IntroductionThe immunoglobulin heavy chain locus (IgH) undergoes multiple changes along B-cell differentiation, affecting transcription and accessibility to V(D)J or class switch recombination (CSR). The iE enhancer upstream of C mostly promotes V(D)J recombination, 1 whereas IgH 3ЈRR enhancers (hs3a, hs1,2, hs3b, and hs4) have controversial roles. A role in CSR was suggested by replacing mouse hs1,2 with a neomycin resistance gene, thus affecting germline transcription and CSR to ␥2a, ␥2b, ␥3, and ⑀. 2 However, deletion of this neo cassette restored CSR. 3 hs3a, hs3b, and hs4 also proved individually dispensable for CSR. 3-5 Enhancer redundancies might explain that their individual deletion only results in minor effect. Indeed, joint hs3b/hs4 deletion impaired germline transcription and CSR to most isotypes except and ␥1. 6 Reporter genes also demonstrated synergies between 3ЈRR enhancers, 7 which altogether promote CSR into large transgenes. 8 The 3ЈRR is followed with DNase hypersensitive sites (hs5-7) lacking enhancer activity but binding CCCTC-binding factor and constituting the 3Ј locus boundary. 9 To reconcile the controversial phenotypes of focal mutations, potentially attenuated by functional redundancies, we evaluated IgH expression and CSR after deleting the whole 30-kb extent of the 3ЈRR.

Burkitt lymphoma (BL) features translocations linking c-myc to an Ig locus. Breakpoints in the H chain locus (IgH) stand either close to JH or within switch regions and always link c-myc to the 3′ IgH locus control region (3′ LCR). To test the hypothesis that the 3′ LCR alone was sufficient to deregulate c-myc, we generated mice carrying a 3′ LCR-driven c-myc transgene and specifically up-regulating c-myc in B cells. Splenic B cells from mice proliferated exaggeratedly in response to various signals had an elevated apoptosis rate but normal B220/IgM/IgD expression. Although all Ig levels were lowered in vivo, class switching and Ig secretion proved normal in vitro. Beginning at the age of 12 wk, transgenic mice developed clonal lymphoblastic lymphomas or diffuse anaplastic plasmacytomas with an overall incidence of 80% by 40 wk. Lymphoblastic lymphomas were B220+IgM+IgD+ with the BL “starry sky” appearance. Gene expression profiles revealed broad alterations in the proliferation program and the Ras-p21 pathway. Our study demonstrates that 3′ IgH enhancers alone can deregulate c-myc and initiate the development of BL-like lymphomas. The rapid and constant occurrence of lymphoma in this model makes it valuable for the understanding and the potential therapeutic manipulation of c-myc oncogenicity in vivo.

Several studies have reported that regulatory elements located 3′ of the IgH locus (namely hs3a, hs1,2, hs3b, and hs4) might play a role during class switch recombination (CSR) and Ig synthesis. While individual deletion of hs3a or hs1,2 had no effect, pairwise deletion of hs3b (an inverted copy of hs3a) and hs4 markedly affected CSR and Ig expression. Among these two elements, hs4 was tentatively presented with the master role due to its unique status within the 3′ regulatory region: distal position outside repeated regions, early activation in pre-B cells, strong activity throughout B cell ontogeny. To clarify its role, we generated mice with a clean deletion of the hs4 after replacement with a floxed neoR cassette. Surprisingly, and as for previous deletion of hs3a or hs1,2, deletion of hs4 did not affect either in vivo CSR or the secretion level of any Ig isotype. In vitro CSR and Ig secretion in response to LPS and cytokines was not affected either. The only noticeable effects of the hs4 deletion were a decrease in the number of B splenocytes and a decreased membrane IgM expression. In conclusion, while dispensable for CSR and Ig transcription in plasma cells, hs4 mostly appears to contribute to Ig transcription in resting B lymphocytes.