Association of germline mutations in the fumarate hydratase gene and uterine fibroids in women with hereditary leiomyomatosis and renal cell cancer

OBJECTIVE

To investigate the risk of uterine fibroids and other reproductive risk factors in women with hereditary leiomyomatosis and renal cell cancer (HLRCC).

DESIGN

Case-control study.

SETTING

National Institutes of Health, Rockville, Maryland. Patients A family-based case-control study was conducted between July 1, 2004, and June 30, 2006, including 105 women from families with HLRCC ascertained throughout North America. A telephone interview was conducted with all participants using a standardized questionnaire that elicited information about their menstrual, pregnancy, uterine fibroid, and hormonal contraceptive use history. Diagnosis of uterine fibroids was confirmed by pathologic diagnosis and by medical record review. DNA was extracted from blood samples and was screened for germline mutations in the fumarate hydratase (FH) gene.

MAIN OUTCOME MEASURES

FH germline mutation status, presence of uterine fibroids, age at diagnosis, and symptoms and treatment of uterine fibroids.

RESULTS

Of 105 women, 77 reported a history of uterine fibroids. Regardless of uterine fibroid status, 75 of 105 women had a germline mutation in FH (FH(mut) positive). The risk of uterine fibroids in FH(mut)-positive women was statistically significantly increased compared with that in FH(mut)-negative women (odds ratio [OR], 7.6; 95% confidence interval [CI], 2.9-20.0), as it was among women clinically affected with HLRCC compared with those clinically unaffected with HLRCC (8.6; 3.1-24.0). The median age at uterine fibroid diagnosis for FH(mut)-positive women (28 years) was significantly younger than that for FH(mut)-negative women (38 years) (P =.03). Women with a germline mutation in FH or clinically affected with HLRCC reported younger age at menarche (P < .004) compared with FH(mut)-negative women (P = .02) or women who were clinically unaffected with HLRCC. Women with HLRCC were more likely to have had treatment for uterine fibroids (OR, 4.6; 95% CI, 1.4-15.8), including hysterectomy (P =.02) at an earlier age compared with women who were clinically unaffected with HLRCC.

CONCLUSIONS

This study provides the first evidence (to our knowledge) that women with germline mutations in FH and with clinical HLRCC have an increased risk of developing uterine fibroids. These women also have a younger age at uterine fibroid diagnosis and are more likely to have treatment for uterine fibroids at a younger age than women without HLRCC in their families.

Differential expression of the regulated catecholamine secretory pathway in different hereditary forms of pheochromocytoma

Pheochromocytomas in patients with von Hippel-Lindau (VHL) syndrome and multiple endocrine neoplasia type 2 (MEN 2) differ in the types and amounts of catecholamines produced and the resulting signs and symptoms. We hypothesized the presence of different processes of catecholamine release reflecting differential expression of components of the regulated secretory pathway among the two types of hereditary tumors. Differences in catecholamine secretion from tumors in patients with VHL syndrome (n = 47) and MEN 2 (n = 32) were examined using measurements of catecholamines in tumor tissue, urine, and plasma, the last of which was under baseline conditions in all subjects and in a subgroup of patients who received intravenous glucagon to provoke catecholamine release. Microarray and proteomics analyses, quantitative PCR, and Western blotting were used to assess expression of tumor tissue secretory pathway components. The rate constant for baseline catecholamine secretion was 20-fold higher in VHL than in MEN 2 tumors (0.359 +/- 0.094 vs. 0.018 +/- 0.009 day(-1)), but catecholamine release was responsive only to glucagon in MEN 2 tumors. Compared with tumors from MEN 2 patients, those from VHL patients were characterized by reduced expression of numerous components of the regulated secretory pathway (e.g., SNAP25, syntaxin, rabphilin 3A, annexin A7, calcium-dependent secretion activator). The mutation-dependent differences in expression of secretory pathway components indicate a more mature regulated secretory pathway in MEN 2 than VHL tumors. These data provide a unique mechanistic link to explain how variations in the molecular machinery governing exocytosis may contribute to clinical differences in the secretion of neurotransmitters or hormones and the subsequent presentation of a disease.

Concurrent robotic partial adrenalectomy and extra-adrenal pheochromocytoma resection in a pediatric patient with von Hippel-Lindau disease

Laparoscopic partial adrenalectomy is a surgical option for select patients with hereditary pheochromocytoma. We present a case of a pediatric patient with von Hippel-Lindau disease (VHL) and both an adrenal pheochromocytoma and an extra-adrenal pheochromocytoma, who underwent concurrent partial adrenalectomy and extra-adrenal pheochromocytoma resection utilizing robotic assistance. To the best of our knowledge, this is the first report of partial adrenalectomy with concurrent extra-adrenal pheochromocytoma resection.

