Myelodysplastic Syndromes Benefit from Iron-Restricted Bone Marrow Transplant in Preclinical Mouse Model

Background: Hematopoietic stem cell transplantation (HSCT) remains the only fully curative therapy for diseases such as b-thalassemia, sickle cell disease, MDS and AML and represents a relevant procedure for gene therapy. Tissue iron overload and non-transferrin bound iron (NTBI) have emerged as adverse prognostic factors for post-transplant survival.

Recently, we showed that – in line with observations in transplanted individuals - preparatory chemotherapy conditioning with a combination of busulfan and fludarabine in wild-type mice resulted in elevated serum iron and transferrin (Tf) saturation and elicited NTBI formation. Tf saturation and NTBI levels increased proportionally with conditioning intensity. Interestingly, we found that pre-conditioning TMPRSS6 silencing through a GalNAc-conjugated siRNA - by increasing hepcidin levels - reduced the peri-transplant elevation of Tf saturation and limited the appearance of NTBI. These results proved that NTBI formation is induced by conditioning chemotherapy in the peri-trasplant period independently of the presence of pre-existing iron overload.

Aims & Methods: Here we asked whether these observations hold true in conditions hallmarked by pre-existing iron overload and elevated NTBI and how iron restriction impacts on bone marrow transplant (BMT) outcome. To this aim, we applied the GalNAc TMPRSS6 siRNA as iron restriction strategy in a mouse model of myelodysplastic syndromes (MDS) and assessed its ability to reduce conditioning-elicited NTBI formation as well as its potential benefit on BMT. After TMPRSS6 siRNA administration, CD45.2⁺ MDS mice received conditioning chemotherapy and transplant of CD45.1⁺ wild-type bone marrow cells. Systemic iron parameters, engraftment and chimerism were then analyzed.

Results: Importantly, conditioning exacerbated the elevated systemic iron levels in MDS mice, which was prevented by pre-conditioning TMPRSS6 silencing. Similar to our observation in wild-type mice, recipient CD45.2⁺ MDS mice showed a superior blood chimerism, resulting from increased bone marrow and spleen engraftment of CD45.1⁺ donor cells. Multilineage engraftment in bone marrow, spleen and peripheral blood was superior in iron-restricted recipient MDS mice compared to controls. Finally, donor chimerism of hematopoietic stem and progenitor cells (HSPCs) was higher in iron-restricted versus control MDS mice undergoing BMT.

Conclusion: These findings demonstrate that pre-transplant conditioning profoundly alters iron homeostasis and restricting iron improves BMT outcome by increasing engraftment efficiency. Overall, these data suggest that conditioning-elicited NTBI is a critical mediator of peri-transplant toxicity and adversely impacts transplant outcome. Specifically, our observations prove that iron restriction improves donor HSPC engraftment in the recipient bone marrow, likely resulting in more efficient reconstitution of the HSPC pool and improved multilineage engraftment. Lastly, our results indicate that the peri-transplant application of novel iron restriction strategies aimed at reducing conditioning-induced NTBI – by positively affecting BMT outcome – is of potential benefit for patients undergoing BMT or gene therapy, presenting either with or without pre-transplant iron overload.