We provide an overview of the evidence for an erythropoietin-fibroblast growth factor 23 (FGF23) signaling pathway directly influencing erythroid cells in the bone marrow. We outline its importance for red blood cell production, which might add, among others, to the understanding of bone marrow responses to endogenous erythropoietin in rare hereditary anemias. FGF23 is a hormone that is mainly known as the core regulator of phosphate and vitamin D metabolism and it has been recognized as an important regulator of bone mineralization. Osseous tissue has been regarded as the major source of FGF23. Interestingly, erythroid progenitor cells highly express FGF23 protein and carry the FGF receptor. This implies that erythroid progenitor cells could be a prime target in FGF23 biology. FGF23 is formed as an intact, biologically active protein (iFGF23) and proteolytic cleavage results in the formation of the presumed inactive C-terminal tail of FGF23 (cFGF23). FGF23-knockout or injection of an iFGF23 blocking peptide in mice results in increased erythropoiesis, reduced erythroid cell apoptosis and elevated renal and bone marrow erythropoietin mRNA expression with increased levels of circulating erythropoietin. By competitive inhibition, a relative increase in cFGF23 compared to iFGF23 results in reduced FGF23 receptor signaling and mimics the positive effects of FGF23-knockout or iFGF23 blocking peptide. Injection of recombinant erythropoietin increases FGF23 mRNA expression in the bone marrow with a concomitant increase in circulating FGF23 protein. However, erythropoietin also augments iFGF23 cleavage, thereby decreasing the iFGF23 to cFGF23 ratio. Therefore, the net result of erythropoietin is a reduction of iFGF23 to cFGF23 ratio, which inhibits the effects of iFGF23 on erythropoiesis and erythropoietin production. Elucidation of the EPO-FGF23 signaling pathway and its downstream signaling in hereditary anemias with chronic hemolysis or ineffective erythropoiesis adds to the understanding of the pathophysiology of these diseases and its complications; in addition, it provides promising new targets for treatment downstream of erythropoietin in the signaling cascade.

Recently, erythropoietin (EPO) was identified as regulator of fibroblast growth factor 23 (FGF23). Proteolytic cleavage of biologically active intact FGF23 (iFGF23) results in the formation of C-terminal fragments (cFGF23). An increase in cFGF23 relative to iFGF23 suppresses FGF receptor signaling by competitive inhibition. EPO lowers the i:cFGF23 ratio, thereby overcoming iFGF23-mediated suppression of erythropoiesis. We investigated EPO-FGF23 signaling and levels of erythroferrone (ERFE) in 90 patients with hereditary hemolytic anemia (www.trialregister.nl [NL5189]). We show, for the first time, the importance of EPO-FGF23 signaling in hereditary hemolytic anemia: there was a clear correlation between total FGF23 and EPO levels (r = +0.64; 95% confidence interval [CI], 0.09-0.89), which persisted after adjustment for iron load, inflammation, and kidney function. There was no correlation between iFGF23 and EPO. Data are consistent with a low i:cFGF23 ratio. Therefore, as expected, we report a correlation between EPO and ERFE in a diverse set of hereditary hemolytic anemias (r = +0.47; 95% CI, 0.14-0.69). There was no association between ERFE and total FGF23 or iFGF23, which suggests that ERFE does not contribute to the connection between FGF23 and EPO. These findings open a new area of research and might provide potentially new druggable targets with the opportunity to ameliorate ineffective erythropoiesis and the development of disease complications in hereditary hemolytic anemias.

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Pyruvate kinase deficiency (PKD) is a rare congenital hemolytic anemia caused by mutations in the PKLR gene. Here, we review pathophysiological aspects of PKD, focusing on the interplay between pyruvate kinase (PK)-activity and reticulocyte maturation in the light of ferroptosis, an iron-dependent process of regulated cell death, and in particular its key player glutathione peroxidase 4 (GPX4). GPX4 plays an important role in mitophagy, the key step of peripheral reticulocyte maturation and GPX4 deficiency in reticulocytes results in a failure to fully mature. Mitophagy depends on lipid oxidation, which is under physiological conditions controlled by GPX4. Lack of GPX4 leads to uncontrolled auto-oxidation, which will disrupt autophagosome maturation and thereby perturb mitophagy. Based on our review, we propose a model for disturbed red cell maturation in PKD. A relative GPX4 deficiency occurs due to glutathione (GSH) depletion, as cytosolic L-glutamine is preferentially used in the form of α-ketoglutarate as fuel for the tricarboxylic acid (TCA) cycle at the expense of GSH production. The relative GPX4 deficiency will perturb mitophagy and, subsequently, results in failure of reticulocyte maturation, which can be defined as late stage ineffective erythropoiesis. Our hypothesis provides a starting point for future research into new therapeutic possibilities, which have the ability to correct the oxidative imbalance due to lack of GPX4.

