Background and Hypothesis: Pulmonary hypertension (PH) is a deadly disease, where Group 1 PAH and Group 3 PH are driven by hypoxia, HIF-2α, and non-coding RNAs. We found that the lncRNA KMT2E-AS1 is up-regulated in Groups 1/3 PH and is induced by HIF-2α. This lncRNA gene neighbors KMT2E, a gene controlling histone 3 lysine 4 trimethylation (H3K4me3) and chromatin remodeling. In pulmonary endothelial cells (ECs), KMT2E-AS1 stabilizes KMT2E to increase H3K4me3, thus driving HIF-2α-specific metabolic and pathogenic alterations. The G-allele of single nucleotide variant (SNV) rs73184087 within KMT2E is associated with risk of developing Group 1 PAH (in discovery/validation cohorts and a meta-analysis of 2,181 PAH vs. 10,060 controls). rs73184087 also displays more avid allele (G)-specific association with HIF-2α leading to induction of this lncRNA-KMT2E pair. A mouse deficient in the conserved lncRNA sequence is protected against Groups 1/3 PH; this is phenocopied by inhibition of histone methylation in PAH rats. We postulate that the KMT2E-AS1/KMT2E axis is a central lynchpin in pathogenic reprogramming in ECs, promoting PH. Aim 1) Define the allele-specific role of the KMT2E SNV rs73184087 in controlling HIF-2α-dependent EC lncRNA-KMT2E expression and PH pathophenotypes. Using ECs derived from genome-edited inducible pluripotent stem cells (iPSC) as well as primary lung ECs carrying rs73184087 A and G alleles, we will determine if (G) increases lncRNA-KMT2E by more HIF-2α binding and drives more severe EC phenotypes. We will also pursue expression quantitative trait loci (eQTL) analysis in blood samples from PAH patients (discovery/validation cohorts) and PAH lung tissues carrying A and G alleles of rs73184087. Aim 2) Define the role of this lncRNA-KMT2E axis and H3K4me3 in promoting PH in vivo. We will quantify Groups 1/3 PH severity in rodents after EC-specific knockdown of this lncRNA vs. lncRNA+KMT2E and after AAV-driven EC- specific expression of lncRNA vs. lncRNA+KMTE2. We will also determine if MM-589, a specific H3K4me3 inhibitor, reverses PAH in rats. Thus, we aim to determine if lncRNA+KMT2E together are necessary and sufficient to drive Group 1/3 PH and if PAH is dependent upon H3K4me3 activity, thus offering a new epigenetic PH therapy. Aim 3) Define the causative role of the G allele of rs73184087 on pulmonary vascular remodeling and PH in vivo. Culturing human precision cut lung slices, we will determine if the rs73184087 G allele drives vascular remodeling via regulation of the lncRNA-KMT2E axis and H3K4me3. We have also inserted the human rs73184087 G vs. A allele in mice and will use these “humanized” mice to study these alleles in vivo. With these 2 unique platforms, we will determine if the G allele drives HIF-2α-specific EC phenotypes and PH. Significance: We plan to shift paradigms of lncRNA biology in PH, via defining the links of hypoxia to epigenetics and metabolism and by introducing new epigenetic therapies. By establishin...