Tyrosine kinase receptor expression in thymomas
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- Henley, J.D., Cummings, O.W. & Loehrer, Sr., P.J. J Cancer Res Clin Oncol (2004) 130: 222. doi:10.1007/s00432-004-0545-z
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Promising new therapies for neoplasia include tyrosine kinase receptor antagonists. Tyrosine kinase oncogenes present an appealing anti-tumor drug target since they play an integral role in a variety of cellular responses including cell proliferation and differentiation. We previously demonstrated a high rate of epidermal growth factor receptor (EGFR) expression in advanced-stage thymic epithelial tumors. More recently, we have examined c-KIT (CD117) expression in a similar series of tumors.
Tumor from 35 patients seen at our institution for treatment of advanced-stage thymoma was available. Twenty thymomas and 15 thymic carcinomas were assessed for c-KIT expression. Tissue sections of tumor were stained immunohistochemically with anti-c-KIT (Oncogene). Either cytoplasmic or membrane staining was considered positive. Appropriate controls were performed. Positive staining for c-KIT was present in 12 tumors (11 thymic carcinomas and 1 thymoma).
In distinction to EGFR, c-KIT is expressed more commonly in thymic carcinomas (73% of carcinomas) than in thymoma (5% of thymomas).
An EGFR negative/c-KIT positive staining pattern is typical of thymic carcinoma, whereas thymomas are generally EGFR positive/c-KIT negative. Possible therapeutic implications of these observations remain to be determined.
Keywordsc-KITCD 117Tyrosine kinaseThymomaThymic carcinoma
Recent advances (Bonner et al. 2000, Hoffmann et al. 1997, Kelloff et al. 1993, Rubin et al. 2000, Yang et al. 2000, 1999] using tyrosine kinase antagonists in the treatment of neoplasia have stimulated our interest in expression of these molecules in epithelial tumors of the thymus. Tyrosine kinase oncogenes present an appealing anti-tumor drug target since they play an integral role in a variety of cellular responses including cell proliferation, differentiation, migration, neovascularization, and apoptosis (Gill et al. 1987, Woodburn1999).
To date, we have investigated the expression of two different tyrosine kinase receptors, specifically c-KIT and epithelial growth factor receptor (EGFR), in a relatively large number of malignant thymic tumors. Both are transmembrane growth factor receptors expressed on a variety of neoplastic and non-neoplastic cells types. While c-KIT is a useful marker in the identification of gastrointestinal stromal tumors, mast cell tumors, and seminomatous germ cell tumors (Gibson and Cooper 2002), EGFR has been implicated in the pathobiology of lung (Fontanini et al. 1995, Veale et al. 1987 and head and neck (Issing et al. 1996, Issing et al. 1993, Stanton et al. 1994, Takes et al. 1998) tumors.
Our results for EGFR, which corroborated the work of others (Gilhus et al. 1995, Hayashi et al. 1995, Pescarmona et al. 1993), were previously reported (Henley et al. 2002). To summarize, we found EGFR expression in large percentage (74%) of malignant thymomas and a much lower rate of positivity in thymic carcinomas (33%). Herein, we report our result for c-KIT, which has potential implications for treatment.
Materials and methods
Thymic epithelial tumors from 35 patients treated at Indiana University Hospitals (IUH) comprised the study material. Most study material was de-identified with respect to patient information. In all cases hematoxylin and eosin-stained slides were reviewed to confirm the referral diagnosis of thymoma or thymic carcinoma; additionally, formalin-fixed, paraffin-embedded tumor was obtained for immunohistochemical study of c-KIT expression. Five-micron sections of tumor were deparaffinized, rinsed with phosphate-buffered saline (PBS), and pretreated with proteinase K (DAKO, Carpinteria, CA) for 5 min. Between rinsings with PBS, sections were incubated with monoclonal anti-c-KIT (Oncogene), prediluted, for 30 min. Bound antibody was detected using a 3,3′-diaminobenzidine+substrate-chromogen system (Dako EnVision+System). A c-KIT expressing gastrointestinal tumor was run in parallel as a positive control.
Staining was considered either positive or negative and was not graded beyond being considered either strong or weak. Membrane and/or cytoplasmic staining was considered positive. Since the majority of our tissue specimens were de-identified, clinical follow-up was not obtained.
EGFR/c-KIT immunohistochemical staining profiles
EGFR (−)c-KIT (−)
EGFR (−) c-KIT (+)
EGFR (+)c-KIT (−)
EGFR (+) c-KIT (+)
Thymoma (WHO type B)
Thymic carcinoma (WHO type C)
The current study is the first to examine c-KIT expression in a large series of malignant thymic tumors. We found c-KIT expression in 73 and 5% of thymic carcinomas and thymomas, respectively. These results contrast our EGFR results, and, thus, thymic carcinomas tend to be EGFR negative/c-KIT positive with the reverse pattern observed in thymoma.
The therapeutic implications of the differential tyrosine kinase expression in thymic epithelial tumors remain to be determined. Studies with EGFR have progressed to clinical trials at our institution using an EGFR inhibitor (Iressa) in the treatment of advanced-stage thymoma. Regarding c-KIT, different types of activating mutations respond differentially to KIT inhibitors, so it will be relevant to attempt to characterize any mutations of the KIT gene (Longley et al. 2001) that may be consistently encountered in thymic carcinoma.
The majority of the tissue used in this study was obtained from outside laboratories and was received as formalin-fixed, paraffin-embedded tissue blocks. This limitation prevented uniform specimen fixation and processing to allow for optimum antigen preservation. This may have imparted an unknown negative staining bias on our results.
We can offer no conclusions on the c-KIT expression and biologic potential given limitations of our study. Virtually all of the study material was obtained from patients referred to our institution for treatment of advanced-stage disease, and, thus, most thymomas studied were malignant type I or cortical-type tumors by the classification schemes of Levine and Rosai (1978) and Marino and Muller-Hermelink (1985), respectively. The histology of our 11 c-KIT-positive thymic carcinomas included 9 lymphoepithelioma-like/undifferentiated carcinomas, 1 squamous cell carcinoma (weak staining), and 1 mixed atypical thymoma/undifferentiated carcinoma. Interestingly, the latter case showed increased staining for c-KIT in areas of carcinoma relative to atypical thymoma.
Examination of tyrosine kinase expression is of timely relevance considering the its potential use as a chemotherapeutic target. Commercially available anti-neoplastic agents directed against tyrosine kinase receptors include trastuzumab (Herceptin, Genentech), imatinib mesylate (Gleevec, Novartis Pharmaceuticals), and gefitinib (Iressa, AstraZeneca Pharmaceuticals). Given the high incidence of EGFR and c-KIT expression in thymoma and thymic carcinoma, respectively, these evolving therapeutic strategies merit further consideration in the treatment of advanced-stage thymoma.
We thank Ms. C. Dodson and Mrs. L. Baldridge for their technical expertise and assistance with immunohistochemical staining.