Global expression profiling of murine MEN1-associated tumors reveals a regulatory role for menin in transcription, cell cycle and chromatin remodelling

CRISPR-Cas9 screens identify regulators of antibody-drug conjugate toxicity

Antibody-drug conjugates (ADCs) selectively deliver chemotherapeutic agents to target cells and are important cancer therapeutics. However, the mechanisms by which ADCs are internalized and activated remain unclear. Using CRISPR-Cas9 screens, we uncover many known and novel endolysosomal regulators as modulators of ADC toxicity. We identify and characterize C18ORF8/RMC1 as a regulator of ADC toxicity through its role in endosomal maturation. Through comparative analysis of screens with ADCs bearing different linkers, we show that a subset of late endolysosomal regulators selectively influence toxicity of noncleavable linker ADCs. Surprisingly, we find cleavable valine-citrulline linkers can be processed rapidly after internalization without lysosomal delivery. Lastly, we show that sialic acid depletion enhances ADC lysosomal delivery and killing in diverse cancer cell types, including with FDA (US Food and Drug Administration)-approved trastuzumab emtansine (T-DM1) in Her2-positive breast cancer cells. Together, these results reveal new regulators of endolysosomal trafficking, provide important insights for ADC design and identify candidate combination therapy targets.

Genetic and epigenetic drivers of neuroendocrine tumours (NET)

Neuroendocrine tumours (NET) of the gastrointestinal tract and the lung are a rare and heterogeneous group of tumours. The molecular characterization and the clinical classification of these tumours have been evolving slowly and show differences according to organs of origin. Novel technologies such as next-generation sequencing revealed new molecular aspects of NET over the last years. Notably, whole-exome/genome sequencing (WES/WGS) approaches underlined the very low mutation rate of well-differentiated NET of all organs compared to other malignancies, while the engagement of epigenetic changes in driving NET evolution is emerging. Indeed, mutations in genes encoding for proteins directly involved in chromatin remodelling, such as DAXX and ATRX are a frequent event in NET. Epigenetic changes are reversible and targetable; therefore, an attractive target for treatment. The discovery of the mechanisms underlying the epigenetic changes and the implication on gene and miRNA expression in the different subgroups of NET may represent a crucial change in the diagnosis of this disease, reveal new therapy targets and identify predictive markers. Molecular profiles derived from omics data including DNA mutation, methylation, gene and miRNA expression have already shown promising results in distinguishing clinically and molecularly different subtypes of NET. In this review, we recapitulate the major genetic and epigenetic characteristics of pancreatic, lung and small intestinal NET and the affected pathways. We also discuss potential epigenetic mechanisms leading to NET development.

Progranulin Stimulates Proliferation of Mouse Pancreatic Islet Cells and Is Overexpressed in the Endocrine Pancreatic Tissue of an MEN1 Mouse Model

OBJECTIVES

Progranulin (PGRN) promotes cell growth and cell cycle progression in several cell types and contributes to tumorigenesis in diverse cancers. We have recently reported PGRN expression in islets and tumors developed in an MEN1 transgenic mouse. Here we sought to investigate PGRN expression and regulation after exposure to hypoxia as well as its effects on pancreatic islet cells and neuroendocrine tumors (NETs) in MEN1(+/−) mice.

METHODS

Gene and protein expression were analyzed by quantitative polymerase chain reaction, immunohistochemistry, and Western blot. We also investigated PGRN expression in samples from patients carrying pancreatic NETs associated or not with the multiple endocrine neoplasia 1 syndrome, using enzyme-linked immunosorbent assay and immunohistochemistry analysis.

RESULTS

Progranulin is upregulated in tumors and islets of the MEN1 mouse as well as in the serum of patients with pancreatic NETs associated with glucagonoma syndrome. In normal mice islets and pancreatic tumors, PGRN expression was strongly potentiated by hypoxia. Progranulin promotes cell proliferation in islet cells and βTC-6 cells, a process paralleled by activation of the mitogen-activated protein kinase signaling cascade.

CONCLUSIONS

Our findings identify PGRN as an effective inducer of pancreatic islet cell proliferation and a possible important factor for pancreatic endocrine tumor development.

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.

