To our knowledge, this is the first study to explore the presence of viral DNA and cellular protein expression in head and neck tumors in West Africa. HPV DNA was detected in 3.4% (95% CI
0.9%-8.5%) of studied cases but further analyses testing for p16INK4a
expression suggested that HPV was not involved in the oncogenic process of the tumors. Expression of p16INK4a
is strongly seen in HPV-associated tumors but nearly absent in HPV-negative carcinomas [4
] and increases in cases where HPV is oncogenically involved due to the interaction of the viral oncoprotein E7 with pRb [5
Similar low HPV prevalences have been reported in the African-American community in the USA. In a prospective study conducted by Settle et al. [6
], overall HPV positivity was 4% in black patients diagnosed with oropharyngeal cancer versus 34% in white patients. Weinberger et al. [7
] found 0% HPV-active HNC in black patients compared to 21% in white patients after immunohistochemical testing with p16INK4a
. Both studies agree on significant ethnic and biological disparities in HPV prevalence, after adjusting for clinical and sociodemographic characteristics, and support the validity of our study. Further studies are needed to elucidate the mechanism of HPV clearance in African-American and African populations, which share common ancestral origins.
These findings are very different from what has been reported previously in other regions of the world. In a systematic review by Kreimer et al. [3
] compiling data on 60 studies focusing on HNSCC, HPV DNA was detected overall in 26% of cases. Reported site-specific HPV prevalence was 23.5% (95% CI
21.9-25.1%) in oral SCC, 35.6% (95% CI
32.6-38.7%) in oropharyngeal SCC and 24.0% (95% CI
21.8-26.3%) in laryngeal SCC [3
]. The highest detection rates were found in Asia, followed by the USA and Nordic countries [3
Low HPV prevalence in invasive HNC in Senegal suggest that other established risk factors such as alcohol and tobacco consumption may play a more significant etiological role than HPV infection in HNC. However, the prevalence of tobacco use in Senegal is relatively low with estimates of 19.9% in men and 1.3% in women [8
]. Alcohol consumption is also negligible, especially in women, since Senegal is a predominantly Muslim country with 95% of the population practicing this religion. Thus, the magnitude of the implication of these two classical risk factors in the development of HNC is probably small given the social norms. Therefore, we hypothesize that environmental factors and eating habits may contribute more importantly to the development of HNC.
For example, indoor air pollution causes 6,300 deaths each year in Senegal due to daily exposure to smoke from open burning of wood and charcoal in homes [9
]. More than 80% of the households use either wood or charcoal as cooking fuels in peri-urban and rural Senegal. Moreover, burning incense to deodorize and heat the indoor is a widely practiced cultural habit. The aforementioned are potential risk factors for laryngeal and nasopharyngeal cancers as they expose individuals to smoke and dangerous emissions of particles such as CO2
and CO [9
]. According to the WHO, 3.7% of the burden of disease in developing countries can be attributed to indoor air pollution [9
Another potential risk factor could be the consumption of “ataya”, a strong and bitter hot tea comparable to “yerba mate” in South America, which has been associated with an increased risk of developing cancer of the oral cavity, larynx and esophagus [10
]. On average, at least three rounds of “ataya” are served twice a day in Senegal. The fact that this tea is drank at very hot temperature and consumed with slurps can contribute to an increased risk of cancer.
Poor oral hygiene is another potential risk factor that has been documented [11
] and could play a non-negligible role in Senegal. A study in 330 Senegalese university students has shown poor dental health and the need to improve prevention programs [12
]. This study was conducted in an educated cohort, which suggests that dental issues may be worse in uneducated, low socio-economic populations with limited access to dental care. Moreover, chewing of kola nuts (Cola acuminate) has also been reported to increase carcinogenesis potential of tobacco in smokers in Nigeria by promoting palatal mucosa keratinization [13
]. Consumption of kola nuts is also widespread in Senegal.
Occupational exposure such as jobs in the construction, metal, textile, ceramic, logging, and food industries has been associated with the development of laryngeal cancer [14
]. In Senegal, protective masks and safety rules are not applied, as workplace safety regulations are not reinforced mainly due to the fact that the informal sector provides most of the jobs. This reality puts workers in a difficult predicament whereby they expose themselves to hazardous materials.
Our study has some methodological limitations. At sample collection stage, some cases were identified eligible for the study but corresponding blocks were not found as they were sent to laboratories abroad for histopathological evaluation. Additionally, several archival records were lost in one of the main laboratories due to a fire. However, these issues affected all cases regardless of patient characteristics or diagnosis. Another major limitation is the lack of individual data on risk factors: no information on patients besides their gender, age and pathological information were available in the registries. Finally, the small sample size (n
5) for oropharyngeal tumors may have affected our positivity rate. It is recognized that the highest associations of HPV in HNC have been shown in the oropharynx, particularly in the tonsils, with a positivity of 57% to 82% compared to 0.8% to 9% in other anatomic sites [15
Our study has several strengths comprising the combined use of highly sensitive assays for HPV DNA detection and a marker of HPV related transforming process such as p16INK4a expression. Additionally, we evaluated the quality of our specimens by means of cellular tubulin detection and subsequently excluded the HPV DNA negative and tubulin negative cases from the statistical analyses. Lastly, all of the study cases were selected from the main anatomy and pathology laboratories in Senegal. Thus, our cohort is representative of the target population as the vast majority of patients are diagnosed in participant centers.