Autosomal dominant siderosis, also called autosomal dominant (or type 4) hemochromatosis, has a distinct clinical picture. Patients with this disorder may have iron deficiency anemia early in life, but later present with increased serum ferritin concentration and macrophage iron accumulation. Ultimately, they can have parenchymal iron deposition in addition to macrophage iron loading, probably because their total body iron exceeds the storage capacity of their macrophages.
This disorder has a very interesting pathogenesis. It is due to missense mutations in ferroportin, the cellular iron exporter. This seems paradoxical at first, because the mutations alter ferroportin function, and ferroportin acts as the basolateral enterocyte transporter involved in intestinal iron absorption (Figure 13.1). However, it is important to consider that fer-roportin also plays a major role in macrophage iron release. Apparently, loss of one functional ferroportin gene results in significant impairment of macrophage iron release, resulting in a decrease in the amount of plasma iron available to developing erythroid precursors. Iron-restricted erythropoiesis probably signals for a compensatory increase in intestinal iron absorption, which is not compromised by the loss of one ferroportin allele in the enterocytes. As a result, total body iron levels gradually rise. In this way, autosomal dominant siderosis exemplifies the meticulous balance involved in normal iron homeostasis.
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