Abstract 2040: Acute myeloid leukemia cells induce new bone formation by expanding osteoprogenitor niche in the bone marrow

Genetic alterations in osteoprogenitor cells have been shown to induce myeloid leukemia in mouse models. We reported recently that acute myeloid leukemia (AML) cells induce osteogenic differentiation in mesenchymal stromal cells (MSC) in the bone marrow (BM) to facilitate faster AML engraftment in mice (Battula et al., JCI Insight, 2017). However, specifics of this osteogenic niche generated by AML are not known. Here, we hypothesize that AML cells induce new bone formation by expanding osteoprogenitor-rich niche in the BM. To investigate the effect of AML cells on osteoprogenitor cells and mature osteoblasts, we generated transgenic mice by crossing Osx-CreERt2 mice with Ocn-GFP; ROSA-tdTomato mice. The resulting triple transgenic mice had the genotype of Osx-CreERt2;Ocn-GFP;ROSA-tdTomato. In these mice the tdTomato (red) positive cells represented cells that originated from Osterix-expressing (Osx+) cells, whereas a GFP+ (green) cell represented an osteocalcin-expressing (Ocn+) mature osteoblast. We implanted murine AML cells with MLL-ENL fusion proteins into these mice. Three weeks after implantation of AML cells, the femurs and tibia of these mice were subjected to histological evaluation using whole-tissue fluorescence microscopy. Interestigly, in the BM of mice implanted with AML cells, we found a 3-4 fold increase in Osx+ cells compared to control animals. However, no significant difference in the number of GFP+ cells on the endosteum and trabecular bone surface was observed, suggesting that AML cells expand osteoprogenitor cells in the BM. Next, to investigate AML-induced alterations in bone, we implanted AML patient-derived xenograft (PDX) cells with the FLT3-ITD translocation into non-obese diabetic scid interleukin-2Rγnull (NSG) mice. When the percentage of AML blasts in peripheral blood increased over 95%, we performed micro-computed tomography (µCT) to analyze changes in the bone architecture. Of note, we observed a massive increase in cortical bone thickness and new medullary bone formation in the diaphysis area. Quantitative analysis revealed that the bone volume and bone mineral content in AML mouse femurs were about twofold higher than those in healthy mouse femurs. To validate these findings, we stained femurs from mice with and without AML with Masson-Goldner's trichrome reagents. Notably, staining of the femurs from mice with AML revealed massive web-like bone formation within the medullary cavity, which is usually not seen in normal BM