Abstract 1910: 3′UTR poly(T/U) tract deletions and altered expression of EWSR1 are a hallmark of mismatch repair-deficient cancers.

Background: Microsatellite instability (MSI), the genome-wide accumulation of DNA replication errors, is the hallmark lesion of DNA mismatch repair (MMR) deficient cancers, present in Lynch syndrome (LS)-related and 10-20% of sporadic colorectal (CRC), gastric and endometrial cancers. MSI testing is widely used to guide clinical management but the functional significance of MSI at distinct genic loci remains largely elusive. Here, we characterize a novel MSI target locus consisting of a mononucleotide (T/U)16 tract located in the 3’untranslated region (UTR) of the Ewing sarcoma break point region 1 (EWSR1) gene (EWS16T). Methods: The diagnostic accuracy of EWS16T to identify MMR-deficient cancers was determined by analyzing 319 cancers (240 colorectal, 72 gastric, 7 endometrial) from two different populations. The functional consequences of EWS16T contractions were assessed in vitro (siRNA-mediated poly(A) site selection and pull-down assays) and in vivo (mRNA and protein expression by qPCR and tissue microarray analysis). Results: The EWS16T locus discriminates MMR-proficient (n=128) from deficient (n=191) cancers with perfect diagnostic sensitivity (100%) and specificity (100%). Biochemical analyses indicate that EWS16T contractions alter poly(A) site selection by promoting SFPQ-mediated distal poly(A) site usage in EWSR1 pre-mRNAs and result in decreased mRNA as well as EWS protein expression. In contrast to their MMR-proficient counterparts (n=64), MMR-deficient, LS-related CRC (n=94) display altered subcellular localization of EWS (p Conclusions: The EWS16T locus represents a novel, quasi-monomorphic MSI target locus to accurately identify both hereditary and sporadic MMR-deficient cancers. Contractions therein affect multiple regulatory mechanisms implicating the RNA-/DNA-binding Ewing sarcoma protein in MSI-associated colorectal tumorigenesis. Citation Format: Salvatore Piscuoglio, Shivendra Kishore, Michal Kovac, Annette Gylling, Friedel Wenzel, Francesca Trapani, Hans Joerg Altermatt, Valentina Mele, Giancarlo Marra, Paivi Peltomaki, Luigi Terracciano, Mihaela Zavolan, Karl Heinimann. 3′UTR poly(T/U) tract deletions and altered expression of EWSR1 are a hallmark of mismatch repair-deficient cancers. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1910. doi:10.1158/1538-7445.AM2013-1910