Abstract 822: Oncogene addiction to GNAS in GNASR201 mutant tumors
Authors: More Aditya , Haridas Valsala , Ito Ichiaki , Chowdhury Saikat , Gu Yue , Fowlkes Natalie W. , Shen John P. 
Published: June 2022

Abstract
Background: The GNASR201 mutation is the single most frequent cancer-causing mutation across all heterotrimeric G proteins. This gain of function mutation in GNAS drives oncogenesis in appendiceal, colorectal, and gastric adenocarcinoma, as well as Intraductal Papillary Mucinous Neoplasms (IPMN) and Small Cell Lung Cancer (SCLC). In this study, we investigated the role of GNAS in tumor growth using peritoneal models of colorectal cancer (CRC).
Methods: GNAS was knocked out in multiple GNASR201C/H mutant colon cell lines (KM12, SNU175, and SKCO1) and overexpressed in GNASWT LS174T cells. Isogenic pairs of KM12 and LS174T cells were injected into the peritoneum of NSG mice to study the role of GNAS in cell line derived xenograft (CDX) models of peritoneal metastasis. Cell lines and CDX were profiled with RNAseq, reverse phase protein assay (RPPA), and immunohistochemistry (IHC) to identify potential pathways regulated by mutant GNAS. Identified mediators of GNAS signaling were then validated using chemical inhibitors of PKA (H-89) and β-catenin (LF3).
Results: GNAS knockout significantly decreased 2D colony formation by KM12 (68%), SNU175 (76%), and SKCO1 (85%) cells (all p &lt; 0.0001), and decreased 3D organoid area of KM12 cells by 62% (p = 0.043). There was significant increase in colony formation by LS174T cells overexpressing GNASR201C (193%, p = 0.016) and GNASR201H (170%, p =0.0037). Mice injected with KM12 GNAS-knockout tumors exhibited a significant 68% reduction in tumor growth (n = 6, p = 0.016) and were more likely to survive at 7 weeks (0% vs. 100%, p = 0.0007) relative to those with parent KM12-GNASR201H tumors. Likewise, mice injected with GNASR201H LS174T cells showed significant increase in tumor growth (934% vs. 100%, n = 3, p = 0.042). Histology of GNAS-knockout tumors showed a marked decrease in mucinous stroma and increased lytic necrosis, suggesting an interaction between oncogenic GNAS signaling and the peritoneal environment. GNAS is known to stimulate adenylate cyclase and GNAS knockout decreased levels of cyclic AMP in KM12 cells, confirming an on-target effect. RPPA and RNAseq profiling of KM12 and LS174T PDX tumors identified phosphorylation of β-catenin and activation of Wnt/β-catenin targets (NES = 1.25, 1.24) as critical downstream effects of mutant GNAS signaling, confirmed by a 4.9-fold (p = 0.001) increase in nuclear β-catenin intensity in LS174T GNASR201H tumors (19% vs. 99% nuclei stained positive, p = 0.0002). Chemical inhibition of both PKA and β-catenin reduced growth of GNAS mutant organoids by 79% (p = 0.004) and 67% (p = 0.007) respectively, supporting the involvement of the cAMP/PKA and β-catenin pathways in mutant GNAS signaling.
Conclusions: Our findings demonstrate oncogene addiction to GNAS in peritoneal models of GNASR201C/H tumors, which signal through the cAMP/PKA and Wnt/β-catenin pathways. Thus, GNAS and its downstream mediators are promising therapeutic targets for GNAS mutant tumors.
Citation Format: Aditya More, Valsala Haridas, Ichiaki Ito, Saikat Chowdhury, Yue Gu, Natalie W. Fowlkes, John P. Shen. Oncogene addiction to GNAS in GNASR201 mutant tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 822.