About 4,300 cases of chronic myelogenous leukemia are diagnosed in the United States each year. CML occurs primarily in adults, and accounts for only 4 percent of childhood leukemia cases. The frequency of the disease increases with age from about one in a million children in the first 10 years of life to one in 100,000 people at age 40, to one in 10,000 people by age 80 and above. The disease progresses in a similar fashion in children and adults, but stem cell transplantation is more successful in younger people.
CML results from an acquired (not inherited) injury to the DNA of a stem cell in the marrow. This leads to an uncontrolled growth of white blood cells, which accumulate in massive amounts in the blood. Unlike the acute form of the disease, chronic myelogenous leukemia allows the development of mature white blood cells that can perform their functions normally.
Chronic myelogenous leukemia is distinguished from other leukemias by the presence of an acquired genetic abnormality in blood cells called the Philadelphia chromosome, also known as chromosome 22. Chromosome 9 is also abnormal in CML patients. Pieces of these genes break off and switch with each other in a process called translocation. The mutation that is formed as a result is called the BCR-ABL gene.
In CML patients, the BCR-ABL gene produces an abnormal or mutated protein, which is responsible for converting the marrow stem cell from normal to leukemic. Although this chromosome breakage may be caused by exposure to high doses of radiation in a few patients, it is not associated with exposure to medical or dental X-rays.
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