p53 in head and neck cancer: Functional consequences and environmental implications of TP53mutations

Carcinomas of the head and neck are among the most common types of cancer [1] and as such represent a major health problem. Although it is difficult to distinguish the effects and risks of individual carcinogens from all other exposures, it is clear that head and neck squamous cell carcinoma (HNSCC) is epidemiologically strongly associated with alcohol consumption and exposure to tobacco smoke [2]. The probability of developing the cancer increases with the amount of tobacco and alcohol consumed [3, 4]. Associations between head and neck cancer risk and exposures to other environmental and occupational factors have also been proposed [5]. Putative occupational risk factors include nickel refining, woodworking, and exposure to textile fibres. Moreover most studies suggest that oral cancer patients have a history of diet low in fruit and vegetables [6]. In addition, human papillomavirus (HPV) infection has been associated with some HNSCC subgroups, mostly cancer in oropharynx [7, 8]. A synergistic effect between exposures is likely, because synergism has been demonstrated between smoking and radon or asbestos in lung cancer and oesophageal cancers [4, 9].

Aberrations of p53 are the most frequent molecular events in human cancers. The TP53 tumour suppressor gene in chromosome 17p13.1 encodes the p53 protein involved in many key events in the cell like regulation of cell cycle and glucose metabolism in cancer cells, DNA-repair, apoptosis, and senescence and induced by various stress signals, including DNA-damage and inflammation [10, 11]. In both mice and humans, germ line mutations in TP53 result in a strong predisposition to cancer [12]. Indeed, Gadea and co workers (2007) showed that a loss of wild-type p53 function was enough by itself to confer an increased migratory capacity to cells [13]. It has been shown that there are less TP53 mutations in the setting of HPV infection [14, 15]. The biological basis for this is provided by the fact that the HPV E6 oncoprotein specifically inactivates wild-type p53. In this way the high-risk HPV E6-mediated degradation of the p53 protein is probably an alternative pathway for a "classical" mutation to knock-out the p53 regulated pathways [15, 16]. Analysis of TP53 mutational patterns has shown its usefulness in at least two main areas [17, 18]. Firstly, knowledge of the position of mutations has helped to better understand the functions of various domains of the p53 protein and their involvement in mediating the suppressive functions that are inactivated in cancer. Secondly, it has been shown that the patterns of mutations may vary according to the nature of etiological agents implicating the use of TP53 mutation spectrum as a biomarker of environmental aetiology. Most of TP53 mutations described in the IARC TP53 mutation database affect exons 5-8, which constitute the site-specific, DNA-binding domain [19]. This region encodes for residues 130-286, also the most important region for folding and stabilization of the tertiary structure of p53 protein. Less than 2% of the mutations are found in the N- and C-terminal regulatory domains. The crystal structure of the core domain, solved in 1994, provides a template for understanding the nature of mutant p53 [20]. The structure contains a β-sandwich scaffold and a DNA-binding surface, including a loop-sheet-helix (LSH) motif and two loops (L2 and L3) tethered by a single zinc atom. Different mutations have very different consequences for the function of p53 protein. However, mostly mutation frequencies in tumours have been reported and less attention has been paid to the connection of functional state of the mutated p53 with clinical and environmental aspects of cancer. Most of the TP53 mutations in human cancers are missense mutations [17], that can either cause a loss of tumour suppressor function (LOF) or, in some cases, a gain of oncogenic function (GOF) [21, 22]. In addition to various degree of LOF, some mutant proteins inhibit the functions of the wt allele by a dominant-negative effect [19]. Recent studies have been carried out in an attempt to provide an explanation for the structural effects of most disease-related TP53 mutations [23, 24] and functional impact of TP53 mutations [25, 26].

In this study, we have analyzed p53 aberrations in primary head and neck cancer patients with information of their chemical exposures. Although the association between smoking and alcohol with head and neck cancers is well-known and quite strong [2], and it is clear that p53 aberrations in general are important in human cancers, it is not known whether p53 aberrations are associated with environmental exposures in head and neck cancers. The 46 head and neck cancers analyzed in this paper provide a significant addition the data in IARC TP53 mutation database, especially due to the known environmental exposures.

The most interesting finding was that we found the same mutation (Asp259Glu most probably leading to a non-functional p53 protein) in four individuals who all had chemical exposure: tobacco and alcohol, and in three cases documented work exposure to chemicals including pesticides, oil and asbestos. In the fourth case the information of work exposure was missing. Although smoking and alcohol in head and neck cancer have been linked with TP53 mutations before [32–36] work exposure has not been included in earlier papers. Furthermore, considering the fact that this mutation has been described only in five cases before in IARC database is certainly an implication to follow-up. Including our cases, out of total of nine Asp259Glu mutations four have been in larynx tumours, which may represent a preferential site for this mutation. Both of our larynx cancer patients with this mutation tumour had a high exposure index (6 and 8) and none of the patients presented a positive family history for cancer. This may justify further studies of TP53 Asp259Glu mutation as a marker of environmental exposure in larynx tumours.

The TP53 mutation frequency in this study is in line with the one reported by IARC in head and neck squamous cell carcinomas (57% in this study; 47.5% in IARC TP53 mutation database [19]. According to the IARC TP53 mutation database, mutations in head and neck cancers occur frequently in codons 238-248, which is a hotspot region. In our material this region was underrepresented with only 3/26 mutations. Among this series, 11/26 (42%) tumours contained multiple TP53 mutations. Although multiple TP53 mutations have earlier been described in the literature in HNSCCs [37–40], they are not as commonly reported as tumours with a single TP53 mutation. Our study is too small to pursue multiple mutations in connection with other parameters.

