p53 codon 72 polymorphism is linked to the development and not the progression of benign and malignant laryngeal tumours

Abstract

The p53 codon 72 polymorphism, resulting in either an arginine or a proline residue has been proposed to affect the susceptibility of p53 protein to human papilloma virus (HPV) E6-mediated degradation in vitro. However, there are controversial results from several clinical studies in various human tumours. The purpose of our study was to investigate the significance of this p53 genotype with respect to the risk of neoplasia development in Greek patients with benign and malignant laryngeal tumours. Furthermore, we searched for an association between p53 alleles and the presence of HPV in the same series of samples. We found a significant statistical association in the distribution of p53 genotypes between laryngeal lesions and normal samples (P<0.001). Allelic analysis of the patients with both benign and malignant tumours revealed a striking over-representation of the homozygous p53Arg allele compared to normal population (P<0.0003). HPV was detected in only 3 laryngeal samples (1 benign and 2 malignant tumours). This is the first study correlating the p53 codon 72 polymophism in laryngeal tumours. Our results provide evidence that this p53 polymorphism may be implicated at the early stages of the disease and concerns predisposition to premalignant laryngeal lesions rather than to progression from benign tumour toward malignancy. Moreover, we demonstrate that the p53Arg homozygous genotype affects the predisposition for laryngeal tumours while the heterozygous status does not. The low incidence of HPV infection suggests that it is not a major oncogenic factor in the development of laryngeal tumours but may have synergistic action with specific genotypes of p53 gene.

Introduction

Laryngeal cancer constitutes up to almost 2% of all human cancers and 90–95% are squamous cell carcinomas. Epidemiological studies suggest a considerable geographical variation of the disease, as well as an association with the smoking and drinking history of the patients [1]. In general the predicted mortality is 30% (thus a survival rate of approximately 70%) [2]. These observations point to the existence of particular mutagens causing various genetic alterations [3], [4], and strongly involved in laryngeal tumorigenesis. Therefore, it is of great importance to identify factors that increase the risk of laryngeal tumours.

Human papilloma virus (HPV) infection has been considered as a potential oncogenic factor for the development of laryngeal cancer. Several of the 60 types of HPVs have been detected in laryngeal tumours [5], [6] although the prevalence of HPV is estimated to range between 3–85% [7], [8], [9]. HPV-16/18 encodes E6 protein, which binds to cellular tumour suppressor protein p53 and directs degradation through the ubiquitin pathway [10], [11]. Therefore, it is widely assumed that p53 is functionally inactivated by the viral E6 protein in HPV-associated cancer cells and that infection with high risk HPV types leads to the same phenotype as a loss of p53 function due to p53 gene mutation.

The p53 gene is frequently mutated in laryngeal tumours either through mutations or via altered expression of the gene product [12], [13], [14]. A polymorphism in the wild-type p53 gene at codon 72 of exon 4 has been described, resulting in either a proline (CCC, p53Pro) or an arginine (CGC, p53Arg) residue [15]. Storey et al. reported a significant overexpression of the Arg/Arg p53 genotype in cervical cancer patients relative to the normal population [16]. Women homozygous for p53Arg were about seven times more susceptible to HPV-associated tumorigenesis than heterozygotes. In addition, the data were supported by results from an in vitro assay demonstrating that the p53Arg form of the protein was more susceptible to degradation by the HPV E6 protein than the p53Pro variant. Similar studies on cervical cancer have shown controversial results [17], [18], [19], [20], [21], [22], [23], [24], [25]. Studies on p53 codon 72 polymorphism and HPV have also been carried out in other human cancers such as esophageal [26], head and neck [27], lung [28], [29], breast [30] and skin [31]. However, the contribution of p53 polymorphism has apparently not been documented. The existence of a similar risk factor for a given p53 genotype in HPV-related laryngeal tumours would facilitate screening of HPV-related individuals.

In the present study, we investigated the genotypic frequency of p53 codon 72 polymorphism in correlation with the presence of HPV in both benign and malignant laryngeal tumours compared to matched healthy controls. Our data provide evidence for the first time that the p53Arg homozygosity present a potential risk factor in laryngeal tumorigenesis and the p53Arg allele, in the absence of p53Pro allele confers a susceptibility for the development rather than the progression of laryngeal tumours.