Year of Publication

2017

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Toxicology and Cancer Biology

First Advisor

Dr. Isabel Mellon

Abstract

Lung cancer is a particularly devastating disease, accounting for the most deaths among all cancer types in the United States. Despite a reduction in the country’s smoking rates, cigarette smoking remains the number one risk factor for lung cancer. Additionally arsenic exposure, which occurs primarily through contaminated drinking water in the U.S., is associated with increased lung cancer incidence. The nucleotide excision repair (NER) pathway is critical for maintenance of genomic fidelity, removing DNA lesions that could otherwise promote DNA mutations and drive carcinogenesis. Tobacco smoking introduces significant amounts of DNA damage and produces characteristic DNA mutations found in lung cancers of smokers, and arsenic increases lung cancer risk in smokers beyond the risk of smoking along. The contributions of these chemicals to DNA damage and cancer have been well documented, but few studies have examined their effects on DNA repair pathways, particularly the nucleotide excision repair (NER) pathway. Arsenic, while not directly mutagenic, promotes the carcinogenicity of other compounds including agents that produce DNA damage that is repaired by the NER pathway. In this dissertation I investigated the effects of cigarette smoke condensate (CSC, a whole-smoke tobacco surrogate) and arsenic on NER. I observed that CSC or arsenic treatment inhibited NER as measured by a slot-blot assay using UV-induced photolesions as model substrates to measure NER. The abundance of Xeroderma Pigmentosum complementation group C (XPC), a critical NER protein, was significantly reduced in all lines treated with either chemical, while XPA protein was unaffected. CSC and arsenic also affected RNA levels of certain NER genes. Finally, proteasome-regulated XPC turnover was affected by CSC and arsenic treatment, suggesting a potential mechanism for XPC protein inhibition. The observed impairment of NER by CSC is critically important in tobacco cancer etiology – CSC introduces DNA damage, some of which is repaired exclusively by NER, and CSC inhibits the NER pathway as well, providing a two-sided assault on cellular genetic fidelity. I then adapted the NER assay to measure repair in lymphocytes isolated from human subjects of a study investigating the high incidence of lung cancer in Appalachian Kentucky. I observed an age-dependent decline in NER efficiency that was modulated by subject smoking status and a reduced NER efficiency among current smokers in the lung cancer patient population compared to control subjects in the youngest age group, suggesting individual DNA repair capacity measured with this repair assay may be a biomarker for lung cancer susceptibility.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2017.030

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