Ethnically Tibetan women in Nepal with low hemoglobin concentration have better reproductive outcomes

Transcriptomic analysis of iPSC-derived endothelium reveals adaptations to high altitude hypoxia in energy metabolism and inflammation

Tibetan adaptation to high-altitude hypoxia remains a classic example of Darwinian selection in humans. Amongst Tibetan populations, alleles in the EPAS1 gene - whose protein product, HIF-2α, is a central regulator of the hypoxia response - have repeatedly been shown to carry some of the strongest signals of positive selection in humans. However, selective sweep signals alone may only account for some of the phenotypes that differentiate high-altitude adapted populations from closely related lowlanders. Therefore, there is a pressing need to functionally probe adaptive alleles and their impact at both the locus-specific and genome-wide levels and across cell types to uncover the full range of beneficial traits. To this end, we established a library of induced pluripotent stem cells (iPSCs) derived from Tibetan and Han Chinese individuals, a robust model system allowing precise exploration of allelic effects on transcriptional responses, and we differentiated them into vascular endothelium. Using this system, we focus first on a hypoxia-dependent enhancer (ENH5) that contributes to the regulation of EPAS1 to investigate its locus-specific effects in endothelium. Then, to cast a wider net, we harness the same experimental system to compare the transcriptome of Tibetan and Han Chinese cells in hypoxia and find evidence that angiogenesis, energy metabolism and immune pathways differ between these two populations with different histories of long-term residence at high altitude. Coupled with evidence of polygenic adaptations targeting the same pathways, these results suggests that the observed transcriptional differences between the two populations were shaped by natural selection.

Archaic introgression contributed to shape the adaptive modulation of angiogenesis and cardiovascular traits in human high-altitude populations from the Himalayas

It is well established that several Homo sapiens populations experienced admixture with extinct human species during their evolutionary history. Sometimes, such a gene flow could have played a role in modulating their capability to cope with a variety of selective pressures, thus resulting in archaic adaptive introgression events. A paradigmatic example of this evolutionary mechanism is offered by the EPAS1 gene, whose most frequent haplotype in Himalayan highlanders was proved to reduce their susceptibility to chronic mountain sickness and to be introduced in the gene pool of their ancestors by admixture with Denisovans. In this study, we aimed at further expanding the investigation of the impact of archaic introgression on more complex adaptive responses to hypobaric hypoxia evolved by populations of Tibetan/Sherpa ancestry, which have been plausibly mediated by soft selective sweeps and/or polygenic adaptations rather than by hard selective sweeps. For this purpose, we used a combination of composite-likelihood and gene network-based methods to detect adaptive loci in introgressed chromosomal segments from Tibetan WGS data and to shortlist those presenting Denisovan-like derived alleles that participate to the same functional pathways and are absent in populations of African ancestry, which are supposed to do not have experienced Denisovan admixture. According to this approach, we identified multiple genes putatively involved in archaic introgression events and that, especially as regards TBC1D1, RASGRF2, PRKAG2, and KRAS, have plausibly contributed to shape the adaptive modulation of angiogenesis and of certain cardiovascular traits in high-altitude Himalayan peoples. These findings provided unprecedented evidence about the complexity of the adaptive phenotype evolved by these human groups to cope with challenges imposed by hypobaric hypoxia, offering new insights into the tangled interplay of genetic determinants that mediates the physiological adjustments crucial for human adaptation to the high-altitude environment.

Higher oxygen content and transport characterize high-altitude ethnic Tibetan women with the highest lifetime reproductive success

