Human genetic adaptation to high altitude

Phenotypic plasticity and genetic adaptation to high-altitude hypoxia in vertebrates

High-altitude environments provide ideal testing grounds for investigations of mechanism and process in physiological adaptation. In vertebrates, much of our understanding of the acclimatization response to high-altitude hypoxia derives from studies of animal species that are native to lowland environments. Such studies can indicate whether phenotypic plasticity will generally facilitate or impede adaptation to high altitude. Here, we review general mechanisms of physiological acclimatization and genetic adaptation to high-altitude hypoxia in birds and mammals. We evaluate whether the acclimatization response to environmental hypoxia can be regarded generally as a mechanism of adaptive phenotypic plasticity, or whether it might sometimes represent a misdirected response that acts as a hindrance to genetic adaptation. In cases in which the acclimatization response to hypoxia is maladaptive, selection will favor an attenuation of the induced phenotypic change. This can result in a form of cryptic adaptive evolution in which phenotypic similarity between high- and low-altitude populations is attributable to directional selection on genetically based trait variation that offsets environmentally induced changes. The blunted erythropoietic and pulmonary vasoconstriction responses to hypoxia in Tibetan humans and numerous high-altitude birds and mammals provide possible examples of this phenomenon. When lowland animals colonize high-altitude environments, adaptive phenotypic plasticity can mitigate the costs of selection, thereby enhancing prospects for population establishment and persistence. By contrast, maladaptive plasticity has the opposite effect. Thus, insights into the acclimatization response of lowland animals to high-altitude hypoxia can provide a basis for predicting how altitudinal range limits might shift in response to climate change.

Partial contributions of developmental hypoxia and undernutrition to prenatal alterations in somatic growth and cardiovascular structure and function

OBJECTIVE

The objective of the study was to compare and contrast the effects of developmental hypoxia vs undernutrition on fetal growth, cardiovascular morphology, and function.

STUDY DESIGN

On day 15 of gestation, Wistar dams were divided into control, hypoxic (10% O(2)), or undernourished (35% reduction in food intake) pregnancy. On day 20, fetal thoraces were fixed, and the fetal heart and aorta underwent quantitative histological analysis. In a separate group, fetal aortic vascular reactivity was determined via wire myography.

RESULTS

Both hypoxic and undernourished pregnancy was associated with asymmetric fetal growth restriction. Pregnancy complicated by hypoxia promoted fetal aortic thickening without changes in cardiac volumes when expressed as a percentage of total heart volume. In contrast, maternal undernutrition affected fetal cardiac morphology without changes in aortic structure. Fetal aortic vascular reactivity was also differentially affected by hypoxia or undernutrition.

CONCLUSION

Developmental hypoxia or undernutrition in late gestation has differential effects on fetal cardiovascular morphology and function.

Genetic variations in Tibetan populations and high-altitude adaptation at the Himalayas

Modern humans have occupied almost all possible environments globally since exiting Africa about 100,000 years ago. Both behavioral and biological adaptations have contributed to their success in surviving the rigors of climatic extremes, including cold, strong ultraviolet radiation, and high altitude. Among these environmental stresses, high-altitude hypoxia is the only condition in which traditional technology is incapable of mediating its effects. Inhabiting at >3,000-m high plateau, the Tibetan population provides a widely studied example of high-altitude adaptation. Yet, the genetic mechanisms underpinning long-term survival in this environmental extreme remain unknown. We performed an analysis of genome-wide sequence variations in Tibetans. In combination with the reported data, we identified strong signals of selective sweep in two hypoxia-related genes, EPAS1 and EGLN1. For these two genes, Tibetans show unusually high divergence from the non-Tibetan lowlanders (Han Chinese and Japanese) and possess high frequencies of many linked sequence variations as reflected by the Tibetan-specific haplotypes. Further analysis in seven Tibetan populations (1,334 individuals) indicates the prevalence of selective sweep across the Himalayan region. The observed indicators of natural selection on EPAS1 and EGLN1 suggest that during the long-term occupation of high-altitude areas, the functional sequence variations for acquiring biological adaptation to high-altitude hypoxia have been enriched in Tibetan populations.

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.

Studying biological responses to global change in atmospheric oxygen

A popular book recently hypothesized that change in atmospheric oxygen over geological time is the most important physical factor in the evolution of many fundamental characteristics of modern terrestrial animals. This hypothesis is generated primarily using fossil data but the present paper considers how modern experimental biology can be used to test it. Comparative physiology and experimental evolution clearly show that changes in atmospheric O(2) over the ages had the potential to drive evolution, assuming the physiological O(2)-sensitivity of animals today is similar to the past. Established methods, such as phylogenetically independent contrasts, as well new approaches, such as adding environmental history to phylogenetic analyses or modeling interactions between environmental stresses and biological responses with different rate constants, may be useful for testing (disproving) hypotheses about biological adaptations to changes in atmospheric O(2).

Sequencing of 50 human exomes reveals adaptation to high altitude

Residents of the Tibetan Plateau show heritable adaptations to extreme altitude. We sequenced 50 exomes of ethnic Tibetans, encompassing coding sequences of 92% of human genes, with an average coverage of 18x per individual. Genes showing population-specific allele frequency changes, which represent strong candidates for altitude adaptation, were identified. The strongest signal of natural selection came from endothelial Per-Arnt-Sim (PAS) domain protein 1 (EPAS1), a transcription factor involved in response to hypoxia. One single-nucleotide polymorphism (SNP) at EPAS1 shows a 78% frequency difference between Tibetan and Han samples, representing the fastest allele frequency change observed at any human gene to date. This SNP's association with erythrocyte abundance supports the role of EPAS1 in adaptation to hypoxia. Thus, a population genomic survey has revealed a functionally important locus in genetic adaptation to high altitude.

Evolution. Genes for high altitudes

Analyses of genomes from Tibetan populations reveal a signaling pathway that may account for high-altitude adaptation.

