A mouse model of juvenile hemochromatosis

Abstract

Hereditary hemochromatosis is an iron-overload disorder resulting from mutations in proteins presumed to be involved in the maintenance of iron homeostasis. Mutations in hemojuvelin (HJV) cause severe, early-onset juvenile hemochromatosis. The normal function of HJV is unknown. Juvenile hemochromatosis patients have decreased urinary levels of hepcidin, a peptide hormone that binds to the cellular iron exporter ferroportin, causing its internalization and degradation. We have disrupted the murine Hjv gene and shown that Hjv-/- mice have markedly increased iron deposition in liver, pancreas, and heart but decreased iron levels in tissue macrophages. Hepcidin mRNA expression was decreased in Hjv-/- mice. Accordingly, ferroportin expression detected by immunohistochemistry was markedly increased in both intestinal epithelial cells and macrophages. We propose that excess, unregulated ferroportin activity in these cell types leads to the increased intestinal iron absorption and plasma iron levels characteristic of the juvenile hemochromatosis phenotype.

Figure 1

Elevated tissue iron content in Hjv^–/– mice. Non-heme tissue iron content (μg/g wet wt) was compared in wild-type (+/+, n = 6), Hjv+/– (+/–, n = 7), and Hjv^–/– (^–/–, n = 4) mice at 6 weeks of age. *P < 0.05 between Hjv^–/– and wild-type mice; Student’s t test.

Figure 2

Histological examination of iron loading. Tissue iron was detected by staining with Perls Prussian blue (blue stain) or DAB-enhanced Perls stain (brown stain). (A) Wild-type liver. (B) Hjv^–/– liver. (C) Wild-type spleen. (D) Hjv^–/– spleen. (E and G) Wild-type proximal duodenum. (F and H) Hjv^–/– proximal duodenum. Duodendal photomicrographs were taken approximately 0.5 cm from the pylorus. Original magnification, ×100 (C–F) and ×400 (A, B, G, and H).

Figure 3

Expression of hepcidin mRNA. Hepatic mRNA was analyzed on a Northern blot probed for hepcidin and actin. Results are shown for 3 wild-type mice (left 3 lanes) and 3 Hjv^–/– mice (right 3 lanes).

Figure 4

Increased ferroportin expression in Hjv^–/– mice. Ferroportin expression was detected by immunohistochemistry. (A and C) Wild-type proximal duodenum and (B and D) Hjv^–/– proximal duodenum, approximately 0.5 cm from the pylorus, which is shown on the right in A and B. Staining is most intense proximally in the wild-type animals but is expressed more intensely and along the distal two-thirds of the villus in the mutant animals. (E) Wild-type liver. (F) Hjv^–/– liver. (G) Wild-type spleen. (H) Hjv^–/– spleen. Note the intense Kupffer cell and macrophage staining in the Hjv^–/– liver (F) and spleen (H). Original magnification, ×20 (A and B), ×100 (C, D, G, and H), and ×400 (E and F).