Year of Publication

2012

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Medicine

Department

Toxicology

First Advisor

Dr. David Orren

Abstract

All human chromosomes end with protective structures called telomeres, which consist of thousands of double-stranded TTAGGG repeats and end in a 3’ guanine-rich overhang. These structures shorten normally during each round of replication, and extremely short telomeres along with telomere dysfunction are thought to contribute to the development of aging and cancer. Although many proteins have roles in telomere maintenance, WRN, which is a 3’ to 5’ helicase that is deficient in the premature aging disorder Werner’s syndrome, has been proposed to play multiple roles at telomeres. In this study, I focus on the effect of telomeric sequences and/or structures formed during DNA replication or recombination and how WRN functions at these sites. This study suggests that WRN may promote proper replication of telomeres by accurately aligning telomeric sequences during replication fork regression, potentially the first step in responding to a blockage, such as DNA damage. However, even in the presence of WRN, replication of telomeric sequences is difficult, possibly due to the ability of G-rich sequences to form secondary structures such as G-quadruplexes. I demonstrate that the translesion polymerase pol η, as well as a variety of other polymerases, is unable to synthesize past an intramolecular G-quadruplex formed from telomeric sequence on the template strand. Furthermore, in physiological salt concentrations, WRN favors binding and unwinding a structure that mimics a strand invasion intermediate over other similar structures especially when it possesses G-telomeric sequence. In addition, WRN promotes unwinding of these structures in a direction that would promote additional annealing and strand invasion, supporting a role for WRN in promoting telomeric recombination and formation of a T-loop, a proposed protective structure specific to telomeres. Overall, the data suggest that telomeres may pose problems in replication due to the G-rich, repeating nature of the structures, while WRN may aid in promoting proper replication at these and other replication blocks. Furthermore, WRN may play a role in promoting additional formation of T-loops and other telomeric recombination, thus supporting the relationship of WRN, telomere maintenance, and potentially development of certain aging characteristics.

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