The impact of altitude on infant health in South America

Several studies report that altitude reduces birth weight. However, much remains unknown about effects in various altitude ranges and about the heterogeneity in altitude effects by fetal health endowments. This study estimates the effects of altitude in South America on the means and quantiles of birth weight and gestational age separately for two large samples born at altitude ranges of 5 to 1,280 m and 1,854 to 3,600 m. The study finds significant negative altitude effects on birth weight and gestational age in the low-altitude sample and on birth weight in the high-altitude sample. Altitude effects are larger for infants with very low fetal health endowments. The study finds differences in the effects of several inputs such as socioeconomic status and maternal fertility history and health between the two altitude samples. The study highlights the importance of adverse altitude effects on infant health when evaluating the costs and returns of policies that change the number of individuals who reside at higher altitude in both low and high altitude ranges.

Cerebral and functional adaptation with chronic hypoxia exposure: a multi-modal MRI study

The current study obtained multi-modal MRI data from 28 immigrant high altitude (HA) young adults who were born and grew up at Qinghai-Tibetan plateau matched with 28 matched sea level (SL) controls. We compared their regional gray matter volumes (VBM) and white matter quality (DAI FA values) as well as resting state brain activity (Regional Homogeneity (ReHo) of BOLD-fMRI). We found that HA residents showed decreased gray matter volume at bilateral anterior insula, bilateral prefrontal cortex, the left precentral, the left cingulate and the right lingual cortex; accompanied by changed FA and ReHo values in relevant and other regions. The resting state activity at the hippocampus and the right insula were increasing with SL relocation. The HA subjects performed worse on a series of working memory tasks, with the ReHo values of several regions as significant predictors of their performance. This study demonstrated the cerebral and functional modifications with chronic high altitude hypoxia.

The genetics of human adaptation: hard sweeps, soft sweeps, and polygenic adaptation

There has long been interest in understanding the genetic basis of human adaptation. To what extent are phenotypic differences among human populations driven by natural selection? With the recent arrival of large genome-wide data sets on human variation, there is now unprecedented opportunity for progress on this type of question. Several lines of evidence argue for an important role of positive selection in shaping human variation and differences among populations. These include studies of comparative morphology and physiology, as well as population genetic studies of candidate loci and genome-wide data. However, the data also suggest that it is unusual for strong selection to drive new mutations rapidly to fixation in particular populations (the 'hard sweep' model). We argue, instead, for alternatives to the hard sweep model: in particular, polygenic adaptation could allow rapid adaptation while not producing classical signatures of selective sweeps. We close by discussing some of the likely opportunities for progress in the field.

Genetic evidence for high-altitude adaptation in Tibet

Tibetans have lived at very high altitudes for thousands of years, and they have a distinctive suite of physiological traits that enable them to tolerate environmental hypoxia. These phenotypes are clearly the result of adaptation to this environment, but their genetic basis remains unknown. We report genome-wide scans that reveal positive selection in several regions that contain genes whose products are likely involved in high-altitude adaptation. Positively selected haplotypes of EGLN1 and PPARA were significantly associated with the decreased hemoglobin phenotype that is unique to this highland population. Identification of these genes provides support for previously hypothesized mechanisms of high-altitude adaptation and illuminates the complexity of hypoxia-response pathways in humans.

Growth and body composition of Peruvian infants in a periurban setting

BACKGROUND

Previous growth studies of Peruvian children have featured high stunting rates and limited information about body composition.

OBJECTIVE

We aimed to characterize anthropometric measures of Peruvian infants 0 to 12 months of age in relation to the international growth references and biological, environmental, and socioeconomic factors.

METHODS

Infants (n = 232) were followed longitudinally from birth through 12 months of age from a prenatal zinc supplementation trial conducted in Lima, Peru, between 1995 and 1997. Anthropometric measures of growth and body composition were obtained at enrollment from mothers and monthly through 1 year of age from infants. Weekly morbidity and dietary intake surveillance was carried out during the second half of infancy.

RESULTS

The prevalence rates of stunting, underweight, and wasting did not exceed 4% based on the World Health Organization growth references. Infants of mothers from high-altitude regions had larger chest circumference (p = .006) and greater length (p = .06) by 12 months. Significant predictors of growth and body composition throughout infancy were age, sex, anthropometric measurements at birth, breastfeeding, maternal anthropometric measurements, primiparity, prevalence of diarrhea among children, and the altitude of the region of maternal origin. No associations were found for maternal education, asset ownership, or sanitation and hygiene factors.

CONCLUSIONS

Peruvian infants in this urban setting had lower rates of stunting than expected. Proximal and familial conditions influenced growth throughout infancy.

Maternal hemoglobin level and fetal outcome at low and high altitudes

Both, low (<7 g/dl) and high (>14.5 g/dl), maternal hemoglobin (Hb) levels have been related to poor fetal outcome. Most studies have been done at low altitude (LA). Here, we have sought to determine whether this relationship exists at both high and low altitude, and also whether there is an adverse effect of high altitude (HA) on fetal outcome independent of level of maternal hemoglobin. The study is based on a retrospective multicenter analysis of 35,449 pregnancies at LA and six other cities above 3000 meters. In analyses of all women at both LA and HA, those with Hb <9 g/dl had odds ratios (ORs) and 95% confidence intervals (CI) of 4.4 (CI: 2.8-6.7), 2.5 (CI: 1.9-3.2), and 1.4 (CI: 1.1-1.9) for stillbirths, preterm, and small for gestational age (SGA) births, respectively, compared with women with 11-12.9 g/dl of Hb, after adjustment for confounders. These risks by hemoglobin level differed little between women at LA and HA, suggesting that no correction of the definition of anemia is necessary for women at HA. Women living at high altitude with hemoglobin >15.5 g/dl had higher risks for stillbirths (OR: 1.3; CI: 1.05-1.3), preterm (OR: 1.5; CI 1.3-1.8), and SGA births (OR: 2.1, CI 1.8-2.3). There was also a significant adverse effect of living at HA, independent of hemoglobin level for all three outcomes (OR: 3.9, 1.7, and 2.3; CI: 2.8-5.2, 1.5-1.9, and 2.1-2.5) for stillbirths, preterms, and SGA respectively, after adjusting for hemoglobin level. Both, high and low maternal hemoglobin levels were related to poor pregnancy outcome, with similar effect of low hemoglobin in both LA and HA. Our data suggest, that maternal hemoglobin above 11 g/dl but below 13 g/dl is the area of minimal risk of poor adverse outcomes. Living at HA had an adverse effect independent of hemoglobin level.

