Overlapping upstream ORFs ending at c.125 lead to reduced Endoglin, contributing to Hereditary Hemorrhagic Telangiectasia.

Hereditary Hemorrhagic Telangiectasia (HHT) is a rare vascular disease mainly caused by pathogenic mutations in ACVRL1 and ENG genes. Despite advances in HHT diagnosis, the molecular origin of some cases remains unclear. Recently, we observed a high prevalence of HHT-causing 5'UTR variants in ENG. These variants commonly introduce upstream AUG codons (uAUGs) at the origin of upstream open reading frames (upORFs) overlapping the coding sequence, all terminating at the same stop codon located at position c.125 (uAUG-c.125). Here, we analyzed all 5'UTR ENG single nucleotide variants that could alter upORFs in silico. Interestingly, we found that 85% of uAUG-c.125 variants alter the protein levels. Furthermore, we identified 2 variants creating uAUG-c.125 and uCUG-c.125 in HHT patients and experimentally demonstrated their association with reduced endoglin levelsThis study provides new elements for the interpretation of upORF-altering variants in the 5'UTR of ENG with new insights for the molecular diagnosis of HHT.