What is chronic myeloid leukaemia?
Chronic myeloid leukaemia (CML) is a rare cancer.
About 600 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. It’s rare in children.
CML usually develops very slowly, which is why it’s described as a ‘chronic’ leukaemia. Many people don’t need treatment for months or years. However, some people need to have treatment straight away.
People with CML make too many of a type of white blood cell called a granulocyte, which is why CML is sometimes called chronic granulocytic leukaemia (CGL). When examined under a microscope, the granulocytes aren’t fully developed (immature). Over time, these abnormal white blood cells collect in the spleen, causing it to enlarge. They also fill the bone marrow reducing the number of normal white blood cells, red blood cells and platelets that are made.
The spleen is an organ on the left side of the tummy underneath the ribs. It produces small numbers of lymphocytes, stores blood cells, and destroys older, damaged blood cells. It’s part of the lymphatic system, which also includes other lymphatic organs such as the bone marrow and the lymph nodes (glands).
To help you understand CML and its treatment, it’s useful to know a bit about your blood, what it does and how it’s made.
Blood is made up of blood cells in a liquid called plasma.
The blood cells are made in the bone marrow. This is a spongy material that’s found in the middle of your bones, particularly in your pelvis and backbone (spine). Normally, millions of new blood cells are made every day to replace the old and worn-out blood cells.
All your blood cells are made from cells called stem cells.
There are two types of stem cell:
lymphoid stem cells, which make white blood cells called lymphocytes
myeloid stem cells, which make all the other types of blood cells: red blood cells, platelets, and white blood cells called granulocytes.
To begin with, the new blood cells are immature. They don’t look like red blood cells, platelets or white blood cells, and they can’t yet do the jobs they’re supposed to. These immature cells are called blast cells. Usually, blast cells stay in the bone marrow until they have matured into red blood cells, platelets or white blood cells.
In the bone marrow, the stem cells divide and grow to form fully developed (mature) red blood cells, platelets and white blood cells.
These are then released into your blood to carry out different functions:
Red blood cells contain haemoglobin, which carries oxygen from your lungs to all the cells in your body.
Platelets are very small cells that help blood to clot, and prevent bleeding and bruising.
White blood cells fight and prevent infection. There are several types of white blood cell. The two most important types are neutrophils and lymphocytes.
The levels of these cells in your blood are measured in a blood test called a full blood count (FBC). The figures below are a guide to the levels usually found in a healthy person.
Full blood count figures
Types of blood cell
Levels found in a healthy person
150–400 x 109/l
White blood cells (WBC)
4.0–11.0 x 109/l
2.0–7.5 x 109/l
1.5–4.5 x 109/l
These figures can vary from hospital to hospital. Your doctor or nurse will be able to tell you what levels they use. They can also be slightly different in people of African-Caribbean and Middle Eastern origin.
The figures might look complicated when they’re written down, but in practice they’re used in a straightforward way. For example, you’ll hear doctors or nurses saying things like ‘your haemoglobin is 14’ or ‘your neutrophils are 4’. Most people with CML soon get used to these figures and what they mean.
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, and others tell it how to mature into an adult cell so it can perform its normal functions in the body. The genes are organised into structures called chromosomes.
CML develops when, by mistake, a gene gets moved from one chromosome to another when a cell is dividing. This causes two genes that are normally completely separate to join together (fuse). This new, abnormal ‘fusion’ gene stops the bone marrow stem cell from maturing into a normal blood cell. These abnormal cells are the leukaemia cells or blast cells.
When doctors look at the leukaemia cells they can often see a chromosome that looks different. This new chromosome, caused by the fusion of the two genes, is called the Philadelphia chromosome.
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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.
How the Philadelphia chromosome develops
Most cells in the body contain 23 pairs of chromosomes. They are numbered from
1–22 and 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.
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 often found in CML. New treatments for CML work by blocking the effects of tyrosine kinase.