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

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.

Diachronic investigations of mitochondrial and Y-chromosomal genetic markers in pre-Columbian Andean highlanders from South Peru

This study examines the reciprocal effects of cultural evolution, and population dynamics in pre-Columbian southern Peru by the analysis of DNA from pre-Columbian populations that lived in the fringe area between the Andean highlands and the Pacific coast. The main objective is to reveal whether the transition from the Middle Horizon (MH: 650-1000 AD) to the Late Intermediate Period (LIP: 1000-1400 AD) was accompanied or influenced by population dynamic processes. Tooth samples from 90 individuals from several archaeological sites, dating to the MH and LIP, in the research area were collected to analyse mitochodrial, and Y-chromosomal genetic markers. Coding region polymorphisms were successfully analysed and replicated for 72 individuals, as were control region sequences for 65 individuals and Y-chromosomal single nucleotide polymorphisms (SNPs) for 19 individuals, and these were compared to a large set of ancient and modern indigenous South American populations. The diachronic comparison of the upper valley samples from both time periods reveals no genetic discontinuities accompanying the cultural dynamic processes. A high genetic affinity for other ancient and modern highland populations can be observed, suggesting genetic continuity in the Andean highlands at the latest from the MH. A significant matrilineal differentiation to ancient Peruvian coastal populations can be observed suggesting a differential population history.

Living the high life: high-altitude adaptation

Genome-wide scans demonstrate that genetic variants associated with high-altitude adaptation in Tibetans and Andeans arose independently as a result of convergent adaptation.

Do anti-angiogenic or angiogenic factors contribute to the protection of birth weight at high altitude afforded by Andean ancestry?

OBJECTIVE

This prospective study was designed to determine whether variation in angiogenic (placental growth factor [PlGF]) and/or anti-angiogenic (soluble fms-like tyrosine kinase [sFlt-1]) factors contribute to the protective effect of highland ancestry (Andean) from altitude-associated reductions in fetal growth.

STUDY DESIGN

Plasma sFlt-1 and PlGF levels, uterine artery (UA) blood flow, and fetal biometry were determined in low-altitude (400 m; Andean n = 27, European n = 28) and high-altitude (3600 m; Andean n = 51, European n = 44) residents during pregnancy (20 and 36 weeks) and 4 months postpartum.

RESULTS

High-altitude decreased sFlt-1 levels in both groups, Andeans had lower sFlt-1, comparable PlGF, lower sFlt-1/PlGF ratios, and higher UA blood flow throughout pregnancy relative to Europeans. Altitude decreased birth weight in Europeans but not Andeans. In high-altitude Europeans sFlt-1/PlGF and sFlt-1 levels were negatively associated with UA diameter and birth weight, respectively.

CONCLUSIONS

Lower sFlt-1 and sFlt-1/PLGF ratio may contribute to or result from variations in maternal vascular adaptation to pregnancy between Andean and Europeans at high altitude. Subsequently, these effects could potentially influence ancestry-associated differences in birth weight.

Evolution. Genes for high altitudes

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

The origin of pre-eclampsia: from decidual "hyperoxia" to late hypoxia

Normal gestation implants on a relatively hypoxic deciduas so that trophoblast deeply invades endometrium and angiogenesis seeks for oxygen supply. If implantation occurs before those hypoxic conditions occur, trophoblast invasion is defective, due to the relatively high oxygen tension in the decidual environment, laying the foundations for subsequent pre-eclampsia.

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.

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.

Hypoglycemia and the origin of hypoxia-induced reduction in human fetal growth

BACKGROUND

The most well known reproductive consequence of residence at high altitude (HA >2700 m) is reduction in fetal growth. Reduced fetoplacental oxygenation is an underlying cause of pregnancy pathologies, including intrauterine growth restriction and preeclampsia, which are more common at HA. Therefore, altitude is a natural experimental model to study the etiology of pregnancy pathophysiologies. We have shown that the proximate cause of decreased fetal growth is not reduced oxygen availability, delivery, or consumption. We therefore asked whether glucose, the primary substrate for fetal growth, might be decreased and/or whether altered fetoplacental glucose metabolism might account for reduced fetal growth at HA.

METHODS

Doppler and ultrasound were used to measure maternal uterine and fetal umbilical blood flows in 69 and 58 residents of 400 vs 3600 m. Arterial and venous blood samples from mother and fetus were collected at elective cesarean delivery and analyzed for glucose, lactate and insulin. Maternal delivery and fetal uptakes for oxygen and glucose were calculated.

