Two Different Missense C1S Mutations, Associated to Periodontal Ehlers-Danlos Syndrome, Lead to Identical Molecular Outcomes

HMGB1 cleavage by complement C1s and its potent anti-inflammatory product

Complement C1s association with the pathogenesis of several diseases cannot be simply explained only by considering its main role in activating the classical complement pathway. This suggests that non-canonical functions are to be deciphered for this protease. Here the focus is on C1s cleavage of HMGB1 as an auxiliary target. HMGB1 is a chromatin non-histone nuclear protein, which exerts in fact multiple functions depending on its location and its post-translational modifications. In the extracellular compartment, HMGB1 can amplify immune and inflammatory responses to danger associated molecular patterns, in health and disease. Among possible regulatory mechanisms, proteolytic processing could be highly relevant for HMGB1 functional modulation. The unique properties of HMGB1 cleavage by C1s are analyzed in details. For example, C1s cannot cleave the HMGB1 A-box fragment, which has been described in the literature as an inhibitor/antagonist of HMGB1. By mass spectrometry, C1s cleavage was experimentally identified to occur after lysine on position 65, 128 and 172 in HMGB1. Compared to previously identified C1s cleavage sites, the ones identified here are uncommon, and their analysis suggests that local conformational changes are required before cleavage at certain positions. This is in line with the observation that HMGB1 cleavage by C1s is far slower when compared to human neutrophil elastase. Recombinant expression of cleavage fragments and site-directed mutagenesis were used to confirm these results and to explore how the output of C1s cleavage on HMGB1 is finely modulated by the molecular environment. Furthermore, knowing the antagonist effect of the isolated recombinant A-box subdomain in several pathophysiological contexts, we wondered if C1s cleavage could generate natural antagonist fragments. As a functional readout, IL-6 secretion following moderate LPS activation of RAW264.7 macrophage was investigated, using LPS alone or in complex with HMGB1 or some recombinant fragments. This study revealed that a N-terminal fragment released by C1s cleavage bears stronger antagonist properties as compared to the A-box, which was not expected. We discuss how this fragment could provide a potent brake for the inflammatory process, opening the way to dampen inflammation.

Degradation of collagen I by activated C1s in periodontal Ehlers-Danlos Syndrome

Periodontal Ehlers-Danlos syndrome (pEDS) is an autosomal dominant disorder characterized by early-onset periodontitis leading to premature loss of teeth, lack of attached gingiva and thin and fragile gums leading to gingival recession. Connective tissue abnormalities of pEDS typically include easy bruising, pretibial plaques, distal joint hypermobility, hoarse voice, and less commonly manifestations such as organ or vessel rupture. pEDS is caused by heterozygous missense mutations in C1R and C1S genes of the classical complement C1 complex. Previously we showed that pEDS pathogenic variants trigger intracellular activation of C1r and/or C1s, leading to extracellular presence of activated C1s. However, the molecular link relating activated C1r and C1s proteases to the dysregulated connective tissue homeostasis in pEDS is unknown. Using cell- and molecular-biological assays, we identified activated C1s (aC1s) as an enzyme which degrades collagen I in cell culture and in in vitro assays. Matrix collagen turnover in cell culture was assessed using labelled hybridizing peptides, which revealed fast and comprehensive collagen protein remodeling in patient fibroblasts. Furthermore, collagen I was completely degraded by aC1s when assays were performed at 40°C, indicating that even moderate elevated temperature has a tremendous impact on collagen I integrity. This high turnover is expected to interfere with the formation of a stable ECM and result in tissues with loose compaction a hallmark of the EDS phenotype. Our results indicate that pathogenesis in pEDS is not solely mediated by activation of the complement cascade but by inadequate C1s-mediated degradation of matrix proteins, confirming pEDS as a primary connective tissue disorder.

Oral characteristics in adult individuals with periodontal Ehlers-Danlos syndrome

AIM

Periodontal Ehlers-Danlos syndrome (pEDS) is a monogenic type of Ehlers-Danlos syndrome characterized by periodontal destruction at a young age. The present study aimed to document the oral phenotype of pEDS based on prospective clinical investigations.

MATERIALS AND METHODS

Thirty-five adult individuals from 13 families with a clinically and genetically confirmed diagnosis of pEDS underwent a systematic oral assessment.

