Polycythemia and oxygen sensing

Polycythemias can be differentiated based on the responsiveness of erythroid progenitors to circulating cytokines. Primary polycythemias are characterized by an augmented response due to acquired somatic or inherited germ-line mutations that are expressed within hematopoietic progenitors causing increased proliferation or decreased apoptosis and resulting in accumulation of red blood cells. In terms of oxygen requirements, primary polycythemias can be viewed as the production of hemoglobin fully dissociated from the tissue oxygen needs and from the oxygen sensing pathway. Polycythemia vera (PV) is the most common primary polycythemia. PV bone marrow progenitors cells can form erythroid colonies in the absence of exogenous erythropoietin in vitro. These endogenous erythroid colonies (EEC) are useful in differentiating PV and secondary polycythemias. They also can differentiate PV where this feature is independent of Epo signalling from primary familial and congenital polycythemia. In this autosomal dominant primary polycythemia, at variance with PV, EEC formation is abolished by anti-Epo and anti-Epo receptor neutralising antibodies. Mutations of the EPOR have been described and resulted in nine cases in truncated EPORs lacking the cytoplasmic carboxy-terminal of the receptor which possesses a negative growth regulatory domain. However, recent data suggest that different mutations may cause PFCP in most cases. Secondary polycythemia can be viewed as either physiological response to satisfy the oxygen needs of the tissues, resulting for instance from high affinity hemoglobins or BPG mutase deficiency, or as the result of germ-line or somatic mutations disturbing the oxygen sensing pathway or its target: Epo. Chuvash polycythemia is a frequently symptomatic disorder with an autosomal recessive inheritance and inappropriately high Epo levels. The erythroid progenitors are hypersensitive to Epo linking this condition to both primary and secondary polycythemia. A germline missense mutation at nucleotide 598 in both alleles of the von Hippel-Lindau gene results in increased hypoxia inducible factor-1 (HIF-1) expression in normoxic conditions. HIF-1 controls the expression of many genes including Epo. Identifying causal defects in other situations like post-renal transplant erythrocytosis and cases of autosomal dominant polycythemia with high Epo levels will help further understanding of the regulation of erythropoiesis.