The HoxC4 homeodomain protein mediates activation of the immunoglobulin heavy chain 3′ hs1,2 enhancer in human B cells: Relevance to class switch DNA recombination

The immunoglobulin heavy chain (IgH) 3′ regulatory region modulates IgH locus transcription, upon induction by specific trans-acting factors, and plays a significant role in class switch DNA recombination (CSR) and, perhaps, somatic hypermutation (SHM). CSR and SHM are central to the maturation of the antibody response. In contrast to the single 5′-hs3a-hs1,2-hs3b-hs4- 3′ mouse IgH 3′ regulatory region, the human IgH 3′ regulatory region exists as a 5′-hs3-hs1,2-hs4-3′ cluster duplicated 3′ of Cα1 and Cα2. We show here that the human hs1,2 element is the strongest enhancer of transcription, as directed by a V_H1 or the ECS-Iγ3 promoter, thereby suggesting a dominant role for hs1,2 over hs3 and hs4 in the overall activity of the 3′ regulatory region. Within hs1,2, we identified three regions (1, 2, and 3) that are all necessary, but individually not sufficient, for enhancement of transcription. In region 2, a HoxC4 site and a HoxC4/embedded octamer (HoxC4/Oct) site are conserved across human, mouse, rat, and rabbit. These two sites recruit HoxC4 and Oct-1/Oct-2, which act synergistically with the Oca-B coactivator to effect the full hs1,2-enhancing activity. HoxC4, Oct-1/Oct-2, and Oca-B recruitment is negligible in pro-B cells, moderate in pre-B cells, and maximal in germinal center B cells and plasma cells, where HoxC4, Oct-2, and Oca-B expression correlates with hs1,2 activation and ongoing CSR. The hs1,2-mediated enhancement of V_H and C_H promoter-driven transcription as induced by HoxC4 and Oct-1/Oct-2 suggests an important role of these homeodomain proteins in the overall regulation of the IgH locus expression.