Menin missense mutants encoded by the MEN1 gene that are targeted to the proteasome: restoration of expression and activity by CHIP siRNA

Hsp70 incompletely disaggregates misfolded K488X-menin to promote tumourigenesis in a family with multiple endocrine neoplasia type 1

Multiple endocrine neoplasia type 1 (MEN1) is caused by germline mutations in the MEN1 gene, including nonsense mutations and missense variants, which result in the formation of truncated inactive menin protein and some of which cause degradation of mutant menin proteins. Here, we describe a c.1462 A > T (p.K488X) mutation in exon 10 of MEN1 as a potential pathogenic mutation in an extended Chinese family with MEN1. We observed that K488X-menin was degraded by ubiquitination modification resulting from the combined actions of carboxy-terminus of Hsc70-interacting protein (CHIP) and Heat Shock Protein Family 70 (Hsp70) in vitro. K488X-menin is a misfolded truncated protein that results in amyloid aggregation in live cells and affected tissues, which is promoted by Hsp70 and/or CHIP. Although Hsp70 can inhibit the aggregation of K488X-menin in vitro, it is not upregulated in the affected tissues in patients with MEN1, and thus cannot completely disaggregate the aggregated K488X-menin. Further, we found that K488X-menin triggers early tumourigenesis in a MEN1 mutant zebrafish model. Moreover, K488X-menin disaggregation was induced by Hsp70 activator and Hsp70 was upregulated in homozygous mutant zebrafish. Our findings provide a novel biophysical mechanism involving Hsp70 underlying MEN1 tumourigenesis in a Chinese family with MEN1.

Targeted protein degradation for cancer therapy

Targeted protein degradation (TPD) aims at reprogramming the target specificity of the ubiquitin-proteasome system, the major cellular protein disposal machinery, to induce selective ubiquitination and degradation of therapeutically relevant proteins. Since its conception over 20 years ago, TPD has gained a lot of attention mainly due to improvements in the design of bifunctional proteolysis targeting chimeras (PROTACs) and understanding the mechanisms underlying molecular glue degraders. Today, PROTACs are on the verge of a first clinical approval and recent structural and mechanistic insights combined with technological leaps promise to unlock the rational design of protein degraders, following the lead of lenalidomide and related clinically approved analogues. At the same time, the TPD universe is expanding at a record speed with the discovery of novel modalities beyond molecular glue degraders and PROTACs. Here we review the recent progress in the field, focusing on newly discovered degrader modalities, the current state of clinical degrader candidates for cancer therapy and upcoming design approaches.

Analysis of Bone Phenotype Differences in MEN1-Related and Sporadic Primary Hyperparathyroidism Using 3D-DXA

Background: The rarity and variability of MEN1-related primary hyperparathyroidism (mPHPT) has led to contradictory data regarding the bone phenotype in this patient population. Methods: A single-center retrospective study was conducted among young age- and sex-matched patients with mPHPT and sporadic hyperparathyroidism (sPHPT). The main parameters of calcium-phosphorus metabolism, bone remodeling markers, and bone mineral density (BMD) measurements were obtained during the active phase of hyperparathyroidism before parathyroidectomy (PTE) and 1 year after. Trabecular Bone Score (TBS) and 3D-DXA analysis of the proximal femur were used to evaluate the differences in bone architecture disruption between groups. Results: Patients with mPHPT had significant lower preoperative BMD compared to sPHPT at lumbar spine-LS (p = 0.002); femur neck-FN (p = 0.001); and total hip-TH (p = 0.002). 3D-DXA analysis showed the prevalence of cortical rather than trabecular bone damage in mPHPT compared to sPHPT: cortical thickness (p < 0.001); cortical surface BMD (p = 0.001); cortical volumetric BMD (p = 0.007); and trabecular volumetric BMD (p = 0.029). One year after, PTE DXA and 3D-DXA parameters were similar between groups, while 3D-visualisation showed more extensive regeneration in cortical sBMD and cortical thickness in mPHPT. Conclusions: mPHPT is associated with lower preoperative BMD values with predominant architecture disruption in the cortical bone. The absence of differences in DXA and 3D-DXA parameters 1 year after PTE between mPHPT/sPHPT combined with significantly lower BMD in mPHPT at the initial stage may indicate faster bone recovery after surgery in mPHPT than in sPHPT.

Multiple Endocrine Neoplasia Type 1 Regulates TGFβ-Mediated Suppression of Tumor Formation and Metastasis in Melanoma

Over the past few decades, the worldwide incidence of cutaneous melanoma, a malignant neoplasm arising from melanocytes, has been increasing markedly, leading to the highest rate of skin cancer-related deaths. While localized tumors are easily removed by excision surgery, late-stage metastatic melanomas are refractory to treatment and exhibit a poor prognosis. Consequently, unraveling the molecular mechanisms underlying melanoma tumorigenesis and metastasis is crucial for developing novel targeted therapies. We found that the multiple endocrine neoplasia type 1 (MEN1) gene product Menin is required for the transforming growth factor beta (TGFβ) signaling pathway to induce cell growth arrest and apoptosis in vitro and prevent tumorigenesis in vivo in preclinical xenograft models of melanoma. We further identified point mutations in two MEN1 family members affected by melanoma that led to proteasomal degradation of the MEN1 gene product and to a loss of TGFβ signaling. Interestingly, blocking the proteasome degradation pathway using an FDA-approved drug and RNAi targeting could efficiently restore MEN1 expression and TGFβ transcriptional responses. Together, these results provide new potential therapeutic strategies and patient stratification for the treatment of cutaneous melanoma.

