Multiple endocrine neoplasia type 1 deletion in pancreatic alpha-cells leads to development of insulinomas in mice

Blood-based Proteomic Signatures Associated With MEN1-related Duodenopancreatic Neuroendocrine Tumor Progression

PURPOSE

Patients with multiple endocrine neoplasia type 1 (MEN1) are predisposed to develop duodenopancreatic neuroendocrine tumors (dpNETs), and metastatic dpNET is the primary cause of disease-related mortality. Presently, there is a paucity of prognostic factors that can reliably identify patients with MEN1-related dpNETS who are at high risk of distant metastasis. In the current study, we aimed to establish novel circulating molecular protein signatures associated with disease progression.

EXPERIMENTAL DESIGN

Mass spectrometry-based proteomic profiling was conducted on plasmas procured through an international collaboration between MD Anderson Cancer Center, the National Institutes of Health, and the University Medical Center Utrecht from a cohort of 56 patients with MEN1 [14 with distant metastasis dpNETs (cases) and 42 with either indolent dpNETs or no dpNETs (controls)]. Findings were compared to proteomic profiles generated from serially collected plasmas from a mouse model of Men1-pancreatic neuroendocrine tumors (Men1fl/flPdx1-CreTg) and control mice (Men1fl/fl).

RESULTS

A total of 187 proteins were found to be elevated in MEN1 patients with distant metastasis compared to controls, including 9 proteins previously associated with pancreatic cancer and other neuronal proteins. Analyses of mouse plasmas revealed 196 proteins enriched for transcriptional targets of oncogenic MYCN, YAP1, POU5F1, and SMAD that were associated with disease progression in Men1fl/flPdx1-CreTg mice. Cross-species intersection revealed 19 proteins positively associated with disease progression in both human patients and in Men1fl/flPdx1-CreTg mice.

CONCLUSIONS

Our integrated analyses identified novel circulating protein markers associated with disease progression in MEN1-related dpNET.

Molecular and Clinical Spectrum of Primary Hyperparathyroidism

Recent data suggest an increase in the overall incidence of parathyroid disorders, with primary hyperparathyroidism (PHPT) being the most prevalent parathyroid disorder. PHPT is associated with morbidities (fractures, kidney stones, chronic kidney disease) and increased risk of death. The symptoms of PHPT can be nonspecific, potentially delaying the diagnosis. Approximately 15% of patients with PHPT have an underlying heritable form of PHPT that may be associated with extraparathyroidal manifestations, requiring active surveillance for these manifestations as seen in multiple endocrine neoplasia type 1 and 2A. Genetic testing for heritable forms should be offered to patients with multiglandular disease, recurrent PHPT, young onset PHPT (age ≤40 years), and those with a family history of parathyroid tumors. However, the underlying genetic cause for the majority of patients with heritable forms of PHPT remains unknown. Distinction between sporadic and heritable forms of PHPT is useful in surgical planning for parathyroidectomy and has implications for the family. The genes currently known to be associated with heritable forms of PHPT account for approximately half of sporadic parathyroid tumors. But the genetic cause in approximately half of the sporadic parathyroid tumors remains unknown. Furthermore, there is no systemic therapy for parathyroid carcinoma, a rare but potentially fatal cause of PHPT. Improved understanding of the molecular characteristics of parathyroid tumors will allow us to identify biomarkers for diagnosis and novel targets for therapy.

Models in Pancreatic Neuroendocrine Neoplasms: Current Perspectives and Future Directions

Pancreatic neuroendocrine neoplasms (pNENs) are a heterogeneous group of tumors derived from multiple neuroendocrine origin cell subtypes. Incidence rates for pNENs have steadily risen over the last decade, and outcomes continue to vary widely due to inability to properly screen. These tumors encompass a wide range of functional and non-functional subtypes, with their rarity and slow growth making therapeutic development difficult as most clinically used therapeutics are derived from retrospective analyses. Improved molecular understanding of these cancers has increased our knowledge of the tumor biology for pNENs. Despite these advances in our understanding of pNENs, there remains a dearth of models for further investigation. In this review, we will cover the current field of pNEN models, which include established cell lines, animal models such as mice and zebrafish, and three-dimensional (3D) cell models, and compare their uses in modeling various disease aspects. While no study model is a complete representation of pNEN biology, each has advantages which allow for new scientific understanding of these rare tumors. Future efforts and advancements in technology will continue to create new options in modeling these cancers.

