Iron availability for erythropoiesis is controlled by the iron‐regulatory hormone hepcidin. Increased erythropoiesis negatively regulates hepcidin synthesis by erythroferrone (ERFE), a hormone produced by erythroid precursors in response to erythropoietin (EPO). The mechanisms coordinating erythropoietic activity with iron homeostasis in erythrocytosis with low EPO are not well defined as exemplified by dominantly inherited (heterozygous) gain‐of‐function mutation of human EPO receptor (mtHEPOR) with low EPO characterized by postnatal erythrocytosis. We previously created a mouse model of this mtHEPOR that develops fetal erythrocytosis with a transient perinatal amelioration of erythrocytosis and its reappearance at 3–6 weeks of age. Prenatally and perinatally, mtHEPOR heterozygous and homozygous mice (differing in erythrocytosis severity) had increased Erfe transcripts, reduced hepcidin, and iron deficiency. Epo was transiently normal in the prenatal life; then decreased at postnatal day 7, and remained reduced in adulthood. Postnatally, hepcidin increased in mtHEPOR heterozygotes and homozygotes, accompanied by low Erfe induction and iron accumulation. With aging, the old, especially mtHEPOR homozygotes had a decline of erythropoiesis, myeloid expansion, and local bone marrow inflammatory stress. In addition, mtHEPOR erythrocytes had a reduced lifespan. This, together with reduced iron demand for erythropoiesis, due to its age‐related attenuation, likely contributes to increased iron deposition in the aged mtHEPOR mice. In conclusion, the erythroid drive‐mediated inhibition of hepcidin production in mtHEPOR mice in the prenatal/perinatal period is postnatally abrogated by increasing iron stores promoting hepcidin synthesis. The differences observed in studied characteristics between mtHEPOR heterozygotes and homozygotes suggest dose‐dependent alterations of downstream EPOR stimulation.

Congenital erythrocytoses represent a heterogenous group of rare defects of erythropoiesis characterized by elevated erythrocyte mass. We performed molecular‐genetic analysis of 21 Czech patients with congenital erythrocytosis and assessed the mutual link between chronic erythrocyte overproduction and iron homoeostasis. Causative mutations in erythropoietin receptor (EPOR), hypoxia‐inducible factor 2 alpha (HIF2A) or Von Hippel–Lindau (VHL) genes were detected in nine patients, including a novel p.A421Cfs*4 EPOR and a homozygous intronic c.340+770T>C VHL mutation. The association and possible cooperation of five identified missense germline EPOR or Janus kinase 2 (JAK2) variants with other genetic/non‐genetic factors in erythrocytosis manifestation may involve variants of Piezo‐type mechanosensitive ion channel component 1 (PIEZO1) or Ten‐eleven translocation 2 (TET2), but this requires further research. In two families, hepcidin levels appeared to prevent or promote phenotypic expression of the disease. No major contribution of heterozygous haemochromatosis gene (HFE) mutations to the erythrocytic phenotype or hepcidin levels was observed in our cohort. VHL‐ and HIF2A‐mutant erythrocytosis showed increased erythroferrone and suppressed hepcidin, whereas no overproduction of erythroferrone was detected in other patients regardless of molecular defect, age or therapy. Understanding the interplay between iron metabolism and erythropoiesis in different subgroups of congenital erythrocytosis may improve current treatment options.

We previously created and characterized a mouse model of congenital erythrocytosis with low erythropoietin (EPO) levels from a gain-of-function mutation of the human erythropoietin receptor gene (mtHEPOR) (Divoky et al. PNAS. 2001; 98:986; Divoky et al. JMM Berl. 2016; 94:597). These mice develop fetal erythrocytosis, followed by transient amelioration of erythrocytosis in perinatal life, and reappearance at 3-6 weeks of age. Similarly, erythrocytosis is observed in heterozygous mtHEPOR patients postnatally but not at birth. We previously reported dynamic changes of the erythron with iron homeostasis during ontogenesis in these mice (Kralova et al. Blood 2017; 130: 170). We observed that while perinatal mtHEPOR mice exhibit relative iron deficiency, aged mice had iron overload. Here, we evaluated developmentally-determined factors associated with hyperactivation of EPOR signaling which could cause a transition from iron deficiency (neonates) to hyperferremia and increased iron deposition (aged mice). To assess the consequences of different levels of EPOR-JAK2-STAT5 signaling, we studied hetero- and homozygous mtHEPOR mice that differ in their severity of erythrocytosis. We found that prenatally and perinatally, mtHEPOR hetero- and homozygous mice have increased erythroferrone (Erfe) transcripts and reduced hepcidin, consistent with the known inverse correlation between Erfe and hepcidin and in accordance with increased numbers of immature erythroid progenitors in the fetal hepatic circulation. At birth, previously normal Epo expression decreased and remained low in adulthood. Iron deficiency, observed in mtHEPOR hetero- and homozygotes at postnatal day 7, was likely related to increased iron consumption by augmented erythropoiesis at this stage. Postnatally, hepcidin levels increased in mutant mice, accompanied by low Erfe induction and iron accumulation in the liver and spleen as reflected by the upregulation of hepatic Bmp6 expression in mature adult (aged ~6.5 months) and old (~16 months) mtHEPOR homozygotes. We hypothesized that this could be a consequence of diminished iron consumption due to a progressive decline of erythropoiesis in mtHEPOR mice, possibly mediated by premature aging of erythroid progenitors with cell-autonomously increased proliferative history and/or increased inflammation. Indeed, young mutant erythrocytes had decreased erythrocyte survival and expression of a senescent marker CD47, an inhibitor of erythrocytes' phagocytosis. Additionally, a progressive decline in the percentage of Ter119-positive bone marrow cells and immature erythroblasts was observed in mtHEPOR hetero- and homozygotes with aging. Clonogenic assays of old mice revealed suppression of early (BFU-E) and late (CFU-E) erythroid progenitors and myeloid bias of hematopoiesis, paralleled by the up-regulation of PU.1 expression, elevation of platelet counts, and an increase in megakaryocytes chiefly in the bone marrow of mtHEPOR homozygotes. Serum levels of inflammatory cytokines did not indicate systemic inflammation; however, induced transcripts of IL-6, Inf-γ, Tgf-β, and Tnf-α, mainly in mtHEPOR homozygotes showed local bone marrow inflammatory stress. These data indicate progressive attenuation of erythroid drive in mtHEPOR homozygotes, and less so in mtHEPOR heterozygotes, paralleled by a decline in hematocrit levels with aging. In response to attenuated erythropoietic activity, iron consumption was reduced in mtHEPOR mice, leading to iron accumulation in the liver and spleen accompanied by markedly increased hepcidin synthesis. Our data suggest that even in the absence of systemic inflammation, albeit with possible paracrine inflammatory signals, known to affect bone marrow remodeling and hematopoietic aging, life-lasting prolonged activation of EPOR-JAK2-STAT5 signaling promoted exhaustion of erythroid progenitors and resulted in an age-related decline of accelerated erythropoiesis in this mouse model of congenital erythrocytosis with human gain-of-function EPOR. Grant support: Czech grant agencies projects GA17-05988S, NV19-07-00412 and LTAUSA17142, Palacky University project IGA_LF_2021_004. Disclosures No relevant conflicts of interest to declare.