Down-Regulation of EPAS1 Transcription and Genetic Adaptation of Tibetans to High-Altitude Hypoxia

Abstract

Tibetans are well adapted to the hypoxic environments at high altitude, yet the molecular mechanism of this adaptation remains elusive. We reported comprehensive genetic and functional analyses of EPAS1, a gene encoding hypoxia inducible factor 2α (HIF-2α) with the strongest signal of selection in previous genome-wide scans of Tibetans. We showed that the Tibetan-enriched EPAS1 variants down-regulate expression in human umbilical endothelial cells and placentas. Heterozygous EPAS1 knockout mice display blunted physiological responses to chronic hypoxia, mirroring the situation in Tibetans. Furthermore, we found that the Tibetan version of EPAS1 is not only associated with the relatively low hemoglobin level as a polycythemia protectant, but also is associated with a low pulmonary vasoconstriction response in Tibetans. We propose that the down-regulation of EPAS1 contributes to the molecular basis of Tibetans' adaption to high-altitude hypoxia.

Keywords: EPAS1; Tibetans; genetic adaptation; high altitude; hypoxia; transgenic mice.

Fig. 1.

Map of linkage disequilibrium among the 32 EPAS1 sequence variants (lower panel) with the largest allelic divergence (F_ST> 0.5) between high altitude Tibetans and low altitude populations (upper panel).

Fig. 2.

In vitro functional test of the EPAS1 variant (rs149594770) using an electrophoretic mobility shift assay (A) and a luciferase reporter gene assay (B). The two-tailed t test was used for statistical assessment.

Fig. 3.

Changes of EPAS1 expression in Tibetan umbilical endothelial cells during prolonged hypoxic treatment (A) and in term Tibetan placentas (B). The adaptive ECU lines are homozygotes of the adaptive EPAS1 haplotype, and the wildtype ECU lines are homozygotes of the wildtype EPAS1 haplotype. TIB Hom, Tibetan homozygote; TIB Het, Tibetan heterozygote; Han Hom, Han Chinese homozygote; WT, wild-type. The one-way ANOVA and two-tailed t tests were conducted for panel A and panel B, respectively.

Fig. 4.

In vivo tests of EPAS1+/− and WT mice under prolonged hypoxia. (A) Weight loss percentage; (B) systolic RV pressure; (C) degree of RV hypertrophy; and (D) hemoglobin concentration. The degree of RV hypertrophy was determined by the weight ratio of right ventricle to heart. Mice were kept in a hypoxic chamber for 4 weeks at two oxygen levels (7 EPAS1+/− vs. 6 wild-type mice at 12%, and 6 EPAS1+/− vs. 5 wild-type mice at 10%). The data at normoxia (4 EPAS1+/− vs. 5 wild-type mice at 21% O₂) was collected as reference. WT, wild-type mice.