Identifying signatures of natural selection in Tibetan and Andean populations using dense genome scan data

High-altitude hypoxia (reduced inspired oxygen tension due to decreased barometric pressure) exerts severe physiological stress on the human body. Two high-altitude regions where humans have lived for millennia are the Andean Altiplano and the Tibetan Plateau. Populations living in these regions exhibit unique circulatory, respiratory, and hematological adaptations to life at high altitude. Although these responses have been well characterized physiologically, their underlying genetic basis remains unknown. We performed a genome scan to identify genes showing evidence of adaptation to hypoxia. We looked across each chromosome to identify genomic regions with previously unknown function with respect to altitude phenotypes. In addition, groups of genes functioning in oxygen metabolism and sensing were examined to test the hypothesis that particular pathways have been involved in genetic adaptation to altitude. Applying four population genetic statistics commonly used for detecting signatures of natural selection, we identified selection-nominated candidate genes and gene regions in these two populations (Andeans and Tibetans) separately. The Tibetan and Andean patterns of genetic adaptation are largely distinct from one another, with both populations showing evidence of positive natural selection in different genes or gene regions. Interestingly, one gene previously known to be important in cellular oxygen sensing, EGLN1 (also known as PHD2), shows evidence of positive selection in both Tibetans and Andeans. However, the pattern of variation for this gene differs between the two populations. Our results indicate that several key HIF-regulatory and targeted genes are responsible for adaptation to high altitude in Andeans and Tibetans, and several different chromosomal regions are implicated in the putative response to selection. These data suggest a genetic role in high-altitude adaption and provide a basis for future genotype/phenotype association studies necessary to confirm the role of selection-nominated candidate genes and gene regions in adaptation to altitude.

Development of a panel of genome-wide ancestry informative markers to study admixture throughout the Americas

Most individuals throughout the Americas are admixed descendants of Native American, European, and African ancestors. Complex historical factors have resulted in varying proportions of ancestral contributions between individuals within and among ethnic groups. We developed a panel of 446 ancestry informative markers (AIMs) optimized to estimate ancestral proportions in individuals and populations throughout Latin America. We used genome-wide data from 953 individuals from diverse African, European, and Native American populations to select AIMs optimized for each of the three main continental populations that form the basis of modern Latin American populations. We selected markers on the basis of locus-specific branch length to be informative, well distributed throughout the genome, capable of being genotyped on widely available commercial platforms, and applicable throughout the Americas by minimizing within-continent heterogeneity. We then validated the panel in samples from four admixed populations by comparing ancestry estimates based on the AIMs panel to estimates based on genome-wide association study (GWAS) data. The panel provided balanced discriminatory power among the three ancestral populations and accurate estimates of individual ancestry proportions (R²>0.9 for ancestral components with significant between-subject variance). Finally, we genotyped samples from 18 populations from Latin America using the AIMs panel and estimated variability in ancestry within and between these populations. This panel and its reference genotype information will be useful resources to explore population history of admixture in Latin America and to correct for the potential effects of population stratification in admixed samples in the region.

Individuals from Latin America are descendants of multiple ancestral populations, primarily Native American, European, and African ancestors. The relative proportions of these ancestries can be estimated using genetic markers, known as ancestry informative markers (AIMs), whose allele frequency varies between the ancestral groups. Once determined, these ancestral proportions can be correlated with normal phenotypes, can be associated with disease, can be used to control for confounding due to population stratification, or can inform on the history of admixture in a population. In this study, we identified a panel of AIMs relevant to Latin American populations, validated the panel by comparing estimates of ancestry using the panel to ancestry determined from genome-wide data, and tested the panel in a diverse set of populations from the Americas. The panel of AIMs produces ancestry estimates that are highly accurate and appropriately controlled for population stratification, and it was used to genotype 18 populations from throughout Latin America. We have made the panel of AIMs available to any researcher interested in estimating ancestral proportions for populations from the Americas.

A genomewide admixture mapping panel for Hispanic/Latino populations

Admixture mapping (AM) is a promising method for the identification of genetic risk factors for complex traits and diseases showing prevalence differences among populations. Efficient application of this method requires the use of a genomewide panel of ancestry-informative markers (AIMs) to infer the population of origin of chromosomal regions in admixed individuals. Genomewide AM panels with markers showing high frequency differences between West African and European populations are already available for disease-gene discovery in African Americans. However, no such a map is yet available for Hispanic/Latino populations, which are the result of two-way admixture between Native American and European populations or of three-way admixture of Native American, European, and West African populations. Here, we report a genomewide AM panel with 2,120 AIMs showing high frequency differences between Native American and European populations. The average intermarker genetic distance is ~1.7 cM. The panel was identified by genotyping, with the Affymetrix GeneChip Human Mapping 500K array, a population sample with European ancestry, a Mesoamerican sample comprising Maya and Nahua from Mexico, and a South American sample comprising Aymara/Quechua from Bolivia and Quechua from Peru. The main criteria for marker selection were both high information content for Native American/European ancestry (measured as the standardized variance of the allele frequencies, also known as "f value") and small frequency differences between the Mesoamerican and South American samples. This genomewide AM panel will make it possible to apply AM approaches in many admixed populations throughout the Americas.

