Chasing Therapeutic Targets in Thymic Malignancies: Finding Needles in the Haystack to Frame a Comprehensive Canvas?
Authors: Nicolas Girard
Journal: Journal of Thoracic Oncology (2016)

In this issue of the Journal of Thoracic Oncology, Alberobello et al. report on the identification of deleterious mutations of regulatory phosphoinositide-3-kinase (PI3K) subunit genes in tissue specimens and cell lines derived from patients with thymic carcinoma.1Alberobello A.T. Wang Y. Beerkens F.J. et al.PI3K as a potential therapeutic target in thymic epithelial tumors.J Thorac Oncol. 2016; 11: 1345-1356Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar The authors first identified such mutations in a tumorigenic thymic carcinoma cell line (MP57), by using targeted exome sequencing. The mutation, which was a homozygous missense mutation in phosphoinositide-3-kinase regulatory subunit 2 gene (PIK3R2) encoding a regulatory subunit of PI3K, had previously been reported in endometrial cancer, and it leads to enhanced PI3K activity. In vitro exposure to the PI3K inhibitor pictilisib inhibited cell viability and proliferation. The authors concluded that targeting PI3K may be an effective strategy in the treatment of thymic carcinomas. The identification of predictive biomarkers in thymic malignancies is highly challenging not only because of their rarity and histologic heterogeneity; in thymomas, it is also challenging given the presence of a significant number of nonneoplastic lymphocytes that tightly intermingle with the neoplastic epithelial cells. This situation precluded large-scale, integrated array-based comparative genomic hybridization and expression profiling approaches being optimally conducted on tissue specimens, as clustering ultimately revealed the genomic and metabolic patterns of the epithelial cells versus lymphocytes.2Girard N. Shen R. Guo T. et al.Comprehensive genomic analysis reveals clinically relevant molecular distinctions between thymic carcinomas and thymomas.Clin Cancer Res. 2009; 15: 6790-6799Crossref PubMed Scopus (149) Google Scholar A way to circumvent these hurdles is to use primary epithelial cells cultures derived from fresh tumor tissue specimens, as Alberobello et al. did to establish the MP57 thymic carcinoma cell line in their present study.1Alberobello A.T. Wang Y. Beerkens F.J. et al.PI3K as a potential therapeutic target in thymic epithelial tumors.J Thorac Oncol. 2016; 11: 1345-1356Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar The establishment of thymic cell lines is even more of an achievement per se in thymomas, which are characterized by a limited number of epithelial cells with a low proliferative index and limited atypia.3Breinig M. Mayer P. Harjung A. et al.Heat shock protein 90-sheltered overexpression of insulin-like growth factor 1 receptor contributes to malignancy of thymic epithelial tumors.Clin Cancer Res. 2011; 17: 2237-2249Crossref PubMed Scopus (22) Google Scholar, 4Gökmen-Polar Y. Sanders K.L. Goswami C.P. et al.Establishment and characterization of a novel cell line derived from human thymoma AB tumor.Lab Invest. 2012; 92: 1564-1573Crossref PubMed Scopus (20) Google Scholar Thymic carcinomas strongly differ from thymomas in this matter, with high and frequent chromosomal aberrations observed at genomic profiling.2Girard N. Shen R. Guo T. et al.Comprehensive genomic analysis reveals clinically relevant molecular distinctions between thymic carcinomas and thymomas.Clin Cancer Res. 2009; 15: 6790-6799Crossref PubMed Scopus (149) Google Scholar, 5Petrini I. Meltzer P.S. Zucali P.A. et al.Copy number aberrations of BCL2 and CDKN2A/B identified by array-CGH in thymic epithelial tumors.Cell Death Dis. 2012; 3: e351Crossref PubMed Scopus (46) Google Scholar Several primary cell lines were previously reported to have been derived from thymic carcinoma specimens, with limited or more comprehensive subsequent molecular characterization. In particular, resistance to apoptosis has been identified as a hallmark of thymic carcinomas by using cell line models5Petrini I. Meltzer P.S. Zucali P.A. et al.Copy number aberrations of BCL2 and CDKN2A/B identified by array-CGH in thymic epithelial tumors.Cell Death Dis. 2012; 3: e351Crossref PubMed Scopus (46) Google Scholar; copy number gain of the antiapoptotic molecule B-cell lymphoma 2 (BCL2) was observed at comparative genomic hybridization, whereas in vitro small interfering RNA knockdown reduced cell proliferation, and in vivo exposure to a pan-BCL2 inhibitor led to inhibition of xenograft growth through a mechanism involving the PI3K/AKT/mammalian target of rapamycin (mTOR) pathway.5Petrini I. Meltzer P.S. Zucali P.A. et al.Copy number aberrations of BCL2 and CDKN2A/B identified by array-CGH in thymic epithelial tumors.Cell Death Dis. 2012; 3: e351Crossref PubMed Scopus (46) Google Scholar Furthermore, escape to growth suppressors related to a deregulation of cell cycle controlling molecules was also observed, with copy number loss of cyclin-dependent kinase inhibitor 2A/B and hypermethylation of its promoter, accompanied by a lack of expression of its related protein p16INK4.5Petrini I. Meltzer P.S. Zucali P.A. et al.Copy number aberrations of BCL2 and CDKN2A/B identified by array-CGH in thymic epithelial tumors.Cell Death Dis. 2012; 3: e351Crossref PubMed Scopus (46) Google Scholar Exposure of thymic carcinoma cells to heat shock protein 90 inhibitors led to cell cycle arrest and apoptosis, and blocked invasiveness through the downregulation of heat shock protein 90 oncogenic clients, including insulin-like growth factor 1 receptor (IGF-1R), a transmembrane tyrosine kinase receptor frequently overexpressed in thymic carcinomas,6Rajan A. Girard N. Marx A. State of the art of genetic alterations in thymic epithelial tumors.J Thorac Oncol. 