We chose the "natural laboratory" provided by high-altitude native ethnic Tibetan women who had completed childbearing to examine the hypothesis that multiple oxygen delivery traits were associated with lifetime reproductive success and had genomic associations. Four hundred seventeen (417) women aged 46 to 86 y residing at ≥3,500 m in Upper Mustang, Nepal, provided information on reproductive histories, sociocultural factors, physiological measurements, and DNA samples for this observational cohort study. Simultaneously assessing multiple traits identified combinations associated with lifetime reproductive success measured as the number of livebirths. Women with the most livebirths had distinctive hematological and cardiovascular traits. A hemoglobin concentration near the sample mode and a high percent of oxygen saturation of hemoglobin raised arterial oxygen concentration without risking elevated blood viscosity. We propose ongoing stabilizing selection on hemoglobin concentration because extreme values predicted fewer livebirths and directional selection favoring higher oxygen saturation because higher values had more predicted livebirths. EPAS1, an oxygen homeostasis locus with strong signals of positive natural selection and a high frequency of variants occurring only among populations indigenous to the Tibetan Plateau, associated with hemoglobin concentration. High blood flow into the lungs, wide left ventricles, and low hypoxic heart rate responses aided effective convective oxygen transport to tissues. Women with physiologies closer to unstressed, low altitude values had the highest lifetime reproductive success. This example of ethnic Tibetan women residing at high altitudes in Nepal links reproductive fitness with trait combinations increasing oxygen delivery under severe hypoxic stress and demonstrates ongoing natural selection.

Sex-biased regulatory changes in the placenta of native highlanders contribute to adaptive fetal development

Compared with lowlander migrants, native Tibetans have a higher reproductive success at high altitude though the underlying mechanism remains unclear. Here, we compared the transcriptome and histology of full-term placentas between native Tibetans and Han migrants. We found that the placental trophoblast shows the largest expression divergence between Tibetans and Han, and Tibetans show decreased immune response and endoplasmic reticulum stress. Remarkably, we detected a sex-biased expression divergence, where the male-infant placentas show a greater between-population difference than the female-infant placentas. The umbilical cord plays a key role in the sex-biased expression divergence, which is associated with the higher birth weight of the male newborns of Tibetans. We also identified adaptive histological changes in the male-infant placentas of Tibetans, including larger umbilical artery wall and umbilical artery intima and media, and fewer syncytial knots. These findings provide valuable insights into the sex-biased adaptation of human populations, with significant implications for medical and genetic studies of human reproduction.

A comparison of quality of life between older adults living in high and low altitude areas

Background

High altitude is known to have a significant impact on human physiology and health, therefore, understanding its relationship with quality of life is an important research area. This study compared the quality of life (QOL) in older adults living in high and low altitude areas, and examined the independent correlates of QOL in those living in a high altitude area.

Methods

Older adults living in three public nursing homes in Xining (high altitude area) and one public nursing home in Guangzhou (low altitude area) were recruited. The WHOQOL-BREF was used to measure the QOL.

Results

644 older adults (male: 39.1%) were included, with 207 living in high altitude and 437 living in low altitude areas. After controlling for the covariates, older adults living in the high altitude area had higher QOL in terms of physical (P = 0.035) and social domains (P = 0.002), but had lower QOL in psychological (P = 0.009) domain compared to their counterparts living in the low altitude area. For older adults living in the high altitude area, smoking status was associated with higher social QOL (P = 0.021), good financial status was associated with higher physical QOL (P = 0.035), and fair or good health status was associated with higher physical (p < 0.001) and psychological QOL (P = 0.046), while more severe depressive symptoms were associated with lower QOL.

Conclusion

Appropriate interventions and support to improve depressive symptoms and both financial and health status should be developed for older adults living in high altitude areas to improve their QOL.

Human genetics: An extreme fitness landscape

A new study aims to identify how genetic and physiological adaptations to altitude affect pregnancy, childbirth and neonatal health in one of the most extreme environments on Earth, the Tibetan Plateau.

Polygenic adaptation leads to a higher reproductive fitness of native Tibetans at high altitude

The adaptation of Tibetans to high-altitude environments has been studied extensively. However, the direct assessment of evolutionary adaptation, i.e., the reproductive fitness of Tibetans and its genetic basis, remains elusive. Here, we conduct systematic phenotyping and genome-wide association analysis of 2,252 mother-newborn pairs of indigenous Tibetans, covering 12 reproductive traits and 76 maternal physiological traits. Compared with the lowland immigrants living at high altitudes, indigenous Tibetans show better reproductive outcomes, reflected by their lower abortion rate, higher birth weight, and better fetal development. The results of genome-wide association analyses indicate a polygenic adaptation of reproduction in Tibetans, attributed to the genomic backgrounds of both the mothers and the newborns. Furthermore, the EPAS1-edited mice display higher reproductive fitness under chronic hypoxia, mirroring the situation in Tibetans. Collectively, these results shed new light on the phenotypic pattern and the genetic mechanism of human reproductive fitness in extreme environments.