Evolutionary adaptation to high altitude: a view from in utero

A primary focus within biological anthropology has been to elucidate the processes of evolutionary adaptation. Frisancho helped to move anthropology towards more mechanistic explanations of human adaptation by drawing attention to the importance of the functional relevance of human variation. Using the natural laboratory of high altitude, he and others asked whether the unique physiology of indigenous high-altitude residents was the result of acclimatization, developmental plasticity, and/or genetic adaptation in response to the high-altitude environment. We approach the question of human adaptation to high altitude from a somewhat unique vantage point; namely, by examining physiological characteristics-pregnancy and pregnancy outcome-which are closely associated with reproductive fitness. Here we review the potent example of high-altitude native population's resistance to hypoxia-associated reductions in birth weight, which is often associated with higher infant morbidity and mortality at high altitude. With the exception of two recent publications, these comparative birth weight studies have utilized surnames, self-identification, and/or linguistic characteristics to assess ancestry, and none have linked 'advantageous' phenotypes to specific genetic variations. Recent advancements in genetic and statistical tools have enabled us to assess individual ancestry with higher resolution, identify the genetic basis of complex phenotypes and to infer the effect of natural selection on specific gene regions. Using these technologies our studies are now directed to determine the genetic variations that underlie the mechanisms by which high-altitude ancestry protects fetal growth and, in turn, to further our understanding of evolutionary processes involved in human adaptation to high altitude.

High serum testosterone levels are associated with excessive erythrocytosis of chronic mountain sickness in men

Chronic mountain sickness (CMS) is characterized by excessive erythrocytosis (EE) secondary to hypoventilation. Erythropoietin (Epo) and testosterone regulate erythrocyte production. Low thyroid hormone levels are also associated to hypoventilation. Hence, these hormones can play a role in etiopathogeny of EE. The purpose of this study was to elucidate the effect of sexual and thyroid hormones and Epo in residents from Lima (150 m) and Cerro de Pasco (4,340 m), Peru, and the response to human chorionic gonadotrophin stimulation (hCG). Three groups, one at low altitude and two at high altitude [1 with hemoglobin values >16-21 g/dl and the second with Hb >or=21 g/dl (EE)], were studied. hCG was administered intramuscularly in a single dose (1,000 IU), and blood samples were obtained at 0, 6, 12, 24, 48, and 72 h after injection. High-altitude natives present similar levels of gonadotropins and thyroid hormones but lower dehydroepiandrosterone sulphate (DHEAS) levels (P < 0.01) and greater Epo (P < 0.01), 17alpha-hydroxyprogesterone (P < 0.01), and testosterone levels (P < 0.01) than those at 150 m. Serum testosterone levels (524.13 +/- 55.91 microg/dl vs. 328.14 +/- 53.23 ng/dl, means +/- SE; P < 0.05) and testosterone/DHEAS ratios are higher (7.98 +/- 1.1 vs. 3.65 +/- 1.1; P < 0.01) and DHEAS levels lower in the EE group (83.85 +/- 14.60 microg/dl vs. 148.95 +/- 19.11 ug/dl; P < 0.05), whereas Epo was not further affected. Testosterone levels were highest and DHEAS levels lowest in the EE group at all times after hCG stimulation. In conclusion, high androgen activity could be involved in the etiopathogeny of CMS. This evidence provides an opportunity to develop new therapeutic strategies.

Genetic bottleneck among daghestan highlanders migrating to lowlands

We present results of Short tandem repeat polymorphisms (STRPs) analysis and epidemiology study of indigenous ethnic highlanders of Daghestan and of the migrants from highlands to the lowland area in 1944, in comparison with native lowlanders. Results obtained show that demographically ancient highland ethnics have achieved a relatively stable equilibrium in their native environment and are characterized by optimal level of the main viability parameters (fertility, mortality, lifespan and morbidity). Migrants from highlands to the lowlands experienced dramatically increased morbidity and mortality in 1944–1947: up to 65–70% of total migrants had suffered malaria, typhus and other new infections and about 35–37% of total migrants had died. Genetic-epidemiological study support that non-survived migrants were characterized by a higher inbreeding rate, lower heterozygosity and higher physiological sensitivity to the environmental stress. This inter-connected complex had advantage for adaptation of the highlanders to the native environment but diminished their adaptability in the new and/or changing environment. A detailed study using STRP we performed in 1995–1999 in one highland isolate of ethnic Avars of whom about 50% were moved to the lowland area. We found significant differences in genetic and demographical structures between these highland and migrant parts of the isolate: the genetic bottleneck among migrants had a great qualitative and quantitative impact on their gene pool, i.e., lost of rare native population alleles, as well as of about 1/3 of total migrants with certain genotypes. Survived migrants demonstrate shorter lifespan and higher morbidity rate that support their still ongoing genetic adaptation to the lowland environment.

Abnormal energy regulation in early life: childhood gene expression may predict subsequent chronic mountain sickness

BACKGROUND

Life at altitude depends on adaptation to ambient hypoxia. In the Andes, susceptibility to chronic mountain sickness (CMS), a clinical condition that occurs to native highlanders or to sea level natives with prolonged residence at high altitude, remains poorly understood. We hypothesized that hypoxia-associated gene expression in children of men with CMS might identify markers that predict the development of CMS in adults. We assessed distinct patterns of gene expression of hypoxia-responsive genes in children of highland Andean men, with and without CMS.

METHODS

We compared molecular signatures in children of highland (HA) men with CMS (n = 10), without CMS (n = 10) and in sea level (SL) children (n = 20). Haemoglobin, haematocrit, and oxygen saturation were measured. Gene expression in white cells was assessed at HA and then, in the same subjects, within one hour of arrival at sea level.

RESULTS

HA children showed higher expression levels of genes regulated by HIF (hypoxia inducible factor) and lower levels of those involved in glycolysis and in the tricarboxylic acid (TCA) cycle. Pyruvate dehydrogenase kinase 1(PDK1) and HIF prolyl hydroxylase 3 (HPH3) mRNA expressions were lowest in children of CMS fathers at altitude. At sea level the pattern of gene expression in the 3 children's groups was indistinguishable.

CONCLUSION

The molecular signatures of children of CMS patients show impaired adaptation to hypoxia. At altitude children of CMS fathers had defective coupling between glycolysis and mitochondria TCA cycle, which may be a key mechanism/biomarker for adult CMS. Early biologic markers of disease susceptibility in Andeans might impact health services and social planning.