PRINCIPAL FINDINGS

The maternal arterial - venous glucose concentration difference was greater at HA. However, umbilical venous and arterial glucose concentrations were markedly decreased, resulting in lower glucose delivery at 3600 m. Fetal glucose consumption was reduced by >28%, but strongly correlated with glucose delivery, highlighting the relevance of glucose concentration to fetal uptake. At altitude, fetal lactate levels were increased, insulin concentrations decreased, and the expression of GLUT1 glucose transporter protein in the placental basal membrane was reduced.

CONCLUSION/SIGNIFICANCE

Our results support that preferential anaerobic consumption of glucose by the placenta at high altitude spares oxygen for fetal use, but limits glucose availability for fetal growth. Thus reduced fetal growth at high altitude is associated with fetal hypoglycemia, hypoinsulinemia and a trend towards lactacidemia. Our data support that placentally-mediated reduction in glucose transport is an initiating factor for reduced fetal growth under conditions of chronic hypoxemia.

A literature review of air medical work hazards and pregnancy

An increased percentage of miscarriages among coworkers at one air medical transport company in 2008 prompted a literature review of selected hazards relevant to the profession of rotor wing air medical flight crew. Because of a lack of known research specific to this population, relevant studies from 1990 to 2008 were chosen to investigate pregnancy risks associated with exposure to vibration, jet fuel, noise, altitude, and fatigue in other occupations. Findings were summarized and recommendations made for future research.

Beyond oxygen: complex regulation and activity of hypoxia inducible factors in pregnancy

In the first trimester the extravillous cytotrophoblast cells occlude the uterine spiral arterioles creating a low oxygen environment early in pregnancy, which is essential for pregnancy success. Paradoxically, shallow trophoblast invasion and defective vascular remodelling of the uterine spiral arteries in the first trimester may result in impaired placental perfusion and chronic placental ischemia and hypoxia later in gestation leading to adverse pregnancy outcomes. The hypoxia inducible factors (HIFs) are key mediators of the response to low oxygen. We aimed to elucidate mechanisms of regulation of HIFs and the role these may play in the control of placental differentiation, growth and function in both normal and pathological pregnancies. The Pubmed database was consulted for identification of the most relevant published articles. Search terms used were oxygen, placenta, trophoblast, pregnancy, HIF and hypoxia. The HIFs are able to function throughout all aspects of normal and abnormal placental differentiation, growth and function; during the first trimester (physiologically low oxygen), during mid-late gestation (where there is adequate supply of blood and oxygen to the placenta) and in pathological pregnancies complicated by placental hypoxia/ischemia. During normal pregnancy HIFs may respond to complex alterations in oxygen, hormones, cytokines and growth factors to regulate placental invasion, differentiation, transport and vascularization. In the ever-changing environment created during pregnancy, the HIFs appear to act as key mediators of placental development and function and thereby are likely to be important contributors to both normal and adverse pregnancy outcomes.

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.

Chapter 10. Methods for evaluating uteroplacental angiogenesis and their application using animal models

In this chapter, we present some of the compelling evidence confirming the importance of placental angiogenesis to fetal growth and development in normal and compromised pregnancies. We then describe the methodology that has been used to evaluate placental angiogenesis throughout pregnancy, including both the practical methods used to obtain reliable samples of the placental microcirculation as well as computerized methods used to analyze and reconstruct it. We then briefly describe the changes in placental angiogenesis and function in the models of normal and compromised pregnancy that we have used, and conclude with what we have learned from these studies and what we believe are the larger questions remaining.

Lessons in hypoxic adaptation from high-altitude populations

An increase in hemoglobin level is seen in virtually all lowlanders who move to or train at altitude; however, studies of high-altitude native populations illustrate that this response is not necessary for successful long-term residence. Indigenous populations living at the same altitude have differences not only in hemoglobin level but also in other traits like oxygen saturation. Support for a genetic causation for differences in features of oxygen transport, namely hemoglobin levels and oxygen saturation, is derived from kindred studies among the highlander populations. Indeed, evidence from Tibet suggests that inferred genes for high oxygen saturation are associated with higher offspring survival. It may be that signaling molecules like nitric oxide and transcription factors such as hypoxia-inducible factor could act as an upstream regulator for highlander traits. However, the preponderance of data suggests that it is unlikely that one process or even a common set of processes is responsible for successful biologic adaptation shown in all three resident high-altitude populations. Future studies will require the ability to identify combinations of genetic variants with outcomes including expression levels, appropriate phenotypes, and functional responses.

