Biomedical subjects

M N Cornforth

Publications and source records attributed to M N Cornforth.

28 records · Page 2Linked to original sources

PubMed1989

On the nature of interactions leading to radiation-induced chromosomal exchange.

Within the conceptual framework of so-called lesion-interaction models, chromosomal interchanges are believed to result from radiation damage to both chromosomes involved. More recently, models of radiation action have been proposed which suggest such exchanges arise from initial damage to only one chromosome, which then associates with an undamaged chromosome. The specific case of 'lesion-nonlesion' chromosomal interaction via telomere-break rejoining was examined through the use of a telomere-specific DNA probe. No evidence was found to support dicentric formation by this mechanism in normal human fibroblasts. To test the more general case (i.e. lesion-nonlesion interaction by some other mechanism) mitotic HeLa cells were fused together to determine whether exchanges would occur between the chromosomes of previously separate genomes, as seen in resulting cell syncytia at the next mitosis. The fusion of irradiated cells (with each other) produced a high frequency of such intergenomic exchanges. However, the frequency of these events was reduced 50-100-fold in syncytia resulting from the fusion of irradiated with unirradiated cells. These results strongly support the view that most radiation-induced exchange aberrations require initial damage to chromatin at both locations involved in the exchange--i.e. they are fundamentally two-hit in nature.

Cells↗

PubMed1989

Telomere staining of human chromosomes and the mechanism of radiation-induced dicentric formation.

The majority of models of radiation action developed over the past half century hold that the curvilinear dose responses exhibited by eukaryotic cells to sparsely ionizing radiations result from the interaction of pairs of lesions produced in sensitive targets of the cell. Within this conceptual framework, chromosomal exchange aberrations (e.g., interchanges) are believed to occur through the interaction of damaged sites on both chromosomes participating in the exchange. In contrast, the model proposed by Chadwick and Leenhouts (as well as some other models) suggests that such exchanges arise from initial radiation damage to only one chromosome, which then becomes associated with an undamaged chromosome. A particular aspect of this theory is that asymmetrical exchanges, such as dicentrics, may be formed from the rejoining of a broken end of one chromosome to the telomere of another. By using a DNA probe that specifically hybridizes to the telomeric region of human chromosomes, we were able to test this assertion directly. After scanning more than 200 dicentrics produced in normal human fibroblasts by 6 Gy of 60Co gamma rays, virtually none were found that contained telomeres located between the centromeres of this aberration type. Therefore, since the proposed telomere-break rejoining process, per se, is not necessarily a central element of the Chadwick-Leenhouts model, we suggest the theory be modified to exclude this mechanism.

Chromosome Aberrations↗

PubMed1989

Radiobiology of ultrasoft X rays. III. Normal human fibroblasts and the significance of terminal track structure in cell inactivation.

Ultrasoft characteristic X rays from carbon (0.28 keV) are severely attenuated as they pass through biological material, causing a nonuniform distribution of dose to cell nuclei. Complications of studying ultrasoft X rays can be minimized in this context by using cells with very thin cytoplasm and nuclei (e.g., less than the attenuation length of the X rays), and which exhibit a more nearly exponential dose response to cell killing, such as normal human fibroblasts compared with V79 cells. Using this cell system, we report the relative biological effectiveness (RBE) of A1-K and C-K X rays to be near unity. Previous studies of cell inactivation by characteristic carbon X rays gave RBEs of 3 to 4, supporting the idea that localized energy depositions from secondary electrons and primary track ends represent the principal mode of biological action for other low-LET radiations. In part, the reported high RBEs result from the use of mean dose to describe energy deposited within the cell nuclei by these poorly penetrating radiations. Implicit in the use of mean dose is that cellular damage varies linearly with dose within a critical target(s), an assumption that is of questionable validity for cells that exhibit pronounced curvilinear dose responses. The simplest interpretation of the present findings is that most energy depositions caused by track-end effects are not necessarily more damaging than the sparsely ionizing component.

Cell Survival↗

PubMed1989

Radiobiology of ultrasoft X rays. IV. Flat and round-shaped hamster cells (CHO-10B, HS-23).

The results reported earlier in this series indicated that the relative biological effectiveness (RBE) of ultrasoft X rays decreases with decreasing cell thickness, approaching unity for the thinnest cells used, plateau-phase human skin fibroblasts (HSF). The possible dependence of RBE on the configuration of the cell nucleus is investigated further in this paper using two CHO cell lines that attach well and have similar intrinsic radiosensitivities to 60Co gamma rays. One of the lines forms monolayers similar to V79 cells, while the other remains more spherical during growth. We find an increasing RBE with decreasing X-ray energy for both of these cell lines, consistent with our results using V79 cells. Also consistent with our results obtained with 10T1/2 and HSF cells, we find an increasing RBE with increasing cell thickness. The possible dependence of RBE on radiosensitivity and the use of the concept of mean dose for ultrasoft X rays is discussed.

Animals↗

PubMed1987

A quantitative comparison of potentially lethal damage repair and the rejoining of interphase chromosome breaks in low passage normal human fibroblasts.

After long postirradiation incubation periods, the residual frequency of prematurely condensed chromosome fragments following X-ray exposure of noncycling diploid human fibroblasts was found to be correlated with the frequency of chromosome aberrations observed under identical treatment conditions when the cells were subcultured and scored after they reached mitosis. Over a wide range of doses, the proportion of such cells without aberrations at their first metaphase was not significantly different from the proportion able to form macroscopic colonies. Further, the rate of rejoining of interphase chromosome breaks was the same as the rate of increase in survival due to the repair of potentially lethal damage (PLD). These results suggest that there is a one-to-one correspondence between the initial breakage and rejoining of G0 chromosomes and the induction and repair of PLD measured by delayed plating from plateau-phase cultures of these cells.

