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Iron Refractory Iron Deficency Anaemia (IRIDA)


Iron Refractory Iron Deficency Anaemia
(IRIDA)

Rare auosomal recessive cases of hypochromic microcytic anaemia have been described caused by inherited mutations of matriptase 2, which allow uninhibited hepcidin secretion, or DMT‐1 genes (Figs 3.4 and 3.5). There may be a haemaological response to intravenous but usually not to oral iron.
(a) Hepcidin reduces iron absorption and release from macrophages by stimulating degradation of ferroportin. Its synthesis is increased by transferrin saturation and inflammation but reduced by increased erythropoiesis, erythropoietin, hypoxia and matriptase. (b) The proposed mechanism by which the degree of transferrin saturation by iron affects hepcidin synthesis. BMP, bone morphogenetic protein; HJV, hemojuvelin; TFR1 and 2, transferrin receptor 1 and 2. BMP stimulates hepcidin synthesis and this is enhanced by HJV binding to BMP. Diferric transferrin competes with TFR1 for binding of HFE. The more diferric transferrin, the less TRF1 is bound to HFE and more HFE is available to bind to TFR2. The HFE/TFR2 complex promotes HJV binding to BMP and so promotes hepcidin synthesis. Low concentrations of diferric transferrin, as in iron defciency, allow HFE binding to TFR1, reducing the amount of HFE able to bind TFR2 and thus reducing HJV binding to BMP and so reducing hepcidin secretion. HFE also enhances BMP expression directly but mutated HFE (see Chapter 4) inhibits its expression.

The regulation of iron absorption. Dietary ferric (Fe3+) iron is reduced to Fe2+ and its entry to the enterocyte is through the divalent cation binder DMT‐1. Its export into portal plasma is controlled by ferroportin. It is oxidized before binding to transferrin in plasma. Haem is absorbed after binding to its receptor protein.