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Biomedical subjects

A T Natarajan

Publications and source records attributed to A T Natarajan.

At least 19 recordsLinked to original sources
PubMed1992

Genotoxic effects of sodium arsenite on human cells.

The effects of sodium arsenite (SA) were studied either alone or in combination with X-rays in peripheral blood lymphocytes, and with short-wave ultraviolet (UV) radiation in primary human fibroblast culture systems. It was found that SA (i) inhibited the cell cycle progression of phytohaemagglutinin (PHA)-responsive lymphocytes, (ii) induced chromatid-type aberrations and sister-chromatid exchanges (SCEs) as a function of concentration and (iii) potentiated the X-ray- and UV-induced chromosomal damage. Our results suggest that SA interferes with the DNA repair process, presumably by inhibiting the ligase activity. This accounted for an increase in the DNA replication-dependent processes, chromatid aberrations and SCEs and synergistic enhancement of the X-ray- and UV-induced chromosomal damage. This ability of arsenite may be responsible for its comutagenic properties with different types of mutagens and hence its carcinogenicity.

Arsenic↗
PubMed1992

Transcription affects the rate but not the extent of repair of cyclobutane pyrimidine dimers in the human adenosine deaminase gene.

To study the relationship between transcription and strand-specific repair of UV-induced cyclobutane pyrimidine dimers, dimer removal was analyzed in a cell line containing two alleles of an inactivated adenosine deaminase (ADA) gene. The cell line was derived from a patient suffering from severe combined immunodeficiency. The disease was caused by a deletion of the complete promoter of the gene as well as the first exon of the ADA gene. This resulted in a true null allele without any detectable transcription (Berkvens, T.M., Gerritsen, E. J. A., Oldenburg, M., Breukel, C., Wijnen, J. T. H., Van Ormondt, H., Vossen, J. M., Van der Eb, A. J., and Meera Khan, P. (1987) Nucleic Acids Res. 15, 9365-9378). Despite this lack of transcription, repair of the ADA gene in this cell line was found to be very efficient with 80% of the dimers being removed within 24 h after UV irradiation. However, the initial rapid repair which is associated with the transcribed strand in normal cells is absent. Dimer removal from two inactive loci, 754 and coagulation factor IX, was much less efficient with only 40% dimers removed after 24 h. From this data, we conclude that transcription is not required for efficient repair of a gene, but forms an additional signal for accelerated repair of the transcribed strand. Furthermore, we suggest that different levels of repair exist between non-transcribed sequences in active genes and those in repressed loci. The results are discussed in terms of the current ideas about the mechanism of preferential DNA repair in human cells.

Adenosine Deaminase↗
PubMed1992

Use of a T-lymphocyte clonal assay for determining HPRT mutant frequencies in individual rats.

Conditions for detection and isolation of HPRT- mutants in cloned rat T-lymphocytes from individual adult Lewis rats were determined. Similar to cloning of human T-cells, best results were obtained with lectin (PHA)-primed T-lymphocytes of rats. High cloning efficiencies, occasionally exceeding 50%, could be obtained when the target cells employed were isolated from cervical lymph nodes. Feeder cells used were splenocytes, irradiated with 40 Gy of X-rays after priming with Con A. Human interleukin-2, present in LAK supernatant, proved to be capable of inducing proliferative activity of rat T-lymphocytes and could replace conditioned medium from primed rat splenocytes. Under the conditions described in this paper, the frequency of mutants in the HPRT gene of T-lymphocytes in Lewis rats was about 80% lower than that found in human T-lymphocytes from adults. The inverse relationship between mutant frequency and cloning efficiency, clearly demonstrated for human data, could not be established for rats. Treatment of rats with N-ethyl-N-nitrosourea, a potent alkylating agent, resulted in a time- and dose-dependent induction of HPRT- mutants, demonstrating the usefulness of this system to study in vivo mutagenesis.

