The Wee1 Inhibitor MK1775 In Combination With Cytarabine (AraC) Has Potent Activity In AML By Completely Abrogating DNA Damage and Cell Cycle Checkpoint Repair Capacity Via The Mrn/NBS1 Complex

Cytarabine (AraC) resistance is a fundamental feature of refractory/relapsed AML. RNA interference (RNAi) screens conducted in our laboratory recently identified WEE1 kinase (WEE1) as one of the top candidate genes and target in leukemias in combination with AraC. WEE1 is a tyrosine kinase belonging to the Ser/Thr family of protein kinases and acts as a negative regulator of mitotic entry by controlling DNA damage (DDR) and cell cycle checkpoint responses. The WEE1 inhibitor MK1775 potently synergizes with AraC ex vivo and in vitro and clinical trials are in preparation. However, the mechanism of action for the anti-leukemic activity of MK1775 with AraC remains unknown. To elucidate genes mediating activity of the combination, we first performed siRNA rescue screens silencing a custom set of 44 genes involved in WEE1 regulation under combined AraC + MK1775 to identify sensitizers and markers of resistance. The MRN (MRE11, Rad51, NBS1) complex and particularly NBS1 were potent modifiers of AraC and MK1775.

Focusing on NBS1 since it is proposed to centrally regulate the defense capacity of leukemic cells, we identified that NBS1 phosphorylation at Ser343 (the ATM regulation site) is significantly altered both in cell lines and primary AML samples under combined AraC+MK1775 treatment as compared to single agent MK1775. In parallel, lower phosphorylation of ATM^S1981(an autophosphorylation site in response to DNA strand breaks), was observed indicating that the ATM-CHEK1 pathway is not activated under co-treatment. Further Homologous recombination (HR)-mediated repair was compromised by AraC+MK1775 shown by DR-GFP expression vector to measure intracellular HR capacity: post-transfection of the I-SceI nuclease which cleaves non-functioning GFP tandem repeats to form a functional GFP unit, the HR was reduced with the combination. Consistently other HR markers decreased as well.

Delayed accumulation of Cyclin A (indicative of S-phase progression) and greater inhibition of phospho-Cdk2^Y15in synchronized cells treated with AraC + MK1775 in comparison to controls was observed. In addition the cell cycle was globally dysregulated by slower S-phase kinetics (progression), a completely abrogated G2/M checkpoint/phase as well as de-regulated DNA replication origin formation and firing as evidenced by Cdt1 and Mus81. As a consequence high single and double strand breaks (ɣH2AX) were observed with an increase in phospho-histone H3 in AraC + MK1775 treated cells compared to untreated cells or MK1775 single agent, confirming faster mitotic entry. Changes were followed by massive induction of apoptosis.

Since WEE1 is implicated in leukemic stem cell maintenance we examined the long term effects of the combination in colony forming assays. AraC + MK1775 treated leukemic cells obtained from patients with AML were re-plated on Methocult after drug washout and colonies counted after 14 days. While MK1775 as a single agent could reduce colony formation by 4 fold compared to controls and lower dose AraC, co-treatment with low to moderate doses of AraC and MK1775 reduced colony formation by more than 7 fold and to almost zero in some primary specimens.

Taken together, these results suggest that leukemia cells co-treated with AraC + MK1775 lost their ability to activate DNA damage and repair pathways mainly by compromising the MRN complex via NBS1 with subsequently reduced HR. The combination (as opposed to single agents) almost complete dysregulated the cell cycle and its checkpoints lead to DNA damage, genomic instability and rapid exit from the cell cycle with cell death via apoptosis. Thus we have molecularly characterized the detailed mechanisms underlying the potent AraC+WEE1 inhibition in AML and describe for the first time a therapeutic combination that has the potential to abrogate the MRN and NBS1 repair capacity which is central for drug resistance in AML. A key implication of our work is to provide a clinical rationale, mechanistic understanding and suggestions for biomarkers to clinically evaluate AraC + MK1775 in patients with AML.