Abstract 4730: The double bromodomain protein Brd2 controls B cell expansion and mitogenesis.

Abstract Background: Bromodomain-containing coregulators of transcription have been identified recently as new epigenetic targets to treat a variety of hematologic malignancies, including acute myeloid leukemias. However, the bromodomain mechanisms that couple histone acetylation to transcription in lymphopoiesis, and that govern mature lymphocyte responsiveness to growth signals, are only partly understood. Brd2 is a transcriptional co-regulator that contains dual Bromodomains and an ExtraTerminal domain (defining the BET family) that couples chromatin modification to cell cycle progression. We previously reported the first functional characterization of a BET protein as an effector of mammalian mitogenic signal transduction: Eμ-Brd2 transgenic mice develop an ‘activated B cell’ type of diffuse large B cell lymphoma. There have been no other animal models reported for genetic or lentiviral expression of BET proteins, which hampers testing of novel anticancer drugs such as JQ1 and its derivatives. Methods: Here we use Brd2 lentivirus transduction of hematopoietic stem cells and reconstitution of lethally irradiated recipient mice to examine the function of Brd2 in hematopoiesis in the bone marrow and mitogenesis in the periphery. Results: Three-fold over-expression of Brd2 in hematopoietic stem cells selectively leads to a doubling of the B cell compartment and increases mature B cell mitogenic responsiveness ex vivo. Knockdown of Brd2 by shRNA totally ablates hematopoietic lineages, likely by inhibiting the lineage-committed progenitor expansion through down-regulation of cyclin A, which reinforces results from Eμ-Brd2 transgenic mice. The novel small molecule BET inhibitor JQ1 reduces B cell mitogenesis by about 60% in parallel experiments, but this effect likely extends to other related BET family members and lacks Brd2 specificity. Conclusion/Significance: The Brd2 pathway integrates lymphopoiesis, leukemogenesis and normal mitogenesis. Targeting this pathway with therapeutic drugs may therefore confer multiple, diverse benefits in the treatment of hematologic malignancy, B cell mediated disease and likely inflammation. Citation Format: Anna C. Belkina, Wanda P. Blanton, Frank L. Lombardi, Gerald V. Denis. The double bromodomain protein Brd2 controls B cell expansion and mitogenesis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4730. doi:10.1158/1538-7445.AM2013-4730