Transcriptional repression of DLEC1 associates with the depth of tumor invasion in oral squamous cell carcinoma.

The objective of this study was to clarify the expression and epigenetic regulation of DLEC1, a candidate tumor suppressor gene (TSG) located at 3p21.3-p22, in oral squamous cell carcinoma (OSCC) and the clinical relevance of its down-expression. Quantitative RT-PCR was performed to exam the expression level of DLEC1 in matched OSCC and normal oral samples from 57 prospectively enrolled patients (with additional matched leukoplakia samples from 9 patients). We defined DLEC1 down-expression as a 2-fold decrease in expression of DLEC1 between normal tissues and tumors, and determined its correlation with clinical characteristics. Methylation-specific PCR (MSP) and bisulfite sequencing were used to evaluate the promoter methylation status of DLEC1 in 19 OSCC, 19 oral leukoplakia (OL), and 17 normal oral tissues. A statistically significant association between DLEC1 down-expression and invasive depth of OSCC was observed (P=0.026). Besides, expression of DLEC1 decreased sequentially from normal tissues to OL and then to OSCC (P<0.05), which was inversely correlated with methylation status of the DLEC1 promoter. Promoter methylation of DLEC1 increased progressively among normal tissues, OL, and OSCC, as revealed by MSP, and confirmed by sequencing. Treatment of OSCC cell lines with 5-aza-2'-deoxycytidine (5-Aza-dC) reversed the methylation and restored DLEC1 expression. Our results demonstrating that down-expression and promoter methylation of DLEC1 increased from normal tissues to premalignancies and then to malignancies. Furthermore, its transcriptional repression is associated with the depth of tumor invasion.