ENHANCER SEQUENCES OF THE DF3 GENE REGULATE EXPRESSION OF THE HERPES-SIMPLEX VIRUS THYMIDINE KINASE GENE AND CONFER SENSITIVITY OF HUMAN BREAST-CANCER CELLS TO GANCICLOVIR

A potentially novel therapeutic strategy for breast cancer treatment i nvolves sensitization of tumor cells to chemotherapy through gene tran sfer. Clinical application of this approach, however, may be limited b y the lack of target cell specificity of currently available gene deli very techniques. Development of vectors with tumor-selective gene expr ession could overcome this problem. The DF3/MUC1 gene encodes a high m olecular weight mucin-like glycoprotein which is overexpressed at the transcriptional level in the majority of human breast cancers. To deve lop a breast tumor-selective enhancer, we cloned the upstream region o f the DF3 gene and have identified a 114-base pair enhancer region tha t can modulate transcription from heterologous promoters. The present studies demonstrate that the DF3 enhancer sequences can direct selecti ve gene expression in DF3-positive breast carcinoma cells. DF3-positiv e breast carcinoma cell lines transfected with herpes simplex virus th ymidine kinase gene expression cassettes modified by the DF3 enhancer were markedly more sensitive to killing by ganciclovir than were the s ame cells transfected with the expression cassettes lacking the DF3 en hancer. DF3-negative cell lines transfected with the DF3 enhancer cons tructs, however, were no more sensitive to ganciclovir than were cells treated with the unmodified expression plasmids. Consistent with an i nnocent bystander effect, nontransfected human breast carcinoma cells were susceptible in a cell density-dependent manner to ganciclovir-ind uced cell killing when adjacent to transfected cells. The results also demonstrate that the DF3 enhancer sequences can be effectively incorp orated into a retroviral vector to mediate selective gene expression f ollowing retroviral infection. These findings suggest that the DF3 pro moter/enhancer may be useful for incorporation into vectors designed f or gene therapy of breast cancer.