Portland State University. Department of Biology
Date of Award
Master of Science (M.S.) in Biology
1 online resource (viii, 77 pages)
DNA interstrand cross-links are a form of genomic damage that cause a block to replication and transcription of DNA in cells and cause lethality if unrepaired. Chemical agents that induce cross-links are particularly effective at inactivating rapidly dividing cells and, because of this, have been used to treat hyperproliferative skin disorders such as psoriasis as well as a variety of cancers. However, evidence for the removal of cross-links from DNA as well as resistance to cross-link-based chemotherapy suggests the existence of a cellular repair mechanism. Characterizing the pathways involved in DNA interstrand cross-link repair has been challenging due to the inherent structure of the damage as it precludes the use of an undamaged, complementary strand of DNA as a template for repair.
A number of models of cross-link repair have been proposed based on the identification of hypersensitive repair mutants as well as biochemical evidence that specific repair enzymes are capable of incising cross-linked structures from DNA. Together, these models have suggested the involvement of multiple repair pathways--such as nucleotide exicision repair, translesion synthesis, recombination of double-strand breaks, and base excision repair--operating in sequential steps to correct the damage. Most of the studies from which these models arose are complicated by the fact that cross-linking agents induce multiple forms of damage or they lack in vivo confirmation of how the repair phenomenon occurs in organisms.
In this study, I use Escherichia coli as a model organism to examine the involvement of the aforementioned pathways in DNA interstrand cross-link repair in vivo. This organism was useful in early cross-link studies and, with its highly conserved repair processes, maintains the potential for delineating how cross-links are removed in higher organisms. In Chapter I, I introduce background information on different cross-linking agents, the complications of studying cross-link repair, and the candidate repair pathways that have been implicated to date.
In Chapter II I demonstrate that there is a limited involvement of the nucleotide excision repair helicase, translesion polymerases, and double-strand break repair enzymes through survival analysis of cells defective in these proteins. For this analysis, I use 8-methoxypsoralen plus UVA as a cross-linking agent and angelicin plus UVA as a monofunctional comparator. The observation that uvrD mutants-- defective in helicase II of nucleotide excision repair--were nearly as resistant to 8-methoxypsoralen-induced damage as wild type cells led me to examine the incision rate of cross-links from endogenous plasmid DNA. Surprisingly, cross-links were not efficiently removed from DNA in uvrD mutants relative to wild type cells. These seemingly contradictory results were rectified when I quantified cross-link formation in cell cultures and revealed that as few as one cross-link per chromosome can inactive wild type cells, a lethal quantity that is lower than what has been previously reported. Taken together, these observations suggest that although cross-links are incised in wild type cells, repair is still not a highly productive event in E. coli.
In Chapter III I examine the involvement of the base excision repair pathway in cross-link repair and demonstrate that Nth and Fpg Glycosylases, Xth and Nfo AP-Endonucleases sensitize Escherichia coli to psoralen-induced DNA damage. This is shown by comparative survival analysis in angelicin plus UVA and 8-methoxypsoralen plus UVA treatment whereby nth-, fpg-, and xth-mutants are each more resistant than wild type cells to either treatment. This suggests that when these gene products are present they impact the production or removal of monoadducts. nfo-mutants were different in that the cells were only hyperresistant to 8-methoxypsoralen monoadducts and cross-links, either implying that the Nfo enzyme interacts specifically with psoralen monoadducts rather than angelicin monoadducts or that the enzyme impedes cross-link removal.
Finally, in Chapter IV a summary of the results is provided as well as future directions that may be explored following this study.
Cole, Jessica Michelle, "The Limitations of DNA Interstrand Cross-link Repair in Escherichia coli" (2018). Dissertations and Theses. Paper 4489.