Cervical cancer affects thousands of women around the world (471,000 cases per year) and is the second leading cause of death from cancer in women. Approximately 80% of cases occur in developing countries, where, in some regions, it is the most common form of cancer in women [1] . Brazil has one of the highest incidence rates of this neoplasia, reaching upwards of 50 - 83 cases per 100,000 women in some regions [1] . For the year 2014, the estimated incidence of cervical cancer was 18,503 new cases, and the estimated risk was 15.3 cases per 100,000 women, with the highest incidence being in the north of the country (23/100,000) [2] .

The etiology of cervical cancer has been studied for over 150 years, and it is often associated with infection. With the recent advent of molecular biology, cervical cancer tumors were determined to arise partially from a sexually transmitted infectious agent, the human papillomavirus (HPV) [3] -[5] . This virus is present in more than 99.7% of cases of squamous-cell carcinoma of the cervix, and the remaining 0.3% may be associated with unknown type of HPV [6] [7] .

Despite the high frequency of infection, the prevalence of intraepithelial lesions and cervical cancer is low. The natural history of HPV infection shows that most infections are transient (80%) and not associated with the detected cytological abnormalities [8] . Therefore, HPV alone is incapable of inducing cancer, although the reasons are poorly understood. One possible explanation is that other factors (cofactors) in addition to HPV are needed for the development of this neoplasm [9] [10] . For example, in addition to sexual activity, multiparity, chronic genital infections, hormonal and immunological alterations, other sexually transmitted diseases, contraceptives and smoking are important factors [8] . Possibly the most important factors are those that affect host immunity. Most of these cofactors alter the ability of the host to respond immunologically; thus, the host is more likely to be a chronic carrier of HPV, to show cellular transformation, or both [11] .

The role of host genetic factors in cervical carcinogenesis is still poorly understood. Investigations are now focusing on new molecular markers associated with susceptibility to the development of cervical carcinoma, known as biomarkers of progression, such as loss of heterozygosity, FHIT, telomerase, DNA ploidy, PCNA, Ki- 67, p16, Mcm and Cdc6. Recently, a polymorphism in the promoter region of the FAS gene was shown to be associated with cervical oncogenesis in some studies, [12] -[15] but not in others [16] -[20] .

The gene for the FAS receptor (also known as APO-1, CD95 or TNFRSF6) with its ligand (FASL) plays an essential role in the physiological regulation of apoptosis [21] [22] and transmission of no apoptotic signals [23] . At least twenty FAS receptor gene mutations have been described in humans and may trigger pathogenesis of many human diseases [24] .

In addition to mutations, genetic elements located in the promoter region, such as polymorphisms, which influence the transcriptional activity of this gene [16] , affect expression of these genes. Huang et al identified two polymorphisms in the FAS promoter region, which were A/G substitutions at position 670 and 1377 of the amplifier region [25] . The presence of the FAS-670 promoter region A/G polymorphism has been described in several diseases, such as rheumatoid arthritis, systemic lupus erythematosus, [26] multiple sclerosis, [27] lung carcinoma, [28] [29] and carcinoma of the nasopharynx, [30] among others. However, few studies have shown an association between FAS gene polymorphisms and cervical carcinogenesis [12] [14] -[16] [20] .

The aims of this study were to evaluate the FAS gene promoter region polymorphism at position-670 in blood samples (control group) and in cervical tissue with precancerous lesions (cervical intraepithelial neoplasia grade 3-CIN 3) and invasive cervical carcinoma to determine whether this polymorphism was associated with the pathogenesis of cervical carcinoma.