Genetic basis for kidney cancer: opportunity for disease-specific approaches to therapy

BACKGROUND

Kidney cancer is not a homogenous entity; it is comprised of many different tumor types, with different biologies and molecular mechanisms leading to disease and therefore different treatment approaches.

OBJECTIVE

To describe the genetic basis and biochemical pathways underlying inherited forms of renal cancer, specifically in four described syndromes (von Hippel-Lindau [VHL], hereditary papillary renal cancer [HPRC], Birt-Hogg-Dubé [BHD] and hereditary leiomyomatosis renal cell carcinoma [HLRCC]), and to elucidate how the understanding of these diseases enables the possibility of disease-specific approaches to therapy.

METHODS

A systematic review of the published literature on inherited and sporadic forms of renal cancer was performed.

CONCLUSION

Understanding of the biology and mechanisms of different forms of kidney cancer provides an opportunity for development of new treatment options.

VHL loss of function and its impact on oncogenic signaling networks in clear cell renal cell carcinoma

Loss of von Hippel-Lindau tumor suppressor gene function occurs in familial and most sporadic clear cell renal cell carcinoma, resulting in the aberrant expression of genes that control cell proliferation, metabolism, invasion and angiogenesis. The molecular mechanisms by which loss of function leads to tumorigenesis are not yet fully defined. The von Hippel-Lindau gene product is part of an ubiquitin ligase complex that targets hypoxia inducible factors for polyubiquitination and proteasomal degradation, linking hypoxia response genes to renal cell carcinoma oncogenesis. Loss von Hippel-Lindau gene function also promotes cell invasiveness in response to hepatocyte growth factor, an important regulator of kidney development and renal homeostasis. Increased cell invasiveness is mediated by another ubiquitin ligase target with relevance to the molecular pathogenesis of renal cell carcinoma: beta-catenin. This discovery and other recent insights into kidney cancer oncogenesis implicate convergent developmental and homeostatic signaling pathways in tumorigenesis, tumor invasiveness and metastasis.

Tissue-based research in kidney cancer: current challenges and future directions

The past several years have seen unprecedented advances in the application of various therapeutic strategies for the treatment of patients with renal cancer. The availability of active immunotherapy, antiangiogenic therapy, and targeted therapy for this disease has brought front and center issues related to choosing the appropriate treatment for particular patient populations. It is increasingly evident that the most promising treatment selection strategies will incorporate identifying specific features of the tumor itself. To facilitate this move toward personalized medicine, it is critically important to establish some standard principles for renal cancer tissue collection, preparation, and analysis for translational research studies. In this article, we identify and discuss some critical issues related to tissue-based kidney cancer research. We focus on five major areas as follows: (a) surgical and image-guided techniques for tissue collection; (b) quality control of specimen collection, processing, storage, and review; (c) issues related to analysis of paraffin embedded tissues; (d) genomic studies; and (e) assessment of reproducibility of assays across institutions. In addition, some practical implementation strategies are proposed. Although many of the topics discussed are specific for renal cancer, several are also relevant to tissue based biomarker investigations in a broad array of malignancies.

Neurologic manifestations of von Hippel-Lindau disease

von Hippel-Lindau disease (VHL) is an autosomal-dominant neoplasia syndrome that is the result of a germline mutation of the VHL tumor suppressor gene on the short arm of chromosome 3. Patients with VHL are predisposed to develop lesions of the central nervous system and viscera. Central nervous system lesions include hemangioblastomas (the most common tumor in VHL) and endolymphatic sac tumors (ELSTs). Visceral manifestations include renal carcinomas and cysts, pancreatic neuroendocrine tumors and cysts, pheochromocytomas, and cystadenomas of the reproductive adnexal organs. Despite their benign pathology, hemangioblastomas and ELSTs are a frequent cause of morbidity and mortality in patients with VHL. Recent molecular biologic investigations into these VHL-associated central nervous system lesions provide new insight into their origin and development. Emerging data from serial imaging and clinical surveillance protocols provide insight into the natural history of these lesions. Because of the dissimilar pathobiology and clinical course between hemangioblastomas and ELSTs, the optimal management strategies for these neurologic manifestations of VHL are very different.

