A New Class of Target Agents against KMT2A-Rearranged and NPM1-Mutated Acute Myeloid Leukemia

Menin inhibitors are new and promising agents currently in clinical development that target the HOX/MEIS1 transcriptional program which is critical for leukemogenesis in histone-lysine N-methyltransferase 2A-rearranged (KMT2Ar) and in NPM1-mutated (NPM1mut) acute leukemias. The mechanism of action of this new class of agents is based on the disruption of the menin-KMT2A complex (consisting of chromatin remodeling proteins), leading to the differentiation and apoptosis of AML cells expressing KMT2A or with mutated NPM1. To date, this new class of drugs has been tested in phase I and II clinical trials, both alone and in combination with synergistic drugs showing promising results in terms of response rates and safety in heavily pre-treated acute leukemia patients. In this brief review, we summarize the key findings on menin inhibitors, focusing on the mechanism of action and preliminary clinical data on the treatment of acute myeloid leukemia with this promising new class of agents, particularly revumenib and ziftomenib.

Menin Deficiency Induces Autism-Like Behaviors by Regulating Foxg1 Transcription and Participates in Foxg1-Related Encephalopathy

FOXG1 syndrome is a developmental encephalopathy caused by FOXG1 (Forkhead box G1) mutations, resulting in high phenotypic variability. However, the upstream transcriptional regulation of Foxg1 expression remains unclear. This report demonstrates that both deficiency and overexpression of Men1 (protein: menin, a pathogenic gene of MEN1 syndrome known as multiple endocrine neoplasia type 1) lead to autism-like behaviors, such as social defects, increased repetitive behaviors, and cognitive impairments. Multifaceted transcriptome analyses revealed that Foxg1 signaling is predominantly altered in Men1 deficiency mice, through its regulation of the Alpha Thalassemia/Mental Retardation Syndrome X-Linked (Atrx) factor. Atrx recruits menin to bind to the transcriptional start region of Foxg1 and mediates the regulation of Foxg1 expression by H3K4me3 (Trimethylation of histone H3 lysine 4) modification. The deficits observed in menin deficient mice are rescued by the over-expression of Foxg1, leading to normalized spine growth and restoration of hippocampal synaptic plasticity. These findings suggest that menin may have a putative role in the maintenance of Foxg1 expression, highlighting menin signaling as a potential therapeutic target for Foxg1-related encephalopathy.

Synergistic Effects of the RARalpha Agonist Tamibarotene and the Menin Inhibitor Revumenib in Acute Myeloid Leukemia Cells with KMT2A Rearrangement or NPM1 Mutation

Inhibition of menin in acute myeloid leukemia (AML) harboring histone-lysine-N-methyltransferase 2A rearrangement (KMT2Ar) or the mutated Nucleophosmin gene (NPM1c) is considered a novel and effective treatment approach in these patients. However, rapid acquisition of resistance mutations can impair treatment success. In patients with elevated retinoic acid receptor alpha (RARA) expression levels, promising effects are demonstrated by the next-generation RARalpha agonist tamibarotene, which restores differentiation or induces apoptosis. In this study, the combination of revumenib and tamibarotene was investigated in various KMT2Ar or NPM1c AML cell lines and patient-derived blasts, focusing on the potential synergistic induction of differentiation or apoptosis. Both effects were analyzed by flow cytometry and validated by Western blot analysis. Synergy calculations were performed using viability assays. Regulation of the relevant key mediators for the MLL complex were quantified by RT-qPCR. In MV4:11 cells characterized by the highest relative mRNA levels of RARA, highly synergistic induction of apoptosis is demonstrated upon combination treatment. Induction of apoptosis by combined treatment of MV4:11 cells is accompanied by pronounced induction of the pro-apoptotic protein BAX and a synergistic reduction in CDK6 mRNA levels. In MOLM13 and OCI-AML3 cells, an increase in differentiation markers like PU.1 or a decreased ratio of phosphorylated to total CEBPA is demonstrated. In parts, corresponding effects were observed in patient-derived AML cells carrying either KMT2Ar or NPM1c. The impact of revumenib on KMT2Ar or NPM1c AML cells was significantly enhanced when combined with tamibarotene, demonstrating synergistic differentiation or apoptosis initiation. These findings propose promising strategies for relapsed/refractory AML patients with defined molecular characteristics.

Menin Reduces Parvalbumin Expression and is Required for the Anti-Depressant Function of Ketamine

Dysfunction of parvalbumin (PV) neurons is closely involved in depression, however, the detailed mechanism remains unclear. Based on the previous finding that multiple endocrine neoplasia type 1 (Protein: Menin; Gene: Men1) mutation (G503D) is associated with a higher risk of depression, a Menin-G503D mouse model is generated that exhibits heritable depressive-like phenotypes and increases PV expression in brain. This study generates and screens a serial of neuronal specific Men1 deletion mice, and found that PV interneuron Men1 deletion mice (PcKO) exhibit increased cortical PV levels and depressive-like behaviors. Restoration of Menin, knockdown PV expression or inhibition of PV neuronal activity in PV neurons all can ameliorate the depressive-like behaviors of PcKO mice. This study next found that ketamine stabilizes Menin by inhibiting protein kinase A (PKA) activity, which mediates the anti-depressant function of ketamine. These results demonstrate a critical role for Menin in depression, and prove that Menin is key to the antidepressant function of ketamine.

Multiple endocrine neoplasia type 1 with Zollinger-Ellison syndrome: clinicopathological analysis of a Japanese family with focus on menin immunohistochemistry

Background

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant disorder characterized by the occurrence of multiple epithelial neuroendocrine tumors (NETs) and non-NETs in various organs. MEN1 encodes a 610-amino acid-long tumor suppressor protein, menin. The optimal treatment for multiple tumors, identification of the most critical tumors for patient prognosis, and menin immunohistochemistry findings remain controversial. Therefore, we aimed to elucidate these issues through a histological analysis of tumors and tumor-like lesions in a Japanese family, comprising a father and his two sons, who had MEN1 with Zollinger-Ellison syndrome (ZES).

Patients and methods

All family members had a germline alteration in exon 10, c.1714-1715 del TC of MEN1, and exhibited multiple synchronous and metachronous tumors. The patients had pulmonary NETs, hyperparathyroidism, hypergastrinemia, pituitary adenomas, pancreaticoduodenal NETs, adrenocortical adenoma with myelolipoma, nodular goiter of the thyroid, lipomas, and angiofibroma. Most tumors were resected and histologically examined. We compared their clinical courses and tumor histology, and conducted menin immunohistochemistry (IHC).

Results

Two patients died of pulmonary NET G2. One patient who underwent pancreaticoduodenectomy was cured of ZES; however, the two other patients who did not undergo pancreaticoduodenectomy suffered persistent ZES despite treatment with octreotide. Menin IHC revealed varying NET intensities, ranging from positive to negative stains.

Conclusion

Pancreaticoduodenectomy is the most effective treatment for ZES. Long-term follow-up is essential for pulmonary NET G2 owing to the risk of distant metastasis and/or multiplicity. Moreover, the variability of menin IHC in MEN1-related tumors may indicate the pattern of tumor formation rather than the diagnostic utility of menin in MEN1.

Menin Maintains Cholesterol Content in Colorectal Cancer via Repression of LXR-Mediated Transcription

BACKGROUND AND AIMS

Menin is a nuclear scaffold protein that regulates gene transcription in an oftentimes tissue-specific manner. Our previous work showed that menin is over-expressed in colorectal cancer (CRC); however, the full spectrum of menin function in colonic neoplasia remains unclear. Herein, we aimed to uncover novel menin-regulated pathways important for colorectal carcinogenesis.

METHODS

RNA-Seq analysis identified that menin regulates LXR-target gene expressions in CRC cell lines. Isolated colonic epithelium from Men1f/f;Vil1-Cre and Men1f/f mice was used to validate the results in vivo. Cholesterol content was quantified via an enzymatic assay.