Cervical cancer is still one of the most common causes of cancer and death among women in developing countries. Although the role of high-risk HPV in cervical carcinogenesis is evident, only a small proportion of these cases progress to an invasive form of the disease [37] , indicating that the development of cervical carcinoma is influenced by other factors, likely including genetic factors [19] . Decreased expression of the FAS gene and/or increased expression of its ligand (FASL) have been demonstrated in many types of human cancer and appear to be associated with human carcinogenesis and more aggressive tumors [38] [39] . Several factors must be considered before addressing the role of FAS gene polymorphisms in cervical cancer susceptibility. First, we should consider the geographical location and ethnic group under study. Although the AA genotype is considered normal [25] , we can see in Table 4 that distribution of the FAS-670 gene polymorphism is similar among the normal population in different countries, with the AG genotype (heterozygote) being the most commonly found (in approximately half the cases), followed by AA (wild-type) and finally GG (mutated) [12] [16] [19] [25] [27] [30] [40] -[43] , just as we found in our study. In contrast, cases in Asian countries showed a much lower frequency of the GG genotype than other countries.

Individual phenotypic variations are often the result of slight differences in the sequence in the coding region

^*AG + GG: 69.8%; Dybikowska et al. Int J Mol Med 2004; 14(3): 475-8; Lai et al. Gynecol Oncol 2005; 99: 113-8; Ueda et al. Int J Gynecol Cancer 2006; 16: 179-82; Kang et al. Cancer Gene Cytogen 2008; 180: 1-5.

of the gene responsible for this phenotypic trait [25] . For example, variation in the promoter region of the FAS gene (position-670) may lead to different levels of expression of this gene. In addition, changes in the expression of the FAS gene can result in loss of or decrease in apoptotic function and, consequently, development of a disease or tumor [16] . This polymorphism has been studied in many diseases of infectious, immune or tumoral origin [26] [31] [38] [44] -[49] . More recently, studies have shown association between the polymorphism and preinvasive and invasive diseases of the cervix [12] [14] -[17] [20] [31] [42] [50] . In this study, we analyzed the FAS-670 gene promoter region polymorphism as a potential genetic marker for cervical carcinogenesis. Analysis of genotypes of this gene in cases of precancerous lesions, such as CIN 3, revealed a significant difference in the prevalence of genotype AG in cases with an adjusted risk estimate of three-fold. Similarly, Ueda et al. examined cervical smears and noted that the G allele frequency increased from low-grade lesions to high-grade lesions (specifically, high-grade cervical squamous intraepithelial lesion [HSIL]) and that there was an increased OR for the G allele in HSIL compared with the control group [31] . The authors concluded that this increased frequency of AG or GG genotypes in HSIL could result in a significant reduction in the expression of the FAS gene and a subsequent decrease in apoptosis of cells with injury. Furthermore, Lai et al. reported a significant difference in the AA genotype in the HSIL and cancer group compared with the control group (AA: 38% vs. 28.6%, respectively), with an estimated risk of 1.7 [42] .

Because HPV infection with high oncogenic risk is inversely correlated with apoptosis of cervical epithelial cells, it is relevant to suggest that the presence of the FAS-670 G allele, which is associated with a lower expression of FAS efficiency, may be a risk factor for cervical carcinogenesis [19] . However, studies investigating the FAS gene and cervical cancer have been very controversial. For example, one study in a Taiwanese population showed that the FAS-670 A allele was associated with an increased risk of squamous intraepithelial lesions and squamous-cell carcinoma [12] . However, Ueda et al. showed that the GG genotype was associated with an increased risk of cervical cancer in the Japanese population [13] . Contrary to these two studies, five other studies [16] -[20] reported that the FAS-670 polymorphism was not associated with an increased risk of cervical carcinoma, similar to our findings. Comparison of the association between the FAS-670 polymorphism and cervical cancer is presented in Figure 2. The risk to develop cervical cancer resulted from this combined analysis was not increased (OR = 1.28; 95% CI = 0.69 - 1.87; p > 0.05). However, the tumor histologic type should be considered, despite the small number of no squamous cervical carcinoma in all studies. Zoodsma et al. observed that the FAS polymorphism was weakly associated with adenocarcinoma of the cervix but not epidermoid carcinoma [18] . Our results showed no association between any FAS genotype and the histological type or degree of tumor differentiation. Similarly, other authors found no significant association between an increased risk of cervical carcinoma, advanced stage or histological type and polymorphisms in the FAS gene [16] [19] [20] . Sun et al.

assessed risk of cervical cancer among Chinese women with FAS and FASL polymorphisms and found that only the FASL -844 gene polymorphism (i.e., switching of C to T) was associated with an increased risk of cancer [51] . Differences in samples, techniques and genomic variation of the FAS-670 gene among different ethnicities may explain the discrepancy of these results.

