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 type-II topoisomerase inhibitor etoposide exhibits significant mutagenicity in primary spermatocytes. Because this is the cell stage during which meiotic 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-pachytene) during which specific-locus deletion mutations can be induced resulted 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 nodules (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 has already disappeared from spermatocytes. Since etoposide blocks religation of the cut made by type II topoisomerases, repair of DNA damage may result 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 predominant action of etoposide is to decrease homologous meiotic recombination, the chemical could be expected to increase nondisjunction, an event associated with human genetic risk. Three periods in spermatogenesis respond to etoposide in different ways. Exposure of (a) late differentiating spermatogonia (and, possibly, preleptotene spermatocytes) results in cell death; (b) early- to mid-pachytene induces specific-locus deletions and crossover reduction; and, (c) late pachytene-through-diakinesis leads to genetically unbalanced conceptuses as a result of clastogenic damage.
Russell
L B
LB
Life Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA. russelllb@bioax1.bio.ornl.gov
Hunsicker
P R
PR
Hack
A M
AM
Ashley
T
T
D000818:Animals; D018797:Cell Cycle Proteins; D018592:Cross-Over Studies; D003433:Crosses, Genetic; D004268:DNA-Binding Proteins / Q000378:metabolism; D005047:Etoposide / Q000494:pharmacology*; D005260:Female; D008297:Male; D008540:Meiosis / Q000187:drug effects*; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D009687:Nuclear Proteins / Q000378:metabolism; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology*; D051135:Rad51 Recombinase; D011995:Recombination, Genetic / Q000187:drug effects*; D051716:Replication Protein A; D013090:Spermatocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D013091:Spermatogenesis / Q000187:drug effects; D013573:Synaptonemal Complex / Q000187:drug effects; D013737:Testis / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D059005:Topoisomerase II Inhibitors
D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D019384:Nucleic Acid Synthesis Inhibitors; D051716:Replication Protein A; C497168:Rpa1 protein, mouse; C490060:SYCP3 protein, human; C490062:Sycp3 protein, mouse; D059005:Topoisomerase II Inhibitors; D005047:Etoposide; D051135:Rad51 Recombinase; C495154:Rad51 protein, mouse