A chimeric fusion of the hASH1 and EZH2 promoters mediates high and specific reporter and suicide gene expression and cytotoxicity in small cell lung cancer cells

Transcriptionally targeted gene therapy is a promising experimental modality for treatment of systemic malignancies such as small cell lung cancer (SCLC). We have identified the human achaete-scute homolog 1 (hASH1) and enhancer of zeste homolog 2 (EZH2) genes as highly upregulated in SCLC compared to a panel of representative normal tissues. Here, we evaluate the use of regulatory regions from the hASH1- and EZH2-promoter regions alone and in combination for suicide gene therapy of SCLC. Two hASH1-promoter regions comprising 0.3 and 0.7 kb immediately upstream of (and including) the transcription start site were tested. Both constructs induced reporter gene activity (up to sevenfold SV40-promoter activity) in all tested classic (hASH1 positive) SCLC and in two hASH1- negative SCLC cell lines, whereas gene activity was low or absent (<4% of SV40 activity) in one hASH1- negative SCLC and in all control cell lines tested. To evaluate its therapeutic potential, the 0.7 kb hASH1 proximal-promoter region was evaluated for cytotoxicity in a suicide gene assay. The construct induced SCLC cytotoxicity at levels equivalent to those observed with the SV40 promoter, while control cells remained unaffected by the treatment. Analogously, a 1.1 kb EZH2-promoter region was evaluated by reporter and suicide gene assays. The EZH2 promoter potently induced reporter gene activity in SCLC (up to 25-fold of SV40 activity) while moderate reporter activity (up to 12% of SV40 activity), was detected in the control cells. However, in the suicide gene assay both control and SCLC cells demonstrated sensitivity indicating lack of promoter specificity. Finally, we fused the 0.7 kb hASH1 promoter to the EZH2 promoter generating a chimeric hASH1EZH2 regulatory construct. The chimeric promoter demonstrated increased activity in SCLC cells compared to the hASH1 promoter alone while retaining specificity in control cells. The hASH1EZH2 promoter thus constitutes a promising transcriptional regulator for SCLC gene therapy.