Molecular Pathogenesis of Multiple Myeloma

Multiple myeloma (MM) is a tumor of mature, isotype switched plasma cells. It is a uniformly fatal malignancy that is frequently preceded by a common (1% of adults), benign pre-clinical phase, monoclonal gammopathy of undetermined significance (MGUS). Our laboratory is focused on understanding the molecular events that lead to the development of MGUS, and to its progression to MM. In the last few of years we have determined that MM is characterized by recurrent chromosome translocations to the immunoglobulin heavy chain gene on 14q32. We have cloned over 35 translocation breakpoints, and identified five frequent translocation partners, that are present in almost one half of patients with MM. The translocations appear to be initiating events in the tumorigenic process, and are present in MGUS. From a detailed analysis of the breakpoints it is clear that the translocations are frequently mediated by aberrant activity of B cell specific mechanisms: somatic hypermutation and isotype switch recombination.

We have cloned six translocation breakpoints on 11q13, from 50-300kb upstream of cyclin D1, associated with the ectopic expression and dysregulation of cyclin D1 mRNA and protein, and translocation breakpoints on 6p21 that dysregulate cyclin D3. We have cloned 7 translocation breakpoints on 4p16.3, from 50-120kb upstream of Fibroblast Growth Factor Receptor 3 (FGFR3). The t(4;14) is a novel, karyotypically silent translocation associated with the ectopic expression and dysregulation of FGFR3 on der14 mediated by the 3' enhancer of the IgH locus. In addition there is dysregulation of a novel SET domain protein that we have called MMSET, mediated by the intronic enhancer on der4. We have cloned five translocation breakpoints on 16q23, within approximately 500 kb of c-maf. The t(14;16) is also a novel, karyotypically silent translocation that is associated with the ectopic expression and dysregulation of c-maf mRNA. In addition we have cloned translocation breakpoints on 20q11 that result in dysregulation of mafB, highlighting the importance of maf family members in MM

We are now using murine and in vitro models to study ectopic gene expression mediated by somatic hypermutation and isotype switch recombination, and to develop faithful mouse models of myeloma. We have generated several different transgenic vectors, both conventional, and using BACs, to ectopically express genes in plasma cells. We are analyzing transgenic mice that express FGFR3, MMSET, c-maf, cyclin D1 in plasma cells. We are studying how the ectopic expression of cyclin D1, FGFR3 and c-maf contribute to plasma cell neoplasia, and how they may be used as therapeutic targets. The novel protein MMSET is expressed in developing T cells, and germinal center B cells. We have generated a conditional murine allele to study it role in normal lymphocyte development, and we are pursuing a biochemical characterization of its function and protein partners in lymphocyte development. We are continuing our analysis and cloning of novel translocation breakpoints and oncogenes and we are using microarray gene expression analysis to identify other molecular targets in myeloma.