Effect of long-term high-altitude hypoxia on fetal pulmonary vascular contractility

Hypoxia in the fetus and/or newborn is associated with an increased risk of pulmonary hypertension. The present study tested the hypothesis that long-term high-altitude hypoxemia differentially regulates contractility of fetal pulmonary arteries (PA) and veins (PV) mediated by differences in endothelial NO synthase (eNOS). PA and PV were isolated from near-term fetuses of pregnant ewes maintained at sea level (300 m) or high altitude of 3,801 m for 110 days (arterial Po(2) of 60 Torr). Hypoxia had no effect on the medial wall thickness of pulmonary vessels and did not alter KCl-induced contractions. In PA, hypoxia significantly increased norepinephrine (NE)-induced contractions, which were not affected by eNOS inhibitor N(G)-nitro-l-arginine (l-NNA). In PV, hypoxia had no effect on NE-induced contractions in the absence of l-NNA. l-NNA significantly increased NE-induced contractions in both control and hypoxic PV. In the presence of l-NNA, NE-induced contractions of PV were significantly decreased in hypoxic lambs compared with normoxic animals. Acetylcholine caused relaxations of PV but not PA, and hypoxia significantly decreased both pD(2) and the maximal response of acetylcholine-induced relaxation in PV. Additionally, hypoxia significantly decreased the maximal response of sodium nitroprusside-induced relaxations of both PA and PV. eNOS was detected in the endothelium of both PA and PV, and eNOS protein levels were significantly higher in PV than in PA in normoxic lambs. Hypoxia had no significant effect on eNOS levels in either PA or PV. The results demonstrate heterogeneity of fetal pulmonary arteries and veins in response to long-term high-altitude hypoxia and suggest a likely common mechanism downstream of NO in fetal pulmonary vessel response to chronic hypoxia in utero.

High-altitude ancestry protects against hypoxia-associated reductions in fetal growth

OBJECTIVE

The chronic hypoxia of high-altitude (>/=2500 m) residence has been shown to decrease birth weight in all populations studied to date. However, multigenerational high-altitude populations appear protected relative to newcomer groups. This study aimed to determine whether such protection exists independently of other factors known to influence fetal growth and whether admixed populations (ie, people having both high- and low-altitude ancestry) show an intermediate level of protection.

DESIGN

3551 medical records from consecutive deliveries to Andean, European or Mestizo (ie, admixed) women at low, intermediate or high altitudes in Bolivia were evaluated for maternal characteristics influencing fetal growth as measured by birth weight and the frequency of small for gestational age births (SGA or </=10th percentile birth weight for gestational age and sex). Two-way analysis of variance and chi(2) tests were used to compare maternal and infant characteristics. The effects of ancestry or altitude on SGA and birth weight were assessed using logistic or linear regression models, respectively.

RESULTS

Altitude decreased birth weight and increased SGA in all ancestry groups. Andean infants weighed more and were less often SGA than Mestizo or European infants at high altitude (13%, 16% and 33% respectively, p<0.01). After accounting for the influences of maternal hypertensive complications of pregnancy, parity, body weight, and number of prenatal visits, European relative to Andean ancestry increased the frequency of SGA at high altitude nearly fivefold.

CONCLUSIONS

Andean relative to European ancestry protects against altitude-associated reductions in fetal growth. The intermediate protection seen in the admixed (Mestizo) group is consistent with the influence of genetic or other Andean-specific protective characteristics.

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.

Human placental hypoxia-inducible factor-1alpha expression correlates with clinical outcomes in chronic hypoxia in vivo

