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

Igor Andrade Pessôa, Fabio P. Estumano da Silva, Nilson Praia Anselmo and Edivaldo Herculano C. de Oliveira

Additional information is available at the end of the chapter

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

**1. Introduction**

#### **1.1.** *TP53* **Mutations and CNS tumors**

Central nervous systems (CNS) malignancies, as others cancers, are formed by the uncontrol‐ led cell growth that involves the sequential accumulation of alterations in genes controlling cell proliferation, lifespan, responses to stress, relationships with neighbors, and gene homeo‐ stasis. These genetic alterations can be achieved by intragenic mutations, chromosome alterations or epigenetics modifications, all playing important role in the activation or inactivation of key genes, such as oncogenes and tumor suppressor genes. Some of these mutations can be most frequently encountered in specific cancers or group of cancers and correlated with tumor biologic behavior and have implications on diagnosis, prognosis or treatment [1].

Biomarkers are important oncology tools in diagnostic, monitoring disease progression, helping in determining prognosis and predicting therapeutic response. Biomarkers vary from specific proteins and antigens to unique genetic, epigenetic or cytogenetic profiles, but common to all markers is that they provide specific information to a disease process. They function as supplementary and rarely supplanting, the histopathologic examination of tissues that is still the mainstay of traditional oncologic pathology [2, 3]. For this reason, we intend to compile the vast information about the important contribution of *TP53* gene as a biomarker in CNS cancer genesis, progression, stratification, prognosis, treatment and its importance to future targeted therapies.

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

CNS cancers are heterogeneous diseases, arbitrarily grouped by the systems that are affected. The "WHO (World Health Organization) Classification of Tumors of the Central Nervous System" discriminates more than one hundred different diseases derived from different cell types, affecting patients of different ages, with a vast biological behavior and clinical implica‐ tions. It is not our intention to describe the features of each CNS tumor. Hence, authors will follow the WHO classification for CNS tumors [4].

*TP53* tumor suppressor gene is the most frequently mutated gene in human tumors and one of the most studied on different kinds of cancer. It is a large and complex gene located on chromosome site 17p13.1 (Figure 1). It has 11 exons along approximately 20.000bp. This gene codifies a protein with 393 amino acids in which different domains are responsible for diverse functions as exhibited on Figure 1. Genetic variations in this gene contribute to human cancers in many different ways. Firstly, somatic mutations are frequent in most cancers [5]: it is estimated that mutations in this gene are present in half of the human cancers. The antiproli‐ ferative role of p53 protein in response to various stresses and during physiological processes such as senescence makes it a primary target for inactivation [6], mainly by a combination of single-base substitution and loss of alleles [7]. Secondly, inheritance of a mutated *TP53* causes predisposition to early-onset cancers including breast carcinomas, sarcomas, brain tumors, and adrenal cortical carcinomas, defining the Li-Fraumeni (LFS) and Li-Fraumeni-like (LFL) syndromes [8, 9]. Thirdly, *TP53* is highly polymorphic in coding and noncoding regions and some of these polymorphisms have been shown to increase cancer susceptibility and to modify cancer phenotypes in *TP53* mutation carriers [10].

Commonly, advanced stage or aggressive behavior cancers have a higher frequency of *TP53* mutations [11, 12]. Moreover, in cancers with low mutation rates, p53 is often inactivated by alternative mechanisms, like protein degradation. *TP53* allelic deletion is also observed in many tumors, resulting in the reduction of expression of tetramers and decreased expression of genes inhibiting cell growth [13]. The cancer-associated somatic mutations in *TP53* are primarily missense substitutions (72.28%) nonrandomly distributed along the molecule, [14]. Over 90% of p53 mutations occur in the central DNA-binding-domain (Figure 1) into exons 4 – 9. These single aminoacid changes affect the transcriptional activity of the gene to various degrees; sometimes missense mutants may even acquire new functions [15, 16]. The *TP53* mutational pattern has proved to be a clinically relevant "molecular sensor" of genotoxic exposure to environmental carcinogens and endogenous mutagens [17].

245, 282, and 306) account for 26% of these mutations. The lack of mutations at other CpG sites may reflect the fact that substitutions at these residues do not generate a dysfunctional protein. Although the same CpG hotspot mutations occur in many cancer types, other types of mutations tend to show differences among different cancers. Some of these differences have been linked to the effect of specific mutagens. This idea is endorsed by geographic differences

distributed. There are nuclear export/localization signals inside and between some domains (NES/NLS).

