Comment on: Reduced digestion of circulating genomic DNA in systemic sclerosis patients with the DNASE1L3 R206C variant Free

Dear Editor, We read with great interest the article by Skaug et al. [1], recently published in Rheumatology, in which the authors focus on the functional consequences of the DNASE1L3 R206C variant, by determining its impact on the digestion of circulating genomic DNA (gDNA) in SSc patients and healthy controls. DNASE1L3, also known as DNAase γ, an endonuclease expressed predominantly by dendritic cells and macrophages, is secreted into the circulation, where it has a unique ability to digest gDNA in apoptosis-derived membrane vesicles (AdMVs). Aiming to elucidate the mechanism by which this variant increases SSc susceptibility, the authors made a new and valuable contribution and clearly confirmed in their paper that circulating gDNA is a physiological DNASE1L3 substrate. Moreover, they showed that its digestion is reduced in SSc patients carrying the R206C (rs35677470) variant. SSc is a chronic, multisystem autoimmune disease, clinically characterized by progressive skin and internal organ fibrosis, producing various clinical manifestations including inflammation, autoimmunity and vasculopathy, and exhibiting one of the highest mortality rates among rheumatic diseases [2].

DNASE1L3 rs35677470 is an important, causal variant, which has been associated previously with an increased risk for SSc as well as for SLE, RA and other autoimmune diseases [1, 3, 4]. We appreciate the opportunity given by the fascinating findings of Skaug et al. [1] to add a piece of further information regarding the functional significance of this variant from the structural/molecular biology viewpoint. Our research group previously performed an evolutionary and a structural analysis of DNASE1L3, by using three-dimension (3D) homology modeling and in silico mutagenesis, and demonstrated that the rs35677470 SNP encodes the non-conservative R206C variation, which disrupts the conserved electrostatic network that holds the elements of secondary protein structure in the appropriate position [5]. Specifically, we had shown that the amino acid R206 to E170 interaction, which is part of a salt bridge network that stabilizes two α-helices, was interrupted in the case of the R206C substitution and, as a consequence, the entire molecular architecture was affected substantially. The authors of the article under discussion mentioned that prior biochemical studies had suggested that R206C substitution reduces DNase activity and enzyme secretion [1, 6]. Consistently, our data nicely complemented these results and comprehensively evaluated further the rs35677470 variant of the shared autoimmune locus DNASE1L3, which leads to the production of an inactive form of DNaseIL3 protein. Moreover, our structural analysis explained the potential role of the R206C substitution through the modification of the placement of structural elements. As a consequence, this substitution led to an aberrant protein folding, thus affecting ultimately the biological function of the protein [5].

Altogether, prompted by the data of Skaug et al. [1], in the present letter we attempted to add new information concerning the role of the R206C variant. Considering that DNASE1L3 gene dysfunction may lead to impaired DNA breakdown and clearance from apoptotic cells, and taking into account that DNA-driven immune complexes are characteristic of SSc, in our opinion targeting the DNASE1L3 gene may be a beneficial therapeutic alternative for the better management of SSc patients. To this end, replenishment of highly efficient DNASE1L3 may be a plausible promising therapeutic strategy in SSc by restoring immune tolerance, which has broken down in SSc patients. Moreover, blockage of autoantibodies secreted against the circulating gDNA, which is a potential self-antigen, can be also a beneficial therapeutic method. Accordingly, Sisirak et al. [7] suggested a treatment with exogenous DNASE1L3 abrogated antibody binding in SLE patients, who have elevated levels of circulating microparticle-associated chromatin, an important self-antigen that may drive loss of tolerance in SLE. Furthermore, the development of a R206C DNASEIL3 replacement therapy could be considered as an alternative future approach. Notably, on a long-term experimental design, a novel murine model carrying the R206C variant of DNASE1L3 can be developed, in which mice exhibit clinical features of SSc. Based on this model, methods to treat the disease by targeting the DNASE1L3 enzyme may be investigated, upon the dissection of mechanisms that lead to the break of tolerance to self-gDNA, which is involved in SSc pathogenesis.

Data availability

Data are available upon reasonable request by any qualified researchers who engage in rigorous, independent scientific research, and will be provided following review and approval of a research proposal and Statistical Analysis Plan (SAP) and execution of a Data Sharing Agreement (DSA). All data relevant to the study are included in the article.

Funding

No specific funding was received from any funding bodies in the public, commercial or not-for-profit sectors to carry out the work described in this manuscript.

Disclosure statement: The authors have declared no conflicts of interest.

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