SERPING1 mutation update: Mutation spectrum and C1 Inhibitor phenotypes.

C1 inhibitor (C1Inh) deficiency is responsible for hereditary angioedema (C1-INH-HAE) and caused by variants of the SERPING1/C1INH/C1NH gene. C1Inh is the major control of kallikrein-kinin system. C1Inh deficiency leads to its uncontrolled activation, with subsequent generation of the vasoactive peptide bradykinin. This update documents 748 different SERPING1 variants, including published variants and additional 120 unpublished ones. They were identified as heterozygous variants (n = 729), as homozygous variants in 10 probands and as compound heterozygous variants (nine combinations). Six probands with heterozygous variants exhibited gonadal mosaicism. Probands with heterozygous (n = 72) and homozygous (n = 1) variants were identified as de novo cases. Overall, 58 variants were found at positions showing high residue conservation among serpins, and have been referred to as a mousetrap function of C1Inh: reactive center loop, gate, shutter, breach, and hinge. C1Inh phenotype analysis identified dysfunctional serpin variants with failed serpin-protease association and a residual 105-kDa species after incubation with target protease. Regarding this characteristic, in conditions with low antigenic C1Inh, 74 C1-INH-HAE probands presented with an additional so-called intermediate C1-INH-HAE phenotype. The present update addresses a comprehensive SERPING1 variant spectrum that facilitates genotype-phenotype correlations, highlighting residues of strategic importance for serpin function and for identification of C1Inh deficiency as serpinopathy.

Hereditary angioedema with normal C1 inhibitor associated with carboxypeptidase N deficiency.

Background: Hereditary angioedema (HAE) is a potentially life-threatening disorder characterized by recurrent episodes of subcutaneous or submucosal swelling. HAE with normal C1 inhibitor (HAE-nC1-INH) is an underdiagnosed condition. Although the association with genetic variants has been identified for some families, the genetic causes in many patients with HAE-nC1-INH remain unknown. The role of genes associated with bradykinin catabolism is not fully understood. Objective: We sought to investigate the biological parameters and the genes related to kallikrein-kinin system in families with a clinical phenotype of HAE-nC1-INH and presenting with a carboxypeptidase N (CPN) deficiency. Methods: This study includes 4 families presenting with HAE-nC1-INH and CPN deficiency. Patients' clinical records were examined, biological parameters of kallikrein-kinin system were measured, and genetics was analyzed by next-generation sequencing and Sanger sequencing. Predictive algorithms (Human Splicing Finder, Sorting Intolerant From Tolerant, Polymorphism Phenotyping v2, MutationTaster, and ClinPred) were used to classify variants as affecting splicing, as benign to deleterious, or as disease-causing. Results: Patients presented with angioedema and urticaria, mainly on face/lips, but also with abdominal pain or laryngeal symptoms. Affected patients displayed low CPN activity-30% to 50% of median value in plasma. We identified 3 variants of the CPN1 gene encoding the catalytic 55-kDa subunit of CPN: c.533G>A, c.582A>G, and c.734C>T. CPN deficiency associated with genetic variants segregated with HAE-nC1-INH symptoms in affected family members. Conclusions: CPN1 gene variants are associated with CPN deficiency and HAE-nC1-INH symptoms in 4 unrelated families. Genetic CPN deficiency may contribute to bradykinin and anaphylatoxin accumulation, with synergistic effects in angioedema and urticarial symptoms.

Editorial: Kinin 2022 Meeting, Annecy, France.

The Kinin 2022 meeting took place at the Imperial Palace, Annecy, France, from 5-8 June 2022 [...].

Recessive SERPING1 Variant Leads to Kinin-Kallikrein System Control Failure in a Consanguineous Brazilian Family with Hereditary Angioedema.

Background: Hereditary angioedema (HAE) is a severe and potentially life-threatening disease. The most common forms are caused by variants in SERPING1, resulting in C1-inhibitor (C1-INH) deficiency (HAE-C1-INH). C1-INH is a serine protease inhibitor (SERPIN) that regulates multiple proteases pathways, including the kallikrein-kinin system (KKS) and its complement. In HAE-C1-INH patients, C1-INH deficiencies affect KKS control, resulting in the development of kallikrein activity in plasma and the subsequent release of bradykinin (BK). While the overwhelming majority of disease-causing SERPING1 variants are dominant, very few recessive variants have been described. We present a large Brazilian HAE-C1-INH family with a recessive form of HAE-C1-INH. Methods: Blood samples of family members were investigated for protein levels of C1-INH, C4, C1q, and C1-INH function. The SERPING1 gene was sequenced. Results: In two severely affected sisters, we identified a homozygous missense variant in SERPING1 (NM_000062.3:c.964G>A;p.Val322Met). Fourteen family members were asymptomatic heterozygous carriers of the variant. Data regarding C1-INH function in the plasma showed that homozygous p.Val322Met strongly impacts C1-INH function to inhibit C1s and kallikrein (PKa). When heterozygously expressed, it affects the C1-INH control of C1s more than that of PKa. Conclusions: These studies of the variant's effects on the structure-function relationship reinforce prior observations suggesting that C1-INH deficiency is a conformational disease.