**Figure 1.** *TP53* gene: Structure, chromosome localization and protein domains distribution. *TP53* is mapped on hu‐ man chromosome site 17p13.1. It is a long gene, with 19, 149 base pair comprising 11 exons that codify a protein with 393 amino acids long, in which the transactivation, proline-rich, DNA binding and the oligomerization domains are

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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All these mutational information about *TP53* are compiled in the International Agency for Research on Cancer (IARC) *TP53* Database [14], which provides structured data and analysis tools to study *TP53* mutations for specific cancers or investigate the functional and clinical impact of some mutations. The existence of this database, dedicated to annotate *TP53* mutations, polymorphism and respective implications in clinical and pathological behavior of human cancers, demonstrates the importance and the necessity of more knowledge to

Several studies have investigated the predictive value of *TP53* mutation status for tumor response to treatment and patient outcome in various cancers. However, different clinical and methodological settings have been used and the results have often been heterogeneous and contradictory [22]. The number and complexity of pathways in which *TP53* participates, the different mutational profiles of each cancer and the diverse environment conditions are

which can be related to different environmental exposures [21].

complete understand its implication on cancer [22].

variables that can contribute to these heterogeneous results.

Among single-base substitutions, about 25% are C:G>T:A substitutions at CpG sites. CpG dinucleotides mutate at a rate 10 times higher than other nucleotides, generating transitions [18]. About 3%–5% of cytosines in the human genome are methylated at position 5' by a postreplicative mechanism that is restricted to CpG dinucleotides and is catalyzed by DNA methyltransferases. The 5' methylcytosine (5mC) is less stable than cytosine and undergoes spontaneous deamination into thymine at a rate five times higher than the unmethylated base. This process is enhanced by oxygen and nitrogen radicals, leading to a higher load of CpG transitions in cancers arising from inflammatory precursors such as Barrett's mucosa or ulcerative colitis [19, 20]. Among the 22 CpG of the DNA-binding domain (DBD), three hotspot codons (175, 248, and 273) represent 60% of CpG mutations and another five residues (196, 213,

Alterations in *TP53* gene – Implications in Tumorigenesis Process and Prognosis in Central Nervous System Cancer http://dx.doi.org/10.5772/58334 129

CNS cancers are heterogeneous diseases, arbitrarily grouped by the systems that are affected. The "WHO (World Health Organization) Classification of Tumors of the Central Nervous System" discriminates more than one hundred different diseases derived from different cell types, affecting patients of different ages, with a vast biological behavior and clinical implica‐ tions. It is not our intention to describe the features of each CNS tumor. Hence, authors will

*TP53* tumor suppressor gene is the most frequently mutated gene in human tumors and one of the most studied on different kinds of cancer. It is a large and complex gene located on chromosome site 17p13.1 (Figure 1). It has 11 exons along approximately 20.000bp. This gene codifies a protein with 393 amino acids in which different domains are responsible for diverse functions as exhibited on Figure 1. Genetic variations in this gene contribute to human cancers in many different ways. Firstly, somatic mutations are frequent in most cancers [5]: it is estimated that mutations in this gene are present in half of the human cancers. The antiproli‐ ferative role of p53 protein in response to various stresses and during physiological processes such as senescence makes it a primary target for inactivation [6], mainly by a combination of single-base substitution and loss of alleles [7]. Secondly, inheritance of a mutated *TP53* causes predisposition to early-onset cancers including breast carcinomas, sarcomas, brain tumors, and adrenal cortical carcinomas, defining the Li-Fraumeni (LFS) and Li-Fraumeni-like (LFL) syndromes [8, 9]. Thirdly, *TP53* is highly polymorphic in coding and noncoding regions and some of these polymorphisms have been shown to increase cancer susceptibility and to modify

Commonly, advanced stage or aggressive behavior cancers have a higher frequency of *TP53* mutations [11, 12]. Moreover, in cancers with low mutation rates, p53 is often inactivated by alternative mechanisms, like protein degradation. *TP53* allelic deletion is also observed in many tumors, resulting in the reduction of expression of tetramers and decreased expression of genes inhibiting cell growth [13]. The cancer-associated somatic mutations in *TP53* are primarily missense substitutions (72.28%) nonrandomly distributed along the molecule, [14]. Over 90% of p53 mutations occur in the central DNA-binding-domain (Figure 1) into exons 4 – 9. These single aminoacid changes affect the transcriptional activity of the gene to various degrees; sometimes missense mutants may even acquire new functions [15, 16]. The *TP53* mutational pattern has proved to be a clinically relevant "molecular sensor" of genotoxic

