p16 (INK4a, MTS-1) gene polymorphism and methylation status in human pituitary tumours

DNA Methylation in Pituitary Adenomas: A Scoping Review

Pituitary adenomas are a diverse group of neoplasms with variable clinical behavior. Despite advances in genetic analysis, understanding the role of epigenetic modifications, particularly DNA methylation, remains an area under investigation. This scoping review aimed to update and synthesize the current body of literature on DNA methylation in pituitary adenomas, focusing on methodological advancements and clinical correlations. A systematic search conducted across multiple databases, including Embase, Scopus, MEDLINE, and CENTRAL, identified 107 eligible studies. Early methods, such as methylation-restricted digestion and methylation-specific PCR (MSP), have evolved into more comprehensive approaches, such as chip-based DNA methylation analysis. Key findings suggest that genes like POMC, SOCS-1, and RASSF1A show a significant association between methylation and clinical behavior. However, methylation patterns alone are insufficient to fully explain tumorigenesis. Emerging data suggest that DNA methylation might serve as a prognostic marker for invasive growth and recurrence, but further longitudinal studies are needed. This review highlights the need for future research to explore the methylome more thoroughly and to better define the clinical impact of epigenetic modifications in pituitary adenomas.

Genetic analysis of the cooperative tumorigenic effects of targeted deletions of tumor suppressors Rb1, Trp53, Men1, and Pten in neuroendocrine tumors in mice

Genetic alterations of tumor suppressor genes (TSGs) are frequently observed to have cumulative or cooperative tumorigenic effects. We examined whether the TSGs Rb1, Trp53, Pten and Men1 have cooperative effects in suppressing neuroendocrine tumors (NETs) in mice. We generated pairwise homozygous deletions of these four genes in insulin II gene expressing cells using the Cre-LoxP system. By monitoring growth and examining the histopathology of the pituitary (Pit) and pancreas (Pan) in these mice, we demonstrated that pRB had the strongest cooperative function with PTEN in suppressing PitNETs and had strong cooperative function with Menin and TRP53, respectively, in suppressing PitNETs and PanNETs. TRP53 had weak cooperative function with PTEN in suppressing pituitary lesions. We also found that deletion of Pten singly led to prolactinomas in female mice, and deletion of Rb1 alone led to islet hyperplasia in pancreas. Collectively, our data indicated that pRB and PTEN pathways play significant roles in suppressing PitNETs, while the Menin-mediated pathway plays a significant role in suppressing PanNETs. Understanding the molecular mechanisms of these genes and pathways on NETs will help us understand the molecular mechanisms of neuroendocrine tumorigenesis and develop effective preclinical murine models for NET therapeutics to improve clinical outcomes in humans.

Epigenetic dysregulation in pituitary tumors

Pituitary tumors are common intracranial neoplasms associated with significant morbidity due to hormonal dysregulation and neurologic symptoms. Somatic mutations are uncommon in sporadic pituitary adenomas, and only few monogenic conditions are associated with pituitary tumors. However, increasing evidence suggests that aberrant epigenetic modifications are found in pituitary tumors. In this review, we describe these mechanisms, including DNA methylation, histone modification and microRNA expression, and the evidence supporting their dysregulation in pituitary tumors, as well as their regulation of pro-tumorigenic genes. In addition, we provide an overview of findings from preclinical studies investigating the use of histone deacetylase inhibitors to treat pituitary adenomas and the need for further studies involving epigenetic drugs and functional characterization of epigenetic dysregulation.

Molecular Network Basis of Invasive Pituitary Adenoma: A Review

Cases with pituitary adenoma comprise 10-25% of intracranial neoplasm, being the third most common intracranial tumor, most of the adenomas are considered to be benign. About 35% of pituitary adenomas are invasive. This review summarized the known molecular basis of the invasiveness of pituitary adenomas. The study pointed out that hypoxia-inducible factor-1α, pituitary tumor transforming gene, vascular endothelial growth factor, fibroblast growth factor-2, and matrix metalloproteinases (MMPs, mainly MMP-2, and MMP-9) are core molecules responsible for the invasiveness of pituitary adenomas. The reason is that these molecules have the ability to directly or indirectly induce cell proliferation, epithelial-to-mesenchymal transition, angiogenesis, degradation, and remodeling of extracellular matrix. HIF-1α induced by hypoxia or apoplexy inside the adenoma might be the initiating factor of invasive transformation, followed with angiogenesis for overexpressed VEGF, EMT for overexpressed PTTG, degradation of ECM for overexpressed MMPs, creating a suitable microenvironment within the tumor. Together, they form a complex interactive network. More investigations are required to further elucidate the mechanisms underlying the invasiveness of pituitary adenomas.

Molecular genetic advances in pituitary tumor development

Pituitary adenomas are a heterogeneous group of tumors that may occur as part of a complex syndrome or as an isolated endocrinopathy and both forms can be familial or non-familial. Studies of syndromic and non-syndromic pituitary adenomas have yielded important insights about the molecular mechanisms underlying tumorigenesis. Thus, syndromic forms, including multiple endocrine neoplasia type 1 (MEN1), MEN4, Carney Complex and McCune Albright syndrome, have been shown to be due to mutations of the tumor-suppressor protein menin, a cyclin-dependent kinase inhibitor (p27Kip1), the protein kinase A regulatory subunit 1-α, and the G-protein α-stimulatory subunit (Gsα), respectively. Non-syndromic forms, which include familial isolated pituitary adenoma (FIPA) and sporadic tumors, have been shown to be due to abnormalities of: the aryl hydrocarbon receptor-interacting protein; Gsα; signal transducers; cell cycle regulators; transcriptional modulators and miRNAs. The roles of these molecular abnormalities and epigenetic mechanisms in pituitary tumorigenesis, and their therapeutic implications are reviewed.

