**5. p53-based gene therapy: GBMs as an example**

Malignant tumors within the brain remain a therapeutic challenge, but current strategies tested in animal models as well as in the clinics have shown promising results. The rapid progress in knowledge of the p53 pathway have led to many different approaches to p53 based cancer therapy as mentioned previously and the field has excited great interest both academically and commercially [215]. The long awaited molecular treatment of GBM and other CNS tumors and utilization of knowledge surrounding p53 may then be foreseeable goals in the future. It will also be important and likely therapeutically be effective to combine gene therapy with other therapeutic modalities, including the standards-of-care [216].

Standard treatment of care for GBM, for example, consists of surgical resection, followed by radiotherapy and chemotherapy [217, 218]. Despite significant advances in current treatment approaches, including the gamma knife (radiation) and TMZ (chemotherapy) [217, 219], GBM continues to present a poor prognosis, with median survival still re‐ mains less than 15 months. It is important to remember that GBMs are the most common and least curable among CNS tumors [220]. Moreover, for this type of tumor, complete resection is practically impossible due to its diffuse nature and the proximity of the tumor to vital brain structures. Moreover, it often recurs in an area close to the original resec‐ tion cavity [221]. The intrinsic resistance of glioblastoma cells to radiotherapy and chemo‐ therapy confers another therapeutic challenge of this disease [222]. On the other hand it has been reported that invading GBM cells, which give rise to recurrences, are resistant to cytotoxic therapies due to the constitutive activation of antiapoptotic signaling pathways [221]. Novel therapeutic approaches and adjuvants to be employed in combination with standard therapeutic strategies are sorely needed for GBM patients, because although isolated traditional therapies allow an increase in the quality of life and survival of these patients, they are not curative and long-term survival is very rare [221, 223, 224].

Gene therapy for CNS tumors is evolving every year, especially for GBM, with the ultimate goal being specific delivery of therapeutic genes or oncolytic viruses to eliminate the tumor. Besides results in cell death, also enhanced immune responses to tumor antigens and disrup‐ tion of the tumor microenvironment [216]. A variety of gene therapy strategies has been examined in GBM preclinical models and clinical trials and includes the use of selective replication-competent oncolytic viruses, non-replicating viral vectors or normal adult stem/ progenitor cells for the delivery of immunostimulatory genes, cytotoxic genes and genes modulating the tumor microenvironment [216].

The number of studies concerning the cellular, molecular, and environmental factors that regulate p53 function in NSCs has increased drastically and brought a better understanding of these factors, and together with the advances in molecular biology techniques, provided much valuable information about the role of p53 in BTSCs. This scenario stimulates future studies exploring the significance of p53 alterations for prognosis and prediction of treatment response that would help development of individual treatment strategies as well as help

Malignant tumors within the brain remain a therapeutic challenge, but current strategies tested in animal models as well as in the clinics have shown promising results. The rapid progress in knowledge of the p53 pathway have led to many different approaches to p53 based cancer therapy as mentioned previously and the field has excited great interest both academically and commercially [215]. The long awaited molecular treatment of GBM and other CNS tumors and utilization of knowledge surrounding p53 may then be foreseeable goals in the future. It will also be important and likely therapeutically be effective to combine gene therapy with other

Standard treatment of care for GBM, for example, consists of surgical resection, followed by radiotherapy and chemotherapy [217, 218]. Despite significant advances in current treatment approaches, including the gamma knife (radiation) and TMZ (chemotherapy) [217, 219], GBM continues to present a poor prognosis, with median survival still re‐ mains less than 15 months. It is important to remember that GBMs are the most common and least curable among CNS tumors [220]. Moreover, for this type of tumor, complete resection is practically impossible due to its diffuse nature and the proximity of the tumor to vital brain structures. Moreover, it often recurs in an area close to the original resec‐ tion cavity [221]. The intrinsic resistance of glioblastoma cells to radiotherapy and chemo‐ therapy confers another therapeutic challenge of this disease [222]. On the other hand it has been reported that invading GBM cells, which give rise to recurrences, are resistant to cytotoxic therapies due to the constitutive activation of antiapoptotic signaling pathways [221]. Novel therapeutic approaches and adjuvants to be employed in combination with standard therapeutic strategies are sorely needed for GBM patients, because although isolated traditional therapies allow an increase in the quality of life and survival of these

patients, they are not curative and long-term survival is very rare [221, 223, 224].

