Bone Abstracts

An IgM monoclonal gammopathy (MGUS) is often the first sign of a lymphomonocytic B-lymphoma (Waldenström macroglobulinemia-WD). Osteolytic lesions can occur in B cell malignancies (WD, hairy cell leukemia, LLC\..) but are less frequent than in myeloma. In addition, bone remodeling in WD is poorly understood. However, an osteoporosis is often observed in MGUS patients. We studied a series of bone biopsies performed in patients with an IgM gammopathy by histomorphometry, microCT and immunohistochemistry. All patients had a double tetracycline labeling; identification and counting of osteoclasts was done after TRAcP staining. 45 patients (9 women and 36 men) with an M-protein level of 16.8±10.9 g/l were studied. There was a majority of IgM κ chain (2/3). 15 patients had vertebral fractures. T-score was <−2 at the femoral upper extremity in 1/3 of patients. Bone marrow invasion (in the form of infiltration or nodules) was associated with B-cells expressing κ or λ chain, CD45, CD138 and IgM which characterized the lymphoma cells. Trabecular volume and cortical thickness were not significantly decreased. The most typical findings were a reduction in bone formation with decreased osteoid parameters, mineralization surfaces (MS/BS) and bone formation rates (Aj.Ar, BFR/TV, BFR/BV, BFR/BS). The eroded surfaces were increased in almost all patients and appeared due to microresorption. The presence of a significant contingent of small mononuclear TRAcP+ osteoclasts was seen in ~80% of patients. MicroCT aspects of trabecular erosion were visible only when the eroded surfaces were considerably increased. During WD, there are marked alterations in bone remodeling characterized by an intense osteoblastic depression associated with microresorption. Bone fragility observed in these patients may reflect the altered bone quality with inability to restore vertebral microdamages due in part to the microresorption associated with a dramatic reduction of the osteoblast activity.