Full-length antithrombin frameshift variant with aberrant C-terminus causes endoplasmic reticulum retention with a dominant-negative effect

The double whammy of ER-retention and dominant-negative effects in numerous autosomal dominant diseases: significance in disease mechanisms and therapy

The endoplasmic reticulum (ER) employs stringent quality control mechanisms to ensure the integrity of protein folding, allowing only properly folded, processed and assembled proteins to exit the ER and reach their functional destinations. Mutant proteins unable to attain their correct tertiary conformation or form complexes with their partners are retained in the ER and subsequently degraded through ER-associated protein degradation (ERAD) and associated mechanisms. ER retention contributes to a spectrum of monogenic diseases with diverse modes of inheritance and molecular mechanisms. In autosomal dominant diseases, when mutant proteins get retained in the ER, they can interact with their wild-type counterparts. This interaction may lead to the formation of mixed dimers or aberrant complexes, disrupting their normal trafficking and function in a dominant-negative manner. The combination of ER retention and dominant-negative effects has been frequently documented to cause a significant loss of functional proteins, thereby exacerbating disease severity. This review aims to examine existing literature and provide insights into the impact of dominant-negative effects exerted by mutant proteins retained in the ER in a range of autosomal dominant diseases including skeletal and connective tissue disorders, vascular disorders, neurological disorders, eye disorders and serpinopathies. Most crucially, we aim to emphasize the importance of this area of research, offering substantial potential for understanding the factors influencing phenotypic variability associated with genetic variants. Furthermore, we highlight current and prospective therapeutic approaches targeted at ameliorating the effects of mutations exhibiting dominant-negative effects. These approaches encompass experimental studies exploring treatments and their translation into clinical practice.

Clinical and Molecular Characterization of Nine Novel Antithrombin Mutations

Antithrombin (AT) is the major plasma inhibitor of thrombin (FIIa) and activated factor X (FXa), and antithrombin deficiency (ATD) is one of the most severe thrombophilic disorders. In this study, we identified nine novel AT mutations and investigated their genotype-phenotype correlations. Clinical and laboratory data from patients were collected, and the nine mutant AT proteins (p.Arg14Lys, p.Cys32Tyr, p.Arg78Gly, p.Met121Arg, p.Leu245Pro, p.Leu270Argfs*14, p.Asn450Ile, p.Gly456delins_Ala_Thr and p.Pro461Thr) were expressed in HEK293 cells; then, Western blotting, N-Glycosidase F digestion, and ELISA were used to detect wild-type and mutant AT. RT-qPCR was performed to determine the expression of AT mRNA from the transfected cells. Functional studies (AT activity in the presence and in the absence of heparin and heparin-binding studies with the surface plasmon resonance method) were carried out. Mutations were also investigated by in silico methods. Type I ATD caused by altered protein synthesis (p.Cys32Tyr, p.Leu270Argfs*14, p.Asn450Ile) or secretion disorder (p.Met121Arg, p.Leu245Pro, p.Gly456delins_Ala_Thr) was proved in six mutants, while type II heparin-binding-site ATD (p.Arg78Gly) and pleiotropic-effect ATD (p.Pro461Thr) were suggested in two mutants. Finally, the pathogenic role of p.Arg14Lys was equivocal. We provided evidence to understand the pathogenic nature of novel SERPINC1 mutations through in vitro expression studies.

SERPINC1 c.1247dupC: a novel SERPINC1 gene mutation associated with familial thrombosis results in a secretion defect and quantitative antithrombin deficiency

BACKGROUND

Antithrombin (AT) is an important anticoagulant in hemostasis. We describe here the characterization of a novel AT mutation associated with clinically relevant thrombosis. A pair of sisters with confirmed type I AT protein deficiency was genetically analyzed on suspicion of an inherited SERPINC1 mutation. A frameshift mutation, c.1247dupC, was identified and the effect of this mutation was examined on the cellular and molecular level.

METHODS

Plasmids for the expression of wild-type (WT) and mutated SERPINC1 coding sequence (CDS) fused to green fluorescent protein (GFP) or hemagglutinin (HA) tag were transfected into HEK293T cells. Subcellular localization and secretion of the respective fusion proteins were analyzed by confocal laser scanning microscopy and Western blot.

RESULTS

The c.1247dupC mutation results in a frameshift in the CDS of the SERPINC1 gene and a subsequently altered amino acid sequence (p.Ser417LysfsTer48). This alteration affects the C-terminus of the AT antigen and results in impaired secretion as confirmed by GFP- and HA-tagged mutant AT analyzed in HEK293T cells.

CONCLUSION

The p.Ser417LysfsTer48 mutation leads to impaired secretion, thus resulting in a quantitative AT deficiency. This is in line with the type I AT deficiency observed in the patients.

Functional analysis of two abnormal antithrombin proteins with different intracellular kinetics

INTRODUCTION

Hereditary antithrombin (AT) deficiency type I causes venous thrombosis due to decreased levels of AT antigen in the blood. We identified one novel and one known abnormal variant in two unrelated Japanese families with venous thrombosis. In this study, we analyzed the mechanism by which these abnormal variants cause type I AT deficiency.

