Acquired hypermethylation of the P16INK4A promoter in abdominal paraganglioma: relation to adverse tumor phenotype and predisposing mutation

Neuroendocrine Neoplasms: Genetics and Epigenetics

Neuroendocrine neoplasms (NENs) are a group of rare, heterogeneous tumors of neuroendocrine cell origin, affecting a range of different organs. The clinical management of NENs poses significant challenges, as tumors are often diagnosed at an advanced stage where overall survival remains poor with current treatment regimens. In addition, a host of complex and often unique molecular changes underpin the pathobiology of each NEN subtype. Exploitation of the unique genetic and epigenetic signatures driving each NEN subtype provides an opportunity to enhance the diagnosis, treatment, and monitoring of NEN in an emerging era of individualized medicine.

Challenges in Paragangliomas and Pheochromocytomas: from Histology to Molecular Immunohistochemistry

Abdominal paragangliomas and pheochromocytomas (PPGLs) are rare neuroendocrine tumors of the infradiaphragmatic paraganglia and adrenal medulla, respectively. Although few pathologists outside of endocrine tertiary centers will ever diagnose such a lesion, the tumors are well known through the medical community-possible due to a combination of the sheer rarity, their often-spectacular presentation due to excess catecholamine secretion as well as their unrivaled coupling to constitutional susceptibility gene mutations and hereditary syndromes. All PPGLs are thought to harbor malignant potential, and therefore pose several challenges to the practicing pathologist. Specifically, a responsible diagnostician should recognize both the capacity and limitations of histological, immunohistochemical, and molecular algorithms to pinpoint high risk for future metastatic disease. This focused review aims to provide the surgical pathologist with a condensed update regarding the current strategies available in order to deliver an accurate prognostication of these enigmatic lesions.

Retrospective application of the pathologic tumor-node-metastasis classification system for pheochromocytoma and abdominal paraganglioma in a well characterized cohort with long-term follow-up

BACKGROUND

A pathologic tumor-node-metastasis staging algorithm for pheochromocytoma and sympathetic paraganglioma was introduced recently in the 8th Edition of the cancer staging manual of the American Joint Committee on Cancer. There is no information, however, as to how this staging correlates to well-established clinical cohorts of pheochromocytoma and sympathetic paraganglioma with extensive follow-up.

METHODS

We applied the pathologic tumor-node-metastasis staging retrospectively to a cohort of 118 patients with pheochromocytoma and sympathetic paraganglioma, in which the majority has been characterized for susceptibility gene mutations and global mRNA expressional patterns as well as histologic risk criteria using the pheochromocytoma of the adrenal gland scaled score (PASS).

RESULTS

The overall tumor stage correlated with the presence of metastases, disease-related death, and PASS scores as well as established mutational and expressional clusters.

CONCLUSION

Stage III to IV pheochromocytomas and sympathetic paragangliomas are associated with increased mortality, increased PASS scores, and mutational and expressional aberrancies in the pseudo-hypoxia pathway cluster. These findings validate the stratification proposed by the American Joint Committee on Cancer staging manual by linking malignancy-associated pheno- and genotypes to more advanced stages. Moreover, because few pheochromocytomas and sympathetic paragangliomas are metastatic at the time of the original presentation, the staging relies heavily on identifying histologic signs of extra-adrenal invasion.

Clinical implications of the oncometabolite succinate in SDHx-mutation carriers

Succinate dehydrogenase (SDH) mutations lead to the accumulation of succinate, which acts as an oncometabolite. Germline SDHx mutations predispose to paraganglioma (PGL) and pheochromocytoma (PCC), as well as to renal cell carcinoma and gastro‐intestinal stromal tumors. The SDHx genes were the first tumor suppressor genes discovered which encode for a mitochondrial enzyme, thereby supporting Otto Warburg's hypothesis in 1926 that a direct link existed between mitochondrial dysfunction and cancer. Accumulation of succinate is the hallmark of tumorigenesis in PGL and PCC. Succinate accumulation inhibits several α‐ketoglutarate dioxygenases, thereby inducing the pseudohypoxia pathway and causing epigenetic changes. Moreover, SDH loss as a consequence of SDHx mutations can lead to reprogramming of cell metabolism. Metabolomics can be used as a diagnostic tool, as succinate and other metabolites can be measured in tumor tissue, plasma and urine with different techniques. Furthermore, these pathophysiological characteristics provide insight into therapeutic targets for metastatic disease. This review provides an overview of the pathophysiology and clinical implications of oncometabolite succinate in SDHx mutations.