Improved identification of von Hippel-Lindau gene alterations in clear cell renal tumors

PURPOSE

To provide a comprehensive, thorough analysis of somatic mutation and promoter hypermethylation of the von Hippel-Lindau (VHL) gene in the cancer genome, unique to clear cell renal cancer (ccRCC). Identify relationships between the prevalence of VHL gene alterations and alteration subtypes with patient and tumor characteristics.

EXPERIMENTAL DESIGN

As part of a large kidney cancer case-control study conducted in Central Europe, we analyzed VHL mutations and promoter methylation in 205 well-characterized, histologically confirmed patient tumor biopsies using a combination of sensitive, high-throughput methods (endonuclease scanning and Sanger sequencing) and analysis of 11 CpG sites in the VHL promoter.

RESULTS

We identified mutations in 82.4% of cases, the highest VHL gene mutation prevalence reported to date. Analysis of 11 VHL promoter CpG sites revealed that 8.3% of tumors were hypermethylated and all were mutation negative. In total, 91% of ccRCCs exhibited alteration of the gene through genetic or epigenetic mechanisms. Analysis of patient and tumor characteristics revealed that certain mutation subtypes were significantly associated with Fuhrman nuclear grade, metastasis, node positivity, and self-reported family history of RCC.

CONCLUSION

Detection of VHL gene alterations using these accurate, sensitive, and practical methods provides evidence that the vast majority of histologically confirmed ccRCC tumors possess genetic or epigenetic alteration of the VHL gene and support the hypothesis that VHL alteration is an early event in ccRCC carcinogenesis. These findings also indicate that VHL molecular subtypes can provide a sensitive marker of tumor heterogeneity among histologically similar ccRCC cases for etiologic, prognostic, and translational studies.

Feasibility and outcomes of repeat partial nephrectomy

PURPOSE

Despite the proven efficacy of nephron sparing surgery, patients with hereditary renal cancer remain at risk for tumor recurrence. Management options for recurrent tumors include completion nephrectomy, ablation and repeat partial nephrectomy. We examine the feasibility and outcomes of repeat partial nephrectomy performed on the same renal unit.

MATERIALS AND METHODS

We retrospectively reviewed the records of 51 attempted repeat partial nephrectomy procedures in 47 patients from 1992 to 2006. Demographic information as well as intraoperative, perioperative and renal functional outcome data were collected. Comparison of preoperative and postoperative renal function was performed using the 2-tailed t test.

RESULTS

Major perioperative complications or reoperations occurred in 10 of 51 (19.6%) cases that included 1 perioperative mortality (1.9%). In cases of successful repeat partial nephrectomy there was a statistically significant increase in postoperative serum creatinine (1.35 vs 1.16 mg/dl, p <0.05), and a significant decrease in creatinine clearance (84.6 vs 95.3 ml per minute, p = 0.05) and renogram split function (52.3% vs 54.8%, p <0.05). Two patients required long-term hemodialysis (3.9%). Of the 51 renal units 10 (19.6%) required subsequent operations for additional local recurrence or de novo tumor formations with a median time to subsequent surgery of 50 months. Of 47 patients 46 are alive at a median followup of 56 months.

CONCLUSIONS

Repeat partial nephrectomy is technically feasible. Although there is a statistically significant decrease in postoperative renal functional studies, most patients retained sufficient function to avoid hemodialysis. Repeat partial nephrectomy may provide acceptable oncological control despite the anticipated development of locally recurrent or de novo tumors.

The UOK 257 cell line: a novel model for studies of the human Birt-Hogg-Dubé gene pathway

The establishment, characterization, and tumorigenicity of a new epithelial cell line (UOK 257) derived from human renal carcinoma of an individual with Birt-Hogg-Dubé (BHD) syndrome are reported. Unlike other established renal tumor cell lines from sporadic renal cell carcinoma, this is the first established renal tumor cell line of BHD, an inheritable neoplastic syndrome. The isolated tumor cells display loss of contact inhibition in vitro, and produce subcutaneous tumors in mouse xenografts. Histopathologic, ultrastructural, and cytogenetic characterizations of the established tumor cells are reported. Cytogenetic analysis using spectral karyotyping on UOK 257 cells revealed 17p loss and a near-triploid and aneuploid karyotype with multiple fluorescence in situ hybridization analysis using a locus-specific gene probe for MYC. The result demonstrates that the established tumor cells consist of two cell populations, one containing four and one containing five copies of the MYC oncogene.

