Tibetans living at sea level have a hyporesponsive hypoxia-inducible factor system and blunted physiological responses to hypoxia

Abstract

Tibetan natives have lived on the Tibetan plateau (altitude ∼ 4,000 m) for at least 25,000 years, and as such they are adapted to life and reproduction in a hypoxic environment. Recent studies have identified two genetic loci, EGLN1 and EPAS1, that have undergone natural selection in Tibetans, and further demonstrated an association of EGLN1/EPAS1 genotype with hemoglobin concentration. Both genes encode major components of the hypoxia-inducible factor (HIF) transcriptional pathway, which coordinates an organism's response to hypoxia. Patients living at sea level with genetic disease of the HIF pathway have characteristic phenotypes at both the integrative-physiology and cellular level. We sought to test the hypothesis that natural selection to hypoxia within Tibetans results in related phenotypic differences. We compared Tibetans living at sea level with Han Chinese, who are Tibetans' most closely related major ethnic group. We found that Tibetans had a lower hemoglobin concentration, a higher pulmonary ventilation relative to metabolism, and blunted pulmonary vascular responses to both acute (minutes) and sustained (8 h) hypoxia. At the cellular level, the relative expression and hypoxic induction of HIF-regulated genes were significantly lower in peripheral blood lymphocytes from Tibetans compared with Han Chinese. Within the Tibetans, we found a significant correlation between both EPAS1 and EGLN1 genotype and the induction of erythropoietin by hypoxia. In conclusion, this study provides further evidence that Tibetans respond less vigorously to hypoxic challenge. This is evident at sea level and, at least in part, appears to arise from a hyporesponsive HIF transcriptional system.

Keywords: EGLN1; EPAS1; erythropoietin; high altitude; hypoxic pulmonary vasoconstriction.

Fig. 1.

Principal component analysis among Asian populations. Data are from the Human Genome Diversity Project data set of Asian ethnicities, Hapmap, reference (5), and this study. The 28 Tibetan DNA samples from our study are represented by light blue, short horizontal lines and cluster together with the Yunnan Tibetan volunteers from reference (5).

Fig. 2.

Pulmonary vascular response to acute isocapnic hypoxia before and after 8 h of sustained isocapnic hypoxia. A and B: end-tidal partial pressure of CO₂ (Pet_CO2) against time for Tibetan and Han Chinese, respectively, during the acute hypoxic protocol. C and D: end-tidal partial pressure of O₂ (Pet_O2) against time for Tibetan and Han Chinese, respectively. E and F: pulmonary arterial systolic pressure (PASP) values against time for 10 Tibetan and 10 Han Chinese volunteers, respectively. Values are means; error bars represent SEM.

Fig. 3.

Pulmonary vascular response during 8 h of sustained isocapnic hypoxia. Closed circles represent PASP results from 10 Tibetan volunteers; open circles are results from 10 Han Chinese volunteers. Values are means; error bars indicate SEM.

Fig. 4.

Percentage change in ΔPmax against percentage change in cardiac output after 8 h of hypoxia for each individual. Closed circles represent data from Tibetan and open circles from Han Chinese volunteers.

Fig. 5.

Example of a volunteer's ventilatory response during the acute cyclical hypoxic challenge of Protocol 1. A and B: end-tidal partial pressure of CO₂ and O₂, respectively, against time. C: corresponding ventilation measurements (breath by breath) against time.

Fig. 6.

Expression of HIF2α mRNA and EGLN1 mRNA (relative to a standard calibrator sample) in PBL from seven Tibetan (filled bars) and seven Han Chinese (open bars) volunteers. Values are means; error bars are SEM.

Fig. 7.

Expression of HIF-target genes (relative to a standard calibrator sample) at graded oxygen tension for seven Tibetan and seven Han Chinese volunteers. A: vascular endothelial growth factor A (VEGFA); B: adrenomedullin; C: aldolase C; D: prolyl-4-hydroxylase-alpha-1 (P4HA1). Filled bars represent results from Tibetan volunteers; open bars from Han Chinese volunteers. Values are means; error bars are SEM.

Fig. 8.

Plasma erythropoietin against time in sustained hypoxia in the chamber for Tibetan volunteers according to EPAS1 genotype (A) and EGLN1 genotype (B). Values are means; error bars are SEM.