Recent insights into the molecular pathogenesis of pheochromocytoma and paraganglioma

XAF1 promotes neuroblastoma tumor suppression and is required for KIF1Bβ-mediated apoptosis

Neuroblastoma is an aggressive, relapse-prone childhood tumor of the sympathetic nervous system. Current treatment modalities do not fully exploit the genetic basis between the different molecular subtypes and little is known about the targets discovered in recent mutational and genetic studies. Neuroblastomas with poor prognosis are often characterized by 1p36 deletion, containing the kinesin gene KIF1B. Its beta isoform, KIF1Bβ, is required for NGF withdrawal-dependent apoptosis, mediated by the induction of XIAP-associated Factor 1 (XAF1). Here, we showed that XAF1 low expression correlates with poor survival and disease status. KIF1Bβ deletion results in loss of XAF1 expression, suggesting that XAF1 is indeed a downstream target of KIF1Bβ. XAF1 silencing protects from NGF withdrawal and from KIF1Bβ-mediated apoptosis. Overexpression of XAF1 impairs tumor progression whereas knockdown of XAF1 promotes tumor growth, suggesting that XAF1 may be a candidate tumor suppressor in neuroblastoma and its associated pathway may be important for developing future interventions.

Pitfalls of Frozen Section Diagnosis for Paraganglioma: A Clinicopathologic Analysis and Review of the Literature

Few paraganglioma (PG) cases include frozen section diagnoses, and therefore, the accuracy of frozen section diagnosis for PG remains unknown. To better understand the histologic characteristics and pitfalls of frozen section findings for PG, 15 PG cases with frozen section diagnoses were selected from 12 articles through PubMed (1984-2015). In addition, we included 3 cases of PG for which intraoperative consultations were requested during a 5-year period (2012-2016) in 2 hospitals. Seven PGs were from the thyroid; 2 from the pancreas; 4 from the mediastinum, retroperitoneum, or mesentery; 1 from the middle ear; and 4 from the urinary bladder (UB). Out of 18 PGs, correct diagnoses were rendered in only 2 cases, and no thyroid or UB PGs were correctly diagnosed on intraoperative consultation. Thyroid PGs were frequently misdiagnosed as medullary thyroid carcinomas (4/7, 57%) and UB PGs were frequently misdiagnosed as malignancies.

Clinicopathologic Characteristics and Mutational Status of Succinate Dehydrogenase Genes in Paraganglioma of the Urinary Bladder: A Multi-Institutional Korean Study

Context.—

Because of the limited number of available primary bladder paraganglioma (PBPG) cases, the rates of succinate dehydrogenase (SDH) mutations and the clinicopathologic characteristics of SDH-deficient tumors have not been fully studied.

Objective.—

To define the clinicopathologic and molecular characteristics of PBPGs.

Design.—

A total of 52 PBPGs were collected retrospectively. SDHA and SDHB immunohistochemical stains were performed. In cases of SDHB expression loss, mutation analyses of SDHB, SDHC, and SDHD were performed.

Results.—

The clinicopathologic features were analyzed for 52 cases (M:F = 27:25), with a mean age of 56 years (range, 22–79 years). Tumor sizes were 0.5 to 8 cm (mean, 2.4 cm). Tumor necrosis was present in 5 of 52 cases (10%), involvement of muscularis propria in 41 (79%), and lymphovascular tumor invasion in 6 (12%). During a mean follow-up period of 41 months (range, 1–161 months), 3 of 52 patients (6%) developed metastases, but no one died from the disease. Immunohistochemistry for SDHA and SDHB showed that all cases were SDHA intact. Among them, 43 cases had intact SDHB, whereas 9 cases were SDHB deficient. Compared with the SDHB-intact cases, the SDHB-deficient cases were characterized by large tumor sizes (4.5 versus 1.9 cm; P < .001), a higher number of mitoses per 10 high-powered fields (2.6 versus 0.1; P = .002), and frequent lymphovascular tumor invasion (33% versus 7%; P = .02) and metastases (22% versus 2%; P = .02). Mutational analyses for SDHB, SDHC, and SDHD were performed in 9 SDHB-deficient cases. Among them, 6 cases were successfully sequenced and revealed SDHB mutations only.