Tibetans retained innate ability resistance to acute hypoxia after long period of residing at sea level

Could the intrinsic characteristics of tolerance to hypoxia be retained in Tibetan high-altitude natives after they had migrated to a low altitude? To answer this question, we undertook a study of 33 healthy male adolescent Tibetans born and raised in a high plateau (3,700 m [12,140 ft] above sea level) who migrated to Shanghai (sea level) for 4 years. Ten age-matched healthy male Han adolescents born and raised in Shanghai were regarded as the control group. Acute hypoxia was induced in a hypobaric chamber for 2 h to simulate the 3,700 m altitude. At sea level, maximal oxygen consumption (VO2 max) was not significantly different between the two groups. During acute hypoxia, the values of VO2 max, tissue oxygen extraction, arterial oxygen pressure, and the arterial oxygen saturation showed markedly higher in Tibetan subjects than in Han subjects (1.41 +/- 0.04 l/min/M2 vs.1.25 +/- 0.04 l/min/M2, 55.0 +/- 4.2% vs. 47.3 +/- 9.1%, 7.2 +/- 0.6 vs. 5.5 +/- 0.2 kPa, and 87.9 +/- 3.3% vs. 78.2 +/- 1.6%, respectively, P < 0.05). The calculated "oxygen reserve capacity" and "cardiac reserve capacity" were better in the Tibetans than in the Han natives (P < 0.05), which suggests that physical work capacity is greater in the Tibetan group. The sympathetic stimulation was less, and there was no noticeable change in cardiac function during acute hypoxia in the Tibetan group. The results indicate that the better tolerance to hypoxia in the Tibetans is retained during the 4-year stay at sea level, implying that the intrinsic hypoxic adaptation still exists in the Tibetan high-altitude natives.

Pathophysiology of anemia and erythrocytosis

An increasing understanding of the process of erythropoiesis raises some interesting questions about the pathophysiology, diagnosis and treatment of anemia and erythrocytosis. The mechanisms underlying the development of many of the erythrocytoses, previously characterised as idiopathic, have been elucidated leading to an increased understanding of oxygen homeostasis. Characterisation of anemia and erythrocytosis in relation to serum erythropoietin levels can be a useful addition to clinical diagnostic criteria and provide a rationale for treatment with erythropoiesis stimulating agents (ESAs). Recombinant human erythropoietin as well as other ESAs are now widely used to treat anemias associated with a range of conditions, including chronic kidney disease, chronic inflammatory disorders and cancer. There is also heightened awareness of the potential abuse of ESAs to boost athletic performance in competitive sport. The discovery of erythropoietin receptors outside of the erythropoietic compartment may herald future applications for ESAs in the management of neurological and cardiac diseases. The current controversy concerning optimal hemoglobin levels in chronic kidney disease patients treated with ESAs and the potential negative clinical outcomes of ESA treatment in cancer reinforces the need for cautious evaluation of the pleiotropic effects of ESAs in non-erythroid tissues.

Growth among Tibetans at high and low altitudes in India

In India, Tibetans have been living at different altitudes for more than 40 years. This study describes physical growth in terms of height, weight, sitting height, skinfold thickness at triceps and upper arm circumference of Tibetans born and raised at three Tibetan refugee settlements (3,521; 970; and 800 m) from the view point of the hypothesis that growth is retarded at high altitude. Samples consist of individuals between the ages of 2 and 40 years. Tibetans at high altitude in India show a growth pattern similar to that previously observed among Tibetans in Tibet. Tibetans at high altitude are taller and heavier compared to Andean highlanders. The general trends show that Tibetan children and adults of both sexes at low altitude in India are advanced in terms of height, weight, skinfold thickness at triceps and upper arm circumference compared to Tibetans at high altitude. Trunk length (sitting height) is similar at the two altitudes but relative sitting height is greater at high altitude. Greater relative sitting height and lesser leg length at high altitude than at low altitudes is discussed in terms of effect of altitude, temperature, and nutritional status.

Prevalence and severity of asthma and allergies in schoolchildren in Lhasa, Tibet

BACKGROUND

The International Study of Asthma and Allergies in Childhood (ISAAC) demonstrated that large variations existed in the prevalence of symptoms of asthma, allergic rhinoconjunctivitis and eczema throughout the world and that environmental factors and lifestyle customs are major determinants of the prevalence and severity of these diseases. However, the relevant data about children living at high-altitude locations were considered to be underreported.

OBJECTIVE

The ISAAC Phase III programme was carried out in Lhasa, the Tibetan Autonomous Region in China, at an elevation of 3658 m above sea level to examine the occurrence of asthma, allergic rhinoconjunctivitis and eczema in schoolchildren aged 13-14 years.

METHODS

All 3196 schoolchildren in eight public junior high schools in urban Lhasa who were confirmed to be 13-14 years old were invited and participated in both written and video questionnaire investigations, among which 3190 pieces of data (49.8% of boys and 50.2% of girls) were validated and analysed.

RESULTS

Among the overall observations, the prevalence of 'having ever experienced wheezing', 'current wheezing' and 'diagnosed to have asthma' was 1.4%, 0.8% and 1.1%, respectively. The prevalence of current exercise-induced asthma and current nocturnal cough was 7.1% and 4.6%, respectively. The current prevalence of allergic rhinoconjunctivitis and eczema was 5.2% and 0.4%, respectively. In addition, the prevalence of rhinoconjunctivitis symptoms during the past 12 months showed no discernable differences throughout the year.

CONCLUSION

The prevalence of asthma, allergic rhinoconjunctivitis and eczema over the past 12 months was the lowest among the centres, that performed ISAAC worldwide.

Detecting natural selection in high-altitude human populations

High-altitude natives have distinctive biological characteristics that appear to offset the stress of hypoxia. Evolutionary theory reasons that they reflect genetic adaptations resulting from natural selection on traits with heritable variation. Furthermore, high-altitude natives of the Andean and Tibetan Plateaus differ from one another, perhaps resulting from different evolutionary histories. Three approaches have developed a case for the possibility of population genetic differences: comparing means of classical physiological traits measured in samples of natives and migrants between altitudes, estimating genetic variance using statistical genetics techniques, and comparing features of species with different evolutionary histories. Tibetans have an inferred autosomal dominant major gene for high oxygen saturation that is associated with higher offspring survival, a strong indicator of ongoing natural selection. New approaches use candidate gene and genomic analyses. Conclusive evidence about population genetic differences and associations with phenotypes remains to be discovered.

