An In Vivo Model To Investigate the Identity of the Cell-of-Origin of Multiple Myeloma.

Multiple myeloma (MM) is characterized by monoclonal expansion of bone marrow plasma cells. However, long-lived plasma cells resident in the marrow are terminally differentiated and possess a limited replicative lifespan; it is puzzling how they could be the source of aggressive and relapsing neoplasms. We postulate that the myeloma clonogenic progenitor may reside in a more immature compartment with greater self-renewal capacity, most probably a cell participating in, or having shortly exited the germinal center reaction. However, it is unclear whether critical mutations occur in the target cell prior to, or following commitment to the plasma cell fate. To investigate the nature of the MM cell-of-origin, we have created a novel flexible mouse model system that enables the delivery of stochastic, sequential, somatic mutations to precisely defined compartments of the germinal center in secondary lymphoid tissues. To this end, we have used BAC transgenic technology to express distinct types of avian leukosis virus (ALV) receptors, TVA and TVB, in the expanding centroblast of the dark zone and the committed plasmablast of the light zone, respectively. Mammalian tissues are refractory to transduction by retroviruses of the ALV family unless they ectopically express the cognate avian-derived receptors. Thus, the coding sequences for the TVA receptor, fused to a fluorescent protein tag were placed under the control of transcription factor A-myb, expressed in centroblasts of the dark zone. Similarly, sequences encoding a fluorescent-tagged TVB receptor were placed under the control of transcription factor Blimp1, expressed in the earliest committed plasmablasts as well as mature plasma cells. Analysis of the Blimp1: TVB mice showed that expression of the avian retroviral receptor in the hematopoietic system is limited to the light zone of germinal centers, extrafollicular collections of CD138+ cells in the spleen and lymph nodes as well as long-lived bone marrow plasma cells. Analysis of A-myb: TVA transgenic mice is currently underway. The system permits the introduction of a variety of molecular lesions to specific plasma cell precursors via retroviral transduction of oncogenes, shRNAs against tumor suppressor genes or inducible regulators of gene expression in an attempt to re-create the sequence of molecular lesions leading to MM in the relevant cellular context.