Recent progress of experimental model in pancreatic neuroendocrine tumors: drawbacks and challenges

The neuroendocrine neoplasm, in general, refers to a heterogeneous group of all tumors originating from peptidergic neurons and neuroendocrine cells. Neuroendocrine neoplasms are divided into two histopathological subtypes: well-differentiated neuroendocrine tumors and poorly differentiated neuroendocrine carcinomas. Pancreatic neuroendocrine tumors account for more than 80% of pancreatic neuroendocrine neoplasms. Due to the greater proportion of pancreatic neuroendocrine tumors compared to pancreatic neuroendocrine carcinoma, this review will only focus on them. The worldwide incidence of pancreatic neuroendocrine tumors is rising year by year due to sensitive detection with an emphasis on medical examinations and the improvement of testing technology. Although the biological behavior of pancreatic neuroendocrine tumors tends to be inert, distant metastasis is common, often occurring very early. Because of the paucity of basic research on pancreatic neuroendocrine tumors, the mechanism of tumor development, metastasis, and recurrence are still unclear. In this context, the representative preclinical models simulating the tumor development process are becoming ever more widely appreciated to address the clinical problems of pancreatic neuroendocrine tumors. So far, there is no comprehensive report on the experimental model of pancreatic neuroendocrine tumors. This article systematically summarizes the characteristics of preclinical models, such as patient-derived cell lines, patient-derived xenografts, genetically engineered mouse models, and patient-derived organoids, and their advantages and disadvantages, to provide a reference for further studies of neuroendocrine tumors. We also highlight the method of establishment of liver metastasis mouse models.

An update on genetically engineered mouse models of pancreatic neuroendocrine neoplasms

Pancreatic neuroendocrine neoplasms (PanNENs) are rare and clinically challenging entities. At the molecular level, PanNENs' genetic profile is well characterized, but there is limited knowledge regarding the contribution of the newly identified genes to tumor initiation and progression. Genetically engineered mouse models (GEMMs) are the most versatile tool for studying the plethora of genetic variations influencing PanNENs' etiopathogenesis and behavior over time. In this review, we present the state of the art of the most relevant PanNEN GEMMs available and correlate their findings with the human neoplasms' counterparts. We discuss the historic GEMMs as the most used and with higher translational utility models. GEMMs with Men1 and glucagon receptor gene germline alterations stand out as the most faithful models in recapitulating human disease; RIP-Tag models are unique models of early-onset, highly vascularized, invasive carcinomas. We also include a section of the most recent GEMMs that evaluate pathways related to cell cycle and apoptosis, Pi3k/Akt/mTOR, and Atrx/Daxx. For the latter, their tumorigenic effect is heterogeneous. In particular, for Atrx/Daxx, we will require more in-depth studies to evaluate their contribution; even though they are prevalent genetic events in PanNENs, they have low/inexistent tumorigenic capacity per se in GEMMs. Researchers planning to use GEMMs can find a road map of the main clinical features in this review, presented as a guide that summarizes the chief milestones achieved. We identify pitfalls to overcome, concerning the novel designs and standardization of results, so that future models can replicate human disease more closely.

Preclinical Models of Neuroendocrine Neoplasia

Neuroendocrine neoplasia (NENs) are a complex and heterogeneous group of cancers that can arise from neuroendocrine tissues throughout the body and differentiate them from other tumors. Their low incidence and high diversity make many of them orphan conditions characterized by a low incidence and few dedicated clinical trials. Study of the molecular and genetic nature of these diseases is limited in comparison to more common cancers and more dependent on preclinical models, including both in vitro models (such as cell lines and 3D models) and in vivo models (such as patient derived xenografts (PDXs) and genetically-engineered mouse models (GEMMs)). While preclinical models do not fully recapitulate the nature of these cancers in patients, they are useful tools in investigation of the basic biology and early-stage investigation for evaluation of treatments for these cancers. We review available preclinical models for each type of NEN and discuss their history as well as their current use and translation.

GFAP-directed Inactivation of Men1 Exploits Glial Cell Plasticity in Favor of Neuroendocrine Reprogramming

BACKGROUND & AIMS

Efforts to characterize the signaling mechanisms that underlie gastroenteropancreatic neoplasms (GEP-NENs) are precluded by a lack of comprehensive models that recapitulate pathogenesis. Investigation into a potential cell-of-origin for gastrin-secreting NENs revealed a non-cell autonomous role for loss of menin in neuroendocrine cell specification, resulting in an induction of gastrin in enteric glia. Here, we investigated the hypothesis that cell autonomous Men1 inactivation in glial fibrillary acidic protein (GFAP)-expressing cells induced neuroendocrine differentiation and tumorigenesis.

