Evidence for distinct human cancer stem cells (CSCs) remains contentious and the degree to which different cancer cells contribute to propagating malignancies in patients remains unexplored. In low- to intermediate-risk myelodysplastic syndromes (MDS), we establish the existence of rare multipotent MDS stem cells (MDS-SCs), and their hierarchical relationship to lineage-restricted MDS progenitors. All identified somatically acquired genetic lesions were backtracked to distinct MDS-SCs, establishing their distinct MDS-propagating function in vivo. In isolated del(5q)-MDS, acquisition of del(5q) preceded diverse recurrent driver mutations. Sequential analysis in del(5q)-MDS revealed genetic evolution in MDS-SCs and MDS-progenitors prior to leukemic transformation. These findings provide definitive evidence for rare human MDS-SCs in vivo, with extensive implications for the targeting of the cells required and sufficient for MDS-propagation.

IntroductionMultiple myeloma (MM) is an incurable plasma cell (PC) malignancy of the bone marrow (BM). Although acquired genetic events and the tumor microenvironment are well-established regulators of myeloma cell survival and proliferation pathways, the identity and functional properties of the myeloma-propagating cells have been a matter of controversy. 1,2 The terminal differentiation of normal mature B lymphocytes to immunoglobulin (Ig)-secreting PCs entails conversion of antigennaive to antigen-experienced B cells in the germinal center of secondary lymphoid organs and their subsequent differentiation to either memory B cells or PCs. 3,4 Each stage of B-cell differentiation can be defined by surface markers with naive and memory B cells expressing CD19 and terminally differentiated normal and malignant PCs, but not B cells, expressing CD138 (Syndecan-1). 5,6 Given this linear B-cell lineage developmental process, it was suggested that myeloma cell growth is sustained by a minority of cells more immature than the PC. This hypothesis is supported by the presence of CD19 ϩ CD138 Ϫ clonotypic B cells (ie, cells sharing the same Ig heavy chain [IgH] complementarity region 3 [CDR3] sequence with the myeloma PCs) in peripheral blood (PB) and BM of patients with MM. 7-10 Indeed, because CD138 Ϫ but not CD138 ϩ PCs were found to lead to myeloma engraftment in NOD/SCID mice, it was proposed that CD138 Ϫ cells were the principal myeloma-propagating or "myeloma stem" cells [11][12][13][14] Earlier studies, For personal use only. on May 9, 2018. by guest www.bloodjournal.org From though, using a huSCID mouse model, had concluded that mature PCs (defined as CD38 hi CD45 Ϫ ), and not the CD19 ϩ B-cell fraction, contained the entire myeloma-propagating activity, 15 whereas more recently, CD19 Ϫ CD138 Ϫ as well as CD138 ϩ cells engrafted SCID-rab mice with myeloma. 16 Furthermore, whereas earlier studies reported that the myeloma side population is enriched in clonogenic activity and identifies with CD138 Ϫ but not CD138 ϩ myeloma cells, 13 recent evidence shows that both CD138 ϩ and CD138 Ϫ cells are included in the highly clonogenic myeloma side population. 17 Whether these discrepancies result from different animal models and phenotypic definitions of PC is not clear. Here, through a detailed phenotypic and genetic analysis of primary human myeloma cells and a prospective, dynamic ex vivo and in vivo study of the constituents of the myeloma cellular architecture, we show that a phenotypic and functional interconvertible state between CD138 ϩ and CD138 Ϫ cells underpins myelomapropagating activity and clinical drug resistance. Methods Patient and normal donor BM and PB samplesPatient BM and PB samples were obtained after written informed consent and appropriate institutional ethics committee approval. Patient characteristics are shown in supplemental Table 1 (available on the Blood Web site; see the Supplemental Materials link at the top of the online article). Diagnosis, remission, and relapse of MM were defined accordi...

No abstract

In the version of this article originally published online on May 15, 2014, there was an error in the presentation of the data in Figure 1G. The colors used to represent myeloid and erythroid colony-forming cells (CFCs) for del(5q) patients were inversed, such that myeloid CFCs were shown in black and erythoid CFCs in gray. In both the print and online versions of the article, this error has now been corrected, with the myeloid CFCs being shown in gray and the erythroid CFCs in black.