B-Cell Receptor Driven MALT1 Activity Regulates MYC Signaling in Mantle Cell Lymphoma

Abstract

Mantle cell lymphoma (MCL) is a mature B-cell lymphoma characterized by poor prognosis. Recent studies revealed the importance of constitutive B-cell receptor (BCR) signaling in maintaining survival of a subset of MCLs. MALT1 is an essential component of the CARD11-BCL10-MALT1 (CBM) complex that links BCR signaling to the nuclear factor kappa-B (NF-κB) pathway. Additionally, MALT1 functions as a protease that cleaves various substrates to promote proliferation and survival. However, its role in the molecular pathogenesis of MCL is unknown. To elucidate the functional role of MALT1 in the biology of MCL, we determined its proteolytic activity in primary MCL cells and in MCL derived cell lines. A large fraction of MCLs displayed constitutive activity of MALT1. This MALT1 activity is driven by constitutive BCR signaling, as we were able to show that RNA interference-mediated knockdown of central components of the BCR cascade abolished MALT1 activity. To gain insights into the functional significance of MALT1 in MCL, we knocked down its expression by different MALT1 shRNAs. Transduction of these shRNAs induced cytotoxicity in models that are characterized by constitutive MALT1 activity, whereas no effect on survival was observable in MCLs without MALT1 activation. To determine if this MALT1 addiction translates into an in vivosetting, we knocked down MALT1 in mouse MCL models and detected a significant inhibition of tumor growth. This indicates that MALT1-activated MCLs are dependent on the function of MALT1. These results were confirmed as pharmacologic inhibition of MALT1 significantly reduced cell viability in MALT1-activated MCLs, implying that MALT1 inhibition might represent a promising therapeutic strategy for MCL patients. To understand which biologic processes are regulated by MALT1 in MCL, we profiled gene expression changes at different time points following MALT1 inhibition. An unbiased gene set enrichment analysis identified various previously described MYC gene expression signatures to be among the top downregulated signatures, suggesting that MALT1 regulates MYC expression. MYC downregulation following MALT1 inhibition or MALT1 knockdown was confirmed at the protein level and various analyses revealed that MALT1 regulates MYC expression posttranslationally by preventing its proteasomal degradation. These results were further confirmed in primary mouse splenocytes, indicating that this novel molecular mechanism of regulating MYC expression is not restricted to MCL. To confirm that MYC is indeed expressed in primary MCLs, we determined MYC expression in 234 primary MCL samples by immunohistochemistry. These analyses revealed that 75 samples (32.1%) displayed an intermediate and 55 samples (23.5%) a high MYC positivity, suggesting that MYC is expressed in a substantial number of primary MCLs. As common alterations such as MYC high-levelamplifications and translocations determined by FISH occurred extremely rarely in our primary MCL samples (1% of samples), it is conceivable that BCR-driven MALT1 signaling is the predominant mechanism of MYC upregulation in MCL. In summary, we report for the first time that a substantial fraction of MCLs is addicted to constitutive MALT1 signaling. Thus, MCLs can be divided based on their MALT1 activation status into two distinct subgroups. We further identified a novel regulatory mechanism of MYC expression by MALT1. Thus, our study provides a strong mechanistic rationale to investigate the therapeutic efficacy in targeting the MALT1-MYC axis in MCL patients. Disclosures Trneny: Janssen Research & Development: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Gilead: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees. Dreyling:Roche: Consultancy, Honoraria, Research Funding, Speakers Bureau. Tzankov:Novartis: Speakers Bureau; Abbott: Speakers Bureau.

Germinal center‐derived lymphomas: The darkest side of humoral immunity

“…[206][207][208][209] MALT1 is a key mediator of the BCR signaling to NF-κB and is essential in the development of ABC-DLBCL and other lymphomas including mantle cell lymphoma, primary effusion lymphoma, and chronic lymphocytic leukemia (CLL). 136,[210][211][212][213][214] Moreover, MALT1 inhibition was shown to be effective in CLL patients, even those with mutations of BTK, PLCy, and CARD11 that cause Ibrutinib resistance. 214 Regarding TLR signaling, IRAK4 inhibitors show activity in preclinical studies in ABC-DLBCL cell lines 144 and PDXs.…”

Genetic landscape and deregulated pathways in B‐cell lymphoid malignancies

“…Recently, MYC protein deregulation has also been related to increased proliferation in MCL [82]. MYC stabilization is induced by MALT1 and a few aggressive MCL cases also harbour MYC rearrangement or amplification, although at low frequencies.…”

Ibrutinib resistance in mantle cell lymphoma: clinical, molecular and treatment aspects

“…In contrast, in MALT1-inactive MCL cell lines, transduction of the BTK C481S mutant or an empty vector did not alter the cells' sensitivity to ibrutinib or to the MALT1 inhibitor. These data indicate that inhibition of MALT1 might be effective as a novel therapeutic option for patients failing ibrutinib therapy (Dai et al, 2017).…”

“…CYLD cleavage may be used as a direct marker of MALT1 proteolytic activity (Staal et al, 2011). Dai et al (2017) showed that MALT1 is constitutively active in a substantial number of MCL models, and that MCLs can be divided into two distinct subgroups based on their MALT1 activation status. To investigate whether inhibition of MALT1 can overcome BTK C481S -induced ibrutinib resistance, they expressed a BTK C481S cDNA or an empty vector in five MALT1-activated and two MALT1-inactive MCL cell lines and subsequently treated these cells with ibrutinib or with a MALT1 inhibitor.…”