Genetically engineered mouse models of pituitary tumors

Animal models constitute valuable tools for investigating the pathogenesis of cancer as well as for preclinical testing of novel therapeutics approaches. However, the pathogenic mechanisms of pituitary-tumor formation remain poorly understood, particularly in sporadic adenomas, thus, making it a challenge to model pituitary tumors in mice. Nevertheless, genetically engineered mouse models (GEMMs) of pituitary tumors have provided important insight into pituitary tumor biology. In this paper, we review various GEMMs of pituitary tumors, highlighting their contributions and limitations, and discuss opportunities for research in the field.

Association between p16(CDKN2A) C540G polymorphism and tumor behavior in prolactinoma: A single-center study

Pituitary tumors usually originate as benign sporadic adenomas and develop into invasive and aggressive tumors such as prolactinomas, which are common functioning pituitary adenomas. The aim of the present study was to examine the association between the tumor behavior in prolactinomas and the p16(CDKN2A) gene polymorphism occurring at the 3'-untranslated region of exon 3 (C540G). A total of 104 patients with prolactinoma were included and assigned to two groups based on invasive vs. non-invasive tumor behavior. Ki67 indices were recorded according to histopathology results. Genotypic analysis of the p16(CDKN2A) C540G polymorphism was carried out using a modified polymerase chain reaction-restriction fragment length polymorphism assay. The corresponding frequencies for CC, CG and GG genotypes in non-invasive vs. invasive tumors were 61.5, 30.8, 7.7 and 64.1, 28.2, 7.7%, respectively (not significant). The observed CG genotype frequency was higher compared with previous studies. In addition, the patients with giant adenomas or a high Ki67 index had a higher frequency of the CG genotype as compared with the other subgroups, although the differences were not significant (46.2 and 42.9%, respectively). In conclusion, a higher frequency of the C540G CG genotype of the CDKN2A gene was found among patients with prolactinoma in comparison with previous studies. These frequencies were also higher in the subgroups with elevated Ki67 or giant adenomas. Further studies are required to improve the definition of the role of the CG genotype in the development and progression of tumors in prolactinomas.

A pilot genome-scale profiling of DNA methylation in sporadic pituitary macroadenomas: association with tumor invasion and histopathological subtype

Pituitary adenomas (PAs) are neoplasms that may cause a variety of neurological and endocrine effects. Although known causal contributors include heredity, hormonal influence and somatic mutations, the pathophysiologic mechanisms driving tumorigenesis and invasion of sporadic PAs remain unknown. We hypothesized that alterations in DNA methylation are associated with PA invasion and histopathology subtype, and that genome-scale methylation analysis may complement current classification methods for sporadic PAs. Twenty-four surgically-resected sporadic PAs with varying histopathological subtypes were assigned dichotomized Knosp invasion scores and examined using genome-wide DNA methylation profiling and RNA sequencing. PA samples clustered into subgroups according to functional status. Compared with hormonally-active PAs, nonfunctional PAs exhibited global DNA hypermethylation (mean beta-value 0.47 versus 0.42, P = 0.005); the most significant site of differential DNA methylation was within the promoter region of the potassium voltage-gated channel KCNAB2 (FDR = 5.11×10⁻¹⁰). Pathway analysis of promoter-associated CpGs showed that nonfunctional PAs are potentially associated with the ion-channel activity signal pathway. DNA hypermethylation tended to be negatively correlated with gene expression. DNA methylation analysis may be used to identify candidate genes involved in PA function and may potentially complement current standard immunostaining classification in sporadic PAs. DNA hypermethylation of KCNAB2 and downstream ion-channel activity signal pathways may contribute to the endocrine-inactive status of nonfunctional PAs.

Frequency of genetic and epigenetic alterations of p14ARF and p16INK4A in head and neck cancer in a Hungarian population

Occurrence of genetic and epigenetic alterations affecting p14ARF and p16INK4A were investigated in tumour samples of 37 oral (OSCC) and 28 laryngeal squamous cell cancer (LSCC) patients, and compared to exfoliated buccal epithelial cells of 68 healthy controls. Presence of deletions and mutations/polymorphisms affecting exons were examined using sequencing. Methylation status of promoters was assessed by methylation-specific PCR. Chi-square and Fisher's exact tests were used to compare frequency of events. Exon deletions were found in four controls, one OSCC and 22 LSCC patients; the latter significantly differed from controls (p < 0.001). Only two mutations (T24610A and C24702A) were in p16 exon 1 of two OSCC patients. Polymorphisms G28575A (Ala140Thr), G31292C (C540G) and G28608A were found in both patient groups. The p14 promoter was unmethylated in 86.7 % of OSCC and in 85.7 % of LSCC patients; for the p16 promoter these rates were 69.0 % and 76.2 % for OSCC and LSCC patients, respectively. Combining the two patient groups, unmethylated promoter was significantly less frequent in case of both p14 and p16 (p = 0.043 and p = 0.001, respectively) compared to the control group. In summary, exon deletion may be important in LSCC, while promoter methylation was relatively frequent in both patient groups.

