June 2015, Vol 4, No 3
The RET Oncogene in Non–Small Cell Lung Cancer: Review of the Current Literature and Directions for the FutureUncategorized
Despite recent advances, lung cancer remains the leading cause of cancer-related death worldwide.1 In the past decade, the treatment of non–small cell lung cancer (NSCLC), which was once a disease with exceedingly few treatment options and historically poor outcomes, has begun to transform. The discovery of targetable cancer-driving mutations has added a promising new dimension to the field, which was once confined to the use of cytotoxic agents that often induced minimal responses that were frequently short-lived. The discovery of the EML4 fusion gene and the success of the ALK mutant–targeting tyrosine kinase inhibitor (TKI) crizotinib has proved that these mutations can have serious clinical impact, and that design of new drugs targeting the mutations can potentially save many lives.2
Importantly, the oncogenic driver mutations in lung adenocarcinoma are generally mutually exclusive, so the discovery of each new oncogenic driver mutation may be helpful to a population of patients who previously were not eligible for available targeted lung cancer therapies.3-6 The list of identifiable drug targetable genetic mutations in lung cancer is growing fast, and one of the newer targets that is just beginning to be explored in human clinical trials is the RET proto-oncogene.
Biology of the Native RET Oncogene
The RET oncogene, located in the pericentromeric region of chromosome 10q11.2, was first identified and mapped in 1985 using cells transfected with human lymphoma DNA.7 RET was first identified as a receptor for the glial cell–derived neurotropic factor family of ligands, which are important for neuron signaling.8,9 RET is expressed at its highest levels during embryogenesis and has been implicated in the process of nephrogenesis, neural crest migration, as well as the maintenance of normal neuron function. RET expression decreases to much lower levels in healthy adult tissues but is still routinely expressed in neurons, parasympathetic ganglia, thyroid C cells, urogenital tract cells, and testis germ cells.10-14
Like many oncogenes, RET encodes a receptor tyrosine kinase (RTK). The RET RTK transmembrane receptor consists of 4 cadherin-like domains in its extracellular region, a calcium binding site, a transmembrane region, and an intracellular region containing the active tyrosine kinase domain.15,16 Upon binding of its ligand and coreceptor, RET autophosphorylates and in turn activates multiple downstream targets, including the RAS-MAPK-ERK1/2 and the PI3K/AKT signaling pathway.17 This leads to activation of multiple essential cell cycle activities, including signaling to induce proliferation and to promote cell survival.18
Oncogenic RET Gene
There have been multiple previously identified mutations to the RET gene implicated in cancer development. For example, germline gain-of-function mutations in RET have long been implicated in the development of multiple endocrine neoplasia, which consists of medullary thyroid cancer, pheochromocytoma, and hyperparathyroidism.19 In addition, many irradiation-induced papillary thyroid cancers contain RET gene rearrangements.20 It is estimated that RET mutations can be found in 30% to 50% of both medullary and papillary thyroid cancers.21,22 Further, germline loss-of-function mutations are now known to be responsible for Hirschsprung disease, a congenital abnormality in which neuroblasts in the developing gut fail to migrate and neural cells do not mature in the gastrointestinal tract, resulting in chronic constipation.23,24
In lung cancer, in a mechanism very similar to that of ALK gene rearrangements, or ROS1, a somatic gain-of-function mutation in RET is responsible for creating a chimeric fusion oncogene, which can then lead to malignant transformation. When the intracellular kinase encoding domain of RET is fused to one of several recently identified gene partners, a fusion gene is created that encodes a constitutively active RTK. In 2011, Ju et al reported the first identified case of adenocarcinoma of the lung containing the RET fusion RTK, RET-KIF5B, in a 33-year-old nonsmoking patient.25 In 2012, several groups independently corroborated these results, identifying multiple different RET gene rearrangements and RET fusion