Defense Date


Document Type


Degree Name

Master of Science


Pharmacology & Toxicology

First Advisor

Lawrence Povirk


DNA DSBs are most toxic to cells because they can lead to genomic rearrangements and even cell death. Most DSBs induced by ionizing radiation or radiomimetic drugs such as calicheamicin and bleomycin, bear 3′-phosphate or 3′- PG moieties that must be removed to allow subsequent gap filling and ligation. DSBs can be repaired by two main pathways: the homologous recombination (HR) pathway and the non-homologous end-joining (NHEJ) pathway, NHEJ is the primary repair pathway in mammalian cells. While HR repairs single strand breaks (SSBs) or DSBs accurately by using an undamaged copy of the sequence mostly at late S phase and G2 phase, the NHEJ pathway repairs DSBs without the requirement for sequence homology in a processing that may be error-free or error- prone and is most active at G1 phase. TDP1 is a DNA repair enzyme in both pathways, It associates with DNA SSB repair proteins XRCC1 and DNA ligase III and plays a role in processing of topoisomerase I- mediated SSBs. Our early results suggested that TDP1 also can remove protruding 3’- PG and other 3’ blocks from DSBs ends in vitro. A homozygous H493R mutation in the active site of TDP1 causes spinocerebellar ataxia with axonal neuropathy (SCAN1), a rare autosomal recessive genetic disease with neurological symptoms including peripheral neuropathy. DNA damage and misrepair can be determined by measuring the incidence of chromosomal aberrations such as rings, breaks, dicentrics, acentric fragments, and translocations in metaphase cells, and micronuclei in interphase cells. To assess the possible role of TDP1 in DSB repair in intact cells, the radiosensitivity of SCAN1 cells was determined by using a dose-fractionation method of irradiation. The data indicated that, when exposed to fractionated radiation doses, the SCAN1 cells were more sensitive than normal cells. Moreover, following treatment of cells with calicheamicin, SCAN1 cells showed a significantly higher incidence of dicentric chromosomes, acentric fragments, and micronuclei compared to normal cells, indicating that calicheamicin-induced DSBs were repaired less accurately and less efficiently, or more slowly in SCAN1 cells than in normal cells. All these results are consistent with a role for TDP1 in repair of 3’-PG DSBs in vivo. Oxidative stress is thought to induce replicative senescence and DNA damage in mouse embryo fibroblasts (MEFs). To determine the possible roles of oxidative stress on Tdp1-deficient MEFs, Tdp1-knockout MEFs and normal MEFs were cultured in 20% oxygen (atmospheric) and 3% (physiological) oxygen. The data from growth assays indicated that normal MEFs showed replicative senescence in 20% oxygen but not in 3% oxygen. Tdp1-knockout MEFs showed very poor growth compared to Tdp1 normal MEFs in both oxygen conditions, clearly suggesting an influence of repair of Tdp1 on oxidative stress induced DNA-DSBs in MEFs. Taken together, our results indicated that TDP1 is capable of removing protruding 3’-PG from DSB ends in intact cells. Moreover, DSBs induced by oxidative stress were repaired more slowly or inefficiently in MEFs when Tdp1 is absent, resulting in cell cycle arrest and poor cell growth.


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Date of Submission

December 2012