RESULTS

RNA-Seq analysis in the HT-29 CRC cell line identified that menin inhibition upregulated LXR-target genes, specifically ABCG1 and ABCA1, with protein products that promote cellular cholesterol efflux. Similar results were noted across other CRC cell lines and with different methods of menin inhibition. Consistent with ABCG1 and ABCA1 upregulation, and similarly to LXR agonists, menin inhibition reduced the total cellular cholesterol in both HT-29 and HCT-15 cells. To confirm the effects of menin inhibition in vivo, we assessed Men1f/f;Vil1-Cre mice lacking menin expression in the colonic epithelium. Men1f/f;Vil1-Cre mice were found to have no distinct baseline phenotype compared to control Men1f/f mice. However, similarly to CRC cell lines, Men1f/f;Vil1-Cre mice showed an upregulation of Abcg1 and a reduction in total cellular cholesterol. Promoting cholesterol efflux, either via menin inhibition or LXR activation, was found to synergistically suppress CRC cell growth under cholesterol-depleted conditions and when administered concomitantly with small molecule EGFR inhibitors.

CONCLUSIONS

Menin represses the transcription of LXR-target genes, including ABCA1 and ABCG1 in the colonic epithelium and CRC. Menin inhibition conversely upregulates LXR-target genes and reduces total cellular cholesterol, demonstrating that menin inhibition may be an important mechanism for targeting cholesterol-dependent pathways in colorectal carcinogenesis.

Diagnostic Utility of Menin Immunohistochemistry in Patients With Multiple Endocrine Neoplasia Type 1 Syndrome

A clinical diagnosis of multiple endocrine neoplasia type 1 (MEN1) syndrome is usually confirmed with genetic testing in the germline. It is expected that menin protein expression is lost in MEN1-related tumors. Therefore, we investigated the potential of menin immunohistochemistry in parathyroid adenomas as an additional tool in the recognition and genetic diagnosis of MEN1 syndrome. Local pathology archives were searched for parathyroid tumors from patients with MEN1 syndrome and without MEN1, including sporadic, patients with multiple endocrine neoplasia type 2A and hyperparathyroidism-jaw parathyroid tumors. Menin immunohistochemistry was performed and its use to identify MEN1-related tumors was assessed. Twenty-nine parathyroid tumors from 16 patients with MEN1 and 61 patients with parathyroid tumors from 32 non-MEN1 were evaluated. Immunohistochemical nuclear menin loss in one or more tumors was found in 100% of patients with MEN1 and 9% of patients with non-MEN1. In patients with multiple tumors, menin loss in at least one tumor was seen in 100% of 8 patients with MEN1 and 21% of patients with 14 non-MEN1. Using a cutoff of at least 2 tumors showing menin loss per patient, the positive and negative predictive values for the diagnosis MEN1 were both 100%. The practical and additional value of menin immunohistochemistry in clinical genetic MEN1 diagnosis is further illustrated by menin immunohistochemistry in 2 cases with a germline variant of unknown significance in the MEN1 gene. Menin immunohistochemistry is useful in the recognition of MEN1 syndrome as well as in the clinical genetic analysis of patients with inconclusive MEN1 germline testing.

Novel Investigational Agents and Pathways That May Influence the Future Management of Acute Myeloid Leukemia

Acute Myeloid leukemia (AML) is a clinically heterogeneous disease with a 5-year overall survival of 32% between 2012 to 2018. The above number severely dwindles with age and adverse risk of disease, presenting opportunities for new drug development and is an area of dire unmet need. Basic science and clinical investigators across the world have been working on many new and old molecule formulations and combination strategies to improve outcomes in this disease. In this review, we discuss select promising novel agents in various stages of clinical development for patients with AML.

Menin represses the proliferation of gastric cancer cells by interacting with IQGAP1

The multiple endocrine neoplasia type 1 gene coding the protein menin was originally identified in patients with multiple endocrine tumors, and is mainly expressed in the cell nucleus. Multiple lines of evidence have indicated that menin acts as a tumor suppressor protein interacting with other various proteins. The mechanism of menin inhibiting tumorigenesis remains unclear. The present study analyzed the expression of menin and IQ motif-containing GTPase-activating protein 1 (IQGAP1) proteins in gastric cancer tissues and cell lines, and investigated the association between these two molecules. Western blotting was used to determine the quantity of target proteins. Cell proliferation was measured using MTT assay. It was found that the protein expression of menin was lower in gastric cancer tissues and AGS cells, while the protein expression of IQGAP1 was higher, compared with the levels observed in normal tissues and GES-1 cells. Ectopic expression of IQGAP1 stimulated the proliferation of gastric cancer cells, but did not affect the expression of menin. However, overexpression of menin inhibited the proliferation of gastric cancer cells. The inhibition was partly achieved through inhibiting the expression of IQGAP1, which was accompanied by inhibition of PI3K and NF-κB expression. Taken together, the present results suggest a novel function for menin and IQGAP1 contributing to suppress the proliferation of gastric cancer cells.

An overview of novel therapies in advanced clinical testing for acute myeloid leukemia

INTRODUCTION

The past decade has seen a sea change in the AML landscape with vastly improved cognizance of molecular pathogenesis, clonal evolution, and importance of measurable residual disease. Since 2017, the therapeutic armamentarium of AML has considerably expanded with the approval of midostaurin, enasidenib, ivosidenib, gilteritinib, and venetoclax in combination with hypomethylating agents and others. Nevertheless, relapse and treatment refractoriness remain the insurmountable challenges in AML therapy. This has galvanized the leukemic research community leading to the discovery and development of agents that specifically target gene mutations, molecularly agnostic therapies that exploit immune environment, apoptotic pathways, leukemic cell surface antigens and so forth.

AREAS COVERED

This article provides an overview of the pathophysiology of AML in the context of non-cellular immune and molecularly targeted and agnostic therapies that are in clinical trial development in AML.

EXPERT OPINION

Ever growing understanding of the molecular pathogenesis and metabolomics in AML has allowed the researchers to identify targets directed at specific genes and metabolic pathways. As a result, AML therapy is constantly evolving and so are the escape mechanisms leading to disease relapse. Therefore, it is of paramount importance to sequentially evaluate the patient during AML treatment and intervene at the right time.

Inhibition of MLL1-menin interaction attenuates renal fibrosis in obstructive nephropathy

Mixed lineage leukemia 1 (MLL1), a histone H3 lysine 4 (H3K4) methyltransferase, exerts its enzymatic activity by interacting with menin and other proteins. It is unclear whether inhibition of the MLL1-menin interaction influences epithelial-mesenchymal transition (EMT), renal fibroblast activation, and renal fibrosis. In this study, we investigated the effect of disrupting MLL1-menin interaction on those events and mechanisms involved in a murine model of renal fibrosis induced by unilateral ureteral obstruction (UUO), in cultured mouse proximal tubular cells and renal interstitial fibroblasts. Injury to the kidney increased the expression of MLL1 and menin and H3K4 monomethylation (H3K4me1); MLL1 and menin were expressed in renal epithelial cells and renal interstitial fibroblasts. Inhibition of the MLL1-menin interaction by MI-503 administration or siRNA-mediated silencing of MLL1 attenuated UUO-induced renal fibrosis, and reduced expression of α-smooth muscle actin (α-SMA) and fibronectin. These treatments also inhibited UUO-induced expression of transcription factors Snail and Twist and transforming growth factor β1 (TGF-β1) while expression of E-cadherin was preserved. Moreover, treatment with MI-503 and transfection with either MLL siRNA or menin siRNA inhibited TGF-β1-induced upregulation of α-SMA, fibronectin and Snail, phosphorylation of Smad3 and AKT, and downregulation of E-cadherin in cultured renal epithelial cells. Finally, MI-503 was effective in abrogating serum or TGFβ1-induced transformation of renal interstitial fibroblasts to myofibroblasts in vitro. Taken together, these results suggest that targeting disruption of the MLL1-menin interaction attenuates renal fibrosis through inhibition of partial EMT and renal fibroblast activation.

