Making a life at nearly 3500 meters above sea level on the Tibetan Plateau is no easy prospect, especially considering that the air contains far less oxygen than most humans are used to. For most, this thin air leads to a markedly low level of oxygen in the bloodstream, but native Tibetans have been living on the plateau as long as 30,000 years with relatively few complications. Recent studies highlight that the Tibetans are well suited to this harsh environment; they possess several interesting adaptations not readily present in other populations, including an elevated resting ventilation, low hypoxic pulmonary vasoconstrictor response, and relatively higher level of blood oxygen saturation and lower hemoglobin levels.
While the Tibetans’ adaptations to their high-altitude home have been illustrated in a variety of studies, the underlying mechanisms for these adaptations are unclear. Previous attempts to explain the genetic bases for high-altitude adaptation have been limited by the relatively small number of analyzed sequence variations. Recently, however, Kun Xiang of the State Key Laboratory of Genetic Resources and Evolution at the Kunming Institute of Zoology (KIZ), Chinese Academy of Sciences (CAS) undertook a study aimed at re-sequencing of the entire genomic region (59.4 kb) of the hypoxic gene EGLN1, one of the leading candidate genes identified in genome-wide scans of Tibetan populations.
Normally, EGLN1 post-translationally regulates the level of EPAS1 and HIF1α (hypoxia-inducible factor 1α) by hydroxylating them on specific proline residues in an oxygen-dependent way, thus enabling recognition of EPAS1 and HIF1α by the VHL ubiquitin ligase complex and subsequent degradation of them by the proteasome. In areas with high-elevation, like the Tibetan Plateau, however, the hydroxylation is significantly decreased, and EPAS1 and HIF1α are stabilized. Since another gene, EPAS1, has been proposed as another leading candidate gene to explain hypoxic adaptation, whether EGLN1, which is a negative regulator of EPAS1, also contributes to hypoxic adaptation has been tested to date.
Without re-sequencing data for the entire gene region of EGLN1 and an extensive survey of Tibetan populations, it is hard to test the signal of selection and identify causal sequence variations. Xiang’s team carried out such a survey, and identified 185 sequence variations including 13 novel variations in this gene, with a non-synonymous mutation (rs186996510, D4E) showing a surprisingly deep divergence between Tibetans and a population of lowlander Han Chinese. This mutation, while highly prevalent in Tibetans (70.9% on average) is extremely rare in Han Chinese, Japanese, Europeans and Africans (0.56%-2.27%), suggesting that this mutation may indeed be the causal mutation of EGLN1 thatcontributes to high-altitude hypoxic adaptation.
Further haplotype network analysis revealed a Tibetan-specific haplotype, which is absent in other world populations. The estimated selective intensity (0.029 for the C allele of rs186996510) puts EGLN1 as one of the genes to have undergone the strongest selection in human populations, likely at the onset of the early Neolithic period (~8,400 years ago), which coincides with one of the major migrations into the Tibetan Plateau. Further tested also revealed
a significant association between rs186996510 and hemoglobin levels in Tibetans, suggesting that EGLN1 contributes to the relatively low hemoglobin level among Tibetans.
Intriguingly, these findings about the relative age for this positive selection of EGLN1offers some interesting possibilities. Earlier research done by colleagues in Xiang’s lab under Principal Investigator Bing Su put forth the idea that the Tibetan Plateau may have been settled by two progressive waves of immigration, one during the Neolithic (~8,400 years ago) and another more ancient migration in the Upper Paleolithic (~30,000 years ago). In the present study, Xiang’s team found that estimated age of selection on the C allele of rs186996510 falls in the early Neolithic (~8,400 years ago), which is much younger than the estimated selection age of EPAS1 (~18,000 years ago). Two possible scenarios may explain this discrepancy. In one model, the adaptive mutations of EGLN1 and EPAS1 were brought in to the Himalayas at different times and the selection age of EGLN1 coincides with the second migration during the early Neolithic. Another potential explanation may be that the mutations of EGLN1 and EPAS1 may have been brought to the Himalayas at the same time during the Upper Paleolithic, but selection might have occurred at different times on these two genes. EPAS1 was likely selected first when people were living at relatively low altitude during the initial colonization in the Upper Paleolithic, while the Neolithic population expansion led to exploration and settlement of progressively higher altitudes (>3,000m), and concurrently selection for EGLN1 led to adaptations useful in higher altitudes. Both alternatives offer exciting possibilities for further exploration, but more data is needed to test either hypotheses.
The full report “Identification of a Tibetan-specific mutation in the hypoxic gene EGLN1 and its contribution to high-altitude adaptation” was recently published in Molecular Biology and Evolution [HYPERLINK NOT YET AVAILABLE—ONLY ADVANCE COPY—UPDATE BEFORE POSTING TO WEBSITE].
(By Andrew Willden)
