|Home | About | Journals | Submit | Contact Us | Français|
Kidney cancer is not a single disease; it is made up of a number of cancers that occur in the kidney, each with a different histology, having a different clinical course, responding differently to therapy and caused by a different gene. Understanding the genetic basis of cancer of the kidney has significant implications for diagnosis and management of this disease. The VHL gene is the gene for clear cell kidney cancer. The VHL protein forms a complex that targets the hypoxia inducible factors for ubiquitin-mediated degradation. Knowledge of this pathway has provided the foundation for the development of a number of novel therapeutic approaches that have been approved by the FDA for treatment of this disease. The MET gene is the gene for the hereditary form of type 1 papillary renal carcinoma and has been found to be mutated in a subset of sporadic type 1 papillary kidney cancers. Clinical trials are currently ongoing with agents targeting the tyrosine kinase domain of MET in sporadic and hereditary forms of papillary kidney cancer. The BHD gene is the gene for the hereditary type of chromophobe kidney cancer. The BHD gene is thought to be involved in energy and/or nutrient sensing through the AMPK and mTOR signaling pathways. Hereditary Leiomyomatosis Renal Cell Carcinoma, a hereditary form of type 2 papillary renal carcinoma, is caused by inactivation of the Krebs cycle enzyme, fumarate hydratase (fumarase, FH). Loss of FH activity has been shown to alter the degradation of hypoxia inducible factor (HIF) in a VHL-independent fashion. Knowledge of these kidney cancer gene pathways has enabled new approaches for the management of this disease and has provided the foundations for the development of targeted therapeutics for this disease.
There are over 57,000 cases of kidney cancer in the United States annually and nearly 13,000 die of this disease each year.(32) Kidney cancer is the seventh most common cancer in men and the ninth most common cancer in women.(32) Unlike other genitourinary malignancies, the incidence of kidney cancer is rapidly increasing at 2.5 percent per year. The incidence of kidney cancer can only partly be explained by the increased use of imaging modalities such as MRI, computed tomography and ultrasound.(8) While there was an increased incidence for localized disease, increases were also noted for advanced disease. The mortality rate for this disease is also increasing, suggesting that the increasing incidence is not driven by increased detection of small tumors alone.(8)
Kidney cancer is not a single disease; it is made up of a number of different types of cancer that occur in the kidney; each with a different clinical course, a different histology, caused by a different gene and responding differently to therapy.(43) Patients who present with a localized kidney tumor (TNM Stage I) have a ninety-six percent five year survival. However, patients who present with advanced disease (TNM Stage IV) have only a twenty-six percent five year survival.(41) The development of novel therapeutic approaches for targeting the kidney cancer disease gene pathways has the potential to significantly improve survival for patients with advanced forms of this disease.
In order to improve the diagnosis and management of kidney cancer and to provide the foundation for the development of better methods for diagnosis as well as targeted therapeutic approaches to this disease, families with kidney cancer were studied in order to identify the genes for this disease.
von Hippel Lindau (VHL) is a hereditary cancer syndrome in which affected individuals are at risk for the development of tumors in a number of organs, including the kidney. Patients with VHL are at risk for the development of early onset, bilateral, multifocal clear cell kidney cancer. It has been estimated that VHL patients are at risk for the development of up to 600 renal tumors and 1100 cysts per kidney.(89) Historically, 35-45% of VHL patients died of metastatic kidney cancer. While removal of both kidneys should decrease the rate of metastasis; the quality of life and long-term survival would be severely impacted. Over a 20-year period an approach has been developed for the clinical management of VHL renal tumors which involves observation of small renal tumors until they reach a 3 cm size threshold. When the tumors reach 3 cm, surgical intervention involving a nephron-sparing approach with partial nephrectomy is utilized.(87, 89) After 10 years of follow-up, no patient with VHL with a renal tumor ≤ 3 cm managed in this fashion had been found to have developed metastatic disease.(25) Renal parenchymal sparing therapy with preservation of renal function with close observation for recurrence is recommended whenever possible.(13)
