Endoplasmic Reticulum Quality Control Is Involved in the Mechanism of Endoglin-Mediated Hereditary Haemorrhagic Telangiectasia

Abstract

Hereditary haemorrhagic telangiectasia (HHT) is an autosomal dominant genetic condition affecting the vascular system and is characterised by epistaxis, arteriovenous malformations and mucocutaneous and gastrointestinal telangiectases. This disorder affects approximately 1 in 8,000 people worldwide. Significant morbidity is associated with this condition in affected individuals, and anaemia can be a consequence of repeated haemorrhages from telangiectasia in the gut and nose. In the majority of the cases reported, the condition is caused by mutations in either ACVRL1 or endoglin genes, which encode components of the TGF-beta signalling pathway. Numerous missense mutations in endoglin have been reported as causative defects for HHT but the exact underlying cellular mechanisms caused by these mutations have not been fully established despite data supporting a role for the endoplasmic reticulum (ER) quality control machinery. For this reason, we examined the subcellular trafficking of twenty-five endoglin disease-causing missense mutations. The mutant proteins were expressed in HeLa and HEK293 cell lines, and their subcellular localizations were established by confocal fluorescence microscopy alongside the analysis of their N-glycosylation profiles. ER quality control was found to be responsible in eight (L32R, V49F, C53R, V125D, A160D, P165L, I271N and A308D) out of eleven mutants located on the orphan extracellular domain in addition to two (C363Y and C382W) out of thirteen mutants in the Zona Pellucida (ZP) domain. In addition, a single intracellular domain missense mutant was examined and found to traffic predominantly to the plasma membrane. These findings support the notion of the involvement of the ER's quality control in the mechanism of a significant number, but not all, missense endoglin mutants found in HHT type 1 patients. Other mechanisms including loss of interactions with signalling partners as well as adverse effects on functional residues are likely to be the cause of the mutant proteins' loss of function.

Intracellular trafficking of transforming growth factor β receptors

“…HHT type 1 and type 2 result from mutations in the genes encoding endoglin, an auxiliary protein for TGFβ binding to signaling TGFβ receptors, and activin-like kinase 1, a type I receptor for TGFβ, BMP-9 and BMP-10, respectively [58]. It has been found that the majority of mutations result in the retention of these proteins in the ER [115,116].…”

Mutant p53 promotes tumor progression and metastasis by the endoplasmic reticulum UDPase ENTPD5

“…To explore whether mutp53 and ENTPD5 enhance folding and maturation of N-glycoproteins, we investigated endoglin/CD105, a coreceptor for TGFβ that is implicated in migration, invasion, and metastasis of breast and pancreatic cancer cells (19,31,32). Endoglin function relies exquisitely on protein folding in the calnexin/calreticulin cycle because endoglin mutants that colocalize with calnexin in the ER cause hereditary hemorrhagic telangiectasia (33,34). Similar to the immature N-glycosylation pattern of hereditary hemorrhagic telangiectasia-associated endoglin mutants (33), we observed an increase in immature N-glycosylated wild-type endoglin in mutp53-and ENTPD5-depleted MDA-MB-231 cells (Fig.…”

Defective cellular trafficking of the bone morphogenetic protein receptor type II by mutations underlying familial pulmonary arterial hypertension

“…We have previously reported that several missense mutations in ALK1 and ENG associated with HHT, lead to ER retention and defective trafficking of the mutant receptors (Ali et al, 2011;Hume et al, 2013). Mutations that cause retention of the mutant protein in ER probably due to misfolding present possibilities for potential therapies involving manipulation of ER quality control.…”