High-level expression of wild-type p53 from the novel p53-responsive pCLPG retrovirus halts the proliferation of tumor cells regardless of their p53 status

One of the barriers to the success of gene therapy is appropriate and adequate expression of the therapeutic gene by the vector. Although retroviral vectors have been widely used in both laboratory and clinical gene transfer studies, many reports in the literature indicate that retroviral vectors suffer silencing of transgene expression in vivo and that modification of the LTR is necessary in order to restore reliable transcription. To this end, we have constructed a retroviral gene transfer system with a modified LTR such that transactivation is mediated by the tumor suppressor p53. Our approach may be beneficial for the achievement of high-level transgene expression as well as inducible transactivation. Ex vivo applications of the pCLPG system would rely on endogenous p53 to drive viral expression implying that stimulation of p53 would result in increased transgene levels (Figure 1A). This transcriptional mechanism, in combination with drug resistance genes, may prove beneficial for the protection of the hematopoietic system from the effects of chemotherapy. In situ applications may benefit from the positive feedback mechanism established by the combination of the p53-responsive LTR plus virally encoded p53, resulting in high-level p53 expression and subsequent suppression of tumor cell proliferation (Figure 1B).