SERPING1 Variants and C1-INH Biological Function: A Close Relationship With C1-INH-HAE.

Hereditary angioedema with C1 Inhibitor deficiency (C1-INH-HAE) is caused by a constellation of variants of the SERPING1 gene (n = 809; 1,494 pedigrees), accounting for 86.8% of HAE families, showing a pronounced mutagenic liability of SERPING1 and pertaining to 5.6% de novo variants. C1-INH is the major control serpin of the kallikrein-kinin system (KKS). In addition, C1-INH controls complement C1 and plasminogen activation, both systems contributing to inflammation. Recognizing the failed control of C1s protease or KKS provides the diagnosis of C1-INH-HAE. SERPING1 variants usually behave in an autosomal-dominant character with an incomplete penetrance and a low prevalence. A great majority of variants (809/893; 90.5%) that were introduced into online database have been considered as pathogenic/likely pathogenic. Haploinsufficiency is a common feature in C1-INH-HAE where a dominant-negative variant product impacts the wild-type allele and renders it inactive. Small (36.2%) and large (8.3%) deletions/duplications are common, with exon 4 as the most affected one. Point substitutions with missense variants (32.2%) are of interest for the serpin structure-function relationship. Canonical splice sites can be affected by variants within introns and exons also (14.3%). For noncanonical sequences, exon skipping has been confirmed by splicing analyses of patients' blood-derived RNAs (n = 25). Exonic variants (n = 6) can affect exon splicing. Rare deep-intron variants (n = 6), putatively acting as pseudo-exon activating mutations, have been characterized as pathogenic. Some variants have been characterized as benign/likely benign/of uncertain significance (n = 74). This category includes some homozygous (n = 10) or compound heterozygous variants (n = 11). They are presenting with minor allele frequency (MAF) below 0.00002 (i.e., lower than C1-INH-HAE frequency), and may be quantitatively unable to cause haploinsufficiency. Rare benign variants could contribute as disease modifiers. Gonadal mosaicism in C1-INH-HAE is rare and must be distinguished from a de novo variant. Situations with paternal or maternal disomy have been recorded (n = 3). Genotypes must be interpreted with biological investigation fitting with C1-INH expression and typing. Any SERPING1 variant reminiscent of the dysfunctional phenotype of serpin with multimerization or latency should be identified as serpinopathy.

Human C3 deficiency associated with impairments in dendritic cell differentiation, memory B cells, and regulatory T cells.

Primary C3 deficiency, a rare autosomal inherited disease (OMIM 120700), was identified in a 2-year-old male suffering from recurrent pyogenic infections from early infancy with undetectable total complement hemolytic activity (CH50) and C3 values. The nonconsanguineous parents and the two patients' two siblings had 50% normal serum C3 concentration. The molecular abnormality associated a paternal allele coding C3 with the missense mutation p.Ser(550)Pro and an apparently null maternal allele, with production of a defective protein that could no longer be secreted. Vaccination of the child did not induce a long-term Ab response. Accordingly, switched memory IgD(-)CD27(+) B cells were barely detected, amounting to only 2.3% of peripheral blood CD19(+) cells. Cells were significantly defective in stimulating alloreactive responses. The in vitro development of immature dendritic cells and their maturation capacity were greatly impaired, with decreased CD1a expression and IL-12p70 secretion ability. These cells were unable to induce autologous B cell proliferation and Ig secretion in the presence of CD40L and C3. Finally, the regulatory T cell development ability of CD4(+) T cells after CD3 and CD46 activation in the presence of IL-2 was significantly impaired. Thus, the association of important functional defects of dendritic cells, acquisition of B cell memory, and regulatory T cells with human C3 deficiency strongly supports a major role for C3 in bridging innate and adaptive immunity in humans.

High-density rafts preferentially host the complement activator measles virus F glycoprotein but not the regulators of complement activation.