METHODS

Transgenic GFAP^ΔMen1 mice were generated by conditional GFAP-directed Men1 deletion in GFAP-expressing cells. Cre specificity was confirmed using a tdTomato reporter. GFAP^ΔMen1 mice were evaluated for GEP-NEN development and neuroendocrine cell hyperplasia. Small interfering RNA-mediated Men1 silencing in a rat enteric glial cell line was performed in parallel.

RESULTS

GFAP^ΔMen1 mice developed pancreatic NENs, in addition to pituitary prolactinomas that phenocopied the human MEN1 syndrome. GFAP^ΔMen1 mice exhibited gastric neuroendocrine hyperplasia that coincided with a significant loss of GFAP expression. Men1 deletion induced loss of glial-restricted progenitor lineage markers and an increase in neuroendocrine genes, suggesting a reprogramming of GFAP⁺ cells. Deleting Kif3a, a mediator of Hedgehog signaling, in GFAP-expressing cells attenuated neuroendocrine hyperplasia by restricting the neuroendocrine cell fate. Similar results in the pancreas were observed when Sox10 was used to delete Men1.

CONCLUSIONS

GFAP-directed Men1 inactivation exploits glial cell plasticity in favor of neuroendocrine differentiation.

A Blood-based Polyamine Signature Associated With MEN1 Duodenopancreatic Neuroendocrine Tumor Progression

CONTEXT

Duodenopancreatic neuroendocrine tumors (dpNETs) frequently occur in patients with multiple endocrine neoplasia type 1 (MEN1), and metastatic dpNET is the primary cause of disease-related mortality. There is a need for biomarkers that can identify patients with MEN1-related dpNETs that are at high risk of developing distant metastasis. Polyamines have tumor-promoting roles in several cancer types.

OBJECTIVE

We hypothesized that MEN1-dpNET-related disease progression is associated with elevated levels of circulating polyamines.

METHODS

Through an international collaboration between The University of Texas MD Anderson Cancer Center, the National Institutes of Health, and the University Medical Center Utrecht, plasma polyamine levels were assessed using mass spectrometry in 84 patients with MEN1 (20 with distant metastatic dpNETs [patients] and 64 with either indolent dpNETs or no dpNETs [controls]). A mouse model of MEN1-pNET, Men1fl/flPdx1-CreTg, was used to test time-dependent changes in plasma polyamines associated with disease progression.

RESULTS

A 3-marker plasma polyamine signature (3MP: N-acetylputrescine, acetylspermidine, and diacetylspermidine) distinguished patients with metastatic dpNETs from controls in an initial set of plasmas from the 3 participating centers. The fixed 3MP yielded an area under the curve of 0.84 (95% CI, 0.62-1.00) with 66.7% sensitivity at 95% specificity for distinguishing patients from controls in an independent test set from MDACC. In Men1fl/flPdx1-CreTg mice, the 3MP was elevated early and remained high during disease progression.

CONCLUSION

Our findings provide a basis for prospective testing of blood-based polyamines as a potential means for monitoring patients with MEN1 for harboring or developing aggressive disease.

Pancreatic Neuroendocrine Tumors: Molecular Mechanisms and Therapeutic Targets

Pancreatic neuroendocrine tumors (pNETs) are unique, slow-growing malignancies whose molecular pathogenesis is incompletely understood. With rising incidence of pNETs over the last four decades, larger and more comprehensive 'omic' analyses of patient tumors have led to a clearer picture of the pNET genomic landscape and transcriptional profiles for both primary and metastatic lesions. In pNET patients with advanced disease, those insights have guided the use of targeted therapies that inhibit activated mTOR and receptor tyrosine kinase (RTK) pathways or stimulate somatostatin receptor signaling. Such treatments have significantly benefited patients, but intrinsic or acquired drug resistance in the tumors remains a major problem that leaves few to no effective treatment options for advanced cases. This demands a better understanding of essential molecular and biological events underlying pNET growth, metastasis, and drug resistance. This review examines the known molecular alterations associated with pNET pathogenesis, identifying which changes may be drivers of the disease and, as such, relevant therapeutic targets. We also highlight areas that warrant further investigation at the biological level and discuss available model systems for pNET research. The paucity of pNET models has hampered research efforts over the years, although recently developed cell line, animal, patient-derived xenograft, and patient-derived organoid models have significantly expanded the available platforms for pNET investigations. Advancements in pNET research and understanding are expected to guide improved patient treatments.

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.

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.

