Leukemia is a cancer of the blood. Leukemia begins when healthy blood cells change and grow out of control. Chronic myeloid Leukemia (CML) is a cancer of the blood-forming cells, called myeloid cells, found in the bone marrow. Bone marrow is the red, spongy tissue in the inner part of long bones. CML often causes an increase in the levels of white blood cells, such as neutrophils or granulocytes, that normally fight infection. It is also sometimes called chronic granulocytic, myelocytic, or myelogenous Leukemia.
The Philadelphia chromosome
People with CML have a genetic mutation in their bone marrow cells. It is called translocation. A translocation is when part of a long strand of genes known as a chromosome breaks off and reattaches to another chromosome. A part of chromosome 9 breaks off in CML and bonds to chromosome 22, forming the Philadelphia or Ph chromosome. The Ph chromosome comprises two BCR and ABL genes that join to form a single fusion gene called BCR-ABL. It is found only in the cells forming blood, not in other body organs. The BCR-ABL gene causes myeloid cells to form an abnormally activated enzyme. Specifically, it is a tyrosine kinase enzyme. This abnormally activated enzyme is a fusion protein and allows white blood cells to grow out of control.
This genetic change develops from damage by chance after a person is born. There is no risk of passing on this gene to their children.
Normally, the levels of white blood cells are tightly controlled by the body, more white blood cells (WBC) are produced during infections or in stress, but then the levels return to standard when the infection is cured. In CML, the abnormal BCR-ABL enzyme is like a switch stuck in the ‘on’ position; it keeps stimulating the WBC to grow and multiply. In addition to white blood cells, the number of blood platelets helping to clot blood often increases. And the number of red blood cells carrying oxygen may decrease.