Among single-base substitutions, about 25% are C:G>T:A substitutions at CpG sites. CpG dinucleotides mutate at a rate 10 times higher than other nucleotides, generating transitions [18]. About 3%–5% of cytosines in the human genome are methylated at position 5' by a postreplicative mechanism that is restricted to CpG dinucleotides and is catalyzed by DNA methyltransferases. The 5' methylcytosine (5mC) is less stable than cytosine and undergoes spontaneous deamination into thymine at a rate five times higher than the unmethylated base. This process is enhanced by oxygen and nitrogen radicals, leading to a higher load of CpG transitions in cancers arising from inflammatory precursors such as Barrett's mucosa or ulcerative colitis [19, 20]. Among the 22 CpG of the DNA-binding domain (DBD), three hotspot codons (175, 248, and 273) represent 60% of CpG mutations and another five residues (196, 213,

exposure to environmental carcinogens and endogenous mutagens [17].

follow the WHO classification for CNS tumors [4].

128 Tumors of the Central Nervous System – Primary and Secondary

cancer phenotypes in *TP53* mutation carriers [10].

**Figure 1.** *TP53* gene: Structure, chromosome localization and protein domains distribution. *TP53* is mapped on hu‐ man chromosome site 17p13.1. It is a long gene, with 19, 149 base pair comprising 11 exons that codify a protein with 393 amino acids long, in which the transactivation, proline-rich, DNA binding and the oligomerization domains are distributed. There are nuclear export/localization signals inside and between some domains (NES/NLS).

245, 282, and 306) account for 26% of these mutations. The lack of mutations at other CpG sites may reflect the fact that substitutions at these residues do not generate a dysfunctional protein. Although the same CpG hotspot mutations occur in many cancer types, other types of mutations tend to show differences among different cancers. Some of these differences have been linked to the effect of specific mutagens. This idea is endorsed by geographic differences which can be related to different environmental exposures [21].

All these mutational information about *TP53* are compiled in the International Agency for Research on Cancer (IARC) *TP53* Database [14], which provides structured data and analysis tools to study *TP53* mutations for specific cancers or investigate the functional and clinical impact of some mutations. The existence of this database, dedicated to annotate *TP53* mutations, polymorphism and respective implications in clinical and pathological behavior of human cancers, demonstrates the importance and the necessity of more knowledge to complete understand its implication on cancer [22].

Several studies have investigated the predictive value of *TP53* mutation status for tumor response to treatment and patient outcome in various cancers. However, different clinical and methodological settings have been used and the results have often been heterogeneous and contradictory [22]. The number and complexity of pathways in which *TP53* participates, the different mutational profiles of each cancer and the diverse environment conditions are variables that can contribute to these heterogeneous results.

The majority of mutations led to protein accumulation in the nucleus of the cells, which can be detected by immunohistochemistry (IHC) assays. Although some studies have shown an association between p53 positive immunostaining and poor outcomes, several studies have produced conflicting results and expectations on the use of p53 as a useful clinical biomarker failed [23]. Therefore, it seems IHC is a poor surrogate for gene mutation detection, as many mutations do not lead to protein accumulation, and because accumulation of wild-type p53 may also occur in the absence of gene mutation, producing a high rate of false negative and positive results. Hence, the use of IHC leads to an unacceptable number of misclassified cases and to a greater inter-study variability [1, 22].

By contrast, the screening for *TP53* mutations by gene sequencing, precisely identifying the mutation, have produced more consistent results, at least for some types of cancers such as breast, head and neck squamous cell carcinoma (HNSCC), and leukemia, in which the presence of a *TP53* mutation is associated with poor outcomes. In other types of cancer such as brain and pancreas, mutations were also found to be associated with both poor and good prognosis, depending on the study and cancer. These results show that the type of tissue and treatment may be important determinants of the prognostic and predictive value of *TP53* mutations [1, 22]. Figure 2 illustrates the use of different techniques in the evaluation of mutational status of *TP53* and expression of p53 protein in gliomas. Fluorescence *in Situ* Hybridization (FISH), sequencing and IHC techniques.
