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Myeloproliferative Neoplasms

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Also known as: MPNs; Myeloproliferative Disorders; MPDs

What are myeloproliferative neoplasms?

Myeloproliferative neoplasms (MPNs) are a subset of bone marrow disorders. They are a group of diseases characterized by the production of too many of one or more types of blood cells in the bone marrow.

Bone marrow is a soft fatty tissue that is located in the center of the body's larger bones. It has a honeycomb or sponge-like structure, consisting of a highly organized meshwork that is filled with liquid. The liquid contains stem cells and a mixture of red blood cells (RBC), white blood cells (WBC), and platelets in various stages of development.

Normally, the body maintains a dynamic but relatively stable number of blood cells in circulation. As cells age, die, or are removed from circulation, new ones are made in the marrow to replace them. When a particular kind of blood cell is needed, some of the stem cells in the bone marrow begin to change. Those stem cells become immature "blast" forms of whatever cell type is in short supply. These blasts mature to become white blood cells, red blood cells, or platelets. Usually only fully mature cells are released into circulation.

With an MPN, too much production of a cell's precursors (e.g., stem cells and blasts) leads to an increase in that type of mature cell. That causes a corresponding increase or decrease in the number of other normal blood cells, which may be inhibited and crowded out of the bone marrow. This results in symptoms related to blood cell overproduction, shortages, and dysfunction throughout the body.

The common types of MPNs include:

  • Chronic myeloid (myelogenous, myelocytic) leukemia (CML)—CML usually occurs in adults, with people 65 and over at greater risk. It rarely occurs in children but is the most common pediatric MPN. People with CML often have no symptoms at first and frequently are diagnosed incidentally during a routine blood test or physical. When symptoms do appear, they are similar to common, less serious illness and include low energy, pale skin, stomach discomfort caused by an enlarged spleen, and unexplained weight loss. CML can be traced to abnormal chromosomes where, inside a stem cell in the bone marrow, pieces from two chromosomes break off and switch places (translocation). This results in an altered, fused gene (called BCR/ABL1) on an abnormal chromosome 22 (also known as the Philadelphia chromosome). This altered gene makes an abnormally functional protein that leads to the overproduction of white blood cells. Left untreated, CML leads to anemia, poor immunity, excessive bruising and bleeding, and a markedly enlarged spleen.
  • Polycythemia vera (also known as PV, primary polycythemia (neoplastic))—a disease in which too many red blood cell precursors are produced in the bone marrow, independent of the mechanisms that normally regulate red blood cell production. This leads to too many red blood cells circulating in the blood. When RBCs build up in the bloodstream, the blood becomes thicker and does not flow smoothly in the blood vessels, causing symptoms such as headache, dizziness, problems with vision, and even excessive clotting or heart attack. A variety of other factors can cause increased red blood cell production; for instance, long-term exposure to low concentrations of oxygen (e.g., emphysema/COPD or living at high altitude). These increases in RBCs are referred to as secondary polycythemia (reactive).
  • Primary myelofibrosis (PMF, previously known as chronic idiopathic myelofibrosis and agnogenic myeloid metaplasia)—a disease where fibrous cells gradually replace normal bone marrow tissue. The dense fiber network impairs bone marrow function and blood cell production and can lead to production of blood cells outside the bone marrow, typically in the liver or spleen (so-called extramedullary hematopoiesis or EMH). The red blood cells that do enter the bloodstream can be malformed, looking like teardrops instead of circles. There may be too few normal, mature red blood cells to carry oxygen, causing anemia.
  • Essential thrombocythemia (ET)—a disease characterized by an increased number of megakaryocytes, precursors to platelets (also called thrombocytes), in the bone marrow as well as sustained and dramatic increases of platelets in the blood. Excess platelets in blood can make it hard for the blood to flow normally and therefore increases a person's risk of developing inappropriate blood clots or of having a stroke. On the other hand, the platelets may not function normally, leading to bleeding. Essential thrombocythemia must be distinguished from secondary or reactive thrombocytosis, which is an increased number of platelets caused by non-neoplastic marrow disorders such as iron deficiency, infection, inflammation (e.g., rheumatoid arthritis), bleeding, or removal of the spleen.

MPNs are generally not curable, but their slow progression can usually be controlled and their symptoms managed. For each MPN, there is a slight chance that the disease will develop into an acute leukemia. If this occurs, the course of the disease will be accelerated, the symptoms will intensify, and more aggressive treatment will be required.

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