Models of Gastroenteropancreatic Neuroendocrine Neoplasms: Current Status and Future Directions

Gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs) are a rare, heterogeneous group of tumors that originate from the endocrine system of the gastrointestinal tract and pancreas. GEP-NENs are subdivided according to their differentiation into well-differentiated neuroendocrine tumors (NETs) and poorly differentiated neuroendocrine carcinomas (NECs). Since GEP-NENs represent rare diseases, only limited data from large prospective, randomized clinical trials are available, and recommendations for treatment of GEP-NEN are in part based on data from retrospective analyses or case series. In this context, tractable disease models that reflect the situation in humans and that allow to recapitulate the different clinical aspects and disease stages of GEP-NET or GEP-NEC are urgently needed. In this review, we highlight available data on mouse models for GEP-NEN. We discuss how these models reflect tumor biology of human disease and whether these models could serve as a tool for understanding the pathogenesis of GEP-NEN and for disease modeling and pharmacosensitivity assays, facilitating prediction of treatment response in patients. In addition, open issues applicable for future developments will be discussed.

Epigenetic landscape of pancreatic neuroendocrine tumours reveals distinct cells of origin and means of tumour progression

Recent data suggest that Pancreatic Neuroendocrine Tumours (PanNETs) originate from α- or β-cells of the islets of Langerhans. The majority of PanNETs are non-functional and do not express cell-type specific hormones. In the current study we examine whether tumour DNA methylation (DNAme) profiling combined with genomic data is able to identify cell of origin and to reveal pathways involved in PanNET progression. We analyse genome-wide DNAme data of 125 PanNETs and sorted α- and β-cells. To confirm cell identity, we investigate ARX and PDX1 expression. Based on epigenetic similarities, PanNETs cluster in α-like, β-like and intermediate tumours. The epigenetic similarity to α-cells progressively decreases in the intermediate tumours, which present unclear differentiation. Specific transcription factor methylation and expression vary in the respective α/β-tumour groups. Depending on DNAme similarity to α/β-cells, PanNETs have different mutational spectra, stage of the disease and prognosis, indicating potential means of PanNET progression.

Daxx maintains endogenous retroviral silencing and restricts cellular plasticity in vivo

Tumor sequencing studies have emphasized the role of epigenetics and altered chromatin homeostasis in cancer. Mutations in DAXX, which encodes a chaperone for the histone 3.3 variant, occur in 25% of pancreatic neuroendocrine tumors (PanNETs). To advance our understanding of physiological functions of Daxx, we developed a conditional Daxx allele in mice. We demonstrate that Daxx loss is well tolerated in the pancreas but creates a permissive transcriptional state that cooperates with environmental stress (inflammation) and other genetic lesions (Men1 loss) to alter gene expression and cell state, impairing pancreas recovery from inflammatory stress in vivo. The transcriptional changes are associated with dysregulation of endogenous retroviral elements (ERVs), and dysregulation of endogenous genes near ERVs is also observed in human PanNETs with DAXX mutations. Our results reveal a physiologic function of DAXX, provide a mechanism associated with impaired tissue regeneration and tumorigenesis, and expand our understanding of ERV regulation in somatic cells.

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.

Men1 maintains exocrine pancreas homeostasis in response to inflammation and oncogenic stress

A more comprehensive understanding of the molecular mechanisms underlying pancreatic diseases, including pancreatitis and cancer, is essential to improve clinical management. MEN1 has established roles in epigenetic regulation and tumor suppression in the endocrine pancreas; however, intriguing recent data suggest MEN1 may also function in the exocrine pancreas. Using physiologically relevant genetic mouse models, we provide direct evidence that Men1 is essential for exocrine pancreas homeostasis in response to inflammation and oncogenic stress. Men1 loss causes increased injury and impaired regeneration following acute caerulein-induced pancreatitis, leading to more severe damage, loss of the normal acinar compartment, and increased cytokeratin 19-positive metaplasias and immune cell infiltration. We further demonstrate the Men1 protein is stabilized in response to insult, and loss of Men1 is associated with the overexpression of proinflammatory Jund target genes, suggesting that loss of Men1-mediated repression of Jund activity is, at least in part, responsible for the impaired response. Finally, we demonstrate that Men1 loss significantly accelerates mutant Kras-dependent oncogenesis. Combined, this work establishes Men1 as an important mediator of pancreas homeostasis in vivo.

