Bivalent Histone Modifications By Reiibp Leads to Transcriptional Reprogramming and TLR7-BTK Driven IL-6 Pro-Inflammatory Response in t(4;14) Myelomagenesis

Multiple Myeloma (MM) is characterized by the uncontrolled proliferation of malignant plasma cells that are incapable of producing functional antibodies. Current treatment regime involves novel drug classes such as proteasome inhibitors, immunomodulatory drugs and monoclonal antibodies, which have significantly improved survival outcomes in patients. Patients with t(4;14) translocation represents ~15% of MM cases, and displays a dysregulation of the MMSET locus. It has one of the worst prognosis when compared to other subgroups, but represents an intermediate-risk group given its response towards bortezomib. REIIBP is a t(4;14)-deregulated isoform arising from alternative promoter usage within the MMSET locus. Despite sharing identical sequence with the C-terminus of MMSET II, we found that REIIBP displayed mutually exclusive expression with the full-length MMSET II arising from MB4-1 breakpoint. Additionally, the expression of REIIBP can be regulated through microRNAs by another histone methyltransferase, EZH2 in a Dicer-dependent manner. We generated a stable cell line that overexpresses REIIBP in the t(4;14)-negative human myeloma cells, RPMI8226, and revealed REIIBP as an epigenetic regulator of repressive H3K27me3 and active H3K4me3 modifications. Transcriptome profiling and ChIP-sequencing identified an upregulation of Toll-like receptor 7 (TLR7) by REIIBP through the enrichment of H3K4me3 on its promoter coupled with decrease in intragenic H3K27me3. This led to a BCR-independent activation of Bruton's tyrosine kinase (BTK) and NF-ĸB signaling in t(4;14) myeloma cells. Using Cancer Cell Line Encyclopedia and DepMap portal (Broad Institute) to compare the expression of BTK and BTK dependency scores across lineages revealed that BTK is an important target in MM, and REIIBP expression correlated with BTK in the CoMMpass dataset. Activation of TLR7-BTK by REIIBP conferred bortezomib resistance through the dysregulation of pro-inflammatory cytokine expression such as IL-6. Importantly, cells with REIIBP overexpression displayed enhanced lethality towards BTK inhibitor Ibrutinib, and combination with Bortezomib potentiated inhibition in myeloma cell lines and mice engrafted with RPMI8226-REIIBP tumors. Altogether, our results indicated that blockade of REIIBP in t(4;14) cells through combining proteasome and BTK inhibitors is a therapeutic strategy in the clinic for further evaluation.