Polycythaemia rubra vera
Aetiology and pathophysiology
Polycythaemia rubra vera (PRV) is a primary neoplastic disorder in which bone marrow erythropoiesis is increased, usually accompanied by increased thrombopoiesis and granulopoiesis. Serum erythropoietin (EPO) levels are low. The JAK2 gene product is a tyrosine kinase that has a key role in signal transduction (see Chapter 1). Mutation of JAK2 is present in >95% of cases of PRV (and also in about 50% of cases with essential thrombocythaemia and primary myelofibrosis) (see Chapter 26). The mutation has the effect of amplifying the growth-promoting action of EPO. It is not clear why the same mutation underlies three different, although clearly related, diseases. The mutation is usually heterozygous, but in a minority it is homozygous and has a more profound effect.