Identifying positive selection candidate loci for high-altitude adaptation in Andean populations

High-altitude environments (>2,500 m) provide scientists with a natural laboratory to study the physiological and genetic effects of low ambient oxygen tension on human populations. One approach to understanding how life at high altitude has affected human metabolism is to survey genome-wide datasets for signatures of natural selection. In this work, we report on a study to identify selection-nominated candidate genes involved in adaptation to hypoxia in one highland group, Andeans from the South American Altiplano. We analysed dense microarray genotype data using four test statistics that detect departures from neutrality. Using a candidate gene, single nucleotide polymorphism-based approach, we identified genes exhibiting preliminary evidence of recent genetic adaptation in this population. These included genes that are part of the hypoxia-inducible transcription factor ( HIF ) pathway, a biochemical pathway involved in oxygen homeostasis, as well as three other genomic regions previously not known to be associated with high-altitude phenotypes. In addition to identifying selection-nominated candidate genes, we also tested whether the HIF pathway shows evidence of natural selection. Our results indicate that the genes of this biochemical pathway as a group show no evidence of having evolved in response to hypoxia in Andeans. Results from particular HIF -targeted genes, however, suggest that genes in this pathway could play a role in Andean adaptation to high altitude, even if the pathway as a whole does not show higher relative rates of evolution. These data suggest a genetic role in high-altitude adaptation and provide a basis for genotype/phenotype association studies that are necessary to confirm the role of putative natural selection candidate genes and gene regions in adaptation to altitude.

Maternal adaptation to high-altitude pregnancy: an experiment of nature--a review

A long and productive history of studies at high altitude has demonstrated that chronic hypoxia plays a key role in the aetiology of intrauterine growth restriction (IUGR) and pre-eclampsia. Susceptibility to altitude-associated IUGR varies among high-altitude populations in relation to their duration of altitude exposure, with multigenerational residents demonstrating one-third the birth weight fall present in shorter-resident groups. Higher uteroplacental blood flow during pregnancy in multigenerational high-altitude residents suggests that such population differences are due, at least in part, to differences in maternal vascular responses to pregnancy. We hypothesize that natural selection acting on hypoxia-inducible factor (HIF)-targeted or -regulatory genes has enabled maternal vascular adaptation to pregnancy in long-resident high-altitude groups. Preliminary evidence in support of this hypothesis demonstrates that the potent HIF-targeted vasoconstrictor, endothelin-1 (ET-1), is differentially regulated by pregnancy and chronic hypoxia in Andean vs European residents of high altitude. Andeans show the normal, pregnancy-associated fall in ET-1 levels previously reported at low altitude, whereas Europeans have higher ET-1 levels and little pregnancy-associated change, like pre-eclamptic women. Single nucleotide polymorphisms (SNPs) in the ET-1 gene also differ in Andeans compared with low-altitude populations. We conclude that high altitude serves as an experiment of nature for elucidating genetic factors underlying susceptibility to complications of pregnancy and fetal life. Such studies may be important for identifying persons at risk for these complications at any altitude.

Pulmonary extravascular fluid accumulation in recreational climbers: a prospective study

BACKGROUND

High altitude pulmonary oedema (HAPE) that is severe enough to require urgent medical care is infrequent. We hypothesised that subclinical HAPE is far more frequent than suspected during even modest climbs of average effort.

METHODS

We assessed 262 consecutive climbers of Monte Rosa (4559 m), before ascent and about 24 h later on the summit 1 h after arriving, by clinical examination, electrocardiography, oximetry, spirometry, carbon monoxide transfer, and closing volume. A chest radiograph was taken at altitude.

FINDINGS

Only one climber was evacuated for HAPE, but 40 (15%) of 262 climbers had chest rales or interstitial oedema on radiograph after ascent. Of 37 of these climbers, 34 (92%) showed increased closing volume. Of the 197 climbers without oedema, 146 (74%) had an increase in closing volume at altitude. With no change in vital capacity, forced expiratory volume in 1 s and forced expiratory flow at 25-75% of forced vital capacity increased slightly at altitude, without evidence of oedema. If we assume that an increased closing volume at altitude indicates increased pulmonary extravascular fluid, our data suggest that three of every four healthy, recreational climbers have mild subclinical HAPE shortly after a modest climb.

INTERPRETATION

The risk of HAPE might not be confined to a small group of genetically susceptible people, but likely exists for most climbers if the rate of ascent and degree of physical effort are great enough, especially if lung size is normal or low.

Iron supplementation improves progressive fatigue resistance during dynamic knee extensor exercise in iron-depleted, nonanemic women

BACKGROUND

Tissue iron depletion may negatively affect endurance performance and muscle fatigability.

OBJECTIVE

We investigated tissue-level iron depletion and progressive fatigue of the quadriceps during dynamic knee-extension exercise in young women.

DESIGN

Twenty iron-depleted (serum ferritin < 20 micro g/L), nonanemic (hemoglobin > 110 g/L) women (macro x +/- SEM age: 29.1 +/- 1.2 y) received iron (iron group) or placebo (placebo group) for 6 wk in a randomized, double-blind trial (n = 10 per group). A protocol integrating 2-3-s maximal voluntary static contractions (MVCs) with dynamic knee extensions was used to assess fatigue.

