Two monoclonal proteins—a urinary κ light chain and a serum γ heavy chain without an identifiable light chain—were discovered in this elderly patient. Absence of a light chain is usually due to truncation of the heavy chain, thereby preventing the binding of light chains.
The patient's presentation with anemia, leukopenia, hypoalbuminemia, and albuminuria raises the possibility of myeloma rather than metastatic prostate cancer; however, malignancies associated with monoclonal γ heavy chains are usually lymphoproliferative disorders that exhibit adenopathy rather than plasma cell myeloma with lytic bone lesions (1).
Both M proteins are low in concentration, and these findings may simply reflect a diclonal gammopathy of unknown significance in an elderly individual with an unrelated neoplasm. Monoclonal gammopathy of unknown significance (MGUS) has been found in 7.5% of patients older than 80 years, and diclonal gammopathy occurred in 3% in a population-based study (2). Individuals with MGUS may have small amounts of urinary free light chains without a serum M protein. These uncommon MGUS clones of light chains have the potential to progress to light chain myeloma (3). In 9% to 17% of patients with monoclonal γ heavy chains, there is no associated pathologic finding, and the truncated protein simply represents a rare type of MGUS (1).
Alternatively, this patient's pathologic fracture may be due to hyposecretory multiple myeloma derived from one or both clones. Rare patients with myeloma have exhibited truncated γ chains. If so, the relationship of the 2 clones to the malignancy may be determined by immunohistochemical or flow cytometry analysis. A recent report described a myeloma patient in whom all of the malignant plasma cells contained a truncated γ chain, whereas a subclone of 30% of the cells also contained light chain (4). Heavy γ chains may also cause immunoglobulin deposition disease or amyloid fibrils (5, 6), which could explain the patient's albuminuria.
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Authors' Disclosures or Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
- Received for publication February 27, 2011.
- Accepted for publication March 10, 2011.
- © 2011 The American Association for Clinical Chemistry