A mutational atlas for Parkin proteostasis

Proteostasis can be disturbed by mutations affecting folding and stability of the encoded protein. An example is the ubiquitin ligase Parkin, where gene variants result in autosomal recessive Parkinsonism. To uncover the pathological mechanism and provide comprehensive genotype-phenotype information, variant abundance by massively parallel sequencing (VAMP-seq) is leveraged to quantify the abundance of Parkin variants in cultured human cells. The resulting mutational map, covering 9219 out of the 9300 possible single-site amino acid substitutions and nonsense Parkin variants, shows that most low abundance variants are proteasome targets and are located within the structured domains of the protein. Half of the known disease-linked variants are found at low abundance. Systematic mapping of degradation signals (degrons) reveals an exposed degron region proximal to the so-called "activation element". This work provides examples of how missense variants may cause degradation either via destabilization of the native protein, or by introducing local signals for degradation.

Pancreatic beta cell regenerative potential of Zanthoxylum chalybeum Engl. Aqueous stem bark extract

ETHNOPHARMACOLOGICAL RELEVANCE

Zanthoxylum chalybeum Engl. is endemic to Africa and has been used traditionally to treat diabetes mellitus. Moreover, its pharmacological efficacy has been confirmed experimentally using in vitro and in vivo models of diabetes. However, the effects of Z. chalybeum extracts and its major constituent compounds on beta cell and islet regeneration are not clear. Further, the mechanisms associated with observed antidiabetic effects at the beta cell level are not fully elucidated.

AIM OF THE STUDY

We determined the beta cell regenerative efficacy of Z. chalybeum aqueous stem bark extract, identified the chemical compounds in Z. chalybeum aqueous stem bark extracts and explored their putative mechanisms of action.

MATERIALS AND METHODS

Phytochemical profiling of the Z. chalybeum extract was achieved using ultra high-performance liquid chromatography hyphenated to high-resolution mass spectrometry. Thereafter, molecular interactions of the compounds with beta cell regeneration targets were evaluated via molecular docking. In vitro, effects of the extract on cell viability, proliferation, apoptosis and oxidative stress were investigated in RIN-5F beta cells exposed to palmitate or streptozotocin. In vivo, pancreas tissue sections from streptozotocin-induced diabetic male Wistar rats treated with Z. chalybeum extract were stained for insulin, glucagon, pancreatic duodenal homeobox protein 1 (Pdx-1) and Ki-67.

RESULTS

Based on ligand target and molecular docking interactions diosmin was identified as a dual specificity tyrosine-phosphorylation-regulated kinase 1A (Dyrk1A) inhibitor. In vitro, Z. chalybeum augmented cell viability and cell proliferation while in palmitate-pre-treated cells, the extract significantly increased cell activity after 72 h. In vivo, although morphometric analysis showed decreased islet and beta cell size and density, observation of increased Pdx-1 and Ki-67 immunoreactivity in extract-treated islets suggests that Z. chalybeum extract has mild beta cell regenerative potential mediated by increased cell proliferation.

CONCLUSIONS

Overall, the mitogenic effects observed in vitro, were not robust enough to elicit sufficient recovery of functional beta cell mass in our in vivo model, in the context of a sustained diabetic milieu. However, the identification of diosmin as a potential Dyrk1A inhibitor merits further inquiry into the attendant molecular interactions.

Clinically Defined Mutations in MEN1 Alter Its Tumor-suppressive Function Through Increased Menin Turnover

Loss of the tumor suppressor protein menin is a critical event underlying the formation of neuroendocrine tumors (NET) in hormone-expressing tissues including gastrinomas. While aberrant expression of menin impairs its tumor suppression, few studies explore the structure-function relationship of clinical multiple endocrine neoplasia, type 1 (MEN1) mutations in the absence of a complete LOH at both loci. Here, we determined whether clinical MEN1 mutations render nuclear menin unstable and lead to its functional inactivation. We studied the structural and functional implications of two clinical MEN1 mutations (R516fs, E235K) and a third variant (A541T) recently identified in 10 patients with gastroenteropancreatic (GEP)-NETs. We evaluated the subcellular localization and half-lives of the mutants and variant in Men1-null mouse embryo fibroblast cells and in hormone-expressing human gastric adenocarcinoma and NET cell lines. Loss of menin function was assessed by cell proliferation and gastrin gene expression assays. Finally, we evaluated the effect of the small-molecule compound MI-503 on stabilizing nuclear menin expression and function in vitro and in a previously reported mouse model of gastric NET development. Both the R516fs and E235K mutants exhibited severe defects in total and subcellular expression of menin, and this was consistent with reduced half-lives of these mutants. Mutated menin proteins exhibited loss of function in suppressing tumor cell proliferation and gastrin expression. Treatment with MI-503 rescued nuclear menin expression and attenuated hypergastrinemia and gastric hyperplasia in NET-bearing mice. Clinically defined MEN1 mutations and a germline variant confer pathogenicity by destabilizing nuclear menin expression.

Significance

We examined the function of somatic and germline mutations and a variant of MEN1 sequenced from gastroenteropancreatic NETs. We report that these mutations and variant promote tumor cell growth and gastrin expression by rendering menin protein unstable and prone to increased degradation. We demonstrate that the menin-MLL (mixed lineage leukemia) inhibitor MI-503 restores menin protein expression and function in vitro and in vivo, suggesting a potential novel therapeutic approach to target MEN1 GEP-NETs.