The fusion (F) protein of measles virus (MeV) activates the alternative pathway of human complement in the presence of both CD46 and CD55 which regulate the complement activation [Devaux, P., Christiansen, D., Plumet, S., Gerlier, D., 2004. Cell surface activation of the alternative complement pathway by the fusion protein of measles virus. J. Gen. Virol. 85, 1665-1673]. The original observation of cold detergent-resistant membranes sedimenting at a higher density than the membrane rafts lead us to analyse the respective distribution of F, CD46 and C55 molecules in what we call heavy rafts (HRs) and in the classical low-density membrane rafts (Rs). Membrane rafts were isolated after cold TX100 solubilization and flotation on a sucrose gradient. The denser fractions collected from the lower part of the gradient could be further separated into a translucent pellet (HR) and a soluble supernatant (S). HR and R were both sensitive to TX100 solubilization after cholesterol depletion and solubilized by octyl-d-glucoside but differed in their lipid and protein composition. A proteomic analysis revealed that the HR fraction was derived from heterogeneous cellular membranes including plasma membrane, early endosomes and rough endoplasmic reticulum. Interestingly, CD55 and CD46 almost exclusively associated with R and S fractions, respectively, while after MeV infection or transient expression, MeV-F distributed almost equally between R, HR and S fractions. However more immature MeV-F(0) than mature MeV-F(1) proteins was associated with the HR fraction whereas this ratio was reverse in R and S fractions. After activation of the alternative pathway of human complement by F expressing cells, both C3b and F protein associated with R, HR and S fractions. When four or five of the five cysteines located in the transmembrane and cytoplasmic tail of F protein were substituted with serine residues, the mutated F distributed almost exclusively in HR fractions and was still efficient in activating the complement. We propose that the partitioning of F, CD46 and CD55 molecules in different membrane microdomains could account for the ability of F to escape complement regulation by the CD55 and CD46 regulators.

Hereditary C1 inhibitor deficiency is associated with high spontaneous amidase activity.

BACKGROUND: Angioedema diagnosis classically targets the complement system (via C1 inhibitor (C1Inh) function and antigenic C4 level) and contact phase activation (via amidase activity). Bradykinin is responsible for angioedema attacks and is produced from contact phase activation secondary to failed C1Inh control. OBJECTIVE: We aimed to compare the diagnostic performances of spontaneous amidase activity and antigenic C4 level in C1Inh hereditary angioedema (C1Inh-HAE) patients. METHODS: Samples from 185 C1Inh-HAE patients (81 men, 104 women; confirmed by SERPING1 gene mutations) and from 99 blood donors (50 men, 49 women) were tested for C1Inh function, antigenic C4 level and spontaneous amidase activity. RESULTS: In the C1Inh-HAE group, antigenic C4 level was decreased (n=135) and amidase activity was increased (n=181). Receiver operating characteristic analyses showed higher diagnostic performance values for the spontaneous amidase assay compared to those of antigenic C4. CONCLUSION: The spontaneous amidase activity assay should replace antigenic C4 level testing and should be tested alongside the C1Inh function for both AE screening and follow up of HAE patients.

C1 Inhibitor as a glycoprotein: The influence of polysaccharides on its function and autoantibody target.

C1 Inhibitor (C1Inh), a member of the Serine proteinase inhibitor family, is the most heavily glycosylated plasma protein. This work investigated the impact of C1Inh glycosylation on its function regarding protease targets and autoantibody binding. C1Inh deglycosylation was found to affect its function with O-linked polysaccharides, but not with N-linked polysaccharides, in controlling the contact phase but not C1s target, thus indicating the N-terminal domain's involvement in C1Inh function. Instructive samples demonstrated that O-deglycosylation strongly suppressed autoantibody binding, suggesting the polysaccharide motif is an antibody target. The autoantibodies did not directly affect C1Inh function.

Kininogen Cleavage Assay: Diagnostic Assistance for Kinin-Mediated Angioedema Conditions.

BACKGROUND: Angioedema without wheals (AE) is a symptom characterised by localised episodes of oedema presumably caused by kinin release from kininogen cleavage. It can result from a hereditary deficiency in C1 Inhibitor (C1Inh), but it can present with normal level of C1Inh. These forms are typically difficult to diagnose although enhanced kinin production is suspected or demonstrated in some cases. OBJECTIVES: We wanted to investigate bradykinin overproduction in all AE condition with normal C1Inh, excluding cases with enhanced kinin catabolism, and to propose this parameter as a disease biomarker. METHODS: We retrospectively investigated high molecular weight kininogen (HK) cleavage pattern, using gel electrophoresis and immunorevelation. Plasma samples were drawn using the same standardised procedure from blood donors or AE patients with normal C1Inh conditions, normal kinin catabolism, and without prophylaxis. RESULTS: Circulating native HK plasma concentrations were similar in the healthy men (interquartile range: 98-175µg/mL, n = 51) and in healthy women (90-176µg/mL, n = 74), while HK cleavage was lower (p<0.001) in men (0-5%) than women (3-9%). Patients exhibited lower native HK concentration (p<10-4; 21-117µg/mL, n = 31 for men; 0-84µg/mL, n = 41 for women) and higher HK cleavage (p<10-4; 10-30% and 14-89%, respectively) than healthy donors. Pathological thresholds were set at: <72µg/mL native HK, >14.4% HK cleavage for men; <38µg/mL; native HK, >33.0% HK cleavage for women, with >98% specificity achieved for all parameters. In plasma from patients undergoing recovery two months after oestrogen/progestin combination withdrawal (n = 13) or two weeks after AE attack (n = 2), HK cleavage was not fully restored, suggesting its use as a post-attack assay. CONCLUSION: As a diagnostic tool, HK cleavage can offer physicians supportive arguments for kinin production in suspected AE cases and improve patient follow-up in clinical trials or prophylactic management.

