Pheochromocytomas (PHEOs) and paragangliomas (PGLs) are rare neuroendocrine (NE) tumors arising from chromaffin cells of the adrenal medulla and extra-adrenal ganglia, respectively. The incidence of PHEOs and PGLs (collectively PPGLs) is estimated at approximately 2–8 cases per million per year ^[1][2][1,2]. However, this is likely an underestimate, based upon the finding of up to 0.05–0.1% incidentally detected cases in an autopsy series [3]. PPGLs may occur at any age and they usually peak between the 3rd and 5th decade of life [4]. Patients with PPGL most commonly present with symptoms of excess catecholamine production including headache, diaphoresis, palpitations, tremors, facial pallor and hypertension. These symptoms are often paroxysmal, although persistent hypertension between these episodes is common and occurs in 50–60% patients with PPGL [5].

The field of genomics in PPGL has rapidly evolved over the past two decades. Approximately 40% of all cases of PPGLs are associated with germline mutations, which makes pheochromocytoma and paraganglioma solid tumors with a high heritability rate.  Currently, more than 20 susceptibility genes have been identified, including at least 12 distinct genetic syndromes, 15 driver genes and an expanding fraction of potential disease modifying genes ^[6][7][11,12]. Thus, the underlying mutations appear to determine the clinical manifestations, such as tumor location, biochemical profile, malignant potential, imaging signature and overall prognosis, that should help to tailor treatment and guidance for follow-up. Moreover, detection of a mutation in an index case and their family members should also help clinicians to implement a pertinent surveillance program to promptly identify tumors and treat patients accordingly ^[8][9][13,14]. Surgical resection remains the mainstay of treatment. In cases where surgery is not feasible or if tumor dissemination limits the probability of curative treatment, the options for treatment are localized radiotherapy, radiofrequency or cryoablation and systemic therapy, which includes chemotherapy or targeted molecular therapies.

There has been increasing interest in radionuclide therapy, which includes ¹³¹I-MIBG therapy and recently PRRT (peptide receptor radionuclide therapy) ¹⁷⁷Lu-DOTATATE ^[10][11][12][15,16,17]. In terms of chemotherapy, CVD (cyclophosphamide, vincristine and dacarbazine) is one of the most traditional chemotherapy regimens and has been used to treat PPGLs over the past 30 years ^[13][18]. New treatments are emerging for patients with advanced/metastatic PPGL. Understanding the molecular signaling and metabolomics of PPGL has led to the development of therapeutic regimens for cluster-specific targeted molecular therapies. Based on TCGA classification for cluster I, antiangiogenic therapy, HIF inhibitors, PARP (polyADP-ribose polymerase) inhibition and immunotherapy are used. For cluster II, mTOR (mammalian target of rapamycin) inhibitors are used. Currently there are no cluster III Wnt signaling targeted therapies for PPGL patients ^[14][19]. At present, clinical genetic testing for patients with a suspected hereditary form of PPGL is carried out using a germline genetic panel rather than using one gene at a time. Based upon its lower financial cost, immunohistochemistry (IHC) can be considered for screening purposes, particularly in patients with suspected succinate dehydrogenase complex (SDHx) mutations. However, IHC should be interpreted with caution as there is likelihood of false-positive and false-negative results ^[15][20].

2. Genetics on What Is Already Known

The identification of the Krebs cycle in the etiology of PPGLs is a milestone in the field of the genetics of PPGLs. The SDH complex plays a pivotal role in energy metabolism in the Krebs cycle, as well as in complex II of the electron transport chain. Mutations in any of the genes encoding the catalytic enzymes of the pathway can lead to an accumulation of their substrates, resulting in hypoxia-inducible factor (HIF) stability and tumorigenesis ^[16][22]. These genes include SDHA, SDHB, SDHC, SDHD, SDHAF2 ^[17][18] [23,24], fumarate hydratase (FH) ^[19][20][25,26], malate dehydrogenase 2 (MDH2) ^[21][22][27,28], hypoxia-inducible factor alpha (HIF2a) ^[23][24][25][29,30,31], prolyl hydroxylase (PHD) ^[26][32] and some newly discovered genes. Mutation of the genes involved in the kinase receptor signaling pathway that are known to cause PPGLs are RET (REarranged during Transfection), neurofibromin 1 (NF1), Myelocytomatosis-Associated factor X (MAX), transmembrane protein 127 (TMEM127), and Harvey rat sarcoma viral gene homologue (HRAS). Genes such as ATRX (Alpha Thalassemia/mental Retardation-X linked) that are involved in chromosomal integrity, are also implicated as drivers in the etiology of PPGLs and are associated with aggressive behavior ^[27][33]. To better understand the genetics based on signaling pathways, The Cancer Genome Atlas (TCGA) has classified PPGLs into three clinically useful molecular clusters: (1) Pseudohypoxic PPGLs, (2) Kinase signaling PPGLs and (3) Wnt signaling PPGLs ^[28][34] (Figure 1).

4. New Screening Guidelines for Asymptomatic SDHx Carriers

Germline mutations in SDHx comprises approximately 20% of cases of PPGL ^[41][42][43][10,57,58]. When a SDHx pathogenic mutation is identified, genetic counselling is proposed for patients’ first-degree relatives. However, there are no established guidelines on how to screen and then follow up asymptomatic mutation carriers.  The penetrance of SDHx-related PPGL is not firmly established. Studies have shown that SDHB has a 8–37% penetrance and SDHD has a 38–64% penetrance ^[44][45][21,59]. Out of all SDHx mutation carriers, patients with an SDHB mutation carry the highest risk for metastatic disease ^[43][46][47][58,60,61]. Patients with SDHD mutations have the highest penetrance. Data regarding SDHC and SDHA mutations are limited but show lower penetrance than SDHD mutation carriers ^[43][48][49][58,62,63]. 

5. Emerging Molecular Genetics and Future Perspectives

The clinical treatment options for patients with PPGL are increasingly based on the underlying molecular biology, genetic and epigenetic analyses of the tumors. Over the last five years, various human- and rodent-derived cell lines and xenografts have been developed. Yet, they do not fully provide subtype classification of tumors and remain challenging for clinical studies. Frankhauser et al. used “immortalized mouse chromaffin cells” (imCCs), MPC/MTT (mouse pheochromocytoma cells/mouse tumor tissue) spheroids, murine pheochromocytoma cell lines and human pheochromocytoma primary cultures, and identified that the PI3Ka inhibitor BYL719 and the MTORC1 inhibitor everolimus are highly effective at tumor shrinkage at clinically relevant doses ^[50][76]. To date, there has only been one human cell line progenitor developed successfully: Pheo1 ^[51][77]. Moreover, the classic approaches to cell line development, such as SV40-mediated immortalization and newer approaches such as patient-derived tumor xenografts and tumor organoids, have become important preclinical models. Induced pluripotent stem cells (iPSCs) are worth exploring further in this field ^[52][53][78,79]. 

6. Conclusions

PPGLs are rare NE tumors with unique molecular landscapes. Cataloging and understanding the germline and somatic mutations associated with PPGLs is a promising approach to understand the clinical behavior and prognosis. Moreover, it can provide guidance on diagnostic strategies and personalized treatments for PPGLs.