Genetic and epigenetic mutations of tumor suppressive genes in sporadic pituitary adenoma

Human pituitary adenomas are the most common intracranial neoplasms. Approximately 5% of them are familial adenomas. Patients with familial tumors carry germline mutations in predisposition genes, including AIP, MEN1 and PRKAR1A. These mutations are extremely rare in sporadic pituitary adenomas, which therefore are caused by different mechanisms. Multiple tumor suppressive genes linked to sporadic tumors have been identified. Their inactivation is caused by epigenetic mechanisms, mainly promoter hypermethylation, and can be placed into two groups based on their functional interaction with tumor suppressors RB or p53. The RB group includes CDKN2A, CDKN2B, CDKN2C, RB1, BMP4, CDH1, CDH13, GADD45B and GADD45G; AIP and MEN1 genes also belong to this group. The p53 group includes MEG3, MGMT, PLAGL1, RASSF1, RASSF3 and SOCS1. We propose that the tumor suppression function of these genes is mainly mediated by the RB and p53 pathways. We also discuss possible tumor suppression mechanisms for individual genes.

The role of epigenetic modification in tumorigenesis and progression of pituitary adenomas: a systematic review of the literature

Background

Pituitary adenomas (PAs) are commonly occurring neoplasms with diverse endocrine and neurological effects. Although somatic gene mutations are uncommon in sporadic PAs, recent studies lend support to epigenetic modification as a potential cause of tumorigenesis and tumor progression.

Methods

A systematic literature review of the PubMed and Google Scholar databases was conducted to identify abstracts (n=1,082) pertaining to key targets and mechanisms implicated in epigenetic dysregulation of PAs published between 1993-2013. Data regarding histopathological subtype, target genes, mode of epigenetic modification, and clinical correlation were recorded and analyzed.

Results

Of the 47 that studies met inclusion criteria and focused on epigenomic assessment of PAs, only 2 were genome-scale analyses. Current evidence supports epigenetic alteration in at least 24 PA genes, which were categorized into four groups based on function and epigenetic alteration: 1) Sixteen tumor suppressor genes silenced via DNA methylation; 2) Two oncogenes overexpressed via histone acetylation and hypomethylation; 3) Three imprinted genes with selective allelic silencing; and 4) One epigenome writer inducing abnormal genome-scale activity and 5) Two transcription regulators indirectly modifying the genome. Of these, 5 genes (CDKN2A, GADD45y, FGFR2, caspase-8, and PTAG) showed particular susceptibility to epigenetic modification, with abnormal DNA methylation in >50% of PA samples. Several genes displayed correlations between epigenetic modification and clinically relevant parameters, including invasiveness (CDKN2A; DAPK; Rb1), sex (MAGE-A3), tumor size (GNAS1), and histopathological subtype (CDKN2A; MEG3; p27; RASSF1A; Rb1).

Conclusions

Epigenetic modification of selected PA genes may play a key role in tumorigenesis and progression, which may translate into important diagnostic and therapeutic applications.

The genetics of pituitary adenomas

Pituitary adenomas are one of the most frequent intracranial tumors with a prevalence of clinically-apparent tumors close to 1:1000 of the general population. They are clinically significant because of hormone overproduction and/or tumor mass effects in addition to the need for neurosurgery, medical therapies and radiotherapy. The majority of pituitary adenomas have a sporadic origin with recognized genetic mutations seldom being found; somatotropinomas are an exception, presenting frequent somatic GNAS mutations. In this and other phenotypes, tumorigenesis could possibly be explained by altered function of genes implicated in cell cycle regulation, growth factors or their receptors, cell-signaling pathways, specific hormonal factors or other molecules with still unclear mechanisms of action. Genetic changes, such as allelic loss or gene amplification, and epigenetic changes, usually by promoter methylation, have been implicated in abnormal gene expression, but alternative mechanisms may be present. Familial cases of pituitary adenomas represent 5% of all pituitary tumors. MEN1 mutations cause multiple endocrine neoplasia type 1 (MEN1), while the Carney complex (CNC) is characterized by mutations in the protein kinase A regulatory subunit-1alpha (PRKAR1A) gene or changes in a locus at 2p16. Recently, a MEN1-like condition, MEN4, was found to be related to mutations in the CDKN1B gene. The clinical entity of familial isolated pituitary adenomas (FIPA) is characterized by genetic defects in the aryl hydrocarbon receptor interacting protein (AIP) gene in about 15% of all kindreds and 50% of homogenous somatotropinoma families. Identification of familial cases of pituitary adenomas is important as these tumors may be more aggressive than their sporadic counterparts.

