Effect of High Levels of Growth Differentiation Factor 15 (GDF15) On Hepcidin Expression in Monocytes of β-Thalassemia Intermedia Patients.

Abstract 4061

Poster Board III-996

Ineffective erythropoiesis in thalassemia has been associated with inappropriate suppression of hepatic synthesis of the key iron regulatory peptide hepcidin, leading to spontaneous iron overload. Hepcidin mRNA was found to be suppressed in hepatocyte cultures by high levels of growth differentiation factor 15 (GDF15) detected in sera from patients with thalassemic syndromes. GDF15 may inhibit hepcidin production by antagonizing positive regulatory cytokines such as bone morphogenic protein 6 (BMP6), shown to stimulate hepatic hepcidin expression in mouse models. Although mainly produced in the liver, human hepcidin production occurs to a lesser extent in circulating monocytes. Studies with monocytes in patients with anemia of chronic disease showed increased hepcidin expression and iron retention, but to the best of our knowledge, monocyte-derived hepcidin has not yet been characterized in iron-loading anemias such as beta-thalassemia intermedia (TI). We evaluated GDF15 plasmatic levels and correlated these to hemoglobin (Hb) levels, reticulocyte counts and gene expressions in monocytes from transfusion-independent, non-chelated TI patients, homozygous for the IVS-I-6 T→C mutation (n=18), healthy, age-matched controls with no iron deficiency or overload (n=10) and transfusion-independent sickle cell anemia (SCA) patients in steady state (n=5), as a positive control group in which hyperexpression of hepcidin in mononuclear cells has been previously demonstrated. Total RNA was extracted from monocytes isolated from peripheral blood mononuclear cells and determination of BMP6 and hepcidin gene expression was performed by Real Time Polymerase Chain Reaction. Plasma GDF15 levels were determined by ELISA. Mean TI patient Hb and serum ferritin levels were 7.05±0.21g/dL and 1846±346.4μg/L, respectively. Mean absolute reticulocyte count in TI was 163.9±21.5×10³/mm³. Mean GDF15 plasma levels differences were statistically significant among TI, SCA and healthy control groups (8390±827, 1780±460 and 196±21pg/mL, p<0.0001, respectively). Hepcidin gene expression did not differ significantly between TI and healthy control groups (0.007±0.006 vs. 0.05±0.03, p>0.05, respectively) but was elevated in our positive control SCA patient group (0.56±0.20; p=0.04). BMP6 gene expression was significantly decreased in TI patients compared to healthy controls (1.17±0.15 vs. 0.51±0.11, p=0.01, respectively). GDF15 concentrations correlated positively with reticulocyte counts (r=0.47; p=0.007) and negatively with hemoglobin levels (r=-0.74; p<0.0001) and BMP6 gene expression (r=-0.62; p=0.006). Our data show very high GDF15 plasma levels in a relatively homogenous population of patients with iron overload secondary to beta-thalassemia intermedia. Correlation of GDF15 with hematimetric parameters reinforces its relation to the degree of erythropoietic activity in beta thalassemia due to ineffective erythropoiesis. In addition, our study demonstrates that monocyte-derived hepcidin, like its hepatic counterpart, is inappropriately suppressed in iron-overloaded beta thalassemia intermedia patients in the presence of increased GDF15 production, correlated to decreased levels of BMP6 expression. This supports the possibility that GDF15/BMP interaction regulates hepcidin production in monocytes and hepatocytes in a similar manner, and further studies of monocyte-derived hepcidin regulation may prove to be a suitable, non-invasive tool for the investigation of human liver-derived hepcidin pathways in thalassemia and other iron-loading anemias.