Acute myeloid leukemias (AMLs) that harbor translocations involving the MLL gene on chromosome 11q23 generate fusion transcripts that give rise novel fusion proteins with potent oncogenic properties and capable to destabilize the normal transcriptional activities. MLL fusion oncoproteins have been shown to initiate leukemic transformation primarily by overexpression of a specific set of genes, including HOXA4, 5, 6, 7, 9, 10 (overall defined as “HOXA-code” genes), MEIS1 (a cofactor of “HOXA-code” proteins) and MYB. It‘s well established that the majority of these genes are involved in normal programs of self-renewal, maintenance and proliferation of hematopoietic stem cells and early progenitors. Therefore is not surprise that the deregulation of the stemness genetic programs due to MLL fusion oncogenes is a crucial step for leukemic transformation. To identify new stemness genes involved in MLL-mediated transformation we performed gene set enrichments analysis (GSEA) using public database of geneset profiles of normal hematopoietic cells in a cohort of pediatric AML previously analyzed. These analyses identified a series of genes more highly expressed in MLL-rearranged AML including the well known HOXA9, HOXA5 and MEIS1, together with an apparently novel gene: ZNF521 or zinc finger protein 521
ZNF521 encodes for a zinc finger protein and, like HOXA9, is strongly expressed by CD34+ hematopoietic stem cells and drastically decreases during differentiation. To evaluate the importance of ZNF521 in MLL-rearranged AML, we performed a series of functional and mechanistic studies to uncover the role of ZNF521 in MLL-rearranged cells. We used lentiviral vectors to silencing the ZNF521 and expression vectors to induce MLL-fusion proteins such as MLL-AF9. These studies, both in vitro and ex vivo, demonstrate that the growth inhibition, reduced clonogenicity and cell cycle arrest induced by ZNF521 depletion is mediated through enhanced myeloid differentiation. Moreover, we demonstrate that ZNF521 is a direct target of MLL-fusion oncoproteins such as MLL-AF9 and MLL-ENL.
Collectively, these findings identify ZNF521 as critical effector of MLL fusion in leukemogenesis that might be targeted to overcome the differentiation block associated with MLL-rearranged AML and thus highlight ZNF521 as potential therapeutic target in treating this subtype of aggressive leukemia
