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Background: MLL (KMT2A)-rearranged acute lymphoblastic leukemia (MLLr ALL) is a rare but aggressive subset that represents 5% of childhood ALL cases, and accounts for about 70% of infant leukemias. While overall survival in these young children is around 50%, after relapse, MLLr ALL becomes an almost incurable disease, highlighting the urgent clinical need for new strategies for this patient group. The histone methyl transferase function of the MLL fusion protein complex requires the methionine metabolite s-adenosylmethionine (SAM) as methyl donor, suggesting a selective sensitivity of MLL-r ALL for perturbations in methionine availability. Recent studies in solid tumor models suggest clinical utility of methionine restricted diets or oral administration of methionine depleting enzyme Methionine Gamma Lyase (MGL) to be safe and effective. Therefore, we explored the effect of methionine restriction (MR) as a potential, new therapy for MLLr ALL. Methods: We compared the effects of MR on metabolic activity and viability between MLLr and non-MLLr pre-BCP ALL cell lines using enzymatic depletion, small molecule inhibitors targeting methionine metabolism, and restrictive culture conditions. To identify intrinsic metabolic differences between MLLr and non-MLLr cells and explore how MR impinges on their metabolic state, we performed global metabolomics on MLLr SEM cells and non-MLLr NALM6 cells cultured with complete depletion of methionine. Additionally, we used RNA sequencing to assess the global effects of MR on gene expression, and a CRISPR/Cas9-based reverse genetic screen to identify sensitizers towards MR. Results were validated in vitro using targeted knockouts and small-molecule inhibitors, as well as in vivo using a 95% methionine restricted diet. Immunocompromised mice were engrafted with MLLr SEM cells and 7 days after transplantation, mice were randomized to control or 95% MR diet. Leukemia progression was monitored by flowcytometric detection of human lymphocytes in the blood. Results: We observed that depletion of methionine reduces metabolic activity in almost all BCP-ALL (B-ALL) cell lines, however, only in MLLr B-ALL cell lines was rapid apoptosis induced (Figure 1A). Global metabolic profiling revealed significant basal metabolic differences, of note being SAM, whose levels were approximately 5-fold higher in MLLr SEM cells compared to non-MLLr NALM6 cells. Consistent with this, addition of SAM completely rescued MLLr cell lines from methionine depletion induced apoptosis, an effect not observed in non-MLLr cells (Figure 1A). Metabolomic profiling also highlighted different salvage mechanisms at play in NALM6 cells, with the folate cycle and polyamine synthesis pathway being activated upon MR. Together, these results indicate that MLLr B-ALL cells are selectively sensitive to MR. In line with this, RNASeq data showed significant decreased expression of several known MLL fusion target genes such as PROM1, HOXA10, and MEIS1 in response to MR. To obtain further insight into the pathways involved in the response to MR and to identify potential therapeutic targets that further sensitize cells to MR, we performed a CRISPR/Cas9-based screen. This identified three members of the Bromodomain- and extra-terminal domain (BET) family as potential modifiers of the response to MR in SEM cells. Indeed, RNAseq analysis showed that Myc activity as a proxy of BRD4 function, was strongly suppressed by MR. Finally, preliminary results show the efficacy of dietary intervention alone on leukemia progression. We observe with 95% MR diet, significant delays on leukemic growth (Figure 1B). Moreover, the MR diet was well tolerated, as indicated by minimal weight loss after two months. Although further studies are needed, we anticipate that targeting epigenetic regulators or use of conventional therapies in combination with MR would further potentiate this effect. Conclusions: MLLr leukemic cells have an increased dependency on S-adenosylmethionine and therefore show increased vulnerability to methionine depletion. Limiting methionine availability, either by enzymatic methionine depletion or dietary restriction could provide a novel therapeutic option for this patient group, particularly when combined with other therapies. The availability of an FDA approved methionine-free formula facilitates rapid translation to clinical practice, particularly in infants. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.