Consistent differences in brain structure and functional connectivity in high-altitude native Tibetans and immigrants

It has been well-established that high-altitude (HA) environments affect the human brain; however, the differences in brain structural and functional networks between HA natives and acclimatized immigrants have not been well clarified. In this study, native HA Tibetans were recruited for comparison with Han immigrants (average of 2.3 ± 0.3 years at HA), with lowland residents recruited as controls. Cortical gray matter volume, thickness, and functional connectivity were investigated using magnetic resonance imaging data. In addition, reaction time and correct score in the visual movement task, hematology, and SpO₂ were measured. In both Tibetans and HA immigrants vs. lowlanders, decreased SpO₂, increased hematocrit and hemoglobin, and increased reaction time and correct score in the visual movement task were detected. In both Tibetans and HA immigrants vs. lowlanders, gray matter volumes and cortical thickness were increased in the left somatosensory and motor cortex, and functional connectivity was decreased in the visual, default mode, subcortical, somatosensory-motor, ventral attention, and subcortical networks. Furthermore, SpO₂ increased, hematocrit and hemoglobin decreased, and gray matter volumes and cortical thickness increased in the visual cortex, left motor cortex, and right auditory cortex in native Tibetans compared to immigrants. Movement time and correct score in task were positively correlated with the thickness of the visual cortex. In conclusion, brain structural and functional network difference in both Tibetan natives and HA immigrants were largely consistent, with native Tibetans only showing more intense brain modulation. Different populations acclimatized to HA develop similar brain mechanisms to cope with hostile HA environmental factors.

Individual variations and sex differences in hemodynamics and percutaneous arterial oxygen saturation (SpO2) in Tibetan highlanders of Tsarang in the Mustang district of Nepal

Background

Many studies have indicated specific low-hemoglobin (Hb) adaptation to high altitude in the Tibetan population, but studies focusing on physiological variations within this population are limited. This study aimed to investigate the relationships between SpO₂ and related factors, including individual variations and sex differences, to assess the generality of high-altitude adaptation in the Tibetan population of Tsarang.

Methods

The participants were 31 male and 41 female community-dwelling people aged ≥18 years living in Tsarang, in the Mustang district of Nepal. Height, weight, SpO₂, Hb concentration, finger temperature, heart rate, and blood pressure were measured. Lifestyle information was obtained by interview.

Results

Men had significantly higher systolic blood pressure (p = 0.002) and Hb (p < 0.001) than women. There was no significant correlation between SpO₂ and other parameters in men. In women, SpO₂ was negatively correlated with heart rate (p = 0.036), Hb (p = 0.004), and finger temperature (p = 0.037). In multiple regression analysis, a higher SpO₂ was marginally correlated with lower age (β = −0.109, p = 0.086) and higher Hb (β = 0.547, p = 0.053) in men. In women, higher SpO₂ was significantly correlated with lower heart rate (β = −0.045, p = 0.036) and Hb (β = −0.341, p = 0.018). Mean hemoglobin (95% confidence interval) was 13.6 g/dl (13.1–14.0 g/dl), which is lower than that found previously in Andeans and almost equal to that in Japanese lowlanders measured using the same device. In some participants of both sexes, hemoglobin was >17.0 g/dl.

Conclusion

Higher SpO₂ was marginally correlated with younger age and higher Hb in men and with lower heart rate and lower Hb in women. Hemoglobin concentration was similar to that found previously in lowlanders, but higher in some individuals. These results indicate individual variation and sex differences in the hemodynamics of high-altitude adaptation in Tibetan highlanders of Tsarang, as well as low-Hb adaptation to high altitude equal to that of other Tibetans.

