The impairment of the alternative complement pathway contributes to rare kidney diseases such as atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathy (C3G). We recently described an aHUS patient carrying an exceptional gain-of-function (GoF) mutation (S250C) in the classical complement pathway component C2 leading to the formation of hyperactive classical convertases. We now report the identification of the same mutation and another C2 GoF mutation R249C in two other patients with a glomerulopathy of uncertain etiology. Both mutations stabilize the classical C3 convertases by a similar mechanism. The presence of R249C and S250C variants in serum increases complement-dependent cytotoxicity (CDC) in antibody-sensitized human cells and elevates deposition of C3 on ELISA plates coated with C-reactive protein (CRP), as well as on the surface of glomerular endothelial cells. Our data justify the inclusion of classical pathway genes in the genetic analysis of patients suspected of complement-driven renal disorders. Also, we point out CRP as a potential antibody-independent trigger capable of driving excessive complement activation in carriers of the GoF mutations in complement C2.
Numerous publications have underlined the link between complement C5a and the clinical course of COVID-19. We previously reported that levels of C5a remain high in the group of severely ill patients up to 90 days after hospital discharge. We have now evaluated which complement pathway fuels the elevated levels of C5a during hospitalization and follow-up. The alternative pathway (AP) activation marker C3bBbP and the soluble fraction of C4d, a footprint of the classical/lectin (CP/LP) pathway, were assessed by immunoenzymatic assay in a total of 188 serial samples from 49 patients infected with SARS-CoV-2. Unlike C5a, neither C3bBbP nor C4d readouts rose proportionally to the severity of the disease. Detailed correlation analyses in hospitalization and follow-up samples collected from patients of different disease severity showed significant positive correlations of AP and CP/LP markers with C5a in certain groups, except for the follow-up samples of the patients who suffered from highly severe COVID-19 and presented the highest C5a readouts. In conclusion, there is not a clear link between persistently high levels of C5a after hospital discharge and markers of upstream complement activation, suggesting the existence of a non-canonical source of C5a in patients with a severe course of COVID-19.
The complement system is one of the first defense lines protecting from invading pathogens. However, it may turn offensive to the body’s own cells and tissues when deregulated by the presence of rare genetic variants that impair physiological regulation and/or provoke abnormal activity of key enzymatic components. Factor B and complement C2 are examples of paralogs engaged in the alternative and classical/lectin complement pathway, respectively. Pathogenic mutations in the von Willebrand factor A domain (vWA) of FB have been known for years. Despite substantial homology between two proteins and the demonstration that certain substitutions in FB translated to C2 result in analogous phenotype, there was a limited number of reports on pathogenic C2 variants in patients. Recently, we studied a cohort of patients suffering from rare kidney diseases and confirmed the existence of two gain-of-function and three loss-of-function mutations within the C2 gene sequences coding for the vWA domain (amino acids 254-452) or nearly located unstructured region (243-253) of C2 protein. Herein, we report the functional consequences of amino acid substitution of glutamine at position 263. The p.Q263G variant resulted in the gain-of-function phenotype, similarly to a homologous mutation p.D279G in FB. Conversely, the p.Q263P variant found in a patient with C3 glomerulopathy resulted in the loss of C2 function. Our results confirm that the N-terminal part of the vWA domain is a hot spot crucial for the complement C2 function.
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