Thyroid cancer

Most poorly differentiated thyroid tumors have lost expression of the normal p53 tumor suppressor gene through inactivating mutations. p53 is a transcription factor mediating critical cellular responses, including cell cycle arrest and apoptosis, after exposure to DNA-damaging stimuli. Mutations in the p53 gene seem to be late genetic events associated with loss of differentiation and are, at least in part, responsible for the aggressive behaviour of advanced and/or dedifferentiated tumors. Restoration of wild-type (wt) p53 expression has recently been used in a variety of experimental cancer models including thyroid cancer and has been tested in human clinical trials. p53 restoration has been shown to be associated with a bystander effect, which means that not only p53-transduced cells are killed, but also that surrounding nontransduced cells are killed by the transduction of their neighbors. A bystander effect is highly desirable for a therapeutic gene, because it reduces the level of transduction efficiency required for successful gene therapy, which represents one of the crucial technical hurdles for in vivo gene therapy applications. The bystander effect of p53 gene therapy results from its antiangiogenic effect, which is the result of downregulation of vascular endothelial growth factor (VEGF) and upregulation of thrombospondin, a potent inhibitor of angiogenesis.

Several studies of p53 gene therapy of thyroid cancer have been reported. Expression of wild-type tumor suppressor gene p53 (wt-p53) in a p53-null thyroid carcinoma cell line (FRO) resulted in decreased cell growth in vitro and inhibition of tumorigenesis in vivo in nude mice. 40% of mice inoculated with p53-transfected FRO were tumor-free and 60% developed small hypovascular tumors indicating suppression of neovascularization. In another study, retroviral p53 gene transfer into p53 mutant papillary thyroid cancer cells (NPA) resulted in a dose-dependent inhibition of tumor cell growth and enhanced chemosensitivity to adriamycin in vitro and in vivo . Using a replication-deficient adenovirus expressing wild-type tumor suppressor gene p53 (wt-p53), Nagayama et al. evaluated the therapeutic efficacy of p53 restoration in four human anaplastic thyroid cancer cell lines harboring p53 mutations (ARO, FRO, NPA, WRO) and normal human thyroid follicular cells in vitro and in vivo . Adenovirus-mediated p53 expression resulted in dose-dependent cell killing in thyroid cancer cell lines, whereas normal thyroid cells were relatively resistant to p53-mediated cell death despite their highest adenovirus infectivity. The mechanism of cell killing was shown to be apoptosis. In addition, wt-p53 expression sensitized some of the cell lines to the chemotherapeutic effect of doxorubicin (FRO and NPA cells) and 5-fluorouracil (FRO cells). In vivo experiments using FRO and NPA cell xenografts in nude mice showed inhibition of tumor growth following direct injection of the adenovirus expressing wt-p53. This effect was augmented by combination with doxorubicin, resulting in tumor regression. To develop an adenoviral gene transfer system that replicates exclusively in wt-p53-deficient cancer cells thereby limiting the cytotoxic effect of a replication-competent adenovirus lacking E1B55K, Nagayama et al. used the geneinactivation strategy using a p53-regulated Cre/loxP system consisting of two recombinant adenoviruses. One contains an expression unit of the synthetic p53-responsive promoter and the Cre recombinase gene, and the other adenovirus contains two expression units: the first consists of the E1A gene flanked by a pair of loxP sites downstream of the constitutive CAG promoter, and the second consists of the E1B19K gene under the control of the cytomegalovirus (CMV) promoter. Coinfection of these two adenoviruses into p53 expressing cells leads to expression of Cre recombinase, which then excises the E1A gene that is flanked by a pair of loxP sites, thereby stopping virus replication. In cells without p53 expression, however, Cre recombinase is not expressed, the E1A gene not excised, and virus replication takes place thereby causing cell lysis. Another, more recent study by Imanishi et al., indicated that the histone deacetylase inhibitor depsipeptide enhances apoptotic killing by p53 gene transfer in anaplastic thyroid cancer cell lines (FRO and WRO cells), suggesting that this combination treatment strategy might also be useful in the treatment of undifferentiated thyroid carcinomas.