Conclusions.—

Large tumor size, a higher number of mitoses, and the presence of lymphovascular tumor invasion and SDHB mutations suggest malignant paraganglioma.

Imaging Features of Various Adrenal Neoplastic Lesions on Radiologic and Nuclear Medicine Imaging

OBJECTIVE

The purpose of this review is to describe the features of diverse adrenal neoplastic lesions on radiologic and nuclear medicine imaging.

CONCLUSION

Various neoplastic lesions with or without malignant potential can occur in the adrenal gland. Knowledge of imaging features of adrenal lesions on radiologic and nuclear medicine imaging will facilitate differential diagnosis and assessment of malignant potential.

An update on the application of newly described immunohistochemical markers in soft tissue pathology

CONTEXT

During the last 5 to 10 years, significant progress has been made in the molecular characterization of soft tissue tumors, predominantly with the identification of recurrent translocations or amplification of certain genes in different tumor types. Alongside this, translational efforts have identified many novel and diagnostically useful immunohistochemical markers for many of these tumor types.

OBJECTIVE

This article reviews a select group of recently described immunohistochemical markers of particular use in the evaluation of mesenchymal neoplasms; the underlying biology of the protein product, practical utility, and limitations of each marker are discussed in detail.

DATA SOURCES

Literature review, authors' research data, and personal practice experience serve as sources.

CONCLUSIONS

There are many diagnostically useful immunohistochemical markers to help confirm the diagnosis of many different soft tissue tumor types, some of which have reduced the need for additional, and more costly, studies, such as fluorescence in situ hybridization. However, no one marker is 100% specific for a given tumor, and knowledge of potential pitfalls and overlap in patterns of staining among other tumor types is crucial to ensure the appropriate application of these markers in clinical practice.

From the radiologic pathology archives: Adrenal tumors and tumor-like conditions in the adult: radiologic-pathologic correlation

Advanced imaging often reveals adrenal tumors and tumor-like conditions in both symptomatic and asymptomatic patients. When adrenal disease is clinically suspected, cross-sectional imaging can be helpful in evaluating the etiology of the patient's symptoms. When adrenal disease is incidentally identified, what the clinician and patient really want to know is whether the findings are benign or malignant, as this ultimately will affect their next step in management. Using radiologic-pathologic correlation, we broadly classify common, uncommon, and rare tumors and tumor-like conditions that can occur in the adrenal as benign or malignant. This classification follows predominant trends in observed biologic behavior while acknowledging those tumors that may behave in the minority in an unpredictable manner. We review the clinical background and presentation of functional adrenal tumors including Conn syndrome, Cushing syndrome, and catecholamine-secreting tumors, as well as their relationship with adrenal anatomy. We discuss a variety of benign tumors, including adrenal cortical adenoma (including oncocytoma) and pheochromocytoma, as well as uncommonly and rarely encountered tumors such as myelolipoma, hemangioma, lymphangioma, schwannoma, ganglioneuroma, and adenomatoid tumor. A variety of tumefactive but nonneoplastic lesions are addressed, including adrenal cortical hyperplasia, adrenal hemorrhage, adrenal cysts, and infections. Malignant tumors discussed include adrenal cortical carcinoma, the rare malignant pheochromocytoma, lymphoma, metastases, and sarcomas. For each tumor and tumor-like lesion, the clinical presentation, epidemiology, key imaging findings, diagnostic differential considerations, and management options are briefly addressed. Finally, an approach to the workup of suspected or incidentally discovered tumors is presented based on a selected literature survey and our clinical experience. Radiologists play an important role in identification and diagnosis of adrenal tumors and tumor-like conditions in both symptomatic and asymptomatic patients.