Genetic linkage analysis was performed in VHL families to identify the clear cell kidney cancer gene. The VHL gene was identified on the short arm of chromosome 3 and mutation of this gene was found in the germline of individuals affected with von Hippel Lindau.(38) With improved detection methods mutation of the VHL gene is now found in nearly 100% of VHL families.(75)
When tumor tissue from patients with kidney cancer was tested for alteration of the VHL gene, mutations were detected in a high percentage of patients with clear cell kidney cancer.(20) No mutations of the VHL gene were detected in tumors from patients with papillary, chromophobe or collecting duct kidney cancer or oncocytoma. Nickerson, et al. detected mutation or methylation of the VHL gene in 91% of tumors from patients with clear cell kidney cancer.(52)
The VHL gene protein (would it be better to say ‘VHL protein’ or ‘VHL gene product’?) forms a complex with elongin B and elongin C and Cul2.(12, 37, 58) and targets the hypoxia inducible factors, HIF1α and HIF2α, for ubiquitin mediated degradation. The hypoxia inducible factors (HIF) are transcription factors that regulate the transcription of a number of other genes such as vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF), epidermal growth factor receptor (EGFR) and the glucose transporter, GLUT1. When there is normoxia, the VHL complex targets HIF for ubiquitin-mediated degradation, while during a hypoxia, the complex does not target and degrade HIF, allowing HIF to accumulate and drive the transcription of the downstream HIF-dependent genes. When there is mutation of the VHL gene in clear cell kidney cancer, either in the alpha domain that binds elongin C/B and Cul2, or in the beta domain that targets HIF for degradation,(73) HIF is not degraded and over-accumulates. The result is the increased transcription of such HIF downstream genes as VEGF, PDGF, EGFR and GLUT1.(31, 35, 45, 54, 55, 73)
The first trial to target the VHL gene pathway in clear cell renal carcinoma was with bevacizumab, an anti-VEGF antibody, which was evaluated in patients with advanced clear cell renal carcinoma. In this three arm randomized trial, patients treated with high dose bevacizumab had a significantly increased progression free survival compared with those treated with placebo.(92) This was the first trial to show a clinical effect in advanced clear cell kidney cancer with an agent targeting the VHL pathway.
Sorafenib is a small molecule inhibitor which targets the tyrosine kinase domain of a number of cell surface receptors, including VEGFR and PDGFR, as well as the intracellular signaling enzyme, raf kinase.(91) When Sorafenib was evaluated in a randomized phase III study as a second line agent in patients with advanced clear-cell renal cell carcinoma, there was a 10% partial response and an increase in disease free progression from 2.8 months to 5.5 months compared to placebo.(14) Final analysis of the study revealed no difference in overall survival between the sorafenib and placebo group; however, when the post-cross-over placebo survival data was censored, there was an increase in survival in the sorafenib arm.(15) Sorafenib is frequently used as a second line agent in patients who progress while on sunitinib.
Sunitinib is another small molecule receptor tyrosine kinase inhibitor which has direct antitumor as well as antiangiogenic effects. This agent has activity against the vascular endothelial growth factor receptor (VEGFR), the platelet derived growth factor receptor (PDGFR) and the KIT receptor.(46) In patients with metastatic clear cell kidney cancer sunitinib has been found to have a response rate of 31%, with a progression free survival of 11 months. In a randomized phase III trial sunitinib has been shown to improve progression free survival compared to treatment with interferon (median 11 months vs. 5 months); a recent analysis of mature data from this trial also suggests that patients randomized to the sunitinib had superior overall survival (median 26.4 months vs. 21.8 months, p=0.051). Treatment with sunitinib is associated with side effects common to this class of agents, such as fatigue, hand-foot syndrome, diarrhea, hypertension and hypothyroidism.(50) Sunitinib is often used as a first line agent in patients with advanced clear cell kidney cancer.