RESULTS

No significant differences between the groups in baseline iron status, MVC at rest, or MVC at the end of the protocol were observed. After treatment, serum iron and transferrin saturation increased significantly in the iron group (P = 0.02 and P = 0.03, respectively). Serum transferrin receptor concentrations increased significantly in the placebo group (P < 0.01) but not in the iron group. After treatment, the rate of decrease in MVC was attenuated in the iron group but not in the placebo group (P = 0.01). In the iron group, MVC at the sixth minute of the fatigue protocol and MVC at the end of the protocol were approximately 15% (P = 0.04) and approximately 27% higher (P < 0.01), respectively, after treatment. These improvements were not related to changes in iron-status indexes or tissue iron stores, although power was low (< 0.50) to detect these relations.

CONCLUSIONS

Iron supplementation was associated with a significant improvement in muscle fatigability. Interpretation regarding the direct role of tissue iron status is limited by the study's low power to detect relations between tissue iron improvement and decreased muscle fatigue.

Large-scale SNP analysis reveals clustered and continuous patterns of human genetic variation

Understanding the distribution of human genetic variation is an important foundation for research into the genetics of common diseases. Some of the alleles that modify common disease risk are themselves likely to be common and, thus, amenable to identification using gene-association methods. A problem with this approach is that the large sample sizes required for sufficient statistical power to detect alleles with moderate effect make gene-association studies susceptible to false-positive findings as the result of population stratification. Such type I errors can be eliminated by using either family-based association tests or methods that sufficiently adjust for population stratification. These methods require the availability of genetic markers that can detect and, thus, control for sources of genetic stratification among populations. In an effort to investigate population stratification and identify appropriate marker panels, we have analysed 11,555 single nucleotide polymorphisms in 203 individuals from 12 diverse human populations. Individuals in each population cluster to the exclusion of individuals from other populations using two clustering methods. Higher-order branching and clustering of the populations are consistent with the geographic origins of populations and with previously published genetic analyses. These data provide a valuable resource for the definition of marker panels to detect and control for population stratification in population-based gene identification studies. Using three US resident populations (European-American, African-American and Puerto Rican), we demonstrate how such studies can proceed, quantifying proportional ancestry levels and detecting significant admixture structure in each of these populations.

Andean and Tibetan patterns of adaptation to high altitude

OBJECTIVES

High-altitude hypoxia, or decreased oxygen levels caused by low barometric pressure, challenges the ability of humans to live and reproduce. Despite these challenges, human populations have lived on the Andean Altiplano and the Tibetan Plateau for millennia and exhibit unique circulatory, respiratory, and hematological adaptations to life at high altitude. We and others have identified natural selection candidate genes and gene regions for these adaptations using dense genome scan data. One gene previously known to be important in cellular oxygen sensing, egl nine homolog 1 (EGLN1), shows evidence of positive selection in both Tibetans and Andeans. Interestingly, the pattern of variation for this gene differs between the two populations. Continued research among Tibetan populations has identified statistical associations between hemoglobin concentration and single nucleotide polymorphism (SNP) genotype at EGLN1 and a second gene, endothelial PAS domain protein 1 (EPAS1).

METHODS

To measure for the effects of EGLN1 and EPAS1 altitude genotypes on hemoglobin concentration among Andean highlanders, we performed a multiple linear regression analysis of 10 candidate SNPs in or near these two genes.

RESULTS

Our analysis did not identify significant associations between EPAS1 or EGLN1 SNP genotypes and hemoglobin concentration in Andeans.

CONCLUSIONS

These results contribute to our understanding of the unique set of adaptations developed in different highland groups to the hypoxia of high altitude. Overall, the results provide key insights into the patterns of genetic adaptation to high altitude in Andean and Tibetan populations.

Pulmonary gas exchange during exercise in women: effects of exercise type and work increment

Exercise-induced arterial hypoxemia (EIAH) has been reported in male athletes, particularly during fast-increment treadmill exercise protocols. Recent reports suggest a higher incidence in women. We hypothesized that 1-min incremental (fast) running (R) protocols would result in a lower arterial PO(2) (Pa(O(2))) than 5-min increment protocols (slow) or cycling exercise (C) and that women would experience greater EIAH than previously reported for men. Arterial blood gases, cardiac output, and metabolic data were obtained in 17 active women [mean maximal O(2) uptake (VO(2 max)) = 51 ml. kg(-1). min(-1)]. They were studied in random order (C or R), with a fast VO(2 max) protocol. After recovery, the women performed 5 min of exercise at 30, 60, and 90% of VO(2 max) (slow). One week later, the other exercise mode (R or C) was similarly studied. There were no significant differences in VO(2 max) between R and C. Pulmonary gas exchange was similar at rest, 30%, and 60% of VO(2 max). At 90% of VO(2 max), Pa(O(2)) was lower during R (mean +/- SE = 94 +/- 2 Torr) than during C (105 +/- 2 Torr, P < 0.0001), as was ventilation (85.2 +/- 3.8 vs. 98.2 +/- 4.4 l/min BTPS, P < 0.0001) and cardiac output (19.1 +/- 0.6 vs. 21.1 +/- 1.0 l/min, P < 0.001). Arterial PCO(2) (32.0 +/- 0.5 vs. 30.0 +/- 0.6 Torr, P < 0.001) and alveolar-arterial O(2) difference (A-aDO(2); 22 +/- 2 vs. 16 +/- 2 Torr, P < 0.0001) were greater during R. Pa(O(2)) and A-aDO(2) were similar between slow and fast. Nadir Pa(O(2)) was </=80 Torr in four women (24%) but only during fast-R. In all subjects, Pa(O(2)) at VO(2 max) was greater than the lower 95% prediction limit calculated from available data in men (n = 72 C and 38 R) for both R and C. These data suggest intrinsic differences in gas exchange between R and C, due to differences in ventilation and also efficiency of gas exchange. The Pa(O(2)) responses to R and C exercise in our 17 subjects do not differ significantly from those previously observed in men.

