Hyperdiploidy impairs fetal hematopoietic progenitor fitness and differentiation enabling persistence of rare preleukemic aneuploid clones.

Aneuploidy is a hallmark of cancer but often reduces cellular fitness. In childhood B cell acute lymphoblastic leukemia (cB-ALL), hyperdiploidy is the most common cytogenetic abnormality and arises in utero from early hematopoietic stem/progenitor cells (HSPCs), yet its impact on early hematopoiesis remains unclear. We model two proposed routes to hyperdiploidy, chromosome mis-segregation and cytokinesis failure, by transiently exposing human fetal liver-derived HSPCs to reversine or cytochalasin D. Induced hyperdiploidy impaired fitness and delayed differentiation in vitro, causing hyperdiploid cells to be rapidly outcompeted by euploid counterparts. Nonetheless, hyperdiploid cells engrafted immunodeficient mice, where rare clones persisted long term and acquired non-random chromosomal gains frequently observed in cB-ALL. Despite this persistence, they did not initiate leukemia. These findings support a two-step model in which hyperdiploid fetal clones require additional perinatal/postnatal events for malignant transformation. Our work establishes a valuable human model for studying early aneuploidy-driven events in childhood leukemia.