Defense Date


Document Type


Degree Name

Doctor of Philosophy


Pharmacology & Toxicology

First Advisor

Dr. Lawrence F. Povirk


DNA double-strand breaks (DSBs) containing unligatable termini are potent cytotoxic lesions leading to growth arrest or cell death. The Artemis nuclease and tyrosyl-DNA phosphodiesterase (TDP1) are each capable of resolving protruding 3′-phosphoglycolate (PG) termini of DNA double-strand breaks (DSBs). Consequently, a knockout of Artemis and a knockout/knockdown of TDP1 rendered cells sensitive to the radiomimetic agent neocarzinostatin (NCS), which induces 3′-PG-terminated DSBs. Unexpectedly, however, a knockdown or knockout of TDP1 in Artemis-null cells did not confer any greater sensitivity than either deficiency alone, indicating a strict epistasis between TDP1 and Artemis. Moreover, a deficiency in Artemis, but not TDP1, resulted in a fraction of unrepaired DSBs, which were assessed as 53BP1 foci. Conversely, a deficiency in TDP1, but not Artemis, resulted in a dramatic increase in dicentric chromosomes following NCS treatment. An inhibitor of DNA-dependent protein kinase, a key regulator of the classical nonhomologous end joining (C-NHEJ) pathway sensitized cells to NCS but eliminated the sensitizing effects of both TDP1 and Artemis deficiencies. Moreover, Polynucleotide Kinase/ Phosphatase (PNKP) is known to process 3′-phosphates and 5′-hydroxyls during DSB repair. PNKP-deficiency sensitized both HCT116 and HeLa cells to 3′-phosphate ended DSBs formed upon radiation and radiomimetic drug treatment. The increased cytotoxicity in the absence of PNKP was synonymous with persistent, un-rejoined 3′-phosphate-ended DSBs. However, DNA-PK deficiency sensitized PNKP-/- cells to low doses of NCS suggesting that, in the absence of PNKP, alternative enzyme(s) can remove 3′-phosphates in a DNA-PK-dependent manner. These results suggest that TDP1 and Artemis perform different functions in the repair of terminally blocked DSBs by the C-NHEJ pathway, and that whereas an Artemis deficiency prevents end joining of some DSBs, a TDP1 deficiency tends to promote DSB mis-joining. In addition, loss of PNKP significantly sensitizes cells to 3′-phosphate-ended DSBs due to a defect in 3′-dephosphorylation.


© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission