Neurofibromatosis type 1 (NF1), also called von Recklinghausen’s disease, is an autosomal dominant cancer syndrome occurring in 1 in 2800 individuals and caused by loss of function mutations in the neurofibromatosis Type 1 (NF1) tumor suppressor gene [24]. NF1 is located on chromosome 17q11.2 and encodes the large 2818 amino acid protein neurofibromin. The most well-defined role of neurofibromin is as a GTPase-inactivating RAS and, thereby, regulating both the PI3K/mTOR (mammalian target of rapamycin) and mitogen-activated protein kinase (MAPK) pathways to control cell proliferation and differentiation [25]. When NF1 is mutated, there is uncontrolled activation of these pathways leading to tumor formation.

Traditionally, NF1 is diagnosed based on clinical criteria rather than genetic testing due to the large size of the gene and lack of mutational hotspots. Up to 50% of patients with NF1 have de novo mutations, and there is variable penetrance and expressivity for the disease even in those family members with the same mutation [26]. The clinical diagnosis is made when patients have two or more of the following characteristics: six or more café au lait spots of varying size based on pubertal status, two or more neurofibromas or one plexiform neurofibroma, Lisch nodules (benign iris hamartomas), optic glioma, skeletal dysplasia (thinning of the long bones), or a first degree family member with NF1 (Table 8.3) [24]. Although not part of the diagnostic criteria, patients with NF1 also are at increased risk of developing PCC, malignant peripheral nerve sheath tumors, and juvenile myelomonocytic leukemia. The prevalence of PCC in patients with NF1 varies from 1 to 15% depending on case detection testing [27–29]. Guidelines for treating patients with NF1 recommend screening for PCC in individuals with hypertension [24]. PCCs in NF1 are usually unilateral, but can be bilateral, and often secrete both epinephrine and norepinephrine. The mean age of diagnosis of PCC in patients with NF1 is 43 years old [30], similar to those with sporadic disease. Although PCCs are rare in individuals with NF1, the rate of malignancy is elevated to approximately 12% [30].

von Hippel-Lindau disease (vHL) is another autosomal dominant cancer syndrome occurring in 1 in 36,000 individuals per year and caused by loss of function mutations in the von Hippel-Lindau (VHL) tumor suppressor gene [31]. VHL is located on chromosome 3p25-26 and encodes two isoforms of the von Hippel-Lindau protein (pVHL), an E3 ubiquitin ligase: one full-length 213 amino acid protein and one smaller protein which lacks the first 53 amino acids [32]. Under normoxic conditions, pVHL binds to a hydroxylated proline residue on hypoxia-inducible factor α (HIFα) and marks it for proteasomal degradation by ubiquitination [33]. Under hypoxic conditions, or if pVHL is inactivated by mutations, pVHL will not bind to HIFα which is then free to dimerize with HIFβ, translocate to the nucleus, and act as a transcription factor for downstream genes involved in the hypoxia signaling pathway [32].

vHL is diagnosed based on clinical features and by identifying the VHL gene alteration through clinical genetic testing. Patients with vHL develop a wide range of benign and malignant tumors including renal cysts and clear cell renal cell carcinomas (RCC), pancreatic cysts and PNETS, hemangioblastomas of the central nervous system, endolymphatic sac tumors, and PCC (Table 8.3) [34]. Up to 20% of all patients with vHL will develop a PCC with the mean age of diagnosis at 30 years, and PCC may be the presenting sign of disease [35]. Interestingly, there are strong genotype-phenotype correlations between PCC and VHL mutations [36–41]. Patients with vHL type 1 have truncating gene mutations or deletions, and these patients have a low risk of PC C and a higher risk of RCC. On the other hand, patients with vHL type 2 have missense mutations in VHL and carry a high risk of PCC or RCC depending on the location of the missense mutation. Missense mutations on the surface of pVHL portend a higher risk of PCC than missense mutations in the core of the protein [42]. Mutations that disrupt the interaction between pVHL and HIF are associated with RCC, while mutations in other parts of pVHL are associated with PCC [37, 40, 43, 44]. This suggests that increased HIF activity is related to renal cell carcinoma formation, while other HIF-independent functions of pVHL may be causative for PCC development. However, by cDNA microarray analysis, VHL-associated PCC display a pseudohypoxia gene expression pattern as expected to occur secondary to increased HIF activity [45], so the exact mechanism by which the genotype-phenotype correlations exist remains to be elucidated.

PCC in vHL can be unilateral or bilateral, and there are rare cases of extra-adrenal PGL [46, 47]. Interestingly, vHL-associated PCC tend to secrete norepinephrine despite forming in the adrenal medulla. The reason for this deviation in the usual adrenal medullary hormonal secretion pattern is due to decreased PNMT expression secondary to promoter hypermethylation, leading to a decreased conversion of norepinephrine to epinephrine [48, 49]. The malignant PCC rate in patients with vHL is about 5% [34]. Guidelines suggest that patients with high-risk VHL mutations be screened annually for PCC starting at age 5 [50].

