Accessible Chromatin Landscape Reveals a Proliferative Osteoprogenitor Transcriptional Program in Bone Marrow Mesenchymal Stromal Cells from Patients with Primary Myelofibrosis

Primary myelofibrosis (PMF) is chronic myeloproliferative neoplasm characterized by clonal hematopoietic alterations contributing to myeloproliferation, egress of CD34 + cells from bone marrow to spleen associated with extra-medullary hematopoiesis, myelofibrosis with neo-ossification, leading to an osteomyelosclerosis as a result of increased CD146 + osteoprogenitor compartment. Recent studies from our laboratory have reported that bone marrow (BM) mesenchymal stromal cells (MSCs) from PMF patients (PMF-MSCs) showed a transcriptome and functional profile in agreement with an imprinted alteration of their osteogenic potential 1&2 . We performed advanced in silico analysis of accessible chromatin landscape of proliferative hFOB cells (human Fetal OsteoBlastic cell line) and identified 1183 target promoters during early phase of osteoblastic differentiation. Integration of these osteoblastic promoters in the transcriptome of PMF versus healthy donors (HD) BM-MSCs 2 revealed an overexpression of 222 genes in the mesenchymal transcriptional program of patients (False discovery rate: FDR We also showed that a majority of transcription factors link to osteoblastic differentiation such as JUN, NFATC1, SP7, DLX5, FOS, RUNX2 were down regulated in PMF-MSC transcriptome (GSEA NES=-2.79, p-value vs. differentiation transcription program in these cells. Erk1/Erk2 was found to be hyperphosphorylated in PMF-MSCs by flow cytometry. Inhibition of the MAPK pathway by PD98059 during the early phase of in vitro MSC osteoblastic differentiation reveals an increase expression of collagen, a marker of differentiation, as shown by confocal microscopy. This is less the case with myc-inhibitor I (Calbiochem), suggesting the participation of the MAPK pathway in the proliferative vs. differentiation balance of MSCs/osteoprogenitors. In conclusion, integration of the open chromatin genomic landscape of the hFOB osteoblast fetal cells in the PMF BM-MSC transcriptome reveals a proliferative program of MSCs/osteoprogenitors associated in vitro with a repression of the differentiation in accordance with an increase proliferative osteoprogenitor priming of PMF-MSCs 1 and with the presence of large size osteoblastic cells in vicinity of the neo-ossification area observed in the BM of PMF patients with osteomyelosclerosis. Martinaud C et al., Cancer Res. 2015 Nov 15;75(22):4753-65 Martinaud C et al., Genom Data. 2015 Sep; 5: 1-2 Disclosures Vannucchi: Novartis: Honoraria, Speakers Bureau.