Elevated frequencies of
chromosomal aberrations have been observed in the lymphocytes of
benzene-exposed workers. Similar changes occurring in the bone marrow may play an important role in the development of
leukemia. The objective of this research has been to characterize chromosomal alterations induced by
benzene in mice and humans and to investigate the potential role of inhibition of
topoisomerase II in the myelotoxic effects of
benzene. The research is presented in three sections corresponding to the specific aims of the project: genotoxicity studies in the mouse,
topoisomerase II studies, and initial studies using a new fluorescence in situ hybridization (FISH) approach to detect chromosome alterations in
benzene-exposed workers. The results of the mouse experiments indicate that both
chromosome breakage and
aneuploidy are induced in the bone marrow of B6C3F1 mice following
benzene administration.
Chromosome breakage is the predominant effect, and this occurs primarily in the mouse
euchromatin. Significant breakage within the mouse
heterochromatin was also observed, as was
aneuploidy. Breakage in the mouse bone marrow erythrocytes increased as a function of both dose and duration of
benzene administration. The
aneuploidy resulting from
benzene exposure in mice was a relatively infrequent event, with increases of both chromosome loss and hyperdiploidy being observed. In the topoisomerase studies,
benzene or its metabolites were shown to inhibit
topoisomerase II enzyme activity in an isolated
enzyme system, in a human bone marrow-derived
leukemia cell line, and in vivo in the bone marrow of treated mice. The decreased activity was probably due to the rapid degradation of the
topoisomerase II protein in the treated cells. In the human biomonitoring studies, the feasibility of using FISH with tandem
DNA probes to detect chromosome alterations in interphase granulocytes and lymphocytes of
benzene-exposed workers was demonstrated. The results from the two worker studies were somewhat inconsistent, however. In the study of Estonian workers, characterized by lower exposures and a smaller sample size, the
benzene-exposed workers exhibited elevated frequencies of breakage in the lq12 region as compared with those seen in controls. A suggestive trend toward increased hyperdiploidy was also seen, although the frequencies in the exposed workers were low and within the range of our laboratory's historical control frequencies. In the larger study of more highly exposed Chinese workers, no increase in breakage affecting the 1q12 region was seen among the exposed workers. A trend toward increased hyperdiploidy of chromosome 1 was seen in the exposed workers when the concentration of urinary
benzene metabolites was used in conjunction with the frequency of hyperdiploidy observed in the lymphocytes of the individual workers. The results of these studies indicate that
benzene exposure is characterized by
chromosome breakage, primarily within the
euchromatin, and modest increases in
aneuploidy. These findings also provide the first direct evidence that
benzene is capable of inhibiting the enzymatic activity of
topoisomerase II in vivo, providing additional support for the hypothesis that inhibition of
topoisomerase II contributes to
benzene-induced toxicity and leukemogenesis.