Axitinib is an oral small molecule tyrosine kinase inhibitor of vascular endothelial receptors VEGFR1, VEGFR2 and VEGFR3. In a Phase II trial of Axitinib in 52 patients with advanced kidney cancer, Rixe, et al. observed a partial response in 40% (21/52), a complete response in 4% (2/52), a median time to progression of 15.7 months and a median survival of 29.9 months. Most patients treated on this study displayed some tumor shrinkage and many had been pre-treated with either IL-2 or interferon. The response profile of these patients compares favorably with those from other antiangiogenesis trials; however, 60% of patients developed diarrhea, 58% hypertension, 52% experienced fatigue, 44% reported nausea, and 37% developed hoarseness. Treatment related proteinuria was observed in 4/52 patients.(62) The approach directed at a single type of growth factor receptor has the potential to provide a strategy for precisely targeting the VHL pathway in clear cell kidney cancer while decreasing the toxicity of multikinase inhibitors.(40)
The mTORC1 pathway regulates the expression and stability of HIF1α. In preclinical models, growth arrest caused by agents which target mTOR correlate with the block in translation of mRNA encoding HIF1α.(28, 77) Hudes, et al. evaluated the effect of temsirolimus, interferon or the combination of both in patients with previously untreated, poor-prognosis metastatic renal cell carcinoma. Eighty percent of the patients in this trial had clear cell kidney cancer. The response rate to temsirolimus was 8.6%, the progression free survival in the temsirolimus treated patients was 3.8 months versus 1.9 months in the interferon treated patients (p=0.0001) and the overall survival was 10.9 months in the temsirolimus treated patients versus 7.3 months in the patients treated with interferon (p=0.0069).(27)
Everolimus, another orally administered small molecule inhibitor of the mammalian target of rapamycin (mTOR), was evaluated in a phase III, randomized, double-blind, placebo-controlled trial in 410 patients with metastatic kidney cancer who had progressed on sunitinib or sorafenib therapy. There was a 1% response rate in the everolimus patients and a progression free survival of 4.0 months in the everolimus arm versus 1.9 months in the placebo arm. The overall survival in the placebo arm was 8.8 months; median overall survival in the everolimus patients had not been reached.(48) Everolimus may be considered in patients who have failed VEGF-targeted agents (Figure 1).
Trials are currently under way to evaluate the role of combinations of therapeutic agents which target the VHL pathway(17, 49, 57) as well as the role of sequential therapy in kidney cancer. The combined toxicity of targeted agents can be significant and dose reduction is often required. While there is evidence that administering targeted agents sequentially provides clinical benefit and prolongs progression-free survival, data on overall survival awaits the results of trials currently in progress.(16)
The standard of care for patients with metastatic kidney cancer has been cytoreductive nephrectomy prior to administration of systemic therapy.(18, 86) The clinical approach involves an assessment of the patient's clinical condition and the extent of the metastatic disease. In general, at the National Cancer Institute, if a patient has more disease in the kidney than outside the kidney, cytoreductive nephrectomy followed by systemic therapy is recommended. If the patient has more disease outside the kidney or has rapidly progressive disease, initial systemic therapy is recommended.(85) This approach has largely been extended to studies involving targeted therapeutics, where the majority of patients on clinical trials have previously had a cytoreductive nephrectomy.
While there are no completed randomized prospective studies addressing the role of the neoadjuvant therapy in setting of metastatic RCC, a few retrospective series have described the use of neoadjuvant targeted therapy in patients with their primary kidney tumor in place. Van der Veldt, et al observed a partial response in 4/17 patients who had been treated with sunitinib prior to cytoreductive nephrectomy. There was a 31% volume reduction in the primary tumor of responding patients.(82) Shuch, et al. reported on four patients who were given neoadjuvant targeted therapy. These patients had shrinkage of tumor thrombus in the inferior vena cava, in retroperitoneal nodes and in a renal fossa recurrence.(70) Karakiewicz et al. reported a dramatic down-staging of a thrombus in the right atrium in a 75 year old woman treated with two cycles of sunitinib.(36)
Margulis, et al. evaluated the surgical parameters and perioperative complications of 44 patients treated with targeted molecular therapies before either cytoreductive nephrectomy or resection of renal cell carcinoma recurrence with a matched cohort of patients with metastatic kidney cancer who had up front cytoreductive nephrectomy. They found no significant difference between the incidence of perioperative mortality, re-exploration, cardiovascular, thromboembolic or other surgical complications between the two groups.(44) Potential advantages of neoadjuvant therapy include tumor downsizing and early institution of systemic therapy, as well as identification of those patients who do not respond to therapy and could be spared a morbidity of a cytoreductive nephrectomy. Alternatively, it is possible that patients who undergo cytoreductive nephrectomy up front will respond better to therapy than those who receive neoadjuvant therapy, followed by cytoreductive nephrectomy and subsequent resumption of systemic therapy. Controlled trials will determine the role of neoadjuvant therapy in the setting of metastatic kidney cancer in patients with their primary kidney cancer in place.
