RUNX1 Alterations in Pediatric Myeloid Malignancies: Divergent Germline and Somatic Trajectories

RUNX1 alterations contribute to pediatric myeloid malignancies through both germline predisposition syndromes and somatic leukemogenic events, but their clinical and biological significance in children remains incompletely defined. This retrospective single-center study evaluated six pediatric patients with myelodysplastic syndromes or acute leukemias harboring RUNX1 variants, integrating clinical, cytogenetic, and targeted next-generation sequencing data, with germline status confirmed using non-hematopoietic tissues. Three patients carried germline RUNX1 variants, characterized by antecedent cytopenias, dysplastic features, and increased treatment-related toxicity, including severe infections, persistent cytopenias, and transplant-related mortality. In contrast, somatic RUNX1 alterations were associated with overt high-risk disease, frequently accompanied by complex cytogenetics or monosomy 7, and demonstrated heterogeneous outcomes ranging from sustained remission to post-transplant relapse. Mixed-phenotype acute leukemia was observed in both groups. These findings support a model of RUNX1-driven leukemogenesis, in which germline and somatic alterations represent distinct yet interconnected trajectories, while highlighting the importance of distinguishing variant origin for risk stratification, donor selection, and therapeutic decision-making in pediatric myeloid malignancies. Given the small cohort size, the findings remain descriptive and require validation in larger prospective studies.