PCC/PGL are associated with a variety of hereditary syndromes, comprising genetic alterations in RET, NF1, VHL, and SDHx genes, the last 2 being involved in regulating the hypoxia pathway.
We hypothesised that PC/PGLs containing SDHx or VHL mutations, and succinate dehydrogenase (SDH)-deficient gastrointestinal stromal tumours (GISTs), would overexpress miR-210 relative to non-SDH or -VHL-mutated counterparts. miR-210 was analysed by quantitative PCR in i) 39 PC/PGLs, according to genotype (one SDHA, five SDHB, seven VHL, three NF1, seven RET, 15 sporadic, one unknown) and pathology (18 benign, eight atypical, 11 malignant, two unknown); ii) 18 GISTs, according to SDHB immunoreactivity (nine SDH-deficient and nine SDH-proficient) and iii) two novel SDHB-mutant neurosphere cell lines. miR-210 was higher in SDHx- or VHL-mutated PC/PGLs (7.6-fold) compared with tumours without SDHx or VHL mutations (P=0.0016). miR-210 was higher in malignant than in unequivocally benign PC/PGLs (P=0.05), but significance was lost when benign and atypical tumours were combined (P=0.08).
RNA expression patterns of HIF2A PGLs (n=6) from 2 patients were compared with normal adrenal medullas (n=8) and other hereditary pseudohypoxic PGLs (VHL: n=13, SDHB: n=15, and SDHD: n=14).
The finding of somatic mutations in VHL and RET in an additional 10-15% of tumors has brought the proportion of all patients with PCC and/or PGL due to a genetic disruption in these genes to approximately one half.
The dysfunction of genes involved in the cellular response to hypoxia, such as VHL, EGL nine homolog 1, and the succinate dehydrogenase (SDH) genes, leads to a direct abrogation of hypoxia inducible factor (HIF) degradation, resulting in a pseudo-hypoxic state implicated in PCC/PGL development.
Analysis of paraganglioma tissue revealed loss of the VHL wild-type allele in both tumors, indicating that in these tumors biallelic VHL gene inactivation occurred.
A large body of evidence supports the absence of mutations in SDH, RET and VHL genes, which suggests the existence of a yet unknown gene at the origin of this particular form of familial PGL.
Within the paediatric cohort, bilateral PCC (60% vs. 5%, p=0.002), PCC+sPGL (30% vs. 0%, p=0.03) and occurrence of a second PCC/PGL (30% vs. 0%, p=0.03) were significantly more frequent among children with VHL mutations than others.
The VHL gene product, pVHL, has multiple functions, but the best documented, and the one most clearly linked to tumor development, relates to its role as the substrate recognition module of a ubiquitin ligase complex that targets hypoxia-inducible factor (HIF) for destruction. pVHL function is often compromised in sporadic kidney cancers, and inhibitors of the HIF-responsive growth factor (vascular endothelial growth factor) are active against this disease. pVHL, by inhibiting atypical protein kinase C and hence JunB, also affects neuronal survival, as do the products of the other genes linked to familial pheochromocytoma or paraganglioma (NF1, RET, SDHB, SDHC, and SDHD).
This allowed for reconstructing the life history of individual tumors, identifying somatic mutations as well as copy-number loss of 3p and 11p (VHL subgroup), 1p (Cluster 2), and 17q (NF1 subgroup) as early events in PPGL tumorigenesis.
Transcriptome studies indeed revealed that pheochromocytomas and paragangliomas can be classified into two major clusters depending on their gene expression profile: Cluster 1 comprises samples associated with a hypoxic signature such as SDHx- and VHL-related tumors and cluster 2 includes RET, NF1, and TMEM127-mutated tumors, as well as most of sporadic tumors.
The current case of carotid body paraganglioma in patient with the 393C>A (N131K) missense mutation in the VHL gene, supports association of this specific mutation and VHL disease type 2, and suggests its correlation with susceptibility to paragangliomas.