Abstract
Human diseases have provided important insights into the function of fundamental physiological processes, and studying erythrocytosis has increased our understanding of the oxygen sensing pathway. In some cases this rare disorder arises specifically from dysregulation of the erythropoietin (Epo) axis. Epo gene transcription is under the control of a negative feedback mechanism involving the kidneys, which sense tissue hypoxia at the molecular level via the hypoxia inducible factor (HIF) transcription complex. Both subunits of the HIF complex, alpha and beta, are constitutively expressed but only the beta subunit is detectable in the presence of oxygen. Key prolines in the oxygen degradation domain of HIFalpha are hydroxylated by a family of prolyl hydroxylase enzymes, PHD1-3. Of these PHD2 has the most widespread tissue distribution. Upon hydroxylation of HIF, the von Hippel Lindau (VHL) protein, a component of an E3 ligase complex, promotes the ubiquitination of the alpha subunit. HIFalpha is then targeted to the proteasome and the transcription of HIF target genes is prevented. In hypoxia the activity of the PHD enzymes is low and the HIF complex assembles causing upregulation of genes such as Epo. Over the last ten years we have maintained a registry of individuals with erythrocytosis. To date 181 patients have been included with clinical details and consented DNA samples have been collected. There is a preponderance of males with ratio of 1.7 males to each female. The mean age of erythrocytosis individuals on the registry is 37 years. The majority of erythrocytosis patients have inappropriately normal (46%) or raised (26%) serum Epo levels as compared to PV, where the Epo level is often undetectable. Thus it can be inferred that dysregulation of the Epo axis via the oxygen sensing pathway would be a significant cause of erythrocytosis. Consequently, PHD2 and VHL have been investigated. We have now identified 2 different mutations, Pro317Arg and Arg371His, in PHD2. In the tertiary structure of PHD2, Pro317and Arg371 are close to essential iron binding residues and furthermore suggest the location of a HIF binding groove. The Arg200Trp VHL mutation, commonly described as the Chuvash mutation, has been detected in 8 Asian families and their large kindred. Two Western European individuals were found to be compound heterozygous for the Arg200Trp and either the novel Pro192Thr or Gly144Arg variants. Intriguingly 2 further individuals possess one wild type and one mutated allele, Arg200Trp or Leu188Val. Screening other genes of the oxygen sensing pathway has failed to reveal further defects in these individuals. In summary, defects in the oxygen sensing pathway are associated with the development of erythrocytosis. Mutations in VHL are more common than PHD2 and highlight the importance of these proteins in the maintenance of red cell homeostasis.
Catalytic Properties of the Asparaginyl Hydroxylase (FIH) in the Oxygen Sensing Pathway Are Distinct from Those of Its Prolyl 4-Hydroxylases