Effect of developmental and ancestral high altitude exposure on chest morphology and pulmonary function in Andean and European/North American natives

Chest depth, chest width, forced vital capacity (FVC), and forced expiratory volume (FEV1) were measured in 170 adult males differing by ancestral (genetic) and developmental exposure to high altitude (HA). A complete migrant study design was used to study HA natives (Aymara/Quechua ancestry, n = 88) and low altitude (LA) natives (European/North American ancestry, n = 82) at both altitude (La Paz, Bolivia, 3,600 m) and near sea level (Santa Cruz, Bolivia, 420 m). HAN and LAN migrant groups were classified as: N(th) generation migrants, born and raised in a non-native environment; child migrants who migrated during the period of growth and maturation (0-18 yrs); and adult migrants who migrated after 18 years of age. Chest depth, FVC, and FEV1 measures were larger with increasing developmental exposure in both HAN migrants at LA and LAN migrants at HA. Developmental responses were similar between HAN and LAN groups. FVC and FEV1 measures were larger in HANs vs LANs born and raised at HA to suggest a genetic effect, but were similar in HANs and LANs born and raised at LA. The similarity of HAN and LAN groups at LA suggests that the genetic potential for larger lung volumes at HA depends upon developmental exposure to HA. Additional data for females (HANs at HA, n = 20, and LAN adult migrants to HA, n = 17) show similar differences as those shown between male HAN and LAN groups. Am. J. Hum. Biol. 11:383-395, 1999. Copyright 1999 Wiley-Liss, Inc.

Ancestry explains the blunted ventilatory response to sustained hypoxia and lower exercise ventilation of Quechua altitude natives

Andean high-altitude (HA) natives have a low (blunted) hypoxic ventilatory response (HVR), lower effective alveolar ventilation, and lower ventilation (VE) at rest and during exercise compared with acclimatized newcomers to HA. Despite blunted chemosensitivity and hypoventilation, Andeans maintain comparable arterial O(2) saturation (Sa(O(2))). This study was designed to evaluate the influence of ancestry on these trait differences. At sea level, we measured the HVR in both acute (HVR-A) and sustained (HVR-S) hypoxia in a sample of 32 male Peruvians of mainly Quechua and Spanish origins who were born and raised at sea level. We also measured resting and exercise VE after 10-12 h of exposure to altitude at 4,338 m. Native American ancestry proportion (NAAP) was assessed for each individual using a panel of 80 ancestry-informative molecular markers (AIMs). NAAP was inversely related to HVR-S after 10 min of isocapnic hypoxia (r = -0.36, P = 0.04) but was not associated with HVR-A. In addition, NAAP was inversely related to exercise VE (r = -0.50, P = 0.005) and ventilatory equivalent (VE/Vo(2), r = -0.51, P = 0.004) measured at 4,338 m. Thus Quechua ancestry may partly explain the well-known blunted HVR (10, 35, 36, 57, 62) at least to sustained hypoxia, and the relative exercise hypoventilation at altitude of Andeans compared with European controls. Lower HVR-S and exercise VE could reflect improved gas exchange and/or attenuated chemoreflex sensitivity with increasing NAAP. On the basis of these ancestry associations and on the fact that developmental effects were completely controlled by study design, we suggest both a genetic basis and an evolutionary origin for these traits in Quechua.

HIF and VEGF relationships in response to hypoxia and sciatic nerve stimulation in rat gastrocnemius

To determine if hypoxia-inducible factor-1 (HIF-1) may regulate skeletal muscle vascular endothelial growth factor (VEGF) expression in response to exercise or hypoxia, rats underwent 1h sciatic nerve electrical stimulation (ES), hypoxic exposure (H) or combined stimuli. HIF-1alpha protein levels increased six-fold with maximal (8V) ES with or without H. Similar HIF-1alpha increases occurred with sub-maximal (6V and 4V) ES plus H, but not in sub-maximal ES or H alone. VEGF mRNA and protein levels increased three-fold in sub-maximal ES or H alone, six-fold in sub-maximal ES plus H, 6.3-fold with maximal ES, and 6.5-fold after maximal ES plus H. These data suggest: (1) intracellular hypoxia during normoxic exercise may exceed that during 8% oxygen breathing at rest and is more effective in stimulating HIF-1alpha; (2) HIF-1 may be an important regulator of exercise-induced VEGF transcription; and (3) breathing 8% O(2) does not alter HIF-1alpha expression in skeletal muscle, implying that exercise-generated signals contribute to the regulation of HIF-1alpha and/or VEGF.

