Pregnancy at high altitude

Introduction and overview: Reproductive success, the driver of evolution, requires the coordinated responses of maternal, placental, and fetal physiology.

Pregnancy requires the coordinated functioning of maternal, placental, and fetal systems yet often one is studied in the absence or at least near exclusion of the others. Of the three, perhaps that least often studied is that of the mother who, clearly, is the one who is pregnant and in whom conception and hence some of the earliest physiological responses that are required. We speculate that the reasons for this oversight go back to ideas of homunculus as well as to the important, early pregnancy physiology studies conducted, of note, at high altitude by Joseph Barcroft, John S Haldane, and Donald Barron. These investigations, stimulated by the recognition that the obligatory fall in atmospheric pO2 made high altitude a natural laboratory for understanding the physiological responses required for defending oxygen and the other nutrient supply required for normal pregnancy outcomes, focused nearly exclusively on fetal and to a lesser extent placental systems. Such limitations are being filled, as the presentations to follow will show, but others remain to be filled by hopefully, you the conference attendees.

Professor Lorna G Moore, University of Colorado, Anschutz Medical Campus, USA

Professor Lorna G Moore, University of Colorado, Anschutz Medical Campus, USA

Doctor Moore’s research has pioneered the use of the chronic hypoxia of residence at high altitude (HA, >2500 m) for studying the factors underlying the pregnancy complications of foetal growth restriction (FGR) and preeclampsia, both of which are more common at HA. Given that humans have lived at HA for varying durations, she has compared groups with shorter vs longer HA residence and shown that the normal uterine artery (UtA) blood flow rise is blunted in shorter compared to multigenerational groups and associated with protection from FGR. Her group was the first to show that natural selection has operated on selected gene regions in multigenerational populations, and that among Andeans the region containing the alpha-1 catalytic subunit of adenosine monophosphate kinase were associated with preservation of normal uterine vascular adaptations to pregnancy and foetal growth. Her and her colleagues’ continuing work focuses on the mechanisms underlying how hypoxia affects such pregnancy complications.

Globally, the likelihood of adverse perinatal outcomes, including low birth-weight, small-for-gestational age, spontaneous preterm birth and pregnancy loss, increases in high-altitude pregnancies. The aim of these studies was to quantify the risk across global settings and relationship with altitude via meta-regression.

59 studies were included in the analysis (n =1,604,770 pregnancies). Data were abstracted according to PRISMA guidelines, and were pooled using random-effects models. The risk of low birth weight (odds ratio [OR] 1.47, 95% confidence interval [CI] 1.33-1.62, P < 0.001), small-for-gestational age (OR 1.88, CI 1.08-3.28, P = 0.026), and spontaneous preterm birth (OR 1.23, CI 1.04-1.47, P = 0.016) were all increased in high- versus low-altitude pregnancies. Birth weight decreases by 54.7 g (±13.0 g, P <0.0001) per 1000 m increase in altitude. Average gestational age at delivery was not significantly different.

Based on the risk of late pregnancy loss was increased in high altitude pregnancies. The likelihood of stillbirth was increased by 63% in pregnancies at high altitude compared with low altitude (odds ratio, 1.63 [1.12-2.35]; P <0.01). Using an observational cohort from Bolivia (n=5386), a strong effect of maternal ancestry on the risk of early miscarriage was shown; Andean women were 24% less likely to have ever experienced a miscarriage compared to European women (OR 0.76; CI:0.62-0.90, P <0.001).

With a growing population residing at high altitude worldwide, it is essential to clearly define the associated risk of adverse pregnancy outcomes for women in different settings.

Professor Catherine Aiken, University of Cambridge, UK

Professor Catherine Aiken, University of Cambridge, UK

Professor Catherine Aiken is an active clinician scientist at the University of Cambridge, and leads a research group that focuses on understanding the long-term outcomes of complex pregnancies. She has published over 80 peer-reviewed papers and is the Chief Investigator of several studies. In her clinical work, she specialises in the care of medically complex pregnancies, and manages labour and deliveries.

