NOD2 in Crohn's Disease-Unfinished Business

Abstract

Studies of Crohn's disease have consistently implicated NOD2 as the most important gene in disease pathogenesis since first being identified in 2001. Thereafter, genome-wide association, next-generation sequencing and functional analyses have all confirmed a key role for NOD2, but despite this, NOD2 also has significant unresolved complexity. More recent studies have reinvigorated an early hypothesis that NOD2 may be a single-gene cause of disease, and the distinct ileal stricturing phenotype seen with NOD2-related disease presents an opportunity for personalized diagnosis, disease prediction and targeted therapy. The genomics of NOD2 has much that remains unknown, including the role of rare variation, phasing of variants across the haplotype block and the role of variation in the NOD2-regulatory regions. Here, we discuss the evidence and the unmet needs of NOD2 research, based on recently published evidence, and suggest methods that may meet these requirements.

Keywords: NOD2; Crohn’s disease; IBD; genetics; genomics.

Figure 1.

Comparison of normal intestinal immune function with NOD2 variant-induced immune dysregulation triggering impaired bacterial clearance and chronic increased alternative inflammatory pathways. MDP, muramyl dipeptide.

Figure 2.

The major NOD2 signalling pathway resulting in activation of NF-κB and additional proinflammatory signalling through the NLRP3 inflammasome and induction of autophagy through ATG16L1. Variants in NOD2 have been demonstrated to impact on: [1] deletions/stop gains prevent transcription or localization, and [2] leucine-rich repeat [LRR] variants such as G908R and L1007fs are located in the LRR and result in defective binding of MDP, probably preventing NOD2 oligomerization and downstream signalling. Similarly, the R702W mutant is in the NOD domain and may also directly result in defective oligomerization: [3] impair ATG16L1-induced autophagy, and [4] CARD domain variants may reduce RIPK2 association with activated NOD2. Precise details of the impact of specific variants are given in Table 2. The normal downstream sequalae of NOD2 activation can be seen in this figure, which are variably impaired when patients harbour NOD2 variants functionally implicated in Crohn’s disease, resulting in impaired immune response to bacteria, barrier breakdown, dysbiosis and chronic inflammation.

Figure 3.

Potential mechanism by which variants inherited on different chromosomes [in trans] will lead to two dysfunctional copies of the gene, whereas the same variants inherited on the same chromosome [in cis] would result in a preserved functional version of NOD2 as there is a still a fully functional copy of the NOD2 gene present. Typically, this would be elucidated through segregation analysis, where sequencing of the mother and father allows ascertainment of whether the variants are inherited on the same chromosome or separately on a maternal or paternal chromosome.