Emerging molecular markers of metastatic pheochromocytomas and paragangliomas

Metastatic pheochromocytoma/paraganglioma (PPGL) represents a major clinical challenge due to limitations in accurate diagnostic tools and effective treatments. Currently, patients classified at high-risk by means of clinical, biochemical and genetic criteria, require a lifelong monitoring, while it remains difficult to determine the metastatic potential of PPGL only on the basis of histopathological features. Thus, tumor molecular markers that improve the risk stratification of these patients are needed. In the past few years, we have witnessed an unprecedented molecular characterization of PPGL, which led to the emergence of promising candidate biomarkers predictive of metastatic behavior. Here, we briefly discuss these breakthroughs and provide some insights for the prospective implementation of molecular markers of metastatic PPGL in the clinical setting in years to come.

Understanding the complexity of antigen retrieval of DNA methylation for immunofluorescence-based measurement and an approach to challenge

Cytosine methylation (5-methylcytosine, 5meC) in the CpG-rich regions of the mammalian genome is an important epigenetic mechanism playing roles in transcription regulation and genomic stability. The abnormalities in DNA methylation can occur in various types of cancer and some genetic diseases. The measurement of DNA methylation is therefore important and there is a range of methodologies used to detect DNA methylation. Many methods based on bisulfite treatment appeared with a lack of specificity after recent discoveries of various modifications of methylated cytosine, however there are new treatments developed to overcome this limitation. Immunofluorescence is currently known to be able to specifically detect DNA methylation as it uses different antibodies against 5meC and its derivatives, but it is a semi-quantitative method. Immunofluorescence protocols commonly include fixation of cells followed by permeabilisation, antigen retrieval, and treatments with antibodies. Establishing the strategy for antigen retrieval of immunofluorescence is important to unmask epitopes (i.e. 5meC) from other proteins, and therefore to access the antigen of interest. There are many approaches used for antigen retrieval induced by acid, enzyme and/or heat. The selection of antigen retrieval method can depend on a variety of such antigen-based or cell-based conditions, since the dynamic structure of DNA and chromatin accounts for the complexity of involved proteins to mask the epitope. This review aims to specifically focus on the complexity of in situ detection of DNA methylation by immunofluorescence-based methods using antigen retrieval with the current understanding of DNA methylation mechanism, and suggests conditions for antigenic retrieval of 5meC epitope.

Epigenetic alterations in endocrine-related cancer

Aberrant epigenetics is a hallmark of cancer, and endocrine-related tumors are no exception. Recent research has been identifying an ever-growing number of epigenetic alterations in both genomic DNA methylation and histone post-translational modification in tumors of the endocrine system. Novel microarray and ultra-deep sequencing technologies have allowed the identification of genome-wide epigenetic patterns in some tumor types such as adrenocortical, parathyroid, and breast carcinomas. However, in other cancer types, such as the multiple endocrine neoplasia syndromes and thyroid cancer, tumor information is limited to candidate genes alone. Future research should fill this gap and deepen our understanding of the functional role of these alterations in cancer, as well as defining their possible clinical uses.

Rare germline mutations identified by targeted next-generation sequencing of susceptibility genes in pheochromocytoma and paraganglioma

CONTEXT

Pheochromocytomas and paragangliomas have a highly diverse genetic background, with a third of the cases carrying a germline mutation in 1 of 14 identified genes.

OBJECTIVE

This study aimed to evaluate next-generation sequencing for more efficient genetic testing of pheochromocytoma and paraganglioma and to establish germline and somatic mutation frequencies for all known susceptibility genes.

