Restriction endonucleases and ionizing radiations have been extensively 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
endonucleases targeted to specific genome sequences or by ionizing radiations could provide additional evidence to clarify this point. Results obtained from the localization of breakpoints induced by AluI, BamHI or
DNase I as well as by neutrons or gamma-rays in G-banded Chinese hamster ovary (CHO) chromosomes are presented. AluI and BamHI were electroporated into CHO cells either during the G1 or S-phase of the cell cycle. A co-localization of breakpoints was found with a preferential occurrence in G-light bands independent of the cell cycle stage in which aberration production took place. Since AluI and BamHI recognition sequences are partitioned in the housekeeping and tissue-specific subgenomes respectively, we postulated that nuclease sensitive sites in active
chromatin 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 these
endonucleases induced G-banding suggesting a higher sensitivity of G-light
chromatin. For comparison purposes, CHO cells were irradiated with neutrons or gamma-rays and breakpoints localized in G-banded
chromosome aberrations. A higher occurrence of breakpoints in G-light bands was also observed. We detected seven breakage-prone G-light bands that were preferentially damaged by the three
endonucleases and by both types of radiation. These results emphasize the possible implication of the
chromatin structure and the nuclear architecture in the localization of chromosome breakpoints induced by
endonucleases, neutrons and gamma-rays.