PROJECT SUMMARY/ABSTRACT Clinical and genetic heterogeneity among the inflammatory bowel diseases (IBD) suggests that IBD is inclusive of a spectrum of mucosal inflammatory diseases. Predicting disease natural history and development of subgroup-specific treatment plans are confounded by profound genetic and patho-biologic heterogeneity, but is essential to the development of targeted therapies. Drug development in unselected patient populations has had only limited success. Novel approaches are needed to define the distinct patient subpopulations likely to benefit from new treatments based on precisely selected targets. Ribonuclease T2, (RNASET2) has been identified as a potential IBD risk gene. Clinically, RNASET2 disease risk variants are associated in CD with a more complicated/resistant disease phenotype defined in part by therapeutic drug failure, increase in length of intestinal resection, a shorter time to reoperation and post-operative endoscopy with high scores in inflammation indices. Recent data demonstrate a functional and biological relationship between RNASET2 and two additional genes, TNFSF15 and ICAM1, which have been implicated by GWAS as potentially involved in IBD pathogenesis. Motif screening of RNASET2 disease risk variants identified rs2149092 as a potential regulatory single nucleotide polymorphism (SNP) predicted to disrupt a consensus ETS-transcription factor (TF) binding site located within an enhancer region. The hypothesis to be addressed in this proposal is that identifying the regulatory mechanisms and molecular components comprising TL1A-mediated down-regulation of RNASET2 expression, and the molecular events contributing to enhancement of pro-inflammatory cytokine expression/secretion related to decreased levels of RNASET2 and subsequent enhanced ICAM1 expression, will yield a more precisely defined molecular signature of a severe form of CD as well as potential targets, that may then be used alone or in combination to optimally mitigate severe disease development in a defined subset of patients with Crohn?s disease (CD). We will approach this hypothesis by the following Specific Aims. 1) Determine the candidate regulatory variants and cis-and trans-regulatory pathways involved in TL1A mediated inhibition of RNASET2 expression 2) Determine the cellular and molecular pathways by which decreased RNASET2 expression drives enhanced LFA1/ICAM1 interaction and subsequent IFNg secretion. 3) Validate the functional impact of the findings from Specific Aims 1 and 2 by: a) using allele specific expression of RNASET2 risk variants in T cells isolated from CD patients and b) testing candidate therapeutic targets in mouse models of TL1A overexpression with many of the characteristics associated with CD to more precisely define molecular signatures and further refine potential therapeutic targets for severe disease.