Background

Oral cancer is ranked as the 11th most common type of cancer worldwide [1], with a higher prevalence in South and Southeast Asian countries such as India, Bangladesh, China, and Sri Lanka [2]. Oral squamous cell carcinoma (OSCC) originates from the squamous cells that cover the surface of the mouth and is a major type of oral cancer, accounting for more than 90% of cases [3]. Tobacco use (chewing with or without smoking), alcohol consumption, and human papillomavirus (HPV) infection are important risk factors for development of OSCC [4, Search strategy

We searched PubMed and Embase databases up to December 10, 2013 with the following search items: [(oral OR tongue OR mouth) AND (cancer OR carcinoma) AND (p53 OR TP53) AND polymorphism]. Reference lists of the included studies and published meta-analyses on related topics were also screened for additional studies.

Data extraction

Two authors independently extracted the following trial data from included studies: last name of the first author, publication year, countries of origin, HPV status of cases, source of control, number and genoty** distribution of cases and controls, diagnostic method for OSCC, genoty** method, and Hardy-Weinberg Equilibrium (HWE) for controls [17]. Disagreements were resolved by discussion.

Statistical analysis

We employed the fixed-effect analytical model first to pool results of the included studies, and the I 2 statistic [18] was used to test for statistical heterogeneity. If I 2 was more than 40%, we switched to a random-effects model. The odds ratios (ORs) and relevant 95% confidence intervals (CIs) were used to quantify the strength of association between the TP53 codon 72 polymorphism and OSCC susceptibility using five genetic models: Arg vs. Pro, ArgArg vs. ProPro, ArgPro vs. ProPro, (ArgPro + ArgArg) vs. ProPro, and ArgArg vs. (ProPro + ArgPro). Additionally, subgroups analyses based on HPV status, source of controls, and HWE status for controls were performed. Publication bias was detected by examination of funnel plots. All statistical analyses were conducted using Review Manager (RevMan) software (version 5.2 for Windows).

Results

Study characteristics

Our systematic literature search identified 278 studies that met the inclusion criteria. After deduplication and exclusion of the clearly irrelevant studies, we eventually included 11 case–control studies [19–29] involving 2,298 OSCC patients and 2,111 controls. Figure 1 shows the study selection process. Of the 11 included studies, two recruited OSCC patients with HPV [20, 28], and three enrolled patients with disrupted HWE [19, 21, 29]. Baseline characteristics of the 11 studies are summarized in Table 1.

Figure 1
figure 1

Study selection flow chart.

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Table 1 Characteristics of included studies
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Meta-analysis

Table 2 illustrates results of the overall and subgroup analyses. Overall, there was no association between the TP53 codon 72 polymorphism and OSCC susceptibility in Asians [(OR = 0.77, 95% CI = 0.48–1.22) for Arg vs. Pro; (OR = 0.67, 95% CI = 0.31–1.43) for ArgArg vs. ProPro; (OR = 1.14, 95% CI = 0.97–1.35) for ArgPro vs. ProPro, Figure 2; (OR = 0.85, 95% CI = 0.53–1.34) for (ArgPro + ArgArg) vs. ProPro; and (OR = 0.34, 95% CI = 0.34–1.23) for ArgArg vs. (ProPro + ArgPro)].

Table 2 Overall and subgroups meta-analysis of TP53 codon 72 polymorphism and OSCC risk in Asians
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Figure 2
figure 2

Forest plot. This represents the OSCC risk associated with the TP53 codon 72 polymorphism in Asians for the ArgPro vs. ProPro genetic model.

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Results of the subgroup analyses stratified by source of controls and HWE status for controls were similar to those of the overall analyses. However, when stratified by HPV status, a correlation between the TP53 codon 72 polymorphism and HPV infection was observed (Table 2).

Publication bias

A funnel plot based on the ArgPro vs. ProPro genetic model showed a relatively symmetrical distribution, enabling us to conclude that there was no publication bias (Figure 3).

Figure 3
figure 3

Funnel plot. This represents the publication bias test based on the ArgPro vs. ProPro genetic model.

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Discussion

Arg and Pro are two distinct functional alleles that are encoded by the TP53 codon 72, and Pro to Arg is the most informative polymorphism in the TP53 gene and have been found to be associated with human cancers [10, 30]. Among the published meta-analyses exploring the association between this polymorphism and cancers, some revealed an increased susceptibility of disease [11–13], while others failed to find any association [31–33]. Results from these meta-analyses indicate an interesting phenomenon, which is that different meta-analyses of the same cancer type could yield opposite results. Although two meta-analyses investigating the relationship of the TP53 codon 72 polymorphism and oral cancer susceptibility both reached the same conclusions [14, 15], such association among the Asian population is unclear. Given that OSCC has a high incidence in this population, we conducted the current meta-analysis to further investigate if the TP53 codon 72 polymorphism plays a role in the development of OSCC.

A total of 2,298 OSCC patients and 2,111 controls were included in our meta-analysis. Results of the overall population demonstrated a negative association of the TP53 codon 72 polymorphism and OSCC, although subgroup analysis revealed a positive correlation between the polymorphism and HPV status in OSCC patients. Our results are in contrast with those reported by Zhou et al. [14], which was based on three studies reporting HPV infection status; however, only one of these focused on an Asian population [20]. Moreover, this earlier meta-analysis is limited by its small sample size and mixed ethnicity. In contrast to the two previous meta-analyses [14, 15], our meta-analysis only focused on OSCC in Asians.

The relationship between HPV and OSCC has been previously established [34]. Our meta-analysis also found that the TP53 codon 72 polymorphism was associated with HPV-related OSCC susceptibility cases. However, because there is no association between this polymorphism and non-HPV OSCC cases, it is currently unclear whether the polymorphism is merely a marker of HPV-related OSCC. Further research is warranted to investigate this relationship.

In 2011, Heah et al. found a significant correlation between p53 expression and TP53 aberration in 26 OSCC cases [35]. This finding is in contrast to the results of our present meta-analysis, although it should be noted that TP53 contains multiple polymorphisms in addition to the one in codon 72.

Our meta-analysis has a number of limitations. First, like all meta-analyses, it is a secondary retrospective study that is limited by various factors including quality of the original studies, study population differences, and the measurement tools used. Second, statistical heterogeneity is substantial, although this is extremely common in meta-analyses of genetic association studies. We therefore performed subgroup analyses to consider the factors that may have contributed to the high degree of heterogeneity. Third, our included studies lacked comprehensive genotype information so the results of our meta-analysis were analyzed using unadjusted data; hence, we could not generate a more accurate analysis based on other adjusted factors. Finally, the sample size of our meta-analysis is relatively small and studies published in languages other than Chinese and English were not considered for inclusion.

Conclusions

Our meta-analysis showed a lack of association between the TP53 codon 72 polymorphism and OSCC susceptibility in Asians, although subgroup analysis demonstrated an association between the polymorphism and HPV-related OSCC patients. Because of the numerous limitations of this meta-analysis including small sample size and substantial statistical heterogeneity, our results should be interpreted with caution and further data from high-quality, well-conducted clinical studies of adequate statistical power are needed.