Gene therapeutic approaches for medullary thyroid carcinoma treatment

Medullary thyroid carcinoma (MTC), a neoplasm of thyroid C-cells, is characterized by dominant activating mutations in the RET proto-oncogene. Currently therapy is restricted to surgical removal of all neoplastic tissue lacking alternative forms of treatment such as chemotherapy or radiotherapy. Therefore MTC is a particularly attractive target for gene therapeutic approaches. Many promising gene therapy strategies have been used in various animal models of MTC, showing enhanced antitumoral efficacy, and these will hopefully extend our current standard of care in the future. These approaches can tentatively be subdivided into four groups: (a) Inhibition of oncogenic RET signaling, (b) suicide gene therapy, (c) immunotherapy, and (d) combination of immunotherapy and suicide approaches. To block oncogenic signal transduction dominant-negative RET mutants were delivered into tumor cells and found to possess strong antineoplastic activity, including tumor growth suppression and increased animal survival. Suicide gene therapeutic approaches applied to MTC treatment featured either gene transfer of herpes simplex virus thymidine kinase with concomitant application of ganciclovir or delivery of nitric oxide synthase II. Here antitumor effects were attributed to the occurrence of substantial bystander activities. Immunotherapy approaches comprised stimulation of immune response by delivery of interleukin 2 or 12. Finally, treatment with herpes simplex virus thymidine kinase/ganciclovir in combination with interleukin 2 was found to be superior over either treatment alone. This review discusses the various gene therapeutic approaches applied to MTC treatment in detail, gives an overview on the diverse vector systems used to achieve efficient transduction of thyroid cancer cells, and points out the strategies employed to accomplish target cell selective gene expression thereby contributing to enhanced safety of gene therapy for MTC.