The role of DNA methylation in human pancreatic neuroendocrine tumours

Pancreatic neuroendocrine tumours (PNETs) are the second most common pancreatic tumour. However, relatively little is known about their tumourigenic drivers, other than mutations involving the multiple endocrine neoplasia 1 (MEN1), ATRX chromatin remodeler, and death domain-associated protein genes, which are found in ~40% of sporadic PNETs. PNETs have a low mutational burden, thereby suggesting that other factors likely contribute to their development, including epigenetic regulators. One such epigenetic process, DNA methylation, silences gene transcription via 5'methylcytosine (5mC), and this is usually facilitated by DNA methyltransferase enzymes at CpG-rich areas around gene promoters. However, 5'hydroxymethylcytosine, which is the first epigenetic mark during cytosine demethylation, and opposes the function of 5mC, is associated with gene transcription, although the significance of this remains unknown, as it is indistinguishable from 5mC when conventional bisulfite conversion techniques are solely used. Advances in array-based technologies have facilitated the investigation of PNET methylomes and enabled PNETs to be clustered by methylome signatures, which has assisted in prognosis and discovery of new aberrantly regulated genes contributing to tumourigenesis. This review will discuss the biology of DNA methylation, its role in PNET development, and impact on prognostication and discovery of epigenome-targeted therapies.

Targeting Menin and CD47 to Address Unmet Needs in Acute Myeloid Leukemia

After forty years of essentially unchanged treatment in acute myeloid leukemia (AML), innovation over the past five years has been rapid, with nine drug approvals from 2016 to 2021. Increased understanding of the molecular changes and genetic ontology of disease have led to targeting mutations in isocitrate dehydrogenase, FMS-like tyrosine kinase 3 (FLT3), B-cell lymphoma 2 and hedgehog pathways. Yet outcomes remain variable; especially in defined molecular and genetic subgroups such as NPM1 (Nucleophosmin 1) mutations, 11q23/KMT2A rearranged and TP53 mutations. Emerging therapies seek to address these unmet needs, and all three of these subgroups have promising new therapeutic approaches. Here, we will discuss the normal biological roles of menin in acute leukemia, notably in KMT2A translocations and NPM1 mutation, as well as current drug development. We will also explore how CD47 inhibition may move immunotherapy into front-line settings and unlock new treatment strategies in TP53 mutated disease. We will then consider how these new therapeutic advances may change the management of AML overall.

miR-3156-5p is downregulated in serum of MEN1 patients and regulates expression of MORF4L2

Multiple endocrine neoplasia type 1 (MEN1), caused by mutations in the MEN1 gene encoding menin, is an autosomal dominant disorder characterised by the combined occurrence of parathyroid, pituitary and pancreatic neuroendocrine tumours (NETs). Development of these tumours is associated with wide variations in their severity, order and ages (from <5 to >80 years), requiring life-long screening. To improve tumour surveillance and quality of life, better circulating biomarkers, particularly for pancreatic NETs that are associated with higher mortality, are required. We, therefore, examined the expression of circulating miRNA in the serum of MEN1 patients. Initial profiling analysis followed by qRT-PCR validation studies identified miR-3156-5p to be significantly downregulated (-1.3 to 5.8-fold, P < 0.05-0.0005) in nine MEN1 patients, compared to matched unaffected relatives. MEN1 knock-down experiments in BON-1 human pancreatic NET cells resulted in reduced MEN1 (49%, P < 0.05), menin (54%, P < 0.05) and miR-3156-5p expression (20%, P < 0.005), compared to control-treated cells, suggesting that miR-3156-5p downregulation is a consequence of loss of MEN1 expression. In silico analysis identified mortality factor 4-like 2 (MOR4FL2) as a potential target of miR-3156-5p, and in vitro functional studies in BON-1 cells transfected with either miR-3156-5p mimic or inhibitors showed that the miR-3156-5p mimic significantly reduced MORF4L2 protein expression (46%, P < 0.005), while miR-3156-5p inhibitor significantly increased MORF4L2 expression (1.5-fold, P < 0.05), compared to control-treated cells, thereby confirming that miR-3156-5p regulates MORF4L2 expression. Thus, the inverse relationship between miR-3156-5p and MORF4L2 expression represents a potential serum biomarker that could facilitate the detection of NET occurrence in MEN1 patients.

MEN1 silencing triggers the dysregulation of mTORC1 and MYC pathways in ER+ breast cancer cells

Menin, encoded by the MEN1 gene, has been identified as a critical factor regulating ESR1 transcription, playing an oncogenic role in ER+ breast cancer (BC) cells. Here, we further dissected the consequences of menin inactivation in ER+ BC cells by focusing on factors within two major pathways involved in BC, mTOR and MYC. MEN1 silencing in MCF7 and T-47D resulted in an increase in phosphor-p70S6K1, phosphor-p85S6K1 and phosphor-4EBP1 expression. The use of an AKT inhibitor inhibited the activation of S6K1 and S6RP triggered by MEN1 knockdown (KD). Moreover, MEN1 silencing in ER+ BC cells led to increased formation of the eIF4E and 4G complex. Clinical studies showed that patients with menin-low breast cancer receiving tamoxifen plus everolimus displayed a trend toward better overall survival. Importantly, MEN1 KD in MCF7 and T-47D cells led to reduced MYC expression. ChIP analysis demonstrated that menin bound not only to the MYC promoter but also to its 5' enhancer. Furthermore, E2-treated MEN1 KD MCF7 cells displayed a decrease in MYC activation, suggesting its role in estrogen-mediated MYC transcription. Finally, expression data mining in tumors revealed a correlation between the expression of MEN1 mRNA and that of several mTORC1 components and targets and a significant inverse correlation between MEN1 and two MYC inhibitory factors, MYCBP2 and MYCT1, in ER+ BC. The current work thus highlights altered mTORC1 and MYC pathways after menin inactivation in ER+ BC cells, providing insight into the crosstalk between menin, mTORC1 and MYC in ER+ BC.

Molecular Classification and Overcoming Therapy Resistance for Acute Myeloid Leukemia with Adverse Genetic Factors

The European LeukemiaNet (ELN) criteria define the adverse genetic factors of acute myeloid leukemia (AML). AML with adverse genetic factors uniformly shows resistance to standard chemotherapy and is associated with poor prognosis. Here, we focus on the biological background and real-world etiology of these adverse genetic factors and then describe a strategy to overcome the clinical disadvantages in terms of targeting pivotal molecular mechanisms. Different adverse genetic factors often rely on common pathways. KMT2A rearrangement, DEK-NUP214 fusion, and NPM1 mutation are associated with the upregulation of HOX genes. The dominant tyrosine kinase activity of the mutant FLT3 or BCR-ABL1 fusion proteins is transduced by the AKT-mTOR, MAPK-ERK, and STAT5 pathways. Concurrent mutations of ASXL1 and RUNX1 are associated with activated AKT. Both TP53 mutation and mis-expressed MECOM are related to impaired apoptosis. Clinical data suggest that adverse genetic factors can be found in at least one in eight AML patients and appear to accumulate in relapsed/refractory cases. TP53 mutation is associated with particularly poor prognosis. Molecular-targeted therapies focusing on specific genomic abnormalities, such as FLT3, KMT2A, and TP53, have been developed and have demonstrated promising results.

Menin Enhances Androgen Receptor-Independent Proliferation and Migration of Prostate Cancer Cells

The androgen receptor (AR) is an important therapeutic target for treating prostate cancer (PCa). Moreover, there is an increasing need for understanding the AR-independent progression of tumor cells such as neuroendocrine prostate cancer (NEPC). Menin, which is encoded by multiple endocrine neoplasia type 1 (MEN1), serves as a direct link between AR and the mixed-lineage leukemia (MLL) complex in PCa development by activating AR target genes through histone H3 lysine 4 methylation. Although menin is a critical component of AR signaling, its tumorigenic role in AR-independent PCa cells remains unknown. Here, we compared the role of menin in AR-positive and AR-negative PCa cells via RNAi-mediated or pharmacological inhibition of menin. We demonstrated that menin was involved in tumor cell growth and metastasis in PCa cells with low or deficient levels of AR. The inhibition of menin significantly diminished the growth of PCa cells and induced apoptosis, regardless of the presence of AR. Additionally, transcriptome analysis showed that the expression of many metastasis-associated genes was perturbed by menin inhibition in AR-negative DU145 cells. Furthermore, wound-healing assay results showed that menin promoted cell migration in AR-independent cellular contexts. Overall, these findings suggest a critical function of menin in tumorigenesis and provide a rationale for drug development against menin toward targeting high-risk metastatic PCa, especially those independent of AR.

