Mutant p53 protein, Bcl-2/Bax ratios and apoptosis in paediatric acute lymphoblastic leukaemia

Abstract

Purpose: The Bcl-2 family of proteins regulates a late step in the apoptosis pathway. Bcl-2 protein is believed to be involved in imparting resistance to or apoptosis induced by chemotherapeutic agents and radiation. The anti-apoptotic function of the Bcl-2 protein appears to be modulated by its ability to heterodimerize with other members of the gene family, predominantly Bax, a protein favouring induction of apoptosis. Susceptibility to undergoing apoptosis may, therefore, be dependent on the ratio between Bcl-2 and Bax. Both Bax and Bcl-2 are regulated by the tumour-suppressor protein p53. The present study therefore aims to study the significance of the Bcl-2:Bax ratio, p53 expression and apoptosis in paediatric acute lymphoblastic leukaemia (ALL). Methods: Expression of Bax, Bcl-2 and p53 was determined by immunocytochemistry, and apoptosis was evaluated by an enzymatic end-labelling technique using biotin-dUTP and further confirmed by annexin binding. The presence of mutant p53 was determined using a mutant-p53-specific enzyme-linked immunosorbent assay (ELISA). Results: A total of 32 cases and 20 controls were evaluated. Bcl-2 was found to be expressed in 22/32 of the ALL cases. Pretreatment (spontaneous) apoptosis was observed in 23/32 cases. The mean pretreatment apoptotic index was 11.34-2.04% with a median value of 7.5%. Conclusions: There was a positive correlation between apoptosis and Bax expression (r=0.5044; P=0.0038). There was good correlation between the immunoreactivity of p53 and detection of mutant p53 by ELISA (r=0.4605; P=0.0079). The apoptosis index showed a negative borderline correlation to the expression of Bcl-2 protein (r=-0.3181; P=0.076). There was an inverse correlation between extent of apoptosis and the presence of mutant p53 protein (r=-0.4732; P=0.006). p53 protein expression also showed a correlation with both Bcl-2 (r=0.4647; P=0.007) and Bax (r=0.4128; P=0.018). The Bcl-2/Bax ratio, however, showed no significant correlation with apoptosis (r=-0.3131; P=0.08) or with p53 expression. No significant association was evident between clinical and laboratory parameters with the Bcl-2/Bax protein expression except lymphadenopathy (r=0.5774; P=0.03). However, Bax expression showed a borderline correlation with the immediate tumour response to chemotherapy (r=-0.338; P=0.0628). These patients are being followed-up to look for any association between clinical outcome, Bcl-2/Bax ratio and apoptosis.