HPV infection is a necessary condition for genital cancer. However, HPV needs to be coupled with other factors for oncogenesis to occur [52] [53] . Among these factors, the most important are smoking, multiparity, immunodeficiency, diet and other concomitant infections (mainly Chlamydia trachomatis and herpes simplex 2). Polymorphisms of the FAS and FASL genes may increase the risk of cancer by interacting with these environmental factors. For example, Zhang et al. reported a significant interaction between FAS gene polymorphisms and smoking and lung cancer [54] . In addition, Sun et al. observed that the polymorphisms of the FAS and FASL genes’ promoter regions were associated with the development of esophageal squamous-cell carcinoma in a Chinese population of tobacco users [55] . The fact that smoking may increase expression of FASL [56] [57] and increase susceptibility to lung cancer and esophageal cancer suggests that this gene-environment interaction may have important implications for the development of human cancer related to tobacco [54] . However, Park et al. evaluated three different types of gene polymorphisms of FAS and FASL in a Korean population and found no association, either alone or in combination, with other risk factors of lung cancer [43] . Several studies in recent years have shown a correlation between smoking and cervical cancer [8] [10] [58] [59] while others have not [9] [60] [61] . The Multicenter Study Program of Cervical Cancer of the International Agency for Research on Cancer (IARC) reviewed the evidence concerning smoking and cervical cancer and concluded that smoking is an independent risk factor for this type of cancer [62] . Mechanisms associated with cervical carcinogenesis and smoking include inhibition of apoptosis by nicotine leading to tumor growth [10] [63] and increased expression of FAS and FASL genes on lymphocytes by metabolites of smoking, which increases lymphocyte apoptosis [57] . However, no studies have assessed the association between smoking, polymorphisms of the FAS gene and cervical cancer, except Kordi et al., which observed high risk of cervical cancer in passive smokers with AG genotype (OR = 4.6) [14] .

Another cofactor that could interact with the FAS gene is hormones. As for tobacco, the IARC Multicenter Study Program of Cervical Cancer has demonstrated that the use of oral contraceptives for more than five years significantly increases the risk of cervical cancer (OR = 2.72), especially after 10 years (OR = 4.48) [62] . Estrogen can trigger apoptosis of T cells by increasing expression of FAS and FASL in cancer cells. Using a nonclassical mechanism, the estrogen receptor can regulate this expression by interacting with other substances [42] [64] . In our study, we attempted to assess the hormonal factor using menopausal status by separating women into age groups of greater or less than 50 years of age, which is the average menopause age among Brazilian women [32] . However, the age factor was not correlated with the frequency of FAS genotypes or the increased risk of cervical cancer.

Our study had some limitations. First, the sample size may have prevented us from finding an association between FAS polymorphisms and susceptibility to cervical cancer. Second, polymorphism of the FAS gene has a variable penetrance over time [65] and perhaps the age of the women may have interfered in these results, since we observed a significant association of genotype AG with CIN 3 (younger women) and not significant association with cancer (older women) compared to the control group. Third, the control group was not proportional to the cases in relation to the region, whereby the racial factor or mainly ancestry [66] could influence the results. Fourth, the HPV type was not assessed. It is possible that some specific type of HPV may suffer interference from any genotype of FAS gene and lead to the persistent infection and cervical oncogenesis. And finally, we did not include information on other exposure factors, which may alter the results and explain the conflict between our findings and those of several studies in the literature.

The authors conclude that the different genotypes of the FAS gene (position-670) promoter region are not associated with cervical carcinogenesis. However, our data suggest a possible relationship between increased risk of CIN 3 and the AG genotype, with risk being increased three-fold when adjusted for the age variable. Future studies should correlate the cofactors associated with carcinogenesis. These studies will show whether this polymorphism may be used as a genetic marker of susceptibility to the development of cervical cancer.