How Does Chronic Myeloid Leukemia
(CML) Happen?
CML is different from other types of leukemia. Almost everyone who has the disease also has an abnormal chromosome. It is called the Philadelphia chromosome (named after the city where it was discovered), or Ph chromosome. People with this type of CML have Philadelphia chromosome-positive CML, which is abbreviated Ph+ CML. The Ph chromosome is only found in the bone marrow and white blood cells (WBCs) of people with Ph+ CML.
Each chromosome is made up of genes. The genes act like a set of instructions to tell the cells of the body what to do. The Ph chromosome forms when:
- Two normal chromosomes break
- Parts from each broken chromosome switch places
- This results in 2 mixed chromosomes; one of these is the Ph+ chromosome
This change is known as translocation.
Translocation makes several unusual things happen:
- First, a new, abnormal gene called bcr-abl forms during translocation
- Then, the abnormal gene makes an abnormal protein called Bcr-Abl tyrosine kinase
- This abnormal protein tells the bone marrow to make more WBCs than it needs to. This abnormal protein is the underlying cause of Ph+ CML
- If this abnormal protein is kept under control, the number of WBCs should decrease, keeping the disease under control
Doctors are not sure why this happens. In a small number of patients, this may happen because they have been exposed to high doses of radiation. Doctors do know that people with Ph+ CML do not pass it on to their children.