2014; 9: S131-S136Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar cyclin-dependent kinase 4, and PI3K/Akt.3Breinig M. Mayer P. Harjung A. et al.Heat shock protein 90-sheltered overexpression of insulin-like growth factor 1 receptor contributes to malignancy of thymic epithelial tumors.Clin Cancer Res. 2011; 17: 2237-2249Crossref PubMed Scopus (22) Google Scholar Meanwhile, general transcription factor IIi, a transcription factor related to the AKT pathway and cyclin D1, was reported to harbor truncating mutations, which are more frequent in type A and AB thymomas (in 80% of cases) than in thymic carcinoma (in 8% of cases).7Petrini I. Meltzer P.S. Kim I.K. et al.A specific missense mutation in GTF2I occurs at high frequency in thymic epithelial tumors.Nat Genet. 2014; 46: 844-849Crossref PubMed Scopus (150) Google Scholar Taken together, these data suggest that escape to apoptosis and cell cycle deregulation may represent a major hallmark of thymic carcinomas through increased activity of BCL2 and deregulation of cyclin-dependent kinase inhibitor 2A, which is possibly related to activation of IGF-1R and the AKT/PI3K/mTOR pathway. Taken together, these data are of significant therapeutic relevance (Fig. 1): although pictilisib is developed mostly in breast cancers, which more frequently harbor PI3K alterations, phase II trials dedicated to thymic epithelial tumors were conducted with the IGF-1R inhibitor cixutumumab,9Rajan A. Carter C.A. Berman A. et al.Cixutumumab for patients with recurrent or refractory advanced thymic epithelial tumours: a multicentre, open-label, phase 2 trial.Lancet Oncol. 2014; 15: 191-200Abstract Full Text Full Text PDF PubMed Scopus (92) Google Scholar the mTOR inhibitor everolimus,10Zucali P.A. De Pas T.M. Palmieri G. et al.Phase II study of everolimus in patients with thymoma and thymic carcinoma previously treated with cisplatin-based chemotherapy.J Clin Oncol. 2014; 32 ([abstract]): 7527Google Scholar and the cyclin-dependent kinases inhibitor milciclib,11Besse B. Garassino M.C. Rajan A. et al.A phase II study of milciclib (PHA-848125AC) in patients (pts) with thymic carcinoma (TC).J Clin Oncol. 2014; 32 ([abstract]): 7526Google Scholar with all reporting on clinical antitumor activity in advanced, refractory cases. Only everolimus is currently available, and it may represent an off-label option for refractory tumors.12Girard N. Ruffini E. Marx A. et al.Thymic epithelial tumours: ESMO clinical practice guidelines for diagnosis, treatment and follow-up.Ann Oncol. 2015; 26: v40-v55Crossref PubMed Scopus (254) Google Scholar On the other hand, the analysis of tumor microenvironment–related hallmarks of thymic carcinomas requires a molecular characterization of tissue specimens. Induction of angiogenesis, which is related to a frequent overexpression of vascular endothelial growth factor receptors,6Rajan A. Girard N. Marx A. State of the art of genetic alterations in thymic epithelial tumors.J Thorac Oncol. 2014; 9: S131-S136Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar has been reported; escape to immune destruction may be involved in carcinogenesis as well, given a frequent expression of programmed death ligand-1 by tumor cells.13Padda S.K. Riess J.W. Schwartz E.J. et al.Diffuse high intensity PD-L1 staining in thymic epithelial tumors.J Thorac Oncol. 2015; 10: 500-508Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar Recent phase II trials with sunitinib14Thomas A. Rajan A. Berman A. et al.Sunitinib in patients with chemotherapy-refractory thymoma and thymic carcinoma: an open-label phase 2 trial.Lancet Oncol. 2015; 16: 177-186Abstract Full Text Full Text PDF PubMed Scopus (173) Google Scholar and pembrolizumab15Giaccone G. Thompson J. Crawford J. et al.A phase II study of pembrolizumab in patients with recurrent thymic carcinoma.J Clin Oncol. 2016; 34 ([abstract]): 8517Google Scholar reported on significant clinical antitumor activity in advanced, refractory thymic carcinomas, with objective responses observed in 26% and 24% of cases, respectively, and with some of them being particularly prolonged.16Remon J. Girard N. Mazieres J. et al.Sunitinib in patients with advanced thymic malignancies: Cohort from the French RYTHMIC network.Lung Cancer. 2016; 97: 99-104Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar Sunitinib is now considered an option in the second-line setting, as reported in the case of the patient from whom the MP57 cell line had been derived. Ultimately, along with PI3K mutations,1Alberobello A.T. Wang Y. Beerkens F.J. et al.PI3K as a potential therapeutic target in thymic epithelial tumors.J Thorac Oncol. 