INTRODUCTION:
Oral cancer constitutes the sixth most common cancer worldwide and third most common cancer in South-East Asia. Oral squamous cell carcinoma (OSCC), the most common type of oral cancer is often preceded by potentially malignant lesions or conditions such as leukoplakia and oral submucous fibrosis (OSF). On biopsy, most leukoplakias show histologic features of epithelial dysplasia. The standardization of histopathological diagnosis and grading of epithelial dysplasia remains subjective as there are many composite histologic criteria of cellular atypia and architectural disturbances. OSF is a well recognized potentially malignant condition that is characterized by rigidity of oral mucosa and development of palpable fibrous bands, resulting from the deposition of collagen in juxtaepithelial and submucosal layers. The malignant transformation rate for leukoplakia ranges from 15-20%, while transformation rates as high as 7.6% over a 10-year period have been reported for OSF.
It has been generally considered that oral carcinogenesis develops through a multistep process of accumulation of genetic mutations related to cell proliferation and differentiation. The principal targets of genetic damage include growth-promoting protooncogenes, growth-inhibiting tumor suppressor genes and genes that regulate apoptosis. Mutations in the tumor suppressor p53 gene and resultant alteration in the protein are the most common abnormalities found in squamous cell carcinoma of the head and neck region. The p53 gene encodes a 53 kD nuclear phosphoprotein that is involved in DNA repair, programmed cell death and negative regulation of cell cycle.
In normal cells, wild type p53 protein has a very short half life (6-20 minutes) and is present in such small quantities that it cannot be detected by immunohistochemical methods. However, mutations in p53 gene often result in a more stable product and prolong the half life of p53 protein, causing it to accumulate within cell nuclei to the extent that it can be easily detected by immunohistochemistry.
The p63 gene mapped on chromosome 3q27-29 is a member of p53 gene family and is responsible for the transcription two groups of p63 protein (TAp63 and ΔNp63), both of which have α, β and γ isoforms. The TAp63 group contains an Nterminal transactivation domain and has functions similar to p53 such as, cell cycle arrest, apoptosis and cell differentiation. The ΔNp63 group lacks TA (Transactivation) domain and acts by inhibiting both p53 and TA p63 and thus favors cell proliferation. It is suggested that p63, possibly in concert with p53 may play a role in the regulation of proliferation and differentiation in potentially malignant disorders and malignant lesions of the oral cavity.
This study is done to evaluate the expression of p53 and p63 proteins in OSCC, oral leukoplakia and OSF by immunohistochemistry.
AIMS AND OBJECTIVES:
l. To evaluate the expression of p53 and p63 proteins in formalin fixed, paraffin embedded sections of OSCC, oral leukoplakia and OSF specimens by immunohistochemistry.
2. To compare the expression of p53 and p63 proteins in formalin fixed, paraffin embedded sections of OSCC, oral leukoplakia and OSF with normal oral mucosa by immunohistochemical methods.
MATERIAL AND METHODS:
Study setting:
The study was conducted in the Department of Oral and Maxillofacial Pathology, Ragas Dental College and Hospital, Chennai.
Study Subjects:
20 consecutive cases of oral squamous cell carcinoma (Group I), 20 consecutive cases of clinically diagnosed oral leukoplakia (Group II), 20 consecutive cases of OSF (Group III), and 10 cases of normal patients (Group IV), were collected over a period of 6 months. A preformatted clinical case sheet was used to record all the cases. Detailed case history including age, sex, and occupation, past medical and dental history along with the history of habits were recorded. This was followed by general examination and intra oral examination.
Selection criteria:
Group I: Clinically and histopathologically confirmed cases of OSCC.
Group II: Clinically appearing white, non-scrapable patch associated with the history of tobacco habit, which was clinically diagnosed as leukoplakia and histopathologically graded as epithelial dysplasia (mild, moderate or severe).
Group III: The criteria for selection of OSF patients were difficulty in opening the mouth, burning sensation of the mouth, palpable vertical fibrous bands in the oral mucosa and history of areca nut chewing.
Group IV: Ten patients who reported to the outpatient department of Oral and Maxillofacial Surgery for removal of impacted third molar constituted the normal control group.
Incisional biopsy of sufficient width and depth to ensure inclusion of connective tissue was taken from the buccal mucosa of the 60 study patients. Normal non-inflammed buccal mucosa adjacent to the site of surgery was biopsied for the control group. Informed consent was obtained from all the patients. The tissues taken were immediately transferred to 10% buffered formalin for further processing. After adequate fixation, paraffin blocks of the tissues were made.
SUMMARY:
1. A total of 70 patients were included in the study, comprising of 20 cases of OSCC (group I), 20 cases of leukoplakia (group II), 20 cases of OSF (Group III) and 10 patients with normal oral mucosa (group IV), showing mean ages of 55.2, 43.6, 36.4 and 29.5 years respectively.
2. In group I, 80% were males and 20% were females. In group II, 100% were males. In group III, 95% were males and 5% were females. In group IV, 40% were males and 60% were females.
3. The p53 mean LI for group I, II, III and IV were 56.9 ± 21.3, 37.6 ± 12.6, 34.6 ± 8.7 and 15.1 ± 9 respectively; the difference in mean LI among the groups was statistically significant (p=0.00). The p63 mean LI for group I, II, III and IV were 56.8 ± 19.6, 42.3 ± 10.5, 32.8 ± 12.1 and 26.4 ± 9.4 respectively and the difference was statistically significant (p=0.00).
4. In group II, p53 staining was seen in the basal layer in 20% and both basal and supra basal layers in 80% of the cases. In group III, 40% exhibited basal staining, while 60% had both basal and supra basal p53 staining. In group IV, 20% of the samples had basal p53 staining and 10% had both basal and supra basal staining. The tissue localization of p53 staining was significantly different among the 3 study groups (p=0.00).
5. In group II, 10% of the cases exhibited p63 staining in the basal layer, while 90% had both basal and supra basal staining. In group III, 30% had basal stain and 70% showed basal and supra basal staining. In group IV, basal staining was seen in 50% of the samples while both basal and supra basal staining was observed in the remaining 50% of the samples. The tissue localization of p63 staining was not statistically significant among the 3 study groups (p=0.05).
6. p53 and p63 staining showed a significant positive correlation in group I and III (p=0.00; p=0.00).
CONCLUSION:
In conclusion, the results of the current study show that there is significantly higher expression of p53 and p63 proteins in OSCC, oral leukoplakia and OSF samples when compared to normal oral mucosa. Overexpression of p63 protein in OSCC, leukoplakia and OSF samples might be due to
1. Compensatory up-regulation of p63, resulting from mutation of p53 gene.
2. Transcriptional dysregulation of p63 gene.
However, the 4A4 antibody against p63 protein, which was used in our study does not discriminate TA or ΔN type of p63, both of which has opposite functions. Further mRNA studies by RT-PCR using isoform-specific primers, will ascertain the exact role of p63 in oral carcinogenesis and also its relationship to p53 protein
