My lab uses yeast genetics, biochemistry and RNAi in human cells to investigate the mechanism of action of DNA topoisomerase I (Top1) and camptothecin chemotherapeutics that targets this enzyme. Top1 plays a critical role in DNA replication, transcription and recombination by catalyzing the relaxation of supercoiledDNA. Camptothecins reversibly stabilize a covalent enzyme-DNA intermediate, which is converted into irreversible DNA lesions during S-phase of the cell cycle. Using yeast as a model system, yeast and human DNA topoisomerase I mutants are examined to define specific alterations in enzyme catalysis and protein architecture necessary for thecytotoxic action of these drugs. Nanomanipulation of single Top1-DNA complexes allow us to probe dynamic changes in enzyme activity induced by camptothecin binding,thereby providing novel insights into the mechanism of drug action in cells. Yeast genetic screens are also developed to identify cellular processes required for the formation and repair of camptothecin-induced DNA damage. Gene products under investigation include the SUMO conjugating enzyme Ubc9, proteins required for the initiation of DNA replication (Cdt1, Cdc45 and Dpb11) and proteins involved in ubiquitinhomeostasis (Doa4). As the majority of these yeast genes have human orthologs, RNAinterference (RNAi) is being used to assess the conserved function of these human pathways in modulating cell sensitivity to DNA topoisomerase I-targeted chemotherapeutics.
A second area of investigation is the TOR signaling pathway. In yeast and human cells, the TOR kinase regulates cellular responses to environmental stress,growth signals and starvation. We have recently shown that inhibition of TOR signaling by rapamycin enhances cell sensitivity to DNA damaging agents in S-phase and delay cell cycle exit from mitosis. Current efforts focus on defining the mechanisms by which the TOR pathway protect cells from genotoxic stress and regulate cell cycle transit through S-phase and mitosis.
Dr. Mary-Ann Bjornsti is the Chair of the Department of Pharmacology and Toxicology. Dr. Bjornsti received her B.S. degree in Biology from NY State University College at Cortland in 1978; and her Ph.D. Degree in Genetics at the University of Minnesota, Minneapolis 1983. Her thesis advisor was Dr. Dwight L. Anderson. Dr. Bjornsti was a Postdoctoral Fellow with Prof. Edward Kellenberger at Biozentrum, University of Basel, Switzerland from 1984-1985. She continued her Postdoctoral training with Prof. James C. Wang in the Dept. of Biochemistry and Molecular Biology at Harvard University in 1985-89. Dr. Bjornsti joined the UAB faculty in 2009.