Patients with hereditary hemochromatosis and non-transfusion-dependent hereditary anemia develop predominantly liver iron-overload. We present a unique method allowing quantification of liver iron retention in humans during first-pass of 59Fe-labeled iron through the portal system, using standard ferrokinetic techniques measuring red cell iron uptake after oral and intravenous 59Fe administration. We present data from patients with iron deficiency (ID; N = 47), hereditary hemochromatosis (HH; N = 121) and non-transfusion-dependent hereditary anemia (HA; N = 40). Mean mucosal iron uptake and mucosal iron transfer (±SD) were elevated in patients with HH (59 ± 18%, 80 ± 15% respectively), HA (65 ± 17%, 74 ± 18%) and ID (84 ± 14%, 94 ± 6%) compared to healthy controls (43 ± 19%, 64 ± 18%) (p < 0.05) resulting in increased iron retention after 14 days compared to healthy controls in all groups (p < 0.01). The fraction of retained iron utilized for red cell production was 0.37 ± 0.17 in untreated HA, 0.55 ± 0.20 in untreated HH and 0.99 ± 0.22 in ID (p < 0.01). Interestingly, compared to red blood cell iron utilization after oral iron administration, red blood cell iron utilization was higher after injection of transferrin-bound iron in HA and HH. Liver iron retention was considerably higher in HH and HA compared to ID. We hypothesize that albumin serves as a scavenger of absorbed Fe(II) for delivering albumin-bound Fe(III) to hepatocytes.

Background: Iron overload is an emerging and underestimated problem in management of patients with hereditary anemia characterized by chronic hemolysis or ineffective erythropoiesis in the absence of regular red cell transfusions. Liver iron overload was present in 65% of the patients with hereditary hemolytic anemia who were never transfused. (Van Straaten et al., Br J Haematol. 2018) Iron-loading results from inadequately high intestinal iron absorption in response to low hepcidin and ultimately leads to iron overload in liver and other organs. Iron chelation treatment is recommended, however drug-related toxicity is considerable, and treatment with iron chelators is expensive. Alternative treatment options for anemic patients not tolerating iron chelating agents are not available. For adequate iron absorption, a low luminal pH is required to solubilize dietary iron salts, and to promote the reduction of dietary Fe(III) to Fe(II) by ferriredutases as only Fe(II) can be absorbed. Proton pump inhibitors (PPI) block gastric acid secretion and thereby impede iron absorption. No studies are available that quantify effects of PPIs on dietary iron absorption in human. However, PPIs have shown to minimize phlebotomy requirements in patients with hereditary hemochromatosis. (Vanclooster et al., Gastroenterology. 2017) Here we report the design of the currently ongoing PPI SHINE AGAIN study that evaluates the efficacy and safety of PPI treatment in patients with non-transfusion dependent hereditary anemias. Methods: The PPI SHINE AGAIN is a phase 3, multicentre, randomized, placebo-controlled, cross-over clinical trial (Netherlands Trial Register, identifier [NL6659]). Transfusion independent adults with a form of hemolytic or dyserythropoietic anemia and mild to moderate iron overload are randomized in a 1:1 ratio to start with either esomeprazole 40 mg (administered orally BID) or placebo. Mild to moderate iron overload is defined as baseline liver iron content (LIC) of 3-15 mg Fe/g dry weight as measured by T2* MRI (MRQuantif Software Université de Rennes, https://imagemed.univ-rennes1.fr/en/mrquantif/quantif.php) without or on stable iron chelation therapy with no expected dose adjustments over the next 2 years. Additional criteria included baseline Hb ≤ 11.3 g/dL, expected to receive less than 4 red cell transfusions in the following 12 months, and no phlebotomies. The trial consists of two treatment periods of 12 months each (Figure), directly starting after baseline MRI. The primary study end-point is effectiveness of PPI treatment defined as difference in delta LIC after one-year treatment with esomeprazole compared to one-year treatment with placebo. Key secondary endpoints include tolerability, quality of life (assessed by EQ5D-5L questionnaire) and cost-effectiveness (assessed by iMTA Productivity Cost Questionnaire and iMTA Medical Consumption Questionnaire). Inclusion in the PPI SHINE AGAIN was completed in April 2019. Thirty patients are enrolled (non-transfusion dependent β-thalassemia n=10; pyruvate kinase deficiency n=8; congenital dyserythropoietic anemia n=3; sideroblastic anemia n=3; SCD n=2; HbH disease n=2; hereditary elliptocytosis n=1; G6PD deficiency n=1) with a median age of 44 years (IQR 28, 54). Results are expected Q3 2020. The trial is funded by ZonMW, The Netherlands Organization for Health and Research Development, and by the Innovatiefonds Zorgverzekeraars. Disclosures Nur: Novartis Pharmaceuticals: Consultancy. van Wijk:Agios Pharmaceuticals: Consultancy, Research Funding; RR Mechatronics: Research Funding. van Beers:Agios Pharmaceuticals, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Research Funding; RR Mechatronics: Research Funding.

