Myelofibrosis osteoclasts are clonal and functionally impaired

Bone marrow (BM) sclerosis is commonly found in patients with late-stage myelofibrosis (MF). Because osteoclasts (OCs) and osteoblasts play a key role in bone remodeling, and MF monocytes, the OC precursors, are derived from the neoplastic clone, we wondered whether decreased OC numbers or impairment in their osteolytic function affects the development of osteosclerosis. Analysis of BM biopsies from 50 MF patients showed increased numbers of multinucleated tartrate-resistant acid phosphatase (TRAP)/cathepsin K¹ OCs expressing phosphorylated Janus kinase 2 (JAK2). Randomly microdissected TRAP¹ OCs from 16 MF patients harbored JAK2 or calreticulin (CALR) mutations, confirming MF OCs are clonal. To study OC function, CD14¹ monocytes from MF patients and healthy individuals were cultured and differentiated into OCs. Unlike normal OCs, MF OCs appeared small and round, with few protrusions, and carried the mutations and chromosomal abnormalities of neoplastic clones. In addition, MF OCs lacked F-actin-rich ring-like structures and had fewer nuclei and reduced colocalization signals, compatible with decreased fusion events, and their mineral resorption capacity was significantly reduced, indicating impaired osteolytic function. Taken together, our data suggest that, although the numbers of MF OCs are increased, their impaired osteolytic activity distorts bone remodeling and contributes to the induction of osteosclerosis.