Yao syndrome (YAOS) is a nucleotide-binding oligomerization domain 2 (NOD2)-associated autoinflammatory disease characterized by periodic fevers, dermatitis, polyarthritis, swelling of distal extremities, and gastrointestinal symptoms in patients with specific NOD2 variants [1]. Gastrointestinal symptoms are a near universal feature of the disease, and abdominal pain and/or diarrhea are included as one of the minor clinical criteria to diagnose YAOS [2]. My colleagues and I recently published the first comprehensive analysis of the gastrointestinal manifestations of YAOS, drawing on both patient-reported symptoms from 14 patients via a validated gastrointestinal symptom survey and an extensive retrospective analysis of gastrointestinal and hepatobiliary testing from 24 patients including blood testing, stool testing, imaging studies, endoscopic evaluations, histopathology from gastrointestinal mucosa, motility testing, and any others [3].

In this study, fourteen patients completed the validated Gastrointestinal Symptoms Rating Survey, revealing 100% prevalence of gastrointestinal symptoms, and symptoms were most commonly severe. Bloating had the highest symptoms scores. Unexpectedly, the majority of the gastrointestinal specific testing including blood and stool tests, imaging studies, and endoscopies with biopsies were normal, specifically showing no mucosal inflammation. Most motility testing was also normal, excluding significant gastrointestinal dysmotility. However, the most common abnormality identified was a heightened stool burden indicative of constipation, which was seen in 86% of abdominal radiographs and very commonly in cross-sectional imaging as well. While most transit studies were normal, 100% of patients who had undergone anorectal physiology testing had abnormal results indicative of a rectal evacuation disorder. There was a minority of patients who had comorbid inflammatory disorders affecting various parts of the gastrointestinal tract, including one with eosinophilic esophagitis, one with lymphocytic esophagitis, one with collagenous gastritis, one with celiac disease, one with an autoimmune-like enteritis, and one with collagenous colitis [3]. This study was limited by a small sample size of 24 patients, with 14 filling out the symptom survey. Furthermore, this was a retrospective study, and the patients were asked to recall symptoms at the time of symptom onset, introducing the possibility of both selection and recall biases. Nonetheless, this was the largest and most comprehensive analysis of the gastrointestinal manifestations of YAOS to date, and should help guide future investigations and research efforts to better understand why gastrointestinal symptoms and dysfunction arise. In this vein, future directions and research priorities are highlighted subsequently.

Clearly, YAOS is a heterogenous condition [4]. It is plausible that some of the heterogeneity of the disease derives from specific NOD2 variants. This is a well described phenomenon in Crohn disease (CD), an inflammatory bowel disease also associated with NOD2 variants. Indeed in CD, specific NOD2 variants hold prognostic implications. For example, the presence of variants R702W, G908R, and 1007fs is associated with more complicated disease phenotypes, specifically featuring earlier onset, stricturing and penetrating behavior, and a higher likelihood of requiring surgeries [5,6]. Similarly, the rs72796353 (IVS4+10 A>C) variant has been identified as a predictor for the development of perianal disease [7]. Accordingly, the presence of two NOD2 variants has 98% specificity for complicated CD [8]. In YAOS, the most common NOD2 variants are IVS8⁺¹⁵⁸ (c.2798 + 158C>T) and R702W (c.2104C>T, p.Arg702Trp) [2,4,9], but novel variants continue to be reported, including a patient in our study with a previously unreported c.902C > T (p.Ala301Val) variant [3]. While there has not been a consistent signal linking specific genotypes to disease activity or severity to date [4], specific genotypes have been associated with different cytokine profiles. For example, the Q902K variant is associated with activation of the NOD2-RIP2-MAPK pathway [10]. Moreover, in the setting of IVS8⁺¹⁵⁸ heterozygosity, there is elevated basal IL-6 expression, which may explain the benefit of IL-6 receptor antagonism (tocilizumab) in these patients [11]. Conversely, compound heterozygotes (IVS8⁺¹⁵⁸ plus R702W) have suppressed NF-κB and TNF-α activity [11]. A deeper understanding of the functional consequences of specific genotypes may help better risk stratify patients and target treatments, as is being actively and enthusiastically pursued in CD [12,13].