Background Recently, erythropoietin (EPO) was identified as an important regulator of production and cleavage of fibroblast growth factor 23 (FGF23). Since erythroid progenitor cells express high levels of FGF23 and carry the FGF receptor, they are involved in the FGF23 metabolic pathway. FGF23 is a bone-derived hormone, a key player in phosphate and vitamin D metabolism and regulator of bone mineralization. FGF23 is formed as intact, biologically active protein (iFGF23). Proteolytic cleavage results in formation of an allegedly assumed inactive C-terminal tail FGF23 (cFGF23). FGF23-knockouts or iFGF23 blocking peptides increase erythropoiesis, reduce erythroid cell apoptosis and increase EPO mRNA and circulating EPO. By competitive inhibition, increase in cFGF23 relative to iFGF23 leads to suppression of FGF receptor signaling. (Recombinant) EPO increases the amount of circulating FGF23 (iFGF23 plus cFGF23) and, more importantly, alters the iFGF23/cFGF23 ratio in favor of cFGF23, thereby overcoming suppression of erythropoiesis by iFGF23. As recently discussed, (van Vuren et al. Front. Physiol. 2019) we hypothesize that EPO-FGF23 signaling plays a role in hereditary hemolytic anemias and is herein related to iron metabolism, inflammation and bone health. We explored this concept further in the current study. Methods FGF23 was measured in 90 patients and 10 healthy controls of the ZEbRA trial, an observational study at the UMC Utrecht on clinical sequelae and pathophysiology of rare congenital hemolytic (Netherlands Trial Register [NL5189]). An overview of relevant patient characteristics is provided in the Table. Two FGF23 assays were used: one that detects the C-terminal and therefore quantifies cFGF23 and (full-length) iFGF23 (total FGF23; Immunotopics/Quidel) and one that only detects iFGF23 (Kainos Laboratories). Chemokines/cytokines were measured simultaneously with a Luminex assay. Results We observed a strong correlation between levels of total FGF23 (cFGF23 and iFGF23) and EPO in patients with various hemolytic anemias (r=0.64, 95% confidence interval [0.09; 0.89]) (Figure). As expected, there was neither a correlation between iFGF23 and EPO values nor between iFGF23 and total FGF23. These data are consistent with a decline of the iFGF23/cFGF23 ratio in response to EPO. iFGF23 and total FGF23 were not correlated to serum calcium, phosphate, creatinine, parathyroid hormone and 25-OH vitamin D (p>0.05). We did not identify a correlation of iFGF23 and total FGF23 with bone health (hip and spine T-scores). EPO was associated with transferrin saturation (r=0.45, 95% CI [0.28; 0.63]) and ferritin (r=0.47, 95% CI [0.06; 0.79]), whereas total FGF23 did not correlate with iron status. The influence of inflammation on the EPO-FGF23 axis was clearly illustrated by higher mean total FGF23 in SCD patients (549 RU/L) compared to other patients (172 RU/L) (95% CI [87; 747]), without a difference in EPO or iFGF23. We identified a correlation of cFGF23 with CCL4 (r=0.49, 95% CI [0.15; 0.72]) and with sICAM (r=0.54, 95% CI [0.16; 0.75]), markers of endothelial dysfunction and immune players in pulmonary hypertension. (Kato et al. Br J Haematol. 2005; Sweatt et al. Circ Res. 2019) Conclusion We show for the first time the importance of EPO in FGF23 signaling in hemolytic anemias and investigated its relation with iron, inflammation and bone health. Absence of a relation between iron and FGF23 suggests that EPO is not simply an intermediary in FGF23 regulation by iron. FGF23 data of SCD patients suggest that inflammation is associated with an additional increase in FGF23 cleavage. Absence of a correlation of FGF23 with bone health might be attributable to treatment for osteoporosis in many patients. Alternatively, systemic FGF23 might underestimate paracrine signaling in the bone. Additionally, we hypothesize on a contribution of decline in the iFGF23/cFGF23 ratio to vasculopathic complications. Our hypothesis aligns with previous observations that too little or too much FGF signaling is related to endothelial dysregulation and endothelial-to-mesenchymal transition, which contributes to development of pulmonary hypertension. (Chen et al. Cell. Rep. 2012; Morrell et al. J. Am. Coll. 2009) Current findings open a new area for research and treatment in hemolytic anemias. Therapeutic targeting of the FGF23 pathway might provide an opportunity to intervene in the development of complications. Disclosures Glenthøj: Agios Pharmaceuticals, Inc.: Consultancy; Celgene: Consultancy; Alexion: Research Funding; Novartis: Consultancy; Novo Nordisk: Honoraria. van Wijk:Agios Pharmaceuticals: Consultancy, Research Funding; RR Mechatronics: Research Funding. van Beers:Novartis: Consultancy, Research Funding; Agios Pharmaceuticals, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Research Funding; RR Mechatronics: Research Funding.

