Mechanisms of osteoclast dysfunction in human osteopetrosis: Abnormal osteoclastogenesis and lack of osteoclast-specific adhesion structures

Osteoclasts from a patient affected by osteopetrosis were examined in vivo and in vitro. Iliac crest biopsy revealed an osteosclerotic pattern, with p rominent numbers of osteoclasts noted for hypernuclearity and incomplete ad herence to the bone surface. A population comprising tartrate-resistant aci d phosphatase (TRAP)-positive, multinucleated and mononuclear cells, and al kaline phosphatase-positive stromal fibroblasts was obtained in vitro from bone marrow. Mononuclear TRAP-positive precursors spontaneously fused in cu lture to form giant osteoclast-like cells. These cells expressed the osteoc last marker MMP-9 and calcitonin receptor, and lacked the macrophage marker , Fc receptor. Expression and distribution of c-src, c-fms, and CD68, and r esponse to steroid hormones relevant to osteoclast differentiation and func tion were apparently normal, whereas cell retraction in response to calcito nin was impaired. TRAP-positive multinucleated cells did not form osteoclas t-specific adhesion structures (clear zone, podosomes, or actin rings). Bon e resorption rate was severely reduced in vitro, Focal adhesions and stress fibers were observed en lieu of podosomes and actin rings. Adhesion struct ures contained low levels of immunoreactive vitronectin receptor, most of t his integrin being retained in cytoplasmic vesicles, These data provide the first characterization of abnormal differentiation and function of human o steopetrotic osteoclastlike cells.