Exploring the Mechanisms of Thalassemic Erythropoiesis Improvement Caused By Bone Marrow Tfr2 Deletion

Transferrin receptor 2 (TFR2), the type 3 hemochromatosis gene, is an activator of the iron hormone hepcidin in the liver and a partner of erythropoietin (EPO) receptor in erythroid cells. The loss of bone marrow (BM) Tfr2 increases erythroblast EPO sensitivity inducing erythrocytosis in mice (Nai et al, Blood 2015). We explored whether deletion of BM Tfr2 improves anemia and ineffective erythropoiesis in β-thalassemias, iron-loading anemias due to recessive β-globin gene mutations. We generated thalassemic mice (Hbb^th3/+) with selective BM inactivation of Tfr2 (Tfr2^BMKO/Hbb^th3/+) through BM transplantation (BMT). Deletion of BM Tfr2 ameliorates RBC morphology with consistent and persistent increase of RBC count and Hb levels in thalassemic mice, accompanied by reduced iron accumulation. Around 22 weeks after BMT the improvement fades in double mutantanimals: Hb levels return comparable to those of Hbb^th3/+mice, while RBC count persists higher. Anemia improvement in double mutant mice reduces serum EPO levels and improves erythropoiesis, in particular 22 weeks after BMT. Overall these data prove that the loss of the beneficial effect of deleting Tfr2 is not accounted for by erythropoiesis failure, but likely by exhaustion of splenic iron consumed by the enhanced erythropoiesis.

In order to elucidate the molecular mechanisms of the phenotype improvement, we investigated whether the EPO-EPOR signaling pathway is overactive, as occurs in Tfr2 null erythroid cells (Nai et al, Blood 2015). Taking into account that Tfr2^BMKO/Hbb^th3/+ mice have lower serum EPO than Hbb^th3/+, the expression levels of target genes of the EPOR-JAK2-STAT5 (Erfe and Bcl-xl) and of EPOR-PI3K-AKT pathway (Fasl, Epor and Ccng2) is consistent with the signaling being inappropriately active in double mutant mice.

To start unraveling the global molecular/cellular processes underlying the remarkable phenotype amelioration, we performed RNAseq analysis on spleen samples from double mutant and Hbb^th3/+ control mice at the time point of maximal erythropoiesis improvement (22 weeks post BMT). Spleens are enlarged in both genotypes with about 80% Ter119+ (erythroid) cells in both. In total we identified 2796 genes (1997 protein coding) differentially regulated between the two genotypes. The analysis of iron-related genes reveals a strong reduction of the expression of the iron exporter Fpn, Hmox1 and Alas2, suggestive of decreased hemolysis and/or of free heme accumulation in double mutants. Gene ontology analysis reveals enrichment of genes involved in cell cycle and proliferation, mitochondrial function, as well as proteasome activity and of most of the antioxidant targets (Sod1, Sod2, Fth1, Txn1, Txn2, Gstpi) of the canonical NF-kB pathway. Underrepresented genes are those involved in lipid handling, leukocyte/lymphocyte differentiation and coagulation.

In summary, the RNAseq patterns indicate an increased spleen erythroid commitment and a mitochondrial metabolic shift, similar to the shift occurring during hematopoietic development to sustain erythroid proliferation and differentiation. We speculate that this effect is mediated by the enhanced EPO sensitivity. Interestingly EPO directly stimulates mitochondrial genes expression in adipocytes. Also the increased proteasome activity may significantly contribute to the improved erythropoiesis, since proteasomal degradation is required in the process of erythroblast enucleation. Finally, the activation of the NF-kB antioxidant response may contrast ROS increase and limit ineffective erythropoiesis.

In conclusion, targeting erythroid TFR2 might become a novel erythropoiesis stimulating approach, worth to be tested in other forms of anemia.