A mutation in transferrin receptor 1 that disrupts iron internalization causes a novel immunodeficiency. S. E. Boyden1,2, H. H. Jabara3,4, J. Chou3,4, N. Ramesh3,4, M. J. Massaad3,4, L. Notarangelo3,4, M. D. Fleming5, W. Al-Herz6, L. M. Kunkel2,4, R. S. Geha3,4 1) National Human Genome Research Institute, National Institutes of Health, Bethesda, MD; 2) Division of Genetics and Genomics, and the Manton Center for Orphan Disease Research, Boston Childrens Hospital, Boston, MA, and Department of Genetics, Harvard Medical School, Boston, MA; 3) Division of Immunology, Boston Childrens Hospital, Boston, MA; 4) Department of Pediatrics, Harvard Medical School, Boston, MA; 5) Department of Pathology, Boston Childrens Hospital and Harvard Medical School, Boston, MA; 6) Department of Pediatrics, Faculty of Medicine, Kuwait University, Kuwait.

   Fourteen patients in a consanguineous Kuwaiti pedigree suffered from a previously undescribed syndrome of combined immunodeficiency, intermittent neutropenia, thrombocytopenia, and mild anemia (CINTA). Patients had normal numbers of T and B cells but impaired T and B cell proliferation and immunoglobulin production, resulting in agammaglobulinemia and recurrent sinopulmonary infections. Several patients died prior to the recognition of a genetic immunodeficiency in the family; subsequently, patients were successfully treated with hematopoietic stem cell transplantation. Linkage analysis and whole genome sequencing revealed patients were homozygous for a c.58T>C missense mutation in TFRC, which encodes transferrin receptor 1 (TFR1), resulting in a p.Y20H substitution. The mutation co-segregated with the CINTA phenotype, altered a perfectly conserved residue, and is absent from variant databases and 396 ancestry-matched control subjects from Kuwait. In one obligate carrier, the mutation was the only heterozygous variant within an extended homozygous background, suggesting a recent de novo origin for the mutation and segregation within the family of both mutant and non-mutant versions of otherwise identical haplotypes. The p.Y20H mutation disrupted the internalization motif critical for TFR1 endocytosis. Correspondingly, TFR1 surface expression was markedly increased and TFR1 internalization was decreased in patient lymphocytes. Transduction of wild-type but not mutant TFRC restored transferrin uptake by patient fibroblasts, and supersaturation of transferrin by iron citrate in vitro, allowing transferrin-independent iron uptake, fully rescued patient T and B cell defects. In contrast to lymphocytes, patient erythroblasts had only a modest increase in surface TFR1 expression. The metalloreductase STEAP3, which possesses an internalization sequence similar to that of TFR1, is selectively expressed in erythroblasts and interacted with TFR1. Overexpression of murine Steap3, but not of an internalization-defective Steap3 mutant, partially rescued transferrin uptake in patient fibroblasts, suggesting that STEAP3 could provide an accessory TFR1 endocytosis signal in erythroblasts that spares patients from severe anemia. Disruption of cellular iron transport represents a novel pathogenic mechanism for primary immunodeficiencies.