Measurement of cardiac output during exercise by open-circuit acetylene uptake

Noninvasive measurement of cardiac output (QT) is problematic during heavy exercise. We report a new approach that avoids unpleasant rebreathing and resultant changes in alveolar PO(2) or PCO(2) by measuring short-term acetylene (C(2)H(2)) uptake by an open-circuit technique, with application of mass balance for the calculation of QT. The method assumes that alveolar and arterial C(2)H(2) pressures are the same, and we account for C(2)H(2) recirculation by extrapolating end-tidal C(2)H(2) back to breath 1 of the maneuver. We correct for incomplete gas mixing by using He in the inspired mixture. The maneuver involves switching the subject to air containing trace amounts of C(2)H(2) and He; ventilation and pressures of He, C(2)H(2), and CO(2) are measured continuously (the latter by mass spectrometer) for 20-25 breaths. Data from three subjects for whom multiple Fick O(2) measurements of QT were available showed that measurement of QT by the Fick method and by the C(2)H(2) technique was statistically similar from rest to 90% of maximal O(2) consumption (VO(2 max)). Data from 12 active women and 12 elite male athletes at rest and 90% of VO(2 max) fell on a single linear relationship, with O(2) consumption (VO(2)) predicting QT values of 9.13, 15.9, 22.6, and 29.4 l/min at VO(2) of 1, 2, 3, and 4 l/min. Mixed venous PO(2) predicted from C(2)H(2)-determined QT, measured VO(2), and arterial O(2) concentration was approximately 20-25 Torr at 90% of VO(2 max) during air breathing and 10-15 Torr during 13% O(2) breathing. This modification of previous gas uptake methods, to avoid rebreathing, produces reasonable data from rest to heavy exercise in normal subjects.

Do high-altitude natives have enhanced exercise performance at altitude?

Natives of high altitude (HA) may have enhanced physical work capacity in hypoxia due to growth and development at altitude or, in the case of indigenous Andean and Himalayan residents, due to population genetic factors that determine higher limits to exercise performance. There is a growing scientific literature in support of both hypotheses, although the specific developmental vs. genetic origins of putative population trait differences remain obscure. Considering whole-body measures of exercise performance, a review of the literature suggests that indigenous HA natives have higher mean maximal oxygen consumption (VO(2) (max)) in hypoxia and smaller VO(2) (max) decrement with increasing hypoxia. At present, there is insufficient information to conclude that HA natives have enhanced work economy or greater endurance capacity, although for the former a number of studies indicate that this may be the case for Tibetans. At the physiological level, supporting the hypothesis of enhanced pulmonary gas exchange efficiency, HA natives have smaller alveolar-arterial oxygen partial pressure difference ((A-a)DO(2)), lower pulmonary ventilation (VE), and likely higher arterial O(2) saturation (SaO(2)) during exercise. At the muscle level, a handful of studies show no differences in fiber-type distributions, capillarity, oxidative enzymes, or the muscle response to training. At the metabolic level, a few studies suggest differences in lactate production/removal and (or) lactate buffering capacity, but more work is needed in this area.

Developmental, genetic, and environmental components of aerobic capacity at high altitude

The aerobic capacity of 268 subjects (158 males and 110 females) was evaluated in La Paz, Bolivia situated at 3,750 m. The sample included 1) 39 high altitude rural natives (all male); 2) 67 high altitude urban natives (32 male, 35 female); 3) 69 Bolivians of foreign ancestry acclimatized to high altitude since birth (37 male, 32 female); 4) 50 Bolivians of foreign ancestry acclimatized to high altitude during growth (25 male, 25 female); and 5) 42 non-Bolivians of either European or North American ancestry acclimatized to high altitude during adulthood (25 male, 18 female). Data analyses indicate that 1) high altitude urban natives, acclimatized to high altitude since birth or during growth, attained higher aerobic capacity than subjects acclimatized to high altitude during adulthood; 2) age at arrival to high altitude is inversely related to maximum oxygen consumption (VO2 max) expressed in terms L/min or ml/min/kg of lean body mass, but not in terms of ml/min/kg of body weight; 3) among subjects acclimatized to high altitude during growth, approximately 25% of the variability in aerobic capacity can be explained by developmental factors; 4) as inferred from evaluations of skin color reflectance and sibling similarities, approximately 20 to 25% of the variability in aerobic capacity at high altitude can be explained by genetic factors; 5) except among the non-Bolivians acclimatized to high altitude during adulthood, the aerobic capacity of individuals with high occupational activity level is equal to the aerobic capacity of high altitude rural natives; and 6) the relationship between occupational activity level and aerobic capacity is much greater among subjects acclimatized to high altitude before the age of 10 years than afterwards. Together these data suggest that the attainment of normal aerobic capacity at high altitude is related to both developmental acclimatization and genetic factors but its expression is highly mediated by environmental factors, such as occupational activity level and body composition.