A Critical Analysis of the Automated Hematology Assessment in Pregnant Women at Low and at High Altitude: Association between Red Blood Cells, Platelet Parameters, and Iron Status

The objectives of the study were to determine differences in the parameters of red blood cells (RBC), white blood cells (WBC), and platelets at low altitude (LA) and at high altitude (HA) and with the gestation being advanced, and to determine correlations between parameters of RBC and platelets. We also studied the association of RBC and platelets with markers of iron status. In addition, markers of iron status and inflammation were measured and compared at each trimester of gestation in pregnant women at LA and HA. A cross-sectional comparative study was conducted at Lima (150 m above sea level) and Cusco at 3400 m above sea level from May to December 2019. Hematological parameters in pregnant women (233 at LA and 211 at HA) were analyzed using an automated hematology analyzer. Serum ferritin levels, soluble transferrin receptor (sTfR), hepcidin, erythropoietin, testosterone, estradiol, and interleukin-6 (IL6) levels were measured by ELISA. One-way ANOVA supplemented with post hoc test, chi-square test, and Pearson correlation test statistical analyses were performed. p < 0.05 was considered significant. Pregnant woman at HA compared to LA had significantly lower WBC (p < 0.01), associated with higher parameters of the RBC, except for the mean corpuscular volume (MCV) that was no different (p > 0.05). Platelets and mean platelet volume (MPV) were higher (p < 0.01), and platelet distribution width (PDW) was lower at HA than at LA (p < 0.01). A higher value of serum ferritin (p < 0.01), testosterone (p < 0.05), and hepcidin (p < 0.01) was observed at HA, while the concentration of sTfR was lower at HA than at LA (p < 0.01). At LA, neutrophils increased in the third trimester (p < 0.05). RBC parameters decreased with the progress of the gestation, except RDW-CV, which increased. The platelet count decreased and the MPV and PDW were significantly higher in the third trimester. Serum ferritin, hepcidin, and serum testosterone decreased, while sTfR and serum estradiol increased during gestation. At HA, the WBC and red blood cell distribution width- coefficient of variation (RDW-CV), PCT, and serum IL-6 did not change with gestational trimesters. RBC, hemoglobin (Hb), hematocrit (Hct), mean corpuscular hemoglobin concentration (MCHC), and platelet count were lower as gestation advanced. MCV, MPV, and PDW increased in the third trimester. Serum ferritin, testosterone, and hepcidin were lower in the third trimester. Serum estradiol, erythropoietin, and sTfR increased as gestation progressed. Direct or inverse correlations were observed between RBC and platelet parameters and LA and HA. A better number of significant correlations were observed at HA. Hb, Hct, and RDW-CV showed a significant correlation with serum ferritin at LA and HA. Of these parameters, RDW-CV and PDW showed an inversely significant association with ferritin (p < 0.05). In conclusion, a different pattern was observed in hematological markers as well as in iron status markers between pregnant women at LA and HA. In pregnant women a significant correlation between several RBC parameters with platelet marker parameters was also observed. Data suggest that pregnant women at HA have adequate iron status during pregnancy as reflected by higher serum ferritin levels, lower sTfR levels, and higher hepcidin values than pregnant women at LA.

Same total normal forms sperm counts of males from Lhasa and Shanghai, China

Male infertility may be caused by genetic and/or environmental factors that impair spermatogenesis and sperm maturation. High-altitude (HA) hypoxic environments represent one of the most serious challenges faced by humans that reside in these areas. To assess the influence of the plateau environment on semen parameters, 2,798 males, including 1,111 native Tibetans and 1,687 Han Chinese individuals living in the plains (HCILP) who underwent pre-pregnancy checkups, were enrolled in this study. The semen samples of males were evaluated to determine conventional sperm parameters, sperm morphology, and sperm movement. Reproductive endocrine hormones (REHs) were detected in 474 males, including 221 Tibetans and 253 HCILP. Due to recurrent abortions in partners, the DNA fragmentation index (DFI) of 133 native Tibetans and 393 HCILP individuals was further compared. Luteinizing hormone (LH) (4.94 ± 2.12 vs. 3.29 ± 1.43 U/L), prolactin (11.34 ± 3.87 vs. 8.97 ± 3.48 nmol/L), E2/T (0.22 ± 0.11 vs 0.11 ± 0.05), median total sperm motility (61.20% vs. 51.56%), and DFI (23.11% vs. 7.22%) were higher in males from plateau areas while median progressive motility (PR) (35.60% vs. 41.12%), total number of PR sperms (51.61 vs. 59.63 mil/ejaculate), percentage of normal form sperms (3.70% vs. 6.00%), curvilinear velocity (36.10 vs. 48.97 μm/s), straight-line (rectilinear) velocity (14.70 vs. 31.52 μm/s), estradiol (103.82 ± 45.92 vs. 146.01 ± 39.73 pmol/L), progesterone (0.29 ± 0.27 vs. 2.22 ± 0.84 nmol/L), testosterone (4.90 ± 1.96 vs. 14.36 ± 5.24 nmol/L), and testosterone secretion index (ratio of testosterone to LH) (33.45 ± 22.86 vs 145.78 ± 73.41) were lower than those in males from the plains. There was no difference in median total sperm number (157.76 vs. 151.65 mil/mL), sperm concentration (52.40 vs. 51.79 mil/mL), volume (3.10 vs. 3.10 mL), total normal form sperms (5.91 vs. 6.58 mil/ejaculate, p50), and follicle-stimulating hormone (FSH) levels (4.13 ± 2.55 U/L vs 3.82 ± 2.35 U/L) between the two groups of males. The REH and sperm parameters of males from HA hypoxic environments were adaptively altered. Although the total number of PR sperm decreased and DFI increased, the Tibetan population that lives at HAs has been found to grown continuously and rapidly. These results supplement prior findings regarding the impact of HA on male reproductive function.

