Abstract 803 Genetic Basis of Disease I Platform, Monday, 5/3

Hereditary hemochromatosis (HH) is a common autosomal recessive disorder characterized by excessive dietary iron absorption and progressive tissue iron deposition. While the mutation in the HFE gene responsible for HH has been identified, the basis for the abnormal feedback regulation of dietary iron absorption remains unknown. We determined that normal HFE protein co-localizes with and is physically associated with transferrin receptor in crypt cells of proximal small intestine, suggesting a role for HFE in uptake of transferrin-bound serum iron by these cells. We propose that loss of normal HFE impairs transferrin-mediated uptake of serum iron by the intestinal crypt cells, leading to an inappropriately low intracellular iron pool and increased expression of dietary iron transporters in daughter villus enterocytes. Based on this model, duodenal expression of the divalent metal transporter (DMT1) would be predicted to be increased in HH. Methods: We tested our prediction using a murine model of HH in which the HFE gene had been disrupted by targeted recombination. Iron status of the HFE -/- mice and HFE +/+ littermates was determined by measuring serum transferrin saturations and hepatic iron concentrations. Northern blot analyses were performed to quantify the duodenal (and hepatic) expression of each of the two classes of DMT1 transcripts: 1) DMT (IRE) which contain an iron responsive element (IRE) in the 3′ untranslated region, and 2) DMT1(non-IRE) which do not. As a positive control, duodenal expression of DMT1 was analyzed in mice with dietary iron deficiency. Results: Mice with low iron status due to dietary iron deficiency showed the expected increase in duodenal DMT1(IRE) expression. By 4 weeks of age the HFE -/- mice had elevated transferrin saturations (68.4 ± 1.8% vs 49.8 ± 4.3%, p < 0.005) and liver iron concentrations (985 ± 171 µg/g vs 381 ± 42 µg/g, p < 0.001) compared with wild-type littermates. The deposition of liver iron was predominantly hepatocellular and in a zonal gradient distribution typical of patients with HH. Despite the elevated body iron status, duodenal expression of DMT1(IRE) transcripts was 7.7 fold greater in the HFE -/- mice than HFE +/+ littermates (p = 0.01). No increase was seen in DMT1(non-IRE) transcripts. Expression of DMT1(IRE) was not increased in the liver of HFE -/- mice. Conclusion: HFE -/- mice have increased duodenal expression of DMT1 despite elevated iron status. This observation supports a model for HH in which HFE mutation leads to impaired transferrin-mediated serum iron uptake by duodenal crypt cells, an inappropriately low intracellular iron pool, increased stability of DMT1(IRE) mRNA, and increased villus expression of DMT1.

Funded by NIH DK53405 and DK41816.