Ataxia Telangiectasia↗

PubMed1987

Relationship between the recovery from sublethal X-ray damage and the rejoining of chromosome breaks in normal human fibroblasts.

Using plateau-phase cultures of AG1522 normal human fibroblasts, we examined relationships between the breakage and rejoining of chromosomes and the induction and repair of sublethal damage (SLD) following fractionated doses of X rays. The rate constant for the rejoining of breaks in prematurely condensed interphase chromosomes, measured previously, accurately predicts both the rate of change in survival due to potentially lethal damage (PLD) repair and the rate of change in survival for dose fractionation due to SLD repair. Further, changes in the frequency of chromosome-type deletions and asymmetrical exchange aberrations measured in the first postirradiation mitosis corresponded closely with changes in cell killing when doses were fractionated, and a dose-fractionation- or dose-rate-independent alpha component of damage was similar for aberration and cell killing end points. These results substantiate the hypothesis that sublethal damage repair results from the rejoining of breaks in interphase chromatin produced by a first dose so they no longer are capable of interacting with those produced by a second dose. The fact that the repair of potentially lethal damage is also readily explained on the basis of chromosome break rejoining (M. N. Cornforth and J. S. Bedford, Radiat. Res. 111, 385-405 (1987)) strongly suggests that PLD and SLD repair are different manifestations of the same basic process operating on the same basic lesions.

Cell Survival↗

PubMed1985

On the nature of a defect in cells from individuals with ataxia-telangiectasia.

The cells and tissues of patients with ataxia-telangiectasia (A-T), an inherited disease characterized by a high degree of proneness to cancer, are abnormally sensitive to ionizing radiation. Noncycling cultures of normal human and A-T fibroblasts were exposed to x-rays so that the breakage and rejoining of prematurely condensed chromosomes in the G1 phase could be compared. After a dose of 6.0 grays, both cell types had the same initial frequency of breaks and the same rate for rejoining of the breaks, but the fraction of breaks that did not rejoin was five to six times greater for the A-T cells. The results also show that progression of cells into the S phase is not a prerequisite for the increased frequency of chromosome fragments that appear in mitosis after A-T cells are irradiated in the G1 or G0 phase.

Ataxia Telangiectasia↗

PubMed1984

Chromosome studies in human subjects chronically exposed to arsenic in drinking water.

A two-year study was carried out on human subjects of various ages and backgrounds who had been drinking water containing more than 0.05 mg/liter (0.05 ppm) arsenic for a period of at least five years. The main aim was to correlate the frequency of chromosome aberrations and sister chromatid exchanges in the lymphocytes with the amount of arsenic in the water. In addition, we explored the incidence of skin cancer, fetal wastage, and genetic or developmental abnormalities. Several other variables--eg, coffee, wine, and cigarette consumption; sex; residence (rural vs urban); and exposure to chemicals, smelters, or pesticides--were also taken into consideration. The data on chromosome aberrations (104 exposed and 86 control individuals) and on sister chromatid exchanges (98 exposed and 83 control individuals) did not show that arsenic at concentrations used by our population (greater than 0.05 mg/liter) has any effect on these parameters. Similarly, no other health effects of arsenic at these concentrations were found.

Arsenic↗

PubMed1983

X-ray--induced breakage and rejoining of human interphase chromosomes.

A method was developed for the high-resolution measurement of breaks in prematurely condensed chromosomes at the G1 phase of the cell cycle. The dose response for fragments (breaks) produced immediately after x-irradiation of confluent cultures of normal human cells was linear down to 10.9 rad (0.109 Gy) and extrapolated to zero effect at zero dose. The curve had a slope of 0.063 breaks per cell per rad, which is at least an order of magnitude greater than that for breaks scored in the same cells after they have progressed to mitosis following subculture. When incubated at 37 degrees C half of the breaks disappeared in 2 hours. A slower, perhaps nonrejoining component was apparent at later incubation times. The initial rate of break rejoining was similar to the rate of increase in survival after incubation because of the repair of potentially lethal damage and is also in close agreement with recently reported values for the rejoining of double-strand breakage in DNA.

Chromosome Aberrations↗

PubMed1983

High-resolution measurement of breaks in prematurely condensed chromosomes by differential staining.

A relatively simple method has been developed to improve the resolution for measuring breaks produced in interphase chromosomes by X rays or other agents following the induction of premature chromosome condensation (PCC). Mitotic HeLa cells, which induce PCC when fused with interphase cells, were obtained from cultures grown for several generations in 5-bromodeoxyuridine (BrdU). These were fused to cells from low-passage confluent cultures of normal human fibroblasts and subsequently stained by a modified fluorescence-plus-Giemsa (FPG) technique. Following this protocol the prematurely condensed chromosomes stain intensely, whereas the mitotic chromosomes of the inducer cell(s), which are intermingled with them, stain very lightly. With this technique the interphase chromosomes and their fragments can be identified unequivocally, making scoring much easier and more accurate. The frequency of breaks produced in G1 phase AG1522 human fibroblasts immediately following X-ray doses of 58 and 117 rad was 3.68 and 7.38 per cell, respectively. Use of this technique should allow the detection of damage from ionizing radiation at doses lower than 10 rad.

Azure Stains↗