Repeatability of adaptive traits among ethnic Tibetan highlanders

OBJECTIVES

Connecting traits to biological pathways and genes relies on stable observations. Researchers typically determine traits once, expecting careful study protocols to yield measurements free of noise. This report examines that expectation with test-retest repeatability analyses for traits used regularly in research on adaptation to high-altitude hypoxia, often in settings without climate control.

METHODS

Two hundred ninety-one ethnic Tibetan women residing from 3500 to 4200 m in Upper Mustang District, Nepal, provided three observations of hemoglobin concentration, percent of oxygen saturation of hemoglobin, and pulse by noninvasive pulse oximetry under conditions designed to minimize environmental noise.

RESULTS

High-intraclass correlation coefficients and low within-subject coefficients of variation reflected consistent measurements. Percent of oxygen saturation had the highest intraclass correlation coefficient and the smallest within-subject coefficient of variability; measurement noise occurred mainly in the lower values. Hemoglobin concentration and pulse presented slightly higher within-subject coefficients of variation; measurement noise occurred across the range of values. The women had performed the same measurements 7 years earlier using the same devices and protocol. The sample means and SD observed across 7 years differed little. Hemoglobin concentration increased substantially after menopause.

CONCLUSIONS

Analyzing repeatability features of traits may improve our interpretation of statistical analyses and detection of variation from measurement or biology. The high levels of measurement repeatability and biological stability support the continued use of these robust traits for investigating human adaptation in this altitude range.

Anti-Mullerian Hormone and Macrophage Migration Inhibitory Factor Determine the Reproductive Health of Ladakhi Women Residing at 3,500 m

Shaw, Snigdha, Himashree Gidugu, Gopinath Bhaumik, Maramreddy Prasanna Kumar Reddy, Usha Panjwani, and Dishari Ghosh. Anti-Mullerian hormone and macrophage migration inhibitory factor determine the reproductive health of Ladakhi women residing at 3,500 m. High Alt Med Biol. 00:000-000, 2021. Background: Reproductive health of Ladakhi high-altitude (HA) native females was investigated for the first time in this study. Available literature suggest that, female reproductive cycle and hormonal profile varies in different HA populations due to heterogeneity. Although these studies illustrate some progress on the role of HA hypoxia, it still leaves scope for evaluation of the remaining mechanisms involved in the maintenance of reproductive health in this contemporary population. Materials and Methods: Menstrual details, phasic variations in circulatory steroid hormones, and gonadotropins along with oxytocin in sea level (SL) and HA (∼3,500 m) native females of India were assessed. Moreover, ovarian reserve marker anti-Mullerian hormone (AMH) and proinflammatory cytokine macrophage migration inhibitory factor (MIF) were measured. Results: A difference in Ladakhi women was registered compared to SL, regarding luteinizing hormone (LH) (2.6 mIU/ml vs. 4.4 mIU/ml, p < 0.05) and progesterone (P) (4.1 ng/ml vs. 9.4 ng/ml, p < 0.05) levels in their luteal phase. Reduced LH might contribute to poor development of the ovarian corpus luteum, subsequently diminish P level. Decreased AMH level in three age groups: 21-30 years (1.4 ng/ml vs. 3.2 ng/ml, p < 0.01), 31-40 years (0.6 ng/ml vs. 2.1 ng/ml, p < 0.01), and >40 years (0.4 ng/ml vs. 1.7 ng/ml, p < 0.01) of Ladakhi women were recorded than their SL counterpart. Elevated oxytocin (83.5 ng/ml vs. 76.3 ng/ml, p < 0.05) and MIF levels (70.2 ng/ml vs. 49.7 ng/ml, p < 0.01) along with low P and AMH levels delineated the reason for recorded early menopause (43.9 years), shorter reproductive span (∼29 years), and history of miscarriage in HA dwellers compared to SL. Conclusion: Therefore, the findings insinuated that the response of the reproductive system to hypoxia in Ladakhi women differs from SL women, and the adaptive response in these women might be in favor of their reproductive health.

