What is myeloma?
Myeloma, also known as multiple myeloma, is a blood cancer arising from the plasma cells. Plasma cells form part of the immune system and are normally found in the bone marrow. They produce immunoglobulin (Ig) molecules (also known as antibodies) to help fight infection.
Unlike many cancers, myeloma does not exist as a solid lump or tumour. Instead, the abnormal plasma cells divide and expand within the bone marrow.
Normally when antigens (such as bacteria or viruses) are recognised by the immune system as foreign, an immune response is triggered which includes the production of many types of Igs by numerous plasma cells (polyclonal response). In myeloma, abnormal malignant plasma cells, known as myeloma cells, replicate uncontrollably. The daughter cells produce only one type of immunoglobulin (monoclonal) called paraprotein (or M-protein) which has no useful function. Paraprotein is a hallmark of myeloma and may be detected in the blood and/or urine. Its presence and amount is often how myeloma is diagnosed and monitored.
The structure and function of immunoglobulins are covered in more detail in Module 2 – Pathogenesis of myeloma.
The following animation shows how a plasma cell can become cancerous and turn into a myeloma cell.
The presence of myeloma cells within the bone marrow can cause a number of problems. They secrete chemical messages (cytokines), or stimulate the secretion of cytokines by other cells that comprise the bone marrow microenvironment, slowing down the production of the different blood cell types (myelosuppression). This can result in anaemia, neutropenia and thrombocytopenia. The myeloma cells can also trigger other complications such as myeloma bone disease.
The volume or amount of myeloma in an individual patient is often referred to as the ‘tumour burden’. This has some prognostic significance, for example, a patient with 70% plasma cells in their bone marrow biopsy would be considered to have a high tumour burden and a potentially poorer prognosis than someone with 40% plasma cells in their bone marrow.
However, a high tumour burden does not necessarily always mean a poorer prognosis: the characteristics of the myeloma cells and the impact they have on the body are as important as the volume or amount of myeloma.
Myeloma cells can also produce excessive amounts of free light chains, a component of the Ig molecule. These are small enough to pass through the renal glomerulus and be excreted in the urine where they are referred to as Bence Jones protein (BJP). These BJP can damage the renal tubules and impair renal function.
In some patients, the myeloma cells do not produce intact whole Ig molecules but light chains only. This is known as ‘Light chain myeloma‘ or ‘Bence Jones myeloma‘ and accounts for around one fifth of myeloma cases.