Despite some progress in understanding the role of the sex hormone estrogen in female susceptibility to pulmonary arterial hypertension (PAH), the female prevalence in PAH incidence remains largely unexplained. Investigations into the genetics of PAH in men and women are hampered by the failure to include the sex chromosomes in genome-wide association studies, to account for sex as variable in targeted genetic analyses, and even the focus on a single gene as cause of PAH. Moreover, an animal model that recapitulates the sex differences of PAH is missing. Our supportive data demonstrate that: a) our EHITSN-transduced intersectin-1s (ITSN) heterozygous mouse (EHITSN-K0ITSN+/-) model of PAH recapitulates most of the sex-specific differences of human disease; b) pulmonary artery endothelial cells (PAECs) are sex dimorphic in the proliferative potential; c) the expression and activity of the long non-coding RNA Xist is upregulated compared to baseline state in both, EHITSN-K0ITSN+/- and human PAH female lung specimens. The EHITSN is the N-terminal fragment of ITSN [a prominent protein of the lung], result of granzyme B cleavage under inflammatory conditions associated with PAH; EHITSN has endothelial cell proliferative potential and triggers a severe vascular arteriopathy in the lungs of EHITSN-K0ITSN+/- mice. Upregulation of Xist, essential for dosage compensation of chromosome X (ChrX)- linked genes expression leads to sex-specific upregulation of Elk1 transcription factor, a ChrX-linked PAH gene, and of cyclin A1, a cell cycle regulatory protein. The augmented Xist expression negatively regulates Klf2, a key transcription factor for endothelial homeostasis and quiescence. These molecular events are more prominent in female EHITSN-K0ITSN+/- mouse and human PAH lung specimens compared to males. Moreover, in vitro knockdown of Xist via siRNA or its inhibition via a specific EHITSN inhibitory peptide abolish the pathological manifestations. Based on these novel findings, we hypothesize that the increase in Xist expression in the female PAH specimens accounts at least in part, for the sex/ ratio imbalance in PAH. Two Specific Aims will address this hypothesis: SA #1 will demonstrate that Xist is a key regulator of the sexual dimorphism of PAECs in the proliferative response. Employing PAECs and lung tissue of PAH, male and female subjects, we will: 1a. Validate the increased Xist expression and transcriptional activity in female PAH specimens; 1b. Demonstrate that sex- specific upregulation of the Elk1/ cyclin A1 axis promotes progression of cell cycle and an increased proliferative response of female PAECs. 1c. Demonstrate that Xist upregulation and its sex-specific interactions with downstream partners (i.e., Klf2) contribute to the increased proliferative response of female PAECs. In SA #2 we propose to: 2a. Investigate the interactions of the EHITSN with nuclear factors; 2b. Explore the therapeutic potential of targeting Xist to inhibit female PAECs...