Epigenetics of pituitary tumours: an update

Epigenetic Mechanisms Modulated by Glucocorticoids With a Focus on Cushing Syndrome

In Cushing syndrome (CS), prolonged exposure to high cortisol levels results in a wide range of devastating effects causing multisystem morbidity. Despite the efficacy of treatment leading to disease remission and clinical improvement, hypercortisolism-induced complications may persist. Since glucocorticoids use the epigenetic machinery as a mechanism of action to modulate gene expression, the persistence of some comorbidities may be mediated by hypercortisolism-induced long-lasting epigenetic changes. Additionally, glucocorticoids influence microRNA expression, which is an important epigenetic regulator as it modulates gene expression without changing the DNA sequence. Evidence suggests that chronically elevated glucocorticoid levels may induce aberrant microRNA expression which may impact several cellular processes resulting in cardiometabolic disorders. The present article reviews the evidence on epigenetic changes induced by (long-term) glucocorticoid exposure. Key aspects of some glucocorticoid-target genes and their implications in the context of CS are described. Lastly, the effects of epigenetic drugs influencing glucocorticoid effects are discussed for their ability to be potentially used as adjunctive therapy in CS.

Integrated multi-omics profiling of nonfunctioning pituitary adenomas

PURPOSE

Genetic and epigenetic alterations are involved in pituitary adenoma pathogenesis, however the molecular basis of proliferative nonfunctioning pituitary adenomas (NFPAs) remains unclear. Here, we analyzed integrated multi-omics profiling including copy number variation (CNV), DNA methylation and gene expression of 8 NFPAs.

METHODS

We collected 4 highly proliferative (hpNFPA, Ki-67 ≥ 3) and 4 lowly proliferative (Ki-67 ≤ 1) NFPAs, and comprehensively assessed CNV, DNA methylation, and gene expression by Illumina HumanMethylation450 BeadChip and Affymetrix GeneChip PrimeView Human Gene Expression Array. We performed Ingenuity Pathway Analysis (IPA) for differentially expressed genes to illustrate aberrant pathways and delineated protein-protein networks of selected key genes in dysregulated pathways.

RESULTS

Aberrant arm level CNV, dysregulated DNA methylation, and associated impacts on gene expressions were observed in 2 early occurring hpNFPAs. Chromosomal losses were associated with attenuated expression of DNA methyltransferases, further altering global methylation in these 2 samples. Correlation analysis between DNA methylation and gene expression in 8 NFPAs indicates methylation in promoter and gene body regions are both involved in gene regulation. IPA showed PPARα/RXRα, dopamine receptor signaling, cAMP-mediated signaling, and calcium signaling were all activated, while p38 MAPK and ERK5 signaling were inhibited in hpNFPAs. Moreover, selected key gene networks in hpNFPAs exhibited concurrent methylation status and expression levels of adenylate cyclase genes, G protein subunits, HLA genes, CXCL12, and CCL2.

CONCLUSION

This study presents comprehensive multi-omics views of CNV, DNA methylation, and gene expression in 8 NFPAs. Pathway analysis and network maps of key genes provide clues to elucidate the molecular basis of hpNFPA.

Exploring the Role of Novel Medical Therapies for Aggressive Pituitary Tumors: A Review of the Literature-"Are We There Yet?"

Aggressive pituitary tumors account for up to 10% of pituitary tumors and are characterized by resistance to medical treatment and multiple recurrences despite standard therapies, including surgery, radiotherapy, and chemotherapy. They are associated with increased morbidity and mortality, particularly pituitary carcinomas, which have mortality rates of up to 66% at 1 year after diagnosis. Novel targeted therapies under investigation include mammalian target of rapamycin (mTOR), tyrosine kinase, and vascular endothelial growth factor (VEGF) inhibitors. More recently, immune checkpoint inhibitors have been proposed as a potential treatment option for pituitary tumors. An increased understanding of the molecular pathogenesis of aggressive pituitary tumors is required to identify potential biomarkers and therapeutic targets. This review discusses novel approaches to the management of aggressive pituitary tumors and the role of molecular profiling.

