Assessing fetal topoisomerase IIα as a mechanism of in utero benzene toxicity in generating double-stranded DNA damage in CD-1 mice

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Holmes, Trent
Benzene, Fetal, Topoisomerase IIα, DNA repair
Benzene is a known human leukemogen and environmental pollutant. Recent epidemiological evidence has shown that benzene exposure during pregnancy is associated with increased infant leukemias after birth. It is hypothesized that these leukemias are being initiated in utero by benzene exposure during pregnancy. The metabolite 1,4-benzoquinone (BQ) specifically inhibits the critical DNA repair enzyme topoisomerase IIα (topo IIα) and is hypothesized to initiate leukemias through the generation of double stranded DNA (dsDNA) breaks in hematopoietic cells. My research examines the role of topo IIα as a target of benzene toxicity to generate dsDNA breaks using complementary in vitro and in vivo mouse models. We hypothesize that benzene exposure in utero will increase dsDNA breaks by perturbing topo IIα, and cause leukemia. First, using a primary cell culture derived from gestational day (GD) 14 fetal livers, exposure to BQ for 24 hours showed a concentration-dependent decrease in topo IIα activity, which was not protected by treatment with N-acetylcysteine. BQ exposure (12.5 µM) is sufficient to increase topo IIα-DNA adducts, alter the population of c-kit+Lin—Sca-1—Il7rα— cells and increases 4-hydroxynonenal (4-HNE). Levels of γH2AX, a marker of dsDNA breaks are also increased in fetal liver cell culture following 12.5 µM BQ exposure. Next, using an in vivo CD-1 mouse model of transplacental benzene carcinogenesis which previously showed that 200 mg/kg IP of benzene increases leukemia in offspring in a sex-dependent manner, topo IIα activity, DNA damage, and DNA repair gene expression were assessed and stratified by sex. Topo IIα activity was not altered in GD14 fetal livers following maternal benzene exposure, but DNA damage was increased differentially by sex. In utero benzene exposure alters fetal DNA repair gene expression for genes involved in non-homologous end joining (NHEJ) and homologous recombination (HR). Overall, this research provides evidence that benzene can induce damage in the developing fetus and supports the hypothesis that childhood leukemias is caused by maternal exposure to benzene.
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