Advancements in genetic research have greatly enhanced our understanding of how humans adapt to high-altitude environments. This progress is primarily due to the development of high-throughput genotyping and powerful genome-wide tests that identify positive selection that has occurred throughout hundreds of generations in humans. Researchers have pinpointed specific genetic markers, including those within the hypoxia-inducible factor (HIF) pathway, that are crucial for adaptation to low oxygen levels in both humans and other highland species. For instance, EPAS1/HIF2A, a key player in the HIF pathway, is linked to haemoglobin concentration in individuals of Tibetan ancestry living at high altitude. Through our recent investigations, we established a connection between a particular adaptive variant of EPAS1 and similar, relatively lower haematocrit levels in a separate high-altitude group in the Andes. This variant is present at a moderate frequency in Andeans and nearly absent in other human populations and vertebrate species. Using advanced gene-editing techniques, we have observed changes in gene expression related to low oxygen levels in human cells with this putatively adaptive variant and provide further functional support from metabolomic analyses. While a relatively low haemoglobin concentration/haematocrit is observed in many Tibetan and some Andean highlanders, the underlying genetic mechanisms likely differ due to distinct non-coding and coding variants identified, respectively, in each group. Our recent studies provide further evidence for genomic signatures of high-altitude adaptation relevant to lowland individuals with implications for understanding hypoxia responses in health and disease.

Tatum S Simonson, University of California San Diego School of Medicine, USA

Tatum S Simonson, University of California San Diego School of Medicine, USA

Tatum Simonson, PhD, Associate Professor, holds the John B West Endowed Chair in Respiratory Physiology within the Division of Pulmonary, Critical Care, Sleep Medicine, and Physiology at the University of California San Diego (UCSD) School of Medicine. Her work focuses on applying integrative physiological genomics approaches to explore systems-level responses to hypoxia. Her research provides evidence for genetic adaptations to high altitude and further investigations into the associations between genetic factors, molecular functions, and physiological traits. Beyond conducting studies in the highlands of Tibet and Peru, Doctor Simonson's team examines variation in human responses to low oxygen levels, aiming to discern the roles of genetic and epigenetic factors in hypoxia-related disease states such as altitude illness, sleep apnoea, and cardiopulmonary diseases. These interdisciplinary endeavours, along with related research initiatives, are coordinated through the Centre for Physiological Genomics of Low Oxygen (CPGLO) she founded and co-directs at UC San Diego.

Human Fetal Growth is a complex process influenced by both the in-utero environment and genetic factors. Hypoxia, specifically HIF oxygen sensing pathways, have been implicated and genetic adaptation is a recognised feature influencing growth. 

High altitude environments provide an ideal natural experiment to study the impact of a low oxygen environment, on pregnancy outcomes. Leh, Ladakh, in the Jammu and Kashmir region of India, lies between the Karakoram and Himalayan Mountain ranges. Genetic selection of the Ladakhi population is relatively poorly studied. The Sonam Norboo Memorial (SNM) Hospital in Leh [situated at 3,540m above sea level], provides maternity care for Ladakh, has an institutional birth rate of >90% making it a unique site to study a pregnant population at high altitude. 

Over the last 10 years a collaborative research group of clinical academics from UCL, AIIMS Hospital, New Delhi and SNM Hospital have been studying pregnancies in Leh. Findings, that will be presented here, support both physical and genetic adaptation in babies born to pregnant women who generationally have long residence at high altitude.  Interestingly the Ladakh population was found to be less homogenous genetically than initially thought. There were clear trends seen in babies born of heavier birth weights, towards enrichment of genes implicated, particularly in height, body mass and skeletal development. 

Dr Sara Hillman, University College London, UK

Dr Sara Hillman, University College London, UK

Sara Hillman is a Clinical Academic at University College London who works as a Fetal Medicine Consultant, managing complex prenatal fetal cases with a suspected genetic diagnosis.

Her research is focused on understanding the aetiology of poor fetal growth in-utero and its associated adverse conditions. She has received extensive funding for pregnancy research from the Medical Research Council and Chan Zuckerberg amongst others. 

With Wellcome SEED funding, she initiated a joint collaboration with colleagues in India to investigate the effect of a hypoxic environment on fetal growth. This involved study of pregnant women delivering at Sonam Norboo Hospital Leh Ladakh, coordinated by Dr Padma Dolma, Obstetrician.

This work has been published and identified new areas to investigate and explore in relation to therapeutic targets to offset hypoxia complications.

