Hematopoietic stem cells (HSC) are regulated by signals from the bone marrow (BM) niche and little is known about their fate in altered hematological conditions associated to non-malignant diseases. In β-thalassemia ineffective erythropoiesis and secondary alterations, as abnormal regulation of bone metabolism, iron overload and hormonal factors, induce changes in the BM homeostasis with a potential impact on HSC-niche interaction. We addressed these unexplored issues in the murine disease model and in patients' cells. We investigated hematopoiesis in thalassemic Hbbth3/+ (th3) mutant mice and we found lower frequency, reduced quiescence and reconstituting potential of HSC. th3 HSC have impaired self-renewal, which is rescued upon transplantation in a normal BM, proving an active role of the niche microenvironment. Both stromal and hematopoietic components of the BM niche are altered in th3 mice. Consistently with the common finding of osteoporosis in patients, we found reduced bone deposition with decreased levels of parathyroid hormone (PTH), which is a key regulator of bone metabolism but also of HSC activity. Low PTH negatively affects bone deposition and expression of the Notch-ligand Jag1 by th3 mesenchymal and osteolineage cells, thus reducing the activation of Notch1 in HSC and consequently impairing their function. In vivo activation of PTH signaling through the reestablished Jag1-Notch1 pathway restores the functional pool of th3 HSC by correcting HSC-niche crosstalk. In addition to the stromal component of the BM, hematopoietic cells with a key role in regulating the fate of HSC, such as megakaryocytes (Mk), were found defective in maturation, possibly due to reduced circulating levels of thrombopoietin (TPO). We are currently investigating the molecular causes of dysmegakaryopoiesis and the Mk-HSC interaction in thalassemic mice. Strikingly, reduced HSC quiescence was confirmed in samples from patients affected by β-thalassemia, along with impaired stromal niche and megakaryopoiesis, thus highlighting the clinical relevance of our findings. Further investigation will unravel the multiple molecular mechanisms that affect in trans HSC functions in the complexity of the stressed thalassemic BM microenvironment. Our results uncover a defect of HSC in β-thalassemia, induced by an altered BM niche and provide new relevant insight for improving transplantation and gene therapy approaches. Disclosures No relevant conflicts of interest to declare.