proteins that are expressed at high levels in certain lung adenocarcinomas.6,25-28 The most frequent rearrangement is the fusion of RET to kinesin family member 5B (KIF5B), which is created by a pericentric inversion of chromosome 10.26 This particular gene fusion appears to be unique to adenocarcinoma of the lung. However, to date, 5 fusion partners of RET have been identified in NSCLC, including KIF5B-RET, CCDC6-RET, NCO4-RET, TRIM33-RET, and, most recently, RUFY2-RET.6,25-27,29 The function of these fusion transcripts appears to be similar to ALK gene fusions in that a coiled-coil domain of the N-terminal of the fusion partner is bound to the kinase domain of RET, which allows RET to dimerize and remain constitutively activated independent of ligand binding.26,30
Rearrangements in RET are reported to be found in approximately 1% to 2% of all adenocarcinomas of the lung, but these mutations may contribute to 6% to 19% of lung adenocarcinomas in never-smokers who are “pan-negative” for other similar oncogenic driver mutations, such as EML4-ALK and ROS1.3,30,31 RET mutations are found with higher frequency in light and never-smokers, younger patients, and in patients of European and Asian descent. The histology of lung cancers harboring RET rearrangements are most commonly moderately to well-differentiated adenocarcinomas; however, there are now rare examples of cancers with adenosquamous histology positive for these mutations as well.31
Preclinical Activity of RET Inhibitors
There are currently no drugs available that selectively inhibit only the RET tyrosine kinase. However, several small-molecule multikinase inhibitors that have already been developed for other targets have been found to have preclinical RET RTK inhibitory activity; this is true for sorafenib and sunitinib, the well-studied vascular endothelial growth factor (VEGF) inhibitors that are commonly used to treat renal cell carcinoma and hepatocellular carcinoma.32,33
Several drugs already approved for use in RET-mutated medullary thyroid cancer are currently being examined for their efficacy in RET-mutated NSCLC. Vandetanib is a potent multikinase inhibitor that was approved by the FDA for the treatment of patients with medullary thyroid carcinoma in 2011.34 In 2012, vandetanib was tested by Matsubara et al in preclinical trials using a patient-derived, RET-mutated lung adenocarcinoma cell line, LC-2/ad, which contained the CCDC6-RET fusion gene. Results from this experiment were the first of its kind to reveal specifically that lung cancer cells with RET mutations were sensitive to inhibition by vandetanib and insensitive to the epidermal growth factor receptor inhibitor gefitinib.35 Suzuki et al were able to corroborate these data in vivo using xenograft models with the same LC-2/ad cell line.36 Finally, Saito et al, using xenograft mice models harboring the most common RET fusion gene in NSCLC, KIF5B-RET, were recently able to prove that vandetanib was effective in significantly reducing tumor burden.37 A second drug with RET inhibitory activity, cabozantinib, was approved for the treatment of medullary thyroid carcinoma in 2012 and is just now beginning to be examined in the setting of NSCLC.38,39
Lenvatinib is an oral multi-TKI that targets VEGF receptors 1-3, fibroblast growth factor receptors 1-3, RET, mast/stem cell growth factor receptor, and platelet-derived growth factor beta. Cell-free kinase assays suggest that lenvatinib inhibits RET kinase-expressing cell lines. A recent preclinical study by Okamoto et al using LC-2/ad lung cancer cells revealed that in vitro lenvatinib inhibited cell growth with an inhibitory concentration of 50% of 48 nM. Lenvatinib was also found to decrease the percentage of cells in S phase, suggesting that the inhibition of signaling led to arrest of the growth phase.40
Finally ponatinib, a drug currently approved for the treatment of Philadelphia chromosome–positive leukemia, has been shown in vitro by Mologni et al to inhibit signaling of both wild-type and mutant forms of RET, with inhibition of RET signaling into the low nanomolar range in cell lines harboring RET-mutated thyroid cancer cells.41,42 At the 2013 Annual Meeting of the American Association for Cancer Research, Gozgit et al presented their data comparing the potency of ponatinib at inhibiting RET in the Ba/F3 cell line engineered to express the most common NSCLC RET fusion gene KIF5B-RET with 4 other TKIs known to inhibit RET. These data are presented in Table 1.43
