Date of Award
Doctor of Philosophy (PhD)
Normal tissue injury resulting from ionizing radiation (IR) during cancer radiotherapy has been attributed to reduced regenerative capacity of stem cell compartments. Utilizing multiple in vivo tissue niches and primary culture models, we demonstrate that normal stem cells are highly radiosensitive while their isogenic, directly differentiated progeny are radioresistant. Stem cell dropout is therefore likely responsible for the resulting radiation injury in these niches. This differential radiosensitivity is independent of proliferation status, and increased IR-induced apoptosis in stem cells is more broadly distributed across the cell cycle.
We elucidate that stem cells exhibit an attenuated DDR resulting in aberrant cell cycle checkpoint activation and severely diminished DNA repair capacity despite normal sensing of DNA double strand breaks. Interestingly, while these stem cells are unable to induce γH2AX foci, apoptosis induces pan-nuclear H2AX S139 phosphorylation together with activation of MST1 and JNK. This apoptotic signaling corresponds to a unique inability of stem cells to dephosphorylate H2AX-Y142 around break sites, which promotes apoptosis through JNK while inhibiting DDR signaling. By investigating these molecular responses for the first time in stem cells, we provide potential mechanisms for IR-induced apoptosis independent of DDR signaling. The abrogated DDR in stem cells is also associated with constitutively elevated histone-3 lysine-56 acetylation, which contributes to IR-induced apoptosis through restriction of the DDR and can be modulated to impart radioprotection on stem cells.
This data establishes that unique epigenetic landscapes among differing cell types can impart heterogeneity in the DDR, resulting in varying radiosensitivities and challenging prior assumptions about the ubiquitous nature of canonical DDR signaling. We thus identify pluralistic molecular and epigenetic mechanisms that collectively contribute to IR hypersensitivity in stem cells, promoting development of future therapeutic strategies for minimizing deleterious sequelae from radiotherapy.
Chair and Committee
Barry Sleckman, Lila Solnica-Krezel, Kristen Kroll, Susana Gonzalo
Jacobs, Keith Michael, "Molecular and Epigenetic Regulation of Stem Cell Radiosensitivity" (2015). Arts & Sciences Electronic Theses and Dissertations. 453.