The 5 year survival rate for patients who present with locally advanced kidney cancer (TNM Stage III) is 40 to 60% and 20 to 40% of patients who undergo surgical resection for clinically localized disease will experience either a local or systemic recurrence.(22) The advent of a number of novel targeted molecular therapeutic approaches in the treatment of clear cell kidney cancer raises the possibility that adjuvant therapy may be beneficial for patients with high risk/locally advanced kidney cancer who have undergone complete surgical resection. A number of clinical trials are underway to determine whether or not adjuvant targeted therapy will be beneficial to patients with high risk localized or locally advanced kidney cancer. The ECOG intergroup trial (ASSURE) randomizes patients with clear and non-clear cell kidney cancer to receive either 1 year of sunitinib, sorafenib or placebo. The MRC trial (SORCE) randomizes patients with clear and non-clear cell kidney cancer to receive either placebo (3 years) or sorafenib (1-3 years). The Pfizer trial (S-TRAC) randomizes patients with clear cell kidney cancer to receive sunitinib versus placebo for 1 year. The Wilex trial (ARISER) randomizes patients to receive G250 monoclonal chimeric antibody versus placebo.
In 1998 investigators in the laboratory of Michael Lerman at the National Cancer Institute, using RNA differential display to study genes in the VHL pathway, found that the carbonic anhydrase genes, CA9 and CA12, were regulated by the VHL protein. Mutation of the VHL gene was found to up-regulate these transmembrane enzymes which are critical for the regulation of pH in the extracellular microenvironment.(30) Kaluz et al. recently reported that carbonic anhydrase 9 is tightly regulated by the transcriptional activity of HIF1α and is one of the most sensitive endogenous sensors of its activity.(34)
G250, an antibody that has high binding affinity for clear cell kidney cancer(74), has been shown to recognize the epitope of carbonic anhydrase 9.(81) Bui et al. showed that CA9 expression is found in more than 94% of clear cell kidney cancers.(5, 6, 81) Divgi et al. conducted a study using an 124I-labelled chimeric G250 antibody (124I-cG250) to perform pre-operative characterization of clear cell kidney cancer and accurately identified 15 of 16 clear cell kidney cancers by 124I-cG250 PET scanning. This study clearly delineated clear cell renal carcinoma from other types of kidney tumors and provides a potential alternative to tumor biopsy.(11) This technique provides a potential molecular diagnostic to differentiate tumors with VHL gene mutations from other types of kidney cancer.
Hereditary Papillary Renal Carcinoma (HPRC) is an inherited cancer syndrome in which affected individuals are at risk for the development of bilateral, multifocal type 1 papillary renal carcinoma.(94) Genetic linkage was performed in HPRC families and the MET gene on chromosome 7 was found to be the HPRC gene.(65) Activating mutations in the tyrosine kinase domain of MET are found in the germline of affected individuals in HPRC families. The renal tumors in HPRC patients are highly penetrant, and it is estimated that an HPRC patient who lives to 80 has a nearly 90% likelihood of developing kidney cancer.(67) These individuals are at risk for the development of up to 3400 microscopic papillary tumors per kidney.(56) The management approach for HPRC patients is similar to that in VHL patients; active surveillance is recommended until the largest tumor reaches the 3 cm threshold.(7, 25)
A multicenter Phase II clinical trial is currently underway evaluating the role of GSK'089 (foretinib), an oral dual kinase small molecule agent which targets the tyrosine kinase domains of MET as well as VEGFR2, in patients with sporadic as well as hereditary papillary kidney cancer. Activity has been observed with this agent in papillary RCC, notably in tumors with MET mutations (Figure 2).(71)
Birt-Hogg-Dubé (BHD) is an inherited cancer syndrome in which affected individuals are at risk for the development of benign cutaneous lesions (fibrofolliculomas), pulmonary cysts and renal tumors.(93) BHD patients are at risk for the development of bilateral, multifocal chromophobe, hybrid oncocytic and clear cell renal cell carcinomas and oncocytomas.(59) Genetic linkage analysis was performed in BHD kindreds to identify the BHD gene on chromosome 17.(53) Germline BHD gene mutations have been found in over 90% of BHD kindreds.(68, 80) The BHD gene has the classic characteristics of a loss of function, tumor suppressor gene.(84)
The product of the BHD gene, folliculin (FLCN), forms a complex with two FLCN interacting proteins, FNIP1 and FNIP2, which bind AMPK, a key molecule for energy sensing that negatively regulates mTOR activity.(2, 23) The BHD gene is thought to be involved in energy sensing through the AMPK and mTOR pathways.