Population genetic aspects and phenotypic plasticity of ventilatory responses in high altitude natives

Highland natives show unique breathing patterns and ventilatory responses at altitude, both at rest and during exercise. For many ventilatory traits, there is also significant variation between highland native groups, including indigenous populations in the Andes and Himalaya, and more recent altitude arrivals in places like Colorado. This review summarizes the literature in this area with some focus on partitioning putative population genetic differences from differences acquired through lifelong exposure to hypoxia. Current studies suggest that Tibetans have high resting ventilation (V (E)), and a high hypoxic ventilatory response (HVR), similar to altitude acclimatized lowlanders. Andeans, in contrast, show low resting V (E) and a low or "blunted" HVR, with little evidence that these traits are acquired via lifelong exposure. Resting V (E) of non-indigenous altitude natives is not well documented, but lifelong hypoxic exposure almost certainly blunts HVR in these groups through decreased chemosensitivity to hypoxia in a process known as hypoxic desensitization (HD). Together, these studies suggest that the time course of ventilatory response, and in particular the origin or absence of HD, depends on population genetic background i.e., the allele or haplotype frequencies that characterize a particular population. During exercise, altitude natives have lower V (E) compared to acclimatized lowland controls. Altitude natives also have smaller alveolar-arterial partial pressure differences P(AO2) - P(aO2) during exercise suggesting differences in gas exchange efficiency. Small P(AO2) - P(aO2) in highland natives of Colorado underscores the likely importance of developmental adaptation to hypoxia affecting structural/functional aspects of gas exchange with resultant changes in breathing pattern. However, in Andeans, at least, there is also evidence that low exercise V (E) is determined by genetic background affecting ventilatory control independent of gas exchange. Additional studies are needed to elucidate the effects of gene, environment, and gene-environment interaction on these traits, and these effects are likely to differ widely between altitude native populations.

Angiotensin-converting enzyme genotype and arterial oxygen saturation at high altitude in Peruvian Quechua

The I-allele of the angiotensin-converting enzyme (ACE) gene insertion/deletion (I/D) polymorphism has been associated with performance benefits at high altitude (HA). In n = 142 young males and females of largely Quechua origins in Peru, we evaluated 3 specific hypotheses with regard to the HA benefits of the I-allele: (1) the I-allele is associated with higher arterial oxygen saturation (Sa(O(2))) at HA, (2) the I-allele effect depends on the acclimatization state of the subjects, and (3) the putative I-allele effect on Sa(O(2)) is mediated by the isocapnic hypoxic ventilatory response (HVR, l/min(1)/% Sa(O(2))(1)). The subject participants comprised two different study groups including BLA subjects (born at low altitude) who were lifelong sea-level residents transiently exposed to hypobaric hypoxia (<24 h) and BHA subjects (born at HA) who were lifelong residents of HA. To control for the possibility of population stratification, Native American ancestry proportion (NAAP) was estimated as a covariate for each individual using a panel of 70 ancestry-informative molecular markers (AIMS). At HA, resting and exercise Sa(O(2)) was strongly associated with the ACE genotype, p = 0.008 with approximately 4% of the total variance in Sa(O(2)) attributed to ACE genotype. Moreover, I/I individuals maintained approximately 2.3 percentage point higher Sa(O(2)) compared to I/D and D/D. This I-allele effect was evident in both BLA and BHA groups, suggesting that acclimatization state has little influence on the phenotypic expression of the ACE gene. Finally, ACE genotype was not associated with the isocapnic HVR, although HVR had a strong independent effect on Sa(O(2)) (p = 0.001). This suggests that the I-allele effect on Sa(O(2)) is not mediated by the peripheral control of breathing, but rather by some other central cardiopulmonary effect of the ACE gene on the renin-angiotensin-aldosterone system (RAAS).

Spanish genetic admixture is associated with larger V(O2) max decrement from sea level to 4338 m in Peruvian Quechua

Quechua in the Andes may be genetically adapted to altitude and able to resist decrements in maximal O2 consumption in hypoxia (DeltaVo2 max). This hypothesis was tested via repeated measures of Vo2 max (sea level vs. 4338 m) in 30 men of mixed Spanish and Quechua origins. Individual genetic admixture level (%Spanish ancestry) was estimated by using ancestry-informative DNA markers. Genetic admixture explained a significant proportion of the variability in DeltaVo2 max after control for covariate effects, including sea level Vo2 max and the decrement in arterial O2 saturation measured at Vo2 max (DeltaSpO2 max) (R2 for admixture and covariate effects approximately 0.80). The genetic effect reflected a main effect of admixture on DeltaVo2 max (P = 0.041) and an interaction between admixture and DeltaSpO2 max (P = 0.018). Admixture predicted DeltaVo2 max only in subjects with a large DeltaSpO2 max (P = 0.031). In such subjects, DeltaVo2 max was 12-18% larger in a subgroup of subjects with high vs. low Spanish ancestry, with least squares mean values (+/-SE) of 739 +/- 71 vs. 606 +/- 68 ml/min, respectively. A trend for interaction (P = 0.095) was also noted between admixture and the decrease in ventilatory threshold at 4338 m. As previously, admixture predicted DeltaVo2 max only in subjects with a large decrease in ventilatory threshold. These findings suggest that the genetic effect on DeltaVo2 max depends on a subject's aerobic fitness. Genetic effects may be more important (or easier to detect) in athletic subjects who are more likely to show gas-exchange impairment during exercise. The results of this study are consistent with the evolutionary hypothesis and point to a better gas-exchange system in Quechua.

