After studying this chapter, you should understand:
The causes and consequences of bone disease, renal dysfunction, and immunodeficiency in multiple myeloma.
The diagnosis and treatment of multiple myeloma.
The major pathologic and clinical features of the other plasma cell and lymphoplasmacytic neoplasms.
Neoplasms composed of plasma cells have special biological and clinical features related to their capacity to secrete complete or partial immunoglobulin (Ig) proteins. By far, the most important of these tumors is multiple myeloma, which is diagnosed in about 15,000 patients per year in the United States. It is a tumor of older adults, with a median age at diagnosis of 69 years. For unknown reasons, it is more common and has an earlier age of onset in people of African descent.
As the name implies, at diagnosis, multiple myeloma typically involves multiple bones of the axial skeleton, particularly the vertebrae, skull, proximal long bones of the extremities, and ribs. The pathophysiology of the disease is primarily related to four factors:
Pathogenic antibodies or antibody fragments. Normal antibodies are composed of two heavy chains encoded by the IgH locus and two light chains, which may be encoded by either the Ig kappa or the Ig lambda locus. Multiple myeloma cells most commonly secrete IgG or IgA antibodies. However, in addition to complete antibodies, neoplastic plasma cells usually also secrete free, unpaired Ig light chains; indeed, in about 20% of cases, only light chains are secreted. The small size of the light chains (around 25 kDa) permits them to pass from the blood through the filtration slits of the renal glomeruli and into the renal tubules. Once in the urinary space, Ig light chains are toxic to renal epithelial cells and tend to form precipitates and obstructive casts, both of which contribute to renal dysfunction (Figure 24-1). Free light chains, particularly lambda light chains, also are prone to form amyloid, fibrillar deposits that may be found in the renal glomeruli (Figure 24-2) and the perivascular spaces of many tissues, including the liver, spleen, and heart. Renal amyloidosis often causes nephrotic syndrome, the spilling of albumin and other plasma proteins into the urine. Alternatively, instead of forming amyloid, free light chains sometimes accumulate in amorphous linear deposits in the kidney and other tissues that produce light chain deposition disease (Figure 24-3). In more than 85% of cases, linear light chain deposits are composed of kappa light chains. Light chain deposition disease most commonly presents as renal dysfunction but also can cause clinically significant hepatic or cardiac failure.
Bone resorption. Bone resorption is caused by tumor-derived factors such as MIP1α and modulators of the Wnt signaling pathway, which act together to suppress osteoblast function and enhance osteoclast function (Figure 24-4). Tilting this balance leads to marked thinning of bone, setting the stage for pathologic fractures, bone ...