MCL1 and BCL2 are pro-survival molecules which inhibit the apoptosis of AML cells treated with cytotoxic chemotherapy. The nuclear chaperone NPM1 is the most frequently mutated gene in AML. Using siRNA, we established that knockdown of NPM1 reduced expression of both MCL1 (P=0.034) and BCL2 (P=0.025) and increased p53 expression (P=0.03) in NPM1-mutant OCI-AML3 cells. These effects were not seen in cells expressing wildtype NPM1. The differentiating agent ATRA also reduced expression of both MCL1 (P=0.005) and BCL2 (P=0.0005). ATRA (3 days, 500 nM) reduced cell number by 50% in OCI-AML3 cells through growth inhibition, inducing features of differentiation and very little apoptosis. Whereas inhibition of BCL2 using up to 3 µM ABT199 did not augment ATRA-mediated cell reduction in OCI-AML3 cells, RNApolymerase II inhibitors, which preferentially target proteins with a short half-life such as MCL1, and which induce apoptosis, acted additively with ATRA to kill OCI-AML3 cells. Primary AML cells with mutant NPM1 (n=38) expressed higher levels of MCL1 but not of BCL2 than NPM1 wildtype samples (n=61). Primary AML cells frequently co-harbour mutant NPM1 and mutations of the receptor tyrosine kinase FLT3. As FLT3 mutation drives aberrant MCL1, we examined the effects of ATRA on MCL1 in NPM1 mutant samples with and without a FLT3 mutation. ATRA (3 days, 500 nM) induced the downregulation (>20%) of MCL1 in 4/4 primary NPM1 mutant/FLT3 wildtype AML samples compared to 0/3 primary NPM1 mutant/FLT3 mutant AML samples. MCL1 was also downregulated by ATRA in 2/5 double wildtype samples. BCL2 was downregulated by ATRA in 1/4 primary NPMmutant/FLT3 wildtype AML samples, 2/3 primary NPM1 mutant/FLT3 mutant AML samples and 2/5 double wildtype samples. In conclusion, we show that MCL1 levels can be downregulated by mutant NPM1 inhibition and by ATRA in NPM1mutant/FLT3wildtype AML cells. RNA polymerase II inhibitors are clinically available and this work shows that ATRA can enhance their cytotoxic activity.

Citation Format: Haitham Qutob, Amina Abdul-Aziz, Amy Proudfoot, Martin Grundy, Nigel H. Russell, Monica Pallis, Claire H. Seedhouse. Targeting MCL1 in NPM1-mutant AML. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 331. doi:10.1158/1538-7445.AM2014-331