Targeted Overexpression of Mutant Activated N-ras Leads to Aberrant Plasma Cell Biology.

The ras oncogene regulates a variety of cellular functions, and its dysregulation has been implicated in a variety of human cancers, including multiple myeloma. Indeed, activating ras mutations have been described in 35 – 50% of myeloma patients, 50% of human myeloma cell lines, and 12.5% of patients with monoclonal gammopathy of undetermined significance (MGUS). Given the higher incidence of activating ras mutations in myeloma compared to MGUS, the current models of myelomagenesis suggest that activating ras mutations are involved in the progression of MGUS to myeloma. While there has been a fairly extensive analysis of activating ras mutations in myeloma patients, there have been few studies to investigate the biology of an activated ras mutation in the context of B- and plasma cell development and tumorigenesis. We previously described a transgenic platform that uses the 3′ kappa immunoglobulin light chain enhancer (3′KE) to target transgene expression to B-cells in late developmental stages, including plasma cells (Blood103: 2779, 2004). To study the potential influence of elevated mutant ras expression on B- and plasma cell survival and proliferation, we used the 3′KE to generate a 3′KE/N-ras V12 transgenic mouse. We hypothesized that the presence of the mutant ras gene would affect normal B- and plasma cell homeostasis. Indeed, samples of mononuclear splenocytes from 4-week-old transgenic mice demonstrate a 70% increase in the number of B220+kappa+ B-cells and a 250% increase in the number of CD138+B220hi plasmablastic cells compared to littermate controls. While survival of the 3′KE/N-ras V12 mice appears similar to littermate controls and transgenic animals do not develop tumors at 35 weeks of age, aberrant lymphocyte biology was noted in multiple founder lines. All aged 3′KE/N-ras V12 transgenic founders demonstrated an immunoglobulinemia. Interestingly, the animal with the highest transgene copy number had the least pronounced immunoglobulinemia, while the animal with the lowest transgene copy number had the most pronounced immunoglobulinemia, suggesting an inversely dose-dependent relationship between over-expression of an activated Ras protein and immunoglobulinemia. We performed extensive necropsies and histopathological analyses on all founder mice and aged-matched littermate controls. While no tumors were found in any of the mice, three of the founder mice demonstrated abnormal accumulations of plasma cells in extramedullary sites, such as the kidney. These data indicate that an activated ras transgene can affect B- and plasma cell homeostasis, and this transgenic model could prove useful in studying the role of activating ras mutations in plasma cell tumorigenesis. We are currently using three targeted c-myc gene expression systems to elicit B- and/or plasma cell tumors by co-expressing c-myc and N-ras V12.