This retrospective study of 125 patients with first primary head and neck tumours evaluated the association between tumour HPV status and patient demographics, clinical risk factors and amplification status at chromosomal band 11q13. We further evaluated the relationship between genetic alterations: TP53 mutation status, 11q13 amp and p16 protein expression in a subpopulation of 69 unselected tumours. Our analysis shows that along with wild-type TP53 and p16 overexpression, HPV-positive tumours were less likely to carry gene amplification of chromosomal band 11q13, which further defines the characteristic of HPV-positive head and neck tumours and suggests a distinct molecular pathway for HNSCC development.
The relationship between LOH at
TP53, 11q13 amp and HPV has been shown in one study including only 37 HNSCC (
Rodrigo et al, 2002). Loss of heterozygosity at
TP53, 11q13 amp and HPV status were reported on only 11 (30%) of their samples and p16 overexpression was not evaluated. The study reported a lower frequency of 11q13 amp in HPV-positive tumours (HPV+ with 11q13 amp=2 out of 4 (50%); HPV− with 11q13 amp=5 out of 7 (71%) but their HPV-positive tumours were more likely to have LOH at
TP53 (three out of four (75%)). Earlier studies utilising immunohistochemistry have shown that the overexpression of p53 and p16 and reduced expression of cyclin D1 were associated with HPV-positive tumours but 11q13 amp was not evaluated and these studies only included tumours arising in the tonsils (
Andl et al, 1998;
Li et al, 2004). One other study evaluated the combined relationship between the three molecular markers and HPV status using immunohistochemistry (
Wilczynski et al, 1998), but similar to the earlier mentioned studies, their analysis was limited to tumours that arose in the tonsils. In addition, the study included tumours that were first primaries, recurrences, lymph node metastases and lung metastases. Despite the differences in sample size and tumour site included in these analyses, the results from these three studies are in agreement with our findings. In our study, the combined characteristics of
TP53 mutation status, p16 protein expression (a protein which is overexpressed as a consequence of HPV E7 expression) and 11q13 amp (a genetic alteration frequently observed in head and neck tumours) were evaluated to further characterise the role of HPV in head and neck tumourigenesis. Tumours that were HPV-positive were more likely to carry all three markers (wild-type
TP53, express p16 and no amplification at 11q13). We included in our analyses tumours that were first primaries from various sites within the oral cavity and pharynx and demonstrate that HPV-positive tumours with this distinct molecular phenotype can arise not only in the oropharynx (50%, four out of eight) but also in the oral cavity (50%, four out of eight).
Our results shed further light on the multiple genetic alterations that are commonly found in of head and neck tumours (
Gollin, 2001). A genetic progression model for HNSCC has been proposed by Califano
et al (
Califano et al, 1996,
2000) and describes a series of ordered genetic changes which may occur during tumour development. It is thought that, for the most part, early genetic alterations include loss of chromosomal bands 9p21, 3p, 17p13 and amplification of 11q13. The candidate genes involved in these genetic alterations are thought to be
CDKN2A/p16, the tumour suppressor gene
FHIT,
TP53 and the
CCND1 oncogene, respectively. Examination of hyperplastic, dysplastic, carcinoma
in situ and invasive carcinoma lesions revealed increasing LOH which corresponded with histopathological progression (
Califano et al, 1996). Loss of heterozygosity at 9p21 was the most frequent alteration in benign hyperplastic lesions, followed by LOH at 3p21 and 17p13. In addition, dysplastic lesions revealed an increased incidence of allelic imbalance at 11q13 (benign hyperplasia: (6%), dysplasia: (29%), carcinoma
in situ: 40%, invasive carcinoma (61%)). Our results show that HPV-positive tumours are less likely to be amplified at 11q13 and at the same time p16 is overexpressed, providing indirect evidence that the infection may have occurred at an earlier time point in the carcinogenic process.
The loss of functional
TP53 by mutation seems not to be necessary in HPV-positive HNSCC, as p53 protein loss results from increased degradation in these tumours (
Wiest et al, 2002;
Hafkamp et al, 2003;
Braakhuis et al, 2004). The HPV E7 oncoprotein binds and degrades the RB1 tumour suppressor protein (
Boyer et al, 1996), which in turn causes the release of E2F (a transcriptional regulator of cell proliferation genes) from pRb/E2F complexes, permitting E2F to transactivate S-phase-related genes. The functional inactivation of pRb by E7 leads to overexpression of the cyclin-dependent kinase inhibitor p16 (
Khleif et al, 1996). The detection of p16 expression therefore is considered to be a surrogate marker for HPV infection, this observation was also confirmed in our studies.
Amplification of chromosomal band 11q13 has been reported in a number of carcinomas (head and neck, breast, lung, pancreatic, prostate, ovarian, bladder and so on.) (
Schwab, 1998). Approximately 45% of HNSCC have amplification of 11q13 (
Schuuring, 1995;
Lese et al, 1995). The amplification status of chromosomal band 11q13 was determined in our study, using a sensitive methodology, FISH with probes for
FGF3/INT2, FGF4/HST1 or
CCND1 (
Lese et al, 1995;
Shuster et al, 2000). A significantly smaller proportion of patients with 11q13 amp were observed in the HPV-positive group (13%, two out of 15) compared to the HPV-negative group (54%, 26 out of 48). One consequence of 11q13 amp is
CCND1 overexpression, which is thought to play a direct role in this disease (
Callender et al, 1994). Other proto-oncogenes which map to the 11q13 core region are amplified and overexpressed (
Huang et al, 2002), but their role in head and neck tumour development has not yet been fully delineated. The
CCND1 gene product, cyclin D1 associates with cyclin-dependent kinase 4 and 6, and this complex promotes RB1 phosphorylation and like the HPV E7 oncoprotein, their interaction leads to the dissociation of pRb from the transcription factor E2F. This results in transition of the cell from G
1 into S-phase. One possible explanation for the lack of 11q13 amp in HPV-positive tumours may be that amplification of chromosomal band 11q13 might be unnecessary, as the resulting interaction of the HPV E7 oncoprotein with pRb might allow the cells to be less dependent on
CCND1 for cell cycle progression. We therefore believe that the low frequency of 11q13 amp observed in our HPV-positive head and neck tumours may be explained by a distinct mechanism for HPV carcinogenesis.
Several studies report that patients with HPV-positive head and neck tumours have an improved prognosis (
Ritchie et al, 2003;
Schlecht, 2005), whereas amplification of 11q13 has been associated with a more rapid and frequent recurrence of disease (
Fujii et al, 2001) and poorer survival (
Akervall et al, 1997;
Rodrigo et al, 2000;
Namazie et al, 2002).
Weinberger et al (2006) has shown that patients with HPV-positive tumours which express high levels of p16 protein and low levels of p53 protein present with a favourable prognosis. We have shown in our study that HPV-positive tumours have high levels of p16 expression and wild-type
TP53 and in addition, a low frequency of amplification at 11q13. Further analyses are warranted to determine whether this lack of 11q13 amp also contributes to the improved prognosis observed in patients with HPV-positive HNSCC.