Dasatinib (BMS-354825), a novel dual SRC/BCR-ABL kinase inhibitor, exhibits greater potency than imatinib mesylate (IM) and inhibits the majority of kinase mutations in IM-resistant chronic myeloid leukemia (CML). We have previously demonstrated that IM reversibly blocks proliferation but does not induce apoptosis of primitive CML cells. Here, we have attempted to overcome this resistance with dasatinib. Primitive IM-resistant CML cells showed only single-copy BCR-ABL but expressed significantly higher BCR-ABL transcript levels and BCR-ABL protein compared with more mature CML cells (P = .031). In addition, CrKL phosphorylation was higher in the primitive CD34⁺CD38^– than in the total CD34⁺ population (P = .002). In total CD34⁺ CML cells, IM inhibited phosphorylation of CrKL at 16 but not 72 hours, consistent with enrichment of an IM-resistant primitive population. CD34⁺CD38^– CML cells proved resistant to IM-induced inhibition of CrKL phosphorylation and apoptosis, whereas dasatinib led to significant inhibition of CrKL phosphorylation. Kinase domain mutations were not detectable in either IM or dasatinib-resistant primitive CML cells. These data confirm that dasatinib is more effective than IM within the CML stem cell compartment; however, the most primitive quiescent CML cells appear to be inherently resistant to both drugs.

Key Points: Targeting RAD52 DNA binding domain I by peptide aptamer induces synthetic lethality in BRCA-deficient leukemias. Individual patients with BRCA-deficient leukemias could be identified by genetic and epigenetic profiling.

Clonal hematopoiesis of indeterminate potential (CHIP) increases rapidly in prevalence beyond age 60 and has been associated with increased risk for malignancy, heart disease and ischemic stroke. CHIP is driven by somatic mutations in hematopoietic stem and progenitor cells (HSPCs). Because mutations in HSPCs often drive leukemia, we hypothesized that HSPC fitness substantially contributes to transformation from CHIP to leukemia. HSPC fitness is defined as the proliferative advantage over cells carrying no or only neutral mutations. If mutations in different genes lead to distinct fitness advantages, this could enable patient stratification. We quantified the fitness effects of mutations over 12 years in older age using longitudinal sequencing and developed a filtering method that considers individual mutational context alongside mutation co-occurrence to quantify the growth potential of variants within individuals. We found that gene-specific fitness differences can outweigh inter-individual variation and, therefore, could form the basis for personalized clinical management.