Role of human papillomavirus in the pathogenesis of oral squamous cell carcinoma

Abstract

Oral cancer is one of the most common cancers and it constitutes a major health problem particularly in developing countries. Oral squamous cell carcinoma (OSCC) represents the most frequent of all oral neoplasms. Several risk factors have been well characterized to be associated with OSCC with substantial evidences. While tobacco and alcohol are the primary risk factors for OSCC development, many epidemiological studies report a strong association with human papillomavirus (HPV) in a subset of OSCC. This article presents our current knowledge on the relationship between HPV and development of OSCC. HPVs are DNA viruses that specifically target the basal cells of the epithelial mucosa. Most experimental data are consistent with the hypothesis that HPV plays a causal role in oral carcinogenesis. Genotypes, such as HPV1 infect epidermal cells, whereas HPV6, 11, 16 and 18 infect epithelial cells of the oral cavity and other mucosal surfaces. Several studies have shown that there is an increased risk of head and neck cancer in the two major HPV 16 oncogenes E6 and E7 -positive patients. The presence of antibodies to HPV E6 and E7 proteins was found to be more associated with tumors of the oro-pharynx than of the oral cavity. However, HPV alone appears to be insufficient as the cause of OSCC but requires other co-factors. Although a viral association within a subset of OSCC has been shown, the molecular and histopathological characteristics of these tumors have yet to be clearly defined.

Key Words: Human papillomavirus; Oral squamous cell carcinoma

INTRODUCTION

Worldwide, oral cancer accounts for 2%-4% of all cancer cases. In some regions, such as Pakistan, oral cancer reaches the 10% of all cancers, and around 45% in India[1,2]. Within the EU the highest oral cancer incidence rates for males are found in France and Hungary (6%) and the lowest rates in Greece and Cyprus[3,4]. In 2009 over 300 000 new cases of oral and oropharyngeal cancer were diagnosed worldwide. During the same time period, over 7000 affected individuals died of these cancers[5]. Oral cancer includes a group of neoplasms affecting any region of the oral cavity, pharyngeal regions and salivary glands. However, this term is almost synonymous with oral squamous cell carcinoma (OSCC), which represents the most frequent of all oral neoplasms. It is estimated that more of 90% of all oral neoplasms are OSCC[6]. It is well established that a strong association exists between genetic factors, environment and oral cancer. Several factors are involved in the pathogenesis of oral cancer, such as age, gender, ethnicity, lifestyle, genetic background, status of health and exposure to one or more oncogenic factors[7]. Tobacco use and alcohol consumption are well-known risk factors. Micronutrient deficiencies and poor oral hygiene[8-10] have also been associated with increased risk. However, the observation that several patients with oral cancer have not been exposed to these risk factors suggests that additional causes may promote oral carcinogenesis. Thus, other agents, such as viruses, are being investigated.

HUMAN PAPILLOMAVIRUS AND OSCC

The human papillomavirus (HPV) family consists of more than 100 genotypes, classified in accordance with the ability to infect and transform epithelial cells. HPVs are DNA viruses that specifically target the basal cells of the epithelial mucosa[11].

Genotypes, such as HPV1 infect epidermal cells, whereas HPV6, 11, 16 and 18 infect epithelial cells of the oral cavity and other mucosal surfaces. The ability of HPV to transform epithelial cells is divided into high-risk and low-risk types. Low-risk types are associated with development of benign lesions such as warts, while infections with high-risk types may progress to malignant lesions[12].

The HPV involvement in oral carcinogenesis was supported on the basis of the following evidences: (1) the strongly established etiological role of HPV in cervical SCC[13,14]; (2) the epithelial tropism of HPV; (3) the similarity between oral and genital epithelia[15]; and (4) the detection of HPV genotypes in samples of OSCC[16-20].

The HPV involvement in oral and oropharyngeal carcinogenesis was first proposed in 1983 by Syrjänen et al[16]. Her results showed that 40% of the laryngeal and oral cancers contained histological and morphological similarities with HPV-infected lesions, and 50% of the samples demonstrated HPV structural proteins by immunohistochemistry[21].

Since then, several studies have focused on HPV detection in oral cancer but results have been conflicting[22-24]. It was found that the prevalence of HPV detection varies broadly, depending on the population, on the location of the cancerous lesion, type of specimen, and detection method. By contrast, HPV was more frequently detected in OSCCs of the oropharynx and tonsil than at other head and neck sites[25-27]. However, in a systematic review that was performed by Syrjänen et al[28] it was suggested that a potentially important causal association between HPV (specifically HPV16) and OSCC exists. In a recent study the overall HPV prevalence was 10.5% in oral cavity carcinomas and was higher in female than in male cases. Ninety five percent of HPV-positive cases were infected by a single HPV type. HPV 16 was the most prevalent type and was found in 95.5% of HPV-positive oral cavity carcinoma cases[29].

Furthermore, other studies have proved the existence of a synergistic effect between HPV and alcohol. The risk of head and neck cancer was statistically significantly increased in heavy alcohol users detected with the virus, compared to that of HPV-negative cancer drinkers. Therefore, it has been proposed that alcohol can biologically modify mucosal tissue, increasing its permeability to viral infection, or by influencing the immune response to HPV[30].

It is believed that one of the major events of HPV-induced carcinogenesis is the integration of the HPV genome into a host chromosome. HPV genome integration often occurs near fragile sites of the human genome, but there are no apparent sites for integration and no evidence for insertional mutagenesis[31].