In order to develop a model to evaluate agents which target the BHD pathway, a mouse model was developed in which the BHD gene was selectively knocked out in the kidneys. Affected mice developed enlarged, polycystic kidneys and died from renal failure by 3 weeks of age. Rapamycin treated mice had significantly smaller kidneys than control animals and their life-expectancy was nearly doubled.(1) These findings may provide a rationale for the development of a targeted approach to BHD-associated chromophobe as well as sporadic chromophobe kidney cancer.
Hereditary Leiomyomatosis Renal Cell Carcinoma (HLRCC) is an inherited cancer syndrome in which affected individuals are at risk for the development of uterine and cutaneous leiomyomas and kidney cancer.(39) HLRCC-associated kidney cancer is an unusually aggressive type of kidney cancer that has a propensity to spread, even when the primary kidney tumor is very small.(21, 47) Surveillance is not recommended in HLRCC kidney cancer; early surgical resection is recommended when an HLRCC renal tumor is detected.
The Krebs cycle enzyme, fumarate hydratase, is the gene for HLRCC.(78) Mutations of the fumarate hydratase gene are found in over 90% of HLRCC families.(79, 90) Isaacs, et al. showed that when fumarate hydratase is inactivated, increased levels of fumarate act as a competitive inhibitor of hypoxia inducible factor (HIF) prolyl hydroxylase. This prevents HIF hydroxylation and results in a VHL-independent mechanism for decreased degradation of HIF.(29) A more recent study has reinforced the glycolytic nature of HLRCC and has identified accumulation of reactive oxygen species as an additional determinant of HIF stabilization.(76) These observations provide the basis for potential therapeutic approaches targeting the HIF pathway and HIF-dependent glycolysis for HLRCC-associated renal cancers.
Succinate dehydrogenase is another Krebs cycle enzyme gene that has been associated with the development of familial tumors. Familial paraganglioma/pheochromocytoma kindreds have been found to have germline mutations in the mitochondrial complex II genes, SDHB, SDHC, and SDHB. Renal carcinoma, along with pheochromocytoma/paraganglioma, has been found to be a component of the familial pheochromocytoma/paraganglioma complex(24, 51, 72, 83) and a recent report described germline SDHB mutations in a family with renal cancer with no history of pheochromocytoma.(61)
The tuberous sclerosis complex is an autosomal dominant disorder associated with mutation of either the TSC1 or TSC2 gene. Affected individuals are at risk for the development of a number of neurologic, dermatologic and pulmonary manifestations as well as renal angiomyolipomas.(9) The TSC1-TSC2 complex interacts with a number of cellular pathways, including modulation of mTOR. When the cell senses energy deprivation, AMPK activates TSC2 and mTOR activity is decreased.(9)
Renal angiomyolipomas are most often managed conservatively. Angioinfarction is often recommended for tumors over 4 cm in size, although surgical resection may also be performed if there is concern about imminent hemorrhage or if renal angiomyolipoma is found in young women of child-bearing age.(69)
Bissler, et al. evaluated the role of sirolimus, an agent which targets the mTOR pathway, in patients affected with tuberous sclerosis and lymphangioleiomyomatosis. In this trial, the renal angiomyolipomas regressed during sirolimus therapy, however, they tended to increase in size after the treatment was stopped.(3)
Since the mid 1980's the management of small renal masses in patients with VHL, HPRC and BHD has involved observation until the largest mass reaches 3 centimeters. As these patients are at risk for the development of bilateral, multifocal tumors, the use of parenchymal sparing surgery was based on a desire to maintain renal function as long as possible while reducing the risk for metastasis.(87, 88) Using this approach, no patient who underwent renal parenchymal sparing surgery required renal replacement and no patient whose tumor ≤ 3 cm was removed was found to have developed metastasis at 10 years.(25)
Over the years organ preservation was combined with minimally invasive techniques. Laparoscopic and then robotic partial nephrectomy was found to provide minimally invasive and nephron sparing surgical options for patients who might otherwise require open surgery or total nephrectomy.(4, 63, 64)
