**3. Genetic basis of NPC**

The identication of genetic changes in pre-malignant lesions and NPC tumors has led to the proposal of a multi-step model for the pathogenesis of NPC. Genome-wide analyses of genetic alterations in NPC have identified consistent genetic losses on several chromosomal arms, including 3p, 9p, 9q, 11q, 14q and 16q. Recurrent chromosomal gains were also identied on chromosomes 1q, 3q, 8q, 12p and 12q (Chan, To et al. 2000; Chan, To et al. 2002; Lo and Huang 2002). The most common genetic change seems to be the loss of chromosome regions on 9p21 and 3p, which is thought to occur early during NPC pathogenesis (Hui, Lo et al. 2002). NPC carcinogenesis seems to develop in a step by step fashion, where the chromosomal alterations, such deletion, tumor suppressor gene inactivation and oncogene promotion, are associated with morphological changes in the cell (Young and Rickinson 2004) (Figure 3).

Fig. 3. NPC carcinogenesis scheme.

Although the precise role of these genetic alterations in NPC carcinogenesis remains to be clarified, these genetic changes may predispose nasopharyngeal cells to facilitate latent EBV infection and this seems to be a crucial event in the multi-step progression towards NPC.

Analysis of EBV in NPC tumors has revealed the presence of clonal EBV genomes, suggesting that these carcinomas arise from the clonal expansion of a single EBV-infected progenitor cell (Raab-Traub and Flynn 1986). Similar to most EBV-associated malignancies, the exact role of EBV in NPC pathogenesis remains poorly understood. Unlike EBVassociated B cell tumors, where the virus is considered to be an initiating factor in the oncogenic process, virus infection in the context of NPC pathogenesis seems to act as a

Recent studies investigated circulating tumor-derived EBV DNA in the blood of NPC patients. Using real-time PCR methodology, studies indicate that the level of pre-treatment EBV DNA has prognostic significance. Furthermore, post-treatment EBV DNA levels seem to be correlated with treatment response and overall survival of NPC patients (Lo, Chan et al. 2000; Lo, Leung et al. 2000; Lin, Chen et al. 2001; Chan, Lo et al. 2002). This methodology

The identication of genetic changes in pre-malignant lesions and NPC tumors has led to the proposal of a multi-step model for the pathogenesis of NPC. Genome-wide analyses of genetic alterations in NPC have identified consistent genetic losses on several chromosomal arms, including 3p, 9p, 9q, 11q, 14q and 16q. Recurrent chromosomal gains were also identied on chromosomes 1q, 3q, 8q, 12p and 12q (Chan, To et al. 2000; Chan, To et al. 2002; Lo and Huang 2002). The most common genetic change seems to be the loss of chromosome regions on 9p21 and 3p, which is thought to occur early during NPC pathogenesis (Hui, Lo et al. 2002). NPC carcinogenesis seems to develop in a step by step fashion, where the chromosomal alterations, such deletion, tumor suppressor gene inactivation and oncogene promotion, are associated

Although the precise role of these genetic alterations in NPC carcinogenesis remains to be clarified, these genetic changes may predispose nasopharyngeal cells to facilitate latent EBV infection and this seems to be a crucial event in the multi-step progression towards NPC.

should be applied in large-scale trials as an approach for early disease diagnosis.

with morphological changes in the cell (Young and Rickinson 2004) (Figure 3).

tumor-promoting agent.

**3. Genetic basis of NPC** 

Fig. 3. NPC carcinogenesis scheme.

The development and progression of NPC involves the accumulation of genetic alterations. Both genetic and epigenetic changes can affect the development of NPC, by altering the function of genes that are critical for proliferation, apoptosis and differentiation.

Host factors previously shown to be associated with NPC development include *HLA* class I and II alleles and polymorphisms of genes responsible for critical cellular functions, as carcinogen metabolism and detoxification, cell cycle control, DNA repair and immune response. Published studies demonstrated that some genetic variations are associated with a higher risk of NPC development (Hildesheim, Anderson et al. 1997; Nazar-Stewart, Vaughan et al. 1999; Cho, Hildesheim et al. 2003; Catarino, Breda et al. 2006; Sousa, Santos et al. 2006). This genetic predisposition to NPC may be modulated by environmental factors and this may have a differential impact in endemic and non-endemic NPC.

Familial clustering of the disease has been widely documented in the Chinese population and several polymorphisms have been associated with NPC development, namely in nitrosamine metabolizing genes cytochrome P450 2A6 (CYP2A6), P450 2E1 (CYP2E1) and P450 2F1 (CYP2F1), Glutathione S- transferase M1 (GSTM1), XRCC1 (codons 194 arg-trp), have been suggested to influence the susceptibility to NPC (Hildesheim and Levine 1993; Pathmanathan, Prasad et al. 1995; Sousa, Santos et al. 2006; Tiwawech, Srivatanakul et al. 2006).

Less in known in non-endemic areas, but it has been suggested that the individual genetic background may influence the onset of NPC, also in these regions. Regarding cell cycle control, some studies investigated the role of polymorphisms in cyclin D1 (CCND1) and TP53 genes in the genetic susceptibility to NPC (Pathmanathan, Prasad et al. 1995; Cho, Hildesheim et al. 2003; Catarino, Pereira et al. 2008). These genetic variants have been associated with increased individual risk of NPC development. Regarding the key regulator of cell cycle, cyclin D1, the study by Catarino et al, demonstrate that individuals carrying the genetic variant present a 2-fold increased risk for the development of NPC, the proportion of cases attributable to the genotype being 14.76% (Catarino, Breda et al. 2006). Moreover, another study indicates that cyclin D1 variants influence the age of onset of oncogenic virusassociated cancers, namely NPC (Catarino, Pereira et al. 2008). The results of this study demonstrate that the waiting time for onset of oncogenic virus-associated neoplasia in patients carrying the cyclin D1 variant was 12 years earlier in comparison with the other patients. Another study by Sousa et al, focus on the polymorphic variants (Arg/Pro) on *TP53* codon 72 in nasopharyngeal cancer development. This study revealed a three-fold risk for carriers of Pro/Pro genotype, suggesting that Pro/Pro genotype represents a stable risk factor for nasopharyngeal carcinoma (Sousa, Santos et al. 2006).

Another study revealed that a MDM2 polymorphism is associated with increased risk to develop NPC adjusted for age and gender, with particular effect in undifferentiated types and early clinical stages. The study also indicates that the median age of onset of NPC was significantly different according to the genetics variants (Sousa, Pando et al. 2011). Regarding the immune response markers, a study revealed an increased frequency of a tumor necrosis factor-alpha (TNF-α) polymorphic variant in patients with NPC. The authors indicate that this variant is associated with a 2-fold increased risk for the development of this disease. Moreover, this effect seems to be stronger in undifferentiated types (Sousa, Breda et al. 2011).

These results performed in low-risk, non-endemic areas indicate that these genetic variations may represent risk markers for NPC and contribute to the definition of genetic susceptibility profiles for the development of this disease. Furthermore, the knowledge of the mechanisms involved in NPC carcinogenesis may help to identify targets for the development of chemoprevention or therapeutic strategies.