Introduction Patients with hereditary hemochromatosis (HH) and non-transfusion-dependent hereditary anemia (HA NTD) both express low hepcidin levels, leading to increased intestinal iron absorption and, ultimately, predominantly parenchymal iron overload. Knowledge about iron absorption in humans stems from iron absorption or utilization studies with radio-labeled iron performed in the 60-70s from the last century. Here, we present unique data of combined absorption and utilization studies in a large cohort of patients with primary and secondary hemochromatosis. Methods We retrospectively analyzed the data from iron absorption and kinetics studies performed from 1972 until 1994 as part of routine clinical practice in patients with iron-related health problems at the University Medical Center Utrecht, the Netherlands. A radioactive tracer dose of oral (1 mg) 59Fe with 51Cr as non-absorbable indicator, or intravenous (10 µCi) 59Fe was administered. Radio-activity was measured with a whole-body counter to assess absorption and with a gamma-counter to determine radio-activity in peripheral blood samples to calculate the amount of iron utilized for red blood cell (RBC) production. Main findings Iron absorption was analyzed in 6 distinct groups with and without iron overload and iron reducing therapy (table). Iron uptake is the percentage of the iron test dose taken up by enterocytes, retention the percentage retained in the body 14 days after ingestion, and transfer the fraction of iron taken up that is retained in the body. Iron uptake, transfer and retention were significantly higher in patients with treated and untreated HH and iron deficiency (ID) compared with healthy controls (p<0.01). Notably, uptake, transfer and retention were also significantly higher in the analyses of patients with treated or untreated HA NTD (including 19 congenital sideroblastic anemia, 6 hereditary spherocytosis, 5 congenital dyserythropoietic anemia, 3 non-transfusion dependent thalassemia, 4 Hb Adana, 2 hexokinase deficiency and 1 PKD) than in analyses of healthy controls (p<0.01). Next, iron retention was used to calculate the percentage of iron utilized for RBC production after 14 days. Mean percentages of RBC iron utilization (RBCIU) after an oral iron test dose were 37% (SD 17%) in untreated HA NTD, 53% (SD 19%) in treated HA NTD, 55% (SD 20%) in untreated HH, 70% (SD 22%) in treated HH, and 99% (SD 22%) in ID patients. Surprisingly, RBCIU was lower after oral than after intravenous iron in patients with HA NTD or HH (figure 1). The difference between oral and intravenous RBCIU was expressed as percentage of intravenous RBCIU, and denominated as the LIR (liver iron retention). The LIR had a mean value of 28% (SD 26%) in untreated HH, 23% (SD 24%) in untreated HA NTD, and 16% (SD 25%) in treated HH patients, all significantly higher than the LIR of 1% (SD 22%) measured in ID patients (p<0.05). The LIR was strongly correlated to iron saturation (r=0.41; p<0.01). Main conclusions Of major interest is the observation that a substantial fraction of oral iron retained in patients with iron overload was not utilized for erythropoiesis. Under circumstances of high transferrin saturation, a part of iron transported from enterocytes into the portal circulation will be non-transferrin-bound iron (NTBI). A part of this NTBI will be available as labile plasma iron (LPI), a form of iron with high redox potential, and the capacity to rapidly cross membranes via transporters and channels. Recently it was shown that in iron-overloaded conditions LPI is almost completely taken up after passage of the liver and this is facilitated by ZIP14 in a non-transferrin-dependent way. (Jenkitkasemwong et al. Cell Metabolism, 2015: 22(1), 138-150). We therefore hypothesize that LPI produced primarily in the portal system (oral dosed iron) is primarily taken up in the liver and that LPI produced elsewhere in the circulation (intravenous dosed iron) may be taken up by other organs as well. In conclusion, our data is suggestive of the existence of significant hepatic scavenging of NTBI/LPI under iron-overloaded conditions. This could explain the distinct patterns of transfusion-dependent and transfusion-independent iron overload and we suggest that ZIP14 could facilitate this. Disclosures van Wijk: Agios Pharmaceuticals: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; RR Mechatronics: Research Funding. van Beers:RR Mechatronics: Research Funding; Bayer: Research Funding; Agios: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Research Funding; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding.