Overexpression of p63 is associated with radiation resistance and prognosis in oral squamous cell carcinoma
Introduction
The p63 gene located on chromosome 3q27–29 encodes a group of six proteins with structural homology to the tumor suppressor p53.1 Physiologically p63 is expressed in progenitor cells within the basal lamina of epithelial tissues and p63-/- Knock-out mice revealed crucial involvement of p63 in epithelial proliferation and differentiation.2 Further analysis of p53- and p63-sequences revealed similar evolutionary conserved DNA binding domains, referring to important biological functions as the target genes govern cell cycle control, apoptosis and cell proliferation.3 In human solid tumors mutation of p53 and p73 resulting in functional deficiency are very common.4 By contrast, the p63 gene most often remains unaffected.5 Its expression is regulated by two different promotor regions entailing two opposing protein groups. TAp63 isoforms with highest analogy to p53 consist of a transactivational domain, a DNA binding domain, and an oligomerization domain. Transcription from a second promotor region in intron three results in three proteins lacking the NH2-terminal transactivational domain (ΔNp63). Investigations on p73 and p63 interaction with the p53 promoter regions suggest a functional role in control of tumor progression in malignancies of squamous cell origin. A model of murine embryonic fibroblasts advocates that p63 and p73 support p53 in pro-apoptotic function and may drive apoptosis in T-cell populations.[6], [7] To date no studies about distinct upstream regulatory elements for p63 in human oral squamous cell carcinoma exist. Solely a hamster model detected a shift from full length transcripts to the ΔNp63 isoforms with increasing grade of dysplasia and carcinoma induction.8 Similarities of p63 and p53 gene code and the restricted expression in basal and parabasal epithelial cell layers of differentiated epithelia point at a role for control of proliferation and differentiation.9 Recent studies revealed an association of clinical aggressiveness and an unfavourable outcome for overexpression of p63 in ovarial carcinoma and squamous cell carcinoma of the head and neck region.[10], [11]
Although TAp63 forms may induce mitochondrial cytochrome c release in vitro12 and therefore may have a potential influence on apoptosis, to the best of our knowledge the linkage of p63 expression and clinical radioresistance has not been explored so far. Thus, we compared p63 expression in oral squamous cell carcinoma to effectiveness of radiation therapy in a clinical setting.
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Patients
33 patients (6 females and 27 males), age 34–72 years (mean 50.8) were enrolled in this retrospective study. Criteria for inclusion were: primary OSCC; pre-operative radiotherapy (36 Gy) followed by surgical resection with clinical safety margin of 1 cm; available follow-up data; antigen preservation in the biopsy confirmed by vimentin control.13 Retrospective investigation of paraffin-embedded biopsy material for the study was performed according to the regulations of the local ethical committee.
Treatment protocol
p63 expression in OSCC
All the squamous cell carcinomas analyzed as part of this study were positive for p63 on immunohistological evaluation. Neither correlation to sex and gender nor to TNM stage or grading was found. The p63 labeling index was highly variable, ranging from 36% to 87%. Representative examples of tumors showing “high” and “low” p63-labeling indices are given in Figure 1.
Association between p63 expression and radiation resistance
After a pre-operative radiation dose of 36 Gy, 22 tumors showed only minimal or partial shrinkage, as the specimen were classified
Discussion
Squamous cell carcinoma of the head and neck (H&NSSC) is a common disease with high mortality rates for patients with stage III and IV cancers.[21], [22], [23], [24] Apart from radical surgical resection, irradiation is the most important treatment modality in curative and palliative management of these tumors. However, in spite of highly sophisticated methods to define target regions and to ensure proper dose delivery, the effectiveness of radiation therapy is still highly variable.
Conclusion
Our comparative analysis of p63 expression and therapeutic effectiveness of irradiation, as measured by histology and survival, suggests, that p63 may be used for rapid and effective identification of patients with radioresistant OSCC, which could not be expected to be cured by radio(chemo)therapy alone. The promising results of this biomarker should now be confirmed in a clinical setting with larger patient counts.
Conflicts of interest statement
None declared.
Acknowledgement
This work was supported in part by a grant of the Stiftung Tumorforschung Kopf-Hals to Maximilian Moergel and Martin Kunkel.
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