Osteogenic potential of mesenchymal stromal cells contributes to primary myelofibrosis

Primary myelofibrosis (PMF) is a myeloproliferative neoplasm which is a precursor to myeloid leukemia. Dysmegakaryopoiesis and extramedullary hematopoiesis characterize PMF, which is also associated with bone marrow (BM) stromal alterations marked by fibrosis, neoangiogenesis and osteomyelosclerosis. In particular, contributions to PMF from mesenchymal stromal cells (MSC) have been suggested by mouse studies, but evidence in humans remains lacking. In this study, we show that BM MSC from PMF patients exhibit unique molecular and functional abnormalities distinct from other myeloproliferative neoplasms and these abnormalities are maintained stably ex vivo in the absence of leukemic cells. PMF-MSC overexpressed heparin-binding cytokines, including pro-inflammatory TGF-β1 and osteogenic BMP-2, as well as glycosaminoglycans such as heparan sulfate and chondroitin sulfate. Transcriptome and functional analyses revealed alterations in MSC differentiation characterized by an increased osteogenic potential and a TGF-β1 signaling signature. Accordingly, phospho-Smad2 levels were intrinsically increased in PMF-MSC along with enhanced expression of the master bone regulator RUNX2, while inhibiting the endogenous TGF-β1 receptor TGFβR1 impaired osteogenic differentiation in these MSC. Taken together, our results define the source of a critical osteogenic function in PMF that supports its pathophysiology, suggesting that combined targeting of both the hematopoietic and stromal cell compartments in PMF patients may heighten therapeutic efficacy.