Response and acquired resistance to everolimus in anaplastic thyroid cancer

Everolimus, an inhibitor of the mammalian target of rapamycin (mTOR), is effective in treating tumors harboring alterations in the mTOR pathway. Mechanisms of resistance to everolimus remain undefined. Resistance developed in a patient with metastatic anaplastic thyroid carcinoma after an extraordinary 18-month response. Whole-exome sequencing of pretreatment and drug-resistant tumors revealed a nonsense mutation in TSC2, a negative regulator of mTOR, suggesting a mechanism for exquisite sensitivity to everolimus. The resistant tumor also harbored a mutation in MTOR that confers resistance to allosteric mTOR inhibition. The mutation remains sensitive to mTOR kinase inhibitors.

Abstract 1724: Genomic mechanisms of exquisite sensitivity and acquired resistance to everolimus in a patient with anaplastic thyroid carcinoma

Understanding genetic mechanisms of sensitivity and resistance to targeted anticancer therapies may improve patient selection and rational treatment designs. One approach to increase this understanding involves the study of exceptional responders: rare patients with unexpected exquisite sensitivity or durable responses to therapy. We identified an exceptional responder on a study of the allosteric mTOR inhibitor everolimus in thyroid cancer: a 57-yr-old woman with refractory metastatic anaplastic thyroid carcinoma (ATC), a highly aggressive neoplasm with no adequate therapies and a median survival of 5 months. After beginning treatment with everolimus, the patient experienced a near complete response that lasted for 18 months, followed by progressive disease, which was then re-biopsied. To date, mechanisms of clinical resistance to mTOR inhibition have not been described.

We performed whole exome sequencing (WES) of both pre-treatment and drug resistant tumor tissue to look for the underlying mechanisms of exquisite sensitivity and acquired resistance to everolimus. WES of the pre-treatment tumor revealed a somatic nonsense mutation in TSC2, a tumor suppressor gene whose inactivation is known to activate the mTOR pathway and result in sensitivity to mTOR inhibition in some cancers. WES of the drug resistant tumor additionally revealed a mutation in mTOR (mTOR-F2108L) not detected in the pre-treatment tumor. Structural modeling demonstrated that this mutation occurs in the FKBP12-rapamycin binding domain of mTOR and is predicted to prevent binding of the drug to the protein. Overexpressing mTOR-F2108L in HEK-293T cells resulted in significant resistance to rapamycin compared to cells expressing wild type (wt) mTOR. In cells expressing the mutant mTOR, rapamycin did not decrease phosphorylation of S6K1, a downstream target of mTOR, compared with cells expressing wt mTOR. Notably, cells expressing mTOR-F2108L remained sensitive to the direct TOR inhibitor torin, suggesting a therapeutic approach to overcome resistance in this patient.

In summary, we add ATC to the growing list of cancers found to be exquisitely sensitive to everolimus when activating mTOR pathway mutations are present. Moreover, we present the first reported, to our knowledge, mechanism of acquired resistance to everolimus identified in patients. The fact that this occurs via a binding domain mutation that blocks allosteric mTOR inhibition suggests that followup therapy with direct TOR inhibitors may still have benefit in some patients who develop resistance to everolimus. The use of precision medicine approaches in ATC to screen for alterations in the mTOR pathway may help identify subsets of patients who would benefit from targeted therapies directed against mTOR. Moreover, the use of serial biopsies to profile patients who develop resistance to everolimus could dictate optimal followup treatment in ATC and other cancers.