DESIGN

A targeted next-generation sequencing approach on an Illumina MiSeq instrument was used for a mutation analysis in 86 unselected pheochromocytoma and paraganglioma tumor samples. The study included the genes EGLN1, EPAS1, KIF1Bβ, MAX, MEN1, NF1, RET, SDHA, SDHB, SDHC, SDHD, SDHAF2, TMEM127, and VHL. RESULTS were verified in tumor and constitutional DNA with Sanger sequencing.

RESULTS

In all cases with clinical syndromes or known germline mutations, a mutation was detected in the expected gene. Among 68 nonfamilial tumors, 32 mutations were identified in 28 of the samples (41%), including germline mutations in EGLN1, KIF1Bβ, SDHA, SDHB, and TMEM127 and somatic mutations in EPAS1, KIF1Bβ, MAX, NF1, RET, and VHL, including one double monoallelic EPAS1 mutation.

CONCLUSIONS

Targeted next-generation sequencing proved to be fast and cost effective for the genetic analysis of pheochromocytoma and paraganglioma. More than half of the tumors harbored mutations in the investigated genes. Notably, 7% of the apparently sporadic cases carried germline mutations, highlighting the importance of comprehensive genetic testing. KIF1Bβ, which previously has not been investigated in a large cohort, appears to be an equally important tumor suppressor as MAX and TMEM127 and could be considered for genetic testing of these patients.

The activating TERT promoter mutation C228T is recurrent in subsets of adrenal tumors

The telomerase reverse transcriptase gene (TERT) encodes the reverse transcriptase component of the telomerase complex, which is essential for telomere stabilization and cell immortalization. Recent studies have demonstrated a transcriptional activation role for the TERT promoter mutations C228T and C250T in many human cancers, as well as a role in aggressive disease with potential clinical applications. Although telomerase activation is known in adrenal tumors, the underlying mechanisms are not established. We assessed C228T and C250T TERT mutations by direct Sanger sequencing in tumors of the adrenal gland, and further evaluated potential associations with clinical parameters and telomerase activation. A total of 199 tumors were evaluated, including 34 adrenocortical carcinomas (ACC), 47 adrenocortical adenomas (ACA), 105 pheochromocytomas (PCC; ten malignant and 95 benign), and 13 abdominal paragangliomas (PGL; nine malignant and four benign). TERT expression levels were determined by quantitative RT-PCR. The C228T mutation was detected in 4/34 ACCs (12%), but not in any ACA (P=0.028). C228T was also observed in one benign PCC and in one metastatic PGL. The C250T mutation was not observed in any case. In the ACC and PGL groups, TERT mutation-positive cases exhibited TERT expression, indicating telomerase activation; however, since expression was also revealed in TERT WT cases, this could denote additional mechanisms of TERT activation. To conclude, the TERT promoter mutation C228T is a recurrent event associated with TERT expression in ACCs, but rarely occurs in PGL and PCC. The involvement of the TERT gene in ACC represents a novel mutated gene in this entity.

Cytocidal activities of topoisomerase 1 inhibitors and 5-azacytidine against pheochromocytoma/paraganglioma cells in primary human tumor cultures and mouse cell lines

There is currently no effective treatment for metastatic pheochromocytomas and paragangliomas. A deficiency in current chemotherapy regimens is that the metastases usually grow very slowly. Drugs that target dividing tumor cells have therefore had limited success. To improve treatment, new strategies and valid experimental models are required for pre-clinical testing. However, development of models has itself been hampered by the absence of human pheochromocytoma/paraganglioma cell lines for cultures or xenografts. Topoisomerase 1 (TOP1) inhibitors are drugs that interfere with mechanisms that maintain DNA integrity during transcription in both quiescent and dividing cells. We used primary cultures of representative human tumors to establish the cytotoxicity of camptothecin, a prototypical TOP1 inhibitor, against non-dividing pheochromocytoma/paraganglioma cells, and then employed a mouse pheochromocytoma model (MPC) to show that efficacy of low concentrations of camptothecin and other TOP1 inhibitors is increased by intermittent coadministration of sub-toxic concentrations of 5-azacytidine, a DNA methylation inhibitor that modulates transcription. We then tested the same drugs against a clonal MPC derivative that expresses CMV reporter-driven luciferase and GFP, intended for in vivo drug testing. Unexpectedly, luciferase expression, bioluminescence and GFP expression were paradoxically increased by both camptothecin and SN38, the active metabolite of irinotecan, thereby masking cell death. Expression of chromogranin A, a marker for neuroendocrine secretory granules, was not increased, indicating that the drug effects on levels of luciferase and GFP are specific to the GFP-luciferase construct rather than generalized cellular responses. Our findings provide proof of principle for use of TOP1 inhibitors against pheochromocytoma/paraganglioma and suggest novel strategies for enhancing efficacy and reducing toxicity by optimizing the combination and timing of their use in conjunction with other drugs. The paradoxical effects of TOP1 inhibitors on luciferase and GFP dictate a need for caution in the use of CMV promoter-regulated constructs for cancer-related imaging studies.