ATRX, DAXX or MEN1 mutant pancreatic neuroendocrine tumors are a distinct alpha-cell signature subgroup

The commonly mutated genes in pancreatic neuroendocrine tumors (PanNETs) are ATRX, DAXX, and MEN1. We genotyped 64 PanNETs and found 58% carry ATRX, DAXX, and MEN1 mutations (A-D-M mutant PanNETs) and this correlates with a worse clinical outcome than tumors carrying the wild-type alleles of all three genes (A-D-M WT PanNETs). We performed RNA sequencing and DNA-methylation analysis to reveal two distinct subgroups with one consisting entirely of A-D-M mutant PanNETs. Two genes differentiating A-D-M mutant from A-D-M WT PanNETs were high ARX and low PDX1 gene expression with PDX1 promoter hyper-methylation in the A-D-M mutant PanNETs. Moreover, A-D-M mutant PanNETs had a gene expression signature related to that of alpha-cells (FDR q-value < 0.009) of pancreatic islets including increased expression of HNF1A and its transcriptional target genes. This gene expression profile suggests that A-D-M mutant PanNETs originate from or transdifferentiate into a distinct cell type similar to alpha cells.

In pancreatic neuroendocrine tumors (PanNETs) ATRX, DAXX, and MEN1 are commonly mutated (A-D-M mutant PanNETs). Here, the authors find in a cohort of PanNETS 58% are A-D-M mutant PanNETs, with a worse clinical outcome and differences in gene expression and methylation compared to A-D-M wild type cases- these gene expression differences suggest that A-D-M mutant PanNETs potentially originate from a cell type similar to alpha cells.

MENIN loss as a tissue-specific driver of tumorigenesis

The MEN1 gene encodes MENIN, a tumor suppressor that plays a role in multiple cellular processes. Germline and somatic mutations in MEN1 have been identified in hereditary and sporadic tumors of neuroendocrine origins suggesting context-specific functions. In this review, we focus on the development of mutational Men1 in vivo models, the known cellular activities of MENIN and efforts to identify vulnerabilities in tumors with MENIN loss.

Gastroenteropancreatic neuroendocrine neoplasms: genes, therapies and models

Gastroenteropancreatic neuroendocrine neoplasms (GEP-NENs) refer to a group of heterogeneous cancers of neuroendocrine cell phenotype that mainly fall into one of two subtypes: gastroenteropancreatic neuroendocrine tumors (GEP-NETs; well differentiated) or gastroenteropancreatic neuroendocrine carcinomas (GEP-NECs; poorly differentiated). Although originally defined as orphan cancers, their steadily increasing incidence highlights the need to better understand their etiology. Accumulating epidemiological and clinical data have shed light on the pathological characteristics of these diseases. However, the relatively low number of patients has hampered conducting large-scale clinical trials and hence the development of novel treatment strategies. To overcome this limitation, tractable disease models that faithfully reflect clinical features of these diseases are needed. In this Review, we summarize the current understanding of the genetics and biology of these diseases based on conventional disease models, such as genetically engineered mouse models (GEMMs) and cell lines, and discuss the phenotypic differences between the models and affected humans. We also highlight the emerging disease models derived from human clinical samples, including patient-derived xenograft models and organoids, which may provide biological and therapeutic insights into GEP-NENs.

Characterization of pancreatic glucagon-producing tumors and pituitary gland tumors in transgenic mice overexpressing MYCN in hGFAP-positive cells

Amplification or overexpression of MYCN is involved in development and maintenance of multiple malignancies. A subset of these tumors originates from neural precursors, including the most aggressive forms of the childhood tumors, neuroblastoma and medulloblastoma. In order to model the spectrum of MYCN-driven neoplasms in mice, we transgenically overexpressed MYCN under the control of the human GFAP-promoter that, among other targets, drives expression in neural progenitor cells. However, LSL-MYCN;hGFAP-Cre double transgenic mice did neither develop neural crest tumors nor tumors of the central nervous system, but presented with neuroendocrine tumors of the pancreas and, less frequently, the pituitary gland. Pituitary tumors expressed chromogranin A and closely resembled human pituitary adenomas. Pancreatic tumors strongly produced and secreted glucagon, suggesting that they derived from glucagon- and GFAP-positive islet cells. Interestingly, 3 out of 9 human pancreatic neuroendocrine tumors expressed MYCN, supporting the similarity of the mouse tumors to the human system. Serial transplantations of mouse tumor cells into immunocompromised mice confirmed their fully transformed phenotype. MYCN-directed treatment by AuroraA- or Brd4-inhibitors resulted in significantly decreased cell proliferation in vitro and reduced tumor growth in vivo. In summary, we provide a novel mouse model for neuroendocrine tumors of the pancreas and pituitary gland that is dependent on MYCN expression and that may help to evaluate MYCN-directed therapies.

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.

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.

