Date of Award

Winter 12-2016

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical Sciences

Advisor

John Pierce Wise, Sr.

Second Committee Member

Kevin Mills

Third Committee Member

J. Christopher States

Additional Committee Members

W. Douglas Thompson

Hong Xie

Abstract

Homologous recombination (HR) repair protects against genomic instability by maintaining high genomic fidelity during DNA double strand break repair. RAD51 nucleofilament formation is the defining step of HR repair, facilitating the search for a homologous sequence. Particulate hexavalent chromium (Cr(VI)), a human lung carcinogen, induces DNA double strand breaks and chromosome instability. Since HR repair has been shown to protect against Cr(VI)-induced chromosome instability, this study investigated the effect of Cr(VI) exposure on HR repair. We showed acute (24 h) Cr(VI) exposure induced a normal HR repair response. In contrast, prolonged (120 h) exposure to particulate Cr(VI) inhibited HR repair and the key HR protein, RAD51. Prolonged Cr(VI) exposure had a profound effect on RAD51, inhibiting RAD51 function, protein levels and its subcellular localization.

RAD51 localization to the nucleus is crucial to its response to DNA damage. Thus, we investigated the mechanism of Cr(VI)-induced RAD51 cytoplasmic accumulation, focusing on the effect of Cr(VI) on RAD51 nuclear transport mediator proteins. We found prolonged Cr(VI) exposure inhibits nuclear localization of both RAD51 nuclear import partners, RAD51C and BRCA2, although RAD51C was more adversely affected. These results suggest prolonged Cr(VI) exposure inhibits HR repair by targeting RAD51 and inhibiting its nuclear import. These data enhance our understanding of the underlying mechanism of Cr(VI)-induced chromosome instability and thus, carcinogenesis.

In contrast to humans, cancer incidence does not correlate with size and lifespan in certain species, including baleen whales (suborder Mysticeti). This suggests cancer etiology differs between humans and baleen whales. In addition, previous studies suggest whale cells are more resistant to Cr(VI)-induced genotoxicity than human cells. Thus, investigating the effect of Cr(VI) in whale cells may provide insight into how to prevent Cr(VI)-induced carcinogenesis. Here, we showed acute and prolonged Cr(VI) exposure induced less cytotoxicity and chromosome damage in whale cells than human cells. In addition, HR repair was not inhibited by prolonged Cr(VI) exposure in whale cells, demonstrating a profound difference in the effect of Cr(VI) on high fidelity DNA repair in whale and human cells.

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