The VHL gene is epigenetically inactivated in pheochromocytomas and abdominal paragangliomas

Pheochromocytoma (PCC) and abdominal paraganglioma (PGL) are neuroendocrine tumors that present with clinical symptoms related to increased catecholamine levels. About a third of the cases are associated with constitutional mutations in pre-disposing genes, of which some may also be somatically mutated in sporadic cases. However, little is known about inactivating epigenetic events through promoter methylation in these very genes. Using bisulphite pyrosequencing we assessed the methylation density of 11 PCC/PGL disease genes in 96 tumors (83 PCCs and 13 PGLs) and 34 normal adrenal references. Gene expression levels were determined by quantitative RT-PCR. Both tumors and normal adrenal samples exhibited low methylation index (MetI) in the EGLN1 (PDH2), MAX, MEN1, NF1, SDHB, SDHC, SDHD, SDHAF2 (SDH5), and TMEM127 promoters, not exceeding 10% in any of the samples investigated. Aberrant RET promoter methylation was observed in two cases only. For the VHL gene we found increased MetI in tumors as compared with normal adrenals (57% vs. 27%; P < 0.001), in malignant vs. benign tumors (63% vs. 55%; P < 0.05), and in PGL vs. PCC (66% vs. 55%; P < 0.0005). Decreased expression of the VHL gene was observed in all tumors compared with normal adrenals (P < 0.001). VHL MetI and gene expressions were inversely correlated (R = −0.359, P < 0.0001). Our results show that the VHL gene promoter has increased methylation compared with normal adrenals (MetI > 50%) in approximately 75% of PCCs and PGLs investigated, highlighting the role of VHL in the development of these tumors.

Neuroendocrine tumours: cracking the epigenetic code

The field of epigenetics has evolved rapidly over recent years providing insight into the tumorigenesis of many solid and haematological malignancies. Determination of epigenetic modifications in neuroendocrine tumour (NET) development is imperative if we are to improve our understanding of the biology of this heterogenous group of tumours. Epigenetic marks such as DNA methylation at RASSF1A are frequent findings in NETs of all origins and may be associated with worse prognosis. MicroRNA signatures and histone modifications have been identified which can differentiate subtypes of NET and distinguish NET from adenocarcinoma in cases of diagnostic uncertainty. Historically, candidate gene-driven approaches have yielded limited insight into the epigenetics of NET. Recent progress has been facilitated by development of high-throughput tools including second-generation sequencing and arrays for analysis of the 'epigenome' of tumour and normal tissue, permitting unbiased approaches such as exome sequencing that identified mutations of chromatin-remodelling genes ATRX/DAXX in 44% of pancreatic NETs. Epigenetic changes are reversible and therefore represent an attractive therapeutic target; to date, clinical outcomes of epigenetic therapies in solid tumours have been disappointing; however, in vitro studies on NETs are promising and further clinical trials are required to determine utility of this class of novel agents. In this review, we perform a comprehensive evaluation of epigenetic changes found in NETs to date, including rare NETs such as phaeochromocytoma and adrenocortical tumours. We suggest priorities for future research and discuss potential clinical applications and novel therapies.