DMT1 Mutation in a Patient with Hypochromic Microcytic Anemia: Functional Consequences and Response to Erythropoietin.

We have previously described a case of severe hypochromic microcytic anemia caused by a homozygous mutation in the divalent metal transporter 1 ( DMT1 1285G>C ). This mutation encodes for an amino acid substitution (E399D) and causes preferential skipping of exon 12 during processing of the DMT1 mRNA. To examine the functional consequences of this mutation, full length DMT1 transcript with the patient’s point mutation or a DMT1 transcript with exon 12 deleted was expressed in Chinese hamster ovary (CHO) cells. Our results demonstrate that the E399D substitution has no effect on protein expression and function. In contrast, deletion of exon 12 led to a decreased expression of the protein and disruption of its subcellular localization and iron uptake activity. We hypothesize that the residual protein in hematopoietic cells represents the functional E399D DMT1 variant, but because of its quantitative reduction, the iron uptake activity of DMT1 in the patient’s erythroid cells is severely suppressed. Because of the positive effect of erythropoietin (EPO) on the growth of patient’s BFU-Es in our in vitro studies, we have treated the patient with recombinant human EPO. The dose of 1.2 μg/kg given once a week did not lead to any improvement in hemoglobin level. However, doubling the dose of EPO (2.4 μg/kg) led to an increase in hemoglobin level from 75 to 91 g/L and to an improvement in the patient’s clinical condition.