The telomerase reverse transcriptase promoter drives efficacious tumor suicide gene therapy while preventing hepatotoxicity encountered with constitutive promoters

In human cells, telomerase activity is regulated by transcriptional control of the telomerase reverse transcriptase gene (hTERT) whose product is the catalytic subunit of the enzyme. The hTERT promoter is active in virtually all types of tumors and immortal cells. but is silent in most adult somatic tissues, in this study, we placed the herpes simplex virus thymidine kinas e gene under the control of the hTERT promoter with the aim of restricting its expression to tumor cells. In transfection experiments, the hTERT promo ter driven thymidine kinase gene (hTERTp/TK) conferred ganciclovir sensitiv ity to all tumor and immortal cell lines tested. whereas normal somatic cel ls remained largely unaffected. Human hTERTp/TK-positive cancer cells impla nted in nude mice developed into tumors that could be eradicated by gancicl ovir treatment. The hTERTp/TK cassette was inserted into an adenovirus vect or and its efficacy in reducing tumor growth was compared with that of an a denovirus carrying the thymidine kinase gene under the control of the cytom egalovirus immediate-early promoter (CMVp/TK). In a xeno-graft model using the human 143B osteosarcoma cell line, a single injection of either virus r esulted in equivalent tumor regression and survival upon ganciclovir treatm ent In animals injected intratumorally with the CMVp/TK adenovirus, express ion of the thymidine kinase gene was detected in tumors, as well as in live r samples. Expression of the suicide gene in combination with ganciclovir r esulted in severe liver histopathology and in an elevation of hepatic enzym es. In sharp contrast, when the hTERT promoter controlled the thymidine kin ase gene, transgene expression was observed in tumors, but not in liver sam ples. Normal liver function in these animals was confirmed by serum levels of hepatic enzymes that were indistinguishable from those of control health y mice. These results indicate that by restricting thymidine kinase express ion to tumor cells, the hTERT promoter allows the tumoricidal effect of the suicidal gene to be exerted without detrimental consequences on healthy ti ssues and vital organs. The tight specificity of expression imparted by the hTERT promoter will assist the development of novel approaches to the trea tment of a broad array of cancer types.