Histone acetyl-lysine reader Brwd1 regulates Igκ accessibility and recombination (HEM2P.260)

Abstract

Abstract The hallmark of B lymphopoiesis is sequential rearrangement of immunoglobulin heavy (Igμ) and the light chain (Igκ and then Igl). We recently demonstrated that downstream of IL-7R signaling, STAT5 represses Igκ transcription and recombination by recruiting Ezh2 and marking Igκ with H3K27me3 (Nat Immunol 12:1139). In addition to Igκ, there was a restricted program of genes similarly repressed by STAT5. Among these, Brwd1 which encodes a BROMO domain containing histone acetyl-lysine reader molecule, demonstrated a similar expression pattern to Igκ. Brwd1-/- mice had defective lymphopoiesis starting at the pre-B cell stage. Strikingly, there was a 10-fold reduction in Igκ recombination in Brwd1-/- small-pre B cells and this was associated with severely reduced H3K16 acetylation. Genome wide analyses of WT pre-B cells indicated that Brwd1 bound at the highest density to Igκ and other loci marked with both H3K9 acetylation and H3S10pK14 acetylation. Phosphorylation of H3S10 was dependent on pre-BCR mediated Erk activation. Interestingly, Brwd1 binding sites were highly enriched in extended GAGA motifs that, in Drosophila, are associated with Trl recruitment and enhanced chromosomal accessibility. These findings identify the epigenetic reader Brwd1 as a critical mediator of Igκ accessibility whose expression and recruitment to the Igκ locus are coordinately controlled by the IL-7R and pre-BCR.