The principal limitation of our gastrointestinal manifestations study was its retrospective nature. There was heterogeneity for when in the disease course patients underwent testing, and many were already on some sort of treatment, which certainly could confound the results. Similarly, the symptom survey required patients to think back before their diagnosis (which in many cases was several years prior), introducing recall bias. And since not all patients in our YAOS cohort responded to the survey, there could also be selection bias. A more definitive approach to characterizing the gastrointestinal manifestations of YAOS is a comprehensive prospective analysis. This should include symptoms (measured from validated gastrointestinal symptom surveys) as well as objective testing such as blood and stool tests, cross-sectional imaging, endoscopy with gastrointestinal mucosal histopathology, and motility studies. This would allow for refinement of the diagnostic criteria for YAOS and earlier identification of patients.

Characterizing the symptoms is of particular importance. Currently, there is discrepancy between the diagnostic criteria and the findings from our analysis, specifically as they relate to diarrhea (one of the minor clinical criteria) [2]. Diarrhea was reported in only two out of the initial seven patients first described to have YAOS [1]. In the largest case series to date of 52 patients with YAOS, 65.4% had abdominal pain or diarrhea, so it is uncertain how frequently just diarrhea was [2]. A previous analysis of our same cohort found diarrhea reported by a striking 90.1% [4]. However, our more comprehensive investigation which identified a high prevalence of constipation and rectal evacuation disorders as well as dolichocolon actually argues for constipation over diarrhea, and that the purported diarrhea reported by patients may actually represent the overflow phenomenon [3]. An alternative explanation is that the constipation and rectal evacuation disorders develop as a compensatory response to chronic diarrhea, as has been demonstrated in other diarrheal disorders such as functional diarrhea and inflammatory bowel disease [14,15]. A prospective evaluation of patient symptoms in conjunction with objective diagnostics including imaging and anorectal physiology testing will help clarify the underlying abnormality, and potentially refine the diagnostic criteria. Related, further anatomical and motility investigations such as transit scintigraphy and imaging evaluation for dolichocolon should be undertaken to identify how and why constipation and rectal evacuation disorders arise.

Since we did not find mucosal inflammation using standard methodology, perhaps more specialized investigations are warranted, such as immunohistochemical investigation of NOD2 expression and evaluation for specific protein and/or cytokine profiles. This approach in one study identified increased small intestinal mucosal NOD2 expression in both YAOS and CD compared with healthy controls, as well as increased phosphorylated RIP2, phosphorylated NF-κB p65, and phosphorylated p38 MAPK, the latter being even greater in YAOS than in CD [10].

Similarly, symptoms potentially suggestive of mast cell activation have been described in YAOS [4]. Additionally, IL-1β receptor antagonism with canakinumab is one of the agents commonly used to treat YAOS, and is particularly efficacious for the management of gastrointestinal symptoms [2,4,10,16,17]. The clinical benefit of canakinumab may relate in part to mast cell physiology, since IL-1 directly activates mast cells and promotes an inflammatory response [18]. An increase in mucosal mast cells has been identified in several gastrointestinal disease states including CD [19] and disorders of gut-brain interaction [20]. Indeed, in CD, there is an expansion of NOD2+ mast cells, which may promote the recruitment of inflammatory cells through CXCL10 and urokinase-type plasminogen activator [21]. Mast cells are not readily visualized with routine hematoxylin and eosin staining, and require immunohistochemical assessment such as with CD117 (c-KIT), tryptase, CD25, and CD2 [22]. Notably, in one of our YAOS patients in whom we specifically requested mucosal mast cell evaluation, there was no increased mast cell density or abnormal morphology. Specific prospective evaluation of mast cells on mucosal biopsies should be considered.