Heterogeneity in Hematological Parameters of High and Low Altitude Tibetan Populations

Introduction

High altitude hypoxia is believed to be experienced at elevations of more than 2500 meters above sea level. Several studies have shed light on the biochemical aspects of high altitude acclimatization, where participants were sojourners to the high altitude from low altitude areas. However, information regarding the difference between the high altitude adapted Tibetans living at high altitude and their counterparts who reside at low altitude are lacking. To understand this, we have measured various hematological parameters in the Tibetan populations, who are residing in both high and low altitudes in India.

Methods

A total of 168 individuals (79 from high altitude (≥4500 meters) and 89 from low altitude (~850 meters) were recruited for this study. Hematological parameters such as red blood cells (RBC) count, hematocrit (HCT), hemoglobin concentration (Hb), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) were measured from the individuals from high and low altitudes. Serum erythropoietin (EPO) was measured by ELISA. Statistical analyses were performed to compare data from both of the altitudes. Gender-wise comparison of data was reported. Correlation analysis was performed within relevant parameters.

Results

Highly significant differences (p <0.0001) between high and low altitude Tibetans were detected in RBC count, HCT, Hb, MCHC in both males and females and in MCV in females. In the case of MCHC, however, age and BMI were potential confounders. Nominally significant differences (p <0.05) were detected in MCV and MCH within males. No significant difference in serum EPO level was found between altitude groups, in any gender. No significant correlation was found between serum EPO with Hb as well as serum EPO with HCT.

Discussion

Our study explores significantly lower RBC count, HCT, Hb, MCH, MCHC and higher MCV in long-term Tibetan residents living at low altitude compared to their high altitude counterparts, which is likely due to the outcome of hematological adaptation to a relatively hyperoxic environment in low altitude areas.

High-Altitude Adaptation: Mechanistic Insights from Integrated Genomics and Physiology

Population genomic analyses of high-altitude humans and other vertebrates have identified numerous candidate genes for hypoxia adaptation, and the physiological pathways implicated by such analyses suggest testable hypotheses about underlying mechanisms. Studies of highland natives that integrate genomic data with experimental measures of physiological performance capacities and subordinate traits are revealing associations between genotypes (e.g., hypoxia-inducible factor gene variants) and hypoxia-responsive phenotypes. The subsequent search for causal mechanisms is complicated by the fact that observed genotypic associations with hypoxia-induced phenotypes may reflect second-order consequences of selection-mediated changes in other (unmeasured) traits that are coupled with the focal trait via feedback regulation. Manipulative experiments to decipher circuits of feedback control and patterns of phenotypic integration can help identify causal relationships that underlie observed genotype–phenotype associations. Such experiments are critical for correct inferences about phenotypic targets of selection and mechanisms of adaptation.

Physiological Genomics of Adaptation to High-Altitude Hypoxia

Population genomic studies of humans and other animals at high altitude have generated many hypotheses about the genes and pathways that may have contributed to hypoxia adaptation. Future advances require experimental tests of such hypotheses to identify causal mechanisms. Studies to date illustrate the challenge of moving from lists of candidate genes to the identification of phenotypic targets of selection, as it can be difficult to determine whether observed genotype-phenotype associations reflect causal effects or secondary consequences of changes in other traits that are linked via homeostatic regulation. Recent work on high-altitude models such as deer mice has revealed both plastic and evolved changes in respiratory, cardiovascular, and metabolic traits that contribute to aerobic performance capacity in hypoxia, and analyses of tissue-specific transcriptomes have identified changes in regulatory networks that mediate adaptive changes in physiological phenotype. Here we synthesize recent results and discuss lessons learned from studies of high-altitude adaptation that lie at the intersection of genomics and physiology.