Background
Exposure to high altitude during pregnancy alters placental metabolism and increases the incidence of placental pathologies associated with fetal growth restriction (FGR), including preeclampsia. High-altitude ancestry protects against altitude-associated FGR, indicating hypoxia tolerance that is genetic in nature. Yet not all babies are protected and preeclampisa occurs in some Andean highlanders. Here, links are presented between genomic selection signals and placental metabolic phenotypes in Andean highlanders as well as phenotypic differences in preeclampisa vs. control.  

Methods
The study cohort included 79 Andeans (18-45y; 39 preeclamptic, 40 normotensive) living in La Paz, Bolivia (3600 - 4100m) delivered by unlabored C-section. Genotyping was performed using a 1.8 million SNP Multiethnic Genotyping Array. Mitochondrial oxidative metabolism was examined in cryopreserved villous placental samples using the Oxygraph-2K. Abundance of fatty acid derivatives acyl carnitines was examined using ultra-high-pressure liquid chromatography-mass spectrometry. 

Results
A maternal haplotype within PTPRD associated with lower placental mitochondrial respiratory capacity. Mitochondrial oxidative capacity associated with fetal oxygen delivery in normotensive but not preeclamptic placenta and was also suppressed in term preeclamptic pregnancy. A fetal haplotype within 200kb of CPT2 associated with increased abundance of placental short chain acyl carnitines. In preeclamptic pregnancy, the abundance of placental  medium and long chain carnitines was increased in preeclampsia vs. control. 

Conclusions
These findings reveal novel associations between putatively adaptive gene regions and phenotypes linked to placental metabolic function in highland Andeans. They also demonstrate maladaptive metabolic mechanisms in the context of preeclampsia, including dysregulation of placental oxygen consumption and disruption of fatty acid oxidation. 

Dr Katie O’Brien, University of Colorado Anschutz, USA

Dr Katie O’Brien, University of Colorado Anschutz, USA

Doctor O’Brien completed her doctorate in 2017 on metabolic profile changes and effects of dietary nitrate supplementation in hypoxia at King’s College London under the direction of Professor Stephen Harridge. She then worked as a postdoctoral researcher investigating drug induced mitochondrial toxicity in the context of hypoxia with Professor Andrew Murray, University of Cambridge and in collaboration with GlaxoSmithKline and AstraZeneca. Following this, Doctor Katie O’Brien received a Marie Curie Global Research Fellowship to investigate high-altitude metabolic phenotype driven by unique Andean genetics between the University of Cambridge with Professor Murray and the University of California, San Diego with Doctor Tatum Simonson.  Currently, she is working as a Postdoctoral Fellow with Doctor Colleen Julian to investigate mechanisms driving human adaptation to hypoxia and the role of disrupted oxygen homeostasis in regulating fetal growth and maternal vascular adaptation to pregnancy.

The maternal cardiovascular system undergoes widespread adaptations during pregnancy that collectively increase uteroplacental blood flow, a process critical for maintaining sufficient fetal oxygen and nutrient supply. Insufficient hemodynamic adaptation to pregnancy is linked to fetal growth restriction and preeclampsia, each a major contributor to maternal and offspring morbidity and mortality across the lifespan. Chronic exposure to the environmental hypoxia of high altitudes ( 2500 m) impairs maternal vascular adaptation to pregnancy, restricting expansion of uterine artery diameter, limiting redistribution of cardiac output to favor the uteroplacental circulation, and blunting the pregnancy-associated fall in blood pressure. Dampened relaxation of uteroplacental resistance vessels has also been identified as an important determinant of restricted uteroplacental blood flow at high altitudes. The increased incidence of fetal growth restriction observed at high altitudes has been attributed to diminished uterine blood flow; however, the lower uterine blood flow alone cannot be solely responsible for reduced fetal growth because uterine oxygen supply exceeds the sum of placental and fetal oxygen consumption. This suggests the involvement of metabolic factors. Integrated genomic, transcriptomic, and physiological studies point to a key role for the adenosine monophosphate kinase signaling pathway, which is hypothesized to serve as a nexus between uteroplacental perfusion and metabolism under conditions of chronic hypoxia, regulating fetal growth through dual roles as a potent vasodilator and regulator of cellular energy homeostasis. Ongoing studies aiming to fill critical knowledge gaps by establishing how uteroplacental oxygenation, perfusion and metabolism interact to regulate fetal growth will also be discussed.

