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 RAS-mutant 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 RAS-mutant 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.

Acute lymphoblastic leukemia in infants (< 1 year-of-age) is characterized by a high incidence of MLL rearrangements. Recently, direct targets of the MLL fusion protein have been identified. However, functional validation of the identified targets remained unacknowledged. In this study, we identify CDK6 as a direct target of the MLL fusion protein and an important player in the proliferation advantage of MLL-rearranged leukemia. CDK6 mRNA was significantly higher expressed in MLL-rearranged infant ALL patients compared with MLL wild-type ALL patients (P < 0.001). Decrease of MLL-AF4 and MLL-ENL fusion mRNA expression by siRNAs resulted in downregulation of CDK6, affirming a direct relationship between the presence of the MLL fusion and CDK6 expression. Knockdown of CDK6 itself significantly inhibited proliferation in the MLL-AF4-positive cell line SEM, whereas knockdown of the highly homologous gene CDK4 had virtually no effect on the cell cycle. Furthermore, we show in vitro sensitivity of MLL-rearranged leukemia cell lines to the CDK4/6-inhibitor PD0332991, inducing a remarkable G 1 arrest, and downregulation of its downstream targets pRB1 and EZH2. We therefore conclude that CDK6 is indeed a direct target of MLL fusion proteins, playing an important role in the proliferation advantage of MLL-rearranged ALL cells.

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1426 MLL-rearranged acute lymphoblastic leukemia represents a highly aggressive and clinically unfavorable type of childhood leukemia, displaying unique gene expression signatures. Nevertheless, the overwhelming number of differentially expressed genes made it difficult to elucidate the actual “drivers” of the leukemia. However, recent advances demonstrated that MLL fusion proteins recruit the histone methyltransferase DOT1L, leading to H3K79 methylation. Hence, genomic regions displaying aberrant enrichment of H3K79 methylation are prone to mark genes transcriptionally activated by the MLL fusion protein itself. Based on this concept, two independent studies recently identified gene signatures consisting of genes likely to represent direct MLL fusion targets. Yet, functional validation of such genes so far remains unacknowledged. In the present study we confirmed that CDK6 (cyclin-dependent protein kinase 6) represents a direct target of MLL-AF4 in t(4;11)-positive ALL cells. In contrast to its functional homologue CDK4, ChIP-sequencing analysis showed the presence of both MLL and AF4, as well as H3K79 methylation at the CDK6. Moreover, CDK6 mRNA appeared significantly (p<0.001) higher expressed in primary MLL- rearranged infant ALL patient samples when compared with other childhood ALL subtypes without translocations of the MLL gene. Next, using RNA interference, we performed MLL-AF4 and MLL-ENL knockdown experiments in ALL cell lines bearing these corresponding fusion transcripts, resulting in CDK6 down-regulation, whereas CDK4 expression was unaffected. These results emphasize that CDK6 is indeed a genuine transcriptional target of the MLL fusion protein itself. Moreover, direct knockdown of CDK6 itself significantly inhibited proliferation in MLL-rearranged ALL cells, whereas knockdown of CDK4 virtually had no effect on the cell cycle in these cells. Taken together we conclude that CDK6 up-regulation in MLL-rearranged ALL is directly mediated by the MLL fusion itself and provides these cells with a proliferation advantage. Disclosures: No relevant conflicts of interest to declare.

Background: Acute Lymphoblastic Leukemia (ALL) in infants is characterized by a high incidence (~80%) of chromosomal rearrangements of the Mixed Lineage Leukemia (MLL) gene, fusing the N-terminal portion of MLL to the C-terminal region of one of its translocation partner genes. MLL-rearranged infant ALL patients are challenged by a very poor prognosis (i.e. 30-40% 5-year EFS), hence the need for better risk stratification and improved therapeutic solutions is evident. We recently screened a relatively large cohort (n=109) infant ALL patients (all enrolled in INTERFANT treatment protocols) for the presence of KRAS and NRAS mutations and found that the incidence of such mutations ranges between 14-24%, depending on the type of MLL translocation. Moreover, these mutations were found to represent independent predictors of exceedingly poor prognosis; patients carrying RAS mutations essentially stand no chance in surviving their malignancy, as all RAS mutation-positive MLL-rearranged infant ALL patients deceased within 3 years from diagnosis. Aims: Here we aimed to identify a therapeutic strategy to improve the prognosis of MLL-rearranged infant ALL patients carrying RAS mutations. Methods: For this, 8 small molecule inhibitors against different RAS-pathway components were selected and initially tested for anti-leukemic activity against the MLL-rearranged ALL cell lines SEM and RS4;11 (RASwt) and KOPN8 (RASmut) using MTS cell viability assays. Next, primary MLL-rearranged infant ALL samples (n=20) all carrying MLL translocation t(4;11) (giving rise to the MLL-AF4 fusion protein) either with (n=6) or without (n=14) RAS mutations were exposed to these inhibitors in MTT cytotoxicity assays. In addition, we assessed the RAS activity in RAS mutated and wild-type MLL-rearranged infant ALL cells, and performed immunoblotting analysis of downstream MEK and ERK both in the absence and presence of the MEK inhibitors. Results: We found that the MEK inhibitors MEK162, Selumetinib and Trametinib effectively reduced the viability of KOPN8 cells (RASmut), whereas SEM and RS4;11 cells (RASwt) largely remained unaffected. In line with this, MLL-AF4+ infant ALL patient samples carrying RAS mutations were significantly more sensitive to these MEK inhibitors when compared with patients carrying wild-type RAS genes: LC50 values for MEK162 were 0.04 vs. 26.9 µM (p<0.01), for Selumetinib 0.04 and 23.7 µM (p<0.01), and for Trametinib 0.01 vs. 26.5 µM (p<0.01), respectively. Furthermore, the presence of RAS mutations in primary MLL-rearranged infant ALL samples was associated with significantly increased RAS activity, as determined by immunoprecipitation of GTP-bound RAS. Remarkably, however, enhanced RAS activation did not manifest itself in the form of increased phosphorylation of the downstream ERK protein, while a slight increase of MEK phosphorylation was observed. Yet, MEK inhibitor exposure in both KOPN8 and SEM cells resulted in nearly complete abrogation of ERK phosphorylation, without affecting total ERK protein levels, suggesting that the loss of ERK activation plays an important role in the observed anti-leukemic effects. Furthermore, MEK162 and Selumetinib seemed to induce accumulation of phosphorylated MEK, while MEK phosphorylation was hardly affected by Trametinib. Interestingly, a subgroup of patient samples (n=5) with wild-type RAS also showed sensitivity towards MEK inhibition, similar to the primary cells with RAS mutations. However, this observation could not be explained by increased RAS activation, nor by the phosphorylation levels of either MEK or ERK. Conclusions: Our data show that MEK inhibition represents a promising therapeutic approach for MLL-rearranged infant ALL patients carrying additional RAS mutations. Furthermore, the mechanism of action provoked by these MEK inhibitors seems to involve abrogation of ERK phosphorylation, but the initial level of ERK phosphorylation did not correlate with MEK inhibitor sensitivity, and has no predictive value. Currently we are in the process of testing the efficacy of the above mentioned MEK inhibitors in vivo using a xenograft mouse model, while further elucidation of the molecular mechanisms underlying the anti-leukemic effects of these inhibitors in MLL-rearranged ALL cells is in progress. Disclosures No relevant conflicts of interest to declare.