Identification of a human mutation of DMT1 in a patient with microcytic anemia and iron overload

Abstract

Divalent metal transporter 1 (DMT1) is a transmembrane protein crucial for duodenal iron absorption and erythroid iron transport. DMT1 function has been elucidated largely in studies of the mk mouse and the Belgrade rat, which have an identical single nucleotide mutation of this gene that affects protein processing, stability, and function. These animals exhibit hypochromic microcytic anemia due to impaired intestinal iron absorption, and defective iron utilization in red cell precursors. We report here the first human mutation of DMT1 identified in a female with severe hypochromic microcytic anemia and iron overload. This homozygous mutation in the ultimate nucleotide of exon 12 codes for a conservative E399D amino acid substitution; however, its predominant effect is preferential skipping of exon 12 during processing of pre-messenger RNA (mRNA). The lack of fulllength mRNA would predict deficient iron absorption in the intestine and deficient iron utilization in erythroid precursors; however, unlike the animal models of DMT1 mutation, the patient is iron overloaded. This does not appear to be due to up-regulation of total DMT1 mRNA. DMT1 protein is easily detectable by immunoblotting in the patient's duodenum, but it is unclear whether the protein is properly processed or targeted. IntroductionThe last 5 years have been marked by an explosion of knowledge in the field of iron metabolism. These discoveries include identification of the proteins involved in iron absorption and trafficking; discovery of a small peptide, hepcidin, which appears to regulate iron uptake; and elucidation of the role of iron in gene expression. One of the newly identified proteins is divalent metal transporter 1 (DMT1), which is expressed at the brush border of enterocytes in the proximal duodenum where it is presumed to mediate pHdependent uptake of ferrous iron from the gut lumen. 1,2 In the erythroblast, the protein is present in the endosomal membrane, where it appears to function in transport of iron released from the transferrin-receptor complex toward the mitochondria, site of heme synthesis, 3 via a pathway that remains to be defined. In rodent models, mutation of a single nucleotide results in substitution of Arg for Gly at position 185 (G185R) of the DMT1 gene and leads to hypochromic/microcytic anemia and iron deficiency. 4,5 The predicted structure of the protein includes 12 transmembrane domains, asparagine-linked glycosylation signals in an extracytoplasmic loop, membrane targeting motifs, and a consensus transport motif that is common to homologous cation transport proteins found in other species. 6,7 Four major mammalian DMT1 isoforms, resulting from alternative splicing at the 5Ј and 3Ј ends of pre-messenger RNA (mRNA) are known. 8 Isoform I has an iron responsive element (IRE) in the 3Ј untranslated region, whereas isoform II lacks the IRE, and the C-terminal 18 amino acids are replaced by a novel 25-amino acid segment. 6 The duodenum expresses primarily isoform I, whereas erythroblasts express chiefly isoform II; o...

Manganese transport in eukaryotes: The role of DMT1

“…Both animal species exhibit severe microcytic anemia that is associated with an impairment of Fe transport as well as an alteration in Mn homeostasis. Similarly, humans with DMT1 mutations at different intron or exon sites, such as an E399D substitution (Mims et al, 2005;Lam-Yuk-Tseung et al, 2005a;Priwitzerova et al, 2005), a R416C substitution Lam-YukTseung et al, 2006) or a G212V substitution (Beaumont et al, 2006) exhibited hypochromic microcytic anemia, proposing a possible commonality in DMT1 function amongst various mammalian species.…”

Functional consequences of the human DMT1 (SLC11A2) mutation on protein expression and iron uptake

“…We previously reported a Czech female with severe hypochromic microcytic anemia and iron overload caused by a homozygous mutation in the DMT1 gene (1285G Ͼ C) that changes Glu 399 to Asp (E399D). 9,10 This single nucleotide substitution also causes preferential skipping of exon 12 during mRNA processing. As a consequence, there are 2 different DMT1 transcripts present in the patient's cells: a full-length transcript containing the point mutation and that missing exon 12; the later version comprising 90% of the total DMT1 mRNA.…”

Iron overload and prolonged ingestion of iron supplements: Clinical features and mutation analysis of hemochromatosis‐associated genes in four cases

“…A mutation in the ironresponsive element of the ferritin heavy-chain gene (FTH1) accounted for iron overload in some members of a Japanese family [37]. In contrast, the sole mutation in DMT1 reported in humans was associated with iron overload and loss of function of DMT1 [38]. Persons with DMT1 mutations could have less risk to develop iron overload due to ingestion of iron supplements than normal persons, although this is unproven.…”