Aberrant N-glycosylation in Crohn's disease: Opportunity for precision medicine

ABSTRACT Crohn’s disease (CD), and the related condition of ulcerative colitis, are chronic inflammatory bowel diseases (IBD) that impact 3.1 million Americans. While anti-cytokine therapies have revolutionized care, clinical remission is only achieved in 40% of patients with CD – a therapeutic ceiling that has not changed in 20 years. These data underscore the need for new treatment strategies. We are focused on understanding the role of defective N-glycosylation in CD as an innovative strategy to study pathophysiology and develop new therapeutics. Depending on the ancestral background, 7-25% of patients with CD carry a pathogenic, missense mutation in the manganese (Mn) transporter ZIP8 (ZIP8 A391T). ZIP8 regulates systemic Mn homeostasis via enteric and hepatobiliary Mn absorption. ZIP8 391-Thr causes a relative Mn insufficiency that impairs Mn-dependent N- glycosylation. Our central hypothesis is that CD is exacerbated by aberrant N-glycosylation in patients with ZIP8 391-Thr and that this defect can be targeted by specific, safe therapy. Supporting this hypothesis, blood Mn levels are 15% lower and complex, branched N-glycans are decreased in plasma in ZIP8 391-Thr carriers. Further, we uncovered an ileal microbiota signature that implicated altered bile acid homeostasis in ZIP8 391- Thr carriers. We generated a knock-in (KI) mouse model (Zip8 393T-KI) that recapitulates findings of these human studies. Promising human trials have shown that defects in N-glycan branching can be safely restored by raising levels of the rate-limiting metabolite UDP-N-acetylglucosamine (GlcNAc) via supplementation, sparing risks related to direct Mn supplementation. Our preliminary data in Zip8 393T-KI mice have demonstrated that GlcNAc supplementation restores N-glycan branching deficits in the gut and rescues defects in bile acid homeostasis, intestinal permeability, and colitis susceptibility. Thus, our objective is to use the Zip8 393T-KI mice to study the molecular impact of decreased N-glycan branching in the ileum and develop aberrant N- glycosylation as a therapeutic target in CD. In Aim 1, we will elucidate how ZIP8 391-Thr impacts systemic Mn homeostasis to impair Mn-dependent processes. We will use study of bile acid homeostasis, determined by the activity of the key ileal glycoprotein, apical sodium-dependent bile acid transporter (Asbt), as a model to determine the impact of decreased N-glycan branching. These mechanistic studies will be complemented by the unbiased approach of glycoproteomics to expand our understanding of the proteins and pathways perturbed by ZIP8 391-Thr and decreased N-glycan branching with studies in Zip8 393T-KI mice and ileal biopies from healthy individuals and patients with active CD. In Aim 2, we will prospectively “glyco-phenotype” patients with CD to inform on the spectrum of N-glycan branching defects in patients with CD, and we will test the efficacy of oral GlcNAc supplementation in patients with CD and ZIP8 391-Thr in a randomized, placebo-controlled, cross-over study. Together, these studies will assess the impact of aberrant N-glycosylation in CD and accelerate progress towards targeted, genotype-specific, glycosylation-directed therapies.