A novel R416C mutation in human DMT1 (SLC11A2) displays pleiotropic effects on function and causes microcytic anemia and hepatic iron overload

Abstract

A patient suffering from microcytic anemia and hepatic iron overload was found to be compound heterozygote for polymorphisms in the iron transporter DMT1 ( Nramp2, SLC11A2), including a 3-bp deletion ( DMT1 delCTT ) in intron 4 that partially impairs splicing and an amino acid substitution ( DMT1 C1246T , R416C) at a conserved residue in transmembrane domain 9 of the protein. The functional properties and possible contribution to disease of the DMT1 R416C mutation were studied in independent mutants at that position (R416C, R416A, R416K, R416E) expressed in LLC-PK 1 kidney cells. Non-conservative substitutions at R416 (C, A, E) cause multiple functional deficiencies including defective protein processing, loss of transport activity, impaired cell surface targeting, and recycling through endosomes, concomitant with retention of the transporter in the endoplasmic reticulum. Conversely, a conservative isoelectric substitution (R416K) was less vulnerable, resulting in a functional transporter that was properly processed and targeted to the cell surface and to recycling endosomes. We propose that DMT1 C1246T (R416C) represents a complete loss-of-function, and that a quantitative reduction in DMT1 expression is the cause of the microcytic anemia and iron overload in the patient.