In thalassemia major (TM), the complications of transfusional iron (Fe) overload and the benefits of chelation therapy are well defined. In sickle cell disease (SCD), by adulthood, the majority of patients will have received multiple transfusions and it is important that the consequences of transfusional iron overload are clearly defined, as they may differ from those of TM or other forms of transfusional iron overload. In the previous grant cycle, a consortium of expert hematology centers completed a natural history study to compare the frequencies of iron-related organ injury in these two disease populations. Using statistical analyses, the Multi Center Study of Iron Overload (MCSIO) established that endocrinopathies were seen almost exclusively in TM and were related to duration of transfusion. In contrast, cardiac failure was demonstrated with similar frequencies between the two populations;although, using this methodology, it was not possible to differentiate cardiac failure related to iron overload from cardiac failure related to other causes. With the recent introduction of MRI technology that directly quantifies cardiac and pituitary iron, it is now possible to utilize the MCSIO population to determine iron distribution in these heavily transfused populations, determine the biological mechanisms controlling this deposition, and ultimately to examine the relationship of iron overload to organ function. The purpose of this study is, therefore, to undertake systematic comparisons of iron distribution in multi- transfused patients with SCD and other forms of transfusionally-dependent anemias using MRI techniques, to evaluate the functional consequences of such differences, and to explore the mechanisms responsible. In particular, this project will explore the hypothesis that a chronic inflammatory state in SCD leads to hepcidin and cytokine mediated iron withholding within the RES, lower plasma NTBI levels and consequently reduced distribution of iron to the heart and pituitary in SCD, resulting in lower cardiac and endocrine morbidity in this disease. This project includes four specific aims: 1) To compare cardiac and pituitary iron content by MRI in heavily transfused SCD vs TM {or Diamond Blackfan Anemia (DBA)};2) To characterize the levels and speciation of NTBI in heavily transfused SCD vs TM (or DBA);3) To examine mediators of iron trafficking including inflammatory and regulatory cytokines (TNF-a, IL-1, IL-6, IL-10, TGF-?) and hepcidin in heavily transfused patients with SCD and TM (or DBA);4) To examine the cellular mechanisms of iron sequestration in the RES in SCD vs TM (or DBA). The study will collaborate with MCSIO Centers to recruit SCD, TM, and DBA patients with similar years of transfusion therapy and age at initiation, complete quantitative MRI to determine heart and pituitary iron overload, and obtain blood under tightly controlled conditions to determine differences in mechanisms important in controlling iron trafficking and storage (including inflammatory cytokines, GDF-15, hepcidin, non-transferrin bound iron, and monocyte/macrophage ferroportin, transferrin receptor and DMT-1 expression). Heart, pituitary and monocyte/macrophage function will be evaluated as a tertiary aim and related to iron parameters.
Despite effective chelation in thalassemia, the patients continue to have cardiac disease, multiple endocrinopathies, and a shortened life span related to iron deposition in target organs. In contrast, SCD patients, even with a similar history of iron exposure, do not show the same levels or frequencies of iron overload in target organs. By comparing iron trafficking mechanisms in these two populations and identifying factors that either limit or exacerbate deposition, it will be possible to develop new therapies that prevent iron deposition from occurring in the heart and endocrine organs, and thereby reduce morbidity and mortality in the thalassemia population.
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