Male Men1 heterozygous mice exhibit fasting hyperglycemia in the early stage of MEN1

Multiple endocrine neoplasia type 1 (MEN1) is an autosomal dominant inherited syndrome characterized by multiple tumors in the parathyroid glands, endocrine pancreas and anterior pituitary. Recent clinical studies have revealed a strong association between MEN1 syndrome and the risk of developing diabetes mellitus; however, the underlying mechanisms remain unknown. In this study, heterozygous Men1 knockout (Men1(+/-)) mice were used as MEN1 models to investigate MEN1-associated glucose metabolic phenotypes and mechanisms. Heterozygous deficiency of Men1 in 12-month-old male mice induced fasting hyperglycemia, along with increased serum insulin levels. However, male Men1(+/-) mice did not show insulin resistance, as evidenced by Akt activation in hepatic tissues and an insulin tolerance test. Increased glucose levels following pyruvate challenge and expression of key gluconeogenic genes suggested increased hepatic glucose output in the male Men1(+/-) mice. This effect could be partly due to higher basal serum glucagon levels, which resulted from pancreatic islet cell proliferation induced by heterozygous loss of Men1 Taken together, our results indicate that fasted male Men1(+/-) mice, in the early stage of development of MEN1, display glucose metabolic disorders. These disorders are caused not by direct induction of insulin resistance, but via increased glucagon secretion and the consequent stimulation of hepatic glucose production.

Animal models of multiple endocrine neoplasia

Multiple endocrine neoplasia (MEN) syndromes are autosomal dominant diseases with high penetrance characterized by proliferative lesions (usually hyperplasia or adenoma) arising in at least two endocrine tissues. Four different MEN syndromes have been so far identified: MEN type 1 (MEN1), MEN2A (also referred to as MEN2), MEN2B (or MEN3) and MEN4, which have slightly varying tumor spectra and are caused by mutations in different genes. MEN1 associates with loss-of-function mutations in the MEN1 gene encoding the tumor suppressor menin. The MEN2A and MEN2B syndromes are due to activating mutations in the proto-oncogene RET (Rearranged in Transfection) and are characterized by different phenotypic features of the affected patients. MEN4 was the most recent addition to the family of the MEN syndromes. It was discovered less than 10 years ago thanks to studies of a rat strain that spontaneously develops multiple endocrine tumors (named MENX). These studies identified an inactivating mutation in the Cdkn1b gene, encoding the putative tumor suppressor p27, as the causative mutation of the rat syndrome. Subsequently, germline mutations in the human ortholog CDKN1B were also found in a subset of patients with a MEN-like phenotype and this led to the identification of MEN4. Small animal models have been instrumental in understanding important biochemical, physiological and pathological processes of cancer onset and spread in intact living organisms. Moreover, they have provided us with insight into gene function(s) and molecular mechanisms of disease progression. We here review the currently available animal models of MEN syndromes and their impact on the elucidation of the pathophysiology of these diseases, with a special focus on the rat MENX syndrome that we have been characterizing.

Animal models of spontaneous pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PNETs) are usually low-grade neoplasms derived from the endocrine pancreas. PNETs can be functioning and cause well-described hormonal hypersecretion syndromes or non-functioning and cause only tumor mass effect. PNETs appear to be more common recently likely due to incidental detection by imaging. Although the diagnosis and management of PNETs have been evolving rapidly, much remains to be studied in the areas of molecular pathogenesis, molecular markers of tumor behavior, early detection, and targeted drug therapy. Unique challenges facing PNETs studies are long disease course, the deep location of pancreas and difficult access to pancreatic tissue, and the variety of tumors, which make animal models valuable tools for PNETs studies. Existing animal models of PNETs have provided insights into the pathogenesis and natural history of human PNETs. Future studies on animal models of PNETs should address early tumor detection, molecular markers of tumor behavior, and novel targeted therapies.

Exploring the tumors of multiple endocrine neoplasia type 1 in mouse models for basic and preclinical studies

Most patients (70-90%) with the multiple endocrine neoplasia type 1 (MEN1) syndrome possess germline heterozygous mutations in MEN1 that predisposes to tumors of multiple endocrine and nonendocrine tissues. Some endocrine tumors of the kinds seen in MEN1 that occur sporadically in the general population also possess somatic mutations in MEN1. Interestingly, the endocrine tumors of MEN1 are recapitulated in mouse models of Men1 loss that serve as a valuable resource to understand the pathophysiology and molecular basis of tumorigenesis. Exploring these endocrine tumors in mouse models using in vivo, ex vivo and in vitro methods can help to follow the process of tumorigenesis, and can be useful for preclinical testing of therapeutics and understanding their mechanisms of action.

