Myeloma protein toxicities include: 

Monoclonal free light chains, free heavy chains, or a mixture of these chains can deposit in the kidney and other organs to cause systemic monoclonal immunoglobulin deposition disease; free κ or λ light chains can deposit selectively in the kidneys proximal tubule to cause light chain proximal tubulopathy or in the kidneys distal tubule to cause light chain myeloma cast nephropathy; and various myeloma proteins can deposit selectively in the kidneys glomeruli to cause various forms of organized deposit and non-organized deposit glomerulonephritis diseases. Free κ or λ light chains can aggregate with each other to cause extracellular amyloid deposits and a disease termed amyloidosis in which the deposits injure and ultimately lead to the failure of such organs as the kidney, heart, liver, stomach, and intestines; these deposits can also cause peripheral and autonomic neuropathies. IgM myeloma proteins or in rare cases other myeloma proteins such as IgA, free κ light chains, or free λ light chains may increase blood viscosity, deposit in peripheral blood vessels, and thereby cause vascular occlusion and gangrene of the extremities in the syndrome termed cryoglobulinemia. Monoclonal IgM myeloma proteins operating through their effects on increasing blood hyperviscosity can reduce blood flow to the central nervous system to cause blurred vision, headaches, vertigo, ataxia, and cold-induced hemolytic anemia. IgM, IgG, and to lesser extents κ and λ free light chain myeloma proteins can cause Immune thrombocytopenic purpura with extensive bleeding tendencies .
In the most malignant form of plasma cell dyscrasias, primary plasma cell leukemia, the plasma cell population contains >1900 distinct DNA alterations in >600 genes.In general, the plasma cell dyscrasias are defined by 1) the presence of these genetically unstable clonal plasma cells, lymphoplasmacytoid cells, or B cells infiltrating the bone marrow or forming distinct masses in bone, and/or other tissues as defined by biopsy of involved tissues and 2) the presence of these cells myeloma proteins (i.e. intact monoclonal antibody, free light chain, free heavy chain, shortened version of these proteins, or any combination of these proteins) in blood and/or urine as defined by various types of gel electrophoresis. Obviously, the latter criterion does not apply to the rare cases of true non-secretory myeloma. Myeloma protein toxicity
Myeloma proteins form as a result of gene mutations rather than physiological gene remodeling responses to an instigating foreign antigen: typically these proteins are non-functional. However, they  sometimes cause serious tissue damage with the kidney being a particularly vulnerable target. The toxic effects of monoclonal proteins may occur at early stages in the plasma cell dyscrasia spectrum and require treatment independently of the mass or tissue-destructive effects of the myeloma protein-producing cells.