MLL (for mixed-lineage leukemia) is a proto-oncogene that is mutated in a variety

Summary Menin displays the unique ability to either promote oncogenic function in the hematopoietic lineage or suppress tumorigenesis in the endocrine lineage, however its molecular mechanism of action has not been defined. We demonstrate here that these discordant functions are unified by menin's ability to serve as a molecular adaptor that physically links the MLL histone methyltransferase with LEDGF, a chromatin-associated protein previously implicated in leukemia, auto-immunity and HIV-1 pathogenesis. LEDGF is required for both MLL-dependent transcription and leukemic transformation. Conversely, a subset of menin mutations in MEN1 patients abrogates interaction with LEDGF while preserving MLL interaction, but nevertheless compromises MLL/menin-dependent functions. Thus, LEDGF critically associates with MLL and menin at the nexus of transcriptional pathways that are recurrently targeted in diverse diseases.

The Mixed-Lineage Leukemia (MLL) protein is a histone methyltransferase that is mutated in clinically and biologically distinctive subsets of acute leukemia. MLL normally associates with a cohort of highly conserved cofactors to form a macromolecular complex that includes menin, a product of the MEN1 tumor suppressor gene, which is mutated in heritable and sporadic endocrine tumors. We demonstrate here that oncogenic MLL fusion proteins retain an ability to stably associate with menin through a high-affinity, amino-terminal, conserved binding motif and that this interaction is required for the initiation of MLL-mediated leukemogenesis. Furthermore, menin is essential for maintenance of MLL-associated but not other oncogene induced myeloid transformation. Acute genetic ablation of menin reverses aberrant Hox gene expression mediated by MLL-menin promoter-associated complexes, and specifically abrogates the differentiation arrest and oncogenic properties of MLL-transformed leukemic blasts. These results demonstrate that a human oncoprotein is critically dependent on direct physical interaction with a tumor suppressor protein for its oncogenic activity, validate a potential target for molecular therapy, and suggest central roles for menin in altered epigenetic functions underlying the pathogenesis of hematopoietic cancers.

Summary AF4 and ENL family proteins are frequently fused with MLL, and comprise a higher order complex (designated AEP) containing the P-TEFb transcription elongation factor. Here, we show that AEP is normally recruited to MLL-target chromatin to facilitate transcription. By contrast, MLL oncoproteins fused with AEP components constitutively form MLL/AEP hybrid complexes to cause sustained target gene expression, which leads to transformation of hematopoietic progenitors. Furthermore, MLL-AF6, an MLL fusion with a cytoplasmic protein, does not form such hybrid complexes, but nevertheless constitutively recruits AEP to target chromatin via unknown alternative mechanisms. Thus, AEP recruitment is an integral part of both physiological and pathological MLL-dependent transcriptional pathways. Bypass of its normal recruitment mechanisms is the strategy most frequently employed by MLL oncoproteins.

The AML1±CBFb transcription factor complex is the most frequent target of speci®c chromosome translocations in human leukemia. The MOZ gene, which encodes a histone acetyltransferase (HAT), is also involved in some leukemia-associated translocations. We report here that MOZ is part of the AML1 complex and strongly stimulates AML1-mediated transcription. The stimulation of AML1-mediated transcription is independent of the inherent HAT activity of MOZ. Rather, a potent transactivation domain within MOZ appears to be essential for stimulation of AML1-mediated transcription. MOZ, as well as CBP and MOZ±CBP, can acetylate AML1 in vitro. The amount of AML1±MOZ complex increases during the differentiation of M1 myeloid cells into monocytes/macrophages, suggesting that the AML1±MOZ complex might play a role in cell differentiation. On the other hand, the MOZ±CBP fusion protein, which is created by the t(8;16) translocation associated with acute monocytic leukemia, inhibits AML1-mediated transcription and differentiation of M1 cells. These results suggest that MOZ±CBP might induce leukemia by antagonizing the function of the AML1 complex.