Methylation of Specific Microrna Genes In MLL-Rearranged Infant Acute Lymphoblastic Leukemia: - Major Matters at a Micro Scale -

Abstract 2483

MLL-rearranged Acute Lymphoblastic Leukemia (ALL) in infants (<1 year) represents one of the most aggressive types of childhood leukemia. In order to develop more suitable treatment strategies, a firm understanding of the biology underlying this disease is of utmost importance. MLL-rearranged ALL displays a unique gene expression profile, partly explained by erroneous histone modifications. We recently showed that t (4;11)-positive infant ALL is also characterized by pronounced promoter CpG hypermethylation. Here we investigated whether this widespread hypermethylation also affected microRNA (miRNA) expression.

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.