Placental hypoxia is causally implicated in fetal growth restriction and preeclampsia, with both occurring more frequently at high altitude (>2700 m; HA). The nuclear transcription factor hypoxia-inducible factor (HIF) may facilitate placental oxygen transport at HA by increasing erythropoiesis and placental angiogenesis. We therefore investigated HIF expression and its regulatory mechanisms in placentas from normal pregnancies at high (3100 m), moderate (1600 m), and sea level (75 m) altitudes. Moderate-altitude and sea level placentas did not differ, but HIF-1alpha and the von Hippel-Lindau tumor suppressor protein were overexpressed in HA placentas. The ability of von Hippel-Lindau tumor suppressor protein to form the E3 ubiquitin protein ligase complex, required for HIF-1alpha degradation, was unaltered in HA placentas. mRNA for factor-inhibiting HIF, a negative modulator of HIF-1alpha transactivation, was increased, but protein levels were diminished. Elevated HIF-1alpha likely contributed to the significant increase we report in HIF-1alpha downstream target proteins, transforming growth factor beta3 in the placenta, and vascular endothelial growth factor and erythropoietin in the maternal circulation. These circulating markers and lowered birth to placental weight ratios correlated with increased HIF-1alpha, thereby linking molecular and systemic physiological data. The HA response to chronic hypoxia resembles preeclampsia in several aspects, illustrating the utility of the HA model in understanding placental pathologies.

Intrauterine growth restriction and circulating neurotrophin levels at term

BACKGROUND

Intrauterine growth restricted (IUGR) fetuses are those with estimated weight <10th customized centile, displaying signs of chronic malnutrition and hypoxia leading to brain sparing effect. Neurotrophins, [Nerve Growth Factor (NGF), Brain Derived Neurotrophic Factor (BDNF), Neurotrophin-3 (NT-3), Neurotrophin-4 (NT-4)] are important for pre- and post-natal brain development.

AIMS

To investigate circulating NGF, BDNF, NT-3 and NT-4 levels in IUGR and appropriate for gestational age (AGA) fullterm fetuses and neonates (day-1 [N1] and day-4 [N4]) and in their mothers.

STUDY DESIGN

Prospective case control study.

SUBJECTS

60 mothers and their single 30 IUGR and 30 AGA fullterm fetuses and neonates.

OUTCOME MEASURES

Determination, by enzyme immunoassays, of NGF, BDNF, NT-3 and NT-4 plasma levels.

RESULTS

No statistically significant differences existed between IUGR and AGA maternal, fetal and neonatal levels of BDNF, NT-3 and NT-4. NGF was significantly higher in AGA than IUGR maternal (p=0.007), fetal (p=0.01), neonatal day 1 (p=0.043) and 4 (p=0.003) plasma, and positively correlated with the infants' centiles and birthweights. IUGR and AGA maternal neurotrophins were higher than the respective fetal and neonatal ones and no correlation with gender or delivery mode in both groups was observed.

CONCLUSIONS

In the perinatal period, circulating levels of BDNF, NT-3 and NT-4 do not differ in IUGR and AGA pregnancies, in contrast to NGF levels, which are higher in the AGA group. NGF is the only neurotrophin correlating with customized centiles and birthweights of the infants. Neurotrophin concentrations are higher in maternal plasma and do not depend on gender.

High-altitude chronic hypoxia during gestation and after birth modifies cardiovascular responses in newborn sheep

Perinatal exposure to chronic hypoxia induces sustained pulmonary hypertension and structural and functional changes in both pulmonary and systemic vascular beds. The aim of this study was to analyze consequences of high-altitude chronic hypoxia during gestation and early after birth in pulmonary and femoral vascular responses in newborn sheep. Lowland (LLNB; 580 m) and highland (HLNB; 3,600 m) newborn lambs were cathetherized under general anesthesia and submitted to acute sustained or stepwise hypoxic episodes. Contractile and dilator responses of isolated pulmonary and femoral small arteries were analyzed in a wire myograph. Under basal conditions, HLNB had a higher pulmonary arterial pressure (PAP; 20.2 ± 2.4 vs. 13.6 ± 0.5 mmHg, P < 0.05) and cardiac output (342 ± 23 vs. 279 ± 13 ml·min−1·kg−1, P < 0.05) compared with LLNB. In small pulmonary arteries, HLNB showed greater contractile capacity and higher sensitivity to nitric oxide. In small femoral arteries, HLNB had lower maximal contraction than LLNB with higher maximal response and sensitivity to noradrenaline and phenylephrine. In acute superimposed hypoxia, HLNB reached higher PAP and femoral vascular resistance than LLNB. Graded hypoxia showed that average PAP was always higher in HLNB compared with LLNB at any Po2. Newborn lambs from pregnancies at high altitude have stronger pulmonary vascular responses to acute hypoxia associated with higher arterial contractile status. In addition, systemic vascular response to acute hypoxia is increased in high-altitude newborns, associated with higher arterial adrenergic responses. These responses determined in intrauterine life and early after birth could be adaptive to chronic hypoxia in the Andean altiplano.

