The tumor suppressive TGF-β/SMAD1/S1PR2 signaling axis is recurrently inactivated in diffuse large B-cell lymphoma

Abstract

The sphingosine-1-phosphate receptor S1PR2 and its downstream signaling pathway are commonly silenced in diffuse large B-cell lymphoma (DLBCL), either by mutational inactivation or through negative regulation by the oncogenic transcription factor FOXP1. In this study, we examined the upstream regulators of S1PR2 expression and have newly identified the transforming growth factor-β (TGF-β)/TGF-βR2/SMAD1 axis as critically involved in S1PR2 transcriptional activation. Phosphorylated SMAD1 directly binds to regulatory elements in the S1PR2 locus as assessed by chromatin immunoprecipitation, and the CRISPR-mediated genomic editing of S1PR2, SMAD1, or TGFBR2 in DLBCL cell lines renders cells unresponsive to TGF-β-induced apoptosis. DLBCL clones lacking any 1 of the 3 factors have a clear growth advantage in vitro, as well as in subcutaneous xenotransplantation models, and in a novel model of orthotopic growth of DLBCL cells in the spleens and bone marrow of MISTRG mice expressing various human cytokines. The loss of S1pr2 induces hyperproliferation of the germinal center (GC) B-cell compartment of immunized mice and accelerates MYC-driven lymphomagenesis in spontaneous and serial transplantation models. The specific loss of Tgfbr2 in murine GC B-cell phenocopies the effects of S1pr2 loss on GC B-cell hyperproliferation. Finally, we show that SMAD1 expression is aberrantly downregulated in >85% of analyzed DLBCL patients. The combined results uncover an important novel tumor suppressive function of the TGF-β/TGF-βR2/SMAD1/S1PR2 axis in DLBCL, and show that DLBCL cells have evolved to inactivate the pathway at the level of SMAD1 expression.