Chronic myeloid leukaemia (CML)

Chronic myeloid leukaemia (CML)

What is chronic myeloid leukaemia?

Chronic myeloid leukaemia (CML) is a blood cancer. Blood is made up of three main types of cells:

  • white blood cells – fight infection
  • red blood cells – carry oxygen around the body
  • platelets – help the blood to clot.

CML develops when the body produces too many of the white blood cells known as granulocytes. The granulocytes are part of the myeloid family of white blood cells. CML is sometimes called chronic granulocytic leukaemia.

The granulocytes that are produced in CML are immature and abnormal. Because they live too long or multiply too quickly, there will be large numbers circulating in the blood. They crowd out normal white blood cells and don’t fight infection themselves, so there can be a higher risk of infection (neutropenia).

As leukaemia progresses, the bone marrow fills with leukaemia cells and there is less room for healthy red cells and platelets to be produced. This may cause various health problems, such as anaemia (from too few red cells) or bleeding or bruising (from thrombocytopenia, too few platelets).

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What is the difference between chronic and acute leukaemia?

While all types of leukaemia start in the bone marrow and affect white blood cell production, they are grouped according to which type of white blood cell is affected (lymphoid or myeloid), whether there are abnormalities in the bone marrow, and how quickly the disease develops.

Chronic leukaemia – This usually affects partly immature cells, appears gradually, and develops slowly over months to years. There are two types of chronic leukaemia: chronic lymphocytic leukaemia (CLL) and chronic myeloid leukaemia (CML).

Acute leukaemia – This affects fully immature cells, occurs suddenly, and develops quickly. There are two types of acute leukaemia: acute lymphoblastic leukaemia (ALL) and acute myeloid leukaemia (AML).

Who gets CML?

Each year in Australia, more than 3200 people are diagnosed with a form of leukaemia, and about 1500 of these cases are chronic leukaemia. CML is the less common type, with about 340 people diagnosed annually. It is rare in children and slightly more common in men than in women.

What causes CML?

Chronic leukaemia is caused by changes to one or more of the genes (DNA) that control the growth and development of blood cells. These changes happen over time, but it is not known why they occur in some people and not others. The exact causes of CML are not yet understood.

Causes of CML

  • Most people with CML have a genetic abnormality known as the Philadelphia chromosome or BCR-ABL gene. This abnormality cannot be passed on from parents to children.
  • Exposure to the chemical benzene or high doses of radiation may cause CML. However, this doesn’t explain the majority of cases.

Blood cells and leukaemia cells

Blood is pumped around your body to provide oxygen and nutrients to your tissues and to remove waste products. It is made up of red blood cells, white blood cells and platelets, all carried in a clear fluid called plasma. 

All three types of blood cells have a limited life span and need to be continually replaced. Most are made in the bone marrow, which is the spongy part in the centre of the bones.

The bone marrow contains stem cells. These are unspecialised blood cells that first develop into immature cells known as blast cells. Normally, the blast cells then become mature red or white blood cells or platelets and carry out their set functions.

There are two families of stem cells:

  • myeloid stem cells – develop into myeloblast cells and then into red blood cells, most types of white blood cells, and platelets
  • lymphoid stem cells – develop into lymphoblast cells and then into lymphocytes, which are a type of white blood cell.

If myeloblast or lymphoblast cells do not mature properly or if there are too many in the blood, it can cause leukaemia. These abnormal cells are also called leukaemia cells.

Blood cell production diagram

This information was last reviewed in March 2016
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