Abstract 499: IgH enhancers hs3b/hs4 are dispensable for c-Myc deregulation in mouse plasmacytomas with T(12;15) translocations

Abstract

Abstract c-Myc deregulating t(12;15) chromosomal translocation is the hallmark cytogenetic abnormality of murine plasmacytomas (PCTs). In the great majority of PCTs, the immunoglobulin heavy chain (IgH) locus is broken between Eμ enhancer and the 3’ regulatory region (3’RR), making the latter the major candidate for orchestrating c-Myc deregulation. To elucidate the role of 3’RR in tumorigenesis, we induced PCTs in mice deficient for the major 3’RR enhancer elements, namely hs3b and hs4 (3’KO). Unexpectedly, and contrary to the previous observations made in a mouse lymphoma model, Hs3b/Hs4 3’KO mice did develop t(12;15)-positive PCTs, although with a lower incidence than wild-type controls. In heterozygous mice, there was no allelic bias in targeting IgH for the t(12;15). Molecular analysis of IgH/Myc junctions revealed dominance of Sμ region breakpoints compared to the prevalence of hits into Sγ or Sα in the controls. Analysis of c-Myc expression as well as Ig secretion in 3’KO PCT cell lines revealed no significant differences from the controls. Our current findings highlight the complexity of the 3’RR and the potential of its components to compensate for each other in the context of differentiation from B-lymphocyte to plasma cell. We next extended our observations to include a PCT model where Cre-mediated deletion can remove all four 3'IgH enhancers: hs3a, hs1,2, hs3b and hs4. In PCT cell lines where c-Myc is translocated to an IgH BAC transgene with a floxed 3’IgH enhancer region, induction of retrovirally-expressed Cre-recombinase causes gradual Myc downregulation and complete cell death within 96 hours. The later observation confirms the requirement of the full 3’RR for c-Myc deregulation by T(12;15). Citation Format: Alexander L. Kovalchuk, Herbert C. Morse. IgH enhancers hs3b/hs4 are dispensable for c-Myc deregulation in mouse plasmacytomas with T(12;15) translocations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 499. doi:10.1158/1538-7445.AM2017-499