The role of menin in bone pathology

Multiple endocrine neoplasia type 1 (MEN1) is the most common cause of hereditary primary hyperparathyroidism (PHPT). Bone disorders are considered one of the key symptoms in PHPT present with the significant reduction in bone mineral density and low-energy fractures. Previously, these bone disorders were believed to be caused solely by the increase in the level of parathyroid hormone and its subsequent effect on bone resorption. The current paradigm, however, states that the mutations in the menin gene, which cause the development of MEN1, can also affect the metabolism of the cells of the osteoid lineage. This review analyzes both the proven and the potential intracellular mechanisms through which menin can affect bone metabolism.

Translating recent advances in the pathogenesis of acute myeloid leukemia to the clinic

Despite FDA approval of nine new drugs for patients with acute myeloid leukemia (AML) in the United States over the last 4 years, AML remains a major area of unmet medical need among hematologic malignancies. In this review, we discuss the development of promising new molecular targeted approaches for AML, including menin inhibition, novel IDH1/2 inhibitors, and preclinical means to target TET2, ASXL1, and RNA splicing factor mutations. In addition, we review progress in immune targeting of AML through anti-CD47, anti-SIRPα, and anti-TIM-3 antibodies; bispecific and trispecific antibodies; and new cellular therapies in development for AML.

Emerging Targeted Therapy for Specific Genomic Abnormalities in Acute Myeloid Leukemia

We describe recent updates of existing molecular-targeting agents and emerging novel gene-specific strategies. FLT3 and IDH inhibitors are being tested in combination with conventional chemotherapy for both medically fit patients and patients who are ineligible for intensive therapy. FLT3 inhibitors combined with non-cytotoxic agents, such as BCL-2 inhibitors, have potential therapeutic applicability. The menin-MLL complex pathway is an emerging therapeutic target. The pathway accounts for the leukemogenesis in AML with MLL-rearrangement, NPM1 mutation, and NUP98 fusion genes. Potent menin-MLL inhibitors have demonstrated promising anti-leukemic effects in preclinical studies. The downstream signaling molecule SYK represents an additional target. However, the TP53 mutation continues to remain a challenge. While the p53 stabilizer APR-246 in combination with azacitidine failed to show superiority compared to azacitidine monotherapy in a phase 3 trial, next-generation p53 stabilizers are now under development. Among a number of non-canonical approaches to TP53-mutated AML, the anti-CD47 antibody magrolimab in combination with azacitidine showed promising results in a phase 1b trial. Further, the efficacy was somewhat better in patients with the TP53 mutation. Although clinical evidence has not been accumulated sufficiently, targeting activating KIT mutations and RAS pathway-related molecules can be a future therapeutic strategy.

Neuronal Menin Overexpression Rescues Learning and Memory Phenotype in CA1-Specific α7 nAChRs KD Mice

The perturbation of nicotinic cholinergic receptors is thought to underlie many neurodegenerative and neuropsychiatric disorders, such as Alzheimer's and schizophrenia. We previously identified that the tumor suppressor gene, MEN1, regulates both the expression and synaptic targeting of α7 nAChRs in the mouse hippocampal neurons in vitro. Here we sought to determine whether the α7 nAChRs gene expression reciprocally regulates the expression of menin, the protein encoded by the MEN1 gene, and if this interplay impacts learning and memory. We demonstrate here that α7 nAChRs knockdown (KD) both in in vitro and in vivo, initially upregulated and then subsequently downregulated menin expression. Exogenous expression of menin using an AAV transduction approach rescued α7 nAChRs KD mediated functional and behavioral deficits specifically in hippocampal (CA1) neurons. These effects involved the modulation of the α7 nAChR subunit expression and functional clustering at the synaptic sites. Our data thus demonstrates a novel and important interplay between the MEN1 gene and the α7 nAChRs in regulating hippocampal-dependent learning and memory.

Antisense Oligonucleotide-Based Therapeutic against Menin for Triple-Negative Breast Cancer Treatment

The tumor suppressor menin has dual functions, acting either as a tumor suppressor or as an oncogene/oncoprotein, depending on the oncological context. Triple-negative breast cancer (TNBC) is characterized by the lack of expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (ERBB2/HER2) and is often a basal-like breast cancer. TNBC is associated with a dismal prognosis and an insufficient response to chemotherapies. Previously, menin was shown to play a proliferative role in ER-positive breast cancer; however, the functions of menin in TNBC remain unknown. Here, we have demonstrated that menin is expressed in various TNBC subtypes with the strongest expression in the TNBC Hs 578T cells. The depletion of menin by an antisense oligonucleotide (ASO) inhibits cell proliferation, enhances apoptosis in Hs 578T cells, highlighting the oncogenic functions of menin in this TNBC model. ASO-based menin silencing also delays the tumor progression of TNBC xenografts. Analysis of the menin interactome suggests that menin could drive TNBC tumorigenesis through the regulation of MLL/KMT2A-driven transcriptional activity, mRNA 3′-end processing and apoptosis. The study provides a rationale behind the use of ASO-based therapy, targeting menin in monotherapy or in combination with chemo or PARP inhibitors for menin-positive TNBC treatments.

Spatiotemporal Patterns of Menin Localization in Developing Murine Brain: Co-Expression with the Elements of Cholinergic Synaptic Machinery

Menin, a product of MEN1 (multiple endocrine neoplasia type 1) gene is an important regulator of tissue development and maintenance; its perturbation results in multiple tumors—primarily of the endocrine tissue. Despite its abundance in the developing central nervous system (CNS), our understanding of menin’s role remains limited. Recently, we discovered menin to play an important role in cholinergic synaptogenesis in the CNS, whereas others have shown its involvement in learning, memory, depression and apoptosis. For menin to play these important roles in the CNS, its expression patterns must be corroborated with other components of the synaptic machinery imbedded in the learning and memory centers; this, however, remains to be established. Here, we report on the spatio-temporal expression patterns of menin, which we found to exhibit dynamic distribution in the murine brain from early development, postnatal period to a fully-grown adult mouse brain. We demonstrate here that menin expression is initially widespread in the brain during early embryonic stages, albeit with lower intensity, as determined by immunohistochemistry and gene expression. With the progression of development, however, menin expression became highly localized to learning, memory and cognition centers in the CNS. In addition to menin expression patterns throughout development, we provide the first direct evidence for its co-expression with nicotinic acetylcholine, glutamate and GABA (gamma aminobutyric acid) receptors—concomitant with the expression of both postsynaptic (postsynaptic density protein PSD-95) and presynaptic (synaptotagamin) proteins. This study is thus the first to provide detailed analysis of spatio-temporal patterns of menin expression from initial CNS development to adulthood. When taken together with previously published studies, our data underscore menin’s importance in the cholinergic neuronal network assembly underlying learning, memory and cognition.

A Box of Chemistry to Inhibit the MEN1 Tumor Suppressor Gene Promoting Leukemia

Targeting protein-protein interactions (PPIs) with small-molecule inhibitors has become a hotbed of modern drug development. In this review, we describe a new class of PPI inhibitors that block menin from binding to MLL proteins. Menin is encoded by the MEN1 tumor suppressor, but acts as an essential cofactor for MLL/KMT2A-rearranged leukemias. The most promising menin-MLL inhibitors belong to the thienopyrimidine class and have recently entered phase I/II clinical trials for treating acute leukemias characterized by MLL/KMT2A translocations or NPM1 mutations. As single agents, thienopyrimidine compounds eradicate leukemia in a xenograft models of primary leukemic cells belonging to the MLL-rearranged or NPM1-mutant subtypes. These compounds are well tolerated with few or no side effects, which is remarkable given the tumor-suppressor function of menin. The menin-MLL inhibitors highlight how leukemia patients could benefit from a targeted epigenetic therapy with novel PPI inhibitors obtained by directed chemical evolution.

