NORD gratefully acknowledges Ayalew Tefferi, MD, Professor of Medicine and Hematology, Mayo Clinic, for assistance in the preparation of this report.
Essential thrombocythemia, also known as ET, is a rare disease. The most important first fact about ET: on average, people with ET have a normal life expectancy. Patients with ET have increased numbers of platelets. Platelets are the smallest of the three types of blood cells and are needed for successful blood clotting after an injury. The two other types of blood cells are red blood cells, which carry oxygen to all tissues in the body, and white blood cells, which help to fight infections. Red blood cell numbers (often measured as a percentage of whole blood, called a hematocrit) are generally normal in ET, while white blood cell numbers are normal or slightly elevated in ET. Importantly, most people with an elevated platelet count do not have ET. Common alternative causes of an elevated platelet count are iron deficiency, infection or generalized inflammation; less common causes are blood disorders such as ET or other related blood diseases (also see below).
In the 1950’s, a pioneering hematologist, William Dameshek, placed ET within a family of blood diseases called myeloproliferative disorders. These were unified by their propensity to lead to abnormal increases in various blood cells, perhaps, in Dr. Dameshek’s words, "due to a hitherto undiscovered stimulus". At the time, it was unclear if these "proliferations" represented a natural response to some external cause, or were the result of an internal defect.
Over time, it became obvious that the myeloproliferative disorders are caused by genetic accidents (an internal defect) in very early blood cells (stem cells), which are then passed along to all of the progeny of that cell, even as they mature into platelets, red cells, or white blood cells (see below). In acknowledgement of this new understanding, myeloproliferative disorders have been renamed myeloproliferative neoplasms (MPN). For this reason, ET is best thought of as a chronic type of leukemia - albeit one with an overall excellent prognosis and often requiring minimal or no treatment.
The greatest health risk in patients with ET is an increased risk of developing blood clots. Blood clots can be in the deep vessels of the legs or lungs; ET patients are also more likely to experience strokes and heart attacks. ET patients can develop clots elsewhere, including within the abdomen, an otherwise rare site for clots to form. The risk of clotting increases with age, and disease-associated risks may be quite different for children than for adults, with children generally being at low risk for clots and other problems related to ET. In addition, (and somewhat counter-intuitively) a subset of ET patients may also be more likely to bleed; this appears to be restricted to a small minority of patients with a very high (over 1.5 million) platelet count. Other symptoms in ET include headaches, fatigue, temporary changes in vision, dizziness, ringing in the ears, vertigo and tingling in the hands.
Very rarely, patients with ET can experience an evolution from ET to a more advanced blood disease. ET can evolve into a related disease called myelofibrosis, or into acute leukemia. This evolution is sufficiently rare (within what is already a rare disease), that the estimates of the risk are imprecise but are thought to be on the order of 1-2% of patients with ET over a lifetime.
Over the ensuing decades, Dr. Dameshek’s predictions about a stimulus triggering the proliferation of blood cells were confirmed. The first came in the 1960s, when the genetic basis for another MPN family member known as chronic myelogenous leukemia (CML) was identified as the Philadelphia chromosome, named after the city in which it was discovered. The Philadelphia chromosome is an abnormal chromosome caused by the fusion of two chromosomes in the leukemia cells. This genetic change causes a specific protein, called a kinase, to be overactive. Because kinases are very powerful drivers of cell growth, this genetic change leads to an elevation in blood counts and enlargement of the liver and spleen. Once the genetic basis of CML was identified, drugs that might interfere with kinase activity (kinase inhibitors) were tested and proved to be extraordinarily successful in treating CML. Collectively these studies radically reversed the natural history of the CML by converting what was previously a fatal disease to one with an excellent prognosis. The majority of patients with CML are diagnosed because they have a high white blood cell count, but on occasion patients with CML will only have a high platelet count, therefore every patient with suspected ET is also evaluated for CML, with a test for the Philadelphia chromosome.