Clinical Activity of RET Inhibitors in Lung Cancer In Vivo
The data surrounding use of the previously described RET inhibitors in patients with NSCLC positive for RET mutations are still extremely limited. Although several drugs with multikinase-inhibiting activity including RET are now in phase 2 clinical trials (Table 2), the majority of these trials are still enrolling patients, so formal reports of results are still very sparse. There are only 2 published reports describing the outcome of patients with NSCLC with RET mutations after treatment with one of the previously described RET-inhibiting drugs. The first, by Drilon et al, is a report of the first 3 patients in their trial testing the drug cabozantinib in patients with RET mutation–positive, advanced NSCLC. The first patient described was a 41-year-old female with a novel TRIM33-RET fusion. This patient had a durable partial response, with a 66% decrease in disease burden as measured by RECIST and was still on the trial with no reported dose reductions at 20 weeks. The second patient, who was significantly older than the first (75-year-old female) also had a confirmed partial response (33% disease reduction). However, this patient suffered several dose-limiting toxicities, including grade 3 fatigue and grade 3 proteinuria, which required 2 dose reductions. Despite dose reductions, she remained progression free and continued on the trial at 16 weeks. The third patient was a 68-year-old female who suffered from grade 3 hypertension requiring dose reduction, but she ultimately had disease stabilization with progression-free survival at 8 months.44 In 2013, Gautschi et al published a report of a 58-year-old male with RET-mutated NSCLC refractory to previous chemotherapy who responded to treatment with vandetanib at a dose of 300 mg per day.45
The field of thoracic oncology has enjoyed considerable growth during the past 5 years since the discovery of oncogenic driver mutations. For the first time in history, a subset of patients with NSCLC that once had a dismal prognosis are being afforded extended survival and improved quality of life on treatment because of the discovery of new “druggable” driver mutations that allow treatment with targeted therapies such as erlotinib or crizotinib, which have fewer toxicities than standard chemotherapy. The success of these therapies has proved that for patients who harbor one of these driver mutations or fusion proteins, treatment with cytotoxic chemotherapy may not be the safest or most effective option for treatment. Previously published trials examining drugs with RET inhibitory activity (sorafenib, sunitinib, and vandetanib) in patients with NSCLC were not performed in populations selected for RET mutations. So although these drugs were previously thought to have little activity in lung cancer, it is possible they were not being tested in the right subset of patients. Whereas it is true that RET mutations are only found in a small percentage of patients with NSCLC, the benefits that RET-inhibiting drugs may have in the right patient population could be, like crizotinib in ALK-mutated patients, fairly dramatic. The results from the various phase 2 clinical trials that are limited to patients predetermined to have RET mutations appear promising and will likely be extremely informative as to whether RET will be another useful drug target to help patients with NSCLC survive longer and with a better quality of life on therapy. What is clear is that the similarities between the RET oncogene and the ALK and ROS1 oncogenes are undeniable, and given the potential benefits to a patient population with too few treatment options, it would be a logical next step to add RET to the list of mutations examined in all new NSCLC cases.
- World Health Organization. Cancer fact sheets. 2015. www.who.int/mediacentre/factsheets/fs297/en/. Accessed October 28, 2014.
- Kwak EL, Bong YJ, Camidge DR, et al. Anaplastic lymphoma kinase inhibition in non-small-cell lung cancer. N Engl J Med. 2010;363:1693-1703.
- Suehara Y, Arcila M, Wang L, et al. Identification of KIF5B-RET and
GOPC-ROS1 fusions in lung adenocarcinomas through a comprehensive mRNA-based screen for tyrosine kinase fusions. Clin Cancer Res. 2012;18:6599-6608.
