Lack of PTEN expression in non-small cell lung cancer could be related to promoter methylation

Purpose: The PTEN gene at chromosome 10q23.3 is a tumor-suppressor gene that is inactivated in several types of human tumors. Although mutation and homozygous deletion are the most common mechanisms of PTEN inactivation, promoter methylation and translational modification can also account for PTEN silencing. The aim of this study was to investigate the expression of PTEN protein in primary non-small cell lung cancer (NSCLC) samples and to investigate the promoter methylation status of the gene in a panel of NSCLC cell lines as well as primary tumors. Experimental Design: We analyzed PTEN expression by immunohistochemistry in tissue samples from 125 patients with early-stage NSCLC. We also evaluated PTEN promoter methylation status by methylation-specific PCR in 20 micro-dissected PTEN-negative primary tumors from among the last specimens as well as in a panel of 16 NSCLC cell lines. Western and Northern blotting were performed in the same panel of NSCLC cell lines. Results: Thirty (24%) of the 125 specimens showed a lack of staining for PTEN. PTEN methylation was detected in 7 (35%) of the 20 PTEN-negative NSCLC samples and in none of the 10 PTEN-positive NSCLC samples that were micro-dissected. Furthermore, PTEN methylation was observed in 11 (69%) of the 16 NSCLC cell lines tested. PTEN mRNA expression was increased in the NCI-H1299 cell line by in vitro treatment with the demethylating agent 5-aza-2′-deoxycytidine. PTEN methylation was well correlated with PTEN expression in NSCLC cell lines by Western and Northern blot (P = 0.025). Conclusions: Although genetic alterations of the PTEN gene are rare in NSCLC, loss of PTEN protein is not an uncommon event in early-stage NSCLC. Lack of PTEN expression may be partially explained by promoter methylation.