MEN1 tumorigenesis in the pituitary and pancreatic islet requires Cdk4 but not Cdk2

Recent studies suggest that physiological and tumorigenic proliferation of mammalian cells is controlled by multiple cyclin-dependent kinases (CDKs) largely in tissue-specific manners. We and others previously demonstrated that adult mice deficient for the Cyclin D partner CDK4 (Cdk4(-/-) mice) exhibit hypoplasia in the pituitary and pancreatic islet due to primary postnatal defects in proliferation. Intriguingly, those neuroendocrine tissues affected in Cdk4(-/-) mice are the primary targets of tumorigenesis in the syndrome of multiple endocrine neoplasia type-1 (MEN1). Mice with heterozygous disruption of the tumor suppressor Men1 gene (Men1(+/-)) develop tumors in the pituitary, pancreatic islets and other neuroendocrine tissues, which is analogous to humans with MEN1 mutations. To explore the genetic interactions between loss of Men1 and activation of CDKs, we examined the impact of Cdk4 or Cdk2 disruption on tumorigenesis in Men1(+/-) mice. A majority of Men1(+/-) mice with wild-type CDKs developed pituitary and islet tumors by 15 months of age. Strikingly, Men1(+/-); Cdk4(-/-) mice did not develop any tumors, and their islets and pituitaries remained hypoplastic with decreased proliferation. In contrast, Men1(+/-); Cdk2(-/-) mice showed pituitary and islet tumorigenesis comparable to those in Men1(+/-) mice. Pituitaries of Men1(+/-); Cdk4(-/-) mice showed no signs of loss of heterozygosity (LOH) in the Men1 locus, whereas tumors in Men1(+/-) mice and Men1(+/-); Cdk2(-/-) mice exhibited LOH. Consistently, CDK4 knockdown in INS-1 insulinoma cells inhibited glucose-stimulated cell cycle progression with a significant decrease in phosphorylation of retinoblastoma protein (RB) at specific sites including Ser780. CDK2 knockdown had minimum effects on RB phosphorylation and cell cycle progression. These data suggest that CDK4 is a critical downstream target of MEN1-dependent tumor suppression and is required for tumorigenic proliferation in the pituitary and pancreatic islet, whereas CDK2 is dispensable for tumorigenesis in these neuroendocrine cell types.

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.

Menin and p53 have non-synergistic effects on tumorigenesis in mice

BACKGROUND

While it is now more than a decade since the first description of the gene mutation underlying the tumour predisposition syndrome multiple endocrine neoplasia type 1 (MEN1), the mechanism by which its protein product menin acts to prevent development of tumours is still poorly understood.

METHODS

We undertook a genetic experiment to assess whether menin synergises with p53. Mice carrying various combinations of Men1 and Trp53 mutations were generated then survival and pathology assessed.

RESULTS

While homozygous loss of Trp53 in mice resulted in early onset, aggressive tumours and profoundly reduced lifespan, heterozygous loss of either Trp53 or Men1 caused later onset disease, with a spectrum of tumours characteristic of each tumour suppressor gene. Loss of one copy of Men1 in animals also lacking both alleles of Trp53 did not exacerbate phenotype, based on survival, animal weight or sites of pathology, compared to Trp53 deletion alone. Dual heterozygous deletion of Men1 and Trp53 resulted in a small reduction in lifespan compared to the individual mutations, without new tumour sites. In the adrenal, we observed development of cortical tumours in dual heterozygous animals, as we have previously seen in Men1+/- animals, and there was loss of heterozygosity at the Men1 allele in these tumours. Median number of pathology observations per animal was increased in dual heterozygous animals compared with heterozygous loss of Trp53 alone.

CONCLUSIONS

Simultaneous heterozygous deletion of Men1 in animals with either heterozygous or homozygous deletion of Trp53 did not result in formation of tumours at any new sites, implying additive rather than synergistic effects of these pathways. Mice that were Men1+/- in addition to Trp53+/- had tumours in endocrine as well as other sites, implying that increase in total tumour burden, at sites typically associated with either Men1 or Trp53 loss, contributed to the slight decrease in survival in Men1+/-: Trp53+/- animals in comparison with their littermates.

Alterations in gene expression in MEN1-associated insulinoma development

OBJECTIVES

To identify gene expression alterations associated with insulinoma formation and progression in 2 mouse models of multiple endocrine neoplasia type 1.

METHODS

Mice were killed at 12 or 16 months, and pancreatic islets were isolated by enzymatic and physical disruption. Islets were separated by size representing control, normal, hyperplastic, and adenomous islets. RNA was isolated from these islets and profiled on Sentrix Mouse-6 Expression version 1 BeadChips. Array data were analyzed in GeneSpring.