Maternal oxygen delivery is not related to altitude- and ancestry-associated differences in human fetal growth

Fetal growth is reduced at high altitude, but the decrease is less among long-resident populations. We hypothesized that greater maternal uteroplacental O(2) delivery would explain increased fetal growth in Andean natives versus European migrants to high altitude. O(2) delivery was measured with ultrasound, Doppler and haematological techniques. Participants (n=180) were pregnant women of self-professed European or Andean ancestry living at 3600 m or 400 m in Bolivia. Ancestry was quantified using ancestry-informative single nucleotide polymorphism. The altitude-associated decrement in birth weight was 418 g in European versus 236 g in Andean women (P<0.005). Altitude was associated with decreased uterine artery diameter, volumetric blood flow and O(2) delivery regardless of ancestry. But the hypothesis was rejected as O(2) delivery was similar between ancestry groups at their respective altitudes of residence. Instead, Andean neonates were larger and heavier per unit of O(2) delivery, regardless of altitude (P<0.001). European admixture among Andeans was negatively correlated with birth weight at both altitudes (P<0.01), but admixture was not related to any of the O(2) transport variables. Genetically mediated differences in maternal O(2) delivery are thus unlikely to explain the Andean advantage in fetal growth. Of the other independent variables, only placental weight and gestational age explained significant variation in birth weight. Thus greater placental efficiency in O(2) and nutrient transport, and/or greater fetal efficiency in substrate utilization may contribute to ancestry- and altitude-related differences in fetal growth. Uterine artery O(2) delivery in these pregnancies was 99 +/- 3 ml min(-1), approximately 5-fold greater than near-term fetal O(2) consumption. Deficits in maternal O(2) transport in third trimester normal pregnancy are unlikely to be causally associated with variation in fetal growth.

Postnatal cardiopulmonary adaptations to high altitude

Postnatal cardiopulmonary adaptations to high altitude constitute a key component of any set of responses developed to face high altitude hypoxia. Such responses are required ultimately to meet the energy demands necessary for adequate functioning at cell and organism level. After a brief insight on general and cardiopulmonary comparative studies in growing and adult organisms, differences and possible explanations for varying cardiopulmonary pathology, pulmonary artery hypertension, persistent right ventricular predominance and subacute high altitude pulmonary hypertension in different populations of children living at high altitude are discussed. Potential long-term implications of early chronic hypoxic exposure on later diseases are also presented. It is hoped that this review will help the practicing physician working at high altitude to make informed decisions concerning individual pediatric patients, specifically with regard to diagnosis and management of altitude-related cardiopulmonary pathology. Finally, plausibility and the knowledge-base of public health interventions to reduce the risks posed by suboptimal or inadequate postnatal cardiopulmonary responses to high altitude are discussed.

Peruvian contributions to the study on human reproduction at high altitude: from the chronicles of the Spanish conquest to the present

Humans have lived in the Peruvian Andes for about 12,000 years providing adequate time for adaptation to high altitude to have occurred. The arrival of the Spanish conquistadors in the 16th century potentially altered this process through genetic admixture. Early records revealed a potential for reduced fertility and a high perinatal and neonatal mortality amongst the early Spanish inhabitants who settled at high altitude when compared to the native Inca population. It appears that fertility is reduced during acute exposure at high altitude but is normal in populations born and living at high altitude. On the other hand, perinatal and neonatal mortality is presently still high at elevated altitudes, even after taking into account socio-economic status. The rates of perinatal and neonatal mortality are, however, lower in populations that have resided at high altitude for longer; populations inhabiting the southern Andes have a longer antiquity at high altitude and lower rates of fetal and neonatal deaths than those in the central Andes with a shorter residence at high altitude. Clearly, antiquity and genetics are important components in determining survival and quality of life at high altitude.

The Heart and Pulmonary Circulation at High Altitudes

More than 140 million people worldwide live >2500 m above sea level. Of them, 80 million live in Asia, and 35 million live in the Andean mountains. This latter region has its major population density living above 3500 m. The primary objective of the present study is to review the physiology, pathology, pathogenesis, and clinical features of the heart and pulmonary circulation in healthy highlanders and patients with chronic mountain sickness. A systematic review of worldwide literature was undertaken, beginning with the pioneering work done in the Andes several decades ago. Original articles were analyzed in most cases and English abstracts or translations of articles written in Chinese were reviewed. Pulmonary hypertension in healthy highlanders is related to a delayed postnatal remodeling of the distal pulmonary arterial branches. The magnitude of pulmonary hypertension increases with the altitude level and the degree of exercise. There is reversal of pulmonary hypertension after prolonged residence at sea level. Chronic mountain sickness develops when the capacity for altitude adaptation is lost. These patients have moderate to severe pulmonary hypertension with accentuated hypoxemia and exaggerated polycythemia. The clinical picture of chronic mountain sickness differs from subacute mountain sickness and resembles other chronic altitude diseases described in China and Kyrgyzstan. The heart and pulmonary circulation in healthy highlanders have distinct features in comparison with residents at sea level. Chronic mountain sickness is a public health problem in the Andean mountains and other mountainous regions around the world. Therefore, dissemination of preventive and therapeutic measures is essential.

Genetic contribution of the endothelial nitric oxide synthase gene to high altitude adaptation in sherpas

The Sherpas' adaptation to high altitude has been hypothesized as being due to a genetic basis since the beginning of the last century, but this has yet to be demonstrated. We randomly enrolled 105 Sherpas in Namche Bazaar (3440 m) and 111 non-Sherpa Nepalis in Kathmandu (1330 m) in Nepal. The genotypes of Glu298Asp and eNOS4b/a polymorphisms of the endothelial nitric oxide synthase (eNOS) gene were identified. The metabolites of nitric oxide (NO( x ): nitrite and nitrate) in serum were measured. The frequencies of the Glu and eNOS4b alleles were significantly higher in Sherpas (Glu: 87.5%; eNOS4b: 96.7%) than in non-Sherpas (Glu: 77.9%, p = 0.036; eNOS4b: 90.5%, p = 0.009). In addition, the combination of the wild types of Glu298Glu and eNOS4b/b was significantly greater in Sherpas (66.7%) than non-Sherpas (47.7%, p = 0.008). However, the serum NO( x ) was significantly lower in Sherpas (53.2 +/- 4.6 micromol/L) than in non-Sherpas (107.3 +/- 9.0 micromol/L, p < 0.0001). The wild alleles of the Glu298Asp and eNOS4b/a polymorphisms of the eNOS gene may be a benefit for the Sherpas' adaptation to high altitude. The nitric oxide metabolites (NO( x )) in serum vary individually, thus it is not a reliable indicator for endogenous nitric oxide production.

