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Chronic myeloid leukaemia (CML) is a rare cancer that causes the body to make too many white blood cells.
About 700 people in the UK are diagnosed with CML each year. It can occur at any age, but is more common in middle-aged and older people and is rare in children.
CML usually develops very slowly. This is why it’s described as a ‘chronic’ leukaemia. It starts in the bone marrow (where our blood cells are made) in a special type of blood cell called a stem cell. To understand how CML develops, it helps to know a bit about blood and how blood cells are made.
Blood is made up of blood cells suspended in liquid called plasma. There are three main types of blood cells:
Blood cells are made in the bone marrow, a spongy material inside the bones. Normally millions of new blood cells are made every day to replace old and worn out blood cells.
All blood cells begin as a special type of cell called a stem cell. There are two types:
Stem cells make new blood cells by copying themselves and then dividing to form two new cells. To begin with the new blood cells made from stem cells are immature. They don’t look like red cells, white cells or platelets and they can’t do the jobs in the body that they can do. These immature cells are called blast cells. Normally blast cells stay in the bone marrow until they have matured into red cells, white cells or platelets.
Blood cell production
All cells have a set of instructions that tell them what to do and when to do it. These instructions are stored inside the cells as genes.
Each gene has its own distinct set of instructions that control a particular aspect of how the cell behaves. For example some genes tell a cell when to rest, others tell it when to grow. The genes are organised into structures called chromosomes.
CML develops when, by mistake, a gene gets moved from one chromosome to another one when a stem cell is dividing. The result is a joining together (fusion) of two genes that are normally completely separate. This new abnormal ‘fusion’ gene is then passed on to other cells. These are the leukaemia cells.
When doctors look at the leukaemia cells under a microscope, they can see a chromosome that looks different. This chromosome is called the Philadelphia chromosome.
Most people with CML (more than 95 out of 100) have the Philadelphia chromosome in all their leukaemia cells. This is known as Philadelphia chromosome positive CML or Ph+CML.
The Philadelphia chromosome isn’t inherited so it’s not something you were born with and it can’t be passed on to your children.
Most cells in the body contain 23 pairs of chromosomes. They are numbered from 1 to 22, (the 23rd pair are the sex chromosomes XX in women and XY in men).
The Philadelphia chromosome is made when the Abl gene on chromosome 9 is mistakenly transferred to chromosome 22 and attaches to the Bcr gene. This creates a new fusion gene called Bcr-Abl.
Development of the Philadelphia chromosome
The Bcr-Abl gene makes a protein called tyrosine kinase. Too much tyrosine kinase makes cells behave abnormally and causes the changes in the blood and bone marrow that are found in CML. Newer treatments| for CML work by blocking the effects of tyrosine kinase.
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