The Perils of Abrogating Myc-dependent Apoptosis
new paradigm is emerging in cancer biology owing to the growing realization that oncogenes, which were once believed to merely provide a growth advantage to incipient cancer cells, can paradoxically also put these cells at grave risk of suicide by apoptosis. The master regulator Myc is the quintessential oncogene in this regard. Although Myc is required in normal and malignant cells for cell growth and cell cycle progression, it effectively kills cells when it is overexpressed or deregulated in stressed environments. Myc-dependent pathways that trigger the apoptotic response have come to be understood as important checkpoints that delete cells that had undergone Myc-induced neoplastic transformation. Not surprisingly, these pathways are often selectively bypassed during tumor development.
More than 20 years ago, a series of studies in the laboratories of Michael Potter, MD, and Frederic Mushinski, MD, at NCI's Laboratory of Genetics, and by others around the world, established that peritoneal mouse plasmacytomas, neoplasms of immunoglobulin-producing plasma cells, are induced by reciprocal chromosomal translocations that result in the deregulated expression of Myc. The most common Myc-activating translocation (~90%) is the T(12;15), which joins the Myc gene to the immunoglobulin heavy-chain locus, Igh, juxtaposing Myc, in approximately 85% of cases, to the most downstream Igh constant gene, Cα. Following chromosomal translocation, Myc expression is driven by the powerful Igh 3'-Cα enhancer, which—in accordance with its physiological function of enhancing immunoglobulin heavy-chain expression in plasma cells—reaches peak activity in plasma cells.
Figure 1. Generation of iMycCα transgenic mice. Shown are the normal mouse Igh locus (top) and the targeted Igh locus with the inserted MycHis gene (bottom). The transcriptional orientations of Igh and MycHis are indicated by black and red arrows, respectively. The 3'-Cα enhancer is depicted as a black diamond. This is the first time gene insertion in mice has been used to reproduce an oncogene-activating chromosomal translocation of great relevance to human cancer.
We hypothesized that mimicking the Myc-Cα juxtaposition by gene insertion in mice might result in a good model of T(12;15) translocation and, thereby, recapitulate the mode of Myc deregulation that is conducive to plasmacytoma development. In collaboration with Lino Tessarollo, PhD, from NCI's Mouse Cancer Genetics Program, we generated Myc transgenics that harbor a single-copy histidine-tagged mouse Myc gene, MycHis, inserted head-to-head into the mouse Cα locus. We refer to these mice as iMycCα (Figure 1). Somewhat disappointingly, the iMycCα mice developed plasma cell tumors infrequently and only after a long latency. This suggested that although the Myc transgene reproduced the requisite molecular changes that initiate neoplastic plasma cell development in mice, Myc’s true oncogenic potential in vivo was tempered, possibly by Myc-dependent apoptosis.
To test this possibility, we crossed the Myc transgenic mice with Bcl2l1 (Bcl-XL) transgenic mice that were recently developed in the laboratory of Brian Van Ness, PhD at the University of Minnesota. In this mouse strain, the expression of the death suppressor Bcl2l1 is driven by the mouse immunoglobulin κ light-chain 3' enhancer, which exhibits, just like the Igh 3'-Cα enhancer, peak activity in plasma cells. Subsequent studies with the research group of Roberto Polakiewicz, PhD at Cell Signaling Technology, Inc., Beverly, MA, showed that similar to the iMycCα mice, single transgenic Bcl2l1 mice demonstrated a weak tumor phenotype. In contrast, the double transgenic Myc-Bcl2l1 mice developed plasma cell tumors with short onset (135 days on average) and full penetrance (100% tumor incidence). These tumors produced monoclonal immunoglobulin, infiltrated the bone marrow, and caused in some cases, osteolytic lesions leading to pathological bone fractures.
Deregulated expression of MYC and death suppressor genes of the BCL2 family, such as BCL2L1 and MCL1, is a consistent feature of human plasma cell neoplasms, including multiple myeloma, which comprises the second most common blood cancer in the United States. Multiple myeloma is further characterized by osteolytic bone lesions and pathological fractures. Our studies showed that the enforced expression of Myc and Bcl2l1 by immunoglobulin enhancers with peak activity in plasma cells generates a mouse model of human multiple myeloma that may be useful in elucidating the mechanism of the Myc-Bcl2l1 collaboration and in designing new approaches for treatment and prevention of human multiple myeloma.