A Transgenic Mouse Model of Multiple Myeloma Shows Heterogeneity in Drug Response with Alterations Due to Selective Gene Modifications.

Abstract

Multiple myeloma (MM) is a type of cancer characterized by malignant plasma cells (PCs) in the bone marrow. One difficulty in treating patients is that there is no single genetic event or abnormality common to all patients, resulting in heterogeneity of genes expressed, tumor aggressiveness, and response to therapy. In addition, mouse models of myeloma are limited. However, deregulation of MYC and BCL-XL are both common in patients' tumors. Our lab has previously shown that a transgenic mouse line targeting overexpression of c-Myc and Bcl-xL to PCs results in an early expansion of non-malignant PCs, which then develop into clonal malignancies in multiple tissue types. Tumors develop quickly (median 16 weeks) in all mice and these cells can be adoptively transferred to syngeneic recipient mice to form tumors in as little as 10 days. Our lab has analyzed cell surface markers and generated genetic profiles on these Bcl-xL/Myc tumor cells, which show genetic heterogeneity in B-cell, PC, and myeloma markers, similar to human MM. Additionally, these tumor cells can be grown in vitro. The purpose of this study was to evaluate different drug sensitivities of our Bcl-xL/Myc primary tumor lines, and to determine if the sensitivities of these Bcl-xL/Myc tumors are dependant on the deregulated expression of specific genes. After treating cells with varying concentrations of dexamethasone, melphalan, and doxorubicin for a number of days, it was discovered that different tumor lines from the Bcl-xL/Myc transgenic mouse line have a range of sensitivity, from highly resistant to extremely sensitive. Three of these cell lines show a dose-response to varying concentrations of dexamethasone. Additionally, one cell line shows evidence of IL-6 protection against dexamethasone-induced apoptosis. Next, Bcl-xL/Myc primary cell lines were treated with BH3I-2′, a Bcl-xL small molecule inhibitor, at varying concentrations. Cell lines overexpressing Bcl-xL were more resistant to BH3I-2′, compared to S194 control cells, which express Bcl-xL at normal levels. One of the Bcl-xL/Myc cell lines did not show an increase in apoptosis or decrease in cell viability, but maintained the same number of cells over four days, whereas in other cell lines, viability started to decrease after 12 hours. Bcl-xL/Myc cell lines were also treated with siRNA targeted against Ezh2 and Cyclin D1, D2 and D3 transcripts, which have been shown to be deregulated in human myeloma as well as our own cell lines. These tumor lines are being treated with dexamethasone, melphalan and doxorubicin to evaluate the cell lines' new sensitivities to chemotherapy. In total, our results show the Bcl-xL/Myc transgenic mouse tumor model demonstrates genetic heterogeneity that may be associated with heterogeneity in tumor responses. This serves as a useful model for human myeloma progression and allows us to examine the role of selective gene regulation in therapeutic response.