MLL1 and menin: not partners in crime?

In this issue of Blood, Li et al report an unexpected but clinically relevant finding. They demonstrate that the mixed lineage leukemia (MLL1) gene acts independently from menin (Men1) in the hematopoietic system.¹ 

The MLL gene encodes a DNA-binding protein that methylates histone H3 lysine 4 (H3K4) and regulates gene expression including multiple Hox genes. Leukemogenic MLL translocations encode MLL fusion proteins (MLL-FPs) that have lost H3K4 methyltransferase activity. An important feature of MLL-FPs is their ability to transform hematopoietic cells into leukemia stem cells. At least 52 functionally diverse MLL-FPs have been described. Fusions with the nuclear proteins AF4, AF9, AF10, ENL, and ELL account for most of MLL-rearranged leukemia. Translocations disrupting the MLL1 gene occur frequently in infant and secondary acute leukemia. MLL1 translocations encode fusion proteins (MLL1-FPs) retaining the N terminus of MLL1, which interacts with the tumor suppressor menin.²  Indeed, several studies have demonstrated that MLL1-FPs require menin interaction for leukemogenesis. Mutations in the menin interaction motif of MLL1-ENL abolish its transforming activity in hematopoietic cells.³  Second, menin is required for MLL1-FPs to bind a PWWP domain–containing protein that participates in targeting of the MLL complex to chromatin.⁴  Finally, Men1-deficient bone marrow progenitors do not sustain leukemogenesis induced specifically by MLL1-FPs,⁴  thus defining Men1 as an essential oncogenic cofactor for MLL-associated leukemogenesis.

Men1 and Mll1 perform several similar functions, and loss of either gene in embryonic stem cells impairs hematopoietic differentiation.⁵  However, because of the discovery of menin in native MLL1 complexes,⁶  and the colocalization of menin, MLL1/MLL2, and H3K4Me3 at most genomic loci, it was predicted that menin would be a necessary cofactor for the function of the MLL1 complex, irrespective of tissue or target genes examined.⁷ 

Nevertheless, until this study, it was unclear whether the normal functions of MLL1 required in hematopoiesis also depend on menin interaction. It was anticipated that disruption of this interaction with a fundamental MLL1 cofactor would abolish normal hematopoiesis because wild-type MLL1 plays a major function in sustaining hematopoietic stem cells (HSCs).

In a very elegant approach, the authors demonstrate the lack of genetic interaction between Men1 and MLL1 in steady-state or regenerative hematopoiesis and in B-cell differentiation despite the fact that MLL1 is critical for these processes. They show that Men1 loss affects only a subset of Mll1 target genes and, more importantly, that these gene expression changes are not sufficient to impact HSC homeostasis. To evaluate whether a single copy of Mll1 in the complete absence of Men1 was sufficient to support hematopoietic regeneration, competitive transplantation experiments using Men1^−/−: Mll1^+/− donor cells, they demonstrate that both proteins function additively through independent pathways, which is opposed to the view that Men1 serves as an essential cofactor for MLL1 in regulation of HSC homeostasis.⁸ 

To determine the effect of disrupting the menin-MLL1 interaction on B-cell differentiation, they introduced a blocking peptide into primitive bone marrow cells and assessed B-cell growth and differentiation. They demonstrate that menin and MLL1 play important roles independently during B-cell differentiation but control largely independent genetic networks. Importantly, disrupting the menin-MLL1 interaction cannot recapitulate the block in B-cell differentiation observed in individual knockouts despite efficient reduction in shared target gene expression.

This study has important clinical implications and elucidates the importance of inhibition of the menin interaction with MLL-FPs.⁹  Specific inhibition could represent a very promising strategy to reverse the oncogenic activity of MLL1-FPs. These new results provide the rational and essential understanding to design safe inhibitors for effectively targeting the menin-MLL interaction in leukemia with minimal effects in the crucial physiological function of MLL1 in maintaining normal hematopoiesis. However, the Men1 tumor suppressive role in neuroendocrine cells could rely on the MLL-Men1 interaction. Disruption of this interaction could lead to the abrogation of Men1 tumor suppression activity in pancreatic cells. Therefore, it will be important to determine whether there is a risk associated with inhibiting the tumor suppression activity achieved by Men1.¹⁰