Abstract:
Chromosomal aberrations giving rise to fusion genes are observed for many different leukemias. The chromosomal translocation t(4;11) marks an infant acute lymphoblastic leukemia associated with a particularly poor prognosis. To define the role of one of the corresponding fusion transcripts, MLL-AF4, in leukemogenesis more precisely, an RNAi approach was applied to inhibit the expression of this oncogene in leukemic cells. RNAi is a cellular process leading to the enzymatic cleavage and breakdown of mRNA. Exogenously added synthetic siRNAs were shown to act as very potent and sequence-specific agents to silence gene expression, demonstrating their great potential not only for the analysis of gene function but also for gene-specific therapeutic approaches. In this work I show that depletion of MLL-AF4 inhibits clonogenicity and proliferation, induces apoptosis in t(4;11)-positive leukemic cells and compromises their engraftment in a severe combined immunodeficiency (SCID) mouse xenotransplantation model. Furthermore, suppression of MLL-AF4 is associated with the decrease of telomerase activity in t(4;11) cells, which, in turn, is caused by the downregulation of the hTERT expression. Studies of the methylation status of hTERT promoter revealed that MLL-AF4 interferes with the genomic DNA methylation process. Thus, along with the significant phenotypical changes mediated by MLL-AF4 siRNA, direct targets and executors of MLL-AF4-mediated leukemogenesis were identified. Targeted inhibition of MLL-AF4 fusion gene expression may lead to an effective and highly specific treatment of this therapy-resistant leukemia.