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

About This Grant

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.