Intranuclear inclusions in myeloma cells in a case of nonsecretory multiple myeloma

Inclusions in the nucleus, compared with those in the cytoplasm, are rare in myeloma cells but have been reported in all electrophoretic varieties of multiple myeloma except the nonsecretory type. In this unusual case, a 54 year old Chinese woman had a pathological fracture of the left femur, and biopsy of the fracture site revealed a round cell tumor compatible with plasmacytoma. A bone marrow aspirate revealed 50% plasma cells, many of which contained intranuclear inclusions. Protein electrophoresis was normal with no paraprotein, and urine was free from Bence-Jones protein. Under electron microscopy, the plasma cells showed electron-dense spherules not circumscribed by a membrane. The absence of a membrane was unusual, because according to all reported cases, these intranuclear inclusions were invariably membrane-bound. The association of nonsecretion of paraprotein in myeloma, which is rare, and the absence of a membrane enclosing the intranuclear inclusions, which is heretofore unreported, is probably not coincidental but causally related in that paraprotein produced in the nucleus of myeloma cells (stored in the form of intranuclear inclusions) fails to be detected in serum and urine because of noninteraction between these inclusions and the membranes of the nucleus and endoplasmic reticulum.