Evidence for a genetic basis for altitude-related illness

Altitude-related illnesses are a family of interrelated pulmonary, cerebral, hematological, and cardiovascular medical conditions associated with the diminished oxygen availability at moderate to high altitudes. The acute forms of these debilitating and potentially fatal conditions, which include acute mountain sickness (AMS), high altitude pulmonary edema (HAPE), and high altitude cerebral edema (HACE), often develop in incompletely acclimatized lowlanders shortly after ascent, whereas, the chronic conditions, such as chronic mountain sickness (CMS) and high altitude pulmonary hypertension (HAPH), usually afflict native or long-term highlanders and may reflect a loss of adaptation. Anecdotal reports of particularly susceptible people or families are frequently cited as evidence that certain individuals have an innate susceptibility (or resistance) to developing these conditions and, in recent decades, there have been a number of studies designed to characterize the physiology of individuals predisposed to these conditions, as well as to identify the specific genetic variants that contribute to this predisposition. This paper reviews the epidemiological evidence for a genetic component to the various forms of altitude-related illness, such as innate susceptibility, familial clustering, and patterns of population susceptibility, as well as the molecular evidence for specific genetic risk factors. While the evidence supports some role for genetic background in the etiology of altitude-related illness, limitations in individual studies and a general lack of corroborating research limit the conclusions that can be drawn about the extent of this contribution and the specific genes or pathways involved. The paper closes with suggestions for future work that could support and expand on previous studies, as well as provide new insights.

Ovine placenta at high altitudes: Comparison of animals with different times of adaptation to hypoxic environment

Fetal growth and newborn weight from ovine gestations at high altitudes (HA) are greater in ewes that live at HA for several generations than in those native to low altitudes (LA) exposed to HA only during pregnancy. Because the placenta is a key regulator of fetal growth, the present study compared placental characteristics in term pregnancies among ewes native to HA and LA. Conception occurred at HA and ewes continued to reside at HA throughout pregnancy or conception occurred at LA and ewes were transported to HA or remained at LA (controls). Ewes native to LA were moved to HA shortly after mating (group LH) and joined with pregnant ewes native to HA (group HH). After parturition, placental cotyledons were counted and measured for total area and histological estimation of surface occupied by vasculature. The total surface of the cotyledons and surface occupied by vasculature were greater at HA, whereas the number of cotyledons was smaller at HA. These changes were more pronounced in ewes of the HH compared with the LH group. The present study showed that exposure to HA induces, in pregnant ewes, placental morphological changes that may improve maternal-fetal exchange. Moreover, because of accentuation of placental changes in ewes with long-term residence at HA, this appears to be an efficient mechanism of adaptation to hypobaric hypoxia.

Human behaviour and development under high-altitude conditions

Although we are far from a universally accepted pattern of impaired function at altitude, there is evidence indicating motor, perceptual, memory and behavioural deficits in adults. Even relatively low altitudes (2500 m) may delay reaction time, and impair motor function. Extreme altitude exposure (>5000 m) may result in more pronounced impairment that can persist after returning to the lowlands. Research into the effects of altitude exposure earlier in development is lacking by comparison. Un-acclimatized children can suffer from acute mountain sickness, and, in native populations born at altitude, subtle cognitive and behavioural deficits suggest incomplete adaptation to hypoxia. The study of neurobehavioural functioning at altitude may provide important information about the effects of clinical hypoxia on the human brain and behavioural development.

Effect of hypobaric hypoxia on lamb intrauterine growth: comparison between high- and low-altitude native ewes

The present studies assessed the effect of hypobaric hypoxia on fetal lamb growth in high-altitude (HA) and low-altitude (LA) native ewes. Growth patterns of fetal biparietal diameter (BPD), abdominal diameter (AD) and thorax height (TH) were described by consecutive ultrasound measurements throughout the entire pregnancy. Three groups of animals were used: (1) pregnant LA ewes kept at LA (control; 'LL' group); (2) pregnant LA ewes moved to HA immediately after confirmation of pregnancy ('LH' group); and (3) pregnant HA ewes kept at HA throughout the entire pregnancy ('HH' group). The slope of the BPD curve was higher in LL fetuses followed by that in LH fetuses. During the last month of pregnancy, TH was higher in LH and HH fetuses, whereas AD was higher in LL than in LH fetuses. The length of gestation was longer in HH ewes (153.2 +/- 4.3 days) than in LH and LL ewes (146.0 +/- 5.5 and 145.0 +/- 3.0 days, respectively). Bodyweight at birth was higher for LL newborns (4.2 +/- 0.3 kg) than for LH and HH newborns (3.0 +/- 0.5 and 3.2 +/- 0.8 kg, respectively), whereas placental weight was higher in the HH group (396 +/- 80 g) than in the LH (303 +/- 64 g) and LL (280 +/- 40 g) groups. In conclusion, an HA environment modifies fetal growth and pregnancy outcome with the magnitude of effects depending on the time of residence at HA.

