Menin is the product of the tumor suppressor gene Men1 that is mutated in the inherited tumor syndrome multiple endocrine neoplasia type 1 (MEN1). Menin has been shown to interact with SET-1 domain-containing histone 3 lysine 4 (H3K4) methyltransferases including mixed lineage leukemia proteins to regulate homeobox (Hox) gene expression in vitro. Using conditional Men1 knockout mice, we have investigated the requirement for menin in hematopoiesis and myeloid transformation. Men1 excision causes reduction of Hoxa9 expression, colony formation by hematopoietic progenitors, and the peripheral white blood cell count. Menin directly activates Hoxa9 expression, at least in part, by binding to the Hoxa9 locus, facilitating methylation of H3K4, and recruiting the methylated H3K4 binding protein chd1 to the locus. Consistent with signaling downstream of menin, ectopic expression of both Hoxa9 and Meis1 rescues colony formation defects in Men1-excised bone marrow. Moreover, Men1 excision also suppresses proliferation of leukemogenic mixed lineage leukemia-AF9 fusionprotein-transformed myeloid cells and Hoxa9 expression. These studies uncover an important role for menin in both normal hematopoiesis and myeloid transformation and provide a mechanistic understanding of menin's function in these processes that may be used for therapy. multiple endocrine neoplasia type 1 ͉ mixed lineage leukemia ͉ Men1 gene ͉ histone lysine methylation

Men1 is a tumor suppressor gene mutated in endocrine neoplasms. Besides its endocrine role, the Men1 gene product menin interacts with the mixed lineage leukemia (MLL) protein, a histone H3 lysine 4 methyltransferase. Although menin and MLL fusion proteins cooperate to activate Homeobox (Hox) gene expression during transformation, little is known about the normal hematopoietic functions of menin. Here, we studied hematopoiesis after Men1 ablation. Menin loss modestly impaired blood neutrophil, lymphocyte, and platelet counts. Without hematopoietic stress, multilineage and myelo-erythroid bone marrow progenitor numbers were preserved, while B lymphoid progenitors were decreased. In contrast, competitive transplantation revealed a marked functional defect of long-term hematopoietic stem cells (HSC) in the absence of menin, despite normal initial homing of progenitors to the bone marrow. HoxA9 gene expression was only modestly decreased in menin-deficient HSCs. These observations reveal a novel and essential role for menin in HSC homeostasis that was most apparent during situations of hematopoietic recovery, suggesting that menin regulates molecular pathways that are essential during the adaptive HSC response to stress. IntroductionMultiple endocrine neoplasia type I (MEN1) is a hereditary tumor syndrome caused by mutations in the MEN1 gene, which encodes the protein menin. 1 MEN1 patients display a spectrum of benign and malignant endocrine lesions, including hyperparathyroidism as well as pituitary and enteropancreatic tumors. Heterozygous loss-of-function mutations are found in MEN1 patients, and additional somatic loss of heterozygosity in the MEN1 locus leads to mutation of both normal MEN1 alleles in tumors. Menin is predominantly a nuclear protein that has been reported to interact with a variety of partners, including nuclear factor kappa B (NF-B), JunD, and SMADs, as well as with proteins involved in DNA repair, such as FANCD2, although it is unclear if these interactions are relevant to menin's activity as a tumor suppressor. 1 Further, chromatin immunoprecipitation coupled with nucleotide array analysis has revealed binding of menin to a large number of genomic sites, suggesting a broad regulatory potential. 2,3 A hematopoietic role for menin was first suggested through its identification in a complex containing the mixed lineage leukemia (MLL) gene product or its homologue MLL2. 4,5 MLL is a homologue of the Drosophila Trithorax gene, an epigenetic regulator that antagonizes the activity of Polycomb family members and is essential for maintaining expression of target genes, such as members of the Homeobox (Hox) family. 6 MLL is a large protein with multiple functional domains, including a C-terminal Su(var)3-9, enhancer-of-zeste, Trithorax (SET) domain that has histone methyltransferase activity, leading to histone H3 lysine 4 (H3K4) methylation, a hallmark of active genes. 4,7 Translocations and rearrangements of the MLL gene arise in acute myeloid and lymphoid leukemias. 8 These translocations genera...