Myeloma : M Component

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Image Description

This image shows serum protein electrophoresis (left half; red bands) and urine protein electrophoresis (right half; blue bands) from a patient with plasma cell myeloma. The right lane in serum protein electrophoresis (red bands) is a normal control showing a thick albumin band at the top followed by alpha1, alpha2, beta1, beta2 (indistinct; merges with gamma), and gamma bands. The left lane shows serum from a patient with plasma cell myeloma. Note the sharply defined band in the gamma region at the bottom. Also note the reduced levels of albumin and other polyclonal immunoglobulins. With immunofixation technique, the M component was identified as IgG kappa.

The right half of this image depicts urine protein electrophoresis from the same patient. It shows the presence of free kappa light chains (Bence-Jones proteins) in urine. The free light chains are reabsorbed by proximal renal tubules and they may be absent from urine if the renal function is preserved.

Plasma cell myeloma patients usually show increased levels of serum immunoglobulins (>3 gm/dl) and/or Bence-Jones proteins in urine (> 6 mg/dl). The M component is usually IgG (about 50% of cases) followed by IgA (about 20% of cases) type. Myelomas producing M component of IgM, IgD, and IgE types are rare. About 20% of myeloma cases show only free light chains, usually kappa subtype (Bence-Jones proteins) in urine and no M component in the serum. About 3% of myelomas are non-secretory; therefore, the absence of M component in the serum or urine does not rule out myeloma. Excessive levels of immunoglobulins produce hyperviscosity syndrome in about 7% of patients.

About the Disease

Plasma cell disorders are B-cell neoplasms composed of plasma cells or lymphocytes that secrete a monoclonal immunoglobulin (Ig) or Ig fragment. They are responsible for about 15% of deaths caused by lymphoid malignancies in the US. The monoclonal Ig, usually identified by serum or urine protein electrophoresis, is known as the M component. Sometimes the M component consists of only light chains or heavy chains. Free light chains are referred to as Bence-Jones proteins when they are excreted in urine. The WHO Classification (2008) of plasma cell neoplasms includes the following: Monoclonal Gammopathy of Undetermined Significance (MGUS): MGUS refers to an asymptomatic patient with an M component (<3 gm/dl) in their blood. It is the most common plasma cell disorder and is seen in about 5% of individuals >70 years of age. About 1% of MGUS patients per year develop multiple myeloma, amyloidosis, macroglobulinemia, or other lymphoproliferative disorder. The median interval between the diagnosis of MGUS and the development of malignancy is about 10.5 years. Plasma Cell Myeloma: Multiple myeloma (MM) is a plasma cell neoplasm of older individuals that is characterized by scattered tumorous masses causing lytic bone lesions in the axial skeleton, hypercalcemia, renal damage, and acquired immune abnormalities. Solitary Plasmacytoma: About 3% to 5% of patients with plasma cell disorders present with solitary bone or soft tissue mass. An M component may be found in serum or urine in a small percentage of cases. Solitary bone plasmacytoma slowly progresses to multiple myeloma in most cases. Extraosseous plasmacytomas involve lungs, nasopharynx, oropharynx, or nasal sinuses and can often be cured by resection. Immunoglobulin Deposition Diseases: This group includes primary amyloidosis (PA) and light- or heavy- chain deposition disease. Immunoglobulins are deposited in various tissues eventually leading to organ failure. About 20% of PA patients have multiple myeloma (MM). The remainder don't meet the criteria for MM but have a small M component and increased plasma cells in bone marrow. Osteosclerotic Myeloma (POEMS Syndrome): Osteosclerotic myeloma is usually associated with polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy, and skin lesions (POEMS syndrome). Some patients also develop Castleman's disease, serous effusions, and thrombocytosis.

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