ROCAMORA, J. L.; Casellas, A.; Figueras, A.; Cerda, P.; Medina-Jover, F.; Torres-Iglesias, R.; Castillo, S.; Graupera, M.; Ola, R.; Riera-Mestre, A.; Vinyals, F.

Hereditary hemorrhagic telangiectasia (HHT) is a rare vascular disorder caused by pathogenic variants in members of the BMP9/ALK1 signaling hub. In the present study we show that, regardless of whether the alterations are caused by reduced BMP9/ALK1 signaling (pathogenic variants in the ENG or ALK1 genes) or by overactivation of this pathway (such as the SMAD6 pathogenic variants), all are associated with increased endothelial cell (EC) proliferation and high levels of ERK MAPK activation in patient biopsies. We reproduced this phenotype in vitro in ECs lacking SMAD6 or after SMAD1 knockdown using siRNA. Loss of SMAD6 leads to dysregulation of the Notch pathway, with downregulation of phosphatases and consequent overstimulation of ERK. In normal ECs, BMP9 and Notch signaling inhibit ERK activity by upregulating PPP1R3C, a regulatory subunit of the PP1 phosphatase. Notably, BMP9-mediated inhibition of ERK is abolished when cells are transfected with siRNA targeting PPP1R3C. ERK hyperactivation was also observed in an HHT2 mouse model (ALK1-2loxP;Cdh5-CreERT2). Loss of both ALK1 alleles in adult mice leads to vascular failure and hemorrhages in the lung and intestine; these injuries are significantly reduced by treatment with the MEK/ERK inhibitor selumetinib. Overall, our work identifies a key role for ERK activation involved in HHT pathogenesis, suggesting that ERK inhibition may represent a promising therapeutic strategy for these patients.