Oxygen sensing: recent insights from idiopathic erythrocytosis.

Idiopathic erythrocytosis (IE) is a rare condition in which there is an increase in red cell mass and hematocrit. As it is typically driven by elevated or inappropriately normal erythropoietin (Epo) levels, it has the potential to reveal the identities of proteins involved in the oxygen sensing pathway that regulates the transcription factor, Hypoxia Inducible Factor (HIF), and hence Epo production in humans. One example of this is provided by Chuvash polycythemia, a form of erythrocytosis due to a mutation in the von Hippel Lindau tumor suppressor protein (VHL), a component of an E3 ubiquitin ligase complex that targets hydroxylated HIF for degradation. A recent report of familial erythrocytosis now implicates a different protein, Prolyl Hydroxylase Domain protein 2 (PHD2), which is an enzyme that hydroxylates HIF.