Nature and role of high sister chromatid exchanges in Bloom syndrome cells. Some cytogenetic and immunological aspects.

The phenomenon of sister chromatid exchange (SCE) is an interesting genetic event in metaphase chromosomes, even though its exact mechanism remains unknown. The fact that SCE can take place, whether "spontaneously" or induced by various agents, is in itself important, for such an event, involving damage and possible repair of bilateral loci in chromosomes, presents opportunities for modification of the chromosomal structure and/or function (e.g., oncogene activation). It has been assumed that under "normal" circumstances, SCE does not lead to any change in the functional genome, although this may not apply to abnormal conditions. The latter may be produced by a number of chemical agents (including various carcinogens) that lead to a significantly increased incidence of SCE in normal and malignant cells either in vitro or in vivo. In fact, SCE has been recognized and advocated as a most sensitive test for potentially mutagenic and/or carcinogenic agents. Thus, the broad field of SCE studies becomes of direct interest for and subject to exploration by those involved with cancer causation and biology. In this review, a synopsis of our experiences with high SCE mechanisms in Bloom syndrome cell lines will be presented, in connection with high SCE mutant cell line derived from ataxia telangiectasia (AT) and/or malignant transformation.