Our long-standing, research interests are in the mechanisms of anti-cancer drug action and in pathways, such as the small ubiquitin-like modifier (SUMO) and mTOR signaling, that regulate tumor growth and cellular responses to replicative stress. My lab pioneered the use of the genetically tractable yeast model system to investigate the mechanism of action of DNA topoisomerase I, SUMO conjugation and experimental chemotherapeutics, with a focus on translating basic mechanisms of chemotherapeutic drug action and cellular pathways that regulate drug responses to human cell lines and mouse models. In genetic screens for mutations that suppress yeast cell sensitivity to camptothecin, we defined novel functions for essential replication fork components (yeast Cdc45 and Dpb11) in promoting replication fork stability in response to replicative DNA damage, and a role for the SUMO pathway in protecting cells from such damage. These findings provided unique insights as to how alterations in replication fork stability directly impact chemosensitivity and novel biomarkers for therapeutic response. For instance, collaborations with Dr. William Plazcek include structural and biochemical studies of the SUMO- conjugating enzyme Ubc9, and how select mutations and small molecules selectively impair the SUMOylation of select substrates to alter cellular responses to replicative stress. With Dr. John Hartman, we use yeast phenomics to investigate mechanisms of drug action, while Dr. Coric and I developed genome-wide siRNA screening capabilities at UAB, that complement state-of-the-art chemistry and small molecule screening platforms at Southern Research, a partner with UAB in the Alabama Drug Discovery Alliance.

Our genetic analyses of TOR signaling in S-phase revealed that TOR-dependent mRNA translation provides a survival pathway for cells exposed to replicative DNA damage, and that persistent TOR signaling under such conditions is inherently mutagenic. In collaboration with Dr. Sunnie Thompson and Dr. David Schneider, we are investigating the role of mTOR-regulated mRNA translation in promoting the survival of breast and ovarian cancer cells treated with cytotoxic chemotherapeutics. These findings provide the rationale for combining mTOR inhibitors and cytotoxics, such as cisplatin, in clinical trials of breast cancer patients.

An area of long-standing interest has been the impact of DNA topoisomerase I (Top1) architecture on enzyme function and camptothecin sensitivity. Eukaryal Top1 is a monomeric protein clamp that completely circumscribes duplex DNA. A flexible linker domain positions the active site tyrosine within the clamp. Cleavage of one strand of duplex DNA forms a covalent Top1-DNA intermediate, allowing for strand rotation to relax DNA supercoils. Camptothecin stabilizes this complex, to induce S-phase-dependent cell death. We have used novel technologies to address Top1 structure, incluuding (1) molecular modeling, to engineer a reversible disulfide bond to lock the Top1 protein clamp around DNA, which induces a novel cytotoxic mechanism of poisoning Top1; (2) molecular tweezers (with the Dekker lab) in single molecule studies with the surprising result that camptothecin selectively prevents the unwinding of overwound DNA - a novel cytotoxic mechanism of fork stalling and collapse, and (3) yeast /human chimeric enzymes to query the role of specific domains in drug sensitivity and enzyme activity.

D'Alfonso A, Di Felice F, Carlini V, Wright CM, Hertz MI, Bjornsti M-A*, Camilloni G*. (2016) Molecular Mechanism of DNA Topoisomerase I-Dependent rDNA Silencing: Sir2p Recruitment at Ribosomal Genes. J Mol Biol. 428:4905-4916.

Nebane M, Coric T, McKellip S, Woods L, Sosa M, Rasmussen L, Bjornsti M-A, E. White EL. (2016) Acoustic Droplet Ejection Technology and its application in High-Throughput RNA Interference Screening. J Lab Automation. 21:198-203

Wright CM, van der Merwe M, DeBrot AH, Bjornsti M-A. (2015) DNA topoisomerase I domain interactions impact enzyme activity and sensitivity to camptothecin. J Biol Chem 290:12068-78 PMID: 25795777.

Ghosh AP, Marshall CB, Coric T, Shim EH, Kirkman R, Ballestas ME, Ikura M, Bjornsti MA, Sudarshan S. (2015) Point mutations of the mTOR-RHEB pathway in renal cell carcinoma. Oncotarget 6:17895-910.

Comeaux EQ, Cuya S, Kojima K, Jafari N, Wanzeck KC, Mobley JA, Bjornsti M-A, van Waardenburg RC. (2015) Tyrosyl-DNA phosphodiesterase I catalytic mutants reveal an alternative nucleophile that can catalyze substrate cleavage. J Biol Chem 290:6203-14.

Garcia PL, Council LN, Christein JD, Arnoletti JP, Heslin MJ, Gamblin TL, Richardson JH, Bjornsti M-A, Yoon KJ. (2013) Development and histopathological characterization of tumorgraft models of pancreatic ductal adenocarcinoma. PLoS ONE 8(10):e781813.

Kohli L, Kaza N, Coric T, Byer SJ, Brossier NM, Klocke BJ, Bjornsti M-A, Carroll SL, Roth KA. (2013) 4-Hydroxy Tamoxifen Induces Autophagic Death Through K-Ras Degradation. Cancer Res. 73(14):4395-405.

Nebane M*, Coric T*, Whig K, McKellip S, Woods LK, Sosa M, Sheppard R, Rasmussen L, Bjornsti M-A, White EL. (2013) High-Throughput RNA Interference Screening: Tricks of the Trade, J Lab Automation 18:334-9.

Koster DA, Crut A, Shuman S, Bjornsti M-A and Dekker NH (2010). Cellular strategies for regulating DNA supercoiling: a single-molecule perspective. Cell 142:519-30.

Shen C, Lancaster CS, Shi B, Guo H, Thimmaiah P and Bjornsti M-A. 2007. TOR signaling is a determinant of cell survival in response to DNA damage. Mol Cell Biol. 27: 7007-17.