- Kris MG, Johnson BE, Berry LD, et al. Using multiplexed assays of oncogenic drivers in lung cancers to select targeted drugs. JAMA. 2014;311:1998-2006.
- Pao W, Girard N. New driver mutations in non-small-cell lung cancer. Lancet Oncol. 2011;12:175-180.
- Lipson D, Capelletti M, Yelensky R, et al. Identification of new ALK and RET gene fusions from colorectal and lung cancer biopsies. Nat Med. 2012;18:382-384.
- Ishizaka Y, Itoh F, Tahira T, et al. Human ret proto-oncogene mapped to chromosome 10q11.2. Oncogene. 1989;4:1519-1521.
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- Esseghir S, Todd SK, Hunt T, et al. A role for glial cell derived neurotrophic factor induced expression by inflammatory cytokines and RET/GFR alpha 1 receptor up-regulation in breast cancer. Cancer Res. 2007;67:11732-11741.
- Mulligan LM. RET revisited: expanding the oncogenic portfolio. Nat Rev Cancer. 2014;14:173-186.
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- Anders J, Kjær S, Ibáñez CF. Molecular modeling of the extracellular domain of the RET receptor tyrosine kinase reveals multiple cadherin-like domains and a calcium-binding site. J Biol Chem. 2001;276:35808-35817.
- Plaza-Menacho I, van der Sluis T, Hollema H, et al. Ras/ERK1/2-mediated STAT3 Ser727 phosphorylation by familial medullary thyroid carcinoma-associated RET mutants induces full activation of STAT3 and is required for c-fos promoter activation, cell mitogenicity, and transformation. J Biol Chem. 2007;282:6415-6424.
- Arighi E, Borrello MG, Sariola H. RET tyrosine kinase signaling in development and cancer. Cytokine Growth Factor Rev. 2005;16:441-467.
- Mulligan LM, Kwok JB, Healey CS, et al. Germ-line mutations of the RET proto-oncogene in multiple endocrine neoplasia type 2A. Nature. 1993;363:458-460.
- Donis-Keller H, Dou S, Chi D, et al. Mutations in the RET proto-oncogene are associated with MEN 2A and FMTC. Hum Mol Genet. 1993;2:851-856.
- Romei C, Elisei R. RET/PTC translocations and clinico-pathological features in human papillary thyroid carcinoma. Front Endocrinol (Lausanne). 2012;3:54.
- Romei C, Fugazzola L, Puxeddu E, et al. Modifications in the papillary thyroid cancer gene profile over the last 15 years. J Clin Endocrinol Metab. 2012;97:E1758-E1765.
- Emison ES, McCallion AS, Kashuk CS, et al. A common sex-dependent mutation in a RET enhancer underlies Hirschsprung disease risk. Nature. 2005;434:857-863.
- Seri M, Yin L, Barone V, et al. Frequency of RET mutations in long- and short-segment Hirschsprung disease. Hum Mutat. 1997;9:243-249.
- Ju YS, Lee WC, Shin JY, et al. A transforming KIF5B and RET gene fusion in lung adenocarcinoma revealed from whole-genome and transcriptome sequencing. Genome Res. 2012;22:436-445.
- Kohno T, Ichikawa H, Totoki Y, et al. KIF5B-RET fusions in lung adenocarcinoma. Nat Med. 2012;18:375-377.
- Takeuchi K, Soda M, Togashi Y, et al. RET, ROS1 and ALK fusions in lung cancer. Nat Med. 2012;18:378-381.
- Li F, Feng Y, Fang R, et al. Identification of RET gene fusion by exon array analyses in “pan-negative lung cancer from never smokers. Cell Res. 2012;22:928-931.
- Zheng Z, Liebers M, Zhelyazkova B, et al. Anchored multiplex PCR for targeted next-generation sequencing. Nat Med. 2014;20:1479-1484.