Pancreatic Neuroendocrine Neoplasms in Multiple Endocrine Neoplasia Type 1

Pancreatic neuroendocrine tumors (pNETs) are a rare group of cancers accounting for about 1-2% of all pancreatic neoplasms. About 10% of pNETs arise within endocrine tumor syndromes, such as Multiple Endocrine Neoplasia type 1 (MEN1). pNETs affect 30-80% of MEN1 patients, manifesting prevalently as multiple microadenomas. pNETs in patients with MEN1 are particularly difficult to treat due to differences in their growth potential, their multiplicity, the frequent requirement of extensive surgery, the high rate of post-operative recurrences, and the concomitant development of other tumors. MEN1 syndrome is caused by germinal heterozygote inactivating mutation of the MEN1 gene, encoding the menin tumor suppressor protein. MEN1-related pNETs develop following the complete loss of function of wild-type menin. Menin is a key regulator of endocrine cell plasticity and its loss in these cells is sufficient for tumor initiation. Somatic biallelic loss of wild-type menin in the neuroendocrine pancreas presumably alters the epigenetic control of gene expression, mediated by histone modifications and DNA hypermethylation, as a driver of MEN1-associated pNET tumorigenesis. In this light, epigenetic-based therapies aimed to correct the altered DNA methylation, and/or histone modifications might be a possible therapeutic strategy for MEN1 pNETs, for whom standard treatments fail.

Menin-regulated Pbk controls high fat diet-induced compensatory beta cell proliferation

Pancreatic beta cells undergo compensatory proliferation in the early phase of type 2 diabetes. While pathways such as FoxM1 are involved in regulating compensatory beta cell proliferation, given the lack of therapeutics effectively targeting beta cell proliferation, other targetable pathways need to be identified. Herein, we show that Pbk, a serine/threonine protein kinase, is essential for high fat diet (HFD)‐induced beta cell proliferation in vivo using a Pbk kinase deficiency knock‐in mouse model. Mechanistically, JunD recruits menin and HDAC3 complex to the Pbk promoter to reduce histone H3 acetylation, leading to epigenetic repression of Pbk expression. Moreover, menin inhibitor (MI) disrupts the menin–JunD interaction and augments Pbk transcription. Importantly, MI administration increases beta cell proliferation, ameliorating hyperglycemia, and impaired glucose tolerance (IGT) in HFD‐induced diabetic mice. Notably, Pbk is required for the MI‐induced beta cell proliferation and improvement of IGT. Together, these results demonstrate the repressive role of the menin/JunD/Pbk axis in regulating HFD‐induced compensatory beta cell proliferation and pharmacologically regulating this axis may serve as a novel strategy for type 2 diabetes therapy.

Multiple Endocrine Neoplasia Type 1: Latest Insights

Multiple endocrine neoplasia type 1 (MEN1), a rare tumor syndrome that is inherited in an autosomal dominant pattern, is continuing to raise great interest for endocrinology, gastroenterology, surgery, radiology, genetics, and molecular biology specialists. There have been 2 major clinical practice guidance papers published in the past 2 decades, with the most recent published 8 years ago. Since then, several new insights on the basic biology and clinical features of MEN1 have appeared in the literature, and those data are discussed in this review. The genetic and molecular interactions of the MEN1-encoded protein menin with transcription factors and chromatin-modifying proteins in cell signaling pathways mediated by transforming growth factor β/bone morphogenetic protein, a few nuclear receptors, Wnt/β-catenin, and Hedgehog, and preclinical studies in mouse models have facilitated the understanding of the pathogenesis of MEN1-associated tumors and potential pharmacological interventions. The advancements in genetic diagnosis have offered a chance to recognize MEN1-related conditions in germline MEN1 mutation-negative patients. There is rapidly accumulating knowledge about clinical presentation in children, adolescents, and pregnancy that is translatable into the management of these very fragile patients. The discoveries about the genetic and molecular signatures of sporadic neuroendocrine tumors support the development of clinical trials with novel targeted therapies, along with advancements in diagnostic tools and surgical approaches. Finally, quality of life studies in patients affected by MEN1 and related conditions represent an effort necessary to develop a pharmacoeconomic interpretation of the problem. Because advances are being made both broadly and in focused areas, this timely review presents and discusses those studies collectively.

Menin and Menin-Associated Proteins Coregulate Cancer Energy Metabolism

The interplay between glycolysis and mitochondrial oxidative phosphorylation (OXPHOS) is central to maintain energy homeostasis. It remains to be determined whether there is a mechanism governing metabolic fluxes based on substrate availability in microenvironments. Here we show that menin is a key transcription factor regulating the expression of OXPHOS and glycolytic genes in cancer cells and primary tumors with poor prognosis. A group of menin-associated proteins (MAPs), including KMT2A, MED12, WAPL, and GATA3, is found to restrain menin's full function in this transcription regulation. shRNA knockdowns of menin and MAPs result in reduced ATP production with proportional alterations of cellular energy generated through glycolysis and OXPHOS. When shRNA knockdown cells are exposed to metabolic stress, the dual functionality can clearly be distinguished among these metabolic regulators. A MAP can negatively counteract the regulatory mode of menin for OXPHOS while the same protein positively influences glycolysis. A close-proximity interaction between menin and MAPs allows transcriptional regulation for metabolic adjustment. This coordinate regulation by menin and MAPs is necessary for cells to rapidly adapt to fluctuating microenvironments and to maintain essential metabolic functions.

Novel Germline c.105_107dupGCT MEN1 Mutation in a Family with Newly Diagnosed Multiple Endocrine Neoplasia Type 1

In multiple endocrine neoplasia type 1 (MEN1), the causative MEN1 gene mutations lead to the reduced expression of menin, which is a tumor suppressor protein. In this study, we present a case of a 16-year-old woman with severe primary hyperparathyroidism and a non-functioning pituitary microadenoma. Genetic testing demonstrated a novel germline heterozygote variant c.105_107dupGCT of MEN1, leading to Leu duplication in position 37 of the menin polypeptide chain. As such a mutation was not reported before as a causative one, confirmation of its pathogenicity required showing the same mutation in a symptomatic first-degree relative. An identical mutation was found in the patient’s father, who was further diagnosed with hyperparathyroidism and a pituitary microadenoma. We observed the presence of the same MEN1-related tumors but an entirely different symptom severity. To the best of our knowledge, this is the first report of MEN1 syndrome caused by the c.105_107dupGCT MEN1 mutation. This case report demonstrates the importance of genetic evaluation towards MEN1. Genetic testing for MEN1 mutations should be performed in all patients with MEN1-related tumors, and in the young patients even with only one such tumor, despite the supposedly negative family history.

Clinicopathological features and menin expression of pancreatic neuroendocrine neoplasm associated with multiple endocrine neoplasia type 1

Background/Purpose

We examined therapeutic strategies for pancreatic neuroendocrine neoplasm (pNEN) associated with MEN1 (M‐pNEN) by investigating clinicopathological features and menin expression.

Methods

Seventy‐seven patients who underwent resection of pNEN at our department from January 2001 to December 2017 were retrospectively analyzed. Immunohistochemical analysis of menin was performed using resected specimens.

Results

Seven patients (9%) met the diagnostic criteria for MEN1. M‐pNEN had more tumors (P < .01), a higher recurrence rate (P = .028), and higher residual pancreatic recurrence (P < .01) than sporadic pNEN (S‐pNEN). There were no significant differences in tumor size, lymph node metastasis, or World Health Organization grade between the two groups. Reduced menin staining in the tumor nuclei was found in 86% of M‐pNEN; whereas only 34% of S‐pNEN showed decreased nuclear staining. The remainder (66%) showed strong nuclear staining similar to normal islet cells (P = .0071). Furthermore, four patients (57%) with MEN1 had many microadenomas with reduced nuclear menin staining. Overall survival of M‐pNEN patients was significantly better than S‐pNEN patients (P = .049).

Conclusion

M‐pNEN patients tend to develop spatially and temporally multifocal pNENs. However, M‐pNEN patient prognosis is good with repeated surgeries at recurrence. Therefore, minimal resection with strict follow‐up is recommended rather than extensive pancreatic resections for consideration of recurrence in M‐pNEN.