Chaperone-assisted E3 ligase CHIP: A double agent in cancer

The carboxy-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin ligase and co-chaperone belonging to Ubox family that plays a crucial role in the maintenance of cellular homeostasis by switching the equilibrium of the folding-refolding mechanism towards the proteasomal or lysosomal degradation pathway. It links molecular chaperones viz. HSC70, HSP70 and HSP90 with ubiquitin proteasome system (UPS), acting as a quality control system. CHIP contains charged domain in between N-terminal tetratricopeptide repeat (TPR) and C-terminal Ubox domain. TPR domain interacts with the aberrant client proteins via chaperones while Ubox domain facilitates the ubiquitin transfer to the client proteins for ubiquitination. Thus, CHIP is a classic molecule that executes ubiquitination for degradation of client proteins. Further, CHIP has been found to be indulged in cellular differentiation, proliferation, metastasis and tumorigenesis. Additionally, CHIP can play its dual role as a tumor suppressor as well as an oncogene in numerous malignancies, thus acting as a double agent. Here, in this review, we have reported almost all substrates of CHIP established till date and classified them according to the hallmarks of cancer. In addition, we discussed about its architectural alignment, tissue specific expression, sub-cellular localization, folding-refolding mechanisms of client proteins, E4 ligase activity, normal physiological roles, as well as involvement in various diseases and tumor biology. Further, we aim to discuss its importance in HSP90 inhibitors mediated cancer therapy. Thus, this report concludes that CHIP may be a promising and worthy drug target towards pharmaceutical industry for drug development.

Multi-omics analyses of MEN1 missense mutations identify disruption of menin-MLL and menin-JunD interactions as critical requirements for molecular pathogenicity

Background

Loss-of-function mutations of the multiple endocrine neoplasia type 1 (MEN1) gene are causal to the MEN1 tumor syndrome, but they are also commonly found in sporadic pancreatic neuroendocrine tumors and other types of cancers. The MEN1 gene product, menin, is involved in transcriptional and chromatin regulation, most prominently as an integral component of KMT2A/MLL1 and KMT2B/MLL2 containing COMPASS-like histone H3K4 methyltransferase complexes. In a mutually exclusive fashion, menin also interacts with the JunD subunit of the AP-1 and ATF/CREB transcription factors.

Results

Here, we applied and in silico screening approach for 253 disease-related MEN1 missense mutations in order to select a set of nine menin mutations in surface-exposed residues. The protein interactomes of these mutants were assessed by quantitative mass spectrometry, which indicated that seven of the nine mutants disrupt interactions with both MLL1/MLL2 and JunD complexes. Interestingly, we identified three missense mutations, R52G, E255K and E359K, which predominantly reduce the MLL1 and MLL2 interactions when compared with JunD. This observation was supported by a pronounced loss of binding of the R52G, E255K and E359K mutant proteins at unique MLL1 genomic binding sites with less effect on unique JunD sites.

Conclusions

Our results underline the effects of MEN1 gene mutations in both familial and sporadic tumors of endocrine origin on the interactions of menin with the MLL1 and MLL2 histone H3K4 methyltransferase complexes and with JunD-containing transcription factors. Menin binding pocket mutants R52G, E255K and E359K have differential effects on MLL1/MLL2 and JunD interactions, which translate into differential genomic binding patterns. Our findings encourage future studies addressing the pathophysiological relevance of the separate MLL1/MLL2- and JunD-dependent functions of menin mutants in MEN1 disease model systems.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-022-00461-8.

Disease-linked mutations cause exposure of a protein quality control degron

More than half of disease-causing missense variants are thought to lead to protein degradation, but the molecular mechanism of how these variants are recognized by the cell remains enigmatic. Degrons are stretches of amino acids that help mediate recognition by E3 ligases and thus confer protein degradation via the ubiquitin-proteasome system. While degrons that mediate controlled degradation of, for example, signaling components and cell-cycle regulators are well described, so-called protein-quality-control degrons that mediate the degradation of destabilized proteins are poorly understood. Here, we show that disease-linked dihydrofolate reductase (DHFR) missense variants are structurally destabilized and chaperone-dependent proteasome targets. We find two regions in DHFR that act as degrons, and the proteasomal turnover of one of these was dependent on the molecular chaperone Hsp70. Structural analyses by nuclear magnetic resonance (NMR) and hydrogen/deuterium exchange revealed that this degron is buried in wild-type DHFR but becomes transiently exposed in the disease-linked missense variants.

Investigating the Effects of Amino Acid Variations in Human Menin

Human menin is a nuclear protein that participates in many cellular processes, as transcriptional regulation, DNA damage repair, cell signaling, cell division, proliferation, and migration, by interacting with many other proteins. Mutations of the gene encoding menin cause multiple endocrine neoplasia type 1 (MEN1), a rare autosomal dominant disorder associated with tumors of the endocrine glands. In order to characterize the structural and functional effects at protein level of the hundreds of missense variations, we investigated by computational methods the wild-type menin and more than 200 variants, predicting the amino acid variations that change secondary structure, solvent accessibility, salt-bridge and H-bond interactions, protein thermostability, and altering the capability to bind known protein interactors. The structural analyses are freely accessible online by means of a web interface that integrates also a 3D visualization of the structure of the wild-type and variant proteins. The results of the study offer insight into the effects of the amino acid variations in view of a more complete understanding of their pathological role.

Gastrin: From Physiology to Gastrointestinal Malignancies

Abetted by widespread usage of acid-suppressing proton pump inhibitors (PPIs), the mitogenic actions of the peptide hormone gastrin are being revisited as a recurring theme in various gastrointestinal (GI) malignancies. While pathological gastrin levels are intricately linked to hyperplasia of enterochromaffin-like cells leading to carcinoid development, the signaling effects exerted by gastrin on distinct cell types of the gastric mucosa are more nuanced. Indeed, mounting evidence suggests dichotomous roles for gastrin in both promoting and suppressing tumorigenesis. Here, we review the major upstream mediators of gastrin gene regulation, including inflammation secondary to Helicobacter pylori infection and the use of PPIs. We further explore the molecular biology of gastrin in GI malignancies, with particular emphasis on the regulation of gastrin in neuroendocrine neoplasms. Finally, we highlight tissue-specific transcriptional targets as an avenue for targetable therapeutics.

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.