A conditional mouse mutant in the tumor suppressor SdhD gene unveils a link between p21(WAF1/Cip1) induction and mitochondrial dysfunction

Mutations in mitochondrial complex II (MCII; succinate dehydrogenase, Sdh) genes cause familiar pheochromocytoma/paraganglioma tumors. Several mechanisms have been proposed to account for Sdh-mutation-induced tumorigenesis, the most accepted of which is based on the constitutive expression of the hypoxia-inducible factor 1α (Hif1α) at normal oxygen tension, a theory referred to as “pseudo-hypoxic drive”. Other molecular processes, such as oxidative stress, apoptosis, or chromatin remodeling have been also proposed to play a causative role. Nevertheless, the actual contribution of each of these mechanisms has not been definitively established. Moreover, the biological factors that determine the tissue-specificity of these tumors have not been identified. In this work, we made use of the inducible SDHD-ESR mouse, a conditional mutant in the SdhD gene, which encodes the small subunit of MCII, and that acts as a tumor suppressor gene in humans. The analysis of the Hif1α pathway in SDHD-ESR tissues and in two newly derived cell lines after complete SdhD loss -a requirement for hereditary paraganglioma type-1 tumor formation in humans- partially recapitulated the “pseudo-hypoxic” response and rendered inconsistent results. Therefore, we performed microarray analysis of adrenal medulla and kidney in order to identify other early gene expression changes elicited by SdhD deletion. Our results revealed that each mutant tissue displayed different variations in their gene expression profiles affecting to different biological processes. However, we found that the Cdkn1a gene was up-regulated in both tissues. This gene encodes the cyclin-dependent kinase inhibitor p21^WAF1/Cip1, a factor implicated in cell cycle, senescence, and cancer. The two SDHD-ESR cell lines also showed accumulation of this protein. This new and unprecedented evidence for a link between SdhD dysfunction and p21^WAF1/Cip1 will open new avenues for the study of the mechanisms that cause tumors in Sdh mutants. Finally, we discuss the actual role of Hif1α in tumorigenesis.

Somatic NF1 inactivation is a frequent event in sporadic pheochromocytoma

Germline mutations in the RET, SDHA, SDHAF2, SDHB, SDHC, SDHD, MAX, TMEM127, NF1 or VHL genes are identified in about 30% of patients with pheochromocytoma or paraganglioma and somatic mutations in RET, VHL or MAX genes are reported in 17% of sporadic tumors. In the present study, using mutation screening of the NF1 gene, mapping of chromosome aberrations by single nucleotide polymorphism (SNP) array, microarray-based expression profiling and immunohistochemistry (IHC), we addressed the implication of NF1 somatic alterations in pheochromocytomas and paragangliomas. We studied 53 sporadic tumors, selected because of their classification with RET/NF1/TMEM127-related tumors by genome wide expression studies, as well as a second set of 11 independent tumors selected on their low individual levels of NF1 expression evaluated by microarray. Direct sequencing of the NF1 gene in tumor DNA identified the presence of an inactivating NF1 somatic mutation in 41% (25/61) of analyzed sporadic tumors, associated with loss of the wild-type allele in 84% (21/25) of cases. Gene expression signature of NF1-related tumors highlighted the downregulation of NF1 and the major overexpression of SOX9. Among the second set of 11 tumors, two sporadic tumors carried somatic mutations in NF1 as well as in another susceptibility gene. These new findings suggest that NF1 loss of function is a frequent event in the tumorigenesis of sporadic pheochromocytoma and strengthen the new concept of molecular-based targeted therapy for pheochromocytoma or paraganglioma.

Signaling pathways in pheochromocytomas and paragangliomas: prospects for future therapies

There is currently no completely effective therapy available for metastatic pheochromocytomas or paragangliomas. Increasing understanding of the germline and somatic mutations leading to pheochromocytoma and paraganglioma development has revealed crucial insights into the molecular pathology of these tumors. A detailed understanding of the molecular pathway alterations giving rise to pheochromocytomas and paragangliomas should allow for the exploration and development of new effective molecular-targeted therapy options for this rare but frequently fatal malignancy. Molecular analysis has shown that pheochromocytoma/paraganglioma-promoting gene mutations can be divided into two major groups-clusters 1 and 2-following two different routes to tumorigenesis. Cluster 1 mutations are associated with pseudohypoxia and aberrant VEGF signaling while cluster 2 mutations are associated with abnormal activation of kinase signaling pathways such as PI3 kinase/AKT, RAS/RAF/ERK, and mTORC1/p70S6K suggesting relevant targets for novel molecular-targeted therapy approaches which will be discussed in detail in this chapter.