RESULTS

One hundred and one genes that were significantly (P ≤ 0.05) altered in hyperplastic islets and insulinomas compared with normal islets were identified. Of these, 64 gene elements showed reduced messenger RNA levels and 37 gene elements had increased gene expression compared with control islets. Altered expression of 3 genes, namely, Gata6, Tspan8, and s100a8, was confirmed by quantitative reverse transcription-polymerase chain reaction, and aberrant levels of Tspan8 and Lmo2 protein measured by Western blot correlated with the changes in messenger RNA levels.

CONCLUSIONS

These results suggest that alterations in gene expression of Gata6, Tspan8, S100a8, and Lmo2 may act via novel pathways that play functionally important roles in Men1-associated tumor progression.

Multiple endocrine neoplasia type 1 (MEN1)

Multiple Endocrine Neoplasia type 1 (MEN1) is an autosomal-dominant disorder characterised by the occurrence of tumours of the parathyroids, pancreas and anterior pituitary. The MEN1 gene, consists of 10 exons that encode a 610-amino acid protein referred to as Menin. Menin is predominantly a nuclear protein that has roles in transcriptional regulation, genome stability, cell division and proliferation. Germ-line mutations usually result in MEN1 or occasionally in an allelic variant referred to as Familial Isolated Hyperparathyroidism (FIHP). MEN1 tumours frequently have loss of heterozygosity (LOH) of the MEN1 locus, which is consistent with a tumour suppressor role of MEN1. Furthermore, somatic abnormalities of MEN1 have been reported in MEN1 and non-MEN1 endocrine tumours. To date, over 1300 mutations have been reported, and the majority (>70%) of these are predicted to lead to truncated forms of Menin. The mutations are scattered throughout the >9 kb genomic sequence of the MEN1 gene. Four, which consist of c.249_252delGTCT (deletion at codons 83-84), c.1546_1547insC (insertion at codon 516), c.1378C>T (Arg460Ter) and c.628_631delACAG (deletion at codons 210-211) have been reported to occur frequently in 4.5%, 2.7%, 2.6% and 2.5% of families, respectively. However, a comparison of the clinical features in patients and their families with the same mutations reveals an absence of phenotype-genotype correlations. The majority of MEN1 mutations are likely to disrupt the interactions of Menin with other proteins and thereby alter critical events in cell cycle regulation and proliferation.

Melanocyte homeostasis in vivo tolerates Rb1 loss in a developmentally independent fashion

There has been uncertainty regarding the precise role that the pocket protein Rb1 plays in murine melanocyte homeostasis. It has been reported that the TAT-Cre mediated loss of exon 19 from a floxed Rb1 allele causes melanocyte apoptosis in vivo and in vitro. This is at variance with other findings showing, either directly or indirectly, that Rb1 loss in melanocytes has no noticeable effect in vivo, but in vitro leads to a semi-transformed phenotype. In this study, we show that Rb1-null melanocytes lacking exon 19 do not undergo apoptosis and survive both in vitro and in vivo, irrespective of the developmental stage at which Cre-mediated ablation of the exon occurs. Further, Rb1 loss has no serious long-term ramifications on melanocyte homeostasis in vivo, with Rb1-null melanocytes being detected in the skin after numerous hair cycles, inferring that the melanocyte stem cell population carrying the Cre-mediated deletion is maintained. Consequently, whilst Rb1 loss in the melanocyte is able to alter cellular behaviour in vitro, it appears inconsequential with respect to melanocyte homeostasis in the mouse skin.

Review paper: origin and molecular pathology of adrenocortical neoplasms

Neoplastic adrenocortical lesions are common in humans and several species of domestic animals. Although there are unanswered questions about the origin and evolution of adrenocortical neoplasms, analysis of human tumor specimens and animal models indicates that adrenocortical tumorigenesis involves both genetic and epigenetic alterations. Chromosomal changes accumulate during tumor progression, and aberrant telomere function is one of the key mechanisms underlying chromosome instability during this process. Epigenetic changes serve to expand the size of the uncommitted adrenal progenitor population, modulate their phenotypic plasticity (i.e., responsiveness to extracellular signals), and increase the likelihood of subsequent genetic alterations. Analyses of heritable and spontaneous types of human adrenocortical tumors documented alterations in either cell surface receptors or their downstream effectors that impact neoplastic transformation. Many of the mutations associated with benign human adrenocortical tumors result in dysregulated cyclic adenosine monophosphate signaling, whereas key factors and/or signaling pathways associated with adrenocortical carcinomas include dysregulated expression of the IGF2 gene cluster, activation of the Wnt/beta-catenin pathway, and inactivation of the p53 tumor suppressor. A better understanding of the factors and signaling pathways involved in adrenal tumorigenesis is necessary to develop targeted pharmacologic and genetic therapies.