GSK-3β protein phosphorylates and stabilizes HLXB9 protein in insulinoma cells to form a targetable mechanism of controlling insulinoma cell proliferation

Insulinomas (pancreatic islet β cell tumors) are the most common type of functioning pancreatic neuroendocrine tumors that occur sporadically or as a part of the MEN1 syndrome that is caused by germ line mutations in MEN1. Tissue-specific tumor predisposition from germ line mutations in ubiquitously expressed genes such as MEN1 could occur because of functional consequences on tissue-specific factors. We previously reported the proapoptotic β cell differentiation factor HLXB9 as a downstream target of menin (encoded by MEN1). Here we show that GSK-3β inactivates the proapoptotic activity of HLXB9 by phosphorylating HLXB9 at Ser-78/Ser-80 (pHLXB9). Although HLXB9 is found in the nucleus and cytoplasm, pHLXB9 is predominantly nuclear. Both pHLXB9 and active GSK-3β are elevated in β cells with menin knockdown, in MEN1-associated β cell tumors (insulinomas), and also in human sporadic insulinomas. Pharmacologic inhibition of GSK-3β blocked cell proliferation in three different rodent insulinoma cell lines by arresting the cells in G2/M phase and caused apoptosis. Taken together, these data suggest that the combination of GSK-3β and pHLXB9 forms a therapeutically targetable mechanism of insulinoma pathogenesis. Our results reveal that GSK-3β and pHLXB9 can serve as novel targets for insulinoma treatment and have implications for understanding the pathways associated with β cell proliferation.

Conditional deletion of p53 and Rb in the renin-expressing compartment of the pancreas leads to a highly penetrant metastatic pancreatic neuroendocrine carcinoma

Efforts to model human pancreatic neuroendocrine tumors (PanNETs) in animals have been moderately successful, with minimal evidence for glucagonomas or metastatic spread. The renin gene, although classically associated with expression in the kidney, is also expressed in many other extrarenal tissues including the pancreas. To induce tumorigenesis within rennin-specific tissues, floxed alleles of p53 and Rb were selectively abrogated using Cre-recombinase driven by the renin promoter. The primary neoplasm generated is a highly metastatic islet cell carcinoma of the pancreas. Lineage tracing identifies descendants of renin-expressing cells as pancreatic alpha cells despite a lack of active renin expression in the mature pancreas. Both primary and metastatic tumors express high levels of glucagon; furthermore, an increased level of glucagon is found in the serum, identifying the pancreatic cancer as a functional glucagonoma. This new model is highly penetrant and exhibits robust frequency of metastases to the lymph nodes and the liver, mimicking human disease, and provides a useful platform for better understanding pancreatic endocrine differentiation and development, as well as islet cell carcinogenesis. The use of fluorescent reporters for lineage tracing of the cells contributing to disease initiation and progression provides an unique opportunity to dissect the timeline of disease, examining mechanisms of the metastatic process, as well as recovering primary and metastatic cells for identifying cooperating mutations that are necessary for progression of disease.

Elimination of von Hippel-Lindau function perturbs pancreas endocrine homeostasis in mice

Mutations in the human homolog of the Vhlh gene [encoding the von-Hippel Lindau (VHL) protein] lead to tumor development. In mice, depletion of Vhlh in pancreatic ß-cells causes perturbed glucose homeostasis, but the role of this gene in other pancreatic cells is poorly understood. To investigate the function of VHL/HIF pathway in pancreatic cells, we inactivated Vhlh in the pancreatic epithelium as well as in the endocrine and exocrine lineages. Our results show that embryonic depletion of Vhlh within the pancreatic epithelium causes postnatal lethality due to severe hypoglycemia. The hypoglycemia is recapitulated in mice with endocrine-specific removal of Vhlh, while animals with loss of Vhlh predominantly in the exocrine compartment survive to adulthood with no overt defects in glucose metabolism. Mice with hypoglycemia display diminished insulin release in response to elevated glucose. Significantly, the glucagon response is impaired both in vivo (circulating glucagon levels) as well as in an in vitro secretion assay in isolated islets. Hypoxia also impairs glucagon secretion in a glucagon-expressing cell line in culture. Our results reveal a novel role for the hypoxia/HIF pathway in islet hormone secretion and maintenance of the fine balance that allows for the establishment of normoglycemia.

