Aim: The ETS transcription factor ERG has been identified as a principal regulator of endothelial function through its ability to repress inflammation in endothelial cells (ECs), and loss of ERG induces endothelial to mesenchymal transition (EndMT) in fibrotic disease. To investigate ERG function in atherosclerosis, we employed an EC-specific Erg knockout ( Erg EC - KO ) in PCSK9-overexpression and apoE-deficient murine atherosclerosis models. Methods: Atherosclerosis was induced in 10-week-old Erg EC - KO mice or wild-type counterparts that had a Cre-inducible EC lineage tag ( Cdh5 CreER ; R26-CAG-LSL-Sun1-sfGFP ). Atherosclerosis was modeled by Apoe deletion or AAV8-PCSK9 with 12 weeks of high-cholesterol diet (1.25%). Aortic intimal cells were assessed with flow cytometry. Plaque burden, morphology, and cellularity were characterized with brightfield, Oil Red O, H&amp;E, Movat, and immunofluorescence imaging. Results: Erg EC-KO in PCSK9-overexpression mice led to a 1.6-fold increase in aortic plaque burden and plaque formation in regions that are usually protected (e.g., greater curvature; p≤0.03; n=9-10). Loss of ERG increased elastin breaks (1.7-fold; p=0.02; n=9) and decreased fibrous cap thickness (0.76-fold; p=0.02; n=9-10) in aortic arch cross sections. Aortic intimal digests from Erg EC- KO mice contained increased Sun1-sfGFP + EC lineage cells (1.9-fold; p=0.01; n=4), which had reduced CD31 positivity (0.79-fold; p=0.03; n=4). Notably, a 5.3-fold increase in EC lineage cells was observed in plaques from brachiocephalic artery cross sections (p=0.0001; n=6-15), which had hallmarks of EndMT (e.g., 31.5-fold increase in GFP + ACTA2 + cells, p=0.02, n=3-5). Increased aortic plaque burden was recapitulated in apoE-deficient E rg EC- KO mice (2.1-fold; p=0.02; n=4-8). Conclusion: Endothelial Erg KO results in increased plaque burden and altered plaque topography in murine atherosclerosis. Loss of ERG induces EndMT and marked ingression of ECs from the luminal endothelium to the plaque interior. These findings highlight loss of ERG as a novel mechanism by which intimal ECs acquire mesenchymal identity in atherosclerosis and implicate EC identity loss as a driver of EC migration into plaques, which is associated with a vulnerable morphology.

Abstract 1041: Loss Of The ETS Transcription Factor ERG Disrupts Fate-defining Programs In The Aortic Endothelium And Promotes Expansion Of Endothelial Lineage Cells In Atherosclerotic Plaque