A valine deletion of ferroportin 1: a common mutation in hemochromatosis type 4?

Hemochromatosis type 4 is an atypical hemochromatosis characterized by dominant inheritance, increased serum ferritin, normal transferrin saturation, and prevalent iron deposition in the reticuloendothelial (RE) cells rather than in hepatocytes. Heterozygousmissense mutations of the iron export protein ferroportin 1(FPN1)1-3 have been reported in 2 large pedigrees.4,5 We have characterized a 3–base pair deletion of FPN1 in 2 related Italians and in 1 unrelated British patient.

The Italian proband (patient 1 in Table1) was a 26-year-old female presenting with mild anemia and normal erythrocyte indices. Serum iron and transferrin saturation were slightly decreased, and serum ferritin levels, increased. Hemolytic anemia and inflammatory chronic diseases were excluded. Bone marrow aspirate showed normal erythroid maturation. Liver biopsy did not reveal fibrosis. Perl staining was positive especially in Kupffer cells, with few hepatocyte granules. Quantitative liver iron concentration (LIC) was obtained by magnetic susceptometry (SQUID).6,7 A low-intensity venesection protocol reduced ferritin without causing anemia. The proband's mother (patient 2) had breast cancer 3 years ago. She is in complete remission, having undergone radical mastectomy and chemotherapy and is being treated with tamoxifen. She has had persistent hyperferritinemia over the last 3 years. Liver function tests were normal. LIC was elevated. The British patient (patient 3) was a 30-year-old asymptomatic male whose father had been diagnosed with hemochromatosis. On presentation he had a raised ferritin but normal transferrin saturation. He had a slight degree of hepatomegaly and had normal liver function test results. On venesection this patient's ferritin concentration returned to the normal range without him becoming anemic. He has had 4.25 g iron removed.

All 3 patients are heterozygotes for the His63Asp mutation inHFE. Sequencing the whole FPN1 gene of the 2 Italian patients identified a heterozygous GTT deletion, corresponding to the loss of a valine in a 3 valine repeat at position 160-162 (Val162del) of the protein, likely due to a slipped strand mispairing. The same deletion in the British patient was first detected by denaturing high performance liquid chromatography (DHPLC) and then confirmed by sequence analysis.

The finding of the same mutation in families of different ethnic origin (see accompanying articles by Devalia et al,8 page 695, and Wallace et al,9 page 692) suggests that Val162del results from multiple deletion events. Due to the mechanism of this deletion, it is likely to be the most common mutation both in hemochromatosis type 4 and possibly in non-Cys282Tyr hemochromatosis. The valine triplet where the deletion occurs is highly conserved in zebrafish, mice, and rats and is in close proximity to the Asn144His substitution,4 in the putative transmembrane domain involved in iron binding or transport. The latter finding supports the hypothesis of a “loss” rather than a “gain” of function as the pathogenic mechanism of iron accumulation in hemochromatosis type 4.10 FPN1 haploinsufficiency could represent a critical threshold for iron export of macrophages, but still be sufficient to allow iron export in enterocytes. Results on venesection indicate that, under forced conditions, iron can be mobilized from RE cells.

Supported by European Union contract QLRT-1999-02237. We thank Elanko Navaratnam for technical assistance.