Genetic and Epigenetic Characterization of Growth Hormone-Secreting Pituitary Tumors

Somatic driver mechanisms of pituitary adenoma pathogenesis have remained incompletely characterized; apart from mutations in the stimulatory Gα protein (Gα_s encoded by GNAS) causing activated cAMP synthesis, pathogenic variants are rarely found in growth hormone-secreting pituitary tumors (somatotropinomas). The purpose of the current work was to clarify how genetic and epigenetic alterations contribute to the development of somatotropinomas by conducting an integrated copy number alteration, whole-genome and bisulfite sequencing, and transcriptome analysis of 21 tumors. Somatic mutation burden was low, but somatotropinomas formed two subtypes associated with distinct aneuploidy rates and unique transcription profiles. Tumors with recurrent chromosome aneuploidy (CA) were GNAS mutation negative (Gsp^- ). The chromosome stable (CS) -group contained Gsp⁺ somatotropinomas and two totally aneuploidy-free Gsp^- tumors. Genes related to the mitotic G₁-S-checkpoint transition were differentially expressed in CA- and CS-tumors, indicating difference in mitotic progression. Also, pituitary tumor transforming gene 1 (PTTG1), a regulator of sister chromatid segregation, showed abundant expression in CA-tumors. Moreover, somatotropinomas displayed distinct Gsp genotype-specific methylation profiles and expression quantitative methylation (eQTM) analysis revealed that inhibitory Gα (Gα_i) signaling is activated in Gsp⁺ tumors. These findings suggest that aneuploidy through modulated driver pathways may be a causative mechanism for tumorigenesis in Gsp^- somatotropinomas, whereas Gsp⁺ tumors with constitutively activated cAMP synthesis seem to be characterized by DNA methylation activated Gα_i signaling. IMPLICATIONS: These findings provide valuable new information about subtype-specific pituitary tumorigenesis and may help to elucidate the mechanisms of aneuploidy also in other tumor types.

65 YEARS OF THE DOUBLE HELIX: Classification of endocrine tumors in the age of integrated genomics

The classification of human cancers represents one of the cornerstones of modern pathology. Over the last century, surgical pathologists established the current taxonomy of neoplasia using traditional histopathological parameters, which include tumor architecture, cytological features and cellular proliferation. This morphological classification is efficient and robust with high reproducibility and has served patients and health care providers well. The most recent decade has witnessed an explosion of genome-wide molecular genetic and epigenetic data for most cancers, including tumors of endocrine organs. The availability of this expansive multi-dimensional genomic data, collectively termed the cancer genome, has catalyzed a re-examination of the classification of endocrine tumors. Here, recent cancer genome studies of various endocrine tumors, including those of the thyroid, pituitary and adrenal glands, pancreas, small bowel, lung and skin, are presented with special emphasis on how genomic insights are impacting endocrine tumor classification.

Protein stabilization by RSUME accounts for PTTG pituitary tumor abundance and oncogenicity

Increased levels of the proto-oncogene pituitary tumor-transforming gene 1 (PTTG) have been repeatedly reported in several human solid tumors, especially in endocrine-related tumors such as pituitary adenomas. Securin PTTG has a critical role in pituitary tumorigenesis. However, the cause of upregulation has not been found yet, despite analyses made at the gene, promoter and mRNA level that show that no mutations, epigenetic modifications or other mechanisms that deregulate its expression may explain its overexpression and action as an oncogene. We describe that high PTTG protein levels are induced by the RWD-containing sumoylation enhancer (RWDD3 or RSUME), a protein originally identified in the same pituitary tumor cell line in which PTTG was also cloned. We demonstrate that PTTG and RSUME have a positive expression correlation in human pituitary adenomas. RSUME increases PTTG protein in pituitary tumor cell lines, prolongs the half-life of PTTG protein and regulates the PTTG induction by estradiol. As a consequence, RSUME enhances PTTG transcription factor and securin activities. PTTG hyperactivity on the cell cycle resulted in recurrent and unequal divisions without cytokinesis, and the consequential appearance of aneuploidies and multinucleated cells in the tumor. RSUME knockdown diminishes securin PTTG and reduces its tumorigenic potential in a xenograft mouse model. Taken together, our findings show that PTTG high protein steady state levels account for PTTG tumor abundance and demonstrate a critical role of RSUME in this process in pituitary tumor cells.