A Novel Germline c.1267T>A MEN1 Mutation in MEN1 Family—from Phenotype to Gene and Back

Primary hyperparathyroidism is a relatively common endocrine disorder, which may be hereditary. This report describes clinical, biochemical, radiographic, and genetic findings, the latter obtained using next generation sequencing (NGS), in three consanguineous patients. Gene panels in NGS consisted of 5 or 70 genes, including MEN1 and RET. The first patient suffered from recurrent primary hyperparathyroidism. Primary hyperparathyroidism and pituitary microadenomas were afterwards diagnosed in two of her daughters. No clinical nor radiological features of gastroenteropancreatic neuroendocrine tumors were found. All three family members were heterozygous for MEN1 NM_130799: c.1267T>A transversion, which is predicted to result in substitution of tryptophan with arginine in position 423. Additionally, the first patient was also a carrier of RET NM_020975: c.1946C>T missense mutation, which was not present in two other family members. We describe a family with a novel heterozygous mutation (NM_130799: c.1267T>A) in MEN1 gene and postulate that it leads to MEN1 syndrome. The study underlies the importance of genetic testing in primary hyperparathyroidism in personalizing patients’ care.

Using Structural Analysis In Silico to Assess the Impact of Missense Variants in MEN1

Despite the rapid expansion in recent years of databases reporting either benign or pathogenic genetic variations, the interpretation of novel missense variants remains challenging, particularly for clinical or genetic testing laboratories where functional analysis is often unfeasible. Previous studies have shown that thermodynamic analysis of protein structure in silico can discriminate between groups of benign and pathogenic missense variants. However, although structures exist for many human disease‒associated proteins, such analysis remains largely unexploited in clinical laboratories. Here, we analyzed the predicted effect of 338 known missense variants on the structure of menin, the MEN1 gene product. Results provided strong discrimination between pathogenic and benign variants, with a threshold of >4 kcal/mol for the predicted change in stability, providing a strong indicator of pathogenicity. Subsequent analysis of seven novel missense variants identified during clinical testing of patients with MEN1 showed that all seven were predicted to destabilize menin by >4 kcal/mol. We conclude that structural analysis provides a useful tool in understanding the effect of missense variants in MEN1 and that integration of proteomic with genomic data could potentially contribute to the classification of novel variants in this disease.

Wild-type menin is rapidly degraded via the ubiquitin-proteasome pathway in a rat insulinoma cell line

Menin is encoded by multiple endocrine neoplasia type 1 (MEN1) gene, the germ line mutations of which are the main cause of pancreatic neuroendocrine tumors (PNETs). To date, a large number of frameshift, nonsense and missense mutations of MEN1 have been identified to be responsible for part of MEN1-defficient PNETs patients due to truncation or rapid degradation of menin protein. However, the stability of the wild-type (WT) menin in PNETs is totally unknown. In the present study, we observed ubiquitination of WT menin in 293T cells by transfection of ectopic WT menin and HA-ubiquitin. As expected, either endogenous or ectopic WT menin is stable in 293T cells, whereas in INS-1 cells, a rat insulinoma cell line derived from PNETs, either endogenous or ectopic WT menin is rapidly degraded through ubiquitin-proteasome pathway. Furthermore, the degradation of WT menin is more rapid in the presence of serum. Our findings suggest that in part of PNETs patients with WT MEN1, a ubiquitin-proteasome system targeting menin is untimely activated.

Molecular pathogenesis of parathyroid tumours

Parathyroid tumors represent an elusive endocrine neoplasia, which lead to primary hyperparathyroidism, pHPT, a common endocrine calcium disorder characterized by hypercalcemia and normal-high parathormone secretion. Parathyroid tumours are benign adenomas or multiple glands hyperplasia in the vast majority (>99% of cases), while malignant neoplasms are rare (less than 1%). Despite pHPT is a common disorder, our knowledge about the genetic predisposition and molecular pathophysiology is limited to the familial syndromic forms of parathyroid tumour, that, however, represent not more than the 10% of all the cases; instead, the pathophysiology of sporadic forms remains an open field, although data about epigenetic mechanisms or private genes have been supposed. Here we present an overview of more recent acquisitions about the genetic causes along with their molecular mechanisms of benign, but also, malignant parathyroid tumours either in sporadic and familial presentation.

Ubiquitin ligase CHIP functions as an oncogene and activates the AKT signaling pathway in prostate cancer

Carboxyl terminus of Hsc-70-interacting protein (CHIP) is an E3 ubiquitin ligase that induces the ubiquitination and degradation of numerous tumor-associated proteins and serves as a suppressor or promoter in tumor progression. To date, the molecular mechanism of CHIP in prostate cancer remains unknown. Therefore, the present study investigated the biological function of CHIP in prostate cancer cells and obtained evidence that CHIP expression is upregulated in prostate cancer tissues. The CHIP vector was introduced into DU145 cancer cells and the cell biological behaviour was examined through a series of experiments, including cell growth, cell apoptosis and migration and invasion assays. The results indicated that the overexpression of CHIP in DU145 prostatic cancer cells promoted cell proliferation through activation of the protein kinase B (AKT) signaling pathway, which subsequently increased cyclin D1 protein levels and decreased p21 and p27 protein levels. The overexpression of CHIP significantly increased the migration and invasion of the DU145 cells, which is possible due to activation of the AKT signaling pathway and upregulation of vimentin. The expression level of CHIP was observed to be increased in human prostate cancer tissues compared with the adjacent normal tissue. Furthermore, the CHIP expression level exhibited a positively association with the Gleason score of the patents. These findings indicate that CHIP functions as an oncogene in prostate cancer.

Pathophysiology of Gastric NETs: Role of Gastrin and Menin

PURPOSE OF REVIEW

Neuroendocrine tumors (NETs) were initially identified as a separate entity in the early 1900s as a unique malignancy that secretes bioactive amines. GI-NETs are the most frequent type and represent a unique subset of NETs, because at least 75% of these tumors represent gastrin stimulation of the enterochromaffin-like cell located in the body of the stomach. The purpose of this review is to understand the specific role of gastrin in the generation of Gastric NETs (G-NETs).

