Evidence for iron overload as determined by serum ferritin concentrations was found in all polytransfused AML patients. Serum ferritin is a practical measure of body iron that can easily be applied and is supposed to represent a reliable marker of body iron stores [19
]. Although factors such as infection, acute and chronic inflammation, and alcohol abuse can exert an influence on individual values, both the stable clinical condition and the concomitant low CRP in all patients suggest that the serum ferritin mainly reflected body iron stores.
It is not surprising that serum ferritin at all time points strongly correlated with the number of blood transfusions as iron overload, commonly observed in recipients of allogeneic HCT, is mainly attributed to multiple blood transfusions. But other factors as overexpression of the growth and differentiation factor (GDF-15) which inhibits the production of hepcidin in the liver have been discussed as possible contributors to excess body iron in thalassemia [22
In this study, patients with AML compared to healthy volunteers demonstrated highly elevated serum hepcidin levels strongly correlating with the number of blood transfusions both in the pre- and post-transplant setting. This finding suggests that transfusional iron remains a dominant regulator of hepcidin in these patients and that hepcidin synthesis and regulation remain intact in polytransfused patients with AML undergoing allogeneic HCT irrespective of the number of prior chemotherapies, type of conditioning regimen, or antifungal treatments.
Usually, hepcidin and serum ferritin respond similarly to inflammation and changes in iron stores, and this is reflected in the strong correlation between hepcidin and ferritin in healthy volunteers with hepcidin responses taking place on the time scale of a few hours, whereas changes in ferritin concentrations are much slower [23
]. However, no correlation of pre- or post-HCT hepcidin levels with serum ferritin values prior to HCT was found despite the fact that both serum ferritin and hepcidin were strongly affected by the number of blood units transfused. Also, pre- and post-transplant serum ferritin and hepcidin values were statistically similar despite significantly higher cumulative blood units received after HCT. These data suggest that hepcidin represents a compensatory mechanism that counteracts transfusional iron input. Hepcidin is known to bind to ferroportin, leading to intracellular retention of iron in macrophages and to a reduction of extracellular iron bound to transferrin or serum ferritin [24
]. This raises the fascinating question of whether elevated hepcidin values in the presence of transfusional iron loading might protect from excessive parenchymal iron overload and subsequent organ damage.
An elevated hepcidin expression as a proposed cause of anemia of chronic inflammation [7
] might explain the inverse correlation of hepcidin levels with Hb values observed in this study and highlight the complexity of hepcidin regulation.
As HFE gene mutations have a high frequency in population of European descent [9
], patients in this study were screened prior to and after HCT for HFE gene mutations to determine whether HFE genotype contributed to excess iron and to assess the impact of HFE gene mutations on hepcidin concentrations. As all patients after HCT expressed full donor chimerism, the HFE genotype after HCT must be considered to reflect donor genotype. HFE deletions or mutations have been associated with a reduction of liver hepcidin expression [12
]. However, it is presently controversial whether HFE is indispensable for hepcidin activation in response to inflammatory stimuli [28
]. Mutations of HFE account for most but not all cases of hemochromatosis [31
]. The most common mutation, C282Y, is associated with disruption of a disulfide bond in HFE that is critical for its binding to β2
]. This interaction is required for the stabilisation, transport, and expression of HFE on the cell surface and endosomal membranes where HFE interacts with the transferrin receptor 1 (TfR1). The H63D mutation, a common HFE mutation whose pathogenic significance is still uncertain, does not impair interaction between HFE and TfR1. As the H63D HFE mutation was the only mutation commonly detected in our cohort, this might in part explain why HFE genotype had no influence on serum hepcidin levels.
Similarly, the missing association between HFE genotype and serum ferritin in our cohort is in accordance with published data. Veneri et al. studied the prevalence of 12 mutations of the HFE gene and its correlation with the iron status in 82 adult patients with acute leukemia of whom 58.5% were affected by acute myeloid leukemia (AML) [33
]. 32.9% of the patients had at least one HFE gene mutation and mean serum ferritin levels were increased at diagnosis. However, there was no difference between patients positive or negative for the HFE mutations in terms of serum ferritin levels. The study therefore highlighted the presence of iron overload in many AML patients but did not support the evidence of an association between HFE mutations and iron overload in acute leukemia [33
Increased pre-transplant iron overload has been demonstrated to represent negative prognostic factors in patients with myelodysplastic syndromes or secondary AML [34
], thalassemia, and other hematologic disorders such as acute leukemias [35
] receiving HCT irrespective of the type of conditioning [35
The impact of an elevated hepcidin level on outcome after HCT is not yet known. But few data suggest that pre-transplant serum hepcidin levels might predict the risk of early infectious bacterial complications after allogeneic HCT [38
Although the aim of this work was not to study the impact of hepcidin values on outcome after HCT, nevertheless no negative impact of an elevated pre-transplant hepcidin concentration on outcome, the incidence or severity of acute or chronic GVHD, or early infectious complications after HCT could be detected in this cohort. It is important to keep in mind that this was a small series of patients surviving the first three months after HCT. Nevertheless, this observation merits further investigation with a higher number of patients.