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

<h4>Abstract</h4>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 epigenetically driven transcriptional and splicing regulation crucial for MM. We recharacterized the transcriptional program induced by interleukin 6 (IL-6)/Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) pathway by integrating chromatin immunoprecipitation sequencing, transcriptomic analyses, and CRISPR knockout screening, identifying the 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 an MM-distinct transcriptional program representing cellular immaturity. The IL-6/JAK/STAT3 pathway augments this program by upregulating and recruiting these factors to 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 trans-acting splicing factors required for MM cell growth. These findings suggest 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 reduced 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.