RECENT FINDINGS

We review here the origin of enterochromaffin cells gut and the role of hypergastrinemia in gastric enteroendocrine tumorigenesis. We describe generation of the first genetically engineered mouse model of gastrin-driven G-NETs that mimics the human phenotype. The common mechanism observed in both the hypergastrinemic mouse model and human carcinoids is translocation of the cyclin-dependent inhibitor p27^kip to the cytoplasm and its subsequent degradation by the proteasome. Therapies that block degradation of p27^kip, the CCKBR2 gastrin receptor, or gastrin peptide are likely to facilitate treatment.

Gastrin Induces Nuclear Export and Proteasome Degradation of Menin in Enteric Glial Cells

BACKGROUND & AIMS

The multiple endocrine neoplasia, type 1 (MEN1) locus encodes the nuclear protein and tumor suppressor menin. MEN1 mutations frequently cause neuroendocrine tumors such as gastrinomas, characterized by their predominant duodenal location and local metastasis at time of diagnosis. Diffuse gastrin cell hyperplasia precedes the appearance of MEN1 gastrinomas, which develop within submucosal Brunner's glands. We investigated how menin regulates expression of the gastrin gene and induces generation of submucosal gastrin-expressing cell hyperplasia.

METHODS

Primary enteric glial cultures were generated from the VillinCre:Men1FL/FL:Sst-/- mice or C57BL/6 mice (controls), with or without inhibition of gastric acid by omeprazole. Primary enteric glial cells from C57BL/6 mice were incubated with gastrin and separated into nuclear and cytoplasmic fractions. Cells were incubated with forskolin and H89 to activate or inhibit protein kinase A (a family of enzymes whose activity depends on cellular levels of cyclic AMP). Gastrin was measured in blood, tissue, and cell cultures using an ELISA. Immunoprecipitation with menin or ubiquitin was used to demonstrate post-translational modification of menin. Primary glial cells were incubated with leptomycin b and MG132 to block nuclear export and proteasome activity, respectively. We obtained human duodenal, lymph node, and pancreatic gastrinoma samples, collected from patients who underwent surgery from 1996 through 2007 in the United States or the United Kingdom.

RESULTS

Enteric glial cells that stained positive for glial fibrillary acidic protein (GFAP+) expressed gastrin de novo through a mechanism that required PKA. Gastrin-induced nuclear export of menin via cholecystokinin B receptor (CCKBR)-mediated activation of PKA. Once exported from the nucleus, menin was ubiquitinated and degraded by the proteasome. GFAP and other markers of enteric glial cells (eg, p75 and S100B), colocalized with gastrin in human duodenal gastrinomas.

CONCLUSIONS

MEN1-associated gastrinomas, which develop in the submucosa, might arise from enteric glial cells through hormone-dependent PKA signaling. This pathway disrupts nuclear menin function, leading to hypergastrinemia and associated sequelae.

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.

The future: genetics advances in MEN1 therapeutic approaches and management strategies

The identification of the multiple endocrine neoplasia type 1 (MEN1) gene in 1997 has shown that germline heterozygous mutations in the MEN1 gene located on chromosome 11q13 predisposes to the development of tumors in the MEN1 syndrome. Tumor development occurs upon loss of the remaining normal copy of the MEN1 gene in MEN1-target tissues. Therefore, MEN1 is a classic tumor suppressor gene in the context of MEN1. This tumor suppressor role of the protein encoded by the MEN1 gene, menin, holds true in mouse models with germline heterozygous Men1 loss, wherein MEN1-associated tumors develop in adult mice after spontaneous loss of the remaining non-targeted copy of the Men1 gene. The availability of genetic testing for mutations in the MEN1 gene has become an essential part of the diagnosis and management of MEN1. Genetic testing is also helping to exclude mutation-negative cases in MEN1 families from the burden of lifelong clinical screening. In the past 20 years, efforts of various groups world-wide have been directed at mutation analysis, molecular genetic studies, mouse models, gene expression studies, epigenetic regulation analysis, biochemical studies and anti-tumor effects of candidate therapies in mouse models. This review will focus on the findings and advances from these studies to identify MEN1 germline and somatic mutations, the genetics of MEN1-related states, several protein partners of menin, the three-dimensional structure of menin and menin-dependent target genes. The ongoing impact of all these studies on disease prediction, management and outcomes will continue in the years to come.

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.

Blocking protein quality control to counter hereditary cancers

Inhibitors of molecular chaperones and the ubiquitin-proteasome system have already been clinically implemented to counter certain cancers, including multiple myeloma and mantle cell lymphoma. The efficacy of this treatment relies on genomic alterations in cancer cells causing a proteostatic imbalance, which makes them more dependent on protein quality control (PQC) mechanisms than normal cells. Accordingly, blocking PQC, e.g. by proteasome inhibitors, may cause a lethal proteotoxic crisis in cancer cells, while leaving normal cells unaffected. Evidence, however, suggests that the PQC system operates by following a better-safe-than-sorry principle and is thus prone to target proteins that are only slightly structurally perturbed, but still functional. Accordingly, implementing PQC inhibitors may also, through an entirely different mechanism, hold potential for other cancers. Several inherited cancer susceptibility syndromes, such as Lynch syndrome and von Hippel-Lindau disease, are caused by missense mutations in tumor suppressor genes, and in some cases, the resulting amino acid substitutions in the encoded proteins cause the cellular PQC system to target them for degradation, although they may still retain function. As a consequence of this over-meticulous PQC mechanism, the cell may end up with an insufficient amount of the abnormal, but functional, protein, which in turn leads to a loss-of-function phenotype and manifestation of the disease. Increasing the amounts of such proteins by stabilizing with chemical chaperones, or by targeting molecular chaperones or the ubiquitin-proteasome system, may thus avert or delay the disease onset. Here, we review the potential of targeting the PQC system in hereditary cancer susceptibility syndromes.