[Molecular aspects of pituitary tumorigenesis]

Pituitary tumors, almost invariably adenomas, are of frequent occurrence, accounting for 10% to 15% of all the intracranial neoplasm. They are classified as microadenomas (< 10 mm) or macroadenomas (> 10 mm) and as secreting or clinically non-secreting (or not functioning) adenomas. These tumors are autonomously capable to release pituitary hormones such as the growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The occurrence of metastases, characterizing a pituitary carcinoma, is exceedingly rare. However tumors with aggressive behavior, leading to local invasion, are relatively common. Although the pathogenesis of pituitary tumors is fully characterized, many molecular mechanisms of pituitary tumorigenesis had already been revealed. This review intends to describe advances in the understanding of the involved advances that have been made in the last decade concerning pituitary tumors progression, including the participation of oncogenes, tumor suppressor genes and growth factors.

Frequent loss of the CDKN2C (p18INK4c) gene product in pituitary adenomas

Genomic alterations of cyclin-dependent kinase inhibitors have been demonstrated in a variety of tumor types including brain tumors. Among them, the cyclin-dependent kinase inhibitor 2A (CDKN2A or p16(INK4a)) gene has been shown to be frequently deleted or inactivated in astrocytic tumors. The CDKN2C (p18(INK4c)) gene is functionally related to CDKN2A. Moreover, mice with targeted disruption of CDKN2C alone or combined CDKN2C and cyclin-dependent kinase inhibitor 1B (CDKN1B or p27(Kip1)), or CDKN2C and TP53 gene disruption develop pituitary adenomas (PA) at high frequencies. The purpose of our study was to investigate genetic alterations of the CDKN2C gene by analysis of loss of heterozygosity (LOH), screening for mutations, analysis of promoter methylation, and protein expression in 38 PAs. In addition, genomic alterations and protein expression of the cell cycle genes CDKN2A and its alternatively spliced form, p14(ARF), as well as the retinoblastoma RB1 gene were investigated. LOH at the CDKN2C gene locus was detected in 25% of pituitary adenomas, whereas the RB1 and CDKN2A loci were altered in only 10%. No mutations were detected within the coding regions of the CDKN2C gene. However, 39.5% of adenomas displayed CDKN2C promoter methylation. The absence of CDKN2C protein was correlated with LOH of the CDKN2C locus on chromosome 1 and with methylation of the CDKN2C promoter. This is the first report to describe that the tumor suppressor gene CDKN2C is frequently targeted by genomic alterations in pituitary adenoma. The most common genetic alteration was promoter methylation suggesting that inactivation of CDKN2C by this mechanism may play an important role in pituitary adenoma development. Additional Supporting Information may be found in the online version of this article.

Acromegaly: re-thinking the cancer risk

Acromegaly is characterized by sustained elevation of circulating growth hormone (GH) and insulin-like growth factor I (IGF-I), and is clearly associated with increased morbidity and overall mortality mainly due to cardiovascular, metabolic, and respiratory diseases. Although cancer-related mortality varies widely amongst retroperspective studies, it appears to be consistently elevated mainly in patients with uncontrolled disease. We review individual tumor types including neoplasms of the colon, breast, prostate, and thyroid where in vitro, animal studies, and studies in non-acromegalic cancer patients have established a role for the GH/IGF-I axis in tumor progression and possibly initiation. We highlight deficiencies in data in acromegalic patients where the evidence is less convincing. Instead, we explore the hypothesis that acromegaly, independent of hormone secretion, is a disease that heralds genetic and/or epigenetic alterations predisposing to cancer risk elsewhere.

Expression of p16(INK4A) gene in human pituitary tumours

Pituitary adenomas comprise 10-15% of primary intracranial tumours but the mechanisms leading to tumour development are yet to be clearly established. The retinoblastoma pathway, which regulates the progression through the cell cycle, is often deregulated in different types of tumours. We studied the cyclin-dependent kinase inhibitor p16(INK4A) gene expression at mRNA level in human pituitary adenomas. Forty-six tumour specimens of different subtypes, 21 clinically non-functioning, 12 growth hormone-secreting, 6 prolactin-secreting, 6 adrenocorticotropin-secreting, and 1 thyrotropin-secreting tumours were studied. All clinically non-functioning and most of the hormone-secreting tumours were macroadenomas (38/46). The RT-PCR assay and electrophoresis of the PCR-products showed that p16(INK4A) mRNA was undetectable in: 62% of non-functioning, 8% of growth hormone-secreting, 17% of prolactin-secreting and 17% of adrenocorticotropin-secreting adenomas. Forty percent of all macroadenomas and 25% of microadenomas had negative p16(INK4A) mRNA, the latter results suggest that the absence of p16(INK4A) product might be an early event in tumours with no expression of this suppressor gene. Within the non-functioning adenomas 63% were "null cell" and 37% were positive for some hormone, both subgroups showed similar percentage of cases with absence of p16(INK4A) mRNA. Our results show that clinically non-functioning macroadenomas have impaired p16(INK4A) expression in a clearly higher proportion than any other pituitary tumour subtype investigated. Other regulatory pathways may be implicated in the development of tumours with positive p16(INK4A) expression.

Oncogenesis and mutagenesis of pituitary tumors

Although pituitary tumors may be present in up to 10% of the population, the pathophysiology of these lesions is not well characterized. Pituitary tumors are composed of monoclonal cell populations with disrupted control of replication pathways. The oncogenes and tumor suppressor genes that are common in other malignancies (i.e. jun, fos, myc, and p53) are rarely involved in the development of these tumors. However, oncogenes, such as gsp, can be present in up to 40% of hormonally active adenomas. The process of pituitary oncogenesis further appears to involve oncogenes such as cyclin E, cyclin D1, and the pituitary tumor transforming gene (PTTG). Finally, the cAMP signaling cascade plays a significant role in generation of both benign and malignant pituitary tumors. In this review, the biology of pituitary adenomas is explored with a special emphasis on potential targets for the development of targeted therapeutics.