Identification and characterization of a novel folliculin-interacting protein FNIP2

Birt-Hogg-Dube' syndrome characterized by increased risk for renal neoplasia is caused by germline mutations in the BHD/FLCN gene encoding a novel tumor suppressor protein, folliculin(FLCN), which interacts with FNIP1 and 5'-AMP-activated protein kinase(AMPK). Here we report the identification and characterization of a novel FNIP1 homolog FNIP2 that also interacts with FLCN and AMPK. C-terminally-deleted FLCN mutants, similar to those produced by naturally-occurring germline mutations in BHD patients, were unable to bind FNIP2. These data taken together with our previous results that demonstrated FNIP1 binding to the C-terminus of FLCN suggest that FLCN tumor suppressor function may be facilitated by interactions with both FNIP1 and FNIP2 through its C-terminus. Furthermore, we demonstrate that FNIP1 and FNIP2 are able to form homo- or heteromeric multimers suggesting that they may function independently or cooperatively with FLCN. Differential expression of FNIP1 and FNIP2 transcripts in some normal tissues may indicate tissue specificity for these homologs. Interestingly FNIP1 and FNIP2 were oppositely expressed in human clear cell renal cell carcinoma (RCC), and coordinately expressed in chromophobe RCC and oncocytoma, suggesting their differential function in different histologic variants of RCC.

BHD mutations, clinical and molecular genetic investigations of Birt-Hogg-Dubé syndrome: a new series of 50 families and a review of published reports

BACKGROUND

Birt-Hogg-Dubé syndrome (BHDS) (MIM 135150) is an autosomal dominant predisposition to the development of follicular hamartomas (fibrofolliculomas), lung cysts, spontaneous pneumothorax, and kidney neoplasms. Germline mutations in BHD are associated with the susceptibility for BHDS. We previously described 51 BHDS families with BHD germline mutations.

OBJECTIVE

To characterise the BHD mutation spectrum, novel mutations and new clinical features of one previously reported and 50 new families with BHDS.

METHODS

Direct bidirectional DNA sequencing was used to screen for mutations in the BHD gene, and insertion and deletion mutations were confirmed by subcloning. We analysed evolutionary conservation of folliculin by comparing human against the orthologous sequences.

RESULTS

The BHD mutation detection rate was 88% (51/58). Of the 23 different germline mutations identified, 13 were novel consisting of: four splice site, three deletions, two insertions, two nonsense, one deletion/insertion, and one missense mutation. We report the first germline missense mutation in BHD c.1978A>G (K508R) in a patient who presented with bilateral multifocal renal oncocytomas. This mutation occurs in a highly conserved amino acid in folliculin. 10% (5/51) of the families had individuals without histologically confirmed fibrofolliculomas. Of 44 families ascertained on the basis of skin lesions, 18 (41%) had kidney tumours. Patients with a germline BHD mutation and family history of kidney cancer had a statistically significantly increased probability of developing renal tumours compared to patients without a positive family history (p = 0.0032). Similarly, patients with a BHD germline mutation and family history of spontaneous pneumothorax had a significantly increased greater probability of having spontaneous pneumothorax than BHDS patients without a family history of spontaneous pneumothorax (p = 0.011). A comprehensive review of published reports of cases with BHD germline mutation is discussed.

CONCLUSION

BHDS is characterised by a spectrum of mutations, and clinical heterogeneity both among and within families.

Kidney-targeted Birt-Hogg-Dube gene inactivation in a mouse model: Erk1/2 and Akt-mTOR activation, cell hyperproliferation, and polycystic kidneys

BACKGROUND

Patients with Birt-Hogg-Dubé (BHD) syndrome harbor germline mutations in the BHD tumor suppressor gene that are associated with an increased risk for kidney cancer. BHD encodes folliculin, a protein that may interact with the energy- and nutrient-sensing 5'-AMP-activated protein kinase-mammalian target of rapamycin (AMPK-mTOR) signaling pathways.

METHODS

We used recombineering methods to generate mice with a conditional BHD allele and introduced the cadherin 16 (KSP)-Cre transgene to target BHD inactivation to the kidney. Kidney cell proliferation was measured by BrdU incorporation and phospho-histone H3 staining. Kidney weight data were analyzed with Wilcoxon's rank-sum, Student's t, and Welch's t tests. Hematoxylin and eosin staining and immunoblot analysis and immunohistochemistry of cell cycle and signaling proteins were performed on mouse kidney cells and tissues. BHD knockout mice and kidney cells isolated from BHD knockout and control mice were treated with the mTOR inhibitor rapamycin. Mouse survival was evaluated by Kaplan-Meier analyses. All statistical tests were two-sided.