Genetic background influences tumour development in heterozygous Men1 knockout mice

Multiple endocrine neoplasia type 1 (MEN1), an autosomal dominant disorder caused by MEN1 germline mutations, is characterised by parathyroid, pancreatic and pituitary tumours. MEN1 mutations also cause familial isolated primary hyperparathyroidism (FIHP), a milder condition causing hyperparathyroidism only. Identical mutations can cause either MEN1 or FIHP in different families, thereby implicating a role for genetic modifiers in altering phenotypic expression of tumours. We therefore investigated the effects of genetic background and potential for genetic modifiers on tumour development in adult Men1+/- mice, which develop tumours of the parathyroids, pancreatic islets, anterior pituitary, adrenal cortex and gonads, that had been backcrossed to generate C57BL/6 and 129S6/SvEv congenic strains. A total of 275 Men1+/- mice, aged 5-26 months were macroscopically studied, and this revealed that genetic background significantly influenced the development of pituitary, adrenal and ovarian tumours, which occurred in mice over 12 months of age and more frequently in C57BL/6 females, 129S6/SvEv males and 129S6/SvEv females, respectively. Moreover, pituitary and adrenal tumours developed earlier, in C57BL/6 males and 129S6/SvEv females, respectively, and pancreatic and testicular tumours developed earlier in 129S6/SvEv males. Furthermore, glucagon-positive staining pancreatic tumours occurred more frequently in 129S6/SvEv Men1+/- mice. Whole genome sequence analysis of 129S6/SvEv and C57BL/6 Men1+/- mice revealed >54,000 different variants in >300 genes. These included, Coq7, Dmpk, Ccne2, Kras, Wnt2b, Il3ra and Tnfrsf10a, and qRT-PCR analysis revealed that Kras was significantly higher in pituitaries of male 129S6/SvEv mice. Thus, our results demonstrate that Kras and other genes could represent possible genetic modifiers of Men1.

Neuron-Specific Menin Deletion Leads to Synaptic Dysfunction and Cognitive Impairment by Modulating p35 Expression

Menin (MEN1) is a critical modulator of tissue development and maintenance. As such, MEN1 mutations are associated with multiple endocrine neoplasia type 1 (MEN1) syndrome. Although menin is abundantly expressed in the nervous system, little is known with regard to its function in the adult brain. Here, we demonstrate that neuron-specific deletion of Men1 (CcKO) affects dendritic branching and spine formation, resulting in defects in synaptic function, learning, and memory. Furthermore, we find that menin binds to the p35 promoter region to facilitate p35 transcription. As a primary Cdk5 activator, p35 is expressed mainly in neurons and is critical for brain development and synaptic plasticity. Restoration of p35 expression in the hippocampus and cortex of Men1 CcKO mice rescues synaptic and cognitive deficits associated with Men1 deletion. These results reveal a critical role for menin in synaptic and cognitive function by modulating the p35-Cdk5 pathway.

Menin Upregulates FOXO1 Protein Stability by Repressing Skp2-Mediated Degradation in β Cells

OBJECTIVES

Menin, a chromatin binding protein, interacts with various epigenetic regulators to regulate gene transcription, whereas forkhead box protein O1 (FOXO1) is a transcription factor that can be regulated by multiple signaling pathways. Both menin and FOXO1 are crucial regulators of β-cell function and metabolism; however, whether or how they interplay to regulate β cells is not clear.

METHODS

To examine whether menin affects expression of FOXO1, we ectopically expressed menin complementary DNA and small hairpin RNA targeting menin via a retroviral vector in INS-1 cells. Western blotting was used to analyze protein levels.

RESULTS

Our current work shows that menin increases the expression of FOXO1. Menin stabilizes FOXO1 protein level in INS-1 cells, as shown by increased half-life of FOXO1 by menin expression. Moreover, menin represses ubiquitination of FOXO1 protein and AKT phosphorylation, We found that menin stabilizes FOXO1 by repressing FOXO1 degradation mediated by S-phase kinase-associated protein 2 (Skp2), an E3 ubiquitin ligase, promoting caspase 3 activation and apoptosis.

CONCLUSIONS

Because FOXO1 upregulates the menin gene transcription, our findings unravel a crucial menin and FOXO1 interplay, with menin and FOXO1 upregulating their expression reciprocally, forming a positive feedback loop to sustain menin and FOXO1 expression.

First Case of Mature Teratoma and Yolk Sac Testis Tumor Associated to Inherited MEN-1 Syndrome

Introduction: Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominantly inherited endocrine tumor syndrome characterized by the development of cancer in various endocrine organs, particularly in the pituitary, parathyroid and pancreas. Moreover, in some cases, also non-endocrine tumors can be diagnosed, developing atypical phenotypes. Case report: We report herein the clinical history of a patient affected by MEN-1 syndrome who developed atypical features for this disease. The patient's clinical history started in August 2015 when he was referred, at the age of 23 years, to the Emergency Department of our Hospital for the occurrence of progressive asthenia, weakness, tremors and syncope. The biochemical test documented hyper-calcemia and severe hypoglycemia. The patient was referred to our Neuroendocrine Tumor and Pituitary Unit and he was diagnosed with pancreatic insulinoma, hypercalcemic hyperparathyroidism, and a prolactin secreting pituitary adenoma. The MEN-1 syndrome was suspected and genetic tests for mutation of menin resulted positive for the pathogenic variant c1548dupG. In January 2016, the patient was diagnosed with intratubular germ cell neoplasia, consisting of a mature teratoma and yolk sac tumor and he underwent a right orchiectomy. Conclusion: This is the first case report showing the clear association of MEN-1 syndrome with yolk sac tumors and teratomas, as in our case, the c1548dupG represents a pathogenic variant rather than a SNP. This case suggests the opportunity of an accurate evaluation of the testis particularly in young MEN-1 affected patients and that a prompt screening for neoplastic disease should involve all the endocrine glands.

MiR-15a/miR-16-1 expression inversely correlates with cyclin D1 levels in Men1 pituitary NETs

Multiple Endocrine Neoplasia type 1 (MEN1) is an autosomal dominant disorder characterised by the combined occurrence of parathyroid, pituitary and pancreatic islet tumours, and is due to mutations of the MEN1 gene, which encodes the tumour suppressor protein menin. Menin has multiple roles in genome stability, transcription, cell division and proliferation, but its mechanistic roles in tumourigenesis remain to be fully elucidated. MicroRNAs (miRNA) are non-coding single stranded RNAs that post-transcriptionally regulate gene expression and have been associated with tumour development, although the contribution of miRNAs to MEN1-associated tumourigenesis and their relationship with menin expression are not fully understood. Alterations in miRNA expression, including downregulation of three putative 'tumour suppressor' miRNAs, miR-15a, miR-16-1 and let-7a, have been reported in several tumour types including non-MEN1 pituitary adenomas. We have therefore investigated the expression of miR-15a, miR-16-1 and let-7a in pituitary tumours that developed after 12 months of age in female mice with heterozygous knock out of the Men1 gene (Men1+/- mice). The miRNAs miR-15a, miR-16-1 and let-7a were significantly downregulated in pituitary tumours (by 2.3-fold, p<0.05; 2.1-fold p<0.01 and 1.6-fold p<0.05, respectively) of Men1+/- mice, compared to normal wild type pituitaries. MiR-15a and miR-16-1 expression inversely correlated with expression of cyclin D1, a known pro-tumourigenic target of these miRNAs, and knock down of menin in a human cancer cell line (HeLa), and AtT20 mouse pituitary cell line resulted in significantly decreased expression of miR-15a (p<0.05), indicating that the decrease in miR-15a may be a direct result of lost menin expression.

