The interplay of leukemia cells and the bone marrow microenvironment

Mesenchymal cell populations contribute to microenvironments regulating stem cells and the growth of malignant cells. Osteolineage cells participate in the hematopoietic stem cell niche. Here, we report that deletion of the miRNA processing endonuclease Dicer1 selectively in mesenchymal osteoprogenitors induces markedly disordered hematopoiesis. Hematopoietic changes affected multiple lineages recapitulating key features of human myelodysplastic syndrome (MDS) including the development of acute myelogenous leukemia. These changes were microenvironment dependent and induced by specific cells in the osteolineage. Dicer1 −/− osteoprogenitors expressed reduced levels of Sbds, the gene mutated in the human bone marrow failure and leukemia predisposition Shwachman-Bodian-Diamond Syndrome. Deletion of Sbds in osteoprogenitors largely phenocopied Dicer1 deletion. These data demonstrate that differentiation stage-specific perturbations in osteolineage cells can induce complex hematological disorders and indicate the central role individual cellular elements of ‘estroma’ can play in tissue homeostasis. They reveal that primary changes in the hematopoietic microenvironment can initiate secondary neoplastic disease.

Acute myelogenous leukemia (AML) is the most common adult leukemia, characterized by the clonal expansion of immature myeloblasts initiating from rare leukemic stem (LS) cells. To understand the functional properties of human LS cells, we developed a primary human AML xenotransplantation model using newborn nonobese diabetic/severe combined immunodeficient/interleukin (NOD/SCID/IL)2r gamma(null) mice carrying a complete null mutation of the cytokine gamma c upon the SCID background. Using this model, we demonstrated that LS cells exclusively recapitulate AML and retain self-renewal capacity in vivo. They home to and engraft within the osteoblast-rich area of the bone marrow, where AML cells are protected from chemotherapy-induced apoptosis. Quiescence of human LS cells may be a mechanism underlying resistance to cell cycle-dependent cytotoxic therapy. Global transcriptional profiling identified LS cell-specific transcripts that are stable through serial transplantation. These results indicate the potential utility of this AML xenograft model in the development of novel therapeutic strategies targeted at LS cells.

Hematopoietic stem cells (HSCs) reside in a perivascular niche but the location remains controversial 1 . HSCs are rare and few can be found in thin tissue sections 2,3 or upon live imaging 4 , making it difficult to comprehensively localize dividing and non-dividing HSCs. We discovered that α-catulinGFP/+ was expressed by only 0.02% of bone marrow hematopoietic cells, including virtually all HSCs. One in 3.5 α-catulin-GFP+c-kit+ cells gave long-term multilineage reconstitution of irradiated mice, indicating that α-catulin-GFP+c-kit+ cells contain HSCs with a purity comparable to the best markers available. We were able to optically clear the bone marrow to perform deep confocal imaging, making it possible to image thousands of α-catulin-GFP+c-kit+ cells and to digitally reconstruct large segments of bone marrow. The distribution of α-catulin-GFP+c-kit+ cells indicated that HSCs were more common in central marrow than near bone surfaces and in the diaphysis relative to the metaphysis. Nearly all HSCs contacted Leptin Receptor+ and Cxcl12high niche cells. Approximately 85% of HSCs were within 10μm of a sinusoidal blood vessel. Most HSCs were distant from arterioles, transition zone vessels, and bone surfaces. This was true of Ki-67+ dividing HSCs and Ki-67− non-dividing HSCs. Dividing and non-dividing HSCs thus reside mainly in perisinusoidal niches with Leptin Receptor+Cxcl12high cells throughout the bone marrow.