Acute lymphoblastic leukemia (ALL) in infants is an aggressive malignancy with a poor clinical outcome, and is characterized by translocations of the Mixed Lineage Leukemia (MLL) gene. Previously, we identified RAS mutations in 14-24% of infant ALL patients, and showed that the presence of a RAS mutation decreased the survival chances even further. We hypothesized that targeting the RAS signaling pathway could be a therapeutic strategy for RAS-mutant infant ALL patients. Here we show that the MEK inhibitors Trametinib, Selumetinib and MEK162 severely impair primary RAS-mutant MLL-rearranged infant ALL cells in vitro. While all RAS-mutant samples were sensitive to MEK inhibitors, we found both sensitive and resistant samples among RAS-wildtype cases. We confirmed enhanced RAS pathway signaling in RASmutant samples, but found no apparent downstream over-activation in the wildtype samples. However, we did confirm that MEK inhibitors reduced p-ERK levels, and induced apoptosis in the RAS-mutant MLL-rearranged ALL cells. Finally, we show that MEK inhibition synergistically enhances prednisolone sensitivity, both in RASmutant and RAS-wildtype cells. In conclusion, MEK inhibition represents a promising therapeutic strategy for MLL-rearranged ALL patients harboring RAS mutations, while patients without RAS mutations may benefit through prednisolone sensitization.

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Keywords Leukemia, MEK inhibitors, MLL-rearrangements, RAS-pathway
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Journal Oncotarget
Kerstjens, M. (Mark), Driessen, E.M.C, Willekes, M, Pinhanços, S.S. (Sandra S.), Schneider, P, Pieters, R, & Stam, R.W. (2017). MEK inhibition is a promising therapeutic strategy for MLL-rearranged infant acute lymphoblastic leukemia patients carrying RAS mutations. Oncotarget, 8(9), 14835–14846. doi:10.18632/oncotarget.11730