Polycythemia vera (PV), the most common form of primary polycythemia, is caused by somatic mutation in a single hematopoietic stem cell leading to clonal hematopoiesis. PV is a myeloproliferative disorder characterized predominantly by erythroid hyperplasia, but also by myeloid leukocytosis, thrombocytosis, and splenomegaly. Familial cases of PV are very rare and usually manifest in elderly patients (Cario, 2005). PV is distinct from the familial erythrocytoses (see, e.g., ECYT1, 133100), which are caused by inherited mutations resulting in hypersensitivity of erythroid progenitors to hormonal influences or increased levels of circulating hormones, namely erythropoietin (EPO; 133170) (Prchal, 2005).

Familial erythrocytosis-2 (ECYT2) is an autosomal recessive disorder characterized by increased red blood cell mass, increased serum levels of erythropoietin (EPO; 133170), and normal oxygen affinity. Patients with ECYT2 carry a high risk for peripheral thrombosis and cerebrovascular events (Cario, 2005). Familial erythrocytosis-2 has features of both primary and secondary erythrocytosis. In addition to increased circulating levels of EPO, consistent with a secondary, extrinsic process, erythroid progenitors may be hypersensitive to EPO, consistent with a primary, intrinsic process (Prchal, 2005). For a general phenotypic description and a discussion of genetic heterogeneity of familial erythrocytosis, see ECYT1 (133100).

Familial erythrocytosis is an inherited condition characterized by an increased number of red blood cells (erythrocytes). The primary function of these cells is to carry oxygen from the lungs to tissues and organs throughout the body. Signs and symptoms of familial erythrocytosis can include headaches, dizziness, nosebleeds, and shortness of breath. The excess red blood cells also increase the risk of developing abnormal blood clots that can block the flow of blood through arteries and veins. If these clots restrict blood flow to essential organs and tissues (particularly the heart, lungs, or brain), they can cause life-threatening complications such as a heart attack or stroke. However, many people with familial erythrocytosis experience only mild signs and symptoms or never have any problems related to their extra red blood cells.

Primary familial and congenital polycythemia (PFCP) is characterized by isolated erythrocytosis in an individual with a normal-sized spleen and absence of disorders causing secondary erythrocytosis. Clinical manifestations relate to the erythrocytosis and can include plethora, the hyperviscosity syndrome (headache, dizziness, fatigue, lassitude, visual and auditory disturbances, paresthesia, myalgia), altered mental status caused by hypoperfusion and local hypoxia, and arterial and/or venous thromboembolic events. Although the majority of individuals with PFCP have only mild manifestations of hyperviscosity such as dizziness or headache, some affected individuals have had severe and even fatal complications including arterial hypertension, intracerebral hemorrhage, deep vein thrombosis, coronary disease, and myocardial infarction. To date 116 affected individuals from 24 families have been reported.