Genes and chromosomes in chronic myeloid leukaemia (CML)

Gene changes happen when something goes wrong when the body is making new blood cells. They are sometimes called gene mutations.
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What are genes and chromosomes?

Chronic myeloid leukaemia (CML) is a cancer of the white blood cells. It develops when some white blood cells start behaving abnormally.

All cells have a nucleus that contains structures called chromosomes. Most cells in the body contain 23 pairs of chromosomes. Chromosomes are made up of DNA, which has a set of instructions that tell the cells how to behave. These instructions are called genes.

Genes and chromosomes

A cell contains pairs of chromosomes, which are made up of genes. If unravelled, the genes are made up of DNA.
Image: The diagram shows, in 3 linked illustrations, how a cell contains pairs of chromosomes, then how the chromosomes are made up of genes, and finally how the genes are made up of DNA. The left-hand illustration shows a cell. The cell contains a nucleus. In this are chromosomes, arranged into pairs. The middle illustration looks more closely at a pair of chromosomes. These are made up of 2 vertical thread-like structures joined together in the middle to make an ‘H’ shape. The right-hand illustration looks even more closely at the thread-like detail. It shows how, if unravelled, the genes are made up of DNA.

 

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The BCR-ABL1 gene

New cells are made when 1 cell divides into 2. Before a cell divides, it makes a copy of all the instructions stored in the genes on the chromosomes. CML develops when something goes wrong during this copying process.

A gene called ABL1, which is on chromosome 9, gets stuck to a gene called BCR on chromosome 22. They combine to make a new abnormal gene called BCR-ABL1. This is called a fusion gene.

This new BCR-ABL1 gene makes too much of a substance called tyrosine kinase. Too much tyrosine kinase can cause the bone marrow to make too many white blood cells. It also stops these cells developing into normal white blood cells or dying when they should. These abnormal cells are the CML cells.

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The Philadelphia (Ph) chromosome

When the BCR-ABL1 gene forms on chromosome 22, it changes how the chromosome looks. Scientists can see it when they look at the leukaemia cells under a special microscope. They call it the Philadelphia chromosome.

You are not born with this chromosome. If you have children, it cannot be passed on to them.

Most people with CML have the Philadelphia chromosome in all the leukaemia cells. It is only in the leukaemia cells. In less than 5 in 100 people with CML , the Philadelphia chromosome cannot be seen in bone marrow samples. But tests on the leukaemia cells can be used to find the BCR-ABL1 fusion gene.

How the Philadelphia chromosome develops

This diagram shows how the Philadelphia chromosome forms when sections of chromosomes 9 and 22 combine, creating a fusion gene (BCR-ABL) on chromosome 22.
Image: This diagram shows how the Philadelphia chromosome is made when sections from chromosome 9 and chromosome 22 join together. The chromosomes are shown as long thin structures with a short upper part and a longer lower part. Chromosome 9 is the longest. Two dots are shown on the lower end of the lower part of chromosome 9. These are labelled as the ABL gene. The lower end of the lower part of chromosome 9 is circled. Chromosome 22 is shorter. Two dots are shown at the upper end of the lower part. These are labelled as the BCR gene. The upper part and the upper end of the lower part of chromosome 22 is circled. There are lines going from the circled areas to the Philadelphia chromosome. The upper part and upper end of the lower part is identical to that of chromosome 22. It contains the BCR gene. The lower end of the lower part is identical to that of chromosome 9. It contains the ABL gene. These genes are labelled as the fusion gene – BCR and ABL combined. Chromosome 22 is a smaller chromosome. It also has two parts. The top part is small, the bottom part is long. The small top part and the top of the longer part have an area labelled as the BCR gene. On the right-hand side of the diagram, the ABL gene from Chromosome 9 and the BCR gene from Chromosome 22 are combined together. This makes the Philadelphia chromosome. This chromosome has a small part at the top and a longer part of the bottom. The bottom part is labelled as a fusion gene, which is BCR and ABL combined.

Rarely, some people develop a different type of leukaemia called atypical chronic myeloid leukaemia (aCML). It does not have the BCR-ABL1 gene and is treated differently to CML. Read more information about aCML on the Leukaemia Care website.

About our information

This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer.

  • References

    Below is a sample of the sources used in our chronic myeloid leukaemia (CML) information. If you would like more information about the sources we use, please contact us at

    informationproductionteam@macmillan.org.uk

     

    National Institute for Health and Care Excellence (NICE). Asciminib for treating chronic myeloid leukaemia after 2 or more tyrosine kinase inhibitors (Published 03 August 2022). Available from: www.nice.org.uk/guidance/ta813 (accessed July 2023).

     

    Smith, G, Apperley, J et al. A British Society for Haematology Guideline on the diagnosis and management of chronic myeloid leukaemia. British Journal of Haematology. 2022. Volume 191. Pages 171-193. Available from: onlinelibrary.wiley.com/doi/10.1111/bjh.16971 (accessed July 2023)

     

    Smith, G, Apperley, J et al. A British Society for Haematology Guideline on the diagnosis and management of chronic myeloid leukaemia. British Journal of Haematology. 2022. Volume 191. Pages 171-193. Available from: onlinelibrary.wiley.com/doi/10.1111/bjh.16971 (accessed July 2023)

Cancer Information Development team

Author

Dr Anne Parker SME

Dr Anne Parker

Reviewer

Consultant Haematologist and Honorary Clinical Senior Lecturer

Queen Elizabeth University Hospital, Greater Glasgow and Clyde

Date reviewed

Reviewed: 01 January 2025
|
Next review: 01 January 2028
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Trusted Information Creator - Patient Information Forum

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