Dual Mechanism of Rag Gene Repression by c-Myb during Pre-B Cell Proliferation

ABSTRACT

Developing B lymphocytes undergo clonal expansion following successful immunoglobulin heavy chain gene rearrangement. During this proliferative burst, expression of the Rag genes is transiently repressed to prevent the generation of double-stranded DNA (dsDNA) breaks in cycling large pre-B cells. The Rag genes are then reexpressed in small, resting pre-B cells for immunoglobulin light chain gene rearrangement. We previously identified c-Myb as a repressor of Rag transcription during clonal expansion using Abelson murine leukemia virus-transformed B cells. Nevertheless, the molecular mechanisms by which c-Myb achieved precise spatiotemporal repression of Rag expression remained obscure. Here, we identify two mechanisms by which c-Myb represses Rag transcription. First, c-Myb negatively regulates the expression of the Rag activator Foxo1, an activity dependent on M303 in c-Myb's transactivation domain, and likely the recruitment of corepressors to the Foxo1 locus by c-Myb. Second, c-Myb represses Rag transcription directly by occupying the Erag enhancer and antagonizing Foxo1 binding to a consensus forkhead site in this cis-regulatory element that we show is crucial for Rag expression in Abelson pre-B cell lines. This work provides important mechanistic insight into how spatiotemporal expression of the Rag genes is tightly controlled during B lymphocyte development to prevent mistimed dsDNA breaks and their deleterious consequences.

KEYWORDS:

• B cell development
• V(D)J recombination
• c-Myb
• chromatin remodeling
• transcriptional regulation

View publisher note:

Articles of Significant Interest Selected from This Issue by the Editors

ACKNOWLEDGMENTS

We thank the members of the Schlissel and Tjian laboratories for helpful advice and discussion. We also thank Hector Nolla, Alma Valeros, and Kartoosh Heydari of the UC Berkeley Flow Cytometry core for their technical assistance. Finally, we thank Shawn Fahl and Tim Bender (University of Virginia) for floxed c-Myb AMuLV cell lines and Lee Krause (UT Southwestern) for p300 cDNA.

This work was supported by NIH RO1 HL48702 awarded to Mark S. Schlissel, a California Institute of Regenerative Medicine (CIRM) postdoctoral fellowship (TG2-01164) awarded to Liangqi Xie, and a CIRM predoctoral fellowship (TG2-01164) awarded to Greg A. Timblin.