A Review of the Scaffold Protein Menin and its Role in Hepatobiliary Pathology

Multiple endocrine neoplasia type 1 (MEN1) is a familial cancer syndrome with neuroendocrine tumorigenesis of the parathyroid glands, pituitary gland, and pancreatic islet cells. The MEN1 gene codes for the canonical tumor suppressor protein, menin. Its protein structure has recently been crystallized, and it has been investigated in a multitude of other tissues. In this review, we summarize recent advancements in understanding the structure of the menin protein and its function as a scaffold protein in histone modification and epigenetic gene regulation. Furthermore, we explore its role in hepatobiliary autoimmune diseases, cancers, and metabolic diseases. In particular, we discuss how menin expression and function are regulated by extracellular signaling factors and nuclear receptor activation in various hepatic cell types. How the many signaling pathways and tissue types affect menin's diverse functions is not fully understood. We show that small-molecule inhibitors affecting menin function can shed light on menin's broad role in pathophysiology and elucidate distinct menin-dependent processes. This review reveals menin's often dichotomous function through analysis of its role in multiple disease processes and could potentially lead to novel small-molecule therapies in the treatment of cholangiocarcinoma or biliary autoimmune diseases.

Novel association of MEN1 gene mutations with parathyroid carcinoma

Inactivating mutations of the multiple endocrine neoplasia 1 (MEN1) gene cause MEN1 syndrome, characterized by primary hyperparathyroidism (pHPT), and parathyroid and gastro-entero-pancreatic pituitary tumors. At present, only 14 cases of malignant parathyroid tumor have been associated with the syndrome, with 6 cases carrying an inactivating mutation of the MEN1 gene. The present study presents the case of a 48-year-old female who presented with multigland pHPT and multiple pancreatic lesions. The patient underwent surgery several times for the excision of parathyroid hyperplasia, carcinoma and adenoma. The MEN1 gene was screened, revealing three variants (in cis) at the intron/exon 3 boundary (IVS2-3G>C, c.497A>T and c.499G>T) detected on the DNA of the proband, not shared by her relatives. RNA sequencing revealed that the IVS2-3C>G variant caused the skipping of the exon 3. Therefore, the present study reports on a novel rare association of MEN1 syndrome and parathyroid carcinoma. The reported splicing mutation was previously identified in subjects who always developed malignant lesions; thus, a possible genotype-phenotype association may be considered.

Degradation of two novel congenital TTP ADAMTS13 mutants by the cell proteasome prevents ADAMTS13 secretion

INTRODUCTION

Over 150 mutations have been identified in the ADAMTS13 gene in patients with congenital thrombotic thrombocytopenic purpura (TTP). The majority of these (86%), lead to reduced (<50%) secretion of mutant recombinant ADAMTS13. The mechanism by which this occurs has not been investigated in vitro. Two novel ADAMTS13 mutations (p.I143T and p.Y570C) identified in two congenital adolescence onset TTP patients were studied, to investigate their effects on ADAMTS13 secretion and subcellular localisation.

MATERIALS AND METHODS

HEK293T cells were transiently transfected with wild type or mutant ADAMTS13 cDNA. Immunofluorescence and confocal microscopy were used to study localisation within the endoplasmic reticulum (ER) and Golgi. The cell proteasome and lysosomes were inhibited in cells stably expressing ADAMTS13 to investigate degradation of ADAMTS13 by either organelle.

RESULTS

Both mutations severely impaired secretion and both mutants localised within the ER and Golgi. Proteasome inhibition led to the intracellular accumulation of both mutants, suggesting proteasome degradation. Lysosome inhibition on the other hand did not lead to increased intracellular accumulation of the mutants.

CONCLUSIONS

Proteasome degradation of these ADAMTS13 mutants contributed to their reduced secretion.

Rapid Proteasomal Degradation of Mutant Proteins Is the Primary Mechanism Leading to Tumorigenesis in Patients With Missense AIP Mutations

CONTEXT

The pathogenic effect of mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene (AIPmuts) in pituitary adenomas is incompletely understood. We have identified the primary mechanism of loss of function for missense AIPmuts.

OBJECTIVE

This study sought to analyze the mechanism/speed of protein turnover of wild-type and missense AIP variants, correlating protein half-life with clinical parameters.

DESIGN AND SETTING

Half-life and protein-protein interaction experiments and cross-sectional analysis of AIPmut positive patients' data were performed in a clinical academic research institution.

PATIENTS

Data were obtained from our cohort of pituitary adenoma patients and literature-reported cases.

INTERVENTIONS

Protein turnover of endogenous AIP in two cell lines and fifteen AIP variants overexpressed in HEK293 cells was analyzed via cycloheximide chase and proteasome inhibition. Glutathione-S-transferase pull-down and quantitative mass spectrometry identified proteins involved in AIP degradation; results were confirmed by coimmunoprecipitation and gene knockdown. Relevant clinical data was collected.

MAIN OUTCOME MEASURES

Half-life of wild-type and mutant AIP proteins and its correlation with clinical parameters.

RESULTS

Endogenous AIP half-life was similar in HEK293 and lymphoblastoid cells (43.5 and 32.7 h). AIP variants were divided into stable proteins (median, 77.7 h; interquartile range [IQR], 60.7-92.9 h), and those with short (median, 27 h; IQR, 21.6-28.7 h) or very short (median, 7.7 h; IQR, 5.6-10.5 h) half-life; proteasomal inhibition rescued the rapid degradation of mutant proteins. The experimental half-life significantly correlated with age at diagnosis of acromegaly/gigantism (r = 0.411; P = .002). The FBXO3-containing SKP1-CUL1-F-box protein complex was identified as the E3 ubiquitin-ligase recognizing AIP.

