Cytochrome c-related caspase-3 activation determines treatment response and relapse in childhood precursor B-cell ALL

Deficient activation