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About chronic myelomonocytic leukaemia (CMML)

CMML is a rare form of blood cancer. It happens when there are too many monocytes, a type of white blood cell in the blood.

What is it

Chronic myelomonocytic leukaemia (CMML) is a rare type of blood cancer. In CMML there are too many monocytes in the blood. Monocytes are a type of white blood cell.

CMML has similar features with two other types of blood disorders: myelodysplastic syndromes (MDS) and myeloproliferative neoplasms (MPN). However, CMML is a separate condition with different treatment options because people with CMML can have features of both MDS and MPN. 

The bone marrow and blood cells

The bone marrow is the soft inner part of our bones that makes the blood cells.

All blood cells start from the same type of cell called a stem cell. The stem cell makes immature blood cells. These immature cells go through various stages of development before they become fully developed blood cells. 

The bone marrow makes different types of blood cells, including:

  • red blood cells to carry oxygen around our bodies
  • white blood cells to fight infection
  • platelets to help the blood clot

The diagram shows how the different types of cells develop from a single blood cell. 

Diagram showing how blood cells are made

What happens

White blood cells called monocytes help the body to fight infection. In CMML the bone marrow produces abnormal monocytes. They are not fully developed and can’t work normally. These abnormal cells are also called leukaemia cells or blast cells. 

These abnormal blood cells either stay in the bone marrow or are destroyed before they get into the bloodstream. As the CMML develops, the bone marrow becomes full of the abnormal monocytes. These abnormal blood cells then spill out into the bloodstream.

Because the bone marrow is full of the abnormal cells it can’t make enough of the other types of blood cells. The low numbers of normal blood cells in the bloodstream eventually cause symptoms.

Types of CMML

There are 2 types of CMML. They are called type 1 and type 2. The type tells you how many abnormal myeloid cells (blasts) you have in your blood samples.

  • Type 1 CMML means you'll have less than 5% blast cells in your blood and less than 10% in your bone marrow
  • Type 2 CMML means you'll have between 5 and 19% blast cells in your blood and bone marrow

Knowing your type of CMML helps your doctor to decide on the best treatment for you.

Transformation to acute myeloid leukaemia (AML)

CMML can develop into an acute myeloid leukaemia (AML) if the number of blast cells in your blood rises above 20%. Doctors call this transformation.

Transformation happens in between 15 and 30 out of every 100 people with CMML (15 to 30%). This might happen after a few months or after several years.

Symptoms of CMML

CMML usually develops slowly and doesn’t cause symptoms at first. When it does start to cause symptoms they include:

  • tiredness (fatigue)
  • infections that don’t get better
  • bruising or bleeding easily – including nosebleeds or bleeding gums
  • abdominal discomfort from a swollen spleen
  • loss of appetite

Risks and causes of CMML

We don't know the cause of most cases of CMML but there are some risk factors that can increase your risk of developing it. A risk factor is something that might make you more likely to develop a particular condition or disease.

Your risk of developing CMML increases with age. It most commonly develops after the age of 60. And it is more common in men than women.

If you have had treatment for cancer you are at an increased risk of developing CMML.

Gene changes

Researchers have found that around half of the people who have CMML (50%) have a change in a gene called TET2. The TET2 gene makes a protein that controls how many monocytes the stem cells make. Between 10 and 30 out of 100 people (10 to 30%) have a change in a gene called RAS. The change makes the cells multiply uncontrollably.

We need more research to find out what causes the changes in these genes.

These genetic changes aren’t inherited, so they can’t be passed on to your children. These gene changes in your bone marrow cells are picked up over the course of your lifetime. 

Last reviewed: 
13 Sep 2017
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    Eric Solary and Raphael Itzykson

    Blood. 2017 July 13;130(2): pages 126-136

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  • Next-generation sequencing technology reveals a characteristic pattern of molecular mutations in 72.8% of chronic myelomonocytic leukemia by detecting frequent alterations in TET2, CBL, RAS, and RUNX1

    Kohlmann A. and others

    Journal of clinical oncology, 2010. Vol. 28, issue 24

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