In vitro functional analysis of human ferroportin (FPN) and hemochromatosis-associated FPN mutations.
Type IV hemochromatosis is associated with dominant mutations in the SLC40A1 gene encoding ferroportin (FPN). Known as the "ferroportin disease," this condition is typically characterized by high serum ferritin, reduced transferrin saturation, and macrophage iron loading. Previously FPN expression in vitro has been shown to cause iron deficiency in human cell lines and mediate iron export from Xenopus oocytes. We confirm these findings by showing that expression of human FPN in a human cell line results in an iron deficiency because of a 3-fold increased export of iron. We show that FPN mutations A77D, V162delta, and G490D that are associated with a typical pattern of disease in vivo cause a loss of iron export function in vitro but do not physically or functionally impede wild-type FPN. These mutants may, therefore, lead to disease by haploinsufficiency. By contrast the variants Y64N, N144D, N144H, Q248H, and C326Y, which can be associated with greater transferrin saturation and more prominent iron deposition in liver parenchyma in vivo, retained iron export function in vitro. Because FPN is a target for negative feedback in iron homeostasis, we postulate that the latter group of mutants may resist inhibition, resulting in a permanently "turned on" iron exporter.
Schimanski
Lisa M
LM
Molecular Immunology Group, Weatherall Institute of Molecular Medicine, Headley Way, Oxford, OX3 9DS, United Kingdom. Schimanski@hammer.imm.ox.ac.uk
Drakesmith
Hal
H
Merryweather-Clarke
Alison T
AT
Viprakasit
Vip
V
Edwards
Jon P
JP
Sweetland
Emma
E
Bastin
Judy M
JM
Cowley
Diana
D
Chinthammitr
Yingyong
Y
Robson
Kathryn J H
KJ
Townsend
Alain R M
AR
D015703:Antigens, CD; D027682:Cation Transport Proteins / Q000235:genetics* / Q000378:metabolism*; D002460:Cell Line; D005293:Ferritins / Q000378:metabolism; D006432:Hemochromatosis / Q000235:genetics*; D006801:Humans; D042541:Intracellular Space / Q000378:metabolism; D007501:Iron / Q000378:metabolism; D000090463:Iron Deficiencies; D009154:Mutation / Q000235:genetics*; D010641:Phenotype; D011485:Protein Binding; D011990:Receptors, Transferrin / Q000378:metabolism; D000098742:Ferroportin
D015703:Antigens, CD; C068126:CD71 antigen; D027682:Cation Transport Proteins; D011990:Receptors, Transferrin; D000098742:Ferroportin; D005293:Ferritins; D007501:Iron