Loss of PTPN2 Activity Alters Iron Handling Protein Expression in IBD Patients and Causes Iron Deficiency in Mice

Abstract

BackgroundAnemia is the most common extraintestinal complication of inflammatory bowel disease (IBD) and is a risk factor for Crohn’s disease (CD) onset. Iron deficiency is the most common cause of anemia in IBD; however, the mechanisms involved are poorly understood. Here, we investigated the role of the IBD risk gene, protein tyrosine phosphatase non‐receptor type 2 (PTPN2), in regulating iron homeostasis.MethodsProteomic analyses were performed on serum from IBD patients genotyped for the IBD‐associated loss‐of‐function rs1893217 PTPN2variant (n=10/genotype). Constitutive Ptpn2 wild type (WT), heterozygous (Het), and knockout (KO) 3‐week‐old mice were analyzed for serum iron content, liver non‐heme iron and liver expression of the iron regulatory hormone, hepcidin (Hamp1). Protein and RNA from duodenal epithelial cells (DECs) were assayed by western blotting and qPCR. Localization of the brush border ferrous iron transporter, DMT1, in duodenal tissue was determined by immunohistochemistry (IHC).ResultsIron homeostasis genes, the iron carrier transferrin (TF) and the transferrin receptor (TFRC), were reduced (‐log p‐value = 10.7) in CD patients with the PTPN2 risk variant. Ptpn2‐KO mice had reduced i) serum iron (p<0.001; n=11); ii) serum TF saturation (p<0.01; n=9); and iii) liver non‐heme iron levels (p<0.01; n=10), vs. Ptpn2‐WT and Het mice. This indicated that PTPN2 loss decreased serum and liver iron storage. Moreover, Ptpn2‐KO mice had reduced liver expression of Hamp1 (p<0.01; n=7), likely suppressed by low serum iron. DEC gene expression of ferritin (Fth1), an intracellular iron storage molecule, was reduced (p=0.048; n=5) while Tfrc1, a mediator of basolateral cellular iron uptake, was significantly increased (p=0.0048; n=6) in Ptpn2‐KO mice. Reduced FTH1 expression (p=0.007; n=12) in DECs of Ptpn2‐KO mice was confirmed by western blot, indicating reduced intracellular iron storage. DMT1 expression was unchanged (n=6), whereas IHC showed reduced apical membrane DMT1 in duodenal epithelium of Ptpn2‐KO mice (n=6), suggesting a possible mechanism of impaired intestinal non‐heme iron uptake.ConclusionsCD patients with the PTPN2 loss‐of‐function rs1893217 SNP display alterations in serum iron handling proteins. Loss of PTPN2 activity in mice causes features of anemia including iron deficiency associated with mislocalization of the duodenal transporter DMT1. These findings identify a major role for PTPN2 in regulating iron homeostasis.