Gene therapy for CNS tumors is evolving every year, especially for GBM, with the ultimate goal being specific delivery of therapeutic genes or oncolytic viruses to eliminate the tumor. Besides results in cell death, also enhanced immune responses to tumor antigens and disrup‐ tion of the tumor microenvironment [216]. A variety of gene therapy strategies has been examined in GBM preclinical models and clinical trials and includes the use of selective replication-competent oncolytic viruses, non-replicating viral vectors or normal adult stem/

clarifying the clinical importance of cancer stem cell biology.

148 Tumors of the Central Nervous System – Primary and Secondary

**5. p53-based gene therapy: GBMs as an example**

therapeutic modalities, including the standards-of-care [216].

The fact that p53 pathway is activated in tumor cells, but not in normal cells, provides a potentially important therapeutic selectivity, indifferent of which signal in the tumor cells activates p53 following its restoration [225]. In this context, the evidences that tumor cells, but not normal cells, have a cellular environment that activates the p53 pathway would create a setting of an advantageous therapeutic index, whose main objective is the development of interventions that selectively kill tumor cells instead normal cells [225].

Different approaches to achieve this goal are already in various stages of development and a diversity of small druglike molecules targeting the p53 system have been developed and several are now in clinical trials. Of critical importance has been the development of: agents which can increase active p53 in tumor cells by interfering with the p53–MDM2 interaction are therefore considered to have therapeutic utility in sensitizing tumor cells for chemo-or radiotherapy, such as the Nutlins [226, 227]; molecules that activate p53 via direct interaction with p53 itself, as PRIMA-1, of which there is evidence of induction of expression of mediators of p53-dependent apoptosis such as Puma, Noxa, and Bax in cells with mutant p53 [228, 229]; small molecules activating p53 family members in a p53 mutant or deficient background; molecules activating p53 by inhibiting class III histone deacetylases, nuclear export, transcrip‐ tional and nucleolar distuption. These screens in combination with RNAi based approaches are of utmost importance for the discovery of new targets for therapy in the p53 pathway [215].

Transfection of wild-type p53 in order to normalize function in mutant p53-containing tumors has been a long-pursued goal of gene therapy. Mercer *et al.* [230] initially demonstrated that plasmid-mediated transfection of the p53 gene is capable of suppressing cell growth in gliomas by inhibition of G0/G1 progression into S phase. Kock *et al.* [231] and Gomez-Manzano *et al.* [232] were among the first to demonstrate that delivering the p53 gene using an adenovirus vector (Ad-p53) resulted in high levels of apoptosis in glioma cell lines, by elevation of the levels of the p21 (cell cycle-related) and Bax (apoptosis-related) proteins. Frederick *et al.*[233] undertook a phase I trial of Ad-p53 in the treatment of patients with recurrent malignant gliomas with the purpose of determine the clinical toxicity of Ad-p53 and obtain molecular information regarding the expression and distribution of the p53 protein after intratumoral treatment of human gliomas with Ad-p53. Thus, their results conclude that Intratumoral injection of Ad-p53 allowed the exogenous transfer of the p53 gene and expression of func‐ tional p53 protein, with minimaltoxicity observed.

