Myeloproliferative neoplasms (MPN) are disorders that affect how the body produces red blood cells, white blood cells, and platelets.

Every day, our bodies create billions of blood cells - red blood cells (RBCs) to carry oxygen round the body, white blood cells (WBCs) to fight infections, and platelets to make the blood clot when we cut ourselves. These blood cells are produced from stem cells inside the bone marrow which grow and mature into blood cells. Usually the stem cells mature into blood cells in a very tightly-regulated way, under the control of a number of chemicals (hormones) in the blood stream. This ensures that we have just the right number of RBCs, WBCs and platelets.

Sometimes the stem cells develop an abnormality, often called a mutation, which means they produce too many (or sometimes too few) blood cells. When this happens, a blood disorder known as a myeloproliferative neoplasm (MPN) can result.

The four commonest MPN subtypes are:

  • essential thrombocythaemia (too many platelets);
  • polycythaemia vera (too many RBCs);
  • chronic myeloid leukaemia (too many neutrophils, which are a type of WBC);
  • myelofibrosis (replacement of the stem cells in the bone marrow by scar and fibrous tissue causing too few blood cells and often an enlarged spleen).

However, there are a number of rarer MPN such as systemic mastocytosis and hypereosinophilic syndrome. Sometimes people can have an MPN which has overlapping features such as both high platelets and high RBCs. It should also be noted that people can have high blood counts which are not caused by an MPN – for instance our platelet counts often go up as a natural response to an infection and return to normal afterwards.

MPN are classed by the World Health Organization (WHO) as blood cancers because the bone marrow is producing blood cells in an uncontrolled way. However, patients with MPN often feel very well and in the majority of cases can live normal, or near-normal lives, with relatively gentle treatment. In fact essential thrombocythaemia and polycythaemia vera are regarded by some specialists as pre-cancerous conditions, rather than actual cancers, because patients with these conditions very rarely develop any cancer-type problems. The most significant problem associated with these two conditions is an increased risk of blood clots.

In most cases, MPN are caused by the development of mutations in the genes of our stem cells, but we don’t yet really understand why some people develop these mutations and others don’t. These mutations are not usually inherited in the same way as, for instance, the mutations which cause haemophilia, so MPN do not normally run in families.