c-Myc-induced B cell and plasma cell neoplasms in mice

我们继续研究由染色体易位T(12；15)引起的小鼠B细胞和浆细胞肿瘤的发病机制，这些易位导致原癌基因Myc的表达失控。T(12；15)易位是人类Burkitt淋巴瘤中最常见的MYC激活t(8；14)(q24；q32)易位的直接对应。在上个财政年度，我们在三个项目领域取得了重大进展： 我们进一步了解了T(12；15)易位的起源，证明了异常免疫球蛋白(Ig)类开关重组(CSR)通过一种间接机制促进易位：Ig重链(IgH)序列转座到Myc。在IL-6转基因BALB/c小鼠B细胞克隆的Myc第一内含子中，发现了5个含有内含子重链增强子Em和截短开关m区(Sm)的IgH转座子。2例Em/Sm易位启动Myc参与跨染色体CSR到CG1，导致T(12；15)。我们的模型认为，Em/Sm转座到Myc使Myc敏化T(12；15)易位，而T(12；15)易位反过来将癌基因并列到重链钙增强子。最终结果是一种有利于B细胞和浆细胞肿瘤形成的Myc去调控模式。 通过在转基因小鼠中插入基因，我们模拟了人类BLt(8；14)易位和同源小鼠浆细胞瘤(PCT)T(12；15)易位的三种不同状态。我们将新开发的基因插入菌株称为iMyc小鼠。这些小鼠容易发生肿瘤性B细胞和浆细胞发展，包括淋巴母细胞B细胞淋巴瘤(LBL)、弥漫性大B细胞淋巴瘤(DLBCL)和浆细胞瘤(PCT)。这些都是成熟的B细胞肿瘤，与人类的B细胞来源的非霍奇金淋巴瘤高度相关。我们最近未发表的工作表明，在iMyc-Em小鼠中出现的大约50%的LBL表现出Myc-Arf-MDM2-P53肿瘤抑制轴的扭曲。一些没有中断这一轴的肿瘤过度表达Survivin，推测是为了抵消Myc依赖的细胞凋亡。PI3K-Akt激活所致的组成性核因子-kB信号在肿瘤进展中起着重要作用。这些发现表明，iMyc模型系统为设计和测试治疗和预防人类B细胞和浆细胞肿瘤的新方法提供了一个极好的机会。 由于MYC和BCL-XL的表达失控是人浆细胞肿瘤(PCN)的共同特征，我们决定研究Myc和BCL-XL在小鼠浆细胞中的靶向表达是否可能导致一种改进的人PCN模型。为此，我们将我们新开发的iMyc-Ca小鼠与新产生的含有由小鼠Igk3‘增强子驱动的小鼠bclxl转基因的转基因小鼠进行了杂交。单个转基因bcl-xl小鼠在380d龄时仍保持无瘤状态，而单个转基因Myc小鼠很少发生B细胞肿瘤(9.3%)。相比之下，双转基因Myc/bclxl小鼠发生浆细胞肿瘤的时间短(平均135天)，外显率高(100%肿瘤发生率)。这些肿瘤产生单克隆性免疫球蛋白，渗透到骨髓，在某些情况下，导致溶骨性损害和病理性骨折。因此，我们证明了在浆细胞中具有最高活性的Ig增强子促进Myc和bclxl的表达，产生了一个概括了人类多发性骨髓瘤关键特征的人PCN的小鼠模型。

We have continued our studies on the pathogenesis of mouse B cell and plasma cell neoplasms that are induced by chromosomal translocations T(12;15) that result in the deregulated expression of the proto-oncogene Myc. The T(12;15) translocation is the direct counterpart of the MYC-activating t(8;14)(q24;q32) translocation most commonly seen in human Burkitt lymphoma (BL). In the past fiscal year we have made significant advances in three project areas: We furthered our understanding of the origin of the T(12;15) translocation by demonstrating that aberrant immunoglobulin (Ig) class switch recombination (CSR) facilitates translocation by an indirect mechanism: transpositon of Ig heavy-chain (Igh) sequences to Myc. Five Igh transposons containing the intronic heavy-chain enhancer Em and a truncated switch m region (Sm) were found inserted in the first intron of Myc in B-cell clones of IL-6 transgenic BALB/c mice. In two cases Em/Sm transposition primed Myc to get involved in trans-chromosomal CSR to Cg1, leading to T(12;15). Our model proposes that Em/Sm transposition to Myc sensitizes Myc to undergo T(12;15) translocation, which, in turn, juxtaposes the oncogene to the heavy-chain Ca enhancer. The final outcome is a mode of Myc deregulation that is conducive to B cell and plasma cell tumor formation in mice. Using gene insertion in transgenic mice, we have mimicked three different states of the human BL t(8;14) translocation and the homologous mouse plasmacytoma (PCT) T(12;15) translocation. We refer to the newly developed gene-insertion strains as iMyc mice. The mice are prone to neoplastic B cell and plasma cell development, including lymphoblastic B-cell lymphoma (LBL), diffuse large B cell lymphoma (DLBCL) and plasmacytoma (PCT). Together these are tumors of mature B cells that are highly relevant for B-cell derived non-Hodgkin's lymphomas in humans. Our most recent unpublished work has shown that approximately 50% of LBL that arise in the iMyc-Em mice exhibit distortions in the Myc-Arf-Mdm2-p53 tumor suppressor axis. Some tumors without interruptions of this axis over-express survivin, presumably to counterbalance Myc-dependent apoptosis. Constitutive NF-kB signaling due to PI3K-Akt activation appears to play an important role in tumor progression. These findings suggest that the iMyc model system affords an excellent opportunity to design and test new approaches for the treatment and prevention of human B cell and plasma cell tumors. Since deregulated expression of MYC and BCL-XL are consistent features of human plasma cell neoplasms (PCN), we decided to investigate whether targeted expression of Myc and Bcl-XL in mouse plasma cells might lead to an improved model of human PCN. To that end, we crossed our newly developed iMyc-Ca mice with newly generated BCL-XL transgenics that contain a mouse BCL-XL transgene driven by the mouse Igk 3' enhancer. Single transgenic BCL-XL mice remained tumor free by 380 days of age, whereas single transgenic Myc mice developed B-cell tumors infrequently (9.3%). In contrast, double transgenic Myc/BCL-XL mice developed plasma cell tumors with short onset (135 days on average) and full penetrance (100% tumor incidence). These tumors produced monoclonal Ig, infiltrated the bone marrow and caused, in some cases, osteolytic lesions with pathological bone fractures. We thus demonstrated that the enforced expression of Myc and BCL-XL by Ig enhancers with peak activity in plasma cells generates a mouse model of human PCN that recapitulates key features of human multiple myeloma.