Glucocorticoids Regulate the Splicing Factor MBNL1, a Potential Control Point for B-Cell Specification

Abstract

Glucocorticoids, such as dexamethasone and prednisone, are mainstays in the treatment of lymphoid malignancies and autoimmune diseases. To understand the mechanism of how glucocorticoids induce cell death, we integrated a comprehensive, genome-wide shRNA screen with differential gene expression analysis in B-cell acute lymphoblastic leukemia (B-ALL). In addition to activating pro-apoptosis genes and repressing anti-apoptosis genes, we found that repression of B-cell development genes contributes to dexamethasone-induced cell death in B-ALL. Similarly, elevated glucocorticoid levels brought on by stress in stroke victims causes lymphocytopenia by inducing of apoptosis in immature B-cells, but not hemopoietic stem cells or mature B-cells. Additionally, chronic stress has been associated with immune suppression and increased risk for autoimmune disease. Together these findings suggest that glucocorticoids play a normal role in B-cell development, and that elevated glucocorticoid levels in response to stress can alter that development. A control point in this mechanism may be MBNL1. MBNL1 is a sequence-specific RNA binding factor that alters mRNA splicing, stability, and localization. In this work, we show that glucocorticoid suppression of MBNL1 contributes to induction of cell death in B-ALL. Deletion of MBNL1 by CRISPR/Cas9 in the B-ALL cell line NALM-6 induces accumulation and depletion of mRNA levels for hundreds of genes. A large number of B-cell specification genes, including CD19, CD79A, SPI1, and LEF1, are repressed upon MBNL1 deletion, whereas the most highly activated gene is CD34, which is a marker for the less mature lymphoid progenitor cells. Depletion of MBNL1 also changed the splicing of B-cell development genes, including quantitative depletion of LEF1 exon 6. These data suggest that depletion of MBNL1 induces de-differentiation of B-cell precursors. Consistent with this model, an analysis of gene expression in differentiating B-cells revealed a rapid elevation of MBNL1 expression during B-cell specification (after the lymphoid precursor stage), and a concomitant repression of CELF2, an RNA-binding splicing factor that has been shown to oppose MBNL1. We therefore propose that glucocorticoids play a physiological role in B-cell development by restraining B-cell specification through repression of key genes, including MBNL1. Figure Disclosures Tasian: Aleta Biotherapeutics: Membership on an entity's Board of Directors or advisory committees; Gilead Sciences: Research Funding; Incyte Corportation: Research Funding.