Reproductive Health Disparities in the USA: Self-Reported Race/Ethnicity Predicts Age of Menarche and Live Birth Ratios, but Not Infertility

Self-identified race/ethnicity and socioeconomic status (SES) contribute to disparities in several health domains, although research on their effects on women's reproductive function has largely focused on links between SES and age of menarche. Here, we assessed whether race/ethnicity, SES, and downstream correlates of SES such as food security and health-insurance security are associated with age of menarche, infertility, and live birth ratios (ratios of recognized pregnancies resulting in live births) in the USA. We used cross-sectional data from 1694 women aged 12-18 years for menarche (2007-2016), 974 women aged 23-45 for infertility (2013-2016), and 1714 women aged 23-45 for live birth ratios (2007-2016) from the National Health and Nutrition Examination Survey. We estimated multiple linear and logistic regressions with survey weights to test these associations. When controlling for lifestyle (activity levels, smoking, alcohol consumption) and physiological factors (diabetes, weight status), non-Hispanic (NH) black and Hispanic girls reported a significantly lower age of menarche by about 4.3 (standard error [SE] = 0.08, p < 0.001), and 3.2 months (SE = 0.09, p < 0.001), respectively, relative to NH white girls. NH black women reported live birth ratios 9% (SE = 0.02, p < 0.001) lower than NH white women. Women with unstable health insurance reported live birth ratios 6% (SE = 0.02, p = 0.02) lower than women with stable health insurance. Race/ethnicity, SES, and its downstream correlates were not associated with infertility. One hypothesized explanation for observed disparities in age of menarche and live birth ratios is the embodiment of discrimination faced by NH black women within the USA. Our findings also underscore the importance of health insurance access for favorable reproductive health outcomes. Future work should elucidate the role of embodied discrimination and other downstream correlates of SES in modulating women's reproductive health outcomes to inform strategies to mitigate health disparities.

The Genomics and Genetics of Oxygen Homeostasis

Human survival is dependent upon the continuous delivery of O₂ to each cell in the body in sufficient amounts to meet metabolic requirements, primarily for ATP generation by oxidative phosphorylation. Hypoxia-inducible factors (HIFs) regulate the transcription of thousands of genes to balance O₂ supply and demand. The HIFs are negatively regulated by O₂-dependent hydrox-ylation and ubiquitination by prolyl hydroxylase domain (PHD) proteins and the von Hippel-Lindau (VHL) protein. Germline mutations in the genes encoding VHL, HIF-2α, and PHD2 cause hereditary erythrocytosis, which is characterized by polycythemia and pulmonary hypertension and is caused by increased HIF activity. Evolutionary adaptation to life at high altitude is associated with unique genetic variants in the genes encoding HIF-2α and PHD2 that blunt the erythropoietic and pulmonary vascular responses to hypoxia.

Population History and Altitude-Related Adaptation in the Sherpa

The first ascent of Mount Everest by Tenzing Norgay and Sir Edmund Hillary in 1953 brought global attention to the Sherpa people and human performance at altitude. The Sherpa inhabit the Khumbu Valley of Nepal, and are descendants of a population that has resided continuously on the Tibetan plateau for the past ∼25,000 to 40,000 years. The long exposure of the Sherpa to an inhospitable environment has driven genetic selection and produced distinct adaptive phenotypes. This review summarizes the population history of the Sherpa and their physiological and genetic adaptation to hypoxia. Genomic studies have identified robust signals of positive selection across EPAS1, EGLN1, and PPARA, that are associated with hemoglobin levels, which likely protect the Sherpa from altitude sickness. However, the biological underpinnings of other adaptive phenotypes such as birth weight and the increased reproductive success of Sherpa women are unknown. Further studies are required to identify additional signatures of selection and refine existing Sherpa-specific adaptive phenotypes to understand how genetic factors have underpinned adaptation in this population. By correlating known and emerging signals of genetic selection with adaptive phenotypes, we can further reveal hypoxia-related biological mechanisms of adaptation. Ultimately this work could provide valuable information regarding treatments of hypoxia-related illnesses including stroke, heart failure, lung disease and cancer.

