Gene therapy of rat medullary thyroid cancer by naked nitric oxide synthase II DNA injection

Background Nitric oxide (NO), produced by NO synthase II (NOS II), is the main mediator of the tumoricidal action of activated macrophages. In the present study we examined the potential of the NOS II gene as a suicide gene for medullary thyroid cancer (MTC) therapy. Methods We orthotopically transplanted rMTC 6-23 cells into the inbred strain of Wag/Rij rats and constructed a plasmid carrying the NOS II gene under the control of the cytomegalovirus (CMV) promoter. Results Successive injections of tumor cells (Day 0) and naked DNA (Day 2) caused strong inhibition of tumor growth (50%, p<0.05). Plasmid injection into established tumors (14-day tumors) resulted in the development of large cavities due to tumor cell destruction, with a significant reduction in tumor tissue volume (35%, p<0.05). Adjacent quiescent tissues were unaffected. Cell death occurred by apoptosis as demonstrated by specific labeling. Macrophages and CD4⁺ lymphocytes were recruited in the treated tumors. However, tumor-specific T lymphocytes were undetectable in the spleen of treated rats. In control experiments using Lac Z as a reporter gene, expression of β-galactosidase was detected in only 1% of the tumor cells. Conclusions Despite a low gene transfer efficiency, NOS II plasmid produced a strong anti-tumor action resulting from its marked 'bystander' effect mainly due to NO production and diffusion. Therefore the NOS II gene appears to be a promising suicide gene therapy of human cancer.