Using Mass Spectrometry to Detect Monoclonal Immunoglobulins in Human Serum
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.
In a study published in the journal Methods, Mayo Clinic researchers John Mills, Ph.D., David Murray, M.D., Ph.D., and David Barnidge, Ph.D., established a mass spectrometry-based method to assign molecular mass to the immunoglobulin light chain of the M-protein and used it to detect the presence of M-proteins.
Mayo Clinic's method enriches serum for immunoglobulins, followed by reduction to separate light chains from heavy chains, and then uses microflow liquid chromatography coupled with electrospray ionization (ESI) and Q-TOF MS (microLC-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, referred to as "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.
While current methodologies used to screen for monoclonal proteins in the clinical laboratory are robust, they suffer from limited sensitivity which makes it impossible to rule out low amounts of residual disease. By having a more sensitive and specific MS serum-based assay to detect residual M-proteins, it may be possible to replace costly and invasive methodologies currently used to establish the absence of minimal residual disease. miRAMM represents a major shift in the way M-proteins are interpreted and will likely transform the way plasma cell disorders will be tracked in the future.