Abstract 4515: Lgr5 mediates positive B-cell selection and is critical for initiation and survival of B-cell malignancies

Abstract Introduction: In B-cell leukemia and lymphoma, leukemia initiating cells occur at a high frequency (Rehe 2013), are phenotypically diverse (Aoki 2015) and can arise from any stage of B-cell development (Le Viseur 2008). Unlike stem cells, where self-renewal is regulated by a developmental hierarchy, in the B-cell lineage, positive selection events, i.e. induced by antigen-receptor (BCR) signaling dictates their ability to self-renew. Leucine-rich repeat containing G-protein coupled receptor 5 (Lgr5) is a Wnt target gene and through binding to its ligand R-spondin, Lgr5 modulates Wnt signaling strength. Lgr5 is widely used as stem cell marker in multiple epithelial tissues, however the role of Lgr5 in hematopoietic cells was not explored. Results: Upon successful completion of immunoglobulin V(D)J gene recombination and first encounter of antigen represent key events in the life of a B-cell that promote survival and positive selection. Here, we found that both events result in upregulation of Lgr5 expression in B cell precursors in the bone marrow and germinal center B cells. Likewise, engagement of BCR signaling on B-cell lymphomas and oncogenic BCR-signaling mimics in leukemia strongly increased LGR5 expression, which was sensitive to inhibition of SYK and BTK kinases in the BCR pathway.In patients with B-cell leukemia, higher than median mRNA levels of LGR5 at the time of diagnosis were associated with poor clinical outcome and higher likelihood of drug-resistance and relapse. Inducible ablation of Lgr5 during earliest stages of B-cell development resulted in a >100-fold reduction of absolute B-cell numbers. Studies in epithelial cells suggest a role of Lgr5 as potentiator of WNT-signaling. However, deletion of Lgr5 in B cells caused cell death in parallel with massive accumulation of nuclear β-catenin and increased expression of β -catenin target genes. Deletion of Lgr5 abolished colony forming capacity and reduced the ability of leukemia cells to initiate fatal disease in transplant recipients. Likewise, inducible activation of a gain-of-function mutant of β-catenin resulted in rapid cell death in normal and malignant B cells. Conclusion: Lgr5-expression and positive B-cell selection is induced by BCR-engagement by antigen or oncogenic mimics of BCR signaling in B-cell malignancies (e.g. transforming oncogenes that engage the BCR pathway). Unlike in epithelial cells, LGR5 expression in B cells restricts the levels of nuclear β-catenin and enables B-cell survival and transformation through negative regulation of Wnt-signaling. Targeting Lgr5 using a novel Lgr5-ADC seems promising to deplete B-cell leukemia- and lymphoma-initiating cells. Citation Format: Kadriye Nehir Cosgun, Anna Hecht, Xin Yang, Maurizio Mangolini,, Ali Aghajanirefah, Gang Xiao, Teresa Sadras, Zhengshan Chen, Lars Klemm, Huimin Geng, Chao Hong, Qingxiao Song, Deye Zeng, Hassan Jumaa, Defu Zeng, Hans Clevers, Markus Muschen. Lgr5 mediates positive B-cell selection and is critical for initiation and survival of B-cell malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4515.