Two routes to functional adaptation: Tibetan and Andean high-altitude natives

Populations native to the Tibetan and Andean Plateaus are descended from colonizers who arrived perhaps 25,000 and 11,000 years ago, respectively. Both have been exposed to the opportunity for natural selection for traits that offset the unavoidable environmental stress of severe lifelong high-altitude hypoxia. This paper presents evidence that Tibetan and Andean high-altitude natives have adapted differently, as indicated by large quantitative differences in numerous physiological traits comprising the oxygen delivery process. These findings suggest the hypothesis that evolutionary processes have tinkered differently on the two founding populations and their descendents, with the result that the two followed different routes to the same functional outcome of successful oxygen delivery, long-term persistence and high function. Assessed on the basis of basal and maximal oxygen consumption, both populations avail themselves of essentially the full range of oxygen-using metabolism as populations at sea level, in contrast with the curtailed range available to visitors at high altitudes. Efforts to identify the genetic bases of these traits have included quantitative genetics, genetic admixture, and candidate gene approaches. These reveal generally more genetic variance in the Tibetan population and more potential for natural selection. There is evidence that natural selection is ongoing in the Tibetan population, where women estimated to have genotypes for high oxygen saturation of hemoglobin (and less physiological stress) have higher offspring survival. Identifying the genetic bases of these traits is crucial to discovering the steps along the Tibetan and Andean routes to functional adaptation.

Intrauterine growth restriction, brain-sparing effect, and neurotrophins

Intrauterine growth restriction (IUGR) is failure of the fetus to achieve his or her intrinsic growth potential, due to anatomical/functional diseases or disorders in the feto-placental-maternal unit. Growth restriction successfully balances reduced oxygen delivery and consumption; however, chronic hypoxia is responsible for fetal blood flow redistribution to cardinal organs (brain, myocardium, and adrenal glands), the so-called brain-sparing effect. The neurotrophin family comprises four structurally related molecules: the nerve growth factor (NGF), the brain-derived neurotrophic factor (BDNF), the neurotrophin-3 (NT-3), and the neurotrophin-4 (NT-4). By exerting neuroprotection, neurotrophins are critical for pre- and postnatal brain development. Based on the assumption that the brain-sparing effect might be activated in full-term IUGR infants, we hypothesized that circulating neurotrophin levels should not differ between IUGR and appropriate for gestational age (AGA) infants. Indeed, we found that in both groups, circulating NT-3, NT-4, and BDNF levels do not differ, and this finding could possibly be attributed to the activation of the brain-sparing effect. In contrast, NGF levels were higher in the AGA compared to the IUGR group. However, only NGF levels positively correlated with the customized centile and the birth weight of the infants, and both of them were lower in the IUGR group.

Finding the genes underlying adaptation to hypoxia using genomic scans for genetic adaptation and admixture mapping

The complete sequencing the human genome and recent analytical advances have provided the opportunity to perform genome-wide studies of human variation. There is substantial potential for such population-genomic approaches to assist efforts to uncover the historical and demographic histories of human populations. Additionally, these genome-wide datasets allow for investigations of variability among genomic regions. Although all genomic regions in a population have experienced the same demographic events, they have not been affected by these events in precisely the same way. Much of the variability among genomic regions is simply the result of genetic drift (i.e., gene frequency changes resulting from the effects of small breeding-population size), but some is also the result of genetic adaptation, which will only affect the gene under selection and nearby regions. We have used a new DNA typing assay that allows for the genotyping of thousands of SNPs on hundreds of samples to identify regions most likely to have been affected by genetic adaptation. Populations that have inhabited different niches (e.g., high-altitude regions) can be used to identify genes underlying the physiological differences. We have used two methods (admixture mapping and genome scans for genetic adaptation) founded on the population-genomic paradigms to search for genes underlying population differences in response to chronic hypoxia. There is great promise that together these methods will facilitate the discovery of genes influencing hypoxic response.

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.