Multiple endocrine neoplasia type 2 (MEN2) is the third classic autosomal dominant cancer syndrome associated with PCC/PGL risk. MEN2 occurs in 1 in 30,000 people and is caused by activating mutations in the RET (rearranged during transfection) proto-oncogene. RET is on chromosome 10q11.2 and encodes the 860 amino acid RET protein, a transmembrane tyrosine kinase required for neural crest development [51]. When bound by ligand, the RET protein dimerizes, autophosphorylates, and signals through the PI3K pathway to regulate proliferation and apoptosis [52]. Activating mutations in RET cause constitutive upregulation of PI3K pathway leading to tumor development.

The MEN2 diagnosis is suspected based on clinical features and confirmed by mutation testing of the RET gene. Patients with MEN2 are divided into two groups per the most recent guidelines, MEN2A and MEN2B [53] (Table 8.3). Classical MEN2A is defined by medullary thyroid cancer (MTC) (95% of patients), primary hyperparathyroidism (15–30% of patients), and PCC (50% of patients). MEN2A also includes three additional subtypes: (1) patients with MTC and cutaneous lichen amyloidosis, (2) patients with MTC and Hirschsprung’s disease, and (3) patients who develop only MTC (the old familial MTC subtype of MEN2). MEN2B is defined by MTC (100% of patients), PCC (50% of patients), mucosal ganglioneuromas, and a marfanoid habitus.

Ninety-five percent of patients with MEN2 have type 2A and 5% have type 2B. There are strong phenotype/genotype correlations based on the mutated RET codon. For example, patients with codon 634 mutations have high risk for classical MEN2A, while patients with mutations at codon 918 almost exclusively have the MEN2B phenotype [54]. The 2015 guide lines suggest that patients with high-risk mutations (codons 918, 634, and 883) be screened annually for PCC starting at age 11 and those with moderate-risk mutations (all other codons) be screened annually for PCC starting at age 16 [53]. The mean age at diagnosis of PCC in patients with MEN2 is 28 years [55], and patients develop bilateral PCC more than 50% of the time [55, 56]. There are rare reports of extra-adrenal PGL [46]. MEN2-associated PCC usually have epinephrine predominance [57], and malignant PCC occurs in less than 5% of cases [56].

The hereditary paraganglioma syndromes are autosomal dominant disorders caused by loss of function mutations in one of the succinate dehydrogenase subunit (SDH) genes (SDHA, SDHB, SDHC, SDHD) or the cofactor SDHAF2 [58–62]. The SDH complex, complex II of the mitochondrial respiratory chain, is a highly conserved heterotetrameric protein and the only respiratory chain complex also involved in the Krebs cycle. The SDH complex couples the oxidation of succinate to fumarate with the electron transfer to the terminal acceptor ubiquinone in the electron transport chain. SDHA and SDHB are the catalytic subunits located in the mitochondrial matrix, and these two proteins are anchored to the inner mitochondrial membrane by subunits SDHC and SDHD. Loss of function mutations in the SDH complex subunits result in accumulation of succinate. Because succinate has structural similarity to α-ketoglutarate (also called 2-oxoglutarate), high levels of succinate competitively inhibit 2-oxoglutarate-dependant dioxygenases such as histone and DNA demethylases as well as HIF prolyl-hydroxylases [63, 64]. This inhibition leads to two main downstream consequences: (1) pseudohypoxia conditions because the inhibition of HIF prolyl-hydroxylase enzymes prevents the hydroxylation of the proline residue on HIFα thereby preventing pVHL binding and subsequent proteasomal degradation and (2) global DNA hypermethylation and other epigenetic changes because of the inhibition of histone and DNA demethylases [65, 66]. Mutations in any of the SDH subunits or cofactor AF2 lead to increased risk of PCC/PGL formation as discussed below (Table 8.3).

The SDHB subunit is the most commonly mutated subunit of the complex associated with PCC/PGL. SDHB is located on chromosome 1p36.1 and encodes the 280 amino acid iron sulfur subunit of the SDH complex. Patients with SDHB mutations most often develop extra-adrenal PGL but also are at risk for HNPGL as well as adrenal PCC. The SDHB-associated tumors tend to secrete norepinephrine and/or dopamine, and the mean age at initial diagnosis ranges from 28.7 to 36.7 years [67–69]. Compared to all of the other PCC/PGL susceptibility genes, the risk of malignancy is highest in patients with SDHB associated PCC/PGL (~23%) [70]. Of those patients with metastatic disease, about half have germline SDHB mutations [18].

SDHD is the next most common subunit of the complex that is mutated in association with PCC/PGL. SDHD is located on chromosome 11q23.1 and encodes the 103 amino acids anchoring subunit with an ubiquinone binding site to which the electrons are transferred from the iron sulfur clusters within the SDHB subunit [71, 72]. SDHD mutations are expressed with a parent-of-origin effect, almost exclusively showing paternal inheritance with extremely rare exception [67, 73–76]. Patients with paternally inherited SDHD mutations develop HNPGL and are at risk to develop multiple primary tumors including adrenal PCC and extra-adrenal PGL as well [75, 77, 78]. The mean age at diagnosis is 35.7 years [67], and the rate of malignancy is less than 5% [70]. The SDHD-associated HNPGLs tend to be biochemically silent or produce only dopamine or methoxytyramine, whereas the PCCs can have norepinephrine secretion [57].

