Tibetan PHD2, an allele with loss-of-function properties.

Tibetans have adapted to the chronic hypoxia of high altitude and display a distinctive suite of physiologic adaptations, including augmented hypoxic ventilatory response and resistance to pulmonary hypertension. Genome-wide studies have consistently identified compelling genetic signatures of natural selection in two genes of the Hypoxia Inducible Factor pathway, PHD2 and HIF2A The product of the former induces the degradation of the product of the latter. Key issues regarding Tibetan PHD2 are whether it is a gain-of-function or loss-of-function allele, and how it might contribute to high-altitude adaptation. Tibetan PHD2 possesses two amino acid changes, D4E and C127S. We previously showed that in vitro, Tibetan PHD2 is defective in its interaction with p23, a cochaperone of the HSP90 pathway, and we proposed that Tibetan PHD2 is a loss-of-function allele. Here, we report that additional PHD2 mutations at or near Asp-4 or Cys-127 impair interaction with p23 in vitro. We find that mice with the Tibetan Phd2 allele display augmented hypoxic ventilatory response, supporting this loss-of-function proposal. This is phenocopied by mice with a mutation in p23 that abrogates the PHD2:p23 interaction. Hif2a haploinsufficiency, but not the Tibetan Phd2 allele, ameliorates hypoxia-induced increases in right ventricular systolic pressure. The Tibetan Phd2 allele is not associated with hemoglobin levels in mice. We propose that Tibetans possess genetic alterations that both activate and inhibit selective outputs of the HIF pathway to facilitate successful adaptation to the chronic hypoxia of high altitude.

Identification of Small-Molecule PHD2 Zinc Finger Inhibitors that Activate Hypoxia Inducible Factor.

The prolyl hydroxylase domain (PHD) protein:hypoxia inducible factor (HIF) pathway is the main pathway by which changes in oxygen concentration are transduced to changes in gene expression. In mammals, there are three PHD paralogues, and PHD2 has emerged as a particularly critical one for regulating HIF target genes such as erythropoietin (EPO), which controls red cell mass and hematocrit. PHD2 is distinctive among the three PHDs in that it contains an N-terminal MYND-type zinc finger. We have proposed that this zinc finger binds a Pro-Xaa-Leu-Glu (PXLE) motif found in proteins of the HSP90 pathway to facilitate HIF-α hydroxylation. Targeting this motif could provide a means of specifically inhibiting this PHD isoform. Here, we screened a library of chemical compounds for their capacity to inhibit the zinc finger of PHD2. We identified compounds that, in vitro, can inhibit PHD2 binding to a PXLE-containing peptide and induce activation of HIF. Injection of one of these compounds into mice induces an increase in hematocrit. This study offers proof of principle that inhibition of the zinc finger of PHD2 can provide a means of selectively targeting PHD2 to activate the HIF pathway.