The search for multiple myeloma stem cells: the long and winding road

Recent years have brought significant breakthroughs in the understanding of tumor biology, related to discovery of cancer stem cells (CSCs) in acute myelogenous leukemia as well as in a number of solid tumors. This finding revealed that not all tumor cells are able to divide indefinitely, and that the bulk of tumor cells are expanded because of divisions and differentiation of CSC fraction. Although the CSCs identified in acute leukemia have a phenotype of early hematopoietic progenitors, it seems that CSCs in multiple myeloma (MM) may resemble the memory B cell fraction. Previous studies in patients with MM have documented the existence of cells without plasma cell characteristics expressing MM-type immunoglobulin genes--so-called "clonotypic" B cells. These cells have been characterized functionally and phenotypically as chemoresistant recirculating B cells. They have been found to self-renew and to be capable of initiating MM growth in immunocompromised animals. Controversy exists as to whether these cells truly belong to an MM clone, however; they may represent only the remaining clones of premalignant B cells. The identification of MM stem cells responsible for the recurrence of MM is of primary importance in designing targeted therapies to definitely cure this disease. This article summarizes the current state of knowledge on these hypothetical "MM stem cells."