To the generation of an effective systemic anti-tumor immune response, it is necessary the development of strategies that promote the GBM tumor cell death, which is essential not only to kill tumor cells and reduce tumor burden, but also to induce the release of inflammatory molecules from dying tumor cells [234]. Drug combinations have been developed to selectively kill cancer cells that lack p53 function while protecting normal cells. The potential to explore the defective checkpoint status of cells with inactive *TP53* genes has also been largely recog‐ nized and in part stimulated the search of drugs that can inhibit PLK1, AURKB, and other proteins that regulate the G 2 /M checkpoint [235]. Shchors et al. [236] used a preclinical model of GBM in combination with a switchable p53 allele to model the therapeutic effect of p53 pathway restoration. It was observed that the therapeutic efficacy of p53 pathway restoration was greatly influenced by both the initial mechanism of p53 pathway-inactivating mutation and the temporal manner in which the selective pressure elicited by p53 pathway restoration was applied. Their results suggested that intermittent dosing regimens of drugs that restore wild-type tumor-suppressor function onto mutant, inactive p53 proteins will prove to be more efficacious than traditional chronic dosing by similarly reducing adaptive resistance.

Epigenetic events in *TP53* gene has been increasingly recognized as an alternative mechanism for inactivation of function of a tumor suppressor gene. Although less frequently, *TP53* epigenetic abnormalities has been found in CNS tumors and several reports shed light on the involvement of mainly DNA methylation and miRNAs in the p53 pathway, suggesting that this process can be involved in the genesis and progression of these tumors. Clearly, additional studies can provide important insights into the central roles of miRNAs in the p53 pathway, as well as *TP53* promoter methylation, which may provide a route to therapeutic intervention

Alterations in *TP53* gene – Implications in Tumorigenesis Process and Prognosis in Central Nervous System Cancer

http://dx.doi.org/10.5772/58334

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Due the difficulty to the use of traditional therapeutic modalities such as chemotherapy and radiotherapy in the CNS tumors, especially in high grade tumors, such as glioblastomas, , it is expected that in a near future molecular treatment that could be obtain more effective control of disease progression will be used, resulting in an improved clinical course of these patients. Over the years, with the increasing advances of molecular biology techniques, much informa‐ tion has been obtained on the role of p53 in carcinogenesis. Because of the critical role p53 plays in a variety of cancers, a diversity of approaches have been undertaken to target p53 and its altered signaling pathways. Different drugs targeting the p53 system in order to activate the p53 pathway have been developed and several are now in clinical trials, and have shown

in CNS tumors.

promising results.

**Nomenclature**

AT/RT Atypical Teratoid/ Rhabdoid Tumor bFGF Basic fibroblast growth factor *BCL2* B-cell CLL/Lymphoma 2 BTSC Brain tumor stem cells CSC Cancer stem cells

*CDK4* Cyclin-dependent kinase 4

CNS Central nervous systems

CPC Choroid plexus carcinomas CPP Choroid plexus papilloma CPT Choroid plexus tumors *EGF* Epidermal growth factor

FISH Fluorescence in Situ Hybridization GFAP Glial fibrillary acidic protein

GBM Glioblastoma

*CDKN2A* Cyclin-dependent kinase inhibitor 2A

CHD5 Chromodomain helicase DNA binding protein 5

This topic focused on GBM because of its poor prognosis and the target for most clinical trials. However, it is important to recognize that there are many other brain tumors which are also targets for gene therapy. Recently, Kunkele *et al.* [227] observed that targeting the p53-MDM2 complex using nutlin-3 significantly reduced cell viability and induced either apoptosis or cell cycle arrest and expression of the p53 target gene p21 in 4 of 6 human medulloblastomas cell lines. However, UW-228 and DAOY cells harboring *TP53* mutations were almost unaffected by nutlin-3, showing that the mutational status of the gene interfere in the efficacy of the treatment. MDM2 knockdown in medulloblastoma cells by siRNA mimicked nutlin-3 treat‐ ment, whereas expression of dominant negative p53 abrogated nutlin-3 effects. Oral nutlin-3 treatment of mice with established medulloblastoma xenografts inhibited tumor growth and significantly increased survival. Hence, the authors suggested that inhibition of the MDM2 p53 interaction with nutlin-3 is a promising therapeutic option for medulloblastomas with functional p53 that should be further evaluated in clinical trials.
