Trimolecular Complexes of λ Light Chain Dimers in Serum of a Patient with Multiple Myeloma

Abstract

Patients with multiple myeloma often have Bence Jones proteins composed of free monoclonal light chains of the kappa or lambda type in their urine. Usually, these light chains exist as monomeric or dimeric forms, but rarely, larger molecules, such as tetramers, have been reported in the serum. We report the presence of trimeric complexes of lambda light chain dimers in a patient who was diagnosed with a free lambda light chain multiple myeloma 2 years earlier and subsequently underwent a stem cell transplant. Recently, the patient presented with a large serum M-spike (23 g/L) by protein electrophoresis. The spike consisted of monoclonal lambda light chains without a heavy chain. The urine contained only 8 mg of lambda light chain in a 24-h specimen. Quantitative analysis of the serum and urinary free light chains (FLCs) indicated the probability of larger aggregates of FLCs. Size-exclusion chromatography, electrophoresis, analytical ultracentrifugation, and mass spectrometric studies of the serum revealed almost exclusively the presence of trimolecular aggregates of lambda light chain dimers without other multimeric species. Monoclonal lambda light chains may present as hexameric aggregates that cannot be cleared by renal excretion.