It took decades longer for the specific genetic basis for ET to be identified. In 2005, four separate groups of investigators discovered a variation (mutation) in the JAK2 gene in 50-60% of patients with ET. This variation, like that in CML, leads to overactivity in a type of enzyme called a kinase, specifically Janus kinase 2 (JAK2). Genes provide instructions for creating proteins that play a critical role in many functions of the body. When a mutation of a gene occurs, the protein product may be faulty, inefficient, absent, or overproduced. Depending upon the functions of the particular protein, this can affect many organ systems of the body.
The liver normally produces a hormone called thrombopoietin. This hormone binds to hematopoietic stem cells, which are immature cells found in the bone marrow that eventually become red cells, white cells, and platelets. When this binding occurs. The JAK2 enzyme makes the hematopoietic cells divide into precursor cells that become platelets (megakaryocytes) and platelets. The platelets are misshapen and can be abnormally large. These extra, abnormally-shaped platelets increase the risk of blood clots.
For the more than 40% of ET patients without the JAK2 mutation, the genetic basis for those patients was unknown until 2013. Two groups of investigators reported that ET patients commonly have a variation in a gene called calreticulin (CALR). CALR mutations were found in approximately 70% of the patients with ET who did not have a JAK2 mutation. Although the function of CALR within a cell is less well understood than that of JAK2, it appears that CALR revs up the same cellular machinery that is abnormally active in ET with a JAK2 mutation. This makes sense, since CALR-mutated patients with ET are virtually indistinguishable from those ET patients who have the JAK2 mutation. Calreticulin is a receptor, a protein found on the surface of hematopoietic cells to which thrombopoietin binds. Much more rarely other variations are found in ET, but mutations either in JAK2 or in CALR are by far the most common, accounting for over 75% of patients with ET.
Fewer than 1 in 100,000 people are diagnosed with ET in any year (the most recent estimates range from 0.38 to 1.7 per 100,000). Women are more likely to be diagnosed with ET than men, although the reason for this is unknown. The average age of onset is mid-fifties, but the range is wide, and includes women in their childbearing years, which makes up an important subset of ET patients with special therapeutic considerations (discussed below). In children ET is exceedingly rare and typically is an inherited genetic disorder. In adults, the genetic mutations typically identified in ET (described below) are not inherited, and instead are acquired genetic accidents (known as an acquired mutation) that happen during an individual’s lifetime. Genetic accidents happen to all of us as we age, although they do not always result in a disease.
Many, if not most ET patients have no symptoms related to their disease when they are diagnosed, and instead have an abnormally high platelet count identified on routine blood tests. Other ET patients are identified when they have symptoms or a complication – often a blood clot – related to ET. Additional tests may be performed to confirm the elevated platelet count, and to elucidate its possible causes. These may include blood tests that evaluate for iron deficiency and/or inflammatory diseases, and genetic tests for mutations that are seen in ET or related diseases. If no other obvious cause for an elevated platelet count is identified, and/or ET or a related blood disorder is suspected, a hematologist will typically recommend a bone marrow biopsy.
A bone marrow biopsy is a safe office procedure where a small piece of bone and a small amount of liquid bone marrow are obtained from the hip bone. Because all blood cells are born in and go through early life in the bone marrow, a bone marrow biopsy is used to directly visualize the bone marrow cells and their architecture within the bone. Many blood disorders are diagnosed by looking at the early blood cells within the bone marrow. Additional genetic and molecular testing on the liquid bone marrow (bone marrow aspirate) also provides valuable information. Together, these findings are used to establish a diagnosis of ET or a related disorder.
Treatment is geared towards two goals: making patients with ET-related symptoms feel better, and reducing the risk of clotting events.
Risk of clotting events is what guides hematologists in their treatment recommendations. Hematologists are doctors that specialize in the diagnosis and treatment of blood disorders. The risk of clots in ET patients increases over time, with patients over the age of 60 having a relatively high risk. In addition, patients who have had a clotting event in the past are at high risk for subsequent events. Other meaningful but less significant risk factors for clots are tobacco use, high blood pressure, diabetes, and the presence of the JAK2 mutation that was discussed above.
For most patients with ET, a low dose aspirin (usually 81-100 mg daily) is recommended to reduce the risk of clots. Aspirin may not be recommended for some patients who are thought to be at a very low risk for blood clots, or who may be at a higher risk for bleeding (a side-effect of aspirin), or in those with an allergy or other sensitivity to aspirin. Likewise, aspirin may not be recommended for patients on other medications such as blood thinners that increase the risk of bleeding.
