Characterization of non-coding Endoglin mutations leading to pathogenic upstream Open Reading Frames in Hereditary Hemorrhagic Telangiectasia – promises for novel therapeutic strategies – ENDOMORF

Hereditary haemorrhagic telangiectasia (HHT) is a rare disease characterized by epistaxis, mucocutaneous telangiectasia and arteriovenous malformations affecting multiple organs. HHT associates with severe complications such as clotting, bleeding, hypoxia and anemia that can be fatal in 10% of cases. About 80% of HHT patients carry rare coding mutations in ACVRL1, ENG or SMAD4 genes. The partners of this project have recently identified 2 non-coding variations in the 5’UTR of the ENG gene in 2 unrelated HHT patients. These 2 variants are predicted to create upstream AUGs in the 5’UTR in frame with the same stop coding located within the Coding Sequence, thus generating Overlapping Open Reading Frames (uoORF). We found that the 2 newly identified variants were associated with a decrease of the Endoglin protein levels and function in vitro using an assay applied for the first time on non-coding variants. Similar results were obtained on 3 additional uoORF-creating variants in the 5’UTR of ENG previously reported in the literature in HHT patients and considered as pathogenic but with no definitive functional evidence. These findings strongly suggest that the 2 new non-coding variants are HHT-causing. In addition, we conducted preliminary investigations suggesting that the ENG expression could be restored in patients carrying these uoORF-creating variants using a common genome editing approach targeting a specific dinucleotide deletion in the 5’UTR.
Based on these results, this project aims at deciphering the exact mechanisms of alteration by uoORFs of the Endoglin protein using molecular tools dedicated to study the mechanism of translation, at ameliorating the molecular diagnosis in HHT by helping variant classification and reducing the number of variants of unknown significance, and at developing new targeted therapeutical strategies based on genome editing and/or antisense oligonucleotide to restore the function of the impaired ENG protein caused by uoORFs.