Activation Levels of b-Catenin Signaling in Osteoblasts Define a Threshold for the Development of Myeloid Neoplasia of Variable Severity

A growing body of evidence indicates that osteoblasts have a direct pathogenetic role in the development of myelodysplastic syndromes (MDS), acute myeloid leukemia (AML) and transformation of MDS to AML. Models of osteoblast-induced MDS or AML (OIAML) include b-catenin activation in osteoblasts inducing AML in mice and seen in more than 30% of MDS and AML patients; deletion of Dicer1 in osteoblast progenitors leading to MDS in mice; Noonan syndrome mutation (PTPN11) in stromal cells driving MPN in mice; and Schwachman-Diamond syndrome mutation in stromal cells driving MDS in mice and predicting AML progression in patients. To further understand the mechanism of OIAML we examined whether a defined threshold of AML-inducing signal by osteoblasts is required for disease development. For this purpose we constitutively activated b-catenin in osteoblasts using three different lines of 2.3Col1 a 1-Cre transgenic mice characterized by different recombination efficiencies in bone. Line 1, in which recombination efficiency was 80%. Line 2 was the same original line in which after several inter-breedings we detected a reduction in recombination efficiency to 20%. Line3, which has 75% recombination efficiency. Activation of b-catenin in osteoblasts using Line 1, led to the development of MDS which rapidly transformed to full-blown AML characterized by multi-organ infiltration with myeloid blasts, hematopoietic deregulation and lethality by 6 weeks of age. In contrast, activation of b-catenin in osteoblasts using Line 2 did not lead to AML development. Mice remained healthy and survived until at least 10 months of age, the entire time they were observed. Activation of b-catenin in osteoblasts using Line 3 led to anemia, with the appearance of myeloid blasts in the blood, marrow and spleen and segmented neutrophils in blood. In addition to what was observed with Line 1, Pelger Huet neutrophils and Howell-Jolly bodies were detected in the blood. The latter are RBCs with inclusions of nuclear chromatin remnants (damaged chromosome fragments) indicating genomic instability and a dysfunctional spleen, unable to clear them from the circulation, due to dense infiltration with blasts and megakaryocytes. A spectrum of atypical immature monocytes with abundant pale blue agranular cytoplasm with vacuoles were observed in the blood. Taken together these features resemble characteristics of human MDS/CMML. Mice die between 4 and 14 weeks of age, when they undergo "blast crisis" indicating transformation to AML. To examine whether and how these observations relate to the human disease, we compared the levels of b-catenin activation in osteoblasts (calculated as a function % osteoblasts (Lin-/CD34-/RUNX2+ cells) expressing activated b-catenin) with the different MDS diseases subtypes as those are defined by the 2008 World Health Organization (WHO) classification system. We found that increased levels of b-catenin activation in osteoblasts correlate with the severity of the MDS type and with conversion to AML. Collectively, these observations suggest that a minimal activation level of a key leukemogenic signal from the stroma, is required to initiate cancer development, and provides mechanistic insight into the formation and progression of preleukemic stem cells in AML and the transformation of MDS to AML.