Inhibition of Fibroblast Growth Factor-23 (FGF-23) Rescues Bone and Hematopoietic Stem Cell Niche Defects in Beta-Thalassemia, Uncovering the Missing Link between Hematopoiesis and Bone

The bone marrow (BM) niche regulation and interactions with hematopoietic stem cells (HSC) have been extensively studied in steady state conditions and malignancies, but are still underexplored in hematological inherited disorders. We provided the first demonstration of impaired HSC function caused by an altered BM niche in a non-malignant disease, beta-thalassemia (BT) (Aprile et al., Blood 2020). BT is a globally widespread congenital hemoglobin disorder, resulting in severe anemia, ineffective erythropoiesis and multi-organ secondary complications, including bone alterations. Correction of the genetic defect is achieved by transplantation of HSC from healthy donors or autologous HSC from patients upon gene therapy. Since the quality and the engraftment of HSC depend on the BM microenvironment, niche-HSC crosstalk plays a crucial role for transplantation outcome. During the analysis of different components of the niche, we found reduced bone density in BT th3 mice, along with a defective HSC supporting activity by the BM stromal niche. Interestingly, osteoporosis is a constant hallmark in BT patients.

We investigated the mechanisms underlying bone and HSC niche defects focusing on the role of fibroblast growth factor-23 (FGF-23), a hormone mainly secreted by osteocytes, but also by erythroid cells, which negatively modulates bone metabolism. Since FGF-23 is stimulated by the anemia-related factor erythropoietin (EPO), we hypothesized that the high EPO levels in BT might contribute to increase FGF-23, potentially affecting bone and BM niche homeostasis.

We found high levels of circulating FGF-23 in th3 mice (wt vs. th3: 290.5±27.3 vs. 1823±136.1 pg/ml, p<0.0001) and in BT patients (HD vs. THAL: 94.7±1.8 vs. 117.2±5.3 RU/ml, p<0.01), associated to its increased expression by bone and BM erythroid cells. In vivo neutralization of EPO axis was sufficient to normalize FGF-23 levels (th3 vs. th3+anti-EPO: 1591±162.2 vs. 496.1±33.3 pg/ml, p<0.001), thus demonstrating the causative role of EPO. EPO stimulation and signaling inhibition strategies highlighted the involvement of Erk1/2 and Stat5 pathways in activating Fgf-23 transcription in bone and BM erythroid cells, respectively.

To provide proof of concept data on the contribution of FGF-23 to BT bone and stromal niche alterations, we inhibited FGF-23 signaling. In vivo administration of FGF-23 blocking peptide rescued the trabecular bone density in th3 mice (th3 vs. th3+FGF23inh: 138.2±4.9 vs. 166.9±5.2 mg/cm ³, p<0.01). Short-term inhibition treatment (38 hours) was sufficient to enhance bone mineralization by acting on Alkaline Phosphatase and on the expression of the main regulators of mineralization Dmp1 and Mepe by osteocyte, whereas long-term administration (12 days) restored also osteoblast number and bone deposition. Importantly, FGF-23 inhibition normalized the expression of key niche molecules, such as Jagged-1 and osteopontin, involved in the functional crosstalk between HSC and the stromal niche. Consistently, the treatment restored the frequency of th3 HSC by expanding the pool of quiescent cells (th3 vs. th3+FGF23inh: 0.026 vs. 0.045% on Lin ^neg BM cells, p<0.01). FGF-23 inhibition had also a positive anti-apoptotic effect on the expanded BM erythroid compartment (th3 vs. th3+FGF23inh: 61.6±1.3 vs. 51.1±2.1% of BM Ter119 ⁺ cells, p<0.001), promoting the maturation of early erythroid precursors (th3 vs. th3+FGF23inh: 8.5±1 vs. 16.4±1.1% of BM Pro-Erythroblasts, p<0.0001), as already shown in models of secondary anemias.

Evidence in BT patients showed negative correlations between FGF-23 levels and markers of bone homeostasis (e.g. osteocalcin R ²=0.88, p<0.05) and positive correlations with makers of ineffective erythropoiesis (e.g. circulating reticulocytes R ²=0.83, p<0.05), thus positioning FGF-23 as the molecule at the crossroads of erythropoiesis and bone metabolism in BT.

Our findings uncover an underexplored role of FGF-23 in bone and BM niche defects in a primary anemia, as a condition of chronic EPO stimulation, and propose FGF-23 as the missing link between hematopoiesis and bone regulation. The inhibition of FGF-23 signaling might provide a novel strategy to ameliorate bone compartment and restore HSC-BM niche interactions in BT by a 'two birds with one stone' approach, with a potential translational relevance in improving HSC transplantation and gene therapy.