Advances in the treatment of prolactinomas

Prolactinomas account for approximately 40% of all pituitary adenomas and are an important cause of hypogonadism and infertility. The ultimate goal of therapy for prolactinomas is restoration or achievement of eugonadism through the normalization of hyperprolactinemia and control of tumor mass. Medical therapy with dopamine agonists is highly effective in the majority of cases and represents the mainstay of therapy. Recent data indicating successful withdrawal of these agents in a subset of patients challenge the previously held concept that medical therapy is a lifelong requirement. Complicated situations, such as those encountered in resistance to dopamine agonists, pregnancy, and giant or malignant prolactinomas, may require multimodal therapy involving surgery, radiotherapy, or both. Progress in elucidating the mechanisms underlying the pathogenesis of prolactinomas may enable future development of novel molecular therapies for treatment-resistant cases. This review provides a critical analysis of the efficacy and safety of the various modes of therapy available for the treatment of patients with prolactinomas with an emphasis on challenging situations, a discussion of the data regarding withdrawal of medical therapy, and a foreshadowing of novel approaches to therapy that may become available in the future.

Tumor suppressor loss in pituitary tumors

The current model of human neoplasia invokes a number of potential genomic alterations that impact cellular phenotype and proliferative rates. In the majority of human tumor models, the transformation from normal cells to neoplastic lesion is a multistep process. This review offers a specific overview of the involvement of tumor suppressor genes (TSGs) in the pathogenesis of human pituitary adenomas. TSG genetic lesions, such as BRCA1 in breast cancer and p53 in Li-Fraumeni Syndrome, have been identified in both sporadic and heritable human endocrine tumors. Familial neoplastic syndromes like multiple endocrine neoplasia type 1 (MEN1) that include pituitary tumor formation as part of a broad clinical spectrum of disease represent a unique opportunity to investigate the general mechanisms of tumorigenesis, and well as genes responsible for sporadic endocrine tumors. Similarly, homologous recombination knockout mice with selectively ablated candidate TSGs have also shed light on the molecular mechanisms of pituitary cell proliferation and tumor suppression. However, despite insights into pituitary tumorigenesis generated by heritable neoplasia syndromes and mouse knockout of critical TSGs that display a pituitary tumor phenotype, the molecular pathogenesis of human pituitary adenomas remains largely an enigma. Thus, the role of TSGs, if any, in sporadic pituitary adenoma formation has yet to be determined, despite our greater understanding of the molecular mechanisms underlying pituitary cell function and phenotype.

The p15(INK4b)/p16(INK4a)/RB1 pathway is frequently deregulated in human pituitary adenomas

Pituitary adenomas are common benign intracranial neoplasms. However, their tumorigenesis is not yet clearly defined. Inactivation of genes involved in the negative cell-cycle regulatory p15(INK4b) - p16(INK4a) -cyclin D/CDK4-RB1-mediated pathway (RB1 pathway) is one of the most common and important mechanisms in the growth advantage of tumor cells. Recently, much attention has been focused on the importance of alternative mechanisms of gene inactivation, particularly promoter hypermethylation in the transcriptional silencing of such tumor-suppressor genes. Based on the rare occurrence of inactivation by gene mutations and deletions of the RB1 pathway in pituitary adenomas, we investigated the deregulation of the RB1 pathway in 42 sporadic human pituitary adenomas, especially focusing on the methylation status of this pathway as determined by a methylation-specific polymerase chain reaction assay. Homozygous deletion of the p15(INK4b) or p16(INK4a) gene was detected in one adenoma each. Amplification of the CDK4 gene was not apparent in any of the pituitary adenomas presently examined. Promoter hypermethylation of the p15(INK4b), p16(INK4a), and RB1 genes was detected in 15 (35.7%), 30 (71.4%), and 12 (28.6%) of the adenomas, respectively. Promoter hypermethylation of the p15(INK4b) gene coincided with p16(INK4a) alteration and/or RB1 methylation, whereas p16(INK4a) and RB1 methylations tended to be mutually exclusive (p = 0.019). Thus, the vast majority of the adenomas (38 of 42, 90.5%) displayed alterations of the RB1 pathway. None of the clinicopathologic features, including the proliferation cell index, was significantly correlated with any particular methylation status. Our results suggest that inactivation of the RB1 pathway may play a causal role in pituitary tumorigenesis, with hypermethylation of the p16(INK4a) gene being the most common deregulation, and further provide evidence that RB1 and p16(INK4a) methylations tend to be mutually exclusive but occasionally coincide with p15(INK4b) methylation.