The SDHC, SDHA, and SDHAF2 genes are mu tated at a lower frequency in association with PCC/PGL. SDHC is located on chromosome 1q23.3, spans 35 kb, and encodes a large subunit with the cytochrome b in the SDH complex. Patients with SDHC mutations develop HNPGL and thoracic PGL most commonly, although PCC/PGL in other locations can be seen [79–81]. The mean age at initial diagnosis is 29–38 years [67, 79], and there is a negligible risk of malignant disease in SDHC mutation carriers [67, 79]. SDHA is located on chromosome 5q15 and encodes the flavoprotein catalytic subunit of SDH complex. Biallelic mutations in SDHA lead to Leigh’s syndrome (an early onset neurodegenerative disorder) [82, 83]. It was subsequently discovered that SDHA mutations also are associated with PCC/PGL and account for about 3% of PCC/PGL cases with low penetrance [60, 84]. The phenotype associated with SDHA-related PCC/PGL is still being uncovered. SDHAF2 is located on chromosome 11q12.2 and encodes the protein needed for flavanation of the SDHA catalytic subunit. Only a few families have been described with mutations in SDHAF2, and those affected mutation carriers tend to have multiple HNPGL [61, 85, 86]. Similar to SDHD mutations, SDHAF2 mutations show a parent-of-origin effect associated with paternal transmission of disease. The average age of onset is 33 years and the rate of malignancy is negligible.

Inherited SDHx gene mutations are associated with increased risk of developing other cancers in addition to PCC/PGL [87] (Table 8.3). Renal cell carcinoma occurs in about 14% of SDHB and 8% of SDHD mutation carriers [75]. SDHC and SDHA mutation carriers also are at higher risk than the general population to develop RCC albeit at a low frequency [88]. In addition, all SDHx mutation carriers are at increased risk for developing gastrointestinal stromal tumors (GISTs) [89], and numerous case reports suggest an association between SDHx mutations and pituitary adenomas [90]. Thus far, no strong genotype/phenotype correlations have been discovered between the specific mutation types and the spectrum of disease.

There are no formal guidelines for screening asymptomatic or unaffected SDHx mutation carriers. PCC/PGL have been reported in children as young as 5 years old, yet the penetrance of PCC/PGL and the other SDHx-associated tumors is not well defined. Most experts suggest screening for SDHx-associated tumors in asymptomatic mutation carrier patients with whole body MRI every 2–5 years starting between ages 5 and 10 along with annual biochemical screening for PCC/PGL.

Several families with inherited PCC/PGL had none of the above susceptibility gene mutations. Applying genomic approaches to DNA and RNA from tumors from these families, identified additional susceptibility genes as causing autosomal dominant transmission of PCC/PGL, including TMEM127, MAX, and FH (Table 8.3), discussed below. In addition, there are rare case reports of families with germline mutations in MDH2, KIF1B, and EGLN1 [91–93].

The TMEM127 gene is located on chromosome 2q11.2 and loss of function mutations lead to PCC/PGL [94]. TMEM127 encodes transmembrane protein 127 believed to play a role in mTORC1 signaling pathway [94]. The patients with germline TMEM127 mutations have a low penetrance of disease. These mutation carriers can develop PCC/PGL in any location, often having bilateral adrenal PCC [95, 96]. The average age of onset is similar to sporadic tumors at 45 years old, and the rate of malignancy is low. Rare cases of RCC have been found in TMEM127 mutation carriers [97].

The MAX gene, located on chromosome 14q23, was found to be a PCC/PGL susceptibility gene through germline exome sequencing [98]. MAX encodes a basic helix-loop-helix leucine zipper protein called MYC-associated protein X (MAX), which heterodimerizes with MYC or MYC repressors to act as a transcription factor for numerous genes involved in cell proliferation, differentiation, and apoptosis [99, 100]. Patients with MAX mutations tend to have adrenal PCC which are often bilateral and can be multifocal within a single adrenal gland; furthermore, there may be an association with an increased risk of malignancy, although this has not been replicated in a second cohort study [98, 101]. The number of patients with MAX mutations identified thus far is quite small making the phenotype difficult to define.

Inactivating mutations in the fumarate hydratase (FH) gene cause hereditary leiomyomatosis and Renal Cell Cancer (HLRCC) syndrome which is an autosomal dominant condition where patients develop smooth muscle tumors (leiomyomas) and type 2 papillary renal cell carcinoma [102]. Rare families with PCC/PGL have been found to carry germline FH mutations, and there may be an associated increased risk of metastatic disease [103, 104]. The FH gene is located on chromosome 1q42.1 and encodes the enzyme fumarate hydratase which converts fumarate to malate in the Krebs cycle. PCC/PGL with FH mutations have similar molecular consequences to PCC/PGL with SDHx mutations including pseudohypoxia and hypermethylation as fumarate shares structural similarities to succinate leading to similar inhibition of the 2-oxoglutarate-dependent dioxygenases [64, 103, 104].