The same techniques developed for patients with inherited, bilateral multifocal renal tumors have been applied to patients with sporadic, non-inherited renal tumors. Gill et al. reported a series of 1800 patients who underwent either open or laparoscopic partial nephrectomy. With either technique, there was excellent functional and oncologic outcome.(19) Huang, et al. recently reviewed the SEER cancer registry database and highlighted the increased cardiovascular morbidity associated with nephrectomy versus partial nephrectomy and recommend partial versus total nephrectomy whenever feasible.(26)
A surveillance approach for the management of small renal tumors in patients with VHL, hereditary papillary renal carcinoma (HPRC) and Birt-Hogg-Dubé (BHD) was adopted in the 1980's. Patients with VHL (clear cell RCC), HPRC (type 1 papillary RCC) or BHD (chromophobe or hybrid oncocytic RCC) were managed expectantly when the largest renal tumor was less than 3 cm in size. When the largest tumor reached 3 cm, surgical intervention was recommended. This approach was found to be safe and preserved renal function over a 20-year period. Active surveillance is not recommended in patients with hereditary leiomyomatosis renal cell cancer (HLRCC), who develop aggressive type 2 papillary renal cancers. In these patients early surgical intervention is recommended.
A rational for surveillance of sporadic renal tumors comes from the experience with management of hereditary renal tumors with known genetic alterations. The biologic behavior of a hereditary renal tumor with a known genetic alteration should be no different from a sporadic renal tumor with a mutation of the same gene. A clear cell renal carcinoma in a patient affected with VHL has a mutation of the VHL gene as well as loss of the second allele (i.e., biallelic inactivation of the VHL gene). A sporadic clear cell renal tumor with a VHL mutation has loss of the second allele as well (i.e., biallelic inactivation of the VHL gene). The histology of the VHL clear cell renal tumor and the histology of the sporadic clear cell renal tumor with a VHL gene mutation are identical.
There have been a number of reports on the role of active surveillance (AS) for clinically localized renal masses.(10, 33) Crispen reported on 172 renal tumors with a median tumor diameter of 2.0 cm in 154 patients under AS. Thirty-nine percent of the tumors underwent delayed intervention and 84% were found to be pathologically malignant. Metastatic disease developed in 2/154 (1.3%) patients.(10) Metastatic disease was also detected in 2% of the patients on active surveillance in the Canadian prospective trial.(33)
A renal biopsy is now increasingly accepted for patients being considered for active surveillance. Although some renal tumors will be found to be benign, some will be high grade malignant tumors (type 2 papillary, collecting duct or medullary RCC) that are not suitable for surveillance. Active surveillance is an appropriate option for selected patients who are elderly or those with significant comorbidities. For example. in a well selected elderly or frail patient with low grade clear cell renal carcinoma, active surveillance might be an appropriate option. In contrast, presence of a high grade type 2 papillary renal carcinoma should warrant intervention, as delay in treatment may not be justified. Further studies will be required to determine whether active surveillance with delayed surgical management is a safe alternative to surgical resection in younger patients.
Understanding the kidney cancer gene pathways has provided the foundation for the development of targeted therapeutic approaches for this disease. Knowledge of the genetic basis of kidney cancer has important implications for diagnosis and management of this disease.
Study of the genes for kidney cancer has revealed that kidney cancer is fundamentally a metabolic disorder. The seven kidney cancer genes represent disorders of energy, nutrient, iron and oxygen sensing (Figure 3). Although current approaches of targeting the downstream HIF upregulated genes has resulted in dramatic response to therapy and increased survival, few complete responses have been reported and most patients progress to succumb to the disease. Targeting the basic metabolic alterations in kidney cancer has the potential to provide a more durable and effective approach to therapy.
This research was supported in part by the Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research. This project has been funded in part with federal funds from the National Cancer Institute, National Institutes of Health, under contract HHSN261200800001E. The content of this publication does not necessarily reflect the views or policies of the Department of Heath and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.
The authors acknowledge the outstanding editorial and/or graphics support by Georgia Shaw and Masaya Baba, M.D., Ph.D.
Disclosure Statement: The authors have no conflicts of interest to disclose.