Pathogenesis of prolactinomas

In recent years the demonstration that human pituitary adenomas are monoclonal in origin provides further evidence that pituitary neoplasia arise from the replication of a single mutated cell in which growth advantage results from either activation of proto-oncogenes or inactivation of tumor suppressor genes. However, with the exception of one RAS mutation identified in a single unusually aggressive prolactinoma resistant to dopaminergic inhibition that resulted to be lethal, no mutational changes have been so far detected in prolactinomas. In the absence of genetic changes, modifications in the level of expression of oncogenes or tumor suppressor genes have been detected in these tumors, although it is unknown whether these changes have a causative role or are a secondary event. Indeed, our knowledge on the molecular events involved in lactotroph proliferation is even more limited in comparison to the other tumor types, since these tumors are very infrequently surgically removed and therefore available for molecular biology studies. In this respect, it is worth noting that the molecular and biological abnormalities so far described in prolactinomas mainly concern aggressive and atypical tumors and likely do not apply to the typical prolactinomas, that are characterized by good response to medical treatment and a very low growth rate.

Gonadotroph tumor associated with multiple endocrine neoplasia type 1

Although anterior pituitary tumors constitute a main clinical feature of multiple endocrine neoplasia type 1 (MEN1), and most types of pituitary tumors have been associated with MEN1, gonadotroph tumors have not previously been recognized clinically as part of this syndrome. We report here a woman who presented with ovarian hyperstimulation due to a gonadotroph tumor that was confirmed biochemically and immunohistochemically. She then developed hyperparathyroidism, and she was found to have three hypercellular parathyroid glands. Subsequently, she developed a temporal lobe metastasis of the gonadotroph tumor, demonstrating that it was a gonadotroph carcinoma. The diagnosis of MEN1 was confirmed by finding a deletion mutation (c.307delC) on the second exon of the MEN1 gene that predicts truncation of the resulting menin protein 15 codons downstream from the deletion (p.Leu103fsX15). This case illustrates that gonadotroph tumors, like other pituitary tumors, can be part of MEN1. The clinical implications of this case are that the clinical and biochemical features of gonadotroph tumors should be considered when evaluating patients for MEN1, and MEN1 should be considered in patients who have gonadotroph tumors.

Isolation and characterization of a novel pituitary tumor apoptosis gene

To determine mechanisms for pituitary neoplasia we used methylation-sensitive arbitrarily primed-PCR to isolate novel genes that are differentially methylated relative to normal pituitary. We report the isolation of a novel differentially methylated chromosome 22 CpG island-associated gene (C22orf3). Sodium bisulfite sequencing of pooled tumor cohorts, used in the isolation of this gene, showed that only a proportion of the adenomas within the pools were methylated; however, expression analysis by quantitative RT-PCR of individual adenoma irrespective of subtype showed the majority (30 of 38; 79%) failed to express this gene relative to normal pituitary. Sodium bisulfite sequencing of individual adenomas showed that 6 of 30 (20%) that failed to express pituitary tumor apoptosis gene (PTAG) were methylated; however, genetic change as determined by loss of heterozygosity and sequence analysis was not apparent in the remaining tumors that failed to express this gene. In those cases where the CpG island of these genes was methylated it was invariably associated with loss of transcript expression. Enforced expression of C22orf3 in AtT20 cells had no measurable effects on cell proliferation or viability; however, in response to bromocriptine challenge (10-40 microm) cells expressing this gene showed a significantly augmented apoptotic response as determined by both acridine orange staining and TUNEL labeling. The apoptotic response to bromocriptine challenge was inhibited in coincubation experiments with the general caspase inhibitor z-VAD-fmk. In addition, in time course experiments, direct measurement of active caspases by fluorochrome-labeled inhibition of caspases, showed an augmented increase (approximately 2.4 fold) in active caspases in response to bromocriptine challenge in cells expressing C22orf3 relative to those harboring an empty vector control. The pituitary tumor derivation and its role in apoptosis of this gene led us to assign the acronym PTAG to this gene and its protein product. The ability of cells, showing reduced expression of PTAG, to evade or show a blunted apoptotic response may underlie oncogenic transformation in both the pituitary and other tumor types.

Lack of correlation for sodium iodide symporter mRNA and protein expression and analysis of sodium iodide symporter promoter methylation in benign cold thyroid nodules

Cold thyroid nodules (CTNs) are characterized by a reduced iodide uptake in comparison to normal thyroid tissue. The sodium iodide symporter (NIS) is the first step in thyroid hormone synthesis and mediates the active iodide transport in the thyroid cells suggesting that decreased iodide uptake could be a result of changes in NIS expression or molecular defects in the NIS gene. In contrast to previous studies, an intraindividual comparison of NIS mRNA expression in CTNs and their corresponding surrounding tissue was performed using direct detection of NIS mRNA. A significant reduction in NIS mRNA expression was detected in 86% of the 14 investigated CTNs. We hypothesized that human sodium iodide symporter (hNIS) transcriptional failure could be caused by primary molecular NIS gene defects and/or methylation of DNA in the NIS promoter. However, no mutation in the NIS cDNA nor in the NIS promoter region upstream up to-443 bp from the ATG start codon was detected. Therefore, primary molecular NIS gene defects were excluded. However, in 50% of CTNs with reduced NIS mRNA expression, the promoter region was hypermethylated. NIS mRNA expression in these hypermethylated CTNs only reached a maximum of 30% of the corresponding surrounding tissue. Hence, methylation of CpG islands in the NIS promotor could be a regulatory mechanism of NIS transcription in CTNs. Immunoblot revealed absent hNIS protein expression in the total cell membrane fraction in 45% of investigated nodules. In the majority of the remaining CTNs NIS protein expression was decreased in the nodule tissue compared to the corresponding surrounding tissue. For investigating protein expression immunhistochemistry has two advantages. First, the whole nodule area can be investigated, and second, NIS expression can be detected in areas where an immunoblot of a cell membrane fraction is negative. Interestingly, immunhistochemistry revealed higher NIS expression in 50% of CTNs compared to their corresponding surrounding tissues and NIS staining was predominantly intracellular. These data demonstrate that NIS protein expression does not reflect NIS mRNA expression. Therefore, factors that affect targeting of NIS to the plasma membrane are likely to be affected.