Autocrine IL-6 mediates pituitary tumor senescence

Cellular senescence is a stable proliferative arrest state. Pituitary adenomas are frequent and mostly benign, but the mechanism for this remains unknown. IL-6 is involved in pituitary tumor progression and is produced by the tumoral cells. In a cell autonomous fashion, IL-6 participates in oncogene-induced senescence in transduced human melanocytes. Here we prove that autocrine IL-6 participates in pituitary tumor senescence. Endogenous IL-6 inhibition in somatotroph MtT/S shRNA stable clones results in decreased SA-β-gal activity and p16INK4a but increased pRb, proliferation and invasion. Nude mice injected with IL-6 silenced clones develop tumors contrary to MtT/S wild type that do not, demonstrating that clones that escape senescence are capable of becoming tumorigenic. When endogenous IL-6 is silenced, cell cultures derived from positive SA-β-gal human tumor samples decrease the expression of the senescence marker. Our results establish that IL-6 contributes to maintain senescence by its autocrine action, providing a natural model of IL-6 mediated benign adenoma senescence.

Expression and Clinical Significance of miR-26a and Pleomorphic Adenoma Gene 1 (PLAG1) in Invasive Pituitary Adenoma

Background

Although pituitary adenoma is a malignant tumor, it can present as invasive growth in some cases. MicroRNA (miR)-26a has been found to be abnormally highly expressed in pituitary adenoma, indicating possible involvement in pathogenesis. As a known target gene of miR-26a, PLAG1 has abnormally low expression in pituitary adenoma. The correlation between miR-26a or PLAG1 expressional abnormality and occurrence of pituitary adenoma is still unknown, as is its association with invasiveness of pituitary adenoma.

Material/Methods

Pituitary adenoma tissues, including both invasive and non-invasive subtypes, were collected from our Neurosurgery Department, in parallel with normal pituitary tissues from postmortem autopsy. qRT-PCR was used to detect mRNA expression of miR-26a and PLAG1, while Western blotting was used to test PLAG1 protein expression. The correlation between miR-26a and PLAG1, and with pathological features, were analyzed. ROC analysis revealed the utility of miR-26a and PLAG1 in differential diagnosis of invasive/non-invasive pituitary tumors and in analyzing their effects on patient prognosis.

Results

MiR-26a was remarkably upregulated in pituitary tumors, while PLAG1 was downregulated, especially in invasive pituitary tumors. miR-26a and PLAG1 had higher diagnostic values for differentiating between invasive and non-invasive pituitary tumors (AUC=0.889 and 0.818, respectively). Those patients with miR-26 overexpression and PLAG1 downregulation had unfavorable prognosis. miR-26 and PLAG1 are independent factors affecting patient diagnosis.

Conclusions

MiR-26a can facilitate occurrence of pituitary tumor and invasiveness, probably via inhibiting PLAG1 expression.