Effect of hypoxia on cerebrovascular and cognitive function during moderate intensity exercise

Exercise in hypoxia places added demands on the brain and cerebrovasculature that can impact cognitive function. The purpose of this study was to investigate the effect of acute hypoxia on cerebrovascular hemodynamics, markers of neuro-steroidal modulation and brain-blood barrier (BBB) integrity, and cognition during exercise. Thirty healthy participants (21±4yrs., BMI 24.0±2.6kg∙m(-2); 15 men) were randomized to both a≈2.5h normoxic (FiO2 20.0%) and hypoxic (FiO2 12.5%) condition on two separate days. After 1.25h, participants underwent 10min of exercise-alone (cycling at 55% HRmax) and 15min of exercise+cognitive testing. Prefrontal cortex (PFC) tissue oxygenation and middle cerebral artery (MCA) mean blood velocity (MnV) were measured using near-infrared spectroscopy and transcranial Doppler respectively at rest, during exercise-alone, and during exercise+cognitive testing. Salivary levels of dehydroepiandosterone [DHEA], DHEA-sulfate [DHEAS]) and neuron specific enolase (NSE) were measured pre and post exercise. Cognition was assessed using standard metrics of accuracy and reaction time (RT), and advanced metrics from drift-diffusion modeling across memory recognition, N-Back and Flanker tasks. MCA MnV increased from rest to exercise (p<0.01) and was unchanged with addition of cognitive testing during exercise in both normoxia and hypoxia. PFC oxygenation increased during exercise (p<0.05) and was further increased with addition of cognitive challenge in normoxia but decreased during exercise in hypoxia (p<0.05) with further reductions occurring with addition of cognitive tasks (p<0.05). DHEA and NSE increased and decreased post-exercise, respectively, in both normoxia and hypoxia (p<0.01). Accuracy on cognitive tasks was similar in normoxia compared to hypoxia, while RT was slower in hypoxia vs normoxia across memory recognition (p<0.01) and Flanker tasks (p=0.04). Drift-diffusion modeling suggested changes in memory RT were due to increases in caution (p<0.01). Overall cognitive performance is maintained during exercise in hypoxia concomitant with slower RT in select cognitive tasks and reduced oxygenation in the PFC. These changes were accompanied by slight increases in neuro-steroidal modulation but appear independent of changes in NSE, a biomarker of BBB integrity. Maintained accuracy and select increases in RT during hypoxic exercise may be related behavioral changes in caution.

What makes a champion?

Variation in human athletic performance is determined by a complex interaction of socio-cultural, psychological, and proximate physiological factors. Human physiological trait variance has both an environmental and genetic basis, although the classic gene-environment dichotomy is clearly too simplistic to understand the full range of variation for most proximate determinants of athletic performance, e.g., body composition. In other words, gene and environment interact, not just over the short term, but also over the lifetime of an individual with permanent effects on the adult phenotype. To further complicate matters, gene and environment may also be correlated. That is, genetically gifted individuals may be identified as children and begin training pulmonary, cardiovascular, and muscle systems at an early critical age. This review covers evidence in support of a genetic basis to human athletic performance, with some emphasis on the recent explosion of candidate gene studies. In addition, the review covers environmental influences on athletic performance with an emphasis on irreversible environmental effects, i.e., developmental effects that may accrue during critical periods of development either before conception (epigenetic effects), during fetal life (fetal programming), or during childhood and adolescence. Throughout, we emphasize the importance of gene-environment interaction (G x E) as a means of understanding variation in human physiological performance and we promote studies that integrate genomics with developmental biology.

Effects of birthplace and individual genetic admixture on lung volume and exercise phenotypes of Peruvian Quechua

Forced vital capacity (FVC) and maximal exercise response were measured in two populations of Peruvian males (age, 18-35 years) at 4,338 m who differed by the environment in which they were born and raised, i.e., high altitude (Cerro de Pasco, Peru, BHA, n = 39) and sea level (Lima, Peru, BSL, n = 32). BSL subjects were transported from sea level to 4,338 m, and were evaluated within 24 hr of exposure to hypobaric hypoxia. Individual admixture level (ADMIX, % Spanish ancestry) was estimated for each subject, using 22 ancestry-informative genetic markers and also by skin reflectance measurement (MEL). Birthplace accounted for the approximately 10% larger FVC (P < 0.001), approximately 15% higher maximal oxygen consumption (VO(2)max, ml.min(-1).kg(-1)) (P < 0.001), and approximately 5% higher arterial oxygen saturation during exercise (SpO(2)) (P < 0.001) of BHA subjects. ADMIX was low in both study groups, averaging 9.5 +/- 2.6% and 2.1 +/- 0.3% in BSL and BHA subjects, respectively. Mean underarm MEL was significantly higher in the BSL group (P < 0.001), despite higher ADMIX. ADMIX was not associated with any study phenotype, but study power was not sufficient to evaluate hypotheses of genetic adaptation via the ADMIX variable. MEL and FVC were positively correlated in the BHA (P = 0.035) but not BSL (P = 0.335) subjects. However, MEL and ADMIX were not correlated across the entire study sample (P = 0.282). In summary, results from this study emphasize the importance of developmental adaptation to high altitude. While the MEL-FVC correlation may reflect genetic adaptation to high altitude, study results suggest that alternate (environmental) explanations be considered.