RESULTS

BHD knockout mice developed enlarged polycystic kidneys and died from renal failure by 3 weeks of age. Targeted BHD knockout led to the activation of Raf-extracellular signal-regulated protein kinase (Erk)1/2 and Akt-mTOR pathways in the kidneys and increased expression of cell cycle proteins and cell proliferation. Rapamycin-treated BHD knockout mice had smaller kidneys than buffer-treated BHD knockout mice had (n = 4-6 mice per group, relative kidney/body weight ratios, mean = 4.64% vs 12.2%, difference = 7.6%, 95% confidence interval = 5.2% to 10.0%; P < .001) and longer median survival time (n = 4-5 mice per group, 41.5 vs 23 days; P = .0065 ).

CONCLUSIONS

Homozygous loss of BHD may initiate renal tumorigenesis in the mouse. The conditional BHD knockout mouse may be a useful research model for dissecting multistep kidney carcinogenesis, and rapamycin may be considered as a potential treatment for Birt-Hogg-Dubé syndrome.

Usefulness of standardized uptake values for distinguishing adrenal glands with pheochromocytoma from normal adrenal glands by use of 6-18F-fluorodopamine PET

6-(18)F-Fluorodopamine ((18)F-FDA) PET is a highly sensitive tool for the localization of pheochromocytoma (PHEO). The aim of this study was to establish cutoff values for pathologic and physiologic adrenal gland tracer uptake.

METHODS

(18)F-FDA PET with CT coregistration was performed in 14 patients (10 men and 4 women; age [mean +/- SD], 42.9 +/- 13.3 y) with unilateral adrenal gland PHEO and in 13 control subjects (5 men and 8 women; age, 51.7 +/- 12.5 y) without PHEO. Standardized uptake values (SUVs) were compared between adrenal glands with PHEO and normal left adrenal glands in control subjects.

RESULTS

(18)F-FDA accumulation was observed in all adrenal glands with PHEO and in 6 of 13 control adrenal glands (P = 0.02). The SUV was higher in adrenal glands with PHEO (mean +/- SD, 16.1 +/- 6.1) than in (18)F-FDA-positive control adrenal glands (7.7 +/- 1.4) (P = 0.005). SUV cutoffs for distinguishing between adrenal glands with PHEO and normal adrenal glands were 7.3 (100% sensitivity) and 10.1 (100% specificity).

CONCLUSION

The SUVs of adrenal foci on (18)F-FDA PET facilitate the distinction between adrenal glands with PHEO and normal adrenal glands.

Salvage partial nephrectomy for hereditary renal cancer: feasibility and outcomes

PURPOSE

Patients with von Hippel-Lindau disease may require repeat partial nephrectomies to avoid the need for renal replacement therapy. We evaluated outcomes in patients who underwent third or fourth partial nephrectomies on the same kidney, which we call salvage partial nephrectomy.

MATERIALS AND METHODS

We retrospectively reviewed the charts of patients who underwent surgery at the National Cancer Institute from 1999 to 2006. We identified 11 of 13 patients in whom salvage partial nephrectomy was performed. Surgical outcomes were assessed by renal preservation as well as intraoperative and postoperative complications. Functional outcomes were assessed by comparing serum creatinine, 24-hour urine creatinine clearance, and nuclear renal scan before and after salvage partial nephrectomy.

RESULTS

All patients were alive and had no radiographic evidence of metastatic disease at a median followup of 25 months (range 3 to 83). Major perioperative complications occurred in 6 of 13 patients (46%) and loss of a renal unit occurred in 3 (23%). None of the patients in whom renal units were preserved required dialysis, although there was an upward trend in serum creatinine and a downward trend in creatinine clearance at postoperative followup (p = 0.07 and 0.08, respectively).

CONCLUSIONS

Although salvage partial nephrectomy is technically demanding and it has a high complication rate, it allows many patients to avoid dialysis. We believe that this experience can be used as a reference for surgeons and patients when considering the risks and benefits of salvage partial nephrectomy.

Robotic partial nephrectomy for complex renal tumors: surgical technique

OBJECTIVES

Laparoscopic partial nephrectomy requires advanced training to accomplish tumor resection and renal reconstruction while minimizing warm ischemia times. Complex renal tumors add an additional challenge to a minimally invasive approach to nephron-sparing surgery. We describe our technique, illustrated with video, of robotic partial nephrectomy for complex renal tumors, including hilar, endophytic, and multiple tumors.

