Abstract 2483 MLL-rearranged Acute Lymphoblastic Leukemia (ALL) in infants ( We performed CpG methylation analyses at 122 miRNA loci using Differential Methylation Hybridization (DMH), and miRNA expression analyses using quantitative real-time PCR on primary t (4;11)-positive infant ALL samples (n= 22) and normal pediatric bone marrows (n= 7). We identified 11 miRNAs that were markedly down-regulated in t (4;11)-positive infant ALL as a consequence of CpG hypermethylation. Seven of these miRNAs were re-activated after exposure to the de-methylating agent Zebularine. Interestingly, 5 of these miRNAs had already been associated either with the MLL gene or with leukemic MLL fusions. For one of the remaining miRNAs, i.e. miR-152, we demonstrate that high degrees of methylation strongly correlate with a poor clinical outcome. Moreover, we identified MLL and DNA methyltransferase 1 (DNMT1) as potential target genes for miR-152. Thus, genome-wide DNA methylation in MLL-rearranged infant ALL not only inactivates numerous protein-coding genes, but also affects several miRNA genes. While inhibition of methylation by Zebularine to certain extents re-activates gene expression, re-activation of miRNAs by this agent restores the suppression of associated target genes. As demethylating agents exert their functions by covalently trapping DNMT1 to the DNA, re-activation of miR-152 by Zebularine further supports demethylation by targeting DNMT1 expression. In summary, our data demonstrates an important role for genome-wide DNA methylation in suppressing miRNA expression and provides additional grounds to initiate efficacy testing of demethylating agents in MLL-rearranged ALL in vivo. Disclosures: No relevant conflicts of interest to declare.