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Molecular Oncology, Markers, Clinical Correlates |
MedImmune, Inc., Gaithersburg, Maryland 20878 [M. S. K.], and Duke University Medical Center, Durham, North Carolina 27710 [M-B. M., D. H. H.]
| ABSTRACT |
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Experimental Design: In our present study, immunohistochemical analyses of a retrospective database of pathologic specimens were used to demonstrate that the EphA2 receptor kinase is frequently overexpressed in NSCLC.
Results: Initial presentation with high levels of EphA2 predicts subsequent survival, overall relapse, and site of relapse. Specifically, high levels of EphA2 in the primary tumor predict brain metastases, whereas low levels of EphA2 relate to disease-free survival or contralateral lung metastasis.
Conclusions: These data suggest that EphA2 may provide a molecular marker to identify and predict patients who have isolated brain metastases. Moreover, the high levels of EphA2 in lung cancer may provide an opportunity for therapeutic targeting.
| Introduction |
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One of the main contributing factors to such a poor survival rate is that a significant number of patients are understaged at the time of resection (4 , 6) . There is an urgent need for a detection method that is more sensitive and reliable than current approaches (4 , 7) . In particular, many of the most dangerous forms of lung cancer arise when a population of tumor cells gains the ability to colonize the brain (8 , 9) . Brain metastases comprise greater than one-quarter of all recurrences in patients with resected NSCLC and are a major source of lung cancer morbidity and mortality. When symptomatic, the median survival of patients with brain lesions is <4 months (5 , 6 , 10) . Thus, it is critical to identify and treat those patients who are at risk for brain metastases before the onset of symptoms.
Recent investigation has sought to identify factors that predict poor prognosis. Unfortunately, the use of conventional histopathological variables (performance status, subtype, size, differentiation, and mitotic rate) to construct a risk model has been limited by low prevalence and the discontinuous nature of individual variables (4 , 11 , 12) . These limitations have underscored recent efforts to ask if molecular markers might improve risk stratification (4) . A series of genetic and epigenetic changes facilitate the transition from normal bronchial epithelium to carcinoma and examples include mutation or overexpression of Ras oncogenes (K-Ras, H-Ras, and N-Ras), receptor tyrosine kinases (epidermal growth factor receptor and HER2) and cell cycle proteins (retinoblastoma and p53; Ref. 4 ). However, none of these markers has been linked with the recurrence of brain metastases.
Our laboratories have sought to identify the causes of metastatic cell behavior, with emphasis upon the identification of markers that predict metastatic recurrence in the clinical setting. Our most recent studies have focused upon the EphA2 receptor tyrosine kinase, which is overexpressed and functionally altered in a variety of different solid tumor types (13, 14, 15, 16, 17, 18) . EphA2 has been linked to the regulation of cellular behaviors that promote a metastatic phenotype and appears to provide an exciting new opportunity for therapeutic targeting (13 , 15) . In this study, we demonstrate that EphA2 provides a new and independent marker of lung cancer recurrence and survival. Importantly, high levels of EphA2 identify those patients that are at risk of lung cancer metastasis to the brain. Thus, EphA2 provides a new and independent marker of aggressive behavior with potential clinical utility in cancer diagnosis and treatment.
| Materials and Methods |
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Histological examination of the specimens was performed by board-certified pathologists at the Duke University Medical Center at the time of resection. Clinical grading of the patients was performed by Dr. Harpole, a board certified thoracic surgeon. Sections (46 µM) sections were obtained from each tissue block by microtome sectioning and deparaffinized as detailed previously (4) . The samples were then stained with EphA2-specific polyclonal (catalogue no. SC-924; 1:100; Santa Cruz Biotechnology, Santa Cruz CA) or monoclonal antibodies (catalogue no. 05-480; 1:500; Upstate Biotechnology, Inc., Lake Placid, NY) as detailed previously (16) . Powerblock Universal Blocking Reagent (catalogue no. HK085-5K; BioGenex Laboratories, San Ramon, CA) was used to block nonspecific binding. The slides were developed using diaminobenzidine as the chromogen and counterstained with hematoxylin. Known positive tumors and normal lung tissue were used as positive and negative controls, respectively.
Data Interpretation and Analysis.
All slides were read independently by two experienced observers who were blinded as to the tissue source. Differences in immunohistochemical scores were rare and resolved by consensus. There were no instances of one-third discordance (2 point differences). Each sample was scored using a 03 scale, with 0 denoting no staining. In general, the tumor cells were found to stain uniformly across the sample, thus the scoring system did not consider the fraction of tumor cells with positive staining. All statistical analyses were performed using statistical software (Microsoft Excel). A two-tailed, homoscedastic Students t test was used for comparisons among primary tumor samples. A two-tailed, paired analysis was used to compare EphA2 immunoreactivity in matched pairs of primary tumor and brain metastases. All analyses defined P < 0.05 as significant.
| Results |
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5-year survival (average staining intensity of 1.8; P = 0.02). However, EphA2 immunoreactivity was not significant in predicting of one year survival (P = 0.099), although this likely resulted from the relatively small sample size with 1-year follow-up (N =).
