Relapse in childhood acute lymphoblastic leukemia is associated with a decrease of the Bax/Bcl-2 ratio and loss of spontaneous caspase-3 processing in vivo

Dysfunction of the p53/Bax/caspase-3 apoptosis signaling pathway has been s hown to play a role in tumorigenesis and tumor progression, ie the developm ent of acquired drug resistance. Low expression of the apoptosis inducer Ba r correlates with poor response to therapy and shorter overall survival in solid tumors. In the present study, we analyzed the p53/Bax/caspase-3 pathw ay in a paired and an unpaired sample series of children with acute lymphob lastic leukemia (ALL) at initial diagnosis and relapse. The data demonstrat e that both Bar expression levels and the Bax/Bcl-2 ratio are significantly lower in samples at relapse as compared with samples at initial diagnosis (P=0.013, Wilcoxon signed rank test (paired samples); P=0.0039, Mann-Whitne y U test (unpaired samples)). The loss of Bar protein expression was not a consequence of Bar frameshift mutations of the G(8) tract and could not be attributed to mutations of the p53 coding sequence (exons 5 to 8) which wer e detected to a similar extent in de novo ALL samples and at relapse. Analy sis of the downstream effector caspase-3 showed loss of spontaneous caspase -3 processing at relapse. Whereas nine out of 14 (64%, paired samples) or 3 7 out of 77 (48%, unpaired samples) ALL patients at initial diagnosis displ ayed spontaneous in vivo processing of caspase-3, this was completely absen t in patients at relapse (paired samples) or detected in only one out of 34 patients at relapse (2.9%, unpaired samples). We therefore conclude that i n ALL relapse a severe disturbance of apoptotic pathways occurs, both at th e level of Bar expression and caspase-3 activation.