An evolutionary model for identifying genetic adaptation to high altitude

Coordinated maternal/fetal responses to pregnancy are required to ensure continuous O2 delivery to the developing organism. Mammals employ distinctive reproductive strategies that afford their young an improved chance of survival through the completion or the reproductive period. Thus, mortality prior to the end of the reproductive period is concentrated in the earliest phases of the lifecycle. At high altitude, fetal growth restriction reduces birth weight and likely compromises survival during the early postnatal period. Population variation in the frequency of the altitude-associated increase in intrauterine growth restriction (IUGR) demonstrates that multigenerational Tibetan and Andean high-altitude populations are protected compared with shorter duration, European or Han (Chinese) residents. This experiment of nature permits testing the hypothesis that genetic factors (a) influence susceptibility to altitude-associated IUGR, (b) act on maternal vascular adjustments to pregnancy determining uteroplacental blood flow, and (c) involve genes which regulate and/or are regulated by hypoxia-inducible factors (HIFs). Serial, studies during pregnancy as well as postpartum in Andean and European residents of high (3600 m) and low (300 m) altitude will permit evaluation of whether uteroplacental O2 delivery is lower in the European than Andean women and, if so, the physiological factors responsible. Comparisons of HIF-targeted vasoactive substances and SNPs in or near HIF-regulatory or targeted genes will permit determination of whether these regions are distinctive in the Andean population. Studies coupling genetic and genomic approaches with more traditional physiological measures may be productively employed for determining the genetic mechanisms influencing physiological adaptation to high altitude.

Epidemiology, Risk Factors, and Genetics of High-Altitude–Related Pulmonary Disease

High-altitude-related pulmonary disease is a spectrum of acute and chronic illnesses with a well-described epidemiology. The risk for these illnesses is related to well-known environmental risk factors and lesser-known but important genetic factors. Prevention of acute high-altitude illness is possible in most visitors from lower elevations. Chronic high-altitude illnesses have an important worldwide impact.

Prenatal hypoxia and cardiac programming

Epidemiologic studies have shown a clear association of adverse intrauterine environment and an increased risk of hypertension and coronary heart disease in the adult. Many studies have been focused on the effects of maternal undernutrition and fetal glucocorticoid exposure on fetal programming and later adult disease. Although it is relatively less clear, there is evidence that fetal exposure to hypoxia, alcohol, tobacco smoking, and cocaine may also cause in utero programming leading to an increased risk of adult disease. Chronic hypoxia during the course of pregnancy is thought to result in fetal intrauterine growth retardation. Among other effects, chronic hypoxia suppresses fetal cardiac function, alters cardiac gene expression, increases myocyte apoptosis, and results in a premature exit of the cell cycle of cardiomyocytes and myocyte hypertrophy. This review discusses recent evidence of an association of prenatal hypoxic exposure with an increased vulnerability of adult heart disease, and the possible mechanisms involved.

Tibetans at Extreme Altitude

Between 1960 and 2003, 13 Chinese expeditions successfully reached the summit of Chomolungma (Mt Everest or Sagarmatha). Forty-five of the 80 summiteers were Tibetan highlanders. During these and other high-altitude expeditions in Tibet, a series of medical and physiological investigations were carried out on the Tibetan mountaineers. The results suggest that these individuals are better adapted to high altitude and that, at altitude, they have a greater physical capacity than Han (ethnic Chinese) lowland newcomers. They have higher maximal oxygen uptake, greater ventilation, more brisk hypoxic ventilatory responses, larger lung volumes, greater diffusing capacities, and a better quality of sleep. Tibetans also have a lower incidence of acute mountain sickness and less body weight loss. These differences appear to represent genetic adaptations and are obviously significant for humans at extreme altitude. This paper reviews what is known about the physiologic responses of Tibetans at extreme altitudes.

Hemoglobin levels in Qinghai-Tibet: different effects of gender for Tibetans vs. Han

The Tibetan population, long a resident on the Qinghai-Tibetan Plateau, has lower hemoglobin concentrations than Han Chinese migrants, but it is incompletely known how gender affects the hemoglobin concentrations in the two populations at various altitudes. Measurements of hemoglobin concentration were obtained in 5,887 healthy male and female Tibetan and Han residents aged 5–60 yr, at altitudes of 2,664, 3,813, 4,525, and 5,200 m. Multiple regression equations showed the β-coefficients for altitude and for age were higher ( P < 0.05) in Han men than in Tibetan men and in Han women than in Tibetan women. Analysis indicated a significant three-way interaction between altitude, gender, and ethnicity (χ2 = 3.72, P = 0.05). With increasing altitude, men progressively had more hemoglobin than women in the Han, but not the Tibetan, population. Above 2,664 m, this gender-related difference in hemoglobin concentration increased from childhood to young adulthood more in Han than in Tibetans. We suggest that the Han-Tibetan ethnic difference in the effect of altitude on hemoglobin concentration depends to a large extent on gender.

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.

Avian embryos in hypoxic environments

Avian embryos at high altitude do not benefit of the maternal protection against hypoxia as in mammals. Nevertheless, avian embryos are known to hatch successfully at altitudes between 4,000 and 6,500 m. This review considers some of the processes that bring about the outstanding modifications in the pressure differences between the environment and mitochondria of avian embryos in hypoxic environments. Among species, some maintain normal levels of oxygen consumption ( VO2) have a high oxygen carrying capacity, lower the air cell-arterial pressure difference ( PAO2 - PaO2 ) with a constant pH. Other species decrease VO2, increase only slightly the oxygen carrying capacity, have a higher PAO2 - PaO2 difference than sea-level embryos and lower the PCO2 and pH. High altitude embryos, and those exposed to hypoxia have an accelerated decline of erythrocyte ATP levels during development and an earlier stimulation of 2,3-BPG synthesis. A higher Bohr effect may ensure high tissue PO2 in the presence of the high-affinity hemoglobin. Independently of the strategy used, they serve together to promote suitable rates of development and successful hatching of high altitude birds in hypoxic environments.

A Tibetan with Chronic Mountain Sickness Followed by High Altitude Pulmonary Edema on Reentry

Chronic mountain sickness (CMS) and high altitude pulmonary edema (HAPE) each occur rarely in Tibetans, and they have previously not been reported in the same person. Here we describe a 37-year-old native Tibetan man with CMS at 4300 m, who developed HAPE after his return home from a 12-day visit to sea level. Possible common pathogenetic factors included a poor ventilatory response to hypoxia, accentuated hypoxemia, pulmonary hypertension, and increased blood volume. In addition, strenuous exercise and high levels (to approximately 1000 ng/L) of plasma atrial natriuretic peptide may have contributed to HAPE.

