JAK2 is a member of the Janus kinase family.In 2013, two groups detected calreticulin mutations in a majority of JAK2-negative/MPL-negative patients with essential thrombocythemia and primary myelofibrosis, which makes CALR mutations the second most common in myeloproliferative neoplasms. All mutations (insertions or deletions) affected the last exon, generating a reading frame shift of the resulting protein, that creates a novel terminal peptide and causes a loss of endoplasmic reticulum KDEL retention signal. Diagnosis
The following revised diagnostic criteria for essential thrombocythemia were proposed in 2005. The diagnosis requires the presence of both A criteria together with B3 to B6, or of criterion A1 together with B1 to B6. The criteria are as follows:
A1. Platelet count > 400 × 103/µL for at least 2 months. A2. Acquired V617F JAK2 mutation present
B1. No cause for a reactive thrombocytosis
normal inflammatory indices
B2. No evidence of iron deficiency
stainable iron in the bone marrow or normal red cell mean corpuscular volume
B3. No evidence of polycythemia vera
hematocrit < midpoint of normal range or normal red cell mass in presence of normal iron stores
B4. No evidence of chronic myeloid leukemia
But the Philadelphia chromosome may be present in up to 10% of cases. Patients with the Philadelphia chromosome have a potential for the development of acute leukemia, especially acute lymphocytic leukemia. B5. No evidence of myelofibrosis
no collagen fibrosis and ≤ grade 2 reticulin fibrosis (using 0–4 scale)
B6. No evidence of a myelodysplastic syndrome
no significant dysplasia
no cytogenetic abnormalities suggestive of myelodysplasia

Treatment
Indications
Not all those affected will require treatment at presentation.
The lifespan of a well-controlled ET person is well within the expected range for a person of similar age but without ET. ET is the myeloproliferative neoplasm least likely to progress to acute myeloid leukemia. Epidemiology
The incidence of ET is 0.6-2.5/100,000 per year, the median age at onset is 65–70 years and it is more frequent in females than in males. The incidence in children is 0.09/100,000 per year. Pregnancy
Hydroxycarbamide and anagrelide are contraindicated during pregnancy and nursing. Essential thrombocythemia can be linked with a three-fold increase in risk of miscarriage. Throughout pregnancy, close monitoring of the mother and fetus is recommended. Low-dose low molecular weight heparin (e.g. enoxaparin) may be used. For life-threatening complications, the platelet count can be reduced rapidly using plateletpheresis, a procedure that removes platelets from the blood and returns the remainder to the patient. References


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