What is the Philadelphia Chromosome?

The Philadelphia chromosome is a chromosomal abnormality which can lead to leukemia. This abnormality is most closely linked with myelogenous leukemia, although it can be present in patients with other leukemias as well. Testing which reveals the presence of the Philadelphia chromosome can be used to determine the best course of treatment for a patient with leukemia.

This abnormality arises as the result of a problem in cell division within the bone marrow. It is a type of translocation, meaning that genetic material from two different chromosomes switches places. In this particular abnormality, chromosomes nine and 22 are involved. All it takes is one bone marrow cell with this abnormality for leukemia to develop, as the cell will replicate itself and perpetuate the abnormality, eventually causing problems with the production of myelogenous blood cells.

Cancer researchers discovered the Philadelphia chromosome in Philadelphia, Pennsylvania in 1960. The discovery was remarkable because it was the first occasion on which an abnormality in the chromosomes was linked with a malignant medical condition, and it paved the way to many future discoveries once people understood how the translocation occurred and what it did. This discovery also changed the approach to treating myelogenous leukemia.

Theoretically, it is possible to develop medications which can target the Philadelphia chromosome, allowing an oncologist to deliver targeted doses of cancer drugs to kill the rogue cells. As of 2009, several experimental drugs which do just that had been developed, and cancer researchers were working to refine them and to develop treatment protocols. At some point, these drugs may make it possible to avoid the pain and risk of a bone marrow transplant in the treatment of myelogenous leukemia.

When patients are diagnosed with leukemia, genetic testing is often recommended to learn more about the origins of the cancer, and because understanding the genetics behind the cancer can help the doctor determine the best course of treatment. Patients with Philadelphia chromosome positive myelogenous leukemia will have a treatment approach which is different from that of patients who lack the abnormality, and the same holds true of people with other leukemias who test positive for this abnormality.

The Philadelphia chromosome cannot be passed on to descendants, because it is an abnormality which occurs during cell division, and it is only present in the bone marrow, not in the germ cells which dictate genetic inheritance. However, genetic testing can reveal the presence of other abnormalities which may play a role in inheritance, making it important to talk with a genetic counselor about the results of any type of genetic testing.