Altogether 13 tumours with TP53 mutations in our series probably harbour a non-functional protein for various reasons. For instance, zinc is essential for the function of p53, because p53 does not adopt the correct conformation in the absence of zinc [41, 42]. Thus mutations in the residues involved in the interaction with zinc, like Cys238Ser in this study, will result in a non-functional p53. According to recent data a mutation that alters the stability of the protein (structural mutants; 7 tumours in our series) is more likely to disrupt all functions of the protein, whereas a mutation within a contact residue (contact mutants; 4 cases in our series) will probably be more selective in affecting the transcriptional activity of p53 [43, 44]. Both classes of mutant p53 proteins commonly accumulate to high levels in tumour cells and are defective for wild-type in p53 functions [43]. It remains to be tested whether any relation of the functional effects of the mutations to exposure types or total exposure exists. Our series is small and the fact that we did not find such correlations does not rule them out.

In accordance with Koch and co workers (1995) our study implicated young age to associate with a higher TP53 mutation frequency in the tumours [45]. Interestingly, all the three patients under 39 had a squamous cell cancer of the oral cavity and all these tumours harboured a TP53 mutation. Petitjean et al. (2007a), on the basis of the IARC TP53 mutation database, reported that the mean age at onset in carriers of a TP53 mutation leading to a functional protein was higher than the age of patients with a non-functional protein [25]. Thus, the penetrance of a mutation may be related to its degree of loss of transcriptional activity, which is not surprising. Similar implications have been described on basis of p53 protein expression. De Paula and co-workers (2009), who evaluated 724 primary HNSCC in young (under 45 years) and older (46-92 years) patients, reported a significantly higher p53 expression (p < 0.05) and a higher incidence of oral cavity tumours in younger patients [46]. On the other hand, Regezi et al. (1999), who evaluated and compared the expression of the cell cycle proteins p53, p21, Rb and MDM2 in tongue cancer patients aged 35 and younger and those aged 75 or older, reported equivalent p53 mutant protein expression [47]. The possible association with age in different head and neck cancers still requires confirmation in larger patient materials.

Positive findings in p53 immunohistochemistry have especially earlier, been interpreted as indicating inactivation of the TP53 gene on the basis of the knowledge that the half-life of the wild-type protein is too short to permit detection, whereas the mutant protein is stable [48]. However, the TP53 gene may also harbour mutations that do not result in its stabilization, or deletions that inhibit transcription altogether. Alternatively, p53 function may be inhibited by epigenetic events, such as enhancing its degradation or by interference with proteins controlling its transcriptional activity [49]. Furthermore, p53 protein may be induced in cells by DNA-damaging chemical exposure (for reviews see [50, 51]) through posttranslational modifications [52]. In agreement with previous studies [48, 53], we found that p53 overexpression was a common event in HNSCC. In newest papers immunohistochemically positive cases associate with TP53 mutations in head and neck cancers [48, 54, 55] and our paper does not contradict this.

In the present study, we found that a positive p53 immunohistochemistry was more common among heavy smokers than among non-smokers, as reported earlier [54, 56]. However, we did not find a correlation between the amount of tobacco consumed and the frequency of TP53 mutations. Vähäkangas and co workers (2001) noticed that in lung cancer TP53 mutations occur more commonly in smokers and ex-smokers than in never-smokers [57]. Other reports pertinent to tobacco exposure, using various methods to detect TP53 mutations, have given conflicting results. Studies showing a positive association between TP53 mutation and tobacco smoke in patients with head and neck carcinoma are, however, more numerous e.g. [32–36] than studies with no association e.g. [38, 58, 59]. Unfortunately, information on alcohol consumption was lacking in eight patients in our material. Interestingly, we also found that the tumours from patients with a negative family history for cancer contained TP53 mutations more often than tumours from patients with a positive a family history. Considering the age correlation and family history data, our results may be interpreted as supporting the environmental aetiology of the TP53 mutations.

We noticed that tobacco and alcohol exposures were statistically significantly higher in laryngeal tumours than in oral cavity tumours and p53 overexpression was more prevalent in laryngeal tumours than in other anatomical sites. Recently De Paula and co-workers (2009) in a large material of over 700 patients noted a correlation between p53 immunohistochemically positive tumours and anatomical site [46]. The exposure to chemical carcinogens e.g. in smoke may not be even in different locations or the sensitivity of locations may vary. Another level of variation is the inter-individual susceptibility according to genetic factors. A small proportion of individuals exposed to potential carcinogens might develop the disease and intrinsic susceptibility to environmental exposure most probably plays a role also in head and neck cancer see e.g. [60, 61]. Furthermore, in our series females with a head and neck cancer had less exposure than males, which supports the reported higher susceptibility of women to carcinogens like cigarette smoke [62].

Our study shows implications of p53 aberrations being associated with the environmental exposure in head and neck cancer. Regardless of how the data are looked at, a trend for a higher frequency of p53 alterations remains among those with higher exposure. In accordance with earlier literature (see e.g. Vähäkangas 2003), our results thus justify further studies of p53 alterations as a biomarker of environmental exposure in head and neck cancers. Especially, the mutation Asp259Glu (GAC > GAA) most probably leading to a non-functional p53 protein, which was found in four tumours in this series, may justify further studies as a marker of environmental exposure in larynx tumours.