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What Is the Philadelphia Chromosome?

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Updated November 03, 2010

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What Is the Philadelphia Chromosome?

The 23 pairs of human chromosomes arranged in order and showing XY chromosomes (male)

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As part of their assessment, doctors will look for the presence of the Philadelphia chromosome to help determine if a patient is affected by particular types of leukemia.

Understanding Chromosomes and DNA

This is a conversation that could go for days, but there are some simple points that can help you better understand the Philadelphia chromosome better.

DNA is the "coding system" that is found in every cell in our bodies. DNA determines everything about that cell -- how it functions, how it looks, where it grows. DNA is also called genetic material.

In our cells, DNA is found in chromosomes. We have 46 chromosomes in each cell, and these chromosomes are paired together and assigned a number.

In most cases of chronic myelogeous leukemia (CML) as well as some cases of acute lymphocytic and myelogenous leukemia (ALL and AML), a piece of chromosome 9 and a piece of chromosome 22 break off and switch places with each other. This type of damage is called translocation.

This new chromosome, which is mostly chromosome 22 with a piece of chromosome 9 stuck to it, is called the Philadelphia chromosome.

The Philadelphia chromosome is only found in the affected blood cells. Because of the damage to the DNA, the Philadelphia chromosome results in the production of an abnormal enzyme called a tyrosine kinase. Along with other abnormalities, this enzyme causes the cancer cell to grow uncontrollably.

Doctors will look for the presence of this abnormality when they are examining samples from your bone marrow aspiration and biopsy to help make a proper diagnosis.

The identification of the Philadelphia chromosome in the 1960s led to major advancements in the treatment of CML. This laid the foundation for a new era of CML therapy called "tyrosine kinase inhibitors," such as Gleevac (imatinib mesylate) and Sprycel (dasatinib)

Sources

Goldman, J. and Daley, G. (2007). Chronic Myeloid Leukemia. In Melo, J., and Goldman, J. (Eds.) Myeloproliferative Disorders (pp.1-13). New York. Springer.

Sherbenou, D. and Druker, B. "Applying the discovery of the Philadelphia chromosome" The Journal of Clinical Investigation August 2007 117:2067-74.

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