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Although the exact cause isn't known, doctors do know that multiple myeloma begins with one abnormal plasma cell in your bone marrow — the soft, blood-producing tissue that fills in the center of most of your bones. This abnormal cell then starts to multiply.

Because abnormal cancerous cells don't mature and then die as normal cells do, they accumulate, eventually overwhelming the production of healthy cells. In healthy bone marrow, less than 5 percent of the cells are plasma cells. But in people with multiple myeloma, more than 10 percent of the cells may be plasma cells.

Because myeloma cells may circulate in low numbers in your blood, they can populate bone marrow in other parts of your body, even far from where they began. That's why the disease is called multiple myeloma. Uncontrolled plasma cell growth can damage bones and surrounding tissue. It can also interfere with your immune system's ability to fight infections by inhibiting your body's production of normal antibodies.

Researchers investigating cause

Researchers are studying the DNA of plasma cells to try to understand what changes occur that cause these cells to become cancer cells. Though they haven't yet discovered the cause of these changes, they have found that almost all people with multiple myeloma have genetic abnormalities in their plasma cells that probably contributed to the cancer.

The genetic abnormalities associated with multiple myeloma include:

  • A defect related to chromosome 14 in which a piece of one chromosome moves to a different chromosome (translocation)
  • Extra copies of certain chromosomes (hyperdiploidy)
  • An abnormality in which part or all of chromosome 13 is missing

A connection with MGUS

Multiple myeloma almost always starts out as a relatively benign condition called monoclonal gammopathy of undetermined significance (MGUS). In the United States, about 3 percent of people older than age 50 have MGUS. Each year, about 1 percent of people with MGUS develop multiple myeloma or a related cancer. MGUS, like multiple myeloma, is marked by the presence of M proteins — produced by abnormal plasma cells — in your blood. However, in MGUS, no damage to the body occurs.

Aug. 16, 2011