Abstract LB-134: The olfactory bulb provides a radioresistant niche for GBM cells
Authors: Timme Cindy R. , Rath Barbara H. , Camphausen Kevin A. , Tofilon Philip J. 
Published: August 2019

Abstract
Brain tumor xenografts initiated from human GBM stem-like cells (GSCs) mimic many of the characteristics of GBM in situ including extensive invasiveness and migration. Using a GSC orthotopic brain tumor model, we show that the CD133+ NSC11 cells, a well-documented GSC line, display extensive migration and invasion along white matter tracts in the right hemisphere (RH) towards the olfactory bulb (OB) by day 35 post-implantation into the right striatum of nude mice. Radioresponse of tumor cells in the right hemisphere and olfactory bulb was assessed by a series of three daily injections of the halogenated thymidine analog CldU (5-chloro-2’-deoxyuridine), which is incorporated into DNA during S-phase and allows for the identification of proliferating cells. The CldU injections delivered to untreated mice resulted in 54% and 56% CldU positive tumor cells in the RH and the OB, respectively. CldU injections initiated 4 days post-10Gy resulted in 2.4% of tumor cells in the RH and 35% in the OB being positive for CldU. CldU injections initiated 12 days post-10Gy resulted in 5.5% of tumor cells in the RH and 50% in the OB being positive for CldU. These data indicate preferential radioresistance of GBM cells located in the olfactory bulb.
These proliferation analyses were followed up with digital droplet PCR (ddPCR) analysis to determine the percentage of human cells remaining in the right hemisphere and olfactory bulb following irradiation. Specifically, NSC11 brain tumor xenografts received a single dose of 10Gy and the right hemisphere and olfactory bulb were collected at day 7 and 14 post-10Gy. Analyses of MKL2 (human specific) and Tfrc (mouse specific) gene sequences revealed that by day 14 post-10Gy, 32% of tumor cells are detectable in the OB and 5% in the RH. This can be compared to sham irradiated controls with 28% of tumor cells in the OB and 25% in the RH detected at the initial time of radiation and 76% in the OB and 73% in the RH following an additional 14 days of growth. These data indicate that following IR, the number of tumor cells in the olfactory bulb remains the same while the number in the right hemisphere decreases, consistent with decreased radiosensitivity of GBM cells in the olfactory bulb.
Finally, RNA was extracted from NSC11 cells located in the right hemisphere and OB taken from formalin-fixed, paraffin embedded (FFPE) tumor cores at day 35 after-implantation. Gene expression profiles were run using Nanostring technology. Analyses revealed that NSC11 cells located in the OB were enriched in cellular processes involved in growth factor signaling, neurotransmitter signaling (i.e. glutamate and calcium), and various cancer-related pathways (i.e., RAS, MAPK, PI3K). These findings suggest intrinsic gene expression differences of GBM cells that have migrated from the right hemisphere to the olfactory bulb and provide a framework for further investigations into the mechanisms and processes mediating the GBM radioresistance.
Citation Format: Cindy R. Timme, Barbara H. Rath, Kevin A. Camphausen, Philip J. Tofilon. The olfactory bulb provides a radioresistant niche for GBM cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-134.