Human complement C3 deficiency: Th1 induction requires T cell-derived complement C3a and CD46 activation.

Human T helper type 1 (Th1) responses are essential in defense. Although T cell receptor (TCR) and co-stimulator engagement are indispensable for T cell activation, stimulation of additional receptor pathways are also necessary for effector induction. For example, engagement of the complement regulator CD46 by its ligand C3b generated upon TCR activation is required for IFN-γ production as CD46-deficient patients lack Th1 responses. Utilizing T cells from two C3-deficient patients we demonstrate here that normal Th1 responses also depend on signals mediated by the anaphylatoxin C3a receptor (C3aR). Importantly, and like in CD46-deficient patients, whilst Th1 induction are impaired in C3-deficient patients in vitro, their Th2 responses are unaffected. Furthermore, C3-deficient CD4(+) T cells present with reduced expression of CD25 and CD122, further substantiating the growing notion that complement fragments regulate interleukin-2 receptor (IL-2R) assembly and that disturbance of complement-guided IL-2R assembly contributes to aberrant Th1 effector responses. Lastly, sustained intrinsic production of complement fragments may participate in the Th1 contraction phase as both C3a and CD46 engagement regulate IL-10 co-expression in Th1 cells. These data suggest that C3aR and CD46 activation via intrinsic generation of their respective ligands is an integral part of human Th1 (but not Th2) immunity.

Contact system activation in patients with HAE and normal C1 inhibitor function.

In addition to hereditary angioedema (HAE) with C1 inhibitor (C1INH) deficiency, a type of HAE with dominant inheritance and normal C1INH function (HAE with normal C1INH) has been described. This relates to contact phase activation with exaggerated kinin formation, and mutations in the coagulation factor XII gene have been identified in some affected families, but the cause of the disease has remained elusive in a majority of families. Several triggering factors are responsible for developing kinin forming system, with participation of endothelium and mast cell component. Angioedema conditions meet the accumulation of kinins with failed kinin catabolism.

Enzymatic assays for the diagnosis of bradykinin-dependent angioedema.

BACKGROUND: The kinins (primarily bradykinin, BK) represent the mediators responsible for local increase of vascular permeability in hereditary angioedema (HAE), HAE I-II associated with alterations of the SERPING1 gene and HAE with normal C1-Inhibitor function (HAE-nC1INH). Besides C1-Inhibitor function and concentration, no biological assay of kinin metabolism is actually available to help physicians for the diagnosis of angioedema (AE). We describe enzymatic tests on the plasma for diagnosis of BK-dependent AE. METHODS: The plasma amidase assays are performed using the Pro-Phe-Arg-p-nitroanilide peptide substrate to evaluate the spontaneous amidase activity and the proenzyme activation. We analyzed data of 872 patients presenting with BK-dependent AE or BK-unrelated diseases, compared to 303 controls. Anti-high MW kininogen (HK) immunoblot was achieved to confirm HK cleavage in exemplary samples. Reproducibility, repeatability, limit of blank, limit of detection, precision, linearity and receiver operating characteristics (ROC) were used to calculate the diagnostic performance of the assays. RESULTS: Spontaneous amidase activity was significantly increased in all BK-dependent AE, associated with the acute phase of disease in HAE-nC1INH, but preserved in BK-unrelated disorders. The increase of the amidase activity was associated to HK proteolysis, indicating its relevance to identify kininogenase activity. The oestrogens, known for precipitating AE episodes, were found as triggers of enzymatic activity. Calculations from ROC curves gave the optimum diagnostic cut-off for women (9.3 nmol⋅min(-1)⋅mL(-1), area under curve [AUC] 92.1%, sensitivity 80.0%, and specificity 90.1%) and for men (6.6 nmol·min(-1)⋅mL(-1), AUC 91.0%, sensitivity 87.0% and specificity 81.2%). CONCLUSION: The amidase assay represents a diagnostic tool to help physicians in the decision to distinguish between BK-related and -unrelated AE.