Tumorigenicity of Ewing sarcoma is critically dependent on the trithorax proteins MLL1 and menin

Developmental transcription programs are epigenetically regulated by the competing actions of polycomb and trithorax (TrxG) protein complexes, which repress and activate genes, respectively. Ewing sarcoma is a developmental tumor that is associated with widespread de-regulation of developmental transcription programs, including HOX programs. Posterior HOXD genes are abnormally over-expressed by Ewing sarcoma and HOXD13, in particular, contributes to the tumorigenic phenotype. In MLL1 fusion-driven leukemia, aberrant activation of HOXA genes is epigenetically mediated by the TrxG complex and HOXA gene expression and leukemogenesis are critically dependent on the protein-protein interaction between the TrxG proteins MLL1 and menin. Based on these data, we investigated whether posterior HOXD gene activation and Ewing sarcoma tumorigenicity are similarly mediated by and dependent on MLL1 and/or menin. Our findings demonstrate that Ewing sarcomas express high levels of both MLL1 and menin and that continued expression of both proteins is required for maintenance of tumorigenicity. In addition, exposure of Ewing sarcoma cells to MI-503, an inhibitor of the MLL1-menin protein-protein interaction developed for MLL1-fusion driven leukemia, leads to loss of tumorigenicity and down-regulated expression of the posterior HOXD gene cluster. Together these data demonstrate an essential role for MLL1 and menin in mediating tumor maintenance and posterior HOXD gene activation in Ewing sarcoma. A critical dependency of these tumors on the MLL1-menin interaction presents a potentially novel therapeutic target.

Enhancer-Mediated Oncogenic Function of the Menin Tumor Suppressor in Breast Cancer

While the multiple endocrine neoplasia type 1 (MEN1) gene functions as a tumor suppressor in a variety of cancer types, we explored its oncogenic role in breast tumorigenesis. The MEN1 gene product menin is involved in H3K4 trimethylation and co-activates transcription. We integrated ChIP-seq and RNA-seq data to identify menin target genes. Our analysis revealed that menin-dependent target gene promoters display looping to distal enhancers that are bound by menin, FOXA1 and GATA3. In this fashion, MEN1 co-regulates a proliferative breast cancer-specific gene expression program in ER⁺ cells. In primary mammary cells, MEN1 exerts an anti-proliferative function by regulating a distinct expression signature. Our findings clarify the cell-type-specific functions of MEN1 and inform the development of menin-directed treatments for breast cancer.

Twenty years of menin: emerging opportunities for restoration of transcriptional regulation in MEN1

Since the discovery of the multiple endocrine neoplasia type 1 (MEN1) gene in 1997, elucidation of the molecular function of its protein product, menin, has been a challenge. Biochemical, proteomics, genetics and genomics approaches have identified various potential roles, which converge on gene expression regulation. The most consistent findings show that menin connects transcription factors and chromatin-modifying enzymes, in particular, the histone H3K4 methyltransferase complexes MLL1 and MLL2. Chromatin immunoprecipitation combined with next-generation sequencing has enabled studying genome-wide dynamics of chromatin binding by menin. We propose that menin regulates cell type-specific transcriptional programs by linking chromatin regulatory complexes to specific transcription factors. In this fashion, the MEN1 gene is a tumor suppressor gene in the endocrine tissues that are affected in MEN1. Recent studies have hinted at possibilities to pharmacologically restore the epigenetic changes caused by loss of menin function as therapeutic strategies for MEN1, for example, by inhibition of histone demethylases. The current lack of appropriate cellular model systems for MEN1-associated tumors is a limitation for compound testing, which needs to be addressed in the near future. In this review, we look back at the past twenty years of research on menin and the mechanism of disease of MEN1. In addition, we discuss how the current understanding of the molecular function of menin offers future directions to develop novel treatments for MEN1-associated endocrine tumors.

Animal models of MEN1

Animal models of cancer have been instrumental in advancing our understanding of the biology of tumor initiation and progression, in studying gene function and in performing preclinical studies aimed at testing novel therapies. Several animal models of the MEN1 syndrome have been generated in different organisms by introducing loss-of-function mutations in the orthologues of the human MEN1 gene. In this review, we will discuss MEN1 and MEN1-like models in Drosophila, mice and rats. These model systems with their specific advantages and limitations have contributed to elucidate the function of Menin in tumorigenesis, which turned out to be remarkably conserved from flies to mammals, as well as the biology of the disease. Mouse models of MEN1 closely resemble the human disease in terms of tumor spectrum and associated hormonal changes, although individual tumor frequencies are variable. Rats affected by the MENX (MEN1-like) syndrome share some features with MEN1 patients albeit they bear a germline mutation in Cdkn1b (p27) and not in Men1 Both Men1-knockout mice and MENX rats have been exploited for therapy-response studies testing novel drugs for efficacy against neuroendocrine tumors (NETs) and have provided promising leads for novel therapies. In addition to presenting well-established models of MEN1, we also discuss potential models which, if implemented, might broaden even further our knowledge of neuroendocrine tumorigenesis. In the future, patient-derived xenografts in zebrafish or mice might allow us to expand the tool-box currently available for preclinical studies of MEN1-associated tumors.

Epigenetic regulation by the menin pathway

There is a trend of increasing prevalence of neuroendocrine tumors (NETs), and the inherited multiple endocrine neoplasia type 1 (MEN1) syndrome serves as a genetic model to investigate how NETs develop and the underlying mechanisms. Menin, encoded by the MEN1 gene, at least partly acts as a scaffold protein by interacting with multiple partners to regulate cellular homeostasis of various endocrine organs. Menin has multiple functions including regulation of several important signaling pathways by controlling gene transcription. Here, we focus on reviewing the recent progress in elucidating the key biochemical role of menin in epigenetic regulation of gene transcription and cell signaling, as well as posttranslational regulation of menin itself. In particular, we will review the progress in studying structural and functional interactions of menin with various histone modifiers and transcription factors such as MLL, PRMT5, SUV39H1 and other transcription factors including c-Myb and JunD. Moreover, the role of menin in regulating cell signaling pathways such as TGF-beta, Wnt and Hedgehog, as well as miRNA biogenesis and processing will be described. Further, the regulation of the MEN1 gene transcription, posttranslational modifications and stability of menin protein will be reviewed. These various modes of regulation by menin as well as regulation of menin by various biological factors broaden the view regarding how menin controls various biological processes in neuroendocrine organ homeostasis.

Menin mediates Tat-induced neuronal apoptosis in brain frontal cortex of SIV-infected macaques and in Tat-treated cells

The molecular mechanisms involved in human immunodeficiency virus (HIV)-associated neurocognitive disorder (HAND) remain poorly understood. It has been recently reported that HIV-1 Tat transactivation requires menin, suggesting that menin may be involved in HAND pathogenesis. But the role of menin is not clear. Here, we found that protein level of menin was increased in simian-human immunodeficiency chimeric virus (SHIV)_-SF162.P4 and simian immunodeficiency virus (SIV) _sm543-3-infected rhesus macaques compared with the controls by immunohistochemistry (IHC) and western blot. Menin mainly expressed in the frontal cortex neurons of the brain, more importantly, the number of menin-staining cells was positively correlated with cleaved-caspase-3-positive cells while it was negatively correlated with a neuron-specific nuclear protein NeuN-positive cells, suggesting that expression of menin may induce neuronal apoptosis. Further studies showed that menin level was significantly increased during Tat-induced apoptosis, while downregulation of menin by pll3.7-MEN1-shRNA attenuated the Tat-induced cleavage of caspase-3 and caspase-8 in SY5Y cells and primary neuron cultures. Together, our findings reveal a pro-apoptotic role of menin in the brains of the SIV-infected macaques and the cultured neurons, indicating that targeting menin may be potential to block the HIV-1 Tat induced neuronal damage in HAND.

