Effect of the topoisomerase-II inhibitor etoposide on meiotic recombination in male mice

Unlike other chemicals that have been tested in mammalian germ cells, the t ype-II topoisomerase inhibitor etoposide exhibits significant mutagenicity in primary spermatocytes. Because this is the cell stage during which meiot ic recombination normally occurs, and because topoisomerases play a role in recombination, we studied the effect of etoposide on crossing-over in male mice. Exposure to those meiotic prophase stages (probably early to mid-pac hytene) during which specific-locus deletion mutations can be induced resul ted in decreased crossing-over in the p-Tyr(c) interval of mouse chromosome 7. Accompanying cytological studies with fluorescent antibodies indicated that while there was no detectable effect on the number of recombination no dules (MLH1 foci), there were marked changes in the stage of appearance and localization of RAD51 and RPA proteins. These temporal and spatial protein patterns suggest the formation of multiple lesions in the DNA after MLH1 h as already disappeared from spermatocytes. Since etoposide blocks religatio n of the cut made by type II topoisomerases, repair of DNA damage may resul t in rejoining of the original DNA strands, undoing the reciprocal exchange that had already occurred and resulting in reduced crossing-over despite a normal frequency of MLH1 foci. Crossing-over could conceivably be affected differentially in different chromosomal regions. If, however, the predomin ant action of etoposide is to decrease homologous meiotic recombination, th e chemical could be expected to increase nondisjunction, an event associate d with human genetic risk; Three periods in spermatogenesis respond to etop oside in different ways. Exposure of (a) late differentiating spermatogonia (and, possibly, preleptotene spermatocytes) results in cell death; (b) ear ly- to mid-pachytene induces specific-locus deletions and crossover reducti on; and, (c) late pachytene-through-diakinesis leads to genetically unbalan ced conceptuses as a result of clastogenic damage. (C) 2000 Elsevier Scienc e B.V. All rights reserved.