Overexpression of Bcl‐2 in squamous cell carcinoma of the larynx: A marker of radioresistance

Abstract

Squamous cell carcinoma of the larynx can be treated using radiotherapy or surgery, either alone or in combination. Radiotherapy is preferred for early‐stage tumours, as it spares the larynx and therefore preserves speech and swallowing. Unfortunately, approximately 15% of tumours treated this way will prove to be radioresistant, as manifest by tumour recurrence within the original radiotherapy field over the ensuing 12 months. By causing extensive DNA damage, radiotherapy aims to induce apoptosis and tumour regression. Our hypothesis was that defects in the mechanisms that recognise DNA damage, induce cell cycle arrest or control apoptosis, either alone or in combination, may be responsible for radioresistance. We therefore undertook an immunohistochemic analysis of pretreatment biopsies of radioresistant (n = 8) and radiosensitive (n = 13) laryngeal tumours. To minimise the impact of confounding factors, strict inclusion criteria were observed; all tumours were of the glottic subsite and all recurrences developed within 12 months of radiotherapy at the site of the original tumour. The expression of key proteins involved in DNA damage recognition (p53), cell cycle arrest (ATM, p16 and p21/WAF1) and apoptosis (Bcl‐2 and BAX) were studied. Ki‐67 was also assessed as a marker of cell proliferation to exclude low mitotic rate as a cause of radioresistance. A statistically significant correlation was observed between overexpression of Bcl‐2 and radioresistance (p = 0.003, Fisher's exact test). We hypothesise that overexpression of the anti‐apoptotic protein Bcl‐2 allows tumour cells with extensive radiation‐induced DNA damage to continue proliferating; the absence of an appropriate apoptotic response manifests clinically as radioresistance.