Cross‐altitude analysis suggests a turning point at the elevation of 4,500 m for polycythemia prevalence in Tibetans
Authors: Hui Zhang, Yaoxi He, Chaoying Cui, None Ouzhuluobu, None Baimakangzhuo, None Duojizhuoma, None Dejiquzong, None Bianba, None Gonggalanzi, Yongyue Pan, None Qula, None Kangmin, None Cirenyangji, None Baimayangji, Caijuan Bai, Wei Guo, None Yangla, Yi Peng, Xiaoming Zhang, Kun Xiang, Zhaohui Yang, Shiming Liu, Xiang Tao, None Gengdeng, Wangshan Zheng, Yongbo Guo, Tianyi Wu, Xuebin Qi, Bing Su
Journal: American Journal of Hematology (2017)

Tibetans are a well-known example of genetic adaptation to hypoxic environments at high altitude.1 Both genetic and archeological data suggest that Tibetans have been living at the Qinghai-Tibetan plateau since late Paleolithic time (as early as ∼30,000 years ago).2, 3 Due to long-time natural selection, Tibetans have accumulated many adaptive mutations in the hypoxia-related genes (e.g., EPAS1 and EGLN1),4-8 though these adaptive mutations have not been fixed in Tibetan populations. The average elevation of the plateau is about 4,000 m, and about 85% Tibetans live in areas with elevations between 2,500 m and 4,500 m, and only 6.2% above 4,500 m,1 implicating that the extreme altitude (>4,500 m) could be a challenge to Tibetans. It is understood that the relatively low hemoglobin (Hb) concentration in Tibetans protects them from polycythemia (red cell overproduction), regarded as a blunted physiological response at high altitude. In contrast, the prolonged exposure of lowlanders to hypobaric hypoxia usually leads to polycythemia, which increases the risk of heart attack, stroke, and fetal loss during pregnancy.9 In general, native Tibetans showed relatively lower Hb concentration compared with Han Chinese immigrants living in Tibet, and there was a linear correlation of Hb concentration with altitude.10-12 The "Qinghai CMS score" was proposed for diagnosis of chronic mountain sickness (CMS), in which a value of Hb ≥ 190 g/L in females or Hb ≥ 210 g/L in males was taken as one of the diagnostic criterion for high altitude polycythemia.13 However, previous studies of Hb were all relied on a linear model and population coverage of altitude was limited, especially those populations from the extreme altitude (>4,500 m). In addition, no detailed analysis was performed to see if Hb concentration would deviate from the linear prediction at certain altitude. We collected blood samples from 8,761 native Tibetan individuals. After filtering (Supporting Information Methods), we had 7,601 individuals (3,526 adult males and 4,075 adult females) from 20 geographic populations across the plateau, with elevations ranging from 2,227 m∼5,018 m. The geographic locations and elevations of the sampled populations are shown in Table 1 and Supporting Information Figure S1. These Tibetan populations are permanent residents at the sample-collection locations. Informed written consents were obtained from all participants. The protocol of this study was reviewed and approved by the Internal Review Board of Kunming Institute of Zoology, Chinese Academy of Sciences (Approval ID: SWYX-2012008) and Tibetan University (Approval ID: 2011-XZDX-001). Geographic location Table 1 lists the average Hb concentration of 20 Tibetan populations from different elevations. Consistent with previous reports,10 we observed a linear increase of Hb level with elevation for populations living below 4,000 m, so was the polycythemia prevalence. But the linear relationship did not hold for higher elevations (Supporting Information Figure S2). In particular, at 5,018 m, the majority of the population (65.3% in males and 65.1% in females) are subject to polycythemia (Table 1), a clear deviation from the linear expectation (Supporting Information Figure S2). This pattern suggests that Tibetans at higher altitudes may have a big change of hemoglobin status. Since the linear regression was not suitable to describe hemoglobin changes at higher altitudes, we therefore performed nonlinear regressions. We tested three different models, including the polynomial model, the exponential growth model, and the Boltzmann sigmoid model, which are commonly used models for physiological data. We adopted the criterion of "goodness of fit" to evaluate how well the nonlinear models fit the observed data, and the statistics of "coefficient of determination" (R2, 0 ≤ R2 ≤ 1) was used. It is known that Hb concentration is different between males and females due to the gender difference of iron status. Hence, model fittings were performed separately for males and females (Supporting Information Methods). As shown in Supporting Information Figures S3 and S4, for Hb concentration, the Boltzmann sigmoid model was the best fit. For polycythemia prevalence, the Boltzmann and the exponential models performed similarly. The Hb levels increased slowly at low elevations, and then sharply after 4,500 m (Figure 1A). The same pattern was observed for the curve of polycythemia prevalence (Figure 1B). We used the non-linear models with the highest R2 values when calculating the elevations of the turning points. As shown in Figure 1A, for Hb concentration, the calculated turning points are 4,466 m for males (R2 = 0.601, the Boltzmann sigmoidal model) and 4,166 m for females (R2 = 0.754, the Boltzmann sigmoidal model). When polycythemia prevalence was considered, the turning points are 4,526 m for males (R2 = 0.754, the Boltzmann sigmoidal model) and 4,430 m for females (R2 = 0.725, the exponential growth model) (Figure 1B). Similar results were obtained when taking the alternative models for polycythemia prevalence, and the calculated turning points are 4,394 m for males (R2 = 0.704, the exponential growth model) and 4,635 m for females (R2 = 0.514, the Boltzmann sigmoidal model). Taken together, the elevation around 4,500 m represents a turning point for Tibetans with a dramatic increase of both Hb concentration and polycythemia prevalence. This implicates that the protective blunting effect on hemoglobin disappears beyond this elevation. Nonlinear correlation of Hb concentration and polycythemia prevalence with altitude in Tibetan populations. A, Hb concentration; B, Polycythemia prevalence. The dash lines indicate the inferred turning points of elevations with a sharp increase of Hb concentration and polycythemia prevalence Our findings of an Hb turning point in Tibetans have several implications. First, although Tibetans are likely the most adapted population for high altitude in the world, the elevation of >4,500 m still serves as a big challenge for them. Second, the protective blunting effect on hemoglobin in Tibetans has an altitude limit and it does not work at elevations above 4,500 m due to the severe hypoxia. This seems to explain the not-yet-fixed adaptive mutations (∼80%) in Tibetans previously reported in the two hypoxic genes (EPAS1 and EGLN1).4–5 In other words, natural selection has not finished its task and may still be in action. Finally, as hemoglobin concentration is only one of the adaptive physiological traits in Tibetans, other traits like resting ventilation, blood oxygen saturation and pulmonary vasoconstriction response should also be studied to further test the proposed turning point of 4,500 m. We are grateful to all participants in this study. The authors declare no competing financial interests. B.S., X.Q. and C.C. conceived the project. H.Z., Y.H., C.C., Ou., Baimak., Duo., Deji., Bianba, Gong., Y.P., Qula, Kangmin, Ciren., Baimay., C.B., W.G., Yangla, Y.P., X.Z., K.X., Z.Y., S.L., X.T., Gengdeng, W.Z., Y.G., T.W. collected the samples; Z.H., Y.H., C.C., Y.P., Ou., X.Q. collected the data; H.Z., Y.H., C.C., X.Q., B.S., conducted data analysis; H.Z., Y.H., C.C., X.Q., B.S. wrote the manuscript with inputs from other authors. Additional Supporting Information may be found online in the supporting information tab for this article. 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