MATERIALS AND METHODS

Wild-type and variant AT expression vectors were constructed and transiently expressed in human embryonic kidney 293 cells, and AT antigen levels and N-glycosylation of cell lysates and culture medium were evaluated by western blot analysis. Subcellular co-localization of AT was also examined using confocal microscopy, and chase experiments with cycloheximide and MG132 were performed to investigate the degradation pathway of AT variants.

RESULTS

Genetic analysis identified a novel variant, c.613delC (p.Leu205Trpfs⁎79), and the known variant c.283T>C (p.Tyr95His). These AT variants exhibited significantly reduced extracellular secretion compared with the wild-type; N-glycosylation of the AT protein was normal. Co-localization analysis suggested that the transport of these abnormal AT proteins to the Golgi apparatus was impaired. The c.613delC variant was degraded early by the proteasome, suggesting that the c.283T>C variant is stored in the endoplasmic reticulum (ER).

CONCLUSIONS

The AT variants identified here synthesize abnormal AT proteins that exhibit suppressed secretion and impaired transport from the ER to the Golgi apparatus. These results provide clues that could help elucidate the mechanism of type I AT deficiency and facilitate therapy development.

Antithrombin Deficiency and Thrombosis: A Wide Clinical Scenario Reported in a Single Institution

Congenital antithrombin (AT) deficiency represents the form of thrombophilia with the highest thrombotic risk. It is characterized by a heterogeneous clinical presentation, depending mostly on the family history of thrombosis and type of genetic mutation. Inherited AT deficiency promotes idiopathic thrombosis at an early age (even in the pediatric population) and at atypical sites. Therefore, a positive family background necessitates ruling out this high-risk thrombophilia at a young age. Studying first-degree relatives, even if they are asymptomatic, is essential to establish thromboprophylaxis and a proper therapeutic approach in case of thrombosis. Patients with congenital AT deficiency require indefinite anticoagulation owing to the high thrombotic recurrence rate. Here, we present four unrelated cases reported in our institution who were diagnosed with hereditary AT deficiency, with a contrasting clinical evolution.

Biochemistry, molecular genetics, and clinical aspects of hereditary angioedema with and without C1 inhibitor deficiency

Hereditary angioedema (HAE) is a rare disorder characterized by cutaneous and submucosal swelling caused mostly by excessive local bradykinin production. Bradykinin is a vasoactive peptide generated by the limited proteolysis of high molecular weight kininogen (HMWK) by plasma kallikrein via the contact activation system. The contact activation system occurs not only in solution but also on the cell surface. Factor XII (FXII), prekallikrein, and HMWK are assembled on the endothelial cell surface via several proteins, including a trimer of a receptor for globular C1q domain in a Zn²⁺-dependent manner, and the reciprocal activation on the cell surface is believed to be physiologically important in vivo. Thus, the contact activation system leads to the activation of coagulation, complement, inflammation, and fibrinolysis. C1-inhibitor (C1-INH) is a plasma protease inhibitor that is a member of the serpin family. It mainly inhibits activated FXII (FXIIa), plasma kallikrein, and C1s. C1-INH hereditary deficiency induces HAE (HAE-C1-INH) due to excessive bradykinin production via the incomplete inhibition of plasma kallikrein and FXIIa through the low C1-INH level. HAE is also observed in patients with normal C1-INH (HAEnCI) who carry pathogenic variants in genes of factor XII, plasminogen, angiopoietin 1, kininogen, myoferlin, and heparan sulfate 3-O-sulfotransferase 6, which are associated with bradykinin production and/or vascular permeability. HAE-causing pathways triggered by pathogenic variants in patients with HAE-C1-INH and HAEnCI are reviewed and discussed.

The conformational landscape of a serpin N-terminal subdomain facilitates folding and in-cell quality control

Many multi-domain proteins including the serpin family of serine protease inhibitors contain non-sequential domains composed of regions that are far apart in sequence. Because proteins are translated vectorially from N- to C-terminus, such domains pose a particular challenge: how to balance the conformational lability necessary to form productive interactions between early and late translated regions while avoiding aggregation. This balance is mediated by the protein sequence properties and the interactions of the folding protein with the cellular quality control machinery. For serpins, particularly α 1 -antitrypsin (AAT), mutations often lead to polymer accumulation in cells and consequent disease suggesting that the lability/aggregation balance is especially precarious. Therefore, we investigated the properties of progressively longer AAT N-terminal fragments in solution and in cells. The N-terminal subdomain, residues 1-190 (AAT190), is monomeric in solution and efficiently degraded in cells. More β -rich fragments, 1-290 and 1-323, form small oligomers in solution, but are still efficiently degraded, and even the polymerization promoting Siiyama (S53F) mutation did not significantly affect fragment degradation. In vitro, the AAT190 region is among the last regions incorporated into the final structure. Hydrogen-deuterium exchange mass spectrometry and enhanced sampling molecular dynamics simulations show that AAT190 has a broad, dynamic conformational ensemble that helps protect one particularly aggregation prone β -strand from solvent. These AAT190 dynamics result in transient exposure of sequences that are buried in folded, full-length AAT, which may provide important recognition sites for the cellular quality control machinery and facilitate degradation and, under favorable conditions, reduce the likelihood of polymerization.