Amplification of the gene that encodes the lineage-specific transcription factor TTF-1, located at 14q13.3, was recently identified as a novel molecular abnormality in 12% of lung adenocarcinomas [24
]. The clinical significance of TTF-1
gene amplification and the relationship with TTF-1 protein expression have not been established. In this study, we investigated TTF-1 expression and TTF-1
amplification in lung adenocarcinomas from a cohort of 89 consecutive patients. We found that TTF-1 is expressed in 72% and the TTF-1
gene is amplified in 7%. We evaluated the prognostic significance of TTF-1
gene amplification and the relationship with TTF-1 protein expression in patients with lung adenocarcinoma.
TTF-1 expression was high in 48%, low in 24%, and absent in 28% of cases. Similar to recently published studies, we found that patients with lung adenocarcinomas with TTF-1 expression had a better outcome than patients with no TTF-1 expression (median overall survival times of 77.8 and 30.5 months respectively, P=0.002) [15
]. We also found that patients with low or high TTF-1 expression have a similar overall survival (P=0.995). Although TTF-1
gene amplification was not a predictor of overall survival in our entire patient group (P=0.508), in patients with adenocarcinomas with TTF-1 expression, TTF-1
gene amplification was associated with poor outcome (median overall survival 39.5 vs 87.5 months, P=0.113). The finding that TTF-1 amplification is a potential indicator of poor prognosis is consistent with the findings of Kendall et al who reported that focal amplification of TTF-1 correlated with a higher stage at diagnosis and recurrence [31
]. The prevalence of TTF-1
amplification in our study focused only on a cohort of patients with well characterized lung adenocarcinomas and excluded other types of non-small cell lung carcinomas, and differences in TTF-1
amplification in different tumor types might explain the reported discrepancies from previously published analyses[24
]. Patients with lung adenocarcinomas with TTF-1 expression and TTF-1
amplification have similar overall survival as patients with adenocarcinomas with no TTF-1 expression (P
In our study there were two cases harboring TTF-1
amplification that had no nuclear TTF-1 expression by immunohistochemical staining. Kendall et al reported amplification of 14q13.3 without TTF-1 protein expression in the squamous cell cancer cell line NCI-H2170 [31
]. They found that the RNA message of TTF-1 was undetectable in this cell line, suggesting gene silencing at the transcription level. In the two cases in our study that had TTF-1
amplification but no nuclear TTF-1 expression, there was cytoplasmic TTF-1 staining suggesting that although TTF-1 is expressed, it is aberrantly localized to the cytoplasm. While TTF-1
gene amplification has been shown to have a role in oncogenesis, we expect that this would be true only for cases that have TTF-1
amplification and nuclear TTF-1 protein expression since TTF-1 functions as a transcription factor. Lack of nuclear expression for two cases with amplification in this study indicates that TTF-1 staining by immunohistochemistry must be performed in conjunction with analysis for TTF-1
gene amplification in evaluating for the biologic or prognostic significance of TTF-1 amplification.
Our study is limited by the small number of patients with TTF-1 gene amplification. In addition, we were able to examine only three randomly selected tissue cores from each tumor. Additional studies are needed to validate the potential diagnostic utility of the characteristics we report, including determination of intraobserver and interobserver variation in their evaluation; their sensitivity, specificity, predictive values for the presence of TTF-1 positivity, and correlation between the different growth patterns, TTF-1 gene expression and TTF-1 amplification; their intratumoral heterogeneity and role of sampling in evaluation; and the findings in lung adenocarcinomas. However, the results of our study support previous findings that TTF-1 expression is a positive prognostic factor in patients with lung adenocarcinomas. Additionally, this is the first report to suggest that not all cases of adenocarcinoma that harbor TTF-1 amplification express nuclear TTF-1 protein, indicating the importance of evaluating protein expression levels in conjunction with TTF-1 amplification status.
Our study provides support for the concept of tumor heterogeneity in lung adenocarcinoma. Therefore, our findings suggest that lung adenocarcinoma with TTF-1 expression in combination with no TTF-1 amplification represents a distinct subgroup of lung adenocarcinoma with characteristic pathologic and clinical features, and improved overall survival.