- Kohno T, Tsuta K, Tsuchihara K, et al. RET fusion gene: translation to personalized lung cancer therapy. Cancer Sci. 2013;104:1396-1400.
- Wang R, Hu H, Pan Y, et al. RET fusions define a unique molecular and clinicopathologic subtype of non-small-cell lung cancer. J Clin Oncol. 2012;30:4352-4359.
- Kim DW, Jo YS, Jung HS, et al. An orally administered multitarget tyrosine kinase inhibitor, SU11248, is a novel potent inhibitor of thyroid oncogenic RET/papillary thyroid cancer kinases. J Clin Endocrinol Metab. 2006;91:4070-4076.
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RET lysosomal degradation independent of proteasomal targeting. J Biol Chem. 2007;282:29230-29240.
- Wells SA Jr, Robinson BG, Gagel RF, et al. Vandetanib in patients with locally advanced or metastatic medullary thyroid cancer: a randomized, double-blind phase III trial. J Clin Oncol. 2012;30:134-141.
- Matsubara D, Kanai Y, Ishikawa S, et al. Identification of CCDC6-RET fusion in the human lung adenocarcinoma cell line, LC-2/ad. J Thorac Oncol. 2012;7:1872-1876.
- Suzuki M, Makinoshima H, Matsumoto S, et al. Identification of a lung adenocarcinoma cell line with CCDC6-RET fusion gene and the effect of RET inhibitors in vitro and in vivo. Cancer Sci. 2013;104:896-903.
- Saito M, Ishigame T, Tsuta K, et al. A mouse model of KIF5B-RET fusion-dependent lung tumorigenesis. Carcinogenesis. 2014;35:2452-2456.
- Elisei R, Schlumberger MJ, Müller SP, et al. Cabozantinib in progressive medullary thyroid cancer. J Clin Oncol. 2013;31:3639-3646.
- Kurzrock R, Sherman SI, Ball DW, et al. Activity of XL184 (cabozantinib), an oral tyrosine kinase inhibitor, in patients with medullary thyroid cancer. J Clin Oncol. 2011;29:2660-2666.
- Okamoto K, Kodama K, Takase K, et al. Antitumor activities of the targeted multi-tyrosine kinase inhibitor lenvatinib (E7080) against RET gene fusion-driven tumor models. Cancer Lett. 2013;340:97-103.
- Mologni L, Redaelli S, Morandi A, et al. Ponatinib is a potent inhibitor of wild-type and drug-resistant gatekeeper mutant RET kinase. Mol Cell Endocrinol. 2013;377:1-6.
- De Falco V, Buonocore P, Muthu M, et al. Ponatinib (AP24534) is a novel potent inhibitor of oncogenic RET mutants associated with thyroid cancer. J Clin Endocrinol Metab. 2013;98:E811-E819.
- Gozgit JM, Chen T, Clackson T, et al. Ponatinib is a highly potent inhibitor of activated variants of RET found in MTC and NSCLC. Paper presented at: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; April 6-10, 2013; Washington, DC. Abstract 2084.
- Drilon A, Wang L, Hasanovic A, et al. Response to cabozantinib in patients with RET fusion-positive lung adenocarcinomas. Cancer Discov. 2013;3:630-635.
- Gautschi O, Zander T, Keller FA, et al. A patient with lung adenocarcinoma and RET fusion treated with vandetanib. J Thorac Oncol. 2013;8:e43-e44.
Dr Shatsky is an Oncology Fellow in the Division of Hematology/Oncology, Department of Medicine at the University of California, San Diego (UCSD).
Dr Bazhenova is Associate Clinical Professor of Medicine in the Division of Hematology/Oncology and is Medical Director of the UCSD Moores Cancer Center Infusion Center. Her clinical practice and research concentrate on lung cancer, particularly as it relates to females and nonsmokers. She actively participates in cooperative group trials and takes an active role in designing and implementing clinical investigations, including phase 2 studies and correlative science projects with several UCSD investigators.
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