GTMB 7 - Gene Therapy & Molecular Biology

Jekunen et al: Strategy of sensitizing tumor cells with adenovirus-p53 transfectioncell cycle, it is critical as a checkpoint that blocksuncontrolled cell division (Levine, 1992). In the nuclei ofnormal cells, the intact p53 protein acts as a transcriptionalactivator for a group of genes involved in cell cycle arrest(p21 cip1/waf1 ), DNA repair (GADD45), and apoptosis (Bax)(O'Connor et al, 1997; Sugrue et al, 1997; Yin et al, 1997;Carrier et al, 1999). In addition to this, p53 is a potentinducer of programmed cell death (apoptosis) within a cellin which the DNA has been damaged. Normally, the p53gene is inactive. When, after DNA damage, the normalp53 is activated, the levels of p21, p27, and GADD 45may become very high (Sherr, 1994). DNA damage incells induces expression of p53 and interruption of the cellcycle in both G1 and G2 (Chu and DeVita, 2001). If DNArepair is successful, the cell continues its cycle. If repairdoes not succeed, the cell undergoes apoptosis.B. Mutation of p53Mutations in the p53 gene are among the mostcommon genetic alterations observed in human tumorsamples (Oren, 1992). The specific cytotoxic treatment,the conditions of treatment, the p53 status, and otherelements of cell-cycle regulation may all contribute to theoutcome of exposure of a cell to DNA-damaging agents(Chu and DeVita, 2001). p53 can activate an apoptoticresponse to DNA damage, especially in hematopoietic andlymphoid cells, which often overrides the G1 checkpointresponse (Fan et al, 1995). In cell types programmed forapoptosis, loss of p53 function decreases their sensitivityto a wide variety of DNA-damaging agents, while in cellapoptosis, it has been more difficult to establish a clearrelationship between p53 gene status and chemosensitivitytypes of some solid tumors not inherently programmed for(Fan et al, 1995). If the DNA is damaged, the cell withintact p53 function will undergo p53-dependent apoptosis(Chu and DeVita, 2001). In tumor cells with mutated p53,the loss of p53 function, is thought to result in resistanceto chemotherapeutic agents (Lowe et al, 1994; Righetti etal, 1996; Blandino et al, 1999). A recent study of ovariancancer shows that women with tumors having the p53 nullmutation have a survival disadvantage over those with p53missense mutations (Shahin et al, 2000).II. Evidence of the role of p53 inchemosensitizingA. p53 and chemotherapeutic agentsDysregulation of the p53 pathway may lead to drugresistance due to overproduction of the gene productsresponsible for entry into the S phase and rapid cellgrowth (Figure 1).Activation of these genes could theoretically increasethe resistance of cells to the following chemotherapeuticagents: methotrexate, 2-chlorodeoxyadenosine,hydroxyurea, fludarabine, cytosine arabinoside, and 5-fluorouracil. Under some experimental circumstances, celldeath in response to exposure to DNA-damaging agentsmay require an intact p53-dependent apoptoticmechanism. Some of the genes that are transcriptionallyactivated by p53 belong to a class of proteins known toinhibit cyclin-dependent kinases (cdk). p21 forms acomplex with proliferating cell nuclear antigen or inhibitscdk’s, e.g. cdk4 (Polyak et al, 1997). Activated p53 cancause a G1 cell cycle arrest by increasing the transcriptionof the cdk inhibitor p21 (Figure 2), which block cdk4activity, preventing reitinoblastoma gene product (RB)phosphorylation (Sherr, 1994) and release of E2F blockingthe transcription of a number of genes, and inhibiting entryinto S phase (Kirsch, 1998). The E2F family oftranscription factors bind to the regulatory regions of anumber of genes that participate in the synthesis of DNA(Figure 2).Figure 1. Effect of chemotherapy via p53 pathway. After chemotherapy has induced DNA damage, p53 protein is activated andtranscription of many genes is increased, resulting in cell cycle arrest and apoptosis. For apoptotically sensitive cells, genotoxic damagecan signal an immediate apoptotic response, while for apoptotically insensitive cells, the primary apoptotic decision point is disabled.Cells that avoid apoptotic or necrotic death after DNA repair can survive and grow. (Kirsch 1998; Brown and Wouters 1999)26