For over 10,000 years Tibetan populations have lived at altitudes above 3500m resulting in a modern population of over 3 million indigenous people adapted to thrive in an environment with just 60% the sea level oxygen concentration. This environment parallels the hypoxia that characterizes or complicates many cardiovascular and hematopoietic disorders. Tibetans display many unique phenotypes, including unelevated hemoglobin levels. Despite the low hemoglobin levels, Tibetans do not have higher oxygen saturation than lowlanders at the same altitude and are therefore severely hypoxemic. Intriguingly, they also experience better reproductive outcomes, and protection against hypoxic pulmonary hypertension and chronic mountain sickness compared to lowlanders at high altitude. Population genetic studies have repeatedly shown strong signals of positive selection and association with unelevated hemoglobin levels in the endothelial PAS domain 1 (EPAS1) gene, whose protein product, Hif-2?, is a central regulator of the hypoxia inducible factors (HIF) pathway. The Di Rienzo lab has conducted a genome-wide association study for hemoglobin levels in Tibetans that allowed narrowing down the region of association to a 17kb segment containing epigenetic signatures of active enhancer elements in endothelial cells, a cell type which has been implicated in both normal hypoxic response and in diseases characterized by hypoxic tissues and HIF pathway dysfunction, such as asthma and pulmonary hypertension. This project seeks to comprehensively examine the effects of these genetic variants on endothelial function and how these genetic effects translate into physiological adaptations to hypoxia. To achieve this goal, in Aim 1, I will examine the changes in chromatin architecture in endothelial cells cultured in normoxia and hypoxia using capture HiC and ATAC-seq. These results will not only help inform my expectations for the EPAS1 locus, but will also provide an invaluable dataset on the impacts of hypoxia on endothelial chromatin architecture.
In Aim 2, I will focus on identifying causal SNPs in EPAS1 by using reporter gene assays and CRISPR-cas9 gene editing technology followed by functional and molecular assays of cellular phenotypes. This will allow me to both identify causal SNPs at the EPAS1 locus, and definitively connect them to EPAS1 expression and physiological outcomes in an endothelial context.
In Aim 3, having validated causal alleles with differential enhancer activity, I will examine their effect in vivo by performing transient LacZ transgenic assays in mice. This will broaden the scope of our understanding of the adaptive alleles by indicating their spatial distribution and the sufficiency of high and low-altitude alleles to drive expression in the endothelium.
Human adaptation to high altitude hypoxia presents a unique opportunity to analyze natural selection's solution to the hypoxic stress that characterizes or complicates most cardiovascular and hematopoietic disorders. In characterizing the functional mechanism of adaptive evolution at the EPAS1 locus in Tibetans, I will generate an abundance of data on both endothelial response to hypoxia and genetic mechanisms of combating chronic hypoxia. Ultimately, this information may lead to targeted therapies and a deeper understanding of the role of the hypoxia response pathway in endothelial function and disease.