Effects of Prolonged Dietary Curcumin Exposure on Skeletal Muscle Biochemical and Functional Responses of Aged Male Rats

Oxidative stress resulting from decreased antioxidant protection and increased reactive oxygen and nitrogen species (RONS) production may contribute to muscle mass loss and dysfunction during aging. Curcumin is a phenolic compound shown to upregulate antioxidant defenses and directly quench RONS in vivo. This study determined the impact of prolonged dietary curcumin exposure on muscle mass and function of aged rats. Thirty-two-month-old male F344xBN rats were provided a diet with or without 0.2% curcumin for 4 months. The groups included: ad libitum control (CON; n = 18); 0.2% curcumin (CUR; n = 18); and pair-fed (PAIR; n = 18) rats. CUR rats showed lower food intake compared to CON, making PAIR a suitable comparison group. CUR rats displayed larger plantaris mass and force production (vs. PAIR). Nuclear fraction levels of nuclear factor erythroid-2 related-factor-2 were greater, and oxidative macromolecule damage was lower in CUR (vs. PAIR). There were no significant differences in measures of antioxidant status between any of the groups. No difference in any measure was observed between CUR and CON rats. Thus, consumption of curcumin coupled with reduced food intake imparted beneficial effects on aged skeletal muscle. The benefit of curcumin on aging skeletal muscle should be explored further.

Regional differences in expression of VEGF mRNA in rat gastrocnemius following 1 hr exercise or electrical stimulation

BACKGROUND

Vascular endothelial growth factor (VEGF) mRNA levels increase in rat skeletal muscle after a single bout of acute exercise. We assessed regional differences in VEGF165 mRNA levels in rat gastrocnemius muscle using in situ hybridization after inducing upregulation of VEGF by treadmill running (1 hr) or electrical stimulation (1 hr). Muscle functional regions were defined as oxidative (primarily oxidative fibers, I and IIa), or glycolytic (entirely IIb or IId/x fibers). Functional regions were visualized on muscle cross sections that were matched in series to slides processed through in situ hybridization with a VEGF165 probe. A greater upregulation in oxidative regions was hypothesized.

RESULTS

Total muscle VEGF mRNA (via Northern blot) was upregulated 3.5-fold with both exercise and with electrical stimulation (P = 0.015). Quantitative densitometry of the VEGF mRNA signal via in situ hybridization reveals significant regional differences (P <or= 0.01) and protocol differences (treadmill, electrical stimulation, and control, P <or= 0.05). Mean VEGF mRNA signal was higher in the oxidative region in both treadmill run (approximately 7%, N = 4 muscles, P <or= 0.05) and electrically stimulated muscles (approximately 60%, N = 4, P <or= 0.05). These regional differences were not significantly different from control muscle (non-exercised, non-stimulated, N = 2 muscles), although nearly so for electrically stimulated muscle (P = 0.056).

CONCLUSIONS

Moderately higher VEGF mRNA signal in oxidative muscle regions is consistent with regional differences in capillary density. However, it is not possible to determine if the VEGF mRNA signal difference is important in either the maintenance of regional capillarity differences or exercise induced angiogenesis.

Higher arterial oxygen saturation during submaximal exercise in Bolivian Aymara compared to European sojourners and Europeans born and raised at high altitude

Arterial oxygen saturation (SaO(2)) was measured at 3,600-3,850 m by pulse oximetry at rest and during submaximal exercise in three study groups: 1) highland Aymara natives of the Bolivian altiplano (n = 25); 2) lowland European/North American sojourners to the highlands with at least 2 months of acclimatization time to 3,600 m (n = 27); and 3) subjects of European ancestry born and raised at 3,600 m (n = 22). Aymara subjects maintained approximately 1 percentage point higher SaO(2) during submaximal work up to 70% of their maximal work capacity, and showed a smaller rate of decline in SaO(2) with increasing work compared to both European study groups. The higher-exercise SaO(2) of Aymara compared to Europeans born and raised at 3,600 m suggests genetic adaptation. The two European study groups, who differed by exposure to high altitude during their growth and development period, did not show any significant difference in either resting or exercise SaO(2). This suggests that the developmental mode of adaptation is less important than the genetic mode of adaptation in determining exercise SaO(2). A weak correlation was detected (across study groups only) between the residual forced vital capacity (FVC) and the residual SaO(2) measured at the highest level of submaximal work output (P = 0.024, R = 0.26). While firm conclusions based on this correlation are problematic, it is suggested that a part of the higher SaO(2) observed in Aymara natives is due to a larger lung volume and pulmonary diffusion capacity for oxygen. Results from this study are compared to similar studies conducted with Tibetan natives, and are interpreted in light of recent quantitative genetic analyses conducted in both the Andes and Himalayas.