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EphA2 Levels in Brain and Contralateral Lung Metastases.
The site of disease recurrence was available for a subset of the clinical specimens (N = 60). An analysis of EphA2 suggested an interesting implication for interpreting the results of EphA2 expression in overall disease recurrence. The average intensity of EphA2 staining was highest in those patients who subsequently developed brain metastases (Table 3)
. In contrast, EphA2 levels were lowest in patients whose metastases were restricted to the contralateral lung. The differences between EphA2 levels in those patients who progressed to brain metastases differed from patients without recurrence (P = 0.01) and those who progressed to have metastases restricted to the contralateral lung (P = 0.02). Perhaps more importantly, removal of patients, whose disease was restricted to contralateral lung (N = 7), was sufficient to provide statistical significance about disease recurrence and present status (P = 0.01 and P = 0.02; respectively). Thus, high levels of EphA2 immunoreactivity appear to identify patients who develop metastases outside of the lung and also predict the recurrence of those metastases.
On the basis of the link between EphA2 and brain metastasis, the immunoreactivity of EphA2 in primary tumors and brain metastases was directly compared (Fig. 4)
. A matched set of clinical materials, obtained from the primary tumor or a brain metastasis of the same donor, was stained with EphA2 monoclonal antibodies (clone D7). The brain metastases demonstrated significantly higher levels of EphA2 than the matched primary tumor (P = 0.0005). These results provide additional evidence linking high levels of EphA2 with brain metastasis.
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| Discussion |
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One novel aspect of this study is a demonstration that EphA2 may convey important diagnostic and predictive information to assist the clinical management of lung cancer. High levels of EphA2 also apply to other tumor types and future studies could address whether EphA2 might similarly convey predictive information about clinical outcome.
As a transmembrane receptor, EphA2 could theoretically serve as a substrate for extracellular proteases and thus be shed into local fluids (e.g., sputum, serum). Alternatively, circulating EphA2 DNA could provide a means of identifying patients with metastatic disease (7) . Thus, accurate and sensitive detection of circulating EphA2 protein or DNA could provide a much-needed diagnostic marker for metastatic disease and also serve as a surrogate marker to assess and fine-tune clinical management of metastatic disease.
The causes of EphA2 overexpression in lung cancer remain largely unclear. What is known suggests that EphA2 expression is tightly regulated during development and in adults and that malignant transformation frequently circumvents these regulatory mechanisms (13 , 16 , 17 , 19, 20, 21, 22, 23, 24) . For example, EphA2 expression is regulated by multiple members of the p53 family of transcription factors, which are generally understood to play critical roles in cancer (25) . Additional studies have linked the up-regulation of EphA2 with intracellular signaling by oncogenic Ras (20 , 22 , 26) . In light of evidence that the Ras signaling pathway is frequently up-regulated in aggressive lung cancers, this particular mechanism may account for much of the observed EphA2 overexpression in metastatic disease.
One of the more intriguing outcomes of this study is a demonstration of differential EphA2 expression in brain versus contralateral lung metastases. EphA2 overexpression does not seem to promote growth under ideal circumstance but instead favors the growth and survival of tumor cells in the context of a foreign microenvironment (13 , 15) . This hypothesis was based on in vitro analyses of EphA2-overexpressing cells, but one interpretation of the present findings is that EphA2 plays a similar role in vivo. Specifically, the relatively low levels of EphA2 in patients who will develop contralateral disease might be consistent with the fact that the contralateral lung represents an accommodating local microenvironment that could facilitate tumor growth and survival without the need for EphA2. In contrast, the foreign microenvironment of the central nervous system might select for those metastatic cells that overexpress EphA2 and thus are capable of thriving under more adverse conditions. It is also tempting to speculate that high levels of EphA2 facilitate homing to the brain. This idea is consistent with the fact that Eph receptors and their ligands are primarily expressed in the CNS (27, 28, 29) . Thus, the relative overexpression of EphA2 within brain metastases could reflect the consequences of the local brain microenvironment.
Antibody targeting of EphA2 can selectively target tumor cells while minimizing toxicities to normal cells (15) . This finding offers an opportunity to target the large number of lung cancers that overexpress EphA2. On the basis of the fact that the highest levels of EphA2 are found in the most aggressive tumors (13 , 14 , 16 , 20) , selective targeting of EphA2 could provide an opportunity for therapeutic intervention against metastatic disease, which is imperative given the poor prognosis of patients with inoperable NSCLC brain metastases.
| FOOTNOTES |
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1 To whom requests for reprints should be addressed, at MedImmune, Inc., 35 West Watkins Mill Road, Gaithersburg, MD 20878. Phone: (240) 632-4639; E-mail: kinchm{at}medimmune.com ![]()
2 The abbreviation used is: NSCLC, non-small cell lung cancer. ![]()
Received 7/15/02; revised 10/ 7/02; accepted 10/11/02.
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