Despite clinical studies suggesting that PPGL is familial in ~10% of patients, the application of molecular genetic studies to PPGL has revealed that around 40% of individuals with PPGL will harbour a germline mutation in a PPGL predisposition gene and that most of these will not have a family history. The most common causes of inherited predisposition to PPGL are germline mutations in the various succinate dehydrogenase subunit genes (SDHX), VHL, RET, and NF1 genes, but mutations in multiple other genes (including MAX, TMEM127, HIF2A/ EPAS1, FH, MDH2, SLC25A11) have also been described to predispose to PPGL. The clinical characteristics of germline mutations in different inherited PPGL genes differ and are described in detail in Chapter 6.12.
The somatic (acquired) genetic and epigenetic alterations associated with PPGL tumourigenesis have been described in both inherited and sporadic forms of PPGL. Most inherited PPGL genes are classed as tumour suppressor genes and the germline mutations are loss of function variants. Initiation of tumourigenesis requires inactivation of the wild- type allele and this can occur through a variety of mechanisms including large chromosomal deletions (most often), somatic mutations, or promoter methylation with transcriptional silencing. For example, PPGL from patients with von Hippel– Lindau disease frequently demonstrate loss of the short arm of chromosome 3p (so the wild- type (non- mutated) VHL allele at 3p25 is lost) and in PPGL from patients with NF1 chromosome 17q loss is found (the NF1 tumour suppressor gene is located at 17q11.2). As germline RET mutations activate the oncogenic function of RET, chromosome 10 deletions are not a feature of phaeochromocytomas from patients with MEN2. Secondary epigenetic alterations (e.g. genome- wide hypermethylation) are a characteristic feature of PPGL associated with germline mutations in SDHX, FH, MDH2, and SLC25A11. The gene products of these genes encode key components of cellular metabolic pathways and their inactivation leads to accumulation of metabolites (e.g. succinate with SDHX and fumarate with FH mutations) that inhibit alpha- ketoglutarate- dependent enzymes including the TET (ten- eleven translocation) proteins that are require for active DNA demethylation and lead to silencing of tumour suppressor genes through aberrant promoter methylation.
Various genome- wide copy number abnormality studies have demonstrated recurrent areas of loss and gain in PPGL including chromosome 1p, 3p, 3q 11p, 17, 21q, and 22q loss. Interestingly 11p loss in PPGL preferentially affects the maternally inherited chromosome suggesting that imprinted genes (e.g. the paternally expressed IGF2 and maternally expressed CDKN1C) are implicated in the pathogenesis of PPGL and the parent- of- origin effects on tumour risks seen with mutations in the chromosome 11q SDHD and SDHAF2 genes.
Large- scale next- generation sequencing studies of somatic mutations in sporadic PPGL have identified mutations in a wide variety of genes including genes known to be associated with inherited PPGL (VHL, NF1, RET) and also genes in which only somatic mutations occur. Though some important inherited PPGL tumour suppressor genes map to regions of recurrent copy number loss in sporadic PPGL (including SDHB to 1p36, VHL to 3p25, and NF1 to 17q11.2) the frequency of somatic mutations in inherited PPGL genes in sporadic PPGL varies markedly. For example, whereas somatic mutations in NF1 and VHL are relatively common, somatic mutations in SDHX subunit genes are infrequent. Gain of function mutations in RET and HIF2A/ EPAS1 can occur as both germline and somatic mutations. The most strongly activating mutations (e.g. RET M918T which is associated with MEN2B) are more often found as somatic than germline variants. Polycythaemia and PPGL can be features of somatic (rarely germline) HIF2A mutations. Importantly, HIF2A variants have been associated with mosaicism and, therefore, if an HIF2A variant is identified in an apparently sporadic tumour, it is recommended that efforts are made to further clarify the presence of the variants in other tissue types, to inform the extent of disease and the appropriate surveillance approach for the patient and relatives. Somatic mutations in HIF2A (which encodes the alpha subunit of the heterodimeric HIF- 2 hypoxia responsive transcription factor) usually occur at or adjacent to the proline residue (P531) that is critical for binding to the VHL gene product (pVHL). The pVHL protein has a key role in targeting the HIF2A protein for proteosomal degradation and somatic inactivating mutations in VHL and activating mutations in HIF2A will both result in stabilization of HIF2A and activation of hypoxic gene response pathway. Furthermore, SDHX and FH mutations are also associated with stabilization of HIF2A as both are associated with an inhibition of Pro531 hydroxylation which is re quired for pVHL binding and there are rare reports of PPGL patients with germline mutations in EGLN1/ PHD2 which encodes one of the proline hydroxylase enzymes that act on HIF2A.
A wide variety of genes have been reported to be somatically mutated in PPGL but not implicated in familial PPGL. These include activating mutations in the HRAS and BRAF proto- oncogenes and CSDE1 (a wnt- pathway regulator). In a comprehensive genomic study, a further wnt- pathway related abnormality, MAML3 fusion genes was identified in PPGL [13]. Other reported targets for somatic mutation include SETD2, FGFR1, TP53, ATRX, ARNT, IDH1, H3F3A, MET (see Figure 5.5.1.1). Though a large number of genes have been reported to harbour germline and/ or somatic mutations in PPGL, the number of driver mutations per tumour is low and in many cases only a single event is detected. Somatic HRAS mutations can be detected in about 10% of sporadic PPGL and, interestingly, it seems that somatic HRAS mutations and germline inherited PPGL gene mutations are usually mutually exclusive. It has therefore been suggested that somatic mutation profiling of PPGL to identify driver gene mutations can facilitate management of patients with PPGL. If a mutation in an inherited PPGL gene is identified, then germline sequencing can be under taken to determine if the mutation is somatic or germline. However, if a HRAS mutation is detected in the tumour then the risk of inherited disease will be low.
Fig1. Summary of findings from a comprehensive genomic analysis of phaeochromocytoma and paraganglioma by Fishbein et al.