Influence of Endurance Exercise and Diet on Human Placental Development and Fetal Growth

The delivery of oxygen and substrate to the maternal-fetal interphase is the major maternal environmental stimulus which either up- or down-regulates feto-placental growth. During pregnancy, sustained exercise sessions cause an intermittent reduction in oxygen and substrate delivery to the interphase that may exceed 50% during the exercise but, it is probable that regular bouts of sustained exercise or exercise training may improve oxygen and substrate delivery at rest. The type of maternal carbohydrate intake (low- versus high-glycemic sources) and food intake frequency also influence substrate availability through their effects on maternal blood glucose levels and insulin sensitivity. As a result, different exercise regimens and/or different types of carbohydrate intake modify feto-placental growth. The magnitude and direction of the effect is determined by their average 24-h effect on oxygen and substrate availability at different time-points in pregnancy. In general, exercise in early and mid pregnancy stimulates placental growth while the relative amount of exercise in late pregnancy determines its effect on late fetal growth. Low-glycemic food sources in the diet decrease growth rate and size at birth while high-glycemic food sources increase it. Thus, it may be possible to improve pregnancy outcomes in both healthy, low-risk women and a variety of high-risk populaces by simply modifying maternal physical activity and dietary carbohydrate intake during pregnancy.

Birth weight at high altitudes in Peru

OBJECTIVE

To determine whether birth weights are lower at high altitudes, and whether gestational age at birth and a population's length of residence mitigate the effect of high altitude.

METHODS

The birth weights of 84,173 neonates recorded in the Peruvian Perinatal Information System Database were analyzed between 1995 and 2002 for the cities of Lima (150 m), Huancayo (3280 m), Cuzco (3400 m), and Juliaca (3800 m).

RESULTS

Birth weight was lower at high altitude, but there was no linear relation between altitude of residence and birth weight. Mean birth weight was higher in Juliaca than in Huancayo. There were no significant differences between the 4 cities regarding birth weights of infants born between 28 and 35 weeks of gestation. However, for infants born between 36 and 42 weeks, birth weight was lower at higher altitudes. This may be due to inadequate maternal oxygenation later in pregnancy at high altitude. In the multivariate analysis, after controlling for maternal age, marital status, parity, body mass index, pre-eclampsia or hemorrhage during pregnancy, and education, as well as sex of the newborn and gestational age at birth, birth weight was lower in all cities located at a higher altitude than Lima. Yet, longer residence at high altitudes may play a protective role. Juliaca (3800 m), where the population has resided the longest, had the lowest reduction in birth weight compared with Lima (150 m); Cuzco had intermediate values; and Huancayo (3280 m), where the population has resided the shortest, had the highest reduction in birth weight.

CONCLUSIONS

Birth weight reduction, which is independent of socioeconomic factors, occurs only in births at term and may be less severe in populations that have resided longer at high altitudes.

Severe respiratory distress in term infants born electively at high altitude

Background

We studied the contribution of elective delivery to severe respiratory distress syndrome (RDS) in term babies born at high altitude.

Methods

We prospectively studied the charts of term babies born in Taif Maternity Hospital (1640 m above sea level) between 1/1/2004 and 31/10/2004 who developed RDS and required mechanical ventilation.

Results

8634 deliveries occurred from 37–<41 weeks; 13 (0.15%) had RDS requiring mechanical ventilation. Seven infants delivered at 37–<38 weeks, (OR for RDS = 26 95%CI -4.6 to 5.8), five delivered at 38–<39 weeks, (OR for RDS = 10 95%CI -4.9 to 5.4) and one delivered at >39 weeks. Six of 13 infants were electively delivered without documented lung maturity.

Conclusion

Infants born at 37 and 38 weeks' gestation remain at significantly increased risk for severe RDS. Elective delivery is responsible for 50% of the potentially avoidable cases. Our data suggest that the altitude does not seem to influence the incidence of severe RDS in term infants born electively.

Evidence for altered placental blood flow and vascularity in compromised pregnancies

The placenta is the organ that transports nutrients, respiratory gases, and wastes between the maternal and fetal systems. Consequently, placental blood flow and vascular development are essential components of normal placental function and are critical to fetal growth and development. Normal fetal growth and development are important to ensure optimum health of offspring throughout their subsequent life course. In numerous sheep models of compromised pregnancy, in which fetal or placental growth, or both, are impaired, utero-placental blood flows are reduced. In the models that have been evaluated, placental vascular development also is altered. Recent studies found that treatments designed to increase placental blood flow can 'rescue' fetal growth that was reduced due to low maternal dietary intake. Placental blood flow and vascular development are thus potential therapeutic targets in compromised pregnancies.

