Non-random distribution of aberrations and identification with C- and G-bindings of the position of breakage points on muntjac chromosomes induced by mitomycin C, bromodeoxyridine and hydroxylamine

Abstract

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 was 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 singe "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 leads 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 decided by the chemical's base-pair affinity and mode of action. A large variety of chemicals induce, in chromosomes, aberrations that are often distributed non-randomly. The non-random distribution of the chemically induced aberrations was noted even before the discovery of banding techniques, but with the use of conventional staining the identification of the exact location of induced break points, except for a few specific landmarks such as centromeres or secondary constrictions, was difficult and often unreliable too. Therefore, by using various banding techniques as a tool, a much more accurate assessment of the induced break points could be made. Several studies suggest that the involvement of specific chromosomes and/or chromosome segments are probably dependent upon the chemical used (Ayrand et al, 1976; Kaina, 1977; Kucerova and Polivkova, 1976; Morad et al., 1973; Morad and Zavahri, 1977; Reeves and Margoles, 1974). Meyne et. (1979) have suggested that, in addition to the nature of the chemicals used, the organization of chromosomes may also be responsible for the localized aberrations. To examine these points further, it was considered of interest to compare the extent of sensitivity of different chromosomal segments identified with banding techniques, after exposure to certain chemicals having different modes of action and base-pair affinities. In the present study, we compared the location of damage points on the chromosomes of the Indian barking deer Muntiacus muntjak, induced by 3 chemicals, namely (1) mitomycin C (MC), an antibiotic that acts by forming cross links with guanines between the complementary strands (Iyer and Szybalski, 1963), (ii) 5-bromodeoxyuridine (BUdR), a base analogue which is incorporated with concomitant thymine replacement into the DNA of mammalian cells (Djordjevic and Szybalski, 1960) and causes mutagenesis through a maispairing mechanism (Drake, 1970; Witkin and Parisi, 1974; Hutchinson and Stein, 1977; Rydberg, 1977), and (ii) hydroxylamine hydrochloride (HA), a reducing agent which reacts mainly with the cytosine moiety of the DNA by aminating only the C-4 atoms (Freese et al., 1961). © 1981.