Author ORCID Identifier
Date of Graduation
Doctor of Philosophy (PhD)
Robert C. Bast Jr., M.D.
George Calin, M.D., Ph.D.
Gordon Mills, M.D., Ph.D.
Sameul Mok, Ph.D.
Heinrich Taegtmeyer, M.D., D. Phil.
Cheryl L. Walker, Ph.D.
DIRAS3 is a maternally imprinted tumor suppressor gene that is downregulated by multiple mechanisms across several tumor types. When re-expressed, DIRAS3 decreases proliferation, inhibits motility, and induces autophagy and tumor dormancy. DIRAS3 encodes a 26 kDa small GTPase with 60% homology to Ras and Rap, differing from oncogenic Ras family members by a 34-amino acid N-terminal extension that is required for its tumor suppressive function in ovarian cancer. By assessing the structure-function relationship, I found that DIRAS3 inhibits Ras-induced transformation and is a natural antagonist of Ras/MAPK signaling. DIRAS3 binds directly to Ras and disrupts cluster formation inhibiting the activation of Raf kinase, which is dependent upon membrane localization and the N-terminal extension. This observation provides a novel approach to target oncogenic Ras and assesses the functional significance of Ras clustering/multimerization.
The N-terminus of DIRAS3 also plays an important role in the mechanism(s) by which DIRAS3 induces autophagy. Expression of DIRAS3 is required for the induction of autophagy in human cells. While DIRAS3 is found in the genome of humans, pigs and cows, it maps to an apparent evolutionary breakpoint in the rodent lineage where chromosomes have been rearranged relative to the human genome since the two species shared a common ancestor. Mice and humans do express two homologous Ras-related GTPases, DIRAS1 and DIRAS2. These 22-kDa GTPases have 30-40% homology with H-Ras and 50-60% homology with DIRAS3 where the major difference is the truncation of the N-terminal extension. DIRAS1 and DIRAS2 have not previously been studied extensively. I compared the roles of the DIRAS family in malignant transformation, proliferation, survival, motility and autophagy. My observations document the role of DIRAS1 and DIRAS2 as ovarian cancer tumor suppressors and demonstrate their role in autophagy and autophagic cell death. Similar to DIRAS3, DIRAS1 and DIRAS2 induce autophagy at several different levels, including transcription-dependent mechanisms. DIRAS1 and DIRAS2 likely serve as surrogates for DIRAS3 in the murine genome, playing an essential role in murine autophagy. These studies are fundamentally important as they explore the functional significance of their N-terminal extensions, helping to explain how members of an oncogenic superfamily acquire tumor suppressor function.
Ras, Autophagy, DIRAS, Ovarian Cancer, Pancreatic Cancer, Lung Cancer, GTPase
Available for download on Wednesday, April 25, 2018