[Global situation of anemia in pregnant women]

Introduction

Detecting past and ongoing natural selection among ethnically Tibetan women at high altitude in Nepal

Adaptive evolution in humans has rarely been characterized for its whole set of components, i.e. selective pressure, adaptive phenotype, beneficial alleles and realized fitness differential. We combined approaches for detecting polygenic adaptations and for mapping the genetic bases of physiological and fertility phenotypes in approximately 1000 indigenous ethnically Tibetan women from Nepal, adapted to high altitude. The results of genome-wide association analyses and tests for polygenic adaptations showed evidence of positive selection for alleles associated with more pregnancies and live births and evidence of negative selection for those associated with higher offspring mortality. Lower hemoglobin level did not show clear evidence for polygenic adaptation, despite its strong association with an EPAS1 haplotype carrying selective sweep signals.

Extreme pregnancy: maternal physical activity at Everest Base Camp

High-altitude natives employ numerous physiological strategies to survive and reproduce. However, the concomitant influence of altitude and physical activity during pregnancy has not been studied above 3,700 m. We report a case of physical activity, sleep behavior, and physiological measurements on a 28-yr-old third-trimester pregnant native highlander (Sherpa) during ascent from 3,440 m to Everest Base Camp (~5,300 m) over 8 days in the Nepal Himalaya and again ~10 mo postpartum during a similar ascent profile. The participant engaged in 250-300 min of moderate to vigorous physical activity per day during ascent to altitude while pregnant, with similar volumes of moderate to vigorous physical activity while postpartum. There were no apparent maternal, fetal, or neonatal complications related to the superimposition of the large volumes of physical activity at altitude. This report demonstrates a rare description of physical activity and ascent to high altitude during pregnancy and points to novel questions regarding the superimposition of pregnancy, altitude, and physical activity in high-altitude natives.

Environmental selection during the last ice age on the mother-to-infant transmission of vitamin D and fatty acids through breast milk

The frequency of the human-specific EDAR V370A isoform is highly elevated in North and East Asian populations. The gene is known to have several pleiotropic effects, among which are sweat gland density and ductal branching in the mammary gland. The former has led some geneticists to argue that the near-fixation of this allele was caused by selection for modulation of thermoregulatory sweating. We provide an alternative hypothesis, that selection instead acted on the allele’s effect of increasing ductal branching in the mammary gland, thereby amplifying the transfer of critical nutrients to infants via mother’s milk. This is likely to have occurred during the Last Glacial Maximum when a human population was genetically isolated in the high-latitude environment of the Beringia.

Because of the ubiquitous adaptability of our material culture, some human populations have occupied extreme environments that intensified selection on existing genomic variation. By 32,000 years ago, people were living in Arctic Beringia, and during the Last Glacial Maximum (LGM; 28,000–18,000 y ago), they likely persisted in the Beringian refugium. Such high latitudes provide only very low levels of UV radiation, and can thereby lead to dangerously low levels of biosynthesized vitamin D. The physiological effects of vitamin D deficiency range from reduced dietary absorption of calcium to a compromised immune system and modified adipose tissue function. The ectodysplasin A receptor (EDAR) gene has a range of pleiotropic effects, including sweat gland density, incisor shoveling, and mammary gland ductal branching. The frequency of the human-specific EDAR V370A allele appears to be uniquely elevated in North and East Asian and New World populations due to a bout of positive selection likely to have occurred circa 20,000 y ago. The dental pleiotropic effects of this allele suggest an even higher occurrence among indigenous people in the Western Hemisphere before European colonization. We hypothesize that selection on EDAR V370A occurred in the Beringian refugium because it increases mammary ductal branching, and thereby may amplify the transfer of critical nutrients in vitamin D-deficient conditions to infants via mothers’ milk. This hypothesized selective context for EDAR V370A was likely intertwined with selection on the fatty acid desaturase (FADS) gene cluster because it is known to modulate lipid profiles transmitted to milk from a vitamin D-rich diet high in omega-3 fatty acids.