RESULTS

Periodontitis stage 3 or 4 or edentulism due to periodontal destruction were diagnosed in 94% of the individuals. First permanent tooth loss was reported at the age of 21.5 years (median; range 13-43 years). Deep periodontal pockets were infrequent, with 94% measuring <4 mm. However, there was increased clinical attachment loss (CAL) averaging 8 mm (range 4-13 mm), and the probability of being edentate between the age of 35 and 44 years was 28-47% compared with less than 0.25% of the general population. Radiographic anomalous findings were only found in a portion of subjects and consisted of fused roots of maxillary second molars (81%), root hypoplasia (57%), taurodontism (26%) and tooth rotation of premolars (67%). As such, radiographic findings are not considered common characteristics of pEDS.

CONCLUSIONS

Characteristic oral traits of pEDS in adults are severe CAL with shallow probing depths and marked gingival recession. This is complemented by a lack of attached gingiva. These indications need to be paralleled by genetic analyses to diagnose pEDS unambiguously.

Complement C1s as a diagnostic marker and therapeutic target: Progress and propective

The molecules of the complement system connect the effectors of innate and adaptive immunity and play critical roles in maintaining homeostasis. Among them, the C1 complex, composed of C1q, C1r, and C1s (C1qr₂s₂), is the initiator of the classical complement activation pathway. While deficiency of C1s is associated with early-onset systemic lupus erythematosus and increased susceptibility to bacteria infections, the gain-of- function variants of C1r and C1s may lead to periodontal Ehlers Danlos syndrome. As C1s is activated under various pathological conditions and associated with inflammation, autoimmunity, and cancer development, it is becoming an informative biomarker for the diagnosis and treatment of a variety of diseases. Thus, more sensitive and convenient methods for assessing the level as well as activity of C1s in clinic samples are highly desirable. Meanwhile, a number of small molecules, peptides, and monoclonal antibodies targeting C1s have been developed. Some of them are being evaluated in clinical trials and one of the antibodies has been approved by US FDA for the treatment of cold agglutinin disease, an autoimmune hemolytic anemia. In this review, we will summarize the biological properties of C1s, its association with development and diagnosis of diseases, and recent progress in developing drugs targeting C1s. These progress illustrate that the C1s molecule is an effective biomarker and promising drug target.

Complement System and Alarmin HMGB1 Crosstalk: For Better or Worse

Our immune system responds to infectious (PAMPs) and tissue damage (DAMPs) signals. The complement system and alarmin High-Mobility Group Box 1 (HMGB1) are two powerful soluble actors of human host defense and immune surveillance. These systems involve molecular cascades and amplification loops for their signaling or activation. Initially activated as alarm raising systems, their function can be finally switched towards inflammation resolution, where they sustain immune maturation and orchestrate repair mechanisms, opening the way back to homeostasis. However, when getting out of control, these defense systems can become deleterious and trigger serious cellular and tissue damage. Therefore, they can be considered as double-edged swords. The close interaction between the complement and HMGB1 pathways is described here, as well as their traditional and non-canonical roles, their functioning at different locations and their independent and collective impact in different systems both in health and disease. Starting from these systems and interplay at the molecular level (when elucidated), we then provide disease examples to better illustrate the signs and consequences of their roles and interaction, highlighting their importance and possible vicious circles in alarm raising and inflammation, both individually or in combination. Although this integrated view may open new therapeutic strategies, future challenges have to be faced because of the remaining unknowns regarding the molecular mechanisms underlying the fragile molecular balance which can drift towards disease or return to homeostasis, as briefly discussed at the end.

Transcriptome Analysis of Monocytes and Fibroblasts Provides Insights Into the Molecular Features of Periodontal Ehlers-Danlos Syndrome

Periodontal Ehlers–Danlos syndrome (pEDS) is a rare hereditary disorder characterized by severe early-onset periodontitis with premature tooth loss, pretibial hyperpigmentation, and skin fragility. It is caused by mutant variants in the C1R and C1S genes that result in C4 cleavage and local complement cascade activation, as well as other possible consequences. However, the exact functional consequences of this activation remain unclear. To shed light on molecular mechanisms underlying pEDS and to identify novel molecular targets that may expand treatment strategies, we performed transcriptome profiling by RNA sequencing of monocytes and gingival fibroblasts from two patients with pEDS. Compared to normal controls, differential expression of genes was found only in monocytes but not gingival fibroblasts. Most of the significant genes were enriched in biological processes such as neutrophil-mediated immunity, response to bacterium, TNF-α and IL-17 pathway which are related to inflammation response and immune response. In disease ontology enrichment analysis, genes related to periodontal host defense, inflammatory response, skin disease, and vascular development, including MMP9, VEGFA, IL10, IL1A, IL1B, IL2RA, and IL6, were significantly enriched and also validated by qPCR and ELISA. Overall, the present study provides the transcriptomic data of pEDS for the first time and the distinct molecular features in monocytes of pEDS might serve as a tool to better understand the disease.

