Exploration of the role of neural-type receptors and the ECM proteins SPARC and Tenascin C in the bone marrow HSC-niche unit

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Myelodysplastic syndromes (MDS) are clonal hematologic disorders characterized by ineffective hematopoiesis, peripheral cytopenias and dysplastic bone marrow cells, with increased risk of evolution to acute myeloid leukemia (AML). MDS is not only a disease of the hematopoietic cell compartment but also alterations in the bone marrow niche influence the development of myeloid neoplasms. Based on this hypothesis, in previous joint collaborative work we have established a robust xenograft model of human MDS based on the co-transplantation of patient derived CD34+ hematopoietic stem cells (HSCs) and bone marrow (BM) derived stromal cells (MSCs). Our work revealed an instructive remodeling of the microenvironment induced by MDS HSCs to preferentially support their own growth. One of the key factors significantly overexpressed in MDS derived MSCs was the extracellular matrix protein secreted protein acidic and rich in cysteine (SPARC). SPARC has been implicated as a central factor in mediating tumor cell interaction with surrounding stroma and metastasis of a variety of solid tumors. In hematologic malignancies, we could show that loss of SPARC in the bone marrow stroma compartment protects hematopoietic cells from chemotherapy toxicity. In the current proposal we aim to use our unique MDS xenograft model to elucidate the molecular mechanisms of SPARC in order to possibly identify new therapeutic strategies that are targeted against the aberrant stroma compartment in myeloid neoplasms. To accomplish this we have crossed previous murine SPARC knockout strains with the NSG model in order to generate NSG SPARC knockout mice. Primary MDS derived and healthy bone marrow samples will be xenotransplanted into this model to study the biologic effect of absence or presence SPARC in the bone marrow niche. As a readout beside engraftment rates, the clonal bone marrow composition of the MDS xenografts will be analyzed via quantitative mutation and immunophenotypic analyses of primary patient samples and MDS xenografts. To analyze the instructive crosstalk between the hematopoietic compartment and the bone marrow stroma, we will perform whole transcriptome RNA sequencing in primary sorted stroma cell fractions, including subfractions such as endothelial cells, osteoblasts and mesenchymal stem cells after exposure to the MDS HSCs in vivo. Beyond this, we will use our model as a pre-clinical platform to test whether by manipulation of the bone marrow niche with either addition of exogenous SPARC or its inhibition, positive effects on MDS biology, clonal composition or improvement of functional blood components can be achieved. The application of pharmacologic substances that have been implicated in the biology of SPARC in the context of other tumor entities such as Bortezomib and nab-Paclitaxel will be evaluated as potential therapies targeting the bone marrow stroma of MDS.