METHODS

Robotic assistance was used to resect 14 tumors in eight patients (mean age: 50.3 yr; range: 30-68 yr). Three patients had hereditary kidney cancer. All patients had complex tumor features, including hilar tumors (n=5), endophytic tumors (n=4), and/or multiple tumors (n=3).

RESULTS

Robotic partial nephrectomy procedures were performed successfully without complications. Hilar clamping was used with a mean warm ischemia time of 31 min (range: 24-45 min). Mean blood loss was 230 ml (range: 100-450 ml). Histopathology confirmed clear-cell renal cell carcinoma (n=3), hybrid oncocytic tumor (n=2), chromophobe renal cell carcinoma (n=2), and oncocytoma (n=1). All patients had negative surgical margins. Mean index tumor size was 3.6 cm (range: 2.6-6.4 cm). Mean hospital stay was 2.6 d. At 3-mo follow-up, no patients experienced a statistically significant change in serum creatinine or estimated glomerular filtration rate and there was no evidence of tumor recurrence.

CONCLUSIONS

Robotic partial nephrectomy is safe and feasible for select patients with complex renal tumors, including hilar, endophytic, and multiple tumors. Robotic assistance may facilitate a minimally invasive, nephron-sparing approach for select patients with complex renal tumors who might otherwise require open surgery or total nephrectomy.

Compensatory alterations in energy homeostasis characterized in uterine tumors from hereditary leiomyomatosis and renal cell cancer

OBJECTIVE

To determine the molecular alterations that maintain energy homeostasis in hereditary leiomyomatosis and renal cell cancer (HLRCC) uterine tumors with disrupted fumarate hydratase, compared with nonsyndromic uterine tumors.

DESIGN

Laboratory study.

SETTING

Tertiary academic university hospital.

PATIENT(S)

Eleven nonsyndromic leiomyoma-myometrium pairs and three HLRCC leiomyoma-myometrium pairs were obtained from patients who were recruited at national and military research centers in the United States.

INTERVENTION(S)

Molecular analysis.

MAIN OUTCOME MEASURE(S)

Hereditary leiomyomatosis and renal cell cancer and nonsyndromic leiomyomas were compared with patient-matched myometrium for relative glycolysis and Krebs cycle gene expression.

RESULT(S)

By microarray analysis, we confirmed that fumarate hydratase messenger RNA (mRNA) was underexpressed in HLRCC fibroids, compared with matched myometrium. Consistent with the possibility that alterations in fumarate hydratase represented a change to a more anaerobic state, we found that HLRCC fibroids overexpressed genes such as phosphofructokinase, aldolase, phosphoglycerate kinase, enolase, and pyruvate kinase. Expression of these genes was not altered in nonsyndromic leiomyomas. Furthermore, there were no overt changes in expression of Krebs cycle enzyme gene expression, with the exception of fumarate hydratase.

CONCLUSION(S)

Our findings demonstrate that alterations in fumarate hydratase are compensated for by increases in glycolysis enzyme expression in HLRCC.

Pseudohypoxic pathways in renal cell carcinoma

Mutations of the von Hippel-Lindau (VHL) or fumarate hydratase (FH) genes lead to morphologically different renal cell carcinomas with distinct clinical courses and outcomes. The VHL protein is a part of an ubiquitin ligase complex that targets proteins for proteosomal degradation. FH is one of the mitochondrial enzymes of the Kreb's cycle. Despite two different functionalities and cellular locations, loss of either VHL or FH products has been shown to alter expression levels of hypoxia-inducible factors (HIF-1alpha and HIF-2alpha) and their downstream targets. HIF proteins are key regulators of oxygen homeostasis. Tight regulation of HIF allows for cell survival and growth at the time of hypoxic stress. HIF acts via transcriptional regulation of vascular endothelial growth factor, platelet derived growth factor, endothelial growth factor receptor, glucose transporter protein 1, erythropoietin, and transforming growth factor-alpha. Loss of VHL or FH is thought to result in a pseudohypoxic state so that cellular response pathways mediated by HIF are activated despite normal oxygen conditions. Understanding of these pseudohypoxic pathways has provided a better appreciation of the molecular mechanisms of carcinogenesis in addition to providing a rationale for targeted therapeutic approaches.