A Trangenic Mouse Model of Plasma Cell Malignancy Shows Cytogenetic and Gene Expression Profiles Similar to Human Multiple Myeloma.

Multiple myeloma is an incurable plasma cell malignancy for which existing animal models are limited. Human plasma cell tumors are genetically diverse, with no single chromosomal abnormality defining the disease, however, dysregulation of the genes c-myc and bcl-xl are both commonly observed. We have previously shown that targeted expression of c-myc and bcl-xl transgenes in mouse plasma cells produces malignancy which displays features of human myeloma such as localization of tumor cells to the bone marrow and lytic bone lesions. Tumors are also present at extramedullary sites (Cheung et al., J. Clin. Invest. 113: 1763, 2004). Tumors rapidly develop (median 16 weeks) in 100% of mice, and can be adoptively transferred to syngeneic controls using as few as 1 million tumor cells to produce tumors in as few as 10 days. Adoptive transfer of similar cell numbers from younger double transgenic mice, without evidence of malignancy, results in increased tumor latency (>8 weeks) or the absence of tumor formation, suggesting that an accumulation of genetic changes is required for tumor development. In order to understand the specific genetic alterations required for tumor progression and for localization of tumors to the bone marrow vs extramedullary sites, we have undertaken a detailed analysis of plasma cell tumors in myc/bcl-xl mice and have begun to compare them with human multiple myeloma. Analysis of cell surface markers shows the majority of tumors have a plasmablast phenotype, expressing CD138+, B220+, CD38+, and CD19+. This result is confirmed by RT-PCR for B cell and plasma cell specific markers Pax5, Xbp1 and Blimp1, which can be detected in tumor samples. In addition, transcripts for Mip1α, EZH2, and Dusp6, genes shown to be upregulated in human myeloma, can also be detected in the mouse myc/bcl-xl tumors. Spectral karyotype analysis of metaphase chromosomes from primary tumor cell cultures demonstrates that a variety of chromosomal abnormalities are present in mouse tumors, including trisomies and translocations, similar to what is observed in human myeloma. The most frequently aberrant chromosomes are 12 and 16, followed by chromosomes 1 and 4. Interestingly, two common sites for translocations were identified; 12F which corresponds to the mouse immunoglobulin heavy chain locus, and 4D, which corresponds to a genomic region containing genes for plasma cell tumor susceptibility (Bliskovsky et al., PNAS 100:14982, 2003). Further characterization of these translocations are being done to identify the precise breakpoints involved, and analysis of gene expression by RT-PCR and microarray analysis will be correlated to specific chromosomal abnormalities. Additionally, global gene expression profiles from myc/bcl-xl tumor cell cultures have been compared to existing profiles of human myeloma (Zhan et al., Blood 99: 1745, 2002). Our preliminary comparison of gene expression profiles from myc/bcl-xl tumors to human myeloma tumors with high myc expression show the mouse tumors are more similar to human tumors than to normal plasma cells. These data suggest the myc/bcl-xl mouse tumors are similar to a subset of human myelomas, and will provide insight into the specific genes and pathways underlying human disease.