Detecting monoclonal immunoglobulins in human serum using mass spectrometry

Abstract

Established guidelines from the International Myeloma Working Group recommend diagnostic screening for patients suspected of plasma cell proliferative disease using protein electrophoresis (PEL), free light chain measurements and immunofixation electrophoresis (IFE) of serum and urine in certain cases. Plasma cell proliferative disorders are generally classified as monoclonal gammopathies given most are associated with the excess secretion of a monoclonal immunoglobulin or M-protein. In clinical practice, the M-protein is detected in a patients' serum by the appearance of a distinct protein band migrating within regions typically occupied by immunoglobulins. Given each M-protein is comprised by a sequence of amino acids pre-defined by somatic recombination unique to each clonal plasma cell, the molecular mass of the M-protein can act as a surrogate marker. We established a mass spectrometry based method to assign molecular mass to the immunoglobulin light chain of the M-protein and used this to detect the presence of M-proteins. Our method first enriches serum for immunoglobulins, followed by reduction to separate light chains from heavy chains, followed by microflow LC-ESI-Q-TOF MS. The multiply charged light chain ions are converted to their molecular mass and reconstructed peak area calculations are used for quantification. Using this method, we term "monoclonal immunoglobulin Rapid Accurate Molecular Mass" or miRAMM, the presence of M-proteins can be reliably detected with superior sensitivity compared to current gel-based PEL and IFE techniques.

Keywords: Clinical diagnostics; Immunoglobulin; Mass spectrometry; Monoclonal gammopathy; Multiple myeloma.