IL-6-driven POU2AF1 and ELL2 are key regulators of multiple myeloma-distinct transcriptional and splicing programs

Multiple myeloma (MM) is a plasma cell neoplasm that depends on the bone marrow (BM) microenvironment; however, the underlying mechanisms of epigenetic contribution to the pathogenesis of MM are incompletely understood. Here, we delineate the epigenetic-driven transcriptional and splicing regulation crucial for MM. We recharacterized transcriptional program induced by IL-6/JAK/STAT3 pathway by integrating ChIP-seq, transcriptomic analyses, and CRISPR knockout screening results, and identified B cell lineage factors, POU2AF1 and ELL2, as crucial IL-6/JAK/STAT3 targets essential for MM cell growth and survival. Genetic depletion of these factors significantly suppressed MM cell growth in vitro and in the xenograft model of IL-6 humanized mice. Mechanistically, POU2AF1 and ELL2 form an autoregulatory loop with IRF4 and establish MM-distinct transcriptional program representing immaturity, and IL-6/JAK/STAT3 pathway augments this program through upregulating and recruiting these factors to the MM-signature genes. Furthermore, POU2AF1 and ELL2 are essential in the regulation of IL-6-dependent alternative RNA splicing. Immunocytochemical and proteomic analyses revealed that POU2AF1 colocalizes and facilitates formation of nuclear speckles, where it interacts with multiple trans-acting splicing factors required for MM cell growth. These findings suggest the dual roles of POU2AF1 and ELL2 in coordinating transcription and RNA splicing to generate MM-associated mRNA isoforms. Finally, we showed that gapmer antisense oligonucleotides targeting POU2AF1 inhibit its expression and MM cell growth in the presence of soluble BM stromal cell factors, including IL-6. Our data demonstrate that IL-6-driven B cell-lineage factors are the vulnerability of MM cells and may represent novel therapeutic targets for this incurable tumor.