Date of Award

5-2013

Level of Access Assigned by Author

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biochemistry and Molecular Biology

Advisor

Stephen C. Pelsue

Second Committee Member

John Pierce Wise

Third Committee Member

W. Douglas Thompson

Abstract

Hexavalent chromium (Cr(VI)) compounds are established human lung carcinogens. Solubility plays a key role in the carcinogenicity of Cr(VI), with the most potent carcinogens being the particulate Cr(VI) compounds; however, their carcinogenic mechanism remains poorly understood. Cr(VI)-induced tumors are characterized by frequent genomic instability markers but show few mutations in key tumor suppressor genes and oncogenes. Little is known about the chromosomes in Cr(VI)-induced lung tumors, lung tumors in general are characterized by severe chromosome instability. Thus, the best mechanistic model for particulate Cr(VI) appears to involve genomic instability. Little is known about the ability of particulate Cr(VI) to induce chromosome instability.

We found that prolonged exposure to particulate Cr(VI) induced a concentrationdependent increase in chromosome damage. We also found that prolonged exposure to particulate Cr(VI) induced concentration- and time-dependent increases in aneuploidy.

In addition, we found particulate Cr(VI)-induced chromosome instability is a permanent phenotypic change. We treated cells with particulate Cr(VI) in three sequential periods, thus modeling clonal expansion and outgrowth of cells. These clonal cell lines acquired progressive changes in phenotype. In the first generation, there were changes in the ability of cells to repair DNA double strand breaks with no changes to chromosomes. In the second generation, there was continued repair deficiency as well as permanent changes in chromosome structure and number. The third generation retained these phenotypes, with some acquiring additional chromosomal abnormalities, and having a neoplastically transformed phenotype.

We also investigated the role of homologous recombination repair (HR) in Cr(VI)- induced chromosome instability. We found that cells deficient in the homologous recombination protein BLM have an increased rate of translocations. In addition, our repair deficient clonal cell lines were unable to form Rad51 foci suggesting a defect in HR. We also saw alterations in the formation of sister chromatid exchanges, further supporting a disruption in HR.

These data indicate that Cr(VI) may induce cancer by disrupting chromosome stability. We propose chronic particulate Cr(V) exposure induces chromosome alterations in the form of structural and numerical alterations and also impairs the HR pathway leading to further genomic instability and, ultimately, to neoplastic transformation and cancer.

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