Biomedical subjects

B D Loucas

Publications and source records attributed to B D Loucas.

5 recordsLinked to original sources

PubMed2004

Complex chromatid-isochromatid exchanges following irradiation with heavy ions?

We describe a peculiar and relatively rare type of chromosomal rearrangement induced in human peripheral lymphocytes that were ostensibly irradiated in G(0) phase of the cell cycle by accelerated heavy ions, and which, to the best of our knowledge, have not been previously described. The novel rearrangements which were detected using mFISH following exposure to 500 MeV/nucleon and 5 GeV/n 56Fe particles, but were not induced by either 137Cs gamma rays or 238Pu alpha particles, can alternatively be described as either complex chromatid-isochromatid or complex chromatid-chromosome exchanges. Different mechanisms potentially responsible for their formation are discussed.

Alpha Particles↗

PubMed2001

Complex chromosome exchanges induced by gamma rays in human lymphocytes: an mFISH study.

Loucas, B. D. and Cornforth, M. N. Complex Chromosome Exchanges Induced by Gamma Rays in Human Lymphocytes: An mFISH Study. Radiat. Res. 155, 660-671 (2001). Combinatorial multi-fluor fluorescence in situ hybridization (mFISH) allows the simultaneous painting of each pair of homologous chromosomes, thereby eliminating many of the difficulties previously associated with the analysis of complex rearrangements. We employed mFISH to visualize exchanges in human lymphocytes and found significant frequencies of these aberrations after gamma-ray doses of 2 and 4 Gy. At 4 Gy, roughly half of the cells contained at least one complex exchange that required anywhere from 3 to 11 initial chromosome breaks. At this dose, more than 40% of gross cytogenetic damage, as measured by the total number of exchange breakpoints, was complex in origin. Both simple and complex exchanges were found to have nonlinear dose responses, although the latter showed significantly more upward curvature. In many cases, it could be deduced that the initial breaks leading to a particular complex exchange were proximate, meaning that the resulting broken chromosome ends all must have been capable of interacting freely during the exchange process. For other complex exchanges, the rearrangement could just as well have resulted from two or more simpler exchanges that occurred sequentially. The results demonstrate the utility of mFISH in visualizing intricacies of the exchange process, but also highlight the various sources of ambiguity concerning cytogenetic analysis that remain despite the power of this approach.

Adult↗

PubMed1998

Postirradiation growth in HAT medium fails to eliminate the delayed appearance of 6-thioguanine-resistant clones in EJ30 human epithelial cells.

The latent effects of radiation-induced damage include "delayed" mutations that arise de novo in the progeny of nonmutant cells. We investigated the early stages of delayed mutagenesis at the HPRT locus of EJ30 human epithelial cells that were exposed to 4 Gy of 137Cs gamma rays. To eliminate directly induced "prompt" HPRT- mutants, cultures were grown in HAT medium before selection in 6-thioguanine was applied. Although irradiated cells were grown in HAT medium throughout the phenotypic expression period, mutant fractions some tenfold above spontaneous levels were observed subsequently; incubation in HAT medium did not cause an increase in mutations in unirradiated cells. We conclude that, in our experimental system, a significant proportion of induced mutation is of a delayed type. We speculate that the delayed induction is caused by an instability process that is a frequent and (typically) transient consequence of exposure of cells to ionizing radiation. The connection, if any, between this process and other manifestations of instability, including the acquisition of a "mutator phenotype," remains to be established.

Cesium Radioisotopes↗

PubMed1994

Initial damage in human interphase chromosomes from alpha particles with linear energy transfers relevant to radon exposure.

To determine the efficiency at which alpha particles at LETs chosen to simulate exposure to radon progeny break chromosomes, the premature chromosome condensation technique was used to measure breaks soon after irradiation. Noncycling human fibroblasts were irradiated with graded doses of monoenergetic alpha particles accelerated to produce LETs of 90, 120, 150, 180 and 200 keV/microns at the midpoint of the cell nuclei. Premature chromosome condensation was initiated immediately after irradiation and cells were scored for the total number of prematurely condensed chromosomes and fragments per cell. Similar experiments were conducted with 250 kVp X rays for comparison. Irradiation with alpha particles produced 8.6 to 13.1 excess fragments per gray, while X rays produced 5.8 excess fragments, resulting in RBEs around 2. Calculations of the number of breaks produced on average by a single particle traversal of a cell nucleus indicated that at the LETs tested more than one break (1.5-2.8) was produced by each traversal, the maximum being that produced by 180 keV/microns alpha particles. When chromosome aberrations are scored at metaphase after high-LET irradiation, RBEs considerably greater than those recorded here (approximately 2) have been reported. These results showing relatively small differences in initial break levels for alpha particles in the LET range of the radon progeny relative to X rays indicate that the greater aberration frequencies are not due principally to an increase in breakage efficiency, but interactions between breaks along the same particle track are important.

Alpha Particles↗

PubMed1994

Kinetics of chromosome rejoining in normal human fibroblasts after exposure to low- and high-LET radiations.

To determine whether chromosome breaks produced by alpha particles are processed differently from those produced by X rays, the premature chromosome condensation technique was used to follow chromosome rejoining after irradiation. Doses of 90 and 200 keV/microns alpha particles (2.7 Gy) and 250 kVp X rays (6 Gy) were chosen to produce approximately the same number of initial chromosome breaks (about 30 excess fragments per cell). Frequencies of excess fragments were assessed at eight times to 24 h after irradiation with the final yields being about 2, 4 and 8 excess fragments per cell for 250 kVp X rays and 200 and 90 keV/microns alpha particles, respectively. For each radiation the time for the initial measured fragment frequency per cell to be halved (i.e. to about 15) was the same (about 100 min). The results were fitted to three models of kinetics of the rejoining, and the initial and residual number of excess chromosome fragments as well as the rate of rejoining were determined. Even with eight times, discrimination between the models of the kinetics was not possible, such that a single-component first-order reaction could not be rejected for either X-ray- or alpha-particle-induced breaks. Although rejoining proceeds at similar rates, the probability of "correct" rejoining is apparently reduced for alpha-particle-irradiated cells.

Chromosomes↗