGBrUABrUACC)(3). In this paper the infrared spectrum of the d(GGTATACC) crystal is presented and curve resolution of both the Raman and IR spectra have been carried out. The percentage of A and B forms have been estimated. The %B form in the crystal has been estimated from the IR spectra to be about 15% and from Raman to be about 20%. Moreover the IR spectrum of the A conformation in the crystal is slightly different from the IR spectrum of the A conformation in polynucleotide fibers in particular in the region of the phosphate stretching vibrations and of the in-plane double bond vibrations of the bases. We show that it is feasible to obtain IR as well as Raman spectra of small crystals of oligonucleotides and that this is a good method of identifying all of the different conformations that may be in the crystal.

Base Sequence↗
PubMed1990

Low frequency Raman spectra of Z-DNA.

We have performed a Raman study of the low frequency modes in three oligo- and polynucleotides in Z-conformation, and we compare the spectra of these samples to those of two polynucleotides in B-conformation. In Z-DNA we find 5 intrahelical modes below 200 cm-1, in addition to the interhelical mode near 30 cm-1 which is only observed in crystalline samples. The most prominent intrahelical mode has a frequency of about 105 cm-1, close to the frequency of the strongest intrahelical mode in A-and B-DNA. The sequence dependence of the frequency of this mode is considerably larger than for the same mode in B-DNA. The other modes are less pronounced, and their frequency variations with base sequence are within the experimental accuracy.

Base Sequence↗
PubMed1989

Loss of heterozygosity for loci on chromosome 17p in human malignant astrocytoma.

Loss of constitutional heterozygosity for specific chromosomal loci, when found consistently in a particular tumor type, suggests that a recessive oncogene important in the genesis of that tumor may be present within the involved chromosomal loci. DNA markers that detect restriction fragment length polymorphisms are powerful tools that have been used to detect loss of chromosomal loci in a growing number of human malignancies. The human brain tumor astrocytoma is usually malignant and virtually incurable. Two types of malignant astrocytomas are recognized histopathologically:anaplastic astrocytoma and glioblastoma multiforme. We carried out a restriction fragment length polymorphism analysis of tumors from 15 patients with anaplastic astrocytoma and 20 patients with glioblastoma using polymorphic DNA markers for loci on chromosome 17. Loss of constitutional heterozygosity for loci on chromosome 17 was found in both anaplastic astrocytoma and glioblastoma patients with equal frequency (40% of cases). Our mapping data revealed a region of loss on chromosome 17p between physical loci p11.2 and pter that was common to both patient groups. Taken together with the previously reported finding of loss of heterozygosity for loci on chromosome 10 in glioblastoma, these results indicate that tumorigenesis in the astrocyte lineage may involve recessive oncogenes on two different chromosomes.

Astrocytoma↗
PubMed1989

The length of a junction between the B and Z conformations in DNA is three base pairs or less.

Recently it has been suggested that double-helical complexes formed between the DNA sequences (CG)n(A)m and their conjugates, (T)m(CG)n, would be candidates for the formation of a B-Z junction in aqueous solution at high salt concentrations [Peticolas et al. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 2579-2583]. The junction was predicted to occur between a B-type helix in the d(A)m.d(T)m section and a Z-type helix in the self-complementary (CG)n.(CG)n sequence. In this paper we report Raman experiments on the deoxyoligonucleotides d(CGCGCGCGCGCGAAAAA) and d(CGCGCGAAAAA) and their complements. It is found the latter compound cannot be induced into the Z form in saturated salt solution but that the former sequence goes into a B-Z junction at 5.5 M salt. From a comparison of the relative intensity of the Raman conformational marker bands for B and Z DNA for both the A-T and C-G base pairs, it is shown that in 5.5 M NaCl solution none of the A-T base pairs are in the Z form, but nine of the C-G base pairs are in the Z form. The remaining three C-G base pairs are either in the junction or in the B form. Thus, the junction is formed from three or less C-G base pairs. If the solution is made 95 microM with NiCl2, then the entire duplex goes into the Z form and the Raman bands of the adenine are completely changed into those of the Z form.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Composition↗
PubMed1989

Environmentally induced conformational changes in B-type DNA: comparison of the conformation of the oligonucleotide d(TCGCGAATTCGCG) in solution and in its crystalline complex with the restriction nuclease EcoRI.

Raman spectroscopic analysis of the secondary structure of the crystalline restriction endonuclease EcoRI, the oligonucleotide d(TCGCGAATTCGCG) in solution, and the corresponding crystalline EcoRI-oligonucleotide complex reveals structural differences between the complexed and uncomplexed protein and oligonucleotide components that appear to be linked to complex formation. Structural differences that are spectroscopically identified include (1) an increase in the population of furanose rings adopting the C3'-endo conformation and (2) spectroscopically observed changes in base stacking which are probably associated with the crystallographically observed distortion of the phosphate backbone about positions C(3)-G(4) and C(9)-G(10) and unwinding between the symmetry-related segments GAA-TTC which make up the central recognition core (McClarin et al., 1986). Changes in base stacking due to distortions and unwinding along the oligonucleotide result in differences in the base vibrational region between the spectra of the complex and the oligonucleotide in solution. The spectroscopic analysis indicates that the C2'-endo population is similar for the oligonucleotide in solution and in the complex. The additional C3'-endo population in the complex appears to arise from the conversion of rings adopting alternative conformations such as C1'-exo and O1'-endo. Analysis of the vibrational bands derived from guanine indicates that the population of guanine residues associated with furanose rings in a C2'-endo conformation is similar for the oligonucleotide in solution and in the crystalline complex. This implies that the increase in C3'-endo population is not associated with guanine residues. Large conformational distortions such as those observed in the crystal distortions are not observed in either the crystal or the solution of the oligomer d(CGCGAATTCGCG).(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallography↗
PubMed1989

A temperature-dependent Z to B to single-strand transition in d(CGCG).

A thermally induced left- to right-handed helical interconversion for the self-complementary tetradeoxynucleotide d(CGCG) has been observed in the presence of NaCl at concentrations from 1.5 to 2.25M. Analysis of the Raman spectrum of d(CGCG) in aqueous solution as a function of temperature indicates that at low temperature a left-handed Z-helical form is the predominant species. An increase in temperature results in a decrease in the population of the left-handed form and an increase in the population of the right-handed form. Further elevation in temperature results in extensive disruption of base stacking and a loss of secondary structure. This unstacking presumably represents dissociation to a single-stranded structure. The data suggest that a temperature-dependent Z to B to single-strand transition occurs under the conditions employed. A direct Z to single-strand conversion was not identified. In contrast to previous examples, the thermally induced left- to right-handed helical conversion for d(CGCG) can occur in the absence of chemically modified nucleic acid bases, alcohol solutions, or divalent ions.

DNA, Single-Stranded↗
PubMed1989

Loss of heterozygosity on chromosome 10 in human glioblastoma multiforme.

Recessive mutations, revealed by loss of the wild-type allele, have been associated with the development of a variety of cancers in children and adults. Polymorphic chromosome 10 markers were used to screen paired tumor and lymphocyte DNA samples in 13 patients with glioblastoma multiforme. Ten patients showed loss of constitutional heterozygosity in the tumor samples. This finding suggests that a recessive gene involved in the development of glioblastoma multiforme is present on chromosome 10.

Alleles↗