Somatic mtDNA mutations at intermediate levels of heteroplasmy are a source of functional heterogeneity among primary leukemic cells.

Somatic mitochondrial DNA (mtDNA) mutations are frequently observed in tumors, yet their role in pediatric cancers remains poorly understood. The heteroplasmic nature of mtDNA-where mutant and wild-type mtDNA coexist-complicates efforts to define its contribution to disease progression. In this study, bulk whole-genome sequencing of 637 matched tumor-normal samples from the Pediatric Cancer Genome Project revealed an enrichment of functionally impactful mtDNA variants in specific pediatric leukemia subtypes. Collectively, the results from single-cell sequencing of five diagnostic leukemia samples demonstrated that somatic mtDNA mutations can arise early in leukemogenesis and undergo positive selection during disease progression, achieving intermediate heteroplasmy-a "sweet spot" that balances mitochondrial dysfunction with cellular fitness. Network-based systems biology analyses link specific heteroplasmic mtDNA mutations to metabolic reprogramming and therapy resistance. We reveal somatic mtDNA mutations as a potential source of functional heterogeneity and cellular diversity among leukemic cells, influencing their fitness and shaping disease progression.