CONCLUSIONS

AIP is a stable protein, driven to ubiquitination by the SKP1-CUL1-F-box protein complex. Enhanced proteasomal degradation is a novel pathogenic mechanism for AIPmuts, with direct implications for the phenotype.

Osteoblast menin regulates bone mass in vivo

Menin, the product of the multiple endocrine neoplasia type 1 (Men1) tumor suppressor gene, mediates the cell proliferation and differentiation actions of transforming growth factor-β (TGF-β) ligand family members. In vitro, menin modulates osteoblastogenesis and osteoblast differentiation promoted and sustained by bone morphogenetic protein-2 (BMP-2) and TGF-β, respectively. To examine the in vivo function of menin in bone, we conditionally inactivated Men1 in mature osteoblasts by crossing osteocalcin (OC)-Cre mice with floxed Men1 (Men1(f/f)) mice to generate mice lacking menin in differentiating osteoblasts (OC-Cre;Men1(f/f) mice). These mice displayed significant reduction in bone mineral density, trabecular bone volume, and cortical bone thickness compared with control littermates. Osteoblast and osteoclast number as well as mineral apposition rate were significantly reduced, whereas osteocyte number was increased. Primary calvarial osteoblasts proliferated more quickly but had deficient mineral apposition and alkaline phosphatase activity. Although the mRNA expression of osteoblast marker and cyclin-dependent kinase inhibitor genes were all reduced, that of cyclin-dependent kinase, osteocyte marker, and pro-apoptotic genes were increased in isolated Men1 knock-out osteoblasts compared with controls. In contrast to the knock-out mice, transgenic mice overexpressing a human menin cDNA in osteoblasts driven by the 2.3-kb Col1a1 promoter, showed a gain of bone mass relative to control littermates. Osteoblast number and mineral apposition rate were significantly increased in the Col1a1-Menin-Tg mice. Therefore, osteoblast menin plays a key role in bone development, remodeling, and maintenance.

A CHIPotle in physiology and disease

The carboxy-terminus of Hsc70 interacting protein (CHIP) is known to function as a chaperone associated E3 ligase for several proteins and regulates a variety of physiological processes. Being a connecting link between molecular chaperones and 26S proteasomes, it is widely regarded as the central player in the cellular protein quality control system. Recent analyses have provided new insights on the biochemical and functional dynamics of CHIP. In this review article, we give a comprehensive account of our current knowledge on the biology of CHIP, which apart from shedding light on fundamental biological questions promises to provide a potential target for therapeutic intervention.

[Various neuroendocrine tumors in a family with multiple endocrine neoplasia type 1]

When multiple endocrine tumors are detected more tests are required to diagnose endocrine tumor syndromes. The authors report the case history of a patient with clinical manifestation of multiplex endocrine neoplasia type 1 (parathyroid adenoma, pancreatic neuroendocrine tumor, pituitary tumor, adrenal gland tumors and thymic neuroendocrine carcinoma). Genetic screening proved a novel stop codon mutation of the MEN1 gene in the patient and in two other members of the family. The son of the index patient showed clinical symptoms of pancreatic neuroendocrine tumor (insulinoma) and parathyroid adenoma. One of the two daughters was also positive for the same mutation, however, she had no clinical symptoms. The authors review current knowledge on the genetic background of multiple endocrine syndrome type 1, the role of menin and the usefulness of gene mutation screening.

Identification and functional characterization of three NoLS (nucleolar localisation signals) mutations of the CDC73 gene

Hyperparathyroidism Jaw-Tumour Syndrome (HPT-JT) is characterized by primary hyperparathyroidism (PHPT), maxillary/mandible ossifying fibromas and by parathyroid carcinoma in 15% of cases. Inactivating mutations of the tumour suppressor CDC73/HRPT2 gene have been found in HPT-JT patients and also as genetic determinants of sporadic parathyroid carcinoma/atypical adenomas and, rarely, typical adenomas, in familial PHPT. Here we report the genetic and molecular analysis of the CDC73/HRPT2 gene in three patients affected by PHPT due to atypical and typical parathyroid adenomas, in one case belonging to familial PHPT. Flag-tagged WT and mutant CDC73/HRPT2 proteins were transiently transfected in HEK293 cells and functional assays were performed in order to investigate the effect of the variants on the whole protein expression, nuclear localization and cell overgrowth induction. We identified four CDC73/HRPT2 gene mutations, three germline (c.679_680delAG, p.Val85_Val86del and p.Glu81_Pro84del), one somatic (p.Arg77Pro). In three cases the mutation was located within the Nucleolar Localisation Signals (NoLS). The three NoLS variants led to instability either of the corresponding mutated protein or mRNA or both. When transfected in HEK293 cells, NoLS mutated proteins mislocalized with a predeliction for cytoplasmic or nucleo-cytoplasmic localization and, finally, they resulted in overgrowth, consistent with a dominant negative interfering effect in the presence of the endogenous protein.

