NCCN Guidelines for Patients® | Chronic Myelogenous Leukemia - page 8

NCCN Guidelines for Patients
: Chronic Myelogenous Leukemia
Version 1.2014
1.2 How does chronic myelogenous leukemia start?
Every cell has 23 pairs of chromosomes. Each pair looks
different from the others and is labeled by a number.
Sometimes pieces of chromosomes break off and switch
with each other. This is called a translocation.
gene is formed by the translocation
between parts of chromosomes 9 and 22. See Figure 4.
The short bottom piece of chromosome 9 has the
gene. It attaches to the short top piece of chromosome
22, which has the
gene. As a result, these two genes
join (fuse) together and form the
fusion gene.
This translocation creates a longer chromosome 9 and a
shorter chromosome 22. The shorter chromosome 22 is
called the Philadelphia chromosome. The Philadelphia
chromosome is the hallmark of CML and it contains the
gene. If you do not have the Philadelphia
chromosome or the
gene, then you do not
have CML.
gene makes the BCR-ABL protein, a type
of protein called a tyrosine kinase. Tyrosine kinases
are proteins that are located on or near the surface of
cells and send signals telling cells when to
grow and divide. The BCR-ABL protein is
abnormal. It is locked in the “on” position
so that it always sends signals for cells to
keep growing and dividing. This causes
blood stem cells to make too many white
blood cells called granulocytes. White
blood cells made by the BCR-ABL protein
are called leukemia cells or CML cells and
they contain the
gene. These
cells aren’t normal. They don’t mature into
healthy, normal cells. They don’t die when
they should. They also may reproduce too
quickly. Over time, these abnormal white
blood cells (CML cells) can overcrowd the
bone marrow so there isn’t room for healthy
white blood cells, red blood cells, and
Figure 4. Philadelphia chromosome and
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