What Are Myeloproliferative Neoplasms?
Myeloproliferative neoplasms (MPN) are a group of diseases that affect blood-cell formation. In all forms of MPN, a bone marrow problem leads to increased levels of blood cells circulating in the bloodstream.
- “Myelo” refers to bone marrow, which is the body’s blood-cell factory.
- “Proliferative” refers to the rapid growth and production of cells.
- “Myeloproliferative” means increased growth and production of bone marrow and blood cells.
- “Neoplasm” means an abnormal growth of cells. A neoplasm can be either benign (noncancerous) or malignant (cancerous). In MPN, the neoplasm starts out benign; over time it may turn into malignant disease.
To understand more about MPN, it’s important to understand some basics about how blood cells normally form.
Healthy Blood-Cell Formation
In a healthy person, the stem cells in the bone marrow make daughter cells. The daughter cells go through several stages of development within the marrow. Eventually they mature into red blood cells (RBCs), white blood cells (WBCs), or platelets. These get released from the marrow to circulate in the bloodstream.
The circulating cells perform important functions.
- RBCs carry oxygen throughout the body.
- WBCs fight infection.
- Platelets protect against easy bleeding by helping your blood to clot.
Blood-Cell Formation in MPN
In a person with MPN, stem cells in the bone marrow develop genetic defects (called acquired defects) that cause them to grow and survive abnormally. This results in unusually high numbers of blood cells in the bone marrow (hypercellular marrow) and in the bloodstream.
- A high level of RBCs is called polycythemia.
- A high level of WBCs is called leukocytosis.
- A high level of platelets is called thrombocytosis.
High levels of blood cells cause some of the symptoms of MPN.
Sometimes in MPN, the abnormal stem cells cause scarring in the marrow, called myelofibrosis. Myelofibrosis may lead to low levels of blood cells, especially low levels of red blood cells (anemia).
In MPN, the abnormal stem cells can also grow in the spleen, causing the spleen to enlarge (splenomegaly), and in other sites outside the marrow, causing enlargement of other organs.
Types of MPN
There are several types of chronic MPN, based on the cells affected. The information about diagnosis and treatment in this section is mainly about the three classic types of MPN.
- Polycythemia vera (PV), in which there are too many RBCs
- Essential thrombocythemia (ET), in which there are too many platelets
- Primary myelofibrosis (PMF), in which fibers and blasts (abnormal stem cells) build up in the bone marrow
Other types of MPN include the following:
- Chronic myeloid leukemia, in which there are too many WBCs
- Chronic neutrophilic leukemia, in which there are too many white blood cells called neutrophils
- Chronic eosinophilic leukemia, not otherwise specified, in which there are too many white blood cells called eosinophils (hypereosinophilia)
- Mastocytosis, also called mast cell disease, in which there are too many mast cells, which are a type of immune system cell found in tissues, like skin and digestive organs, rather than in the bloodstream
- Myeloproliferative neoplasms, unclassifiable—meaning your MPN does not fit into one of the other types
- Myeloid and lymphoid neoplasms with eosinophilia and abnormalities of the PDGFRA, PDGFRB, and FGFR1 genes
Some problems with blood-cell formation are not only myeloproliferative (having to do with overproduction of cells in the marrow) or only myelodysplastic (having to do with abnormal production of cells in the marrow). Instead, these problems have features of both MPN and myelodysplastic syndrome (MDS). They are described in the section about MDS subtypes.