Blau syndrome NOD2 mutations result in loss of NOD2 cross-regulatory function

Abstract

The studies described here provide an analysis of the pathogenesis of Blau syndrome and thereby the function of NOD2 as seen through the lens of its dysfunction resulting from Blau-associated NOD2 mutations in its nucleotidebinding domain (NBD). As such, this analysis also sheds light on the role of NOD2 risk polymorphisms in the LRR domain occurring in Crohn's disease. The main finding was that Blau NOD2 mutations precipitate a loss of canonical NOD2 signaling via RIPK2 and that this loss has two consequences: first, it results in defective NOD2 ligand (MDP)-mediated NF-kB activation and second, it disrupts NOD2-mediated cross-regulation whereby NOD2 downregulates concomitant innate (TLR) responses. Strong evidence is also presented favoring the view that NOD2-mediated cross-regulation is under mechanistic control by IRF4 and that failure to up-regulate this factor because of faulty NOD2 signaling is the proximal cause of defective cross-regulation and the latter's effect on Blau syndrome inflammation. Overall, these studies highlight the role of NOD2 as a regulatory factor and thus provide additional insight into its function in inflammatory disease. Mutations in the nucleotide binding domain of the CARD15 (NOD2) gene underlie the granulomatous inflammation characterizing Blau syndrome (BS). In studies probing the mechanism of this inflammation we show here that NOD2 plasmids expressing various Blau mutations in HEK293 cells result in reduced NOD2 activation of RIPK2 and correspondingly reduced NOD2 activation of NF-kB. These in vitro studies of NOD2 signaling were accompanied by in vivo studies showing that BS-NOD2 also exhibit defects in cross-regulation of innate responses underlying inflammation. Thus, whereas over-expressed intact NOD2 suppresses TNBS-colitis, over-expressed BS-NOD2 does not; in addition, whereas administration of NOD2 ligand (muramyl dipeptide, MDP) suppresses DSS-colitis in Wild Type (WT) mice it fails to do so in homozygous or heterozygous mice bearing a NOD2 Blau mutation. Similarly, mice bearing a Blau mutation exhibit enhanced anti-collagen antibody-induced arthritis. The Frontiers in Immunology frontiersin.org 01

Functional genomics in inborn errors of immunity

“…These editors are now able to make almost all possible edits and have the advantage of being compatible with highly multiplexed and massively parallel readout assays 107 . However, there are still limitations for base editors, including that multiple different variants may be introduced by a single guide and not all mutations are targetable or can be targeted at the same editing efficiency 10,86 …”

“…9 Mutations in NOD2 can lead to Blau syndrome, an IEI characterized by granulomatous arthritis, uveitis, and rash. 10 Variants of this gene are also involved in more common conditions like Crohn's disease. 11 Understanding these nuanced genetic determinants can yield significant insights into immune regulation and open avenues for targeted therapies tailored to the specific nature of the mutation and the resultant immune disorder.…”

“…Indeed, several genes implicated in IEI also play a role in more common autoimmune diseases, offering a glimpse into the shared genetic architecture of these disorders 9 . Mutations in NOD2 can lead to Blau syndrome, an IEI characterized by granulomatous arthritis, uveitis, and rash 10 . Variants of this gene are also involved in more common conditions like Crohn's disease 11 .…”

Receptor Interacting Ser/Thr-Protein Kinase 2 as a New Therapeutic Target

“…Blau syndrome (BS) and early-onset sarcoidosis (EOS) usually manifest before age 5 with skin rash, arthritis, and uveitis and are characterized by non-caseating granulomas on biopsy of affected tissues . Inhibiting the NOD2–RIPK2 pathway is a promising therapeutic strategy for autoinflammatory granulomatous diseases like BS and EOS since these disorders are related to gain-of-function mutations located in the NOD central domain of NOD2, ,− but also with loss of NOD2 negative regulation of TLR inflammatory pathways, thus causing its overactivation …”

“…88 Inhibiting the NOD2−RIPK2 pathway is a promising therapeutic strategy for autoinflammatory granulomatous diseases like BS and EOS since these disorders are related to gain-of-function mutations located in the NOD central domain of NOD2, 33,89−93 but also with loss of NOD2 negative regulation of TLR inflammatory pathways, thus causing its overactivation. 94 Other studies have mentioned RIPK2 as having a role in diseases such as leprosy, 95 experimental and rheumatoid arthritis, 96,97 and multiple sclerosis. 98…”