Heterochromatin and chromosomal aberration in microtus agrestis: role of chromosomal association

Recent developments in the assessment of chromosomal damage

Ionizing radiation and restriction endonucleases are very efficient in inducing chromosomal aberrations (CAs). These aberrations are mainly consequences of misrepair of DNA double-strand breaks (DSBs). The fast repairing component of DSBs induced by ionizing radiation seems to be responsible for exchange aberration. Use of premature chromosome condensation technique in combination with DNA repair inhibitors such as ara A has given valuable information on the assessment of the frequencies of initial chromosome breaks and the kinetics of their repair following low LET radiation. The recently developed 'chromosome painting' technique using chromosome-specific libraries has also increased considerably the resolution of identifying and scoring of CAs. After low LET radiation, stable chromosome exchanges (translocations) are induced more frequently than unstable chromosome exchanges (dicentrics). Fluorescence in situ hybridization employing telomeric probe has made it possible to score efficiently exchange aberrations involving the acrocentric chromosomes of mouse. Chinese hamster cells have several intercalary telomeric sequences present in most of the chromosomes. These telomeric blocks have been found to be associated with chromosomal aberrations induced by restriction endonucleases and short wave UV and evidence has been obtained for apparent amplification of telomeric sequences at the break points.

A comparative analysis of data on the clastogenicity of 951 chemical substances tested in mammalian cell cultures

A literature review was conducted using original papers published during 1964-1985 on the in vitro clastogenicity of chemical substances. Results of tests on 951 chemical substances were abstracted from over 240 reports to form the database. The evaluation of these data relied on each author's original conclusion on a positive or negative outcome. Of these 951 substances, 447 (47%) were consistently positive either with or without activation; 417 (44%) were negative in the direct test but not tested with metabolic activation systems; 4 were negative but tested only with activation; and 30 (3%) were clearly negative both with and without activation. The remaining 53 substances gave variable results when tested under different experimental protocols or in different cell types, but were positive in at least one test. Although discrepant results were found associated with some cell types, the addition of metabolic activation systems tended to eliminate such variability. No one cell appeared to be superior in response to all clastogens. For screening purposes, the choice of cell may thus depend more on the general usefulness and reliability of a cell type than on a strong response to a particular chemical. However, the use of a suitable metabolic activation system does appear to be of critical importance. The concentration at which clastogenic effects were detected varied extensively for different test substances, ranging from a minimum of 4.3 X 10(-8) to 6.9 X 10(2) mM. Possible mechanisms of action for substances active at only high levels are discussed, but no satisfactory explanation is available at this time. The relevance of tests conducted at concentrations high enough to alter significantly the osmolarity and other culture conditions is considered, and caution urged in the interpretation of test results obtained under physiologically stressful conditions. The clastogenic potential was compared quantitatively using an index of effective concentration (D20) and one which estimates the number of cells with exchange aberrations expected per mg/ml (TR) for data obtained by using a uniform protocol and cultures of Chinese hamster lung (CHL) cells. Both values were distributed over a wide range, demonstrating the variety of genotoxic potential in chemicals. In general, a substance which was active at only high concentrations produced fewer exchange-type aberrations. In vivo activity, as measured by tumourigenic effect and formation of micronuclei in bone marrow, tended to be greater for substances with a D20 below 10(-2) mg/ml and a TR value over 10(3).(ABSTRACT TRUNCATED AT 400 WORDS)

Non-random distribution of aberrations and identification with C- and G-bandings of the position of breakage points on Muntjac chromosomes induced by mitomycin c, bromodeoxyuridine and hydroxylamine

The analysis of chromosomes from muntjac after treatment of its lymphocyte cultures with 3 chemical mutagens having different base-pair affinities and modes of action, namely mitomycin C (MC), 5-bromodeoxyuridine (BUdR) and hydroxylamine hydrochloride (HA), with G- and C-band staining displayed non-random distribution of chemically specific damage points on them. The randomness of the involvement of each site on the chromosomes were examined by assuming an expected value calculated on the basis of its relative mitotic length. The observation revealed that a large fraction of MC-induced aberrations was preferentially located in the C-band positive constitutive heterochromatin, especially in the long "neck-like" centromeric region of the X-chromosome. On the chromosomal arms, the light G-bands were involved in aberrations either in proportion to or higher than that expected. When the cells were treated with BUdR, the dark G-bands on all the chromosomes of the complement were the preferred sites, displaying statistically significant higher numbers of aberrations. A single "hot-spot" for induced damage on 1 mid-q was also recorded. HA induced a very high frequency of damage in the secondary constriction regions of the chromosome pairs 1, X and Y2, and the frequency was slightly lower than this in the centromeres of 1, 2 and X chromosomes. The observation of specific distribution of damage points induced by the 3 chemicals lead to the suggestion that, though the effect of a chemical on chromosome segments depends on several factors, each being partially responsible for the end result, it is perhaps primarily depended by the chemical's base-pair affinity and mode of action.

The action of N-methyl-N-nitrosourea on non-established human cell lines in vitro. II. Non-random distribution of chromatid aberrations in diploid and Down's cells

Chromatid gaps, breaks and aberrations involved in interchanges induced by N-methyl-N-nitrosourea (MNU) were found non-randomly distributed on individual chromosomes and chromosome segments (G bands) both in human diploid fibroblasts with trisomy 21 cultured in vitro. Aberration events were located exclusively in pale G bands. Considering cells in the first post-treatment mitosis, the pattern of aberration distribution, as revealed by the position of hot spots, varied with recovery time and was different in diploid and Down's cells. In comparison with diploid cells, the X chromosomes of Down's cells were not involved in aberrations. Despite the higher aberration frequencies of Down's cells, the number of hot spots and the proportion of aberrations located in hot spots were not increased in this cell type. Therefore, the increased chromosomal sensitivity to MNU of Down's cells does not reflect an increased sensitivity of special chromosomes or chromosome sites.

Giemsa banding and heterochromatin distribution in Ornithogalum

In Ornithogalum virens, following Giemsa staining, the mitotic chromosomes revealed distinct C-banding pattern. Interphase nucleus also showed 12 chromocentres corresponding to the number of C-bands. Based on the measurements of C-bands, about 13.5% of the genome is heterochromatic.