In this case-control analysis, more patients with oral cancer than cancer-free controls had a history of smoking, alcohol consumption, or HPV infection. We found that smoking, alcohol consumption, and HPV16 infection independently increased the risk of oral cancer, particularly for oropharyngeal cancer. Smoking and alcohol use as the traditional oral cancer risk factors were independently associated with a moderately increase in risk of oral cancer, and HPV16 seropositivity was associated with a great increase in risk of it, which were consistent with results of prior studies [10, 24]. However, the results of previous investigations have been unclear regarding the combined effect of HPV exposure with tobacco and alcohol use on risk of oral cancer. In this study of the combination of HPV exposure with traditional oral cancer risk factors, we found that among patients who were HPV16-seropositive, ever-smokers and -drinkers had a markedly greater risk of oral cancer than did never-smokers and -drinkers, particularly among those with oropharyngeal cancer patients.
In a study of HPV DNA in exfoliated oral cells, Smith et al. reported a synergistic effect of heavy alcohol consumption and HPV infection on risk of oral cancer but did not observe such an effect of smoking and HPV infection [16]. Alternatively, others observed an increased risk of oropharyngeal cancer in patients who were seropositive for anti-HPV16 antibodies and had tobacco exposure [15, 25, 26]. Other studies of the interactions among tobacco and alcohol use and HPV16 infection demonstrated no evidence of synergistic interactions [10, 24-29]. In the present study, we observed a strong combined effect of HPV16 seropositivity with tobacco or alcohol use on risk of oral cancer, particularly for oropharyngeal cancer. This suggests that the carcinogenic effects of smoking and alcohol consumption and the genomic instability caused by HPV infection and p53 inactivation act at different stages of carcinogenesis, resulting in a multiplicative rather than additive effect on tumorigenesis.
One possible reason for recent disparate conclusions in the literature regarding the interaction among risk factors for oral cancer is use of different methodologies of HPV detection combined with different types of HPV. HPV markers can be detected as HPV DNA in exfoliated oral cells and biopsy specimens and as anti-HPV antibodies in serum. Smith et al. examined HPV DNA in exfoliated oral cells and found that detection of HPV DNA in them was associated with increased risk of oral cancer [16], whereas Herrero et al. found no association between HPV DNA in exfoliated cells and detection of HPV DNA in oral tumor biopsy specimens [27]. Other studies support the findings described by Herrero and colleagues, suggesting that HPV DNA in exfoliated cells is a poor marker of tumor HPV status [15]. In addition, Gillison et al. stratified head and neck squamous cell carcinomas according to tumor HPV status and found no associations among tobacco and alcohol use and HPV16-positive tumor biopsy specimens [28]. Detection of anti-HPV16 antibodies in serum suggests exposure with HPV at any mucosal site, not specifically the oral mucosa. Also, whereas anti-HPV antibodies generally indicate invasive HPV infection, some individuals may not undergo seroconversion. Additionally, antibody levels may not be maintained over time, making anti-HPV16 antibodies in serum difficult markers to interpret. Nonetheless, antibodies are minimally invasive markers predictive of exposure to HPV and invasive HPV infection [30].
Discrepancies in the findings describing the interactions among HPV infection, smoking, and alcohol consumption may also be related to different subsites of oral cancer, such as the oral cavity (tongue, floor of mouth, palate, buccal mucosa, and gingivae) and oropharynx (base of tongue and tonsils), which are associated with different cancer risk factors. In a pooled case-control series, Lubin et al. demonstrated similarly increased risk of oral cavity and oropharyngeal cancer with smoking and drinking [8]. This is consistent with historical evidence that smoking and alcohol consumption are strongly associated with oral cancer at all subsites. Conversely, the rate of infection with HPV has varied depending on the head and neck cancer subsite. Consistent with previous meta-analyses [31, 32]. For example, In a study of 92 head and neck squamous cell carcinomas patients, their findings demonstrated that patients with cancer in the oropharynx had a higher HPV infection rate than did patients with cancer in the oral cavity [33]. These results suggest that stratification of patients according to head and neck cancer subsite is important in classifying specific cancer risk and associated risk factors.
In this case-control study, we controlled for the confounding variables of, age and sex by frequency-matched on these variables and adjusted with these variables with additional ethnicity to HPV exposure between the controls and cases, who had not undergone treatment. The limitations of this hospital-based study include possible selection bias due to the selection of cases and controls from different populations. Also, small sample sizes may also limit the statistical power of data analysis, which could restrict the generalizability of the findings. Furthermore, although the use of HPV serology enabled us to use a cancer-free control group, HPV serology we used is not specific to oral cancer tumor status but is rather a reasonable indication of HPV exposure [27, 30]. Infection with high-risk HPV types other than HPV16 may also be associated with oral cancer, but we limited this study to anti-HPV16 antibodies. Finally, patients’ self-reporting of previous alcohol and tobacco use may have resulted in recall bias, in which case the estimates of association between smoking and alcohol consumption exposure alone or in combination with HPV exposure and risk of oral cancer may have been underestimated.