Abstract 3914: BTK and MALT1 are critical for cell adhesion and dissemination in mantle cell lymphoma

Abstract Bruton’s tyrosine kinase (BTK) is a great target in mantle cell lymphoma (MCL). This is evident by multiple FDA approvals of covalent BTK inhibitors (BTKi, e.g. ibrutinib, acalabrutinib and zanubrutinib) and recent exciting clinical data on non-covalent BTKi pirtobrutinib. However, resistance to BTKi is a major clinical challenge and the resistance mechanism is not yet fully understood. To address this, we generated JeKo BTK KD cells via BTK knockdown (KD) through by CRISPR Cas9 in BTKi-sensitive JeKo-1 cells, which resulted in superior resistance to BTKi and cell growth defects in vitro. Interestingly, JeKo BTK KD cells demonstrated early tumor cell engraftment and growth in subcutaneous xenograft models, while parental JeKo-1 cells showed much later engraftment but with much faster growth kinetics. To understand this and the BTKi-resistance mechanism, we first performed bulk RNA sequencing analysis and identified MALT1, but not its well-known binding partners CARD11 and BCL10, as one of the top overexpressed genes in BTKi-resistant MCL cells, including JeKo BTK KD cells. Genetic knockout (KO) of MALT1 or CARD11 by CRISPR Cas9 in JeKo-1 resulted in defects in cell growth in vitro and delayed tumor engraftment and growth in vivo. In contrast, MALT1 KO, but not CARD11 KO, in JeKo BTK KD cells remarkably suppressed cell growth in vitro, and tumor engraftment and growth in vivo. These data demonstrate that MALT1 overexpression can drive ibrutinib resistance via bypassing BTK-CARD11 signaling. BTKi-resistant cells including JeKo BTK KD cells showed much higher potency in adhesion to extracellular matrix or stromal cells compared to BTKi-sensitive cells. BTK inhibition or MALT1 inhibition significantly suppressed cell adhesion and migration to extracellular matrix or stromal cells. Furthermore, BTK KD and MALT1 KO but not CARD11 KO in JeKo-1 cells remarkably suppressed tumor cell dissemination and growth in spleen, liver, bone marrow and peripheral blood. MALT1 KO in JeKo BTK KD cells further suppressed tumor cell dissemination. Consistent with this, MALT1 inhibition greatly suppressed tumor cell dissemination and growth in spleen, bone marrow and peripheral blood of an ibrutinib-resistant patient-derived xenograft model. Therefore, both BTK and MALT1 are critical for tumor cell adhesion and dissemination in vivo in a CARD11-independent manner. Furthermore, co-targeting MALT1 with safimaltib and BTK with pirtobrutinib induced potent anti-MCL activity in BTKi-resistant MCL cell lines and patient-derived xenografts. Therefore, we conclude that (1) BTK and MALT1 are key molecules that control MCL cell growth and dissemination, (2) MALT1 overexpression drives resistance to BTKi in MCL, (3) targeting MALT1 is a promising therapeutic strategy to overcome BTKi resistance, and (4) co-targeting of BTK and MALT1 improves efficacy and durability beyond single agents. Citation Format: Vivian Changying Jiang, Yang Liu, Qingsong Cai, Joseph M. McIntosh, Yijing Li, Zhihong Chen, Heng-Huan Lee, Wei Wang, Yixin Yao, Lei Nie, Michael Wang. BTK and MALT1 are critical for cell adhesion and dissemination in mantle cell lymphoma. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3914.