Colleen G Julian, University of Colorado, USA

Colleen G Julian, University of Colorado, USA

Colleen Julian is an associate professor in the Department of Biomedical Informatics at the University of Colorado Denver. She is a faculty member in the Integrative Physiology and Human Medical Genetics and Genomics graduate programs and curriculum director for the University of Colorado Program to Increase Diversity among Individuals Engaged in Health-Related Research (PRIDE) academy. The Julian lab focuses on human physiologic responses and genetic adaptations to hypoxia and how these processes modify maternal vascular adaptation to pregnancy, fetal growth, and the long-term health of affected offspring. Using the naturally occurring variability between populations in the frequency of hypoxia-associated vascular disorders of pregnancy at high altitudes, this work integrates in vivo and in vitro studies of vascular function with molecular and clinical data to identify biological pathways regulating fetal growth and hypertensive disorders of pregnancy.

The chronic hypoxia of residence at high altitude (HA, >2500m) reduces uterine artery (UtA) blood flow, contributing to an increased frequency of fetal growth restriction (FGR). FGR pregnancies at low altitudes (LA, <1700m) have reduced UtA blood flow, partially due to impaired myometrial artery (MyoA) vasodilation. Studies in Andeans, who are protected from HA-associated reductions in fetal growth, detected an association between genetic variants predicted to activate the AMP-activated protein kinase (AMPK) pathway and increased birth weight. Our wire myography studies have shown that AMPK-dependent vasodilator responses are augmented in MyoA from women at HA with uncomplicated pregnancies compared to LA and reduced in FGR pregnancies regardless of altitude. We also observed an increase in the phosphorylation levels of AMPK, as an indicator of augmented activation, and the expression of downstream targets in placentas from uncomplicated pregnancies at HA and LA compared to sea-level residents. These findings suggest a role for AMPK in vasodilating MyoA at HA and in the impaired vasodilation of MyoA in FGR pregnancies. Furthermore, placental AMPK seems to play a role in adapting placental function in uncomplicated HA pregnancy. Future studies will aim to elucidate the mechanisms underlying the altitude-dependent activation of AMPK in uncomplicated pregnancies and its reduction in FGR.

Doctor Ramón A Lorca, University of Colorado Anschutz Medical Campus, USA

Doctor Ramón A Lorca, University of Colorado Anschutz Medical Campus, USA

Doctor Lorca received his PhD in Physiology from the Catholic University of Chile. He completed postdoctoral trainings at the University of Iowa and Washington University in St Louis in Doctor Sarah England’s lab, where he studied the role of potassium channels in vascular and uterine function during pregnancy. He is currently an Assistant Professor in the Department of Obstetrics and Gynecology at the University of Colorado Anschutz Medical Campus. His research interests focus on the regulation of vascular smooth muscle cell excitability during pregnancy and how impaired maternal vascular function impacts fetal growth.

Ascent to high altitude is accompanied by physiological responses that, to an extent, mitigate the challenge of hypobaric hypoxia, maintaining arterial oxygen content and convective oxygen delivery. Nevertheless, arterial oxygen tension remains low and tissue hypoxia persists, posing a challenge for metabolic and redox homeostasis, as well as function. At high altitude, a suppression of placental respiratory capacity and switch to glycolytic ATP production occurs downstream of hypoxia-inducible factor (HIF) stabilization. This may play an adaptive role to protect oxygen tension in the fetal circulation, albeit at the cost of greater placental glucose utilization. As such, this can result in compromised cellular energetics and is associated with lower birth weight. Studies in human populations of highland ancestry have revealed physiological traits, underpinned by genetic variants, that have undergone selection, and which allow people to live, work, and reproduce at high altitude. Notably, placental metabolic adaptations form a vital component of an integrated response that supports healthy pregnancies in highlander populations at altitude. Moreover, in the case of preeclampsia, both at sea level and in high-altitude populations, mitochondrial abnormalities are associated with the redox stress of ischemia/reperfusion, and may play a role in the pathology. Indeed, maternal and fetal genetic signals associated with outcomes implicate a vital role for mitochondria and substrate metabolism in the relative protection against preeclampsia in highlander populations. Placental metabolic responses to tissue hypoxia therefore have important implications for pregnancies both at high altitude and more generally, being associated with outcomes in healthy pregnancies and in those with common complications.

