Hypoxia: adapting to high altitude by mutating EPAS-1, the gene encoding HIF-2α.

Living at high altitude is demanding and thus drives adaptational mechanisms. The Tibetan population has had a longer evolutionary period to adapt to high altitude than other mountain populations such as Andeans. As a result, some Tibetans living at high altitudes do not show markedly elevated red blood cell production as compared to South American high altitude natives such as Quechuas or Aymaras, thereby avoiding high blood viscosity creating cardiovascular risk. Unexpectedly, the responsible mutation(s) reducing red blood cell production do not involve either the gene encoding the blood hormone erythropoietin (Epo), or the corresponding regulatory sequences flanking the Epo gene. Similarly, functional mutations in the hypoxia-inducible transcription factor 1α (HIF-1α) gene that represents the oxygen-dependent subunit of the HIF-1 heterodimer, the latter being the main regulator of over 100 hypoxia-inducible genes, have not been described so far. It was not until very recently that three independent groups showed that the gene encoding HIF-2α, EPAS-1 (Wenger et al. 1997), represents a key gene mutated in Tibetan populations adapted to living at high altitudes (Beall et al. 2010 , Yi et al. 2010 , Simonson et al. 2010). Hypoxia-inducible transcription factors were first identified by the description of HIF-1 (Semenza et al. 1991 , 1992), which was subsequently found to enhance transcription of multiple genes that encode proteins necessary for rescuing from hypoxic exposure, including erythropoietic, angiogenic and glycolytic proteins. Then HIF-2 was identified (Ema et al. 1997 ; Flamme et al. 1997 ; Hogenesch et al. 1997 ; and Tian et al. 1997) and although it is highly similar to HIF-1 and has the potential to bind (Camenisch et al. 2001) and mediate (Mole et al. 2009) many of the same genes as HIF-1, its biological actions in response to hypoxia are distinct from those of HIF-1 (reviewed by Loboda et al. 2010). By now, several of these HIF-2 mediated processes have been implicated in the human response to high altitude exposure including erythropoiesis (Kapitsinou et al. 2010), iron homeostasis (Peyssonnaux et al. 2008), metabolism (Shohet et al. 2007; Tormos et al. 2010; Biswas et al. 2010 ; Rankin et al. 2009) and vascular permeability (Chen et al. 2009; Tanaka et al. 2005), among others. Clearly, mutation of EPAS-1 has the potential to bring far more advantage when adapting to high altitude than solely mutating the Epo gene.