What is chronic myeloid leukaemia? (CML)

CML is more common in middle-aged and older people.

People who have CML make too many granulocytes, a type of white blood cell. These are not fully developed and fill the bone marrow so that it can’t make healthy blood cells. The abnormal granulocytes can also collect in the spleen, an organ that stores blood cells and destroys older damaged ones.

All cells in our body contain a set of instructions that tell them how to behave. These instructions are stored inside our cells as genes. Our genes are arranged in 23 pairs of chromosomes.

CML happens when a gene from one chromosome (the ABL gene) wrongly attaches to a gene on another chromosome (the BCR gene). This means that 2 genes that should be completely separate become attached. The new gene is called the BCR-ABL gene and the altered chromosome is called the Philadelphia chromosome.

Over 95% (95 out 100) of people who have CML have the Philadelphia chromosome.

Understanding chronic myeloid leukaemia

CML is a rare cancer. About 700 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.

CML usually develops very slowly, which is why it’s described as a chronic leukaemia.

People with CML make too many granulocytes. These are a type of white blood cell. This is why CML is sometimes called chronic granulocytic leukaemia (CGL). In a person with CML, when the granulocytes are looked at under a microscope, they are immature (not fully developed). The granulocytes fill the bone marrow and stop it making enough normal white blood cells, red blood cells and platelets.

Over time, these abnormal white blood cells collect in the spleen, making it enlarge. The spleen is an organ on the left side of the tummy, underneath the ribs. The spleen:

  • produces small numbers of lymphocytes 
  • stores blood cells
  • destroys older, damaged blood cells. 

The spleen is part of the lymphatic system. The bone marrow and the lymph nodes (glands) are also part of the lymphatic system.

The lymphatic system with spleen
The lymphatic system with spleen

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How CML develops

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. These control how the cell behaves. For example, some genes tell a cell to rest, others tell it to grow, and others tell it to develop into a mature cell so it can perform its functions in the body. The genes are organised into structures called chromosomes. Most cells in the body contain 23 pairs of chromosomes.

CML develops when a cell is dividing and, by mistake, a gene gets moved from one chromosome to another. This means two genes that are normally completely separate join together and create a new gene. This new, abnormal fusion gene stops the bone marrow stem cell from developing into a normal blood cell. These abnormal cells are the leukaemia cells (blast cells).

When doctors look at the leukaemia cells under a microscope, they can often see a chromosome that looks different. This new chromosome is caused by two genes joining together. It is called the Philadelphia chromosome.


Philadelphia chromosome

Most people with CML (more than 95% or 95 out of 100) have the Philadelphia chromosome in all their leukaemia cells. This is called Philadelphia chromosome-positive CML, or Ph+ CML.

The Philadelphia chromosome isn’t inherited, so it is not something you were born with and it can’t be passed on to your children.

How the Philadelphia chromosome develops

There are 23 pairs of chromosomes in a cell. 22 of them are numbered from 1–22. The 23rd pair is responsible for our gender – it is called XX in women and XY in men. The Philadelphia chromosome is made when a gene on chromosome 9 (the ABL gene) wrongly attaches to a gene on chromosome 22 (the BCR gene). This creates a new gene called BCR-ABL.

How the Philadelphia chromosome develops
How the Philadelphia chromosome develops

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The BCR-ABL gene makes a substance called tyrosine kinase. Too much tyrosine kinase makes cells develop and behave abnormally.

Back to Understanding chronic myeloid leukaemia

The blood

To understand chronic myeloid leukaemia, it helps to understand a bit about your blood.

Symptoms of CML

Not everyone has symptoms. Any symptoms that occur are usually mild and similar to common illnesses such as flu.