Citation Format: Nikhil Wagle, Brian C. Grabiner, Eliezer M. Van Allen, Ali Amin-Mansour, Scott C. Carter, Nathanael Gray, Justine A. Barletta, Scott J. Swanson, Daniel Ruan, David J. Kwiatkowski, Glenn J. Hanna, Robert I. Haddad, David Sabatini, Pasi A. Janne, Levi A. Garraway, Jochen H. Lorch. Genomic mechanisms of exquisite sensitivity and acquired resistance to everolimus in a patient with anaplastic thyroid carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1724. doi:10.1158/1538-7445.AM2014-1724

A diverse array of cancer-associated MTOR mutations are hyperactivating and can predict rapamycin sensitivity

Genes encoding components of the PI3K-AKT-mTOR signaling axis are frequently mutated in cancer, but few mutations have been characterized in MTOR, the gene encoding the mTOR kinase. Using publicly available tumor genome sequencing data, we generated a comprehensive catalog of mTOR pathway mutations in cancer, identifying 33 MTOR mutations that confer pathway hyperactivation. The mutations cluster in six distinct regions in the C-terminal half of mTOR and occur in multiple cancer types, with one cluster particularly prominent in kidney cancer. The activating mutations do not affect mTOR complex assembly, but a subset reduces binding to the mTOR inhibitor DEPTOR. mTOR complex 1 (mTORC1) signaling in cells expressing various activating mutations remains sensitive to pharmacologic mTOR inhibition, but is partially resistant to nutrient deprivation. Finally, cancer cell lines with hyperactivating MTOR mutations display heightened sensitivity to rapamycin both in culture and in vivo xenografts, suggesting that such mutations confer mTOR pathway dependency.

Activating mTOR mutations in a patient with an extraordinary response on a phase I trial of everolimus and pazopanib

Understanding the genetic mechanisms of sensitivity to targeted anticancer therapies may improve patient selection, response to therapy, and rational treatment designs. One approach to increase this understanding involves detailed studies of exceptional responders: rare patients with unexpected exquisite sensitivity or durable responses to therapy. We identified an exceptional responder in a phase I study of pazopanib and everolimus in advanced solid tumors. Whole-exome sequencing of a patient with a 14-month complete response on this trial revealed two concurrent mutations in mTOR, the target of everolimus. In vitro experiments demonstrate that both mutations are activating, suggesting a biologic mechanism for exquisite sensitivity to everolimus in this patient. The use of precision (or "personalized") medicine approaches to screen patients with cancer for alterations in the mTOR pathway may help to identify subsets of patients who may benefit from targeted therapies directed against mTOR.

A Tumor suppressor complex with GAP activity for the Rag GTPases that signal amino acid sufficiency to mTORC1

The mTOR complex 1 (mTORC1) pathway promotes cell growth in response to many cues, including amino acids, which act through the Rag guanosine triphosphatases (GTPases) to promote mTORC1 translocation to the lysosomal surface, its site of activation. Although progress has been made in identifying positive regulators of the Rags, it is unknown if negative factors also exist. Here, we identify GATOR as a complex that interacts with the Rags and is composed of two subcomplexes we call GATOR1 and -2. Inhibition of GATOR1 subunits (DEPDC5, Nprl2, and Nprl3) makes mTORC1 signaling resistant to amino acid deprivation. In contrast, inhibition of GATOR2 subunits (Mios, WDR24, WDR59, Seh1L, and Sec13) suppresses mTORC1 signaling, and epistasis analysis shows that GATOR2 negatively regulates DEPDC5. GATOR1 has GTPase-activating protein (GAP) activity for RagA and RagB, and its components are mutated in human cancer. In cancer cells with inactivating mutations in GATOR1, mTORC1 is hyperactive and insensitive to amino acid starvation, and such cells are hypersensitive to rapamycin, an mTORC1 inhibitor. Thus, we identify a key negative regulator of the Rag GTPases and reveal that, like other mTORC1 regulators, Rag function can be deregulated in cancer.