Altered MENIN expression disrupts the MAFA differentiation pathway in insulinoma

The protein MENIN is the product of the multiple endocrine neoplasia type I (MEN1) gene. Altered MENIN expression is one of the few events that are clearly associated with foregut neuroendocrine tumours (NETs), classical oncogenes or tumour suppressors being not involved. One of the current challenges is to understand how alteration of MENIN expression contributes to the development of these tumours. We hypothesised that MENIN might regulate factors maintaining endocrine-differentiated functions. We chose the insulinoma model, a paradigmatic example of well-differentiated pancreatic NETs, to study whether MENIN interferes with the expression of v-MAF musculoaponeurotic fibrosarcoma oncogene homologue A (MAFA), a master glucose-dependent transcription factor in differentiated β-cells. Immunohistochemical analysis of a series of human insulinomas revealed a correlated decrease in both MENIN and MAFA. Decreased MAFA expression resulting from targeted Men1 ablation was also consistently observed in mouse insulinomas. In vitro analyses using insulinoma cell lines showed that MENIN regulated MAFA protein and mRNA levels, and bound to Mafa promoter sequences. MENIN knockdown concomitantly decreased mRNA expression of both Mafa and β-cell differentiation markers (Ins1/2, Gck, Slc2a2 and Pdx1) and, in parallel, increased the proliferation rate of tumours as measured by bromodeoxyuridine incorporation. Interestingly, MAFA knockdown alone also increased proliferation rate but did not affect the expression of candidate proliferation genes regulated by MENIN. Finally, MENIN variants with missense mutations detected in patients with MEN1 lost the WT MENIN properties to regulate MAFA. Together, our findings unveil a previously unsuspected MENIN/MAFA connection regarding control of the β-cell differentiation/proliferation balance, which could contribute to tumorigenesis.

Menin: a scaffold protein that controls gene expression and cell signaling

The protein menin is encoded by the MEN1 gene, which is mutated in patients with multiple endocrine neoplasia type 1 (MEN1) syndrome. Although menin acts as a tumor suppressor in endocrine organs, it is required for leukemic transformation in mouse models. Menin possesses these dichotomous functions probably because it can both positively and negatively regulate gene expression, as well as interact with a multitude of proteins with diverse functions. Here, we review the recent progress in understanding the molecular mechanisms by which menin functions. The crystal structures of menin with different binding partners reveal that menin is a key scaffold protein that functionally crosstalks with various partners to regulate gene transcription and interplay with multiple signaling pathways.

The embryonic transcription factor Hlxb9 is a menin interacting partner that controls pancreatic β-cell proliferation and the expression of insulin regulators

The multiple endocrine neoplasia type 1 (MEN1) syndrome is caused by germline mutations in the MEN1 gene encoding menin, with tissue-specific tumors of the parathyroids, anterior pituitary, and enteropancreatic endocrine tissues. Also, 30-40% of sporadic pancreatic endocrine tumors show somatic MEN1 gene inactivation. Although menin is expressed in all cell types of the pancreas, mouse models with loss of menin in either pancreatic α-cells, or β-cells, or total pancreas develop β-cell-specific endocrine tumors (insulinomas). Loss of widely expressed tumor suppressor genes may produce tissue-specific tumors by reactivating one or more embryonic-specific differentiation factors. Therefore, we determined the effect of menin overexpression or knockdown on the expression of β-cell differentiation factors in a mouse β-cell line (MIN6). We show that the β-cell differentiation factor Hlxb9 is posttranscriptionally upregulated upon menin knockdown, and it interacts with menin. Hlxb9 reduces cell proliferation and causes apoptosis in the presence of menin, and it regulates genes that modulate insulin level. Thus, upon menin loss or from other causes, dysregulation of Hlxb9 predicts a possible combined mechanism for β-cell proliferation and insulin production in insulinomas. These observations help to understand how a ubiquitously expressed protein such as menin might control tissue-specific tumorigenesis. Also, our findings identify Hlxb9 as an important factor for β-cell proliferation and insulin regulation.

Recent results of basic and clinical research in MEN1: opportunities to improve early detection and treatment

Due to the variable expression of multiple endocrine neoplasia type 1 (MEN1), it is difficult to predict the course of the disease. However, knowledge about the normal function of the MEN1 gene product, together with the effects of cellular derangement by subsequent genetic events, has increased considerably. At first, the possible existence of a genotype-phenotype correlation is discussed. Thus, mild- and late-onset phenotypes may be distinguished from more malignant phenotypes depending on the character of the primary MEN1 disease gene mutation. Subsequently, tumor-promoting factors such as gender, additional genetic mutations and ecogenetic factors may contribute to the course of the disease. New developments in management are based on the knowledge and experience of the multidisciplinary teams involved. Finally, the metabolic effects of MEN1 mutations in aged patients are discussed. Early identification of predisposition to the disease, together with knowledge about the natural history of specific mutations, risks of additional mutations and periodic clinical monitoring, allow early treatment and may improve life expectancy and quality of life.

Impaired transforming growth factor-β (TGF-β) transcriptional activity and cell proliferation control of a menin in-frame deletion mutant associated with multiple endocrine neoplasia type 1 (MEN1)

Multiple endocrine neoplasia type 1 (MEN1) is characterized by tumors of the parathyroid, enteropancreas, and anterior pituitary. The MEN1 gene encodes the tumor suppressor menin of 610 amino acids that has multiple protein partners and activities. The particular pathways that, when lost, lead to tumorigenesis are not known. We demonstrated that members of a three-generation MEN1 kindred are heterozygous for a donor splice site mutation at the beginning of intron 3 (IVS3 + 1G→A). Lymphoblastoid cells of a mutant gene carrier had, in addition to the wild-type menin transcript, an aberrant transcript resulting from use of a cryptic splice site within exon III that splices to the start of exon IV. The predicted menin Δ(184-218) mutant has an in-frame deletion of 35 amino acids but is otherwise of wild-type sequence. The transfected menin Δ(184-218) mutant was well expressed and fully able to mediate the normal inhibition of the activity of the transcriptional regulators JunD and NF-κB. However, it was defective in mediating TGF-β-stimulated Smad3 action in promoter-reporter assays in insulinoma cells. Importantly, lymphoblastoid cells from an individual heterozygous for the mutation had reduced TGF-β-induced (Smad3) transcriptional activity but normal JunD and NF-κB function. In addition, the mutant gene carrier lymphoblastoid cells proliferated faster and were less responsive to the cytostatic effects of TGF-β than cells from an unaffected family member. In conclusion, the menin mutant exhibits selective loss of the TGF-β signaling pathway and loss of cell proliferation control contributing to the development of MEN1.