You may hear your doctor talk about a ‘Philadelphia chromosome’ or refer to your ‘Philadelphia status’. If you have this chromosome it almost always means you have CML, and it’ll be inside all of your leukaemia cells.
You can’t inherit this chromosome, or pass it on to your children.
All cells in your body contain a set of instructions which tell the cell what to do and when to do it, stored inside the cells in structures called chromosomes. The chromosomes are made up of a chemical known as DNA.
The DNA is arranged in sections called genes. There are 23 pairs of chromosomes in each cell in your body.
When cells divide to form new cells, normally the chromosomes stay the same in each new cell.
CML is thought to begin when an event takes place which doesn’t usually happen. A small part (the ABL1 gene) of chromosome 9 gets stuck next to a small part (the BCR gene) of chromosome 22 by mistake when the cell divides. They form a new fusion gene called BCR-ABL1 and make chromosome 22 shorter than normal. This new, shorter chromosome is called the Philadelphia chromosome, because that’s where it was discovered.
This swapping of genetic material is called a translocation, or chromosomal translocation. This particular translocation is sometimes called t(9;22).
The new BCR-ABL1 gene is important in the development of CML because it makes a new protein (also called BCR-ABL1). This protein is a type of enzyme known as tyrosine kinase, that causes the leukaemia stem cells to divide more often and to live longer than usual.
If doctors can detect the BCR-ABL1 fusion gene, it can help them to diagnose CML – and also monitor your response to treatment. The standard treatment for CML is to use drugs that inhibit the tyrosine kinase enzyme and block its effects.