Hereditary angioedema: the plasma contact system out of control: reply

Quantification of C1 inhibitor activity using a chromogenic automated assay: analytical and clinical performances

OBJECTIVES

The quantification of functional C1 inhibitor activity (fC1-INH) is an important tool to diagnose bradykinin-mediated angioedema (AE), whether hereditary or acquired. For that an accurate assay is necessary, therefore we evaluated the analytical performances of a fC1-INH chromogenic assay (Berichrom®, Siemens) performed utilizing an Optilite turbidimeter (Binding Site).

METHODS

fC1-INH was quantified by means of the chromogenic assay Berichrom®. Internal quality controls were used to determine the precision of the assay. Stability under various storage and matrix conditions, uncertainty, linearity, interference (of hemolysis, lipemia, and icterus), agreement with the manual Technochrom® assay, and diagnostic performances were further evaluated on samples from patients and healthy donors.

RESULTS

The fC1-INH Berichrom® assay presented good performances regarding intra- and inter-assay precision (CV: 1.3-4.5 % and 3.0-6.0 %, respectively), expanded uncertainty (5.5 % at normal level and 12.5 % at the clinical threshold) and linearity (rho2>0.99: range 7-130 % activity). Addition of interfering substances (hemoglobin <16 g/L, intralipid® <12 g/L, and bilirubin <1 g/L) did not affect fC1-INH quantification. fC1-INH activity from healthy donors remained stable in citrate whole blood until 4 days at room temperature, and 7 days when plasma was collected. Agreement between the automated Berichrom® assay and the manual Technochrom® assay (n=47) was excellent as obtained with both quantitative (Deming regression and Bland-Altman difference plot) and qualitative (Kappa index=1) analyses. Finally, the diagnostic performance of the quantification of fC1-INH for AE evaluated on 81 patients revealed a sensitivity of 100 %, a specificity of 97.2 %, a positive predictive value of 83.3 % and a negative predictive value of 100 %.

CONCLUSIONS

The automated fC1-INH Berichrom® assay showed good performance, both at the analytical and diagnostic/clinical levels that allowed its usage in a clinical laboratory for C1-INH-dependent bradykinin-mediated AE research in combination with quantitative C1-INH and C4 determinations.

Mutant plasminogen in hereditary angioedema is bypassing FXII/kallikrein to generate bradykinin

Hereditary angioedema (HAE) is characterized by recurrent localized edema in various organs, which can be potentially fatal. There are different types of hereditary angioedema, which include genetic deficiency of C1 inhibitor (C1-INH) and hereditary angioedema with normal C1-INH (HAEnCI). In HAEnCI patients mutations have been identified in the F12, PLG, KNG1, ANGPT1, MYOF, and HS3ST6 genes. The release of bradykinin from kininogen via the kallikrein-kinin system (KKS) has been shown to be the main mediator in HAE-FXII, but for HAE-PLG there are only first indications how the PLG mutations can result in bradykinin release. Here we identified in a multi-generation HAE-PLG family an additional F12 mutation, resulting in the loss of one F12 allele. There were no differences in the clinical presentation between HAE-PLG patients with and without the additional F12 mutation, thus we concluded that the kallikrein-kinin system is bypassed in HAE-PLG. Structural modeling and in vitro assays using purified proteins confirmed the PLG mutation c.988A>G; p.K330E to be a gain of function mutation resulting in an increased bradykinin release by direct cleavage of high molecular weight kininogen (HMWK). Thus, we can provide clinical and experimental evidence that mutant plasminogen in HAE-PLG is bypassing FXII/kallikrein to generate bradykinin.

The central role of endothelium in hereditary angioedema due to C1 inhibitor deficiency

An impairment of the endothelial barrier function underlies a wide spectrum of pathological conditions. Hereditary angioedema due to C1-inhibitor deficiency (C1-INH-HAE) can be considered the "pathophysiological and clinical paradigm" of Paroxysmal Permeability Diseases (PPDs), conditions characterized by recurrent transient primitively functional alteration of the endothelial sieving properties, not due to inflammatory-ischemic-degenerative injury and completely reversible after the acute flare. It is a rare yet probably still underdiagnosed disease which presents with localized, non-pitting swelling of the skin and submucosal tissues of the upper respiratory and gastrointestinal tracts, without significant wheals or pruritus. The present review addresses the pathophysiology of C1-INH-HAE with a focus on the crucial role of the endothelium during contact and kallikrein/kinin system (CAS and KKS) activation, currently available and emerging biomarkers, methods applied to get new insights into the mechanisms underlying the disease (2D, 3D and in vivo systems), new promising investigation techniques (autonomic nervous system analysis, capillaroscopy, flow-mediated dilation method, non-invasive finger plethysmography). Hints are given to the binding of C1-INH to endothelial cells. Finally, crucial issues as the local vs systemic nature of CAS/KKS activation, the episodic nature of attacks vs constant C1-INH deficiency, pros and cons as well as future perspectives of available methodologies are briefly discussed.

Polyphosphates and Complement Activation

To sustain life in environments that are fraught with risks of life-threatening injury, organisms have developed innate protective strategies such that the response to wounds is rapid and localized, with the simultaneous recruitment of molecular, biochemical, and cellular pathways that limit bleeding and eliminate pathogens and damaged host cells, while promoting effective healing. These pathways are both coordinated and tightly regulated, as their over- or under-activation may lead to inadequate healing, disease, and/or demise of the host. Recent advances in our understanding of coagulation and complement, a key component of innate immunity, have revealed an intriguing linkage of the two systems. Cell-secreted polyphosphate promotes coagulation, while dampening complement activation, discoveries that are providing insights into disease mechanisms and suggesting novel therapeutic strategies.

A mechanism for hereditary angioedema with normal C1 inhibitor: an inhibitory regulatory role for the factor XII heavy chain

The plasma proteins factor XII (FXII) and prekallikrein (PK) undergo reciprocal activation to the proteases FXIIa and kallikrein by a process that is enhanced by surfaces (contact activation) and regulated by the serpin C1 inhibitor. Kallikrein cleaves high-molecular-weight kininogen (HK), releasing the vasoactive peptide bradykinin. Patients with hereditary angioedema (HAE) experience episodes of soft tissue swelling as a consequence of unregulated kallikrein activity or increased prekallikrein activation. Although most HAE cases are caused by reduced plasma C1-inhibitor activity, HAE has been linked to lysine/arginine substitutions for Thr³⁰⁹ in FXII (FXII-Lys/Arg309). Here, we show that FXII-Lys/Arg309 is susceptible to cleavage after residue 309 by coagulation proteases (thrombin and FXIa), resulting in generation of a truncated form of FXII (δFXII). The catalytic efficiency of δFXII activation by kallikrein is 15-fold greater than for full-length FXII. The enhanced rate of reciprocal activation of PK and δFXII in human plasma and in mice appears to overwhelm the normal inhibitory function of C1 inhibitor, leading to increased HK cleavage. In mice given human FXII-Lys/Arg309, induction of thrombin generation by infusion of tissue factor results in enhanced HK cleavage as a consequence of δFXII formation. The effects of δFXII in vitro and in vivo are reproduced when wild-type FXII is bound by an antibody to the FXII heavy chain (HC; 15H8). The results contribute to our understanding of the predisposition of patients carrying FXII-Lys/Arg309 to angioedema after trauma, and reveal a regulatory function for the FXII HC that normally limits PK activation in plasma.