Complement C1s and C4d as Prognostic Biomarkers in Renal Cancer: Emergence of Noncanonical Functions of C1s

The complement system plays a complex role in cancer. In clear cell renal cell carcinoma (ccRCC), local production of complement proteins drives tumor progression, but the mechanisms by which they do this are poorly understood. We found that complement activation, as reflected by high plasma C4d or as C4d deposits at the tumor site, was associated with poor prognosis in two cohorts of patients with ccRCC. High expression of the C4-activating enzyme C1s by tumor cells was associated with poor prognosis in three cohorts. Multivariate Cox analysis revealed that the prognostic value of C1s was independent from complement deposits, suggesting the possibility of complement cascade-unrelated, protumoral functions for C1s. Silencing of C1s in cancer cell lines resulted in decreased proliferation and viability of the cells and in increased activation of T cells in in vitro cocultures. Tumors expressing high levels of C1s showed high infiltration of macrophages and T cells. Modification of the tumor cell phenotype and T-cell activation were independent of extracellular C1s levels, suggesting that C1s was acting in an intracellular, noncanonical manner. In conclusion, our data point to C1s playing a dual role in promoting ccRCC progression by triggering complement activation and by modulating the tumor cell phenotype and tumor microenvironment in a complement cascade-independent, noncanonical manner. Overexpression of C1s by tumor cells could be a new escape mechanism to promote tumor progression.See related Spotlight by Magrini and Garlanda, p. 855. See article by Daugan et al., p. 909 (40).

Functional recombinant human complement C1q with different affinity tags

Complement C1q is a multifunctional protein able to sense pathogens and immune molecules such as immunoglobulins and pentraxins, and to trigger the classical complement pathway through activation of its two associated proteases, C1r and C1s. C1q is a multimeric protein composed of three homologous yet distinct polypeptide chains A, B, and C, each composed of an N-terminal collagen-like sequence and a C-terminal globular gC1q module, that assemble into six heterotrimeric (A-B-C) subunits. This hexameric structure exhibits the characteristic shape of a bouquet of flowers, comprising six collagen-like triple helices, each terminating in a trimeric C-terminal globular head. We have produced previously functional recombinant full-length C1q in stably transfected HEK 293-F cells, with a FLAG tag inserted at the C-terminal end of C1qC chain. We report here the generation of additional recombinant C1q proteins, with a FLAG tag fused to the C-terminus of C1qA or C1qB chains, or to the N-terminus of the C1qC chain. Two other variants harboring a Myc or a 6-His tag at the C-terminal end of C1qC were also produced. We show that all C1q variants, except for the His-tagged protein, can be produced at comparable yields and are able to bind with similar affinities to either IgM, a ligand of the globular regions, or to the C1r₂-C1s₂ tetramer, and to trigger IgM-mediated serum complement activation. These new recombinant C1q variants provide additional tools to investigate the multiple functions of C1q.

Prospective clinical investigations of children with periodontal Ehlers-Danlos syndrome identify generalized lack of attached gingiva as a pathognomonic feature

PURPOSE

We report prospective clinical investigations of children affected with periodontal Ehlers-Danlos syndrome (pEDS). The main clinical features of pEDS in adults are early severe periodontitis, generalized lack of attached gingiva, and pretibial hemosiderin plaques due to dominant pathogenic variants in the C1R or C1S genes.

METHODS

Nineteen children with a parent diagnosed with molecularly confirmed pEDS underwent physical examination including oral and radiological investigations followed by genetic testing.

RESULTS

The only consistent manifestation of pEDS in childhood was a characteristic gingival phenotype: generalized lack of attached gingiva. All children with this gingival phenotype had inherited the familial pathogenic variant (n = 12) whereas the gingival phenotype was absent in children without the familial pathogenic variant (n = 7). Easy bruising was reported in eight affected and zero unaffected children. Other manifestations of pEDS were rarely present in children. Only 2/12 affected children aged 8 and 13 years fulfilled the clinical criteria for pEDS.

CONCLUSION

Generalized lack of attached gingiva is a pathognomonic feature of pEDS and the only clinical finding that is consistently present in affected adults and children. This is important because an early diagnosis may facilitate better dental hygiene in childhood, which may be essential to prevent early dental loss.