Abstract A055: RSPO3 overexpression is a characteristic feature of signet ring gastrointestinal malignancies
Authors: Morris Van , Sorokin Alexey , Blum Mariela , Korphaisarn Krittiya , Estrella Jeannelyn , Faoro Leonardo , Wang Min , Hamilton Stanley , Kapoun Ann M. , Kopetz Scott 
Published: August 2018

Abstract
Introduction: The Wnt/β-catenin pathway is critical in the pathogenesis of many gastrointestinal malignancies, and, when disrupted, promotes aberrant cell proliferation and loss of cell stemness. Mutations in APC are the most common etiology of Wnt/β-catenin signaling dysfunction in colorectal (CRC) cancers. However, in the approximately 20% of CRCs that are APC wild-type, abnormalities in cell-surface genes like RNF43 and RSPO drive pathologic Wnt/β-catenin activity. We sought to characterize further gastrointestinal tumors harboring RSPO alterations. Methods: Under an IRB-approved protocol at MD Anderson, tumors from 665 patients with metastatic CRC at MD Anderson were evaluated by next-generation sequencing for APC mutations and for the presence of signet ring cells. Mutation profiling and gene expression data from the TCGA (www.cbioportal.org) were analyzed in 223 patients with CRC for APC mutations and for RSPO overexpression for a correlation using a Fisher’s exact test. RNA extracted from 19 separate frozen tumors from patients with confirmed signet ring metastatic CRC was next analyzed by RNA-seq for the presence for RSPO fusions and for RSPO gene expression levels. In a validation set of FFPE samples derived from patients with signet ring metastatic CRC or signet ring gastric adenocarcinomas, RSPO3 gene expression was analyzed using RT-PCR in a CLIA-validated setting (Oncomed Pharmaceuticals). Results: APC mutations were detected less frequently in signet ring CRC tumors relative to non-signet ring counterparts (18% vs. 56%, P&lt; .001). RSPO overexpression was also observed more commonly in APC wild-type CRC tumors (16% vs. 7%, P= .04). Given these associations, we investigated further a correlation between signet ring adenocarcinomas and RSPO overexpression. Gene expression analysis of signet ring CRC tumors revealed high RSPO3 expression in 13 of 19 cases (68%). Here, a trend towards higher RSPO3 expression was observed in tumors with &gt; 50% signet ring cells present (P=.20). In a validation cohort of patients with signet ring adenocarcinomas, RSPO3 overexpression was noted in 20/38 (52%) CRCs and 21/22 (96%) gastric adenocarcinomas. RSPO fusions were detected in 0 of 19 patients with signet ring CRC analyzed by RNA-seq. Conclusions: In molecularly specified populations of patients with colorectal and gastric malignancies, RSPO3 overexpression is detected in high frequencies in signet ring adenocarcinomas. Given the cell surface localization of RSPO3 and the importance of Wnt/β-catenin signaling in the tumorigenesis of gastrointestinal malignancies, our findings warrant further evaluation for anti-RSPO3 therapies as novel treatment options for patients with signet ring adenocarcinomas.
Citation Format: Van Morris, Alexey Sorokin, Mariela Blum, Krittiya Korphaisarn, Jeannelyn Estrella, Leonardo Faoro, Min Wang, Stanley Hamilton, Ann M. Kapoun, Scott Kopetz. RSPO3 overexpression is a characteristic feature of signet ring gastrointestinal malignancies [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A055.