Pheochromocytomas and paragangliomas: assessment of malignant potential

Pheochromocytomas and paragangliomas (PPGLs) are rare catecholamine-secreting tumors which arise from the adrenal glands or sympathetic neuronal tissue. Malignant transformation of these tumors occurs in a significant proportion and may therefore lower overall survival rates. In patients with PPGLs it is impossible to identify malignant disease without the presence of metastatic disease, something which can occur as long as 20 years after initial surgery. Early identification of malignant disease would necessitate a more aggressive treatment approach, something which may result in better disease outcome. We have therefore reviewed possible predictors of malignancy and current developments in order to help clinicians to swiftly assess malignant potential in patients with PPGLs. Currently, there is no absolute marker which can objectively reflect malignant potential. Tumor size is the most reliable predictor and should therefore be used as the baseline characteristic. The combination of various clinical markers (extra-adrenal disease and post-operative hypertension), biochemical markers (high dopamine, high norepinephrine and epinephrine to total catecholamine ratio) and/or histological markers (SNAIL, microRNAs and/or microarray results) can raise or lower the suspicion of malignancy. Furthermore, we discuss how clinical markers may affect biochemical results linked to malignancy, how biochemical results may distinguish hereditary syndromes, the role of imaging in determining malignant potential and tumor detection, and recent results of proposed histological markers.

Cancer and altered metabolism: potential importance of hypoxia-inducible factor and 2-oxoglutarate-dependent dioxygenases

Hypoxia-inducible factor (HIF) deregulation contributes to the Warburg effect. HIF consists of an unstable α subunit and a stable β subunit. In the presence of oxygen, HIFα becomes prolyl hydroxylated by members of the EglN (also called PHD) family, leading to its proteasomal degradation. Under hypoxic conditions, EglN activity is diminished and HIF levels rise. EglN1 is the primary HIF prolyl hydroxylase with EglN2 and EglN3 playing compensatory roles under certain conditions. EglN2 and EglN3 also appear to play HIF-independent roles in regulating cell proliferation and apoptosis, respectively. The EglNs belong to a large family of 2-oxoglutarate-dependent dioxygenases that includes the TET DNA hydroxymethylases and JmjC-containing histone demethylases. Members of this superfamily can be inhibited by endogenous metabolites, including fumarate and succinate, which accumulate in tumors that have fumarate hydratase (FH) or succinate dehydrogenase (SDH) mutations, respectively, as well as by the 2-hydroxyglutarate detected in isocitrate dehydrogenase (IDH) mutant tumors. 2-Oxoglutarate-dependent dioxygenases therefore provide a link between altered metabolism and cancer.

JunB promotes cell invasion and angiogenesis in VHL-defective renal cell carcinoma

Inactivation of the von Hippel-Lindau (VHL) tumor-suppressor gene causes both hereditary and sporadic clear-cell renal-cell carcinoma (ccRCC). Although the best-characterized function of the VHL protein (pVHL) is regulation of hypoxia-inducible factor-α (HIFα), pVHL also controls the development of pheochromocytoma through HIF-independent pathways by regulating JunB. However, it is largely unknown how these pathways contribute to the development and progression of ccRCC. In the present study, we confirmed that JunB was upregulated in VHL-defective ccRCC specimens by immunostaining. Short-hairpin RNA (shRNA)-mediated knockdown of JunB in 786-O and A498 VHL null ccRCC cells suppressed their invasiveness. In addition, JunB knockdown significantly repressed tumor growth and microvessel density in xenograft tumor assays. Conversely, forced expression of wild-type, but not dimerization-defective, JunB in a VHL-restored 786-O subclone promoted invasion in vitro and tumor growth and vessel formation in vivo. Quantitative PCR array analysis revealed that JunB regulated multiple genes relating to tumor invasion and angiogenesis such as matrix metalloproteinase-2 (MMP-2), MMP-9 and chemokine (C-C motif) ligand-2 (CCL2) in 786-O cells. JunB knockdown in these cells reduced the proteolytic activity of both MMPs in gelatin zymography and the amount of CCL2 in the culture supernatant. Moreover, shRNA-mediated knockdown of MMP-2 or inhibition of CCL2 activity with a neutralizing antibody repressed xenograft tumor growth and angiogenesis. Collectively, these results suggest that JunB promotes tumor invasiveness and enhances angiogenesis in VHL-defective ccRCCs.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