For patients considered at high risk for clotting, such as those with a prior clot, or a combination of other risks, a medication known as hydroxyurea or hydroxycarbamide is often recommended. This medication is recommended because it has been proven to meaningfully reduce the risk of ET-related complications such as clots. Patients at high risk for events have a risk of experiencing a significant clotting event that can exceed 3.5% per year. Hydroxyurea is an oral chemotherapy, and the most common effect of hydroxyurea is the lowering of blood counts. Less common side effects include mouth sores, and leg ulcers. Hypersensitivity reactions, such as fevers, rash or other allergic-type symptoms are uncommon. Long term use of hydroxyurea can also increase the risk of non-melanoma skin cancers, thus patients on hydroxyurea should be particularly mindful about sun exposure. There also remains an unresolved debate among hematologists as to whether hydroxyurea may marginally increase the risk of evolution of ET to acute leukemia. Although available data on the safety of hydroxyurea in ET and other MPN are reassuring and no studies have demonstrated an increased risk for leukemia, there have not been definitive studies to resolve this issue. Generally, if a hematologist recommends hydroxyurea it is because she or he feels that the benefits of this therapy outweigh its risks.
Other medications used to treat ET include an oral medication called anagrelide, the chemotherapy medication busulfan, and an injectable medication called interferon; some patients with ET (such as those who have had a clot in liver veins) take blood-thinning medications such as warfarin. For those ET patients not considered at high risk for clots, but who are experiencing symptoms related to ET, the same treatments options discussed above are available.
Several special considerations should be given in women with ET who are pregnant or attempting to become pregnant. Because of the risks to the developing fetus, many medications used to treat ET should be avoided, including hydroxyurea, anagrelide and warfarin. Pregnancy in general increases a woman’s risk of clots, and women with ET are particularly vulnerable. An injectable blood thinner that is safe in pregnancy – such as heparin or low molecular weight heparin – may be recommended to ET patients during and/or for a short period after pregnancy. If additional ET therapy is needed, interferon can also be safely used during pregnancy.
Smoking puts patients with ET at a particularly high risk for clots, so it is always recommended that an ET patient stop smoking, as difficult as that may be. There are no known additional diets or special lifestyle recommendations for patients with ET.
People with ET can live a long life with an excellent quality of life.
Establishing a diagnosis of ET requires evaluating the bone marrow and a thorough evaluation for other disorders.
Patients with ET should periodically see a hematologist who is experienced in treating patients with this disorder.
Low dose aspirin is commonly recommended to treat symptoms and to reduce the risk of clots in patients with ET.
Patients with uncontrolled symptoms related to ET or patients at high risk for clots may need additional medical therapy
A detailed and continuously updated listing of clinical studies for patients with ET is available on the website clinicaltrials.gov. Of interest are:
Ruxolitinib and related JAK inhibitors: In 2011 the Food and Drug Administration approved an oral medication, ruxolitinib, for the treatment of another MPN known as myelofibrosis. Ruxolitinib inhibits the JAK2 kinase, and since JAK2 is abnormally active in ET patients, ruxolitinib and other JAK inhibitors are currently being tested in clinical trials for rare ET patients with difficult-to-treat disease.
Interferon: The effectiveness of interferon (specifically the longer acting form, pegylated interferon) is also being studied in ET.
Interferon-alfa and pegylated interferon (a longer acting formulation), are approved medications for other diseases, and are used in special situations to treat some individuals with ET.
Another therapy rarely used for people with ET is plateletpheresis, which involves the removal of platelets from the circulating blood (this is also the process used by the Red Cross to collect platelets from normal donors). Plateletpheresis is used for short term control in rare special acute situations encountered by ET patients. Plateletpheresis has not been extensively studied for ET.
Information on current clinical trials is posted on the Internet at https://clinicaltrials.gov/. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.
For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:
Toll-free: (800) 411-1222
TTY: (866) 411-1010
Some current clinical trials also are posted on the following page on the NORD website:
For information about clinical trials sponsored by private sources, contact:
For information about clinical trials conducted in Europe, contact:
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