Animals↗
PubMed1992

The mouse splenocyte assay, an in vivo/in vitro system for biological monitoring: studies with X-rays, fission neutrons and bleomycin.

A modified mouse splenocyte culture system was standardized after testing different mitogens (i.e., phytohemagglutinin (PHA), concanavalin A (Con A)). The mitotic index was determined for comparison between different mitogens. Following selection of appropriate mitogen (PHA 16, Flow), a series of experiments were conducted to evaluate the application of a cytokinesis-block for scoring micronuclei and assays for chromosomal aberrations produced by treatment in G0 and G2 for the purposes of biological dosimetry following in vivo and/or in vitro exposure to X-rays, fission neutrons and bleomycin. In the X-irradiation studies, the frequencies of micronuclei and chromosomal aberrations (i.e., dicentrics and rings) increased in a dose-dependent manner. These data could be fitted to a linear-quadratic model. No difference was observed between irradiation in vivo and in vitro, suggesting that measurement of dicentrics and micronuclei in vitro after X-irradiation can be used as an in vivo dosimeter. Following in vivo irradiation with 1 MeV fission neutrons and in vitro culturing of mouse splenocytes, linear dose-response curves were obtained for induction of micronuclei and chromosomal aberrations. The lethal effects of neutrons were shown to be significantly greater than for a similar dose of X-rays. The relative biological effectiveness (RBE) was 6-8 in a dose range of 0.25-3 Gy for radiation-induced asymmetrical exchanges (dicentrics and rings), and about 8 for micronuclei in a dose range of 0.25-2 Gy. Furthermore, the induction of chromosomal aberrations by bleomycin was investigated in mouse G0 splenocytes (in vitro) and compared with X-ray data. Following bleomycin treatment (2 h) a similar pattern of dose-response curve was obtained as with X-rays. In this context a bleomycin rad equivalent of 20 micrograms/ml = 0.50 Gy was estimated.

Animals↗
PubMed1992

Frequencies of X-ray-induced chromosome translocations in human peripheral lymphocytes as detected by in situ hybridization using chromosome-specific DNA libraries.

In situ hybridization with chromosome-specific DNA libraries was used to analyse radiation-induced stable translocations in human peripheral blood lymphocytes. These data were compared with radiation-induced unstable-type aberrations (dicentrics) in the same samples. The results indicate that far more stable aberrations are induced by radiation in comparison to unstable aberrations.

Female↗
PubMed1992

Chromosomal aberrations in human lymphocytes induced in vitro by very low doses of X-rays.

This paper presents results of a collaborative experiment between six laboratories which examined the yields of unstable chromosomal aberrations in human lymphocytes induced in vitro by X-rays over the dose range 0-300 mGy. The work included data points of nominal doses of 0, 3, 5, 6, 10, 20, 30, 50 and 300 mGy. Cells from 24 donors were examined and a total of about 300,000 metaphases were scored. The work was undertaken to determine the limits of sensitivity of the system taking into account variations in scoring data due to inter-donor sample and inter-laboratory effects. Despite the existence of these effects, aberration yields significantly in excess of control values were seen at doses greater than 20 mGy and these were consistent with a linear extrapolation from higher doses. Below 20 mGy the observed dicentric yields were generally lower than background, but not significantly so. Excess acentric aberrations, on the other hand, and centric rings, were higher than the controls but the increase was usually not significant. It is concluded that the statistical uncertainties are such that below 20 mGy this technique cannot distinguish between a linear or a threshold model.

Chromosome Aberrations↗
PubMed1992

Enhanced hprt mutant frequency but no significant difference in mutation spectrum between a smoking and a non-smoking human population.

