**Abstract**

Paragangliomas and pheochromocytoma (PPGLs) are hereditary tumors in about 40% of cases. Mutations in the genes encoding for components of the mitochondrial succinate dehydrogenase protein complex (*SDHB*, *SDHD*, *SDHC*) are among the most prevalent. Most PPGLs have a benign behavior, but patients with germline *SDHB* mutations may develop metastatic PPGLs in up to 30% of cases. This suggest that the SDH substrate, succinate, is key for the activation of the metastatic cascade. The last decade has witnessed significant advances in our understanding of how succinate may have oncogenic properties. It is now widely accepted that succinate is an oncometabolite that modifies the epigenetic landscape of SDHdeficient tumors via modulating the activities of DNA and histone modification enzymes. In this chapter, we summarize recent discoveries linking SDH-deficiency and metastasis in SDH-deficient PPGLs via inhibition of DNA methylcytosine dioxygenases, histone demethylases and modified expression of non-coding RNAs. We also highlight promising therapeutic avenues that may be used to counteract epigenetic deregulations.

**Keywords:** paraganglioma, pheochromocytoma, metastasis, epigenetic, DNA methylation, histone methylation, succinate

### **1. Introduction**

Paragangliomas and pheochromocytomas (PPGLs) are rare neuroendocrine tumors that originate in the diffuse paraganglionic tissue and the adrenal gland, respectively. Approximately 40% of these tumors are hereditary and related to germline mutations in *SDHB, SDHC, SDHA, SDHD* and *SDHAF2* (collectively called *SDHx*), as well as *RET, VHL, NF1, TMEM127, MAX, FH, KIF1B* and *EGLN1* among others [1]. Mutations in genes encoding different subunits of the succinate dehydrogenase (SDH) complex are the most prevalent in hereditary PPGLs being present in about 50% of cases. Among these genes, *SDHB*, *SDHD* and *SDHC* are the most frequently affected. Somatic mutations affecting *SDHx* genes can be also detected in non-hereditary PPGLs [2]. The strong association of *SDHx* mutations and PPGLs reveals that the activity of this mitochondrial complex plays an

essential, and likely unique, role in the neuroendocrine tissues conforming human paraganglia such that its deregulation cause development of neoplasia in these tissues that can, eventually, become metastatic.

One of the peculiarities of PPGLs is that they are generally slow growing, indolent tumors that are not life-threatening. However, 10–30% (according to different studies) of the PPGLs metastasize and once metastasis occurs, treatment options are rather limited and patients have poor prognosis, often with less than 50% surviving at 5 years [3]. Surgery can improve the prognosis but standard chemotherapeutic regimen with cyclophosphamide, vincristine, and dacarbazine, or radionuclide therapy with 131 Iodine-radiolabelled metaiodobenzylguanidine result in only partial responses. Thus, there is still a long road to reach therapeutic improvements. Further challenges for clinicians come from the fact that, in half of the cases, metastases are not present during the initial treatment of the patient but emerge over a period of undetermined time, which may even exceed 10 years after diagnosis of the primary tumor. For this reason, these patients receive longterm, post-treatment surveillance. However, the duration as well as the interval of the follow-up screening is poorly defined. Following these reasonings, the WHO 2017 Classification of Tumors of Endocrine Organs stated that PPGLs should be considered as tumors of undetermined biologic potential and should not be termed benign but should be classified as metastatic or not metastatic [4]. Given that all PPGLs are recognized as exhibiting malignant potential to some extent, the risk for malignant behavior must be determined to be able to pinpoint cases at risk of future metastases directly in the early post-operative period, a knowledge that would have a significant clinical impact.

Despite overwhelming advances in understanding the molecular mechanisms of PPGL development made in the last decade, the factors governing the emergence of metastasis are still very poorly understood. Considerable efforts have been made in identifying histopathological features suggestive of metastatic behavior using pre-defined algorithms. The Pheochromocytoma of the Adrenal Gland Scaled Score (PASS) and the Grading System for Adrenal Pheochromocytoma and Paraganglioma (GAPP), rely on different histopathologic features or on a combination of histopathologic, immunohistochemical (Ki-67 index) and biochemical (catecholamine production) parameters, respectively, as tools to distinguish PPGLs with potential for aggressive behavior [5]. However, these algorithms lack accuracy and have a high degree of inter-observer variability thus complicating their clinical roll-out. Hence, the guiding of therapeutic decision-making by using predictive biomarkers in PPGL patients require in-depth knowledge of the biology of this neoplasia.
