Molecular basis of inherited microcytic anemia due to defects in iron acquisition or heme synthesis

Abstract

Microcytic anemia is the most commonly encountered anemia in general medical practice. Nutritional iron deficiency and beta thalassemia trait are the primary causes in pediatrics, whereas bleeding disorders and anemia of chronic disease are common in adulthood. Microcytic hypochromic anemia can result from a defect in globin genes, in heme synthesis, in iron availability or in iron acquisition by the erythroid precursors. These microcytic anemia can be sideroblastic or not, a trait which reflects the implications of different gene abnormalities. Iron is a trace element that may act as a redox component and therefore is integral to vital biological processes that require the transfer of electrons as in oxygen transport, oxidative phosphorylation, DNA biosynthesis and xenobiotic metabolism. However, it can also be pro-oxidant and to avoid its toxicity, iron metabolism is strictly controlled and failure of these control systems could induce iron overload or iron deficient anemia. During the past few years, several new discoveries mostly arising from human patients or mouse models have highlighted the implication of iron metabolism components in hereditary microcytic anemia, from intestinal absorption to its final inclusion into heme. In this paper we will review the new information available on the iron acquisition pathway by developing erythrocytes and its regulation, and we will consider only inherited microcytosis due to heme synthesis or to iron metabolism defects. This information could be useful in the diagnosis and classification of these microcytic anemias.

Figure 1.

The iron acquisition pathway in developing erythroblasts The Fe(III)-Tf complex binds to specific receptors present on the cell surface, inducing the formation of an endocytic vesicle. Acidification of the endosome will release iron from transferrin and Steap3, an erythroid-specific reductase, will reduce iron to its ferrous Fe(II) state. DMT1/Nramp2, a proton and Fe(II) co-transporter present in the endosomal membrane (see putative structure in lower insert) will export iron into the cytosol. Most of the iron will be targeted to mitochondria to participate in heme synthesis or in Fe/S cluster assembly. Mitoferrin is part of the mitochondrial iron transport machinery. Ferrochelatase, the last enzyme of the heme biosynthetic pathway, catalyzes the insertion of Fe(II) into PPIX to form heme (right insert) Illustration by Jean-Pierre Laigneau.

Figure 2.

Iron and heme sensing pathway in developing erythroblasts. The left part of the figure shows that when iron supply is adequate, heme and ISC are normally synthesized. Available Fe/S clusters will inactivate IRP1, stimulating ALAS2 synthesis and heme will inactivate HRI, allowing a full rate of globin synthesis and hemoglobin formation. Uncommitted heme or free iron will contribute to ubiquitination and degradation of IRP2 to prevent excessive iron uptake. The right part of the figure shows the effect of low iron supply on IRP2 activation and stabilization of TfR1 mRNA as well as activation of HRI that will phosphorylate eIF2α, leading to repression of globin chain translation.

Figure 3.

Decisional tree for the choice of candidate genes to sequence for the molecular diagnosis of genetic microcytic anemia. This decisional tree is based on the authors’ own experience and liable to modifications following the discovery of new forms of genetic microcytic anemia. Some genes are proposed based on mouse models but have not yet been found to be implicated in human diseases (Table 1). DMT1 has several isoforms and according to the position of the mutation can theoretically give rise solely to defective intestinal iron absorption or to defective iron uptake by erythroid precursors. Some additional diagnostic tools are recommended such as sTfRs, e-PPIX and serum hepcidin. The soluble form of TfR1 (sTfR) is formed by proteolytic cleavage of TfR1 ectodomain and sTfR levels correlate with the total mass of erythroid precursors and increase during iron deficiency. Similarly, accumulation of Zn-PPIX in erythrocytes is a good marker of defective heme synthesis due to limited iron supply or acquisition. Finally, several hepcidin assays are now available and likely to be helpful for the choice of genes to sequence. MRI: Magnetic Resonance Imaging.