Recently, we have observed a small (36%), but significant, enhancement of the frequency of 6-thioguanine (6-TG)-resistant T-lymphocytes in blood from smokers. The molecular nature of 43 hypoxanthine-guanine phosphoribosyltransferase (hprt) mutant T-lymphocyte clones from nine smoking individuals was determined to investigate whether the increase in hprt mutant frequency would lead to a changed mutation spectrum. The types and distribution of hprt mutations in smokers was compared with those found in 55 6-TGr T-lymphocyte clones from 12 members of a control group of non-smokers. From this control group 25 hprt mutants were novel, whereas 31 have been described previously. Among smokers and non-smokers, a similar proportion of base substitutions (approximately 35%), mutations causing aberrant splicing (approximately 37%), frameshifts (approximately 16%) and deletions (approximately 9%) was found. In both groups, GC----AT base pair changes were found to be predominant among transitions. However, whereas all types of transversions were about equally represented in non-smokers, GC----TA transversions were not recovered among smokers. Investigation of the distribution of base substitutions over the hprt coding region showed no differences between the two groups. These data provide no clues on the nature of DNA adducts induced by smoking, which are thought to be responsible for the increased mutation frequency at the hprt locus in T-lymphocytes from smokers.

Adult↗
PubMed1992

A Chinese hamster ovary cell mutant (EM-C11) with sensitivity to simple alkylating agents and a very high level of sister chromatid exchanges.

We have isolated a Chinese hamster ovary cell mutant hypersensitive to monofunctional alkylating agents. The mutant, designed as EM-C11, showed hypersensitivity to ethyl methanesulfonate (EMS), methyl methanesulfonate and ethylnitrosourea (8-, 7- and 2-fold, respectively, based on D10 values). About 2-fold increased sensitivity towards 4-nitroquinoline-1-oxide and only slightly increased sensitivity to X-rays (1.4-fold) and mitomycin C treatment (1.6-fold) were found in this mutant. EM-C11 was not hypersensitive to UV irradiation nor to adriamycin. The EM-C11 cells showed approximately 10-fold higher level of spontaneous sister chromatid exchange. The level of spontaneous chromosomal aberrations was 2- to 3-fold higher, but the frequency of EMS-induced chromosomal aberrations was approximately 10-fold higher in the mutant cells, in agreement with the increased sensitivity to killing. As measured by alkaline elution, EM-C11 cells showed a defect in the rejoining of single-strand DNA breaks after exposure to X-rays and even more so after the EMS treatment. Genetic analysis revealed that the EM-C11 mutant belongs to the same complementation group as the EM9 mutant described earlier. The XRCC1 gene which complements the defect in EM9 also complements the defect in EM-C11, confirming that these two independently isolated mutants are defective in the same gene.

Alkylating Agents↗
PubMed1991

Increased frequency of chromosomal damage in peripheral blood lymphocytes up to nine years following curative chemotherapy of patients with testicular carcinoma.

The presence of micronuclei (MN) in binucleated peripheral blood lymphocytes of 37 testicular carcinoma patients was studied 1.5 to 9.3 years following curative chemotherapy. All patients received cisplatinum, 35 patients also received bleomycin. In addition, most patients were treated with another cytotoxic drug, i.e., vinblastine (n = 23) and/or etoposide (n = 24). The mean time interval between cessation of chemotherapy and the micronucleus assay was 4.6 years. The median frequency of MN in binucleated cells in treated patients (0.059) was significantly higher than that in 12 untreated cancer patients (0.036; P = 0.003) or that in 26 healthy age-matched controls (0.034; P less than 0.001). Frequencies of MN in the 12 untreated cancer patients (including 7 patients with disseminated testicular carcinoma) did not differ from those in the 26 healthy controls (P = 0.890), suggesting that chromosomal damage in lymphocytes of treated testicular cancer patients must be attributed to the chemotherapy. Results indicate that lymphocytes containing chemically induced chromosomal damage persist for up to 9.3 years following cessation of chemotherapy. The implications of these findings with regard to the increased risk for secondary tumors are discussed.

Adult↗
PubMed1991

A cytogenetic follow-up study of the victims of a radiation accident in Goiania (Brazil).