Menin and PRMT5 suppress GLP1 receptor transcript and PKA-mediated phosphorylation of FOXO1 and CREB

Menin is a scaffold protein that interacts with several epigenetic mediators to regulate gene transcription, and suppresses pancreatic β-cell proliferation. Tamoxifen-inducible deletion of multiple endocrine neoplasia type 1 (MEN1) gene, which encodes the protein menin, increases β-cell mass in multiple murine models of diabetes and ameliorates diabetes. Glucagon-like-peptide-1 (GLP1) is another key physiological modulator of β-cell mass and glucose homeostasis. However, it is not clearly understood whether menin crosstalks with GLP1 signaling. Here, we show that menin and protein arginine methyltransferase 5 (PRMT5) suppress GLP1 receptor (GLP1R) transcript levels. Notably, a GLP1R agonist induces phosphorylation of forkhead box protein O1 (FOXO1) at S253, and the phosphorylation is mediated by PKA. Interestingly, menin suppresses GLP1-induced and PKA-mediated phosphorylation of both FOXO1 and cAMP response element binding protein (CREB), likely through a protein arginine methyltransferase. Menin-mediated suppression of FOXO1 and CREB phosphorylation increases FOXO1 levels and suppresses CREB target genes, respectively. A small-molecule menin inhibitor reverses menin-mediated suppression of both FOXO1 and CREB phosphorylation. In addition, ex vivo treatment of both mouse and human pancreatic islets with a menin inhibitor increases levels of proliferation marker Ki67. In conclusion, our results suggest that menin and PRMT5 suppress GLP1R transcript levels and PKA-mediated phosphorylation of FOXO1 and CREB, and a menin inhibitor may reverse this suppression to induce β-cell proliferation.

miR-762 can negatively regulate menin in ovarian cancer

Ovarian cancer accounts for the major part of the mortality attributable to female reproductive system malignant tumors worldwide. Recently, the incidence of ovarian cancer has been increasing annually, and there remains a lack of suitable treatment methods that can significantly improve the 5-year survival rates of patients. Therefore, it is necessary to identify more effective treatments for ovarian cancer. It is established that microRNAs (miRNAs) have important roles in the diagnosis and treatment of ovarian cancer and a specific miRNA, miR-762, can promote the development of a variety of tumors. Menin is encoded by MEN1, a tumor suppressor gene, that is usually downregulated in ovarian cancer. In this study, we evaluated the expression levels of miR-762 and menin in ovarian cancer tissues and demonstrated that they were correlated. In addition, we found that miR-762 can downregulate the expression of menin through a binding site in its 3′-UTR and consequently upregulate the Wnt cell signaling pathway to promote the development of ovarian cancer. These results indicate that miR-762 is a promising potential target for the treatment of ovarian cancer.

EXPRESSION OF MENIN IN THE HUMAN THYROID GLAND

INTRODUCTION

The expression of menin in the thyroid gland has long been debated. Animal models with targeted inactivation of menin in the thyroid gland have shown that its inactivation might play a role in the progression to a more aggressive type of cancer. Human studies are conflicting, some have identified mutations in the MEN1 gene in a sub-type of oncocytic thyroid carcinomas, while others have not identified a higher prevalence of thyroid cancer in MEN1 patients.

OBJECTIVE

To analyze the immunohistochemical expression of menin in different types of thyroid carcinomas.

MATERIALS AND METHODS

48 thyroid tumours (12 papillary thyroid carcinomas (PTC), 6 anaplastic thyroid carcinomas (ATC), 12 poorly differentiated thyroid carcinomas (PDTC), 5 medullary thyroid carcinomas (MTC), 5 oncocytic follicular carcinomas (OC), 3 oncocytic adenomas (OA) and 5 goiters (G)) were tested for nuclear expression of menin using an anti-menin antibody. The expression was considered positive, negative or decreased.

RESULTS

The expression of menin was positive, identical to normal tissue, in 39 cases (81.25%). The expression was decreased (n=8) or absent (n=1) in 9 tumours (18.75% - 2 PTC, 5 PDTC, 2 OC) accounting for 42% (5/12) of the PDTC and 40% (2/5) of the OC.

CONCLUSIONS

Our results show that the expression of menin is generally preserved in human thyroid carcinomas, but it can be decreased or absent in certain types of thyroid cancer. Further molecular studies are needed to evaluate to potential of menin protein in tumorigenesis.

Foxa2, a novel protein partner of the tumour suppressor menin, is deregulated in mouse and human MEN1 glucagonomas

Foxa2, known as one of the pioneer factors, plays a crucial role in islet development and endocrine functions. Its expression and biological functions are regulated by various factors, including, in particular, insulin and glucagon. However, its expression and biological role in adult pancreatic α-cells remain elusive. In the current study, we showed that Foxa2 was overexpressed in islets from α-cell-specific Men1 mutant mice, at both the transcriptional level and the protein level. More importantly, immunostaining analyses showed its prominent nuclear accumulation, specifically in α-cells, at a very early stage after Men1 disruption. Similar nuclear FOXA2 expression was also detected in a substantial proportion (12/19) of human multiple endocrine neoplasia type 1 (MEN1) glucagonomas. Interestingly, our data revealed an interaction between Foxa2 and menin encoded by the Men1 gene. Furthermore, using several approaches, we demonstrated the relevance of this interaction in the regulation of two tested Foxa2 target genes, including the autoregulation of the Foxa2 promoter by Foxa2 itself. The current study establishes menin, a novel protein partner of Foxa2, as a regulator of Foxa2, the biological functions of which extend beyond the pancreatic endocrine cells. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Menin and Daxx Interact to Suppress Neuroendocrine Tumors through Epigenetic Control of the Membrane Metallo-Endopeptidase

Neuroendocrine tumors (NET) often harbor loss-of-function mutations in the MEN1 and DAXX tumor suppressor genes. Here, we report that the products of these genes, menin and Daxx, interact directly with each other to suppress the proliferation of NET cells, to a large degree by inhibiting expression of the membrane metallo-endopeptidase (MME). Menin and Daxx were required to enhance histone H3 lysine9 trimethylation (H3K9me3) at the MME promoter, as mediated partly by the histone H3 methyltransferase SUV39H1. Notably, the menin T429K mutation associated with a NET syndrome reduced Daxx binding, MME repression, and proliferation of NET cells. Conversely, inhibition of MME in NET cells repressed proliferation and tumor growth in vivo Our findings reveal a previously unappreciated cross-talk between two crucial tumor suppressor genes thought to work by independent pathways, focusing on MME as a common target of menin/Daxx to treat NET. Cancer Res; 77(2); 401-11. ©2016 AACR.

Menin localization in cell membrane compartment

Menin is encoded by the MEN1 gene, which is mutated in an inherited human syndrome, multiple endocrine neoplasia type 1(MEN1). Menin is primarily nuclear protein, acting as a tumor suppressor in endocrine organs, but as an oncogenic factor in the mixed lineage leukemia, in a tissue-specific manner. Recently, the crystal structures of menin with different binding partners reveal menin as a key scaffold protein that functionally interacts with various partners to regulate gene transcription in the nucleus. However, outside the nucleus, menin also regulates multiple signaling pathways that traverse the cell surface membrane. The precise nature regarding to how menin associates with the membrane fraction is poorly understood. Here we show that a small fraction of menin associates with the cell membrane fraction likely via serine palmitoylation. Moreover, the majority of the membrane-associated menin may reside inside membrane vesicles, as menin is protected from trypsin-mediated proteolysis, but disruption of the membrane fraction using detergent abolishes the detection. Consistently, cellular staining for menin also reveals the distribution of menin in the cell membrane and the punctate-like cell organelles. Our findings suggest that part of intracellular menin associates with the cell membrane peripherally as well as resides within the membrane vesicles.

MLL1 and MLL1 fusion proteins have distinct functions in regulating leukemic transcription program

Mixed lineage leukemia protein-1 (MLL1) has a critical role in human MLL1 rearranged leukemia (MLLr) and is a validated therapeutic target. However, its role in regulating global gene expression in MLLr cells, as well as its interplay with MLL1 fusion proteins remains unclear. Here we show that despite shared DNA-binding and cofactor interacting domains at the N terminus, MLL1 and MLL-AF9 are recruited to distinct chromatin regions and have divergent functions in regulating the leukemic transcription program. We demonstrate that MLL1, probably through C-terminal interaction with WDR5, is recruited to regulatory enhancers that are enriched for binding sites of E-twenty-six (ETS) family transcription factors, whereas MLL-AF9 binds to chromatin regions that have no H3K4me1 enrichment. Transcriptome-wide changes induced by different small molecule inhibitors also highlight the distinct functions of MLL1 and MLL-AF9. Taken together, our studies provide novel insights on how MLL1 and MLL fusion proteins contribute to leukemic gene expression, which have implications for developing effective therapies in the future.