Transplantation in neonate mouse recipients enhances umbilical cord blood CD34+ cells permissiveness to ETO2::GLIS2-driven transformation

Acute megakaryoblastic leukemia (AMKL) is a rare and aggressive pediatric acute myeloid leukemia subtype, frequently driven by the ETO2::GLIS2 (EG) gene fusion in de novo AMKL patients. The EG fusion confers poor prognosis and high relapse risk. In a previous study, we demonstrated that human fetal hematopoietic cells are intrinsically permissive to EG-driven transformation and that microenvironmental cues critically enhance leukemogenesis in less permissive human cord blood (CB) cells. However, most pediatric AML models are established in adult recipient mice, potentially overlooking developmental niche-specific effects. Here, we investigated whether mouse neonates provide a more permissive environment for EG-driven leukemogenesis from human CB hematopoietic stem and progenitor cells (HSPCs). Comparison of xenotransplants of EG-transduced CB CD34+ cells into sublethally irradiated NSG neonate or adult mice show that neonatal recipients are more supportive of EG-driven leukemia when cell doses are adjusted for recipient size. Neonates exhibited a higher frequency of leukemia-initiating cells (LIC), enhanced expansion of EG/GFP+ leukemic cells, and increased disease penetrance compared with adults at similar dose/size ratio, despite similar overall human hematopoietic engraftment. Leukemic cells arising in neonates and adults displayed comparable phenotypes and secondary transplantation capacities, indicating functional equivalence. Both CD34+CD38- and CD34+CD38+ CB subpopulations were sensitive to EG-driven transformation, with neonates consistently showing higher LIC frequencies than adults. Together, our findings demonstrate that the developmental stage of the host microenvironment is a critical extrinsic determinant of EG-mediated leukemogenesis. Neonatal recipients constitute a physiologically relevant and highly sensitive model for studying pediatric AMKL initiation with low cell inputs, enabling refined investigation of microenvironmental signals that promote leukemia development and may reveal novel therapeutic vulnerabilities