Tbe benzodiazepines are a class of d.rugs that are widely used in the treatment of various psychiatric disorders. One member of um ~' oxazepam, is also a common metabolite of sevmd other benzod.iazepines. Since the evidence for the genetic toxicity and carcinogenic properties of these compounds is incol:lsb1ent, we investigated the oxazepam-induced fonnation of micronuclei in Syrian Hamster embryo fibroblast (SHE) cells, human amniotic fluid fibroblast-like (AFFL) cells and LS178Y mouse cells. A dose-dependent increase in micronucleus fractions was found in all tbree ceU llnes. The time course of micronucleus induction in L5178Y cells showed a maximum at 5 h after treatment, suggesting that the micronuclei were fonned in the first mitosis after treatment.Kinetochore staining (CREST -antiserum) revealed the presence of kinetochores in -SO% of the micronuclei in aU tbree ceU types. ThJs resu1t was further confinned by in situ bybridization in LS178Y cells and indicates tbe presence of wbole Chromosomes or centric fragments as weU as acentric fragments in the oxazepam-induced micronuclei. The LS178Y cells did not show a mutagenic response to oxazepam at any of the doses or expression times used.

Analysis of 122 spontaneous large and small colony mutants derived from L5178Y tk +l~ mouse lymphoma cells at 28 heteromorphic microsatellite loci on chromosome 11 showed that extensive loss of heterozygosity (LOH) is common in both large colony and small colony mutants, eliminating most chromosome 11 loci as candidates for a putative growth control locus. These results, in conjunction with historical cytogenetic data, suggest that a putative growth control locus lies distal to the thymidine kinase (Tkl) gene, near the telomere. Thirty seven mutants were hybridized with a chromosome 11-specific whole chromosome painting probe for analysis of rearrangements. Generally, painting confirmed earlier observations that large colony mutants are karyotypically normal, whereas small colony mutants frequently have detectable rearrangements. A point probe distal to Tkl revealed no evidence of chromosome breakage in small colony mutants that appeared normal on whole 11 painting and had no LOH. Therefore, the molecular difference between large and small colony mutants remains unknown. Models to explain large and small colony mutants consistent with our findings are presented, including loss of a putative growth control gene, differential mechanisms of chromosome breakage/ repair and second site mutations as explanations for small colony mutants. Painting revealed translocations and aneuploidy and showed that non-disjunction was not a common explanation for complete LOH. The most common finding was that large regions of LOH do not result from deletions, demonstrating that these cells can detect recombination events as well as previously observed chromosomal rearrangements, deletions and point mutations.

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