Novel uAUG creating variants in the 5’UTR of ENG causing Hereditary Hemorrhagic Telangiectasia

Abstract

AbstractHereditary Hemorrhagic Telangiectasia (HHT) is a rare vascular disorder causing abnormal vessel formation and characterized by autosomal dominant transmission. The associated considerable variability in symptoms and clinical severity complicate the management of the disease. In clinical routine, 3 main genes,ACVRL1(also known asALK1),ENGandSMAD4are screened for pathogenic variants at the origin of HHT. About 80% of HHT cases are caused by pathogenic coding variants inACVRL1andENG. However, at least 15% remain with no molecular explanations in the 3 main genes. We here report the identification of 2 never reported variants, c.-79C>T and c.-68G>A, in the 5’UTR ofENGin 2 unrelated HHT patients. These 2 variants are predicted to create upstream AUGs (uAUGs) in the 5’UTR, which are in frame with a stop codon located within the CoDing Sequence (CDS), thus generating Overlapping upstream Open reading frames (uoORFs). In other cases, uAUGs can lead to fully upstream ORFs (uORFs) when associated with stop codons located within the 5’UTR or to elongated CDS (eCDS) when in frame with the CDS and associated with the main stop codon.In order to assess the pathogenicity of theseENGvariants, we performedin vitrofunctional assays based on the expression of wild-type and mutant constructs in human cells. We found that these 5’UTRENGvariants were associated with a decrease of protein levels in HeLa and HUVEC cells. They were also associated with a decreased ability to activate BMP9-stimulated ALK1 receptor in a BMP-response element (BRE) assay. This assay is a mandatory element before providing a definitive molecular diagnosis and has been so far applied only on codingENGvariants. We applied the same experimental workflow on 3 additional uoORF-creating variants (c.-142A>T, c.-127C>T and c.-10C>T) and one eCDS-creating variant (c.-9G>A) in the 5’UTR ofENGpreviously reported in HHT patients. We found that all the analysed uoORF-creating variants alter endoglin levels and function. A comparison of our experimental results with patients’ clinical characteristics suggests that uoORF-creating variants leading to ENG protein levels ≤ 40%in vitrowould be associated with HHT. Additional experiments relying on an artificial deletion in our mutated constructs show that created uAUGs predicted to create uoORFs are able to initiate the translation indicating that the associated effect is likely caused by an alteration of the translation mechanism.Overall, we here identified two never reported 5’UTRENGvariations in HHT patients and shed new lights on the role of upstream ORFs on ENG regulation. Our findings contribute to the amelioration of molecular diagnosis in HHT.