Loss of expression of the growth inhibitory gene GADD45gamma, in human pituitary adenomas, is associated with CpG island methylation

Inappropriate expression of cell-cycle regulatory genes and/or their protein products are a frequent finding in pituitary tumours; however, genetic changes associated with or responsible for their dysregulation are in general uncommon. In a search for novel genes, and employing cDNA-representational difference analysis, the gene encoding GADD45gamma was recently isolated and identified as being under-represented in pituitary adenomas. GADD45gamma is a member of a family of genes that are induced by DNA damage and function in the negative regulation of cell growth. In this study, we further confirm this initial report that the majority of pituitary adenomas (22 of 33; 67%) do not express GADD45gamma as determined by RT-PCR analysis. Loss of expression was not associated with either loss of heterozygosity or mutations within the coding region of this gene. In marked contrast, epigenetic change, namely methylation of the GADD45gamma genes CpG island, was a frequent finding (19 of 33 adenoma; 58%) and was significantly associated with tumours in which GADD45gamma transcript was not expressed (18 of 22; 82%; P=0.002). In common with the primary tumours, methylation-associated gene silencing of the GADD45gamma gene was also found in the pituitary tumour cell line AtT20. The treatment of AtT20 cells with the demethylating agent, 5-Aza-2'-deoxycytidine, induced the re-expression of this gene. These findings show that silencing of the GADD45gamma gene in pituitary tumours is primarily associated with methylation of the genes CpG island. Methylation has functional importance since reversal of this epigenetic change in a pituitary-derived cell line is associated with re-expression. Silencing of GADD45gamma, a negative regulator of cell growth, is most likely responsible for conferring a selective growth advantage during tumour evolution and outgrowth.

Genetics of pituitary tumors: Focus on G-protein mutations

In recent years the demonstration that human pituitary adenomas are monoclonal in origin has provided further evidence that pituitary neoplasia arise from the replication of a single mutated cell in which growth advantage results from either activation of proto-oncogenes or inactivation of tumor suppressor genes. While common oncogenes, such as Ras, are only exceptionally involved, the only mutations identified in a significant proportion of pituitary tumors, and particular in GH-secreting adenomas, occur in the Gsalpha gene (GNAS1) and cause constitutive activation of the cAMP pathway (gsp oncogene). Moreover, pituitary tumors overexpress hypothalamic releasing hormones, growth factors, and their receptors as well as cyclins involved in cell cycle progression. As far as the role of tumor suppressor genes in pituitary tumorigenesis is concerned, reduced expression of these genes seems to frequently occur in pituitary tumors as a consequence of abnormal methylation processes. Although the only mutational change so far identified in pituitary tumors is the gsp oncogene, this oncogene is not associated with a clear phenotype in patients bearing positive tumors. Mechanisms able to counteract the cAMP pathway, such as high sensitivity to somatostatin, and induction of genes with opposite actions, such as phosphodiesterases, CREB end ICER, or instability of mutant Gsalpha, have been proposed to account for the lack of genotype/phenotype relationships.

Epigenetic and genetic analysis of p16 in dermal fibroblasts from type 1 diabetic patients with nephropathy

BACKGROUND

Several studies have shown that cultured skin fibroblasts from patients with diabetic nephropathy (DN) exhibit a hyperplastic growth phenotype. Increased DNA synthesis in cells from patients with DN may ultimately involve alterations in cell cycle regulatory proteins. p16 protein is a member of INK4 family of cyclin-dependent kinase inhibitors, which plays an important role in cell cycle regulation. In this study, we examined the correlation between p16 protein expression in cultured dermal fibroblasts from type 1 diabetic patients and the presence of DN.

METHOD

Western blot analysis was performed to compare p16 protein expression in skin fibroblasts from patients with DN as compared to control subjects, diabetic patients without DN, and nondiabetic patients with nephropathy. Transcriptional regulation of the p16 gene was assessed using competitive reverse transcription-polymerase chain reaction (RT-PCR). Methylation status of the promoter region of p16 was examined using methylation-specific PCR, and we used single-stranded conformational polymorphism (SSCP)-PCR to assess p16 single-nucleotide polymorphism.

RESULTS

Cells from diabetic patients with DN had nondetectable to significantly lower protein expression of p16. Similarly, mRNA expression of p16 was significantly lower in diabetic patients with DN. No hypermethylation of p16 gene was detected, and no abnormal migrating bands were noticed on SSCP-PCR analysis in cells from patients with DN.

