Cancer Cancer Basics

Understanding Chronic Myelogenous Leukemia

CML is a cancer of the blood. In people with CML, there is a disruption in the process that governs the normal life cycle of white blood cells. As a result, too many white blood cells develop. CML is called myelogenous because it affects a particular white blood cell called the myeloid cell, and it is considered a chronic leukemia because the disease tends to grow slowly over a long period of time.

CML is relatively uncommon.  It is estimated that, globally, there is one case of CML for every 100,000 people.  CML is usually diagnosed in adults over the age of 40.  This form of leukemia rarely occurs in children and is slightly more common in men than in women.

Blood cells start out as stem cells in the bone marrow. These are called hemopoietic stem cells. Different hemopoietic stem cells mature into red blood cells, white blood cells and platelets. 

Doctors believe CML starts when a genetic mistake or mutation occurs in a single hemopoietic stem cell. What causes this mutation is unknown, and probably differs for different people. Doctors report an increase in CML and other leukemias following the nuclear bombings of Japan during World War II, so there may be some correlation between high levels of radiation and the development of CML. In very rare instances, doctors believe there may be a slight genetic predisposition to CML.

The Philadelphia Chromosome
While the specific trigger for CML is unknown, doctors understand much of the process by which the disease unfolds. In the vast majority of cases of CML, the disease starts when two chromosomes swap parts during cell division. This is called a translocation. One of the abnormal chromosomes that results is called the Philadelphia chromosome, and this is the distinguishing feature in most cases of CML.

The Philadelphia chromosome contains a unique gene called BCR-ABL which produces an abnormal enzyme, also referred to as BCR-ABL. The abnormal BCR-ABL enzyme interferes with the programmed life cycle of white blood cells. In CML, white blood cells do not die as they normally should. This, and other mechanisms that are only partially understood, leads to the over-production and accumulation of these blood cells.

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