Animal models and cell lines of pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (PNETs), also known as islet cell tumors, are mostly indolent neoplasms that probably arise from a network of endocrine cells that includes islet cells and pluripotent precursors in the pancreatic ductal epithelium. The incidence and prevalence of PNETs continue to rise in recent years because of more sensitive detection. The molecular pathogenesis, early detection, molecular predictors of tumor behavior, and targeted drug therapy of PNETs are not well understood and require additional basic and translational research. The rarity and indolent nature of these tumors, difficulty of access to appropriate patient tissue samples, and varying histopathology and secreted hormones pose particular challenges to PNET researchers. Animal models and cell lines are indispensable tools for investigating the pathogenesis, pathophysiology, mechanisms for tumor invasion and metastasis, and therapeutics of PNETs. This review summarizes currently available animal models and cell lines of PNETs, which have provided valuable insights into the pathogenesis and natural history of human PNETs. In the future, animal models and cell lines of PNETs should also be used to study early tumor detection and molecular predictors of tumor behavior and to test the responses to, and mechanisms for, novel targeted drug therapies.

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.

Deconstructing pancreas developmental biology

The relentless nature and increasing prevalence of human pancreatic diseases, in particular, diabetes mellitus and adenocarcinoma, has motivated further understanding of pancreas organogenesis. The pancreas is a multifunctional organ whose epithelial cells govern a diversity of physiologically vital endocrine and exocrine functions. The mechanisms governing the birth, differentiation, morphogenesis, growth, maturation, and maintenance of the endocrine and exocrine components in the pancreas have been discovered recently with increasing tempo. This includes recent studies unveiling mechanisms permitting unexpected flexibility in the developmental potential of immature and mature pancreatic cell subsets, including the ability to interconvert fates. In this article, we describe how classical cell biology, genetic analysis, lineage tracing, and embryological investigations are being complemented by powerful modern methods including epigenetic analysis, time-lapse imaging, and flow cytometry-based cell purification to dissect fundamental processes of pancreas development.

Genome-wide characterization of menin-dependent H3K4me3 reveals a specific role for menin in the regulation of genes implicated in MEN1-like tumors

Inactivating mutations in the MEN1 gene predisposing to the multiple endocrine neoplasia type 1 (MEN1) syndrome can also cause sporadic pancreatic endocrine tumors. MEN1 encodes menin, a subunit of MLL1/MLL2-containing histone methyltransferase complexes that trimethylate histone H3 at lysine 4 (H3K4me3). The importance of menin-dependent H3K4me3 in normal and transformed pancreatic endocrine cells is unclear. To study the role of menin-dependent H3K4me3, we performed in vitro differentiation of wild-type as well as menin-null mouse embryonic stem cells (mESCs) into pancreatic islet-like endocrine cells (PILECs). Gene expression analysis and genome-wide H3K4me3 ChIP-Seq profiling in wild-type and menin-null mESCs and PILECs revealed menin-dependent H3K4me3 at the imprinted Dlk1-Meg3 locus in mESCs, and all four Hox loci in differentiated PILECs. Specific and significant loss of H3K4me3 and gene expression was observed for genes within the imprinted Dlk1-Meg3 locus in menin-null mESCs and the Hox loci in menin-null PILECs. Given that the reduced expression of genes within the DLK1-MEG3 locus and the HOX loci is associated with MEN1-like sporadic tumors, our data suggests a possible role for menin-dependent H3K4me3 at these genes in the initiation and progression of sporadic pancreatic endocrine tumors. Furthermore, our investigation also demonstrates that menin-null mESCs can be differentiated in vitro into islet-like endocrine cells, underscoring the utility of menin-null mESC-derived specialized cell types for genome-wide high-throughput studies.

Mouse models for inherited endocrine and metabolic disorders

In vivo models represent important resources for investigating the physiological mechanisms underlying endocrine and metabolic disorders, and for pre-clinical translational studies that may include the assessments of new treatments. In the study of endocrine diseases, which affect multiple organs, in vivo models provide specific advantages over in vitro models, which are limited to investigation of isolated systems. In recent years, the mouse has become the popular choice for developing such in vivo mammalian models, as it has a genome that shares ∼85% identity to that of man, and has many physiological systems that are similar to those in man. Moreover, methods have been developed to alter the expression of genes in the mouse, thereby generating models for human diseases, which may be due to loss- or gain-of-function mutations. The methods used to generate mutations in the mouse genome include: chemical mutagenesis; conventional, conditional and inducible knockout models; knockin models and transgenic models, and these strategies are often complementary. This review describes some of the different strategies that are utilised for generating mouse models. In addition, some mouse models that have been successfully generated by these methods for some human hereditary endocrine and metabolic disorders are reviewed. In particular, the mouse models generated for parathyroid disorders, which include: the multiple endocrine neoplasias; hyperparathyroidism-jaw tumour syndrome; disorders of the calcium-sensing receptor and forms of inherited hypoparathyroidism are discussed. The advances that have been made in our understanding of the mechanisms of these human diseases by investigations of these mouse models are described.