Professor Andrew Murray, University of Cambridge, UK

Professor Andrew Murray, University of Cambridge, UK

Andrew Murray grew up in South Wales, and studied Biochemistry at the University of Oxford, before a British Heart Foundation-supported DPhil studying diabetic cardiomyopathy, and later work on ketone body metabolism as a postdoctoral fellow. In 2017, Andrew moved to the University of Cambridge as Research Councils UK Academic Fellow. He is currently Professor of Metabolic Physiology, and his group’s research interests lie in the mitochondrial responses to altered oxygen and nutrient supply, including the implications for function and health at the level of the tissue, organ and organism. Andrew has a long-standing interest in high-altitude physiology, and is co-Principal Investigator of the Xtreme Everest Oxygen Research Consortium. Andrew and his group have studied cardiac and skeletal muscle energetics in climbers on Everest, and mitochondrial adaptations in highlander populations. A major current interest lies in placental metabolic function in healthy and pathological pregnancies at altitude, including in adapted populations.

Hypoxia is one of the most common and severe stresses to an organism's homeostatic mechanisms, and hypoxia during gestation has profound adverse effects on maternal health and developmental plasticity. Gestational hypoxia is associated with high incidence of clinical complications including preeclampsia and intrauterine growth restriction (IUGR). Both human and animal studies have revealed a causative role of increased uterine vascular resistance and lowered uterine blood flow in preeclampsia and IUGR. Dr Zhang's research strategy takes advantage of a unique animal model of pregnant sheep acclimatised to high altitude (3801 m/12,470 ft) hypoxia. This model has been well-established and extensively studied by one of the kind team of investigators at Loma Linda University over the past 40 years, and it is a unique animal model to investigate cardiovascular and metabolic complications of pregnancy and developmental plasticity of cardiovascular disease associated with chronic hypoxia during gestation. We demonstrated that high altitude hypoxia suppressed pregnancy-induced uterine arterial adaptation and increased uterine vascular resistance and systemic blood pressure in pregnant sheep. In addition, we showed that gestational hypoxia at high altitude elevated pulmonary vascular resistance and increased pulmonary artery pressure and pressure response to acute hypoxia in newborn lambs. Furthermore, Dr Zhang and their team revealed that fetal hypoxia impaired cerebral blood flow autoregulation in the neonate. Their study is broadly based, multidisciplinary, integrated research program and provides new insights into the understanding of pathophysiologic mechanisms underlying pregnancy complications including preeclampsia and increased risk of pulmonary hypertension and intraventricular haemorrhage in newborns associated with gestational hypoxia.

Professor Lubo Zhang, Loma Linda University School of Medicine, USA

Professor Lubo Zhang, Loma Linda University School of Medicine, USA

Doctor Zhang has a broad background in maternal physiology and developmental biology with specific expertise in key research areas of molecular and epigenetic modulations of developmental plasticity in programming of health and disease. Over the past 30 years, he has published over 320 peer-reviewed publications in Journals with high impact factors on the subject, which have led to significant advances in the understanding of molecular and epigenetic mechanisms of maternal adaptation and fetal and neonatal development in response to stress during gestation.  Doctor Zhang has been continuously funded by over a dozen NIH R01 grants and 4 terms of five-year NICHD Program Project grants (PPG) consecutively for 30 years, and has demonstrated track records of leadership, creativity, consistent productivity, and significant scientific impact in the cardiovascular and neurobehavioral fields. The research activities in Doctor Zhang’s laboratory have been directed to investigate the cellular and molecular mechanisms underlying the cardiovascular disease, and the role of sex and sex-defining steroids in determining vascular and heart function and “ischemic-sensitive” phenotype in the heart and brain. More recently, his novel approach of multi-omics integration of transcriptome, proteome and metabolome, as well as single cell RNAseq provides an unbiased means to understand and accurately predict biological behavior in a given condition, and reveals a fundamental mechanism of mitochondria in stress-mediated inflammation and cardiovascular disease.