Genetics, gene expression and bioinformatics of the pituitary gland

Genetic cases of congenital pituitary hormone deficiency are common and many are caused by transcription factor defects. Mouse models with orthologous mutations are invaluable for uncovering the molecular mechanisms that lead to problems in organ development and typical patient characteristics. We are using mutant mice defective in the transcription factors PROP1 and POU1F1 for gene expression profiling to identify target genes for these critical transcription factors and candidates for cases of pituitary hormone deficiency of unknown aetiology. These studies reveal critical roles for Wnt signalling pathways, including the TCF/LEF transcription factors and interacting proteins of the groucho family, bone morphogenetic protein antagonists and targets of notch signalling. Current studies are investigating the roles of novel homeobox genes and pathways that regulate the transition from proliferation to differentiation, cell adhesion and cell migration. Pituitary adenomas are a common human health problem, yet most cases are sporadic, necessitating alternative approaches to traditional Mendelian genetic studies. Mouse models of adenoma formation offer the opportunity for gene expression profiling during progressive stages of hyperplasia, adenoma and tumorigenesis. This approach holds promise for the identification of relevant pathways and candidate genes as risk factors for adenoma formation, understanding mechanisms of progression, and identifying drug targets and clinically relevant biomarkers.

Discovery of transcriptional regulators and signaling pathways in the developing pituitary gland by bioinformatic and genomic approaches

We report a catalog of the mouse embryonic pituitary gland transcriptome consisting of five cDNA libraries including wild type tissue from E12.5 and E14.5, Prop1(df/df) mutant at E14.5, and two cDNA subtractions: E14.5 WT-E14.5 Prop1(df/df) and E14.5 WT-E12.5 WT. DNA sequence information is assembled into a searchable database with gene ontology terms representing 12,009 expressed genes. We validated coverage of the libraries by detecting most known homeobox gene transcription factor cDNAs. A total of 45 homeobox genes were detected as part of the pituitary transcriptome, representing most expected ones, which validated library coverage, and many novel ones, underscoring the utility of this resource as a discovery tool. We took a similar approach for signaling-pathway members with novel pituitary expression and found 157 genes related to the BMP, FGF, WNT, SHH and NOTCH pathways. These genes are exciting candidates for regulators of pituitary development and function.

Somatostatin stimulates menin gene expression by inhibiting protein kinase A

Somatostatin is a potent inhibitor of gastrin secretion and gene expression. Menin is a 67-kDa protein product of the multiple endocrine neoplasia type 1 (MEN1) gene that when mutated leads to duodenal gastrinomas, a tumor that overproduces the hormone gastrin. These observations suggest that menin might normally inhibit gastrin gene expression in its role as a tumor suppressor. Since somatostatin and ostensibly menin are both inhibitors of gastrin, we hypothesized that somatostatin signaling directly induces menin. Menin protein expression was significantly lower in somatostatin-null mice, which are hypergastrinemic. We found by immunohistochemistry that somatostatin receptor-positive cells (SSTR2A) express menin. Mice were treated with the somatostatin analog octreotide to determine whether activation of somatostatin signaling induced menin. We found that octreotide increased the number of menin-expressing cells, menin mRNA, and menin protein expression. Moreover, the induction by octreotide was greater in the duodenum than in the antrum. The increase in menin observed in vivo was recapitulated by treating AGS and STC cell lines with octreotide, demonstrating that the regulation was direct. The induction required suppression of protein kinase A (PKA) since forskolin treatment suppressed menin protein levels and octreotide inhibited PKA enzyme activity. Small-interfering RNA-mediated suppression of PKA levels raised basal levels of menin protein and prevented further induction by octreotide. Using AGS cells, we also showed for the first time that menin directly inhibits endogenous gastrin gene expression. In conclusion, somatostatin receptor activation induces menin expression by suppressing PKA activation.