Intrauterine programming of physiological systems: causes and consequences

The intrauterine conditions in which the mammalian fetus develops have an important role in regulating the function of its physiological systems later in life. Changes in the intrauterine availability of nutrients, oxygen, and hormones program tissue development and lead to abnormalities in adult cardiovascular and metabolic function in several species. The timing, duration, severity, and type of insult during development determines the specific physiological outcome. Intrauterine programming of physiological systems occurs at the gene, cell, tissue, organ, and system levels and causes permanent structural and functional changes, which can lead to overt disease, particularly with increasing age.

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.

Placental angiogenesis in sheep models of compromised pregnancy

Because the placenta is the organ that transports nutrients, respiratory gases and wastes between the maternal and fetal systems, development of its vascular beds is essential to normal placental function, and thus in supporting normal fetal growth. Compromised fetal growth and development have adverse health consequences during the neonatal period and throughout adult life. To establish the role of placental angiogenesis in compromised pregnancies, we first evaluated the pattern of placental angiogenesis and expression of angiogenic factors throughout normal pregnancy. In addition, we and others have established a variety of sheep models to evaluate the effects on fetal growth of various factors including maternal nutrient excess or deprivation and specific nutrients, maternal age, maternal and fetal genotype, increased numbers of fetuses, environmental thermal stress, and high altitude (hypobaric) conditions. Although placental angiogenesis is altered in each of these models in which fetal growth is adversely affected, the specific effect on placental angiogenesis depends on the type of 'stress' to which the pregnancy is subjected, and also differs between the fetal and maternal systems and between genotypes. We believe that the models of compromised pregnancy and the methods described in this review will enable us to develop a much better understanding of the mechanisms responsible for alterations in placental vascular development.

Association of hsp70-2 and hsp-hom gene polymorphisms with risk of acute high-altitude illness in a Chinese population

High-altitude illness (HAI) is a potentially fatal condition involving genetic and environmental components. Accumulated experimental evidence suggests that heat shock proteins (Hsps), especially HSP70, can protect cells and organs against different types of damage. We investigated whether genetic variation in constitutive and inducible hsp70 genes could be associated with risk of HAI. The association between polymorphisms of the HSP70 family genes and risk of HAI was determined in 56 patients with HAI and in 100 matched controls by genotyping for the polymorphisms +190 G/C, +1267 A/G, 2437 G/C in the hsp70-1, hsp70-2, and hsp70-hom genes by using polymerase chain reaction-restriction fragment length polymorphism. The data showed that there was no statistically significant difference in the genotype and allele distributions of hsp70-1, in hsp70-2 allele and hsp70-2 A/A and A/B genotypes, and in allele distribution of hsp70-hom among patients with HAI and controls (chi2 test, P > 0.05). However, there was a significantly higher frequency of hsp70-2 B/B and hsp70-hom A/A and B/B genotypes and a significantly lower frequency of the hsp70-hom A/B genotype in the HAI patients compared with the controls (P < 0.05 for all). The risk associated with the hsp70-2 B/B and hsp70-hom A/A, A/B, and B/B genotypes were 4.017 (95% CI = 1.496-10.781; P = 0.004), 2.434 (95% CI = 1.184-5.003; P = 0.012), 0.299 (95% CI = 0.148-0.602, P = 0.001), and 5.880 (95% CI =1.145-30.196, P = 0.026), respectively. Our results suggest that individuals with hsp70-2 B/B and hsp70-hom A/B and B/B genotypes may be more susceptible to HAI, whereas those with hsp70-hom A/B genotype may be tolerant to HAI. Further studies in individuals of different age and sex are warranted to elucidate the underlying mechanisms of this association and the possible functions of different genotypes of hsp70-2 and hsp70-hom under hypoxic stress.

Cellular life histories and bow tie biology

Considering the life-long influences of fetal growth biology, it is of interest to further elucidate the nature of the fetal growth process itself. Previous analyses of longitudinal fetal ultrasound data led to the hypothesis that hypoxia signals were important aspects of normal growth biology and directed attention to the place of oxygen as a basic nutrient. From the perspective of the cell, both hypoxia and lack of energy substrate trigger a common adaptive pathway through their effects on ATP availability. Comparative data from animal studies and cell culture provide evidence for an integrated energy/oxygen signaling system that acts redundantly and hierarchically with cellular differentiation programs, providing opportunities for developmental flexibility in response to variable ecologic or environmental challenge. The multinodal and interactive design of the fetal growth process suggests that it follows what has been described as the "bow tie" model of metabolism, with implications for robust and inventive approaches to cell, organ, and whole organism construction.