What is chronic myeloid leukaemia (CML)?

Chronic myeloid leukaemia (CML) is a type of cancer that affects the blood and bone marrow. The leukaemia develops from early blood cells called myeloid blood stem cells. CML usually develops very slowly. 

There are different types of leukaemia. The treatment you need depends on the type you have.

What is leukaemia?

Leukaemia is a blood cancer. Your bone marrow makes blood cells. 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. For example, they don't give you the protection from infection that they should. There are too many of these abnormal white blood cells. They might stop the bone marrow producing enough healthy blood cells.

They can also build up in the lymph nodes and spleen and cause swelling. 

There are different 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 quickly it progresses.

Acute leukaemias tend to develop quickly and get rapidly worse if they are not treated. 

There are 2 main types of acute leukaemia:

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

Chronic leukaemias develop slowly and tend to get worse slowly, over a long time. 

In chronic leukaemia the white blood cells are almost fully developed, but are not completely normal. They still work, but not as well as they should do at fighting infection. Your body makes too many of these abnormal white blood cells. 

There are 3 main types of chronic leukaemia:

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

Blood cells and leukaemia

To understand how and why leukaemia affects you as it does, it helps to know more about blood cells.

Your body makes blood cells in the bone marrow. The bone marrow is the soft inner part of your bones. You make blood cells in a controlled way, as your body needs them.

All blood cells start as the same type of cell, called a stem cell. This stem cell then develops into:

  • myeloid stem cells, which become white blood cells called monocytes and neutrophils (granulocyte), red blood cells and platelets
  • lymphoid stem cells, which become white blood cells called lymphocytes

The diagram below helps to explain this.

A simplified 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, the abnormal leukaemia cells develop from early blood cells called the myeloid blood stem cells. They become myelocytes. These cells are sometimes called granulocytes. So you may hear this type of leukaemia called chronic granulocytic leukaemia or CGL.

Diagram showing which cells CML can start in

What are blasts?

Blast cells are immature blood cells. They have not yet fully developed into mature, fully functioning blood cells.

In CML, the disease can enter a phase where it suddenly begins to develop more quickly. There is a sudden increase in blast cells in the bone marrow and blood. This is called blast crisis.

How does leukaemia affect 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 you make too many white blood cells, 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, easy bruising, very heavy periods or a fine rash of red spots caused by bleeding into the skin (petechiae).

Genes and chromosomes in CML

This information about genes and chromosomes in CML might help you understand more about the tests and treatment for CML. 

What are genes and chromosomes?

Most 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. 

Sometimes a change happens in the genes when a cell divides. This happens by chance when a cell is dividing. It is not something you inherit. So you were not born with it and it can't be passed on to your children. 

CML develops when an abnormal change happens to chromosome 9 and and chromosome 22.

The BCR-ABL1 gene

A gene called the ABL1 gene is on chromosome 9. The ABL1 gene breaks off and sticks to a gene called the BCR gene which is on chromosome 22. This creates a completely new gene called BCR-ABL1. This is known as a fusion gene.

The BCR-ABL1 gene causes the cell to make too much of a protein called tyrosine kinase. This protein encourages leukaemia cells to grow and multiply. The main treatment for CML are drugs called tyrosine kinase inhibitors (TKIs). These drugs work by switching off (inhibiting) the tyrosine kinase that the BCR-ABL1 gene makes. 

The Philadelphia chromosome

The BCR-ABL1 gene changes chromosome 22. It looks shorter than normal. It is called the Philadelphia chromosome. Most people with CML have the Philadelphia chromosome. 

Doctors can see the Philadelphia chromosome when they look at blood and bone marrow under the microscope. So they use it to help diagnose CML and to monitor response to treatment.

Watch this 2 minute video to explain what Philadelphia positive leukaemia is. 

How common is chronic myeloid leukaemia?

Around 830 people are diagnosed with CML in the UK each year. Although it can be diagnosed at any age, it's more common in older people. 

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