Second generation Tibetan lowlanders acclimatize to high altitude more quickly than Caucasians

Tibetan highlanders develop at altitude peak aerobic power levels close to those of Caucasians at sea level. In order to establish whether this feature is genetic and, as a consequence, retained by Tibetan lowlanders, altitude-induced changes of peak aerobic performance were assessed in four groups of volunteers with different ethnic, altitude exposure and fitness characteristics, i.e. eight untrained second-generation Tibetans (Tib 2) born and living at 1300 m; seven altitude Sherpas living at approximately 2800-3500 m; and 10 untrained and five trained Caucasians. Measurements were carried out at sea level or at Kathmandu (1300 m, Nepal) (PRE), and after 2-4 (ALT1), 14-16 (ALT2), and 26-28 (ALT3) days at 5050 m. At ALT3, of untrained and trained Caucasians was -31% and -46%, respectively. By contrast, of Tib 2 and Sherpas was -8% and -15%, respectively. At ALT3, peak heart rate (HR(peak)) of untrained and trained Caucasians was 148 +/- 11 and 149 +/- 7 beats min(-1), respectively; blood oxygen saturation at peak exercise was 76 +/- 6% and 73 +/- 6%, and haemoglobin concentration ([Hb]) was 19.4 +/- 1.0 and 18.6 +/- 1.2 g dl(-1), respectively. Compared to Caucasians, Tib 2 and Sherpas exhibited at ALT3 higher HR(peak) (179 +/- 9 and 171 +/- 4 beats min(-1), P < 0.001), lower [Hb] (16.6 +/- 0.6 and 17.4 +/- 0.9 g dl(-1), respectively, P < 0.001), and slightly but non-significantly greater average values (82 +/- 6 and 80 +/- 7%). The above findings and the time course of adjustment of the investigated variables suggest that Tibetan lowlanders acclimatize to chronic hypoxia more quickly than Caucasians, independent of the degree of fitness of the latter.

Optimal hemoglobin concentration and high altitude: a theoretical approach for Andean men at rest

The beneficial role of erythrocytosis for O2 transport has been questioned by evidence from bloodletting and hemodilution research as well as by studies suggesting the existence of an "optimal" hematocrit (Hct) or hemoglobin concentration ([Hb]) value. To assess to what extent erythrocytosis is beneficial in Andean men at high altitude, we examined and discussed optimal [Hb] using a mathematical approach by modeling the mixed (mean) venous Po2 (Pv(O2)) and arterial O2 content, considering for both the relation between [Hb] and arterial Po2. Relations of [Hb] to other physiological variables such as cardiac output and convective arterial O2 transport were also discussed, revealing the importance of Pv(O2) in this model. Our theoretical analysis suggests that increasing [Hb] allows increase and maintenance of Pv(O2) with only moderate declines in arterial Po2 as a consequence of moderate increases in altitude, reaching its maximum at the optimal [Hb] of 14.7 g/dl. Our analysis also shows that [Hb] corresponding to high arterial O2 content and O2 transport values is apparently not quite advantageous for improvement of oxygenation. Furthermore, chronic mountain sickness is discussed as an insightful example of the effects of excessive erythrocytosis at high altitude.

Fetal growth restriction and maternal oxygen transport during high altitude pregnancy

High altitude reduces birth weights, averaging a 100-g fall per 1000 m elevation gain, as the result of restriction of third trimester fetal growth. Intrauterine growth restriction (IUGR) raises neonatal or infant mortality at low as well as at high altitude, but existing studies are unclear as to whether IUGR-specific mortality at high altitude is similar to, less than, or greater than at low altitude. Pregnancy increases maternal ventilation and raises arterial O(2) saturation at high altitude, which helps to protect against altitude-associated IUGR. Chronic hypoxia interferes with the maternal circulatory adjustments to pregnancy such that blood volume is lower and the rise in cardiac output diminished compared with sea level. The growth and remodeling of the uterine artery and other uteroplacental vessels is incomplete at high compared with low altitude, with the result that there is less redistribution of common iliac flow from the external iliac to the uterine arteries and lower uterine artery blood flow near term. Adaptations in multigenerational high altitude populations (e.g., Andeans and Tibetans) permit higher uterine artery blood flows and protect against altitude-associated IUGR.

Sleep structure and periodic breathing in Tibetans and Han at simulated altitude of 5000 m

Tibetans are the oldest population living permanently at high altitude. They possess several adaptations to low oxygen pressure that improve oxygen transport. We hypothesised that native Tibetans have mechanisms allowing them to maintain a better sleep structure and oxygenation during sleep at high altitude than newcomers from lower altitudes acclimatised to living at high altitude. We studied eight healthy young Tibetans, aged 26+/-7 years, and six healthy young Han aged 30.5+/-4 years. All subjects were living on the Tibetan plateau at an altitude of around 4000 m. Investigations were performed in Xining at an altitude of 2261 m, PB=581 mmHg. Two full polysomnographies (PSG) were performed in a hypobaric chamber, one at the ambient altitude, the second during acute exposure to the simulated altitude of 5000 m (PB=405 mmHg). Both PSG were done on the same night using split night design. At 2261 m no differences in sleep structure, breathing pattern during sleep or oxygenation were found, except a higher number of arousals and awakenings in Han (P<0.002). At 5000 m Tibetans had a longer sleep time (P=0.002), shorter stage 1 non-REM sleep (P<0.001) and longer stage 2 non-REM sleep than Han (P<0.001). Tibetans showed a trend to have more periodic breathing (PB) and higher mean arterial blood saturation than Han (NS). Our data suggest that Tibetans preserved better sleep structure and arterial blood oxygenation than Han during acute exposure to the simulated altitude of 5000 m.

Functional genomics and the comparative physiology of hypoxia

Comparative physiology has proven a powerful approach to our understanding of how animals function under hypoxic conditions and to identifying potential adaptations to environmental oxygen levels. This review considers the potential for using a similar comparative approach with functional genomics to understand the genetic basis of such physiological processes and evolutionary adaptations. Comparative functional genomics is currently limited by genome data, which are available for only a few model organisms. However, comparative studies between model organisms of the same species having slightly different genomes (e.g., in-bred strains of laboratory rodents, transgenic mice, and consomic rats) demonstrate the types of results, as well as the analytical challenges, that are possible if comparative functional genomics is applied to more species. Results from wild and domestic animal studies suggest new models to investigate physiological and evolutionary responses to oxygen levels with functional genomics.