2016; 11: 1345-1356Abstract Full Text Full Text PDF PubMed Scopus (32) Google Scholar rare KRAS mutations,2Girard N. Shen R. Guo T. et al.Comprehensive genomic analysis reveals clinically relevant molecular distinctions between thymic carcinomas and thymomas.Clin Cancer Res. 2009; 15: 6790-6799Crossref PubMed Scopus (149) Google Scholar the only oncogenic predictive drivers identified in thymic carcinomas so far, are KIT proto-oncogene receptor tyrosine kinase gene (KIT) activating mutations6Rajan A. Girard N. Marx A. State of the art of genetic alterations in thymic epithelial tumors.J Thorac Oncol. 2014; 9: S131-S136Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar; KIT mutations are found only in 9% of cases, consisting of mutations observed in other malignancies (V560del and L576P) or mutations unique to thymic carcinomas (H697Y and D820E).3Breinig M. Mayer P. Harjung A. et al.Heat shock protein 90-sheltered overexpression of insulin-like growth factor 1 receptor contributes to malignancy of thymic epithelial tumors.Clin Cancer Res. 2011; 17: 2237-2249Crossref PubMed Scopus (22) Google Scholar, 6Rajan A. Girard N. Marx A. State of the art of genetic alterations in thymic epithelial tumors.J Thorac Oncol. 2014; 9: S131-S136Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar KIT mutants are not uniformly sensitive to imatinib according to the clinical and/or the preclinical evidence in thymic carcinoma and/or other KIT-mutant malignancies. Responses and possibly prolonged survival were reported with the use of several KIT inhibitors—imatinib, sunitinib, or sorafenib—mostly in single-case observations.6Rajan A. Girard N. Marx A. State of the art of genetic alterations in thymic epithelial tumors.J Thorac Oncol. 2014; 9: S131-S136Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar KIT sequencing (exons 9–17) has been considered an option for refractory thymic carcinomas in the setting of possible access to off-label use of such inhibitors.16Remon J. Girard N. Mazieres J. et al.Sunitinib in patients with advanced thymic malignancies: Cohort from the French RYTHMIC network.Lung Cancer. 2016; 97: 99-104Abstract Full Text Full Text PDF PubMed Scopus (43) Google Scholar Taken together, those data frame a more comprehensive canvas of the biology of thymic carcinomas that is relevant for the assessment and the use of targeted agents (see Fig. 1). Recent progress and better access to high-throughput, next-generation sequencing represents a momentum for basic science and translational research using tissue specimens and derived cells, as well as for clinical care in the setting of precision medicine approaches. A Thymoma Cancer Genome Atlas project was initiated in 2013 to perform comprehensive molecular analysis of a series of 124 specimens of thymic epithelial tumor. The analyses are ongoing, and the results will be published soon; in particular, the high number of cases in this project should allow an assessment of coexistent alterations in single thymic carcinoma tumors. Meanwhile, raw data are already available to researchers on the Cancer Genome Atlas portal (https://tcga-data.nci.nih.gov/tcga/) and the National Cancer Institute Genomic Data Commons (https://gdc-portal.nci.nih.gov). Meanwhile, high-throughput, next-generation sequencing (in particular, targeted exome sequencing) is now becoming available in the clinic, especially in dedicated programs such as SPECTA-lung, an initiative of the European Organization for Research and Treatment of Cancer and the European Thoracic Oncology Platform, which aims at screening patients with thoracic malignancies for downstream clinical trials using the Oncomine Focus 318 assay (Thermo Fisher Scientific, Waltham, MA). In the setting of rare thoracic malignancies, collaboration between expert centers is key, such as in the French RYTHMIC network, a nationwide network dedicated to the management of patients with thymic tumors that was appointed in 2012 by the French National Cancer institute as part of its rare cancer program. The objectives of the network include territorial coverage by regional expert centers, dissemination of the highest standards of diagnostic and therapeutic management of patients, and promotion of collaborative and translational research through systematic biobanking. Ultimately, ensuring that patients with thymic malignancies have equal access to highly specialized and most innovative therapeutic strategies, is a daily challenge for clinicians. The author thanks Dr. Lara Chalabreysse for providing the pathologic image for the figure and Dr. Caroline Leroux (from the Joint Research Unit “Viral Infection and Compared Pathology”) for fruitful discussions. PI3K as a Potential Therapeutic Target in Thymic Epithelial TumorsJournal of Thoracic OncologyVol. 11Issue 8PreviewThymic epithelial tumors (TETs) are rare tumors originating from the epithelium of the thymus with limited therapeutic options beyond surgery. The pathogenesis of TETs is poorly understood, and the scarcity of model systems for these rare tumors makes the study of their biology very challenging. Full-Text PDF Open Archive