CONCLUSION

Our data indicate that cells from patients with DN exhibit significantly lower protein and mRNA expression of p16. This study could have not only important implications for the understanding of the pathogenesis of DN, but also the absence of p16 may ultimately serve as an early marker for DN.

p16(INK4a) and p15(INK4b) gene methylations in plasma cells from monoclonal gammopathy of undetermined significance

p15(INK4b) and p16(INK4a) proteins are cell cycle regulators involved in the inhibition of G1 phase progression. High frequency of methylation of both genes has been reported in multiple myeloma (MM), but it remains to be determined how and when these alterations contribute to tumorigenesis. Monoclonal gammopathy of undetermined significance (MGUS) represents an early disease stage in a fraction of MMs. Plasma cells from 33 patients with MGUS and 33 patients with MM were isolated and analyzed for p15(INK4b) and p16(INK4a) methylation by methylation-specific polymerase chain reaction. Selective methylation was found in 19% for p16(INK4a), 36% for p15(INK4b), and 6.5% for both genes in MGUS, and frequencies were similar in MM suggesting that methylation of these genes is an early event, not associated with transition from MGUS to MM. p15(INK4b) and p16(INK4a) gene methylation might contribute to immortalization of plasma cells rather than malignant transformation in the natural history of MM.

CDKN2A/p16 inactivation is related to pituitary adenoma type and size

p16 (CDKN2A, MTS1, INK4A) status at genomic and protein levels was analysed and correlated with clinico-pathological features in 72 pituitary adenomas. Methylation of CpG islands of promoter/exon 1 sequences was found in most gonadotroph, lactotroph, plurihormonal, and null cell adenomas (36 of 44, 82%), but it was rare in somatotroph (1 of 13 cases, 8%) and corticotroph adenomas (1 of 15 cases, 7%). Homozygous CDKN2A deletion was restricted to rare somatotroph (15%) and corticotroph adenomas (13%). Immunohistochemical p16 protein expression was observed in the normal adenohypophysis, whereas it was absent in 60 of 72 (83%) tumours and reduced in another ten (14%) tumours. Staining for p16 was only seen in 5 of 15 (33%) corticotroph, 3 of 13 (23%) somatotroph, 3 of 5 (60%) plurihormonal, and 1 of 19 (5%) null cell adenomas. p16 immunonegativity without CDKN2A methylation or deletion occurred in 22 tumours, including most somatotroph and corticotroph adenomas (15 of 28, 54%). Both CDKN2A alterations and p16 negativity were related to larger tumour size. Patients with p16-negative tumours were older than patients with p16-positive tumours. These data suggest that p16 down-regulation is common in all adenoma types. The mechanisms of p16 down-regulation probably involve CDKN2A methylation in most types, but remain to be determined in somatotroph and corticotroph adenomas. These findings also suggest that p16 down-regulation is usually not an initial event, but is acquired during adenoma progression.

p16(MTS-1/CDKN2/INK4a) in cancer progression

Since its discovery as an inhibitor of cyclin-dependent kinases 4 and 6, the tumor suppressor p16 has continued to gain widespread importance in cancer. The high frequency of deletions of p16 in tumor cell lines first suggested an important role for p16 in carcinogenesis. This initial genetic evidence was subsequently strengthened by numerous studies documenting p16 inactivation in kindreds with familial melanoma. Moreover, a high frequency of p16 gene alterations was found in primary tumors, while recent studies have identified p16 promoter methylation as a major mechanism of tumor-suppressor-gene silencing. Additional insight into p16's role in cancer has come from the genetic analysis of precancerous lesions and various tissue culture models. It is now believed that loss of p16 is an early and often critical event in tumor progression. Consequently, p16 is a major tumor-suppressor gene whose frequent loss occurs early in many human cancers.

Genetic heterogeneity and alterations in chromosome 9 loci in a localized region of a functional pituitary adenoma

The molecular alterations reported in pituitary adenomas include mutations at the G(s)alpha in somatotrophinomas, and hypermethylation of the p16 tumor suppressor gene. There are, however, no reports of genomic instability or intratumor genetic heterogeneity in pituitary adenomas. We have studied the microsatellite loci on the short arm of chromosome 9 (9p) and the DNA fingerprinting pattern, of adjacent compartments, about 2 mm across, in a functional chromophobe pituitary adenoma secreting growth hormone and prolactin. The microsatellite loci were studied by PCR amplification using locus specific primers, while the DNA fingerprinting pattern was studied by randomly amplified polymorphic DNA (RAPD) analysis. Normal leukocyte DNA was taken as control. Only one compartment (Ta) showed alterations in several of the microsatellite loci and in the RAPD pattern vis a vis corresponding normal DNA and also the other two compartments of the tumor. This provides evidence for the localized nature of genomic instability in this tumor.

Chromosomal abnormalities in pituitary adenomas

Cytogenetic studies were conducted on 30 pituitary adenomas, using both direct and/or short-term in vitro culture methods. An apparently normal chromosome complement was found in 14 tumors; 5 adenomas were characterized by hyperdiploid or near-triploid modal chromosome numbers. Recurrent numerical deviations were identified in 12 samples, which primarily involved gains of chromosomes 4, 7, 8, 9, 12, and 20 by gains, and losses of chromosomes 10, 14, 19, and 22. Four adenomas were shown to have structural chromosome rearrangements with no apparent recurrent pattern of involvement.