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About chronic myeloid leukaemia (CML)

Who gets chronic myeloid leukaemia (CML), where it starts and how common it is.

What leukaemia is

Leukaemia is a blood cancer. Blood cells are made in your bone marrow. The bone marrow is the soft inner part of some of your bones.

In most types of leukaemia, abnormal white blood cells are made in the bone marrow. These cells can get into the bloodstream and circulate round the body. They do not develop properly and so do not work normally. They don't give you the protection from infection that they should. Because there are too many of these abnormal white blood cells, they stop the bone marrow producing enough healthy blood cells.

They can also build up in the lymph nodes and spleen and cause swelling. They may also cause problems in the liver and central nervous system.

There are several types of leukaemia. They are divided into two main groups:

  • acute leukaemia
  • chronic leukaemia

Acute and chronic leukaemia

Leukaemia is acute or chronic depending on how fast it develops and gets worse.

Chronic leukaemia tends to develop very slowly. You may have a chronic leukaemia for months or years without having many symptoms. It may be stable for months or years before it gets worse. 

There are 3 main types of chronic leukaemia:

  • chronic lymphocytic leukaemia (CLL)
  • chronic myeloid leukaemia (CML)
  • hairy cell leukaemia

The word acute means that the leukaemia can develop fairly quickly. There are 2 main types of acute leukaemia:

  • acute myeloid leukaemia (AML) 
  • acute lymphoblastic leukaemia (ALL)

Blood cells and leukaemia

To understand why leukaemia affects you the way it does, it helps to know how blood cells are normally produced and what they do.

Normally, blood cells are produced in the bone marrow. The body makes them in a controlled way. All blood cells start as the same type of cell, called a stem cell. This earliest stem cell then develops into: 

  • red blood cells (erythrocytes)
  • platelets (thrombocytes)
  • white blood cells (granulocytes, monocytes or lymphocytes)
Diagram showing how blood cells are made

The type of chronic leukaemia you have depends on what type of white blood cell has become cancerous. In CML, it’s the granulocyte white blood cells that are cancerous. You may also hear CML referred to chronic granulocytic leukaemia (CGL).

Diagram showing which cells CML can start in

What blasts are

Immature blood cells are called blasts. In CML, the disease can enter a phase where it suddenly begins to develop more quickly. There is a sudden increase of leukaemia cells, with a lot of myeloid blasts in the bone marrow and blood. Doctors call this blast crisis.

How leukaemia affects the blood cells

White blood cells help to fight infection. If you have abnormal white blood cells they cannot fight infection so well. You may get a lot of infections, which may be difficult to get rid of.

When too many white blood cells are made, they take up much more room in the bone marrow than they would normally. This means that there is not enough space for making normal red blood cells and platelets. Red blood cells carry oxygen round the body. If you don't have enough of these, you have anaemia. This can make you tired and breathless.

Platelets are vital for normal blood clotting. If you do not have enough platelets, you will have bleeding problems such as nosebleeds, very heavy periods or a fine rash of red spots caused by bleeding into the skin (petechiae).

Philadelphia chromosome

All body cells contain chromosomes. Chromosomes are made up of thousands of genes. There are 23 pairs of chromosomes in human cells and each chromosome has a number from 1 to 23. These are a bit like an instruction manual for building the body and keeping it healthy. 

Most people with CML have an abnormal chromosome called the Philadelphia chromosome. You get a Philadelphia chromosome when a gene called the ABL gene on chromosome 9 breaks off and sticks to a gene called the BCR gene on chromosome 22.

This produces a new gene called BCR-ABL known as a fusion gene. The changed chromosome 22 with the new BCR-ABL gene on it is the Philadelphia chromosome. 

This process is called chromosomal translocation. It is a known type of genetic abnormality but not something that is inherited. So you were not born with it and it can't be passed on to your children. 

The Philadelphia chromosome makes the cell produce a protein that encourages leukaemic cells to grow and multiply. 

How common it is

About 8,600 people are diagnosed with leukaemia each year in the UK. Around 750 of these have CML. So it’s quite a rare condition and is more common in men than women.

Last reviewed: 
11 Nov 2014
  • Essential haematology
    Hoffbrand and Moss, 6th Edition, 2011

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