This article by Hayun et al. investigates how ERK signaling activity in immature leukemic cells drives clonal selection during induction therapy for acute myeloid leukemia (AML). Clonal selection after standard “7+3” chemotherapy (cytarabine/Ara-C plus daunorubicin/DNR) is a major cause of relapse, yet resistant subclones do not consistently share a single genetic mutation. The authors hypothesized that the differentiation stage, reflected by CD34 expression, rather than genotype alone, influences chemotherapy resistance.
Using the Kasumi-1 AML cell line, sorted into CD34⁺ (immature) and CD34⁻ (more mature) fractions, they showed that CD34⁺ cells were significantly more resistant to Ara-C and DNR. The CD34⁺ subset exhibited higher baseline ERK1/2 phosphorylation (p-ERK) and elevated BCL-2 expression compared to CD34⁻ cells. Upon chemotherapy exposure, ERK activation increased further, particularly in the CD34⁺ fraction. Pharmacologic inhibition of MEK1/2 with U0126 sensitized CD34⁺ cells to chemotherapy-induced apoptosis, while PI3K/AKT inhibition had no significant effect, identifying the MEK/ERK pathway as a key survival signal in immature leukemic cells. Similar findings were observed in the chemotherapy-resistant OCI-AML3 cell line, where MEK inhibition markedly enhanced Ara-C– and DNR-induced apoptosis.
In primary AML samples, CD34⁺ and CD34⁻ blast populations differed not only in ERK activity and apoptosis rates but also in subclonal mutational composition (including FLT3-ITD, NPM1, and DNMT3A). However, no single mutation consistently explained resistance. Instead, CD34⁺ subsets showed higher ERK activation and distinct allelic ratios of mutations, suggesting that differentiation stage and survival signaling influence which subclones persist under chemotherapy pressure.
During induction therapy, dynamic shifts in CD34⁺ blast fractions were observed, particularly among responders, indicating real-time clonal selection. Relapsed patients frequently exhibited a dominant CD34⁺ population.
Overall, the study demonstrates that ERK activity is closely linked to leukemic cell maturation stage and promotes survival of immature (CD34⁺) AML cells during chemotherapy. Rather than specific mutations alone dictating relapse, subclones with enhanced MEK/ERK-driven survival signaling and limited differentiation capacity are preferentially selected. These findings suggest that combining standard induction chemotherapy with MEK/ERK inhibition may reduce chemoresistance and limit clonal selection, offering a potential strategy to improve AML treatment outcomes.
Journal Club: continue your reading here:
Hayun, M., Zaatra, M., Itzkovich, C. et al. ERK Activity in Immature Leukemic Cells Drives Clonal Selection during Induction Therapy for Acute Myeloid Leukemia
Sci Rep 10, 8349 (2020). DOI: 10.1038/s41598-020-65061-6