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Liver disease due to alpha-1-antitrypsin deficiency (A1ATD) is associated with hepatic iron overload in a subgroup of patients. The underlying cause for this association is unknown. The aim of the present study was to define the genetics of this correlation and the effect of alpha-1-antitrypsin (A1AT) on the expression of the iron hormone hepcidin. Full exome and candidate gene sequencing were carried out in a family with A1ATD and hepatic iron overload. Regulation of hepcidin expression by A1AT was studied in primary murine hepatocytes. Cells co-transfected with hemojuvelin (HJV) and matriptase-2 (MT-2) were used as a model to investigate the molecular mechanism of this regulation. Observed familial clustering of hepatic iron overload with A1ATD suggests a genetic cause, but genotypes known to be associated with hemochromatosis were absent. Individuals homozygous for the A1AT Z-allele with environmental or genetic risk factors such as steatosis or heterozygosity for the HAMP non-sense mutation p.Arg59* presented with severe hepatic siderosis. In hepatocytes, A1AT induced hepcidin mRNA expression in a dose-dependent manner. Experiments in overexpressing cells show that A1AT reduces cleavage of the hepcidin inducing bone morphogenetic protein co-receptor HJV via inhibition of the membrane-bound serine protease MT-2. The acute-phase protein A1AT is an inducer of hepcidin expression. Through this mechanism, A1ATD could be a trigger of hepatic iron overload in genetically predisposed individuals or patients with environmental risk factors for hepatic siderosis.

Background and Aims: The genetic contribution to inflammatory bowel disease (IBD) is only partially understood and it has been suggested that rare variants that escape detection by genome-wide association studies contribute to the genetic risk in IBD. To identify novel, rare high-penetrance variants involved in disease etiology, we performed exome sequencing in cases of familial and sporadic early-onset IBD. Methods and Results: Exome sequencing revealed a novel hemizygous nonsense mutation (E99X) in the gene encoding for the Xlinked inhibitor of apoptosis protein (XIAP) in a three-year old patient with a one-year history of colonic, stricturing and fistulizing CD refractory to treatment with corticosteroids, immunosuppressants, and anti-TNFα antibodies. XIAP deficiency is associated with X-linked lymphoproliferative syndrome (XLP), a primary immunodeficiency characterized by severe susceptibility to Epstein-Barr virus (EBV) infection. In accordance with absence of XLPrelated clinical symptoms, EBV-specific antibodies and EBV DNA could not be detected in the CD patient. In addition, while XIAP has been implicated in various immunological pathways related to IBD including T cell apoptosis, Natural Killer T cell homeostasis, and inflammasome activation, relative and absolute numbers of PBMC subsets, stimulationand restimulation-induced T cell proliferation and apoptosis, and NLRP3-dependent IL-1β release were unimpaired in the CD patient. However, while XIAP-deficient PBMCs and monocytes exhibited unaltered responses to toll-like receptor 2, 3, 4, 7, and 9 stimulation, they failed to respond to NOD2 stimulation by muramyl dipeptide. Accordingly, co-immunoprecipitation studies revealed that wildtype but not E99X XIAP associated with NOD2 in a RIP2-dependent manner. Lentiviral reconstitution of XIAP expression in primary monocyte-derived dendritic cells restored NOD2 signaling and confirmed a critical role of XIAP deficiency in the observed NOD2 defects. Conclusion: Our studies reveal a novel XIAP mutation associated with early onset IBD and suggest that XIAP deficiency may contribute to disease pathogenesis via impaired NOD2 signaling. In addition, given the central role of XIAP deficiency in XLP, these results support the concept that at least a subset of IBD cases may result from congenital or acquired immunodeficiency.