Abstract
Myc-deregulating T(12;15) chromosomal translocations are the hallmark cytogenetic abnormalities of murine plasmacytomas (PCTs). In most PCTs, the immunoglobulin heavy chain (Igh) locus is broken between the Eμ enhancer and the 3’ regulatory region (3’RR), making the latter the major candidate for orchestrating Myc deregulation. To elucidate the role of the Igh3’RR in tumorigenesis, we induced PCTs in Bcl-xL-transgenic mice deficient for the major Igh3’RR enhancer elements, hs3b and hs4 (hs3b-4-/-). Contrary to previous observations using a mouse lymphoma model, which showed no tumors with peripheral B-cell phenotype in hs3b-4-/- mice, these animals developed T(12;15)-positive PCTs, although with a lower incidence than hs3b-4+/+ (wild-type, WT) controls. In heterozygous hs3b-4+/- mice there was no allelic bias in targeting Igh for T(12;15). Molecular analyses of Igh/Myc junctions revealed dominance of Sμ region breakpoints versus the prevalence of Sγ or Sα in WT controls. Myc expression and Ig secretion in hs3b-4-/- PCTs did not differ from WT controls. We also evaluated the effect of a complete Igh3’RR deletion on Myc expression in the context of an established Igh/Myc translocation in ARS/Igh11-transgenic PCT cell lines. Cre-mediated deletion of the Igh3’RR resulted in gradual reduction of Myc expression, loss of proliferative activity and increased cell death, confirming the necessity of the Igh3’RR for Myc deregulation by T(12;15).
Highlights
The two major regulatory elements in the immunoglobulin heavy chain (Igh) locus are the Eμ intronic enhancer that promotes V(D)J recombination in developing B-cells, and an enhancer cluster located downstream of all constant gene segments, termed the 3’ regulatory region (3’RR), and recently classified as a superenhancer, which has a crucial functional role [1, 2]
In most PCTs, the immunoglobulin heavy chain (Igh) locus is broken between the Eμ enhancer and the 3’ regulatory region (3’RR), making the latter the major candidate for orchestrating Myc deregulation
We evaluated the effect of a complete Igh3’RR deletion on Myc expression in the context of an established Igh/Myc translocation in ARS/Igh11transgenic PCT cell lines
Summary
The two major regulatory elements in the Igh locus are the Eμ intronic enhancer that promotes V(D)J recombination in developing B-cells, and an enhancer cluster located downstream of all constant gene segments, termed the 3’ regulatory region (3’RR), and recently classified as a superenhancer, which has a crucial functional role [1, 2]. It was shown that mice prone to lymphoma development (due to a combined p53 and DNA Ligase 4 defect) and simultaneously lacking hs3b4 enhancers overwhelmingly succumbed to pro-B cell lymphomas [21] In such pro-B cell lymphomas, Myc expression was likely deregulated by the Eμ enhancer since the Igh3’RR is poorly active at this stage. We conclude that hs3b-4 enhancer is crucial for synapsis with various parts of the Igh locus at the initiation of translocations in mature B-cells and for subsequent Myc deregulation, but becomes dispensable once a translocation-positive precursor cell differentiates to the plasma cell stage If such a “hypomorphic” Igh3’RR with a partial deletion can support Myc deregulation in plasma cell tumors, is it possible that the complete hs1,2,3a,3b,4 combination of Igh3’RR enhancers is no longer essential in established PCTs? When the whole hs1,2,3a,3b,4-containing region was deleted from a Myc-translocated chromosome using Cre recombinase, deregulated Myc expression could no longer be sustained
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