Transcriptionally targeted retroviral vector for combined suicide and immunomodulating gene therapy of thyroid cancer

Gene therapy may be an effective approach to thyroid carcinoma refractory to conventional treatment. A transcriptionally targeted retroviral vector for gene therapy of thyroid carcinomas was generated replacing the viral enhancer with the enhancer sequence of the human thyroglobulin (TG) gene, yielding a chimeric long-terminal repeat. The TG enhancer was used to drive the expression of either a reporter gene (beta-galactosidase) or two therapeutic genes, i.e. the prodrug-activating enzyme thymidine kinase of herpes simplex virus (HSV-TK) and human IL-2, separated by an internal ribosome entry site. The corresponding vector having an unmodified long-terminal repeat was used as control. The targeted vector allowed selective transgene expression and cell killing in differentiated thyroid tumor cells but not in anaplastic thyroid carcinoma cells and nonthyroid cells, as demonstrated by quantitative RT-PCR and cytotoxicity assays. Nude mice injected with tumor cells underwent near complete or complete regression of tumors transduced with the control vector after ganciclovir treatment. On the other hand, infection with the thyroid-specific vector led to regression only of TG-expressing tumors. In addition, tumors expressing human IL-2 showed significant growth retardation, compared with non-transduced tumors while exhibiting signs of necrosis and presence of an inflammatory infiltrate. However, HSV-TK/IL-2 plus ganciclovir was significantly more efficient than HSV-TK/IL-2 alone in eradicating tumor masses. Our results indicate that replacement of viral enhancer with TG enhancer confers selectivity of transgene expression in thyroid cells. Thus, the combined thyroid-specific expression of two therapeutic genes (cytokine and suicide genes), although a safe tumor-targeted treatment, would allow an increased anticancer effect.