Little is known at present about the pathogenesis of YAOS. It is considered a genetically transitional disease, where NOD2 variants are necessary but not sufficient for disease development. Drawing on evidence from other autoinflammatory diseases, potential triggers include infections, toxins and pollutants, physical and psychosocial stress, and lifestyle habits [23]. Accordingly, infections and vaccination have been reported as triggers of YAOS flares [4]. Further investigation into the exposome should be undertaken to understand risk factors, exposures, and epigenetic contributors to the development of YAOS. Anecdotally, many YAOS patients report infections preceding the onset of their symptoms. In CD, there has been increasing appreciation that infections may trigger the disease in a subset of patients, most notably with Epstein-Barr virus [24,25], a pathogen associated with the subsequent development of numerous autoimmune and immune-mediated inflammatory conditions including multiple sclerosis, systemic lupus erythematosus, Sjögren syndrome, rheumatoid arthritis, and others [26,27]. Preliminary findings from a phenome-wide association study at our institution have identified that NOD2 variants are associated with an increased risk of inflammatory disorders, gastroenteritis and colitis, infections, and requiring long-term antibiotics [28].

Future research should also seek to better characterize the role of the gut microbiota and microbial metabolites in YAOS. Mechanistically, the NOD2 protein functions primarily as a pattern recognition receptor expressed principally within monocytes, macrophages, dendritic cells, and the intestinal endothelium. It contributes to immune surveillance, specifically to recognize and bind muramyl dipeptide from bacterial peptidoglycan through leucine-rich repeats (LRR) [29]. NOD2 variants, especially those which affect the LRR region, can impair pathogen recognition. YAOS-associated variants have demonstrated decreased signaling in response to muramyl dipeptide, as well as an amplified and aberrant cytokine response such as increased IL-6 elaboration [11]. Dysfunction of NOD2 is strongly linked to dysbiosis, or unfavorable shifts in the structure, function, and composition of the gut microbiota, best described in CD. NOD2 has been described as the “Intestinal Gatekeeper,” [30] given its central role in regulation of the intestinal microbiota [31,32]. Dysbiosis and epithelial barrier dysfunction are hallmarks of CD, mediated in large part by NOD2 [33,34]. Taken together, dysfunctional NOD2 impairs host ability to sense and response to pathogenic microbes. This leads to reduced microbial killing and impaired production of antimicrobial compounds, allowing for overgrowth and colonization of pathobionts and pathogens. Ultimately this creates a shift in the microbiota toward a pro-inflammatory milieu characterized by destabilization of the gut epithelial barrier, metabolic endotoxemia, and incitement of an inflammatory cascade. A previous study found an association between YAOS and Whipple disease (a rare multi-system, though primarily gastrointestinal infectious disease due to infection by the Gram positive actinobacterium Tropheryma whipplei featuring intestinal malabsorption, chronic diarrhea, weight loss, abdominal pain, and extraintestinal symptoms such as fever, lymphadenopathy, endocarditis, pleuritis, and neurologic symptoms) [35]; however, our cohort did not identify any patients with Whipple disease. In light of the crucial role of NOD2 in regulation of the intestinal microbiota and the fact that many of the NOD2 variants in YAOS are also seen in CD, future research should leverage microbiome sequencing techniques to elucidate the role of the microbiota in the pathogenesis of YAOS.

YAOS is a NOD2-associated autoinflammatory disease which almost universally features gastrointestinal symptoms. There is still much unknown about the disease, but building on the knowledge of other NOD2-associated diseases, particularly CD, should help inform future research efforts. A clearer understanding of YAOS pathogenesis may be achieved with better characterization of the functional consequences of specific NOD2 variants, a prospective investigation of gastrointestinal symptoms and manifestations, and a comprehensive multi-omics evaluation to determine the role and interaction of genetics, epigenetics, intestinal microbiota, and environmental factors such as infections.

None.

Dr. Damianos has received clinical trial research funding from ExeGi Pharma as well as consulting fees, and speakers bureau fees from i-Health.