Myelodysplastic syndrome is the name given to a group of closely related diseases that arise in the bone marrow from hematopoietic stem cells, the immature cells from which all blood cells develop. (Myelo refers to bone marrow and dysplasia means abnormal.) In patients with MDS, the bone marrow stops making healthy blood cells and instead produces abnormal, poorly functioning blood cells. MDS can affect red blood cells (which carry oxygen), white blood cells (which fight infection), or platelets (which prevent bleeding), or any combination of the three.
MDS used to be referred to as a preleukemic condition (and it is sometimes called preleukemia), but many patients with MDS never develop acute leukemia.
Each year between 15,000 and 20,000 new cases of MDS are diagnosed in the United States. Patients tend to be a median age of 70 at the time of diagnosis, although people of any age can develop the disease. MDS is classified as either primary (also called de novo) -- disease that has no known cause -- or secondary (also called treatment-related) disease that is linked to previous treatment with chemotherapy. In the United States, the number of new cases of MDS is rising, both because older people make up a growing segment of the population and because people now live longer after treatment for their first cancer. Secondary MDS accounts for about 10 to 20 percent of the cases now diagnosed.
Hematopoietic stem cells are produced in the bone marrow and mature (or differentiate) there into functioning blood cells. MDS arises when one of these stem cells undergoes a transformation from a normal cell into a malignant cell, one capable of uncontrolled proliferation. The malignant cell begins producing identical copies of itself, or clones, sometimes referred to as "dysplastic clones." (These cells malfunction, in part, because different parts of the cell -- the nucleus and the cytoplasm -- mature at different rates.) In MDS patients, the malignant cells (which can include very immature cells called "blasts") are incapable of maturing into functional adult blood cells, and over time they may fill the bone marrow, displacing the normal red and white blood cells and platelets that are produced there. As the number of healthy cells decline in a patient's body, he or she may be prone to anemia (a lack of oxygen-carrying red blood cells), infections caused by low counts of disease-fighting white blood cells, and bruising and bleeding, resulting from low levels of blood platelets.