The E3 ligase TRAF6 regulates Akt ubiquitination and activation

Akt signaling plays a central role in many biological functions, such as cell proliferation and apoptosis. Because Akt (also known as protein kinase B) resides primarily in the cytosol, it is not known how these signaling molecules are recruited to the plasma membrane and subsequently activated by growth factor stimuli. We found that the protein kinase Akt undergoes lysine-63 chain ubiquitination, which is important for Akt membrane localization and phosphorylation. TRAF6 was found to be a direct E3 ligase for Akt and was essential for Akt ubiquitination, membrane recruitment, and phosphorylation upon growth-factor stimulation. The human cancer-associated Akt mutant displayed an increase in Akt ubiquitination, in turn contributing to the enhancement of Akt membrane localization and phosphorylation. Thus, Akt ubiquitination is an important step for oncogenic Akt activation.

MEKK3 is required for lysophosphatidic acid-induced NF-kappaB activation

Lysophosphatidic acid (LPA) is a potent agonist that exerts various cellular functions on many cell types through binding to its cognate G protein-coupled receptors (GPCRs). Although LPA induces NF-kappaB activation by acting on its GPCR receptor, the molecular mechanism of LPA receptor-mediated NF-kappaB activation remains to be well defined. In the present study, by using MEKK3-, TAK1-, and IKKbeta-deficient murine embryonic fibroblasts (MEFs), we found that MEKK3 but not TAK1 deficiency impairs LPA and protein kinase C (PKC)-induced IkappaB kinase (IKK)-NF-kappaB activation, and IKKbeta is required for PKC-induced NF-kappaB activation. In addition, we demonstrate that LPA and PKC-induced IL-6 and MIP-2 production are abolished in the absence of MEKK3 but not TAK1. Together, our results provide the genetic evidence that MEKK3 but not TAK1 is required for LPA receptor-mediated IKK-NF-kappaB activation.

CARMA3 deficiency abrogates G protein-coupled receptor-induced NF-{kappa}B activation

G protein-coupled receptors (GPCRs) play pivotal roles in regulating various cellular functions. Although many GPCRs induce NF-kappaB activation, the molecular mechanism of GPCR-induced NF-kappaB activation remains largely unknown. CARMA3 (CARD and MAGUK domain-containing protein 3) is a scaffold molecule with unknown biological functions. By generating CARMA3 knockout mice using the gene targeting approach, here we show CARMA3 is required for GPCR-induced NF-kappaB activation. Mechanistically, we found that CARMA3 deficiency impairs GPCR-induced IkappaB kinase (IKK) activation, although it does not affect GPCR-induced IKKalpha/beta phosphorylation, indicating that inducible phosphorylation of IKKalpha/beta alone is not sufficient to induce its kinase activity. We also found that CARMA3 is physically associated with NEMO/IKKgamma, and induces polyubiquitination of an unknown protein(s) that associates with NEMO, likely by linking NEMO to TRAF6. Consistently, we found TRAF6 deficiency also abrogates GPCR-induced NF-kappaB activation. Together, our results provide the genetic evidence that CARMA3 is required for GPCR-induced NF-kappaB activation.

Constitutively active type I insulin-like growth factor receptor causes transformation and xenograft growth of immortalized mammary epithelial cells and is accompanied by an epithelial-to-mesenchymal transition mediated by NF-kappaB and snail

Type I insulin-like growth factor receptor (IGF-IR) can transform mouse fibroblasts; however, little is known about the transforming potential of IGF-IR in human fibroblasts or epithelial cells. We found that overexpression of a constitutively activated IGF-IR (CD8-IGF-IR) was sufficient to cause transformation of immortalized human mammary epithelial cells and growth in immunocompromised mice. Furthermore, CD8-IGF-IR caused cells to undergo an epithelial-to-mesenchymal transition (EMT) which was associated with dramatically increased migration and invasion. The EMT was mediated by the induction of the transcriptional repressor Snail and downregulation of E-cadherin. NF-kappaB was highly active in CD8-IGF-IR-MCF10A cells, and both increased levels of Snail and the EMT were partially reversed by blocking NF-kappaB or IGF-IR activity. This study places IGF-IR among a small group of oncogenes that, when overexpressed alone, can confer in vivo tumorigenic growth of MCF10A cells and indicates the hierarchy in the mechanism of IGF-IR-induced EMT.