Myeloproliferative neoplasms (MPN)
What is MPN?
Myeloproliferative neoplasms (MPN) are a group of disorders in which the bone marrow stem cells grow and reproduce abnormally. In MPN abnormal stem cells produce excess numbers of one or more types of blood cells (red cells, white cells and/or platelets). These abnormal cells cannot function properly and can cause serious health problems unless properly treated and controlled. MPNs are chronic diseases that, in most cases, remain stable for many years and progress gradually over time.
MPNs are sometimes described as being clonal blood stem cell disorders. This means that they result from a change, or mutation, in the DNA of a single blood stem cell. This change (or changes) results in abnormal blood cell development and in this case the overproduction of blood cells.
In MPN the original mutation is preserved when the affected stem cell divides (proliferates) and produces a ‘clone’: a group of identical stem cells all with the same defect. Mutations in dividing cells occur all the time and healthy cells have sophisticated mechanisms within them to stop these abnormalities persisting. But the longer we live, the more chance we have of acquiring mutations that manage to escape these safeguards.
MPNs are usually described according to the type of blood cell which is most affected. MPNs are closely related diseases, so it’s not uncommon for people to have features of more than one MPN when they are first diagnosed, or during the course of their illness. In some cases, one disorder may transform over time to another, or to a type of leukaemia called acute myeloid leukaemia.
How common is MPN?
Myeloproliferative neoplasms are a rare group of diseases. Polycythaemia vera is diagnosed in an estimated 250 Australians each year, essential thrombocythaemia around 200 and myelofibrosis an estimated 150. The rarer sub types of MPN, as a group, are diagnosed in less than 50 Australians per year.
Who gets MPN?
Most people with an MPN have no family history of the disease. MPN is more commonly diagnosed in people over the age of 50 although it can rarely occur in younger people, even very rarely in children.
What causes MPN?
The exact cause of MPNs remain unknown but there are likely to be a number of factors involved. That’s why MPNs, like most leukaemias and other cancers, become more common as we get older. A mutation of a particular gene (a segment of DNA that makes proteins) known as Janus kinase 2 (JAK2) is found in a large proportion of people with MPNs. The exact meaning of this mutation remains unclear but it appears to play a role in the overproduction of blood cells seen in these disorders. The discovery of a mutation in the JAK2 gene is important because it is likely to have a significant impact on the way MPNs are diagnosed and treated.
Long-term exposure to high levels of benzene or very high doses of ionising radiation may increase the risk of myelofibrosis in a small number of cases. Around one third of people with myelofibrosis have been previously diagnosed with polycythaemia or essential thrombocythaemia.
What are the symptoms of MPN?
Many people have no symptoms when they are first diagnosed with an MPN and the disease is picked up accidentally during a routine blood test or physical examination. In other cases, people go to see their GP because they have some troubling symptoms of their disease. When symptoms do occur, they develop gradually over time. Common symptoms include:
- blurred vision
- itchiness (pruritus)
- night sweats
- raised blood pressure (hypertension).
Other symptoms experienced in MPN are a result of the affected cell involved with the MPN.
Polycythaemia (rubra) vera (PV) symptoms
As the blood is thicker than normal it cannot flow as easily, especially through the smaller blood vessels. If left untreated, this increases the risk of the formation of a blood clot within a blood vessel. Blood clots are a common complication of PV and occur in around 30% of people, even before they are diagnosed. Bleeding and easy bruising can also occur. This is usually minor and occurs in around one quarter of all patients.
Essential thrombocythaemia (ET) symptoms
A major aim of treatment in ET is to reduce your platelet count, and therefore your risk of thrombosis. Less commonly, people experience symptoms of abnormal bleeding including bruising for no apparent reason, or exaggerated or prolonged bleeding following minor cuts or injury. In pregnancy, uncontrolled ET can reduce the blood supply to the placenta or fetus. This can cause problems with fetal growth and may in some cases lead to miscarriage. An enlarged spleen is common and occurs in around 30% of cases of ET. In some cases the liver may also be enlarged (hepatomegaly).