A radiation accident involving a cesium-137 therapy source occurred in Goiania (Brazil) in September 1987, in which more than 50 individuals were exposed to moderate to high doses (0.2-7 Gy) of gamma-radiation. A cytogenetic technique (i.e., frequencies of dicentrics and rings in peripheral lymphocytes) was employed to estimate the absorbed radiation dose. The follow-up study extending over more than 1 year indicated a decline in the frequencies of dicentrics in the lymphocytes. Using chromosome-specific biotinylated library probes for chromosomes 1, 2, 8 and 19, we studied the frequencies of chromosomal translocations and deletions and the incidence of aneuploidy in the lymphocytes of exposed individuals. In some individuals there was a significant increase in the frequency of translocations and aneuploidy. In other experiments, in which the frequencies of HPRT mutations were determined in lymphocytes using the BrdU-labeling method, some individuals showed an increase (from about 2- to 50-fold) in mutant frequencies.

Accidents↗
PubMed1991

Cytogenetic characterization of radiosensitive mouse mutants.

In order to develop mouse models for human mutagen-sensitive syndromes, we carried out cytogenetic characterization of several mouse mutants and MS/Ae mice showing enhanced radiosensitivities. The applied cytogenetic techniques include chromosomal analysis of in vitro cell cultures and lymphocyte cultures as well as in vivo UDS in hepatocytes, induction of micronuclei in polychromatic erythrocytes and translocation induction in spermatogonial stem cells. Among the mutations studied, namely the contrasted allele of steel (Slcon), viable dominant spotting (Wc), wasted (wst), varitint-waddler (Va) and dystonia musculorum (dt) as well as MS/Ae mice, various iso-, hyper- or hypo-sensitive conditions were recorded. Only Va and dt appear to be associated with some deficiency in DNA repair.

Animals↗
PubMed1991

Radiation-induced chromosomal breakage and rejoining in interphase-metaphase chromosomes of human lymphocytes.

The premature chromosome condensation (PCC) technique and conventional chromosome analysis were applied to examine the kinetics of radiation-induced primary chromosome breaks, their rejoining and formation of dicentrics in human peripheral blood lymphocytes (PBLs). Numbers of chromosomal elements and dicentrics per cell were analyzed for each dose. Dose-dependent increases were observed for chromosome fragments (linear) and dicentrics (linear/quadratic). For an assessment of repair kinetics, numbers of breaks and dicentrics were estimated immediately and at several recovery periods after irradiation, using the PCC technique. It was found that chromosome fragments restitute with time, whereas the dicentrics are formed very quickly and their frequency remains the same, despite the decline in the number of chromosome breaks at later recovery times. Fractionation experiments were conducted to study the time-dependent interaction of primary breaks in the formation of dicentric chromosomes. PBLs were irradiated with 3 Gy X-rays split into 2 equal fractions separated by different intervals up to 5 h. No marked difference was observed in the yield of dicentrics following the different fractionation protocols, except that the mean levels of dicentrics declined when the fraction interval was 4 h or more. It appears that most of the dicentrics are formed by misrepair of strand breaks, produced directly by radiation and not resulting from rejoining of existing breaks during the slow repair process which follows. We also studied the role of the chromatin configuration at the time of irradiation on the yield of chromosome fragments and dicentrics. Highly condensed chromatin due to pretreatment with 0.3 M NaCl was found to reduce the frequency of radiation-induced chromosomal aberrations.

Animals↗
PubMed1991

Frequency of chromosomal aberrations in a subject accidentally exposed to 137Cs in the Goiania (Brazil) radiation accident: intercomparison among four laboratories.

After the Goiania radiation accident which occurred in Brazil in September 1987, an intercomparison was performed to determine whether different cytogenetic laboratories would score similar frequencies of chromosomal aberrations in cultures of lymphocytes of a highly exposed patient. For this purpose 2 chromosome slides from the subject were scored by 4 laboratories in total. The results were consistently close and confirmed the high frequency of chromosome-type aberrations observed initially in the patient.

Accidents, Occupational↗