Chronic myeloid leukaemia (CML) is a type of leukaemia and therefore a type of blood cancer. The word ‘chronic’ in the name indicates that the disease may develop slowly. The word ‘myeloid’ refers to the type of blood cells affected, which are called myeloid cells.

In CML, the myeloid cells in the bone marrow multiple uncontrollably and the cells may not be fully developed and mature. The myeloid cells cannot be considered completely normal and may function to fight infection but not as well as they should.

In the case of myeloblasts, these derive from myeloid stem cells, and normally develop into fully functioning myeloid cells, platelets or red blood cells. There are several types of myeloid cells which have different functions, such as fighting viral and bacterial infections and preventing the spread of tissue damage.

Myeloid cells mature and develop to form red blood cells, platelets and other types of white blood cells such as monocytes and granulocytes. A myeloid stem cell becomes one of three types of mature blood cell,

• Red blood cells that carry oxygen to all tissues of the body.
• Platelets that form blood clots to stop bleeding.
• Granulocytes (white blood cells, including neutrophils and monocytes) that fight infection and disease.

CML occurs when a myeloid cell develops changes/alterations (mutations) in its DNA. A cell’s DNA controls how a cell develops and functions, to multiply at a set rate and to die at a set time. A cell with altered DNA may be unable to fully develop and mature properly to fulfil its function to protect against infection. In addition, the usual controls for growth and dying may be lost, thereby allowing the resulting abnormal cells to continue to multiply quickly. The abnormal myeloid cells could now be called leukaemia cells or CML cells.

When CML develops, any of the types of myeloid cell may multiply uncontrollably however, this is often granulocytes. These CML cells may multiply and survive better than normal cells. As the CML cells accumulate and begin to fill up the bone marrow, they prevent it from producing healthy blood cells. As a result, the number of healthy blood cells may be lower than normal, red cells, platelets and white blood cells.

This can lead to:

As the amount of immature myeloid cells/ blast cells increases in the blood and bone marrow, there is less room for healthy white blood cells, red blood cells, and platelets. The number of blast cells in the blood and bone marrow and the severity of signs or symptoms are used to determine the phase of the disease.

The result is a new gene called BCR-ABL1 which results in the cells making too much of a protein called tyrosine kinase. The extra protein encourages the leukaemia cells to grow and multiply.

There are targeted cancer drugs called tyrosine kinase inhibitors (usually imatinib) which are used to block the protein which helps stop the leukaemia cells from multiplying.

Only 5% of CML patients do not have the Philadelphia Chromosome/ BCR-ABL1 gene and this is called Atypical CML.