LOCALIZATION OF CHROMOSOME BREAKPOINTS - IMPLICATION OF THE CHROMATINSTRUCTURE AND NUCLEAR ARCHITECTURE

Restriction endonucleases and ionizing radiations have been extensivel y used to study the origin of chromosomal aberrations. Although a non- random distribution of chromosome breakpoints induced by these agents has been claimed by several authors, the significance of the chromatin structure and nuclear architecture in the localization of breakpoints is still not well understood. Breakpoint patterns produced by endonuc leases targeted to specific genome sequences or by ionizing radiations could provide additional evidence to clarify this point. Results obta ined from the localization of breakpoints induced by AluI, BamHI or DN ase I as well as by neutrons or gamma-rays in G-banded Chinese hamster ovary (CHO) chromosomes are presented, AluI and BamHI were electropor ated into CHO cells either during the G1 or S-phase of the cell cycle. A co-localization of breakpoints was found with a preferential occurr ence in G-light bands independent of the cell cycle stage in which abe rration production took place. Since AluI and BamHI recognition sequen ces are partitioned in the housekeeping and tissue-specific subgenomes respectively, we postulated that nuclease sensitive sites in active c hromatin could be the main targets for the induction of breakpoints by these endonucleases. This assumption is supported by the finding that DNase I-induced breakpoint patterns in CHO cells are similar to those produced by AluI and BamHI. Digestion of fixed CHO chromosomes with t hese endonucleases induced G-banding suggesting a higher sensitivity o f G-light chromatin. For comparison purposes, CHO cells were irradiate d with neutrons or gamma-rays and breakpoints localized in G-banded ch romosome aberrations. A higher occurrence of breakpoints in G-light ba nds was also observed. We detected seven breakage-prone G-light bands that were preferentially damaged by the three endonucleases and by bot h types of radiation. These results emphasize the possible implication of the chromatin structure and the nuclear architecture in the locali zation of chromosome breakpoints induced by endonucleases, neutrons an d gamma-rays. (C) 1998 Elsevier Science B.V. All rights reserved.