What is chronic myeloid leukaemia (CML)?
CML is a type of blood cancer that affects your myeloid cells (a type of white blood cell).
Your body needs new blood cells all the time and they are made in the bone marrow (the spongy material inside some of your bones). In a healthy person, ‘starter cells’ called myeloid stem cells develop into myeloid blasts, before eventually turning into mature (fully functioning) blood cells.
If you have CML, your body produces too many myeloid blasts and too many white blood cells called granulocytes. These cells overcrowd the bone marrow, meaning there isn’t enough room for other important blood cells to be made. Some myeloid blasts also enter the bloodstream and, because they haven’t developed properly, stop your body from fighting infection properly.
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Phases of CML
There are three phases of CML: the chronic phase, the accelerated phase and the blast phase.
Most people with CML (9 in 10) are diagnosed in the early (chronic) phase. In this phase your body makes too many granulocytes, but the disease is developing slowly.
If CML is left untreated, it begins to develop more quickly and reaches the accelerated phase. In this phase, you have more myeloid blasts in your blood or bone marrow than in the chronic phase, but this number is still relatively small.
If the leukaemia continues to develop, it will eventually reach the blast phase and transform into an acute form of leukaemia (one that develops more quickly). In this phase, you have too many myeloid blasts in your blood and bone marrow.
Philadelphia chromosome
All cells in your body contain a set of instructions that tell the cell what to do and when to do it. These instructions are stored in structures called chromosomes inside the cells. There are 23 pairs of chromosomes in each cell in your body.
Your chromosomes are made up of a chemical called DNA, which is arranged in sections called genes. Each gene is a code that helps the body make different proteins.
CML is thought to begin when chromosomes 9 and 22 get mixed up during cell division (when a cell divides in order to replicate itself). This creates a new, shorter chromosome called the Philadelphia chromosome. During this mix-up, a small part of chromosome 9 (containing the ABL1 gene) gets stuck next to a small part of chromosome 22 (containing the BCR gene). In the process, they form a new fusion gene called BCR-ABL1.
The new fusion gene makes a new protein (also called BCR-ABL1). This protein is known as tyrosine kinase, and it causes leukaemia stem cells to divide more often and to live longer than normal blood cells.
We don’t know why this happens, but we do know that you aren’t born with this chromosome and can’t pass it on to your children.
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