Multiple myeloma (MM) is a universally fatal disease characterized by the accumulation of malignant plasma cells (PC) in the bone marrow. It is, therefore, crucial to identify the changes that occur during the progression from preneoplastic states, i.e., monoclonal gammopathy of undetermined significance (MGUS), to more aggressive disease states, e.g., MM. The growth characteristics of MM cells, and to a lesser extent PCs from MGUS patients, are in striking contrast with those of normal end-stage PCs. Despite this important phenotypic difference between MM/MGUS cells and normal PCs, the overall proliferative index of MM is quite low. The inability of current therapeutic strategies to eradicate this disease has fueled the notion that there is a subpopulation within the myeloma cell pool that is relatively resistant to agents that target more differentiated tumor cells with little or no further growth potential. Given the considerable phenotypic, morphologic, and genetic heterogeneity observed among tumor cells, we hypothesize there is a subpopulation of MM cells with extensive renewal capability that exists within the monoclonal PC pool in this disease. Similarly, this compartment is also likely to exist in MGUS and understanding clonal evolution and the differences in growth regulation of this cellular compartment in MM vs. MGUS is key to understanding disease progression. We further hypothesize that the mechanisms of growth control in this subset of monoclonal PCs from MM and perhaps MGUS patients will overlap, at least in part with mechanisms utilized by hematopoietic stem cells and/or early B cell progenitors that possess intrinsic self-renewal capabilities. There is growing evidence of the striking parallels between normal cells with self-renewal capability and cancer cells, including transformation-related reactivation of expression of transcription factors that are typically active only during normal early B lineage development. Because of our preliminary data and recent literature linking the Wnt/frizzled/13-catenin/LEF-1 and the Polycomb Group genes, Bmi-1 and EZH2, to early hematopoietic cell self-renewal and oncogenesis, we have hypothesized that genes in these two pathways have been inappropriately activated in this disease, particularly in the subset of tumor cells that has self-renewal properties. To address these hypotheses, we propose four specific aims: 1) analyze PC intraclonal homogeneity or heterogeneity (as defined by Ig VDJ or VJ sequence) in MGUS patients that do or do not progress to myeloma; 2) assess the growth potential of MM and MGUS cell subpopulations; 3) characterize the genetic and phenotypic differences that exist between non-proliferating MM/MGUS cells and MM/MGUS cells with self-renewal capability; and 4) specifically study the role of the Wnt/frizzled/13-catenin/LEF-1 pathway and the Polycomb Group genes Bmi-1 and EZH2 in MM 9rowth control.