Oncogene-Induced Senescence in Pituitary Adenomas--an Immunohistochemical Study

Oncogene-induced senescence (OIS) serves as an initial barrier to cancer development, being proposed as a possible explanation for the usually benign behavior of the pituitary adenomas. We aimed to explore the immunohistochemical expression of the OIS markers, senescence-associated lysosomal β-galactosidase (SA-β-GAL), p16, and p21 in different types of 345 pituitary adenomas and compared it with the expression in the normal pituitary and in the specimens from the repeated surgeries. SA-β-GAL was overexpressed in the pituitary adenomas, compared to the normal pituitaries. Growth hormone (GH) producing adenomas showed the strongest SA-β-GAL, with densely granulated (DG)-GH adenomas more reactive than the sparsely granulated (SG). Nuclear p21 was decreased in the adenomas, except for the SG-GH adenomas that had higher p21 than the normal pituitaries and the other adenomas. p16 was significantly lower in the adenomas, without type-related differences. SA-β-GAL was slightly lower and p16 slightly higher in the recurrences. Our findings indicate alterations of the senescence program in the different types of pituitary adenomas. Activation of senescence in the pituitary adenomas presents one possible explanation for their usually benign behavior, at least in the GH adenomas that show a synchronous increase of two OIS markers. However, subdivision into GH adenoma subtypes reveals differences that reflect complex regulatory mechanisms influenced by the interplay between the granularity pattern and the hormonal factors, with possible impact on the different clinical behavior of the SG- and DG-GH adenoma subtypes. p16 seems to have a more prominent role in the pituitary tumorigenesis than in the senescence. Recurrent growth in a subset of the pituitary adenomas is not associated with consistent changes in the senescence pattern.

EZH2 is highly expressed in pituitary adenomas and associated with proliferation

Enhancer of zeste homolog 2 (EZH2) is a core epigenetic regulator, playing a crucial role in cell cycle regulation. The protein is known to be associated with proliferation and worse outcome in several tumor entities. In this study, we immunohistochemically investigated the expression pattern of EZH2 in a large cohort of pituitary tumors. These results were correlated with clinical features and double immunofluorescence stainings (DIS) were conducted to evaluate co-expression of EZH2 and proliferation marker Ki-67. Furthermore, we analyzed the effect of EZH2 inhibition on cell proliferation in vitro using the pituitary cell line AtT-20. While in the normal anterior pituitary EZH2 was almost absent, the cohort of tumors showed enhanced expression levels (p ≤ 0.0005). This was positively associated with Ki-67 indices (r = 0.834, p ≤ 0.0005) and DIF confirmed a predominant co-expression of both markers. In vitro experiments revealed a significant (p ≤ 0.05) decrease of tumor cell proliferation using the EZH2 inhibitor GSK126. Our results further support that epigenetic events are involved in the pathogenesis and biology of pituitary adenomas (PA). Therefore, EZH2 may function as a new potential target for therapeutic interventions in PA.

hTERT promoter methylation in pituitary adenomas

Telomerase reverse transcriptase (TERT) expression is a hallmark in tumorigenesis and upregulated due to mutations and methylation of the human (h)TERT promoter. As mutations are rare but methylation is common in pituitary adenomas (PA), we determined promoter methylation and its clinical impact in 85 primary and 15 recurrent PA by methylation-specific PCR. 40 females (47%) and 45 males (53%) with a median age of 53 years harboring micro-, macro-, and giant adenomas in 12, 82, and 6% were included (prolactinomas, corticotroph, somatotroph, gonadotroph, thyreotroph, plurihormonal, and null cell adenomas in 11, 18, 10, 29, 1, 10, and 21%, respectively). In primary diagnosed tumors, methylation rate was 27% and higher in males than in females (40 vs. 13%, p = 0.001) after uni- and multivariate analyses. Methylation differed among PA subtypes (0-42%, p = n.s.) and was not significantly correlated with tumor size, cavernous sinus invasion, or serum hormone levels. Ki67 labeling index and recurrence (N = 16, 19%) were independent of methylation. In recurrent tumors, methylation was similar to primary PA (N = 5/15, 33%) and remained unchanged along follow-up. Thus, while being commonly observed in PA, hTERT promoter methylation is stable along follow-up and independent of most clinical variables, PA subtype, proliferation, and without prognostic value.