Phaeochromocytomas and sympathetic paragangliomas

BACKGROUND

About 24 per cent of phaeochromocytomas (PCCs) and sympathetic paragangliomas (sPGLs) appear in familial cancer syndromes, including multiple endocrine neoplasia type 2, von Hippel-Lindau disease, neurofibromatosis type 1 and PCC-paraganglioma syndrome. Identification of these syndromes is of prime importance for patients and their relatives. Surgical resection is the treatment of choice for both PCC and sPGL, but controversy exists about the management of patients with bilateral or multiple tumours.

METHODS

Relevant medical literature from PubMed, Ovid and Embase websites until 2009 was reviewed for articles on PCC, sPGL, hereditary syndromes and their treatment.

DISCUSSION

Genetic testing for these syndromes should become routine clinical practice for those with PCC or sPGL. Patients should be referred to a clinical geneticist. Patients and family members with proven mutations should be entered into a standardized screening protocol. The preferred treatment of PCC and PGL is surgical resection; to avoid the lifelong consequences of bilateral adrenalectomy, cortex-sparing adrenalectomy is the treatment of choice.

An immunohistochemical procedure to detect patients with paraganglioma and phaeochromocytoma with germline SDHB, SDHC, or SDHD gene mutations: a retrospective and prospective analysis

BACKGROUND

Phaeochromocytomas and paragangliomas are neuro-endocrine tumours that occur sporadically and in several hereditary tumour syndromes, including the phaeochromocytoma-paraganglioma syndrome. This syndrome is caused by germline mutations in succinate dehydrogenase B (SDHB), C (SDHC), or D (SDHD) genes. Clinically, the phaeochromocytoma-paraganglioma syndrome is often unrecognised, although 10-30% of apparently sporadic phaeochromocytomas and paragangliomas harbour germline SDH-gene mutations. Despite these figures, the screening of phaeochromocytomas and paragangliomas for mutations in the SDH genes to detect phaeochromocytoma-paraganglioma syndrome is rarely done because of time and financial constraints. We investigated whether SDHB immunohistochemistry could effectively discriminate between SDH-related and non-SDH-related phaeochromocytomas and paragangliomas in large retrospective and prospective tumour series.

METHODS

Immunohistochemistry for SDHB was done on 220 tumours. Two retrospective series of 175 phaeochromocytomas and paragangliomas with known germline mutation status for phaeochromocytoma-susceptibility or paraganglioma-susceptibility genes were investigated. Additionally, a prospective series of 45 phaeochromocytomas and paragangliomas was investigated for SDHB immunostaining followed by SDHB, SDHC, and SDHD mutation testing.

FINDINGS

SDHB protein expression was absent in all 102 phaeochromocytomas and paragangliomas with an SDHB, SDHC, or SDHD mutation, but was present in all 65 paraganglionic tumours related to multiple endocrine neoplasia type 2, von Hippel-Lindau disease, and neurofibromatosis type 1. 47 (89%) of the 53 phaeochromocytomas and paragangliomas with no syndromic germline mutation showed SDHB expression. The sensitivity and specificity of the SDHB immunohistochemistry to detect the presence of an SDH mutation in the prospective series were 100% (95% CI 87-100) and 84% (60-97), respectively.

INTERPRETATION

Phaeochromocytoma-paraganglioma syndrome can be diagnosed reliably by an immunohistochemical procedure. SDHB, SDHC, and SDHD germline mutation testing is indicated only in patients with SDHB-negative tumours. SDHB immunohistochemistry on phaeochromocytomas and paragangliomas could improve the diagnosis of phaeochromocytoma-paraganglioma syndrome.

FUNDING

The Netherlands Organisation for Scientific Research, Dutch Cancer Society, Vanderes Foundation, Association pour la Recherche contre le Cancer, Institut National de la Santé et de la Recherche Médicale, and a PHRC grant COMETE 3 for the COMETE network.

Review: the role of neural crest cells in the endocrine system

The neural crest is a pluripotent population of cells that arises at the junction of the neural tube and the dorsal ectoderm. These highly migratory cells form diverse derivatives including neurons and glia of the sensory, sympathetic, and enteric nervous systems, melanocytes, and the bones, cartilage, and connective tissues of the face. The neural crest has long been associated with the endocrine system, although not always correctly. According to current understanding, neural crest cells give rise to the chromaffin cells of the adrenal medulla, chief cells of the extra-adrenal paraganglia, and thyroid C cells. The endocrine tumors that correspond to these cell types are pheochromocytomas, extra-adrenal paragangliomas, and medullary thyroid carcinomas. Although controversies concerning embryological origin appear to have mostly been resolved, questions persist concerning the pathobiology of each tumor type and its basis in neural crest embryology. Here we present a brief history of the work on neural crest development, both in general and in application to the endocrine system. In particular, we present findings related to the plasticity and pluripotency of neural crest cells as well as a discussion of several different neural crest tumors in the endocrine system.

The von Hippel-Lindau tumour suppressor protein: O2 sensing and cancer

The von Hippel-Lindau disease is caused by inactivating germline mutations of the VHL tumour suppressor gene and is associated with an increased risk of a variety of tumours in an allele-specific manner. The role of the heterodimeric transcription factor hypoxia-inducible factor (HIF) in the pathogenesis of VHL-defective tumours has been more firmly established during the past 5 years. In addition, there is now a greater appreciation of HIF-independent VHL functions that are relevant to tumour development, including maintenance of the primary cilium, regulation of extracellular matrix formation and turnover, and modulation of cell death in certain cell types following growth factor withdrawal or in response to other forms of stress.

Development of differential diagnosis for benign and malignant pheochromocytomas

Unlike common malignant tumors, malignant pheochromocytomas cannot be definitely diagnosed using histological features. This unique nature of pheochromocytomas provides a valuable model that may promote the investigation of the mechanism of other common malignant tumors where similar frameworks are not available. Studies on malignant pheochromocytomas should benefit not only the individuals with pheochromocytomas but those with other tumors. A review on the development of differentiating diagnosis between malignant and benign pheochromocytomas in imaging studies, biological fluid examinations, pathological examinations, molecular markers and genome studies, was updated in the hopes of guiding the next studies of pheochromcytomas.

The kinesin KIF1Bbeta acts downstream from EglN3 to induce apoptosis and is a potential 1p36 tumor suppressor

VHL, NF-1, c-Ret, and Succinate Dehydrogenase Subunits B and D act on a developmental apoptotic pathway that is activated when nerve growth factor (NGF) becomes limiting for neuronal progenitor cells and requires the EglN3 prolyl hydroxylase as a downstream effector. Germline mutations of these genes cause familial pheochromocytoma and other neural crest-derived tumors. Using an unbiased shRNA screen we found that the kinesin KIF1Bbeta acts downstream from EglN3 and is both necessary and sufficient for neuronal apoptosis when NGF becomes limiting. KIF1Bbeta maps to chromosome 1p36.2, which is frequently deleted in neural crest-derived tumors including neuroblastomas. We identified inherited loss-of-function KIF1Bbeta missense mutations in neuroblastomas and pheochromocytomas and an acquired loss-of-function mutation in a medulloblastoma, arguing that KIF1Bbeta is a pathogenic target of these deletions.

The von hippel-lindau tumor suppressor protein: an update

Inactivation of the von Hippel-Lindau (VHL) tumor suppressor has been linked to a variety of tumors, including clear cell renal carcinoma, retinal and cerebellar hemangioblastoma, and pheochromocytoma. The best documented function of VHL protein (pVHL) relates to its ability to target the hypoxia-inducible transcription factor (HIF) for polyubiquitylation and proteasomal degradation. This chapter focuses on studies published over the past 2 years related to pVHL. These studies include those describing genetically engineered mice that were used to interrogate the relationship between pVHL and HIF in vivo and cell culture studies that underscore the importance of pVHL in epithelial differentiation and maintenance of the primary cilium. In addition, recent work suggests that pVHL regulates neuronal apoptosis in an HIF-independent manner, and this activity is linked to the risk of developing pheochromocytoma.