Slightly modifying Koch’s postulates, in order to establish a relationship between a causative virus and a disease, four criteria are needed: (1) viral genome to be present in tumor lesions or in tumor cells; (2) virus must be isolated from a pathologic lesion and grown in culture; (3) cultured virus should cause disease when inoculated into a healthy organism; and (4) virus must be reisolated from the inoculated host and identified as being identical to the original specific causative factor. However, the use of Koch’s postulates to establish disease causation does not fully apply to these phenomena, since the etiology of cancer is multifactorial[32]. In the contrary, other authors suggest that the incidence of HPV infection in the oral cavities of healthy population is very low and therefore other risk factors are most likely responsible to promote oral carcinogenesis[33].

GENITAL HPV INFECTION AND RISK OF OSCC

Several studies examined the incidence of second cancers after an initial diagnosis of ano-genital cancers[34,35] and have showed that there is an increased risk of head and neck cancer as well as other HPV-associated ano-genital cancers in the two major HPV 16 oncogenes E6 and E7 -positive patients. This association between HPV-associated anogenital cancers and head and neck cancer was further strengthened by two larger studies[36,37].

Additionally, the presence of antibodies to HPV E6 and E7 proteins was found to be more associated with tumors of the oro-pharynx than of the oral cavity[12].

A recent study has shown that besides the classical horizontal transmission during the sexual life, a vertical transmission occurs in approximately 20% of case HPV positive people. In these individuals, HPV-DNA is detected in amniotic fluid, foetal membranes, blood and placental trophoblastic cells, all suggesting HPV infection in utero, i.e. prenatal transmission[38].

HPV TESTING

HPV testing is critical for the estimation of HPV prevalence in various oral diseases. HPV testing is usually based on PCR method. General or consensus primers targeting L1 gene are most frequently used for HPV detection because they are able to identify several HPV genotypes at the same time. Sampling techniques together with widely divergent PCR methods in different studies explain most of the variability in HPV prevalence among OSCC and control samples[29]. In situ hybridization and in situ oncogenic protein staining techniques have also increased sensitivity and specificity and are used for HPV testing. These techniques have allowed not only the detection of HPV in cytological smears or histopathological immune-sections but also the determination of the topographic site of the infection[39]. According to recent studies, HPV-positive squamous cell carcinomas have intact p16 gene and wild type p53 compared to HPV negative ones[40]. Other authors have noted that a distinctive mark of the presence of HPV in oral cancer could be found in p16 nuclear or cytoplasmic overexpression[41,42]. However, one goal of the scientific research is to find new biological markers able to identify the set(s) of genes involved in oral carcinogenesis.

HPV SEROLOGY

The immune response to HPV infection involves both the cell-mediated and humoral responses. However, serological evidence is circumstantial since it provides only data on prior exposure to HPV. Since not all patients with HPV-associated cancers have detectable HPV antibodies, serum antibody determination may be a limited biomarker for HPV infection and carcinogenesis. Serum antibodies to HPV capsid proteins (virus-like particles) are thought to be a marker of lifetime HPV infection[43,44]. Antibodies against HPV E6 and E7 proteins are associated with increased risk of HPV-associated cancer[45,46] but are rather linked more with tumors from the oro-pharynx than from the oral cavity[12].

The use of HPV viral load in oral biopsies in conjunction with serological markers may serve to identify a subset of HPV-associated oral cancers in which HPV is biologically active.

PROGNOSIS AND FAVORABLE OUTCOME

Several lines of evidence suggest that HPV-positive and HPV-negative HNSCC represent distinct subgroups with different biological, epidemiological and prognostic profiles[7,47]. Recent data suggest that a positive HPV status represents an important prognostic factor and is associated with a favorable outcome in head and neck cancer. Many studies confirmed that HPV-positive OSCC have a better prognosis compared with those that are HPV negative[48-51]. There is an approximate 30% absolute survival difference at 5 years (HPV-positive = 60% vs HPV-negative = 30%)[52]. The favorable outcome for patients harboring HPV-positive cancer cannot be easily explained. It has been proposed that HPV-positive cancer arises through a different mechanism or exhibits less genetic instability i.e., shows a lower degree of aneuploidy and a tendency to have fewer chromosomal aberrations, when compared to HPV-negative cancer[53]. In contrast, there appears to be a subgroup of HPV-positive patients whose clinical prognosis is worse than the typical HPV-positive patient. This subgroup has higher smoking rates, higher rates of p53 mutations and higher expression of EGFR and Bcl-xL[27].

EXPERIMENTAL EVIDENCE

Experimental evidence regarding the role of HPV in oral carcinogenesis is limited both in vitro and in animal experimentation. The lack of suitable experimental animal models has hindered research into HPV cancers for many years. In one of the most significant studies it has been shown that oral keratinocytes could not be transformed by HPV alone but required further mutations in other oncogenes[54].

CONCLUSION

The vast amounts of epidemiological, molecular pathological and in vitro experimental data are consistent with the hypothesis that HPV does indeed have a causal role in oral carcinogenesis. However, HPV alone appears to be insufficient as the cause of OSCC but requires other co-factors. Although a viral association within a subset of OSCC has been shown, the molecular and histopathological characteristics of these tumors have yet to be clearly defined. Since HPV16 is involved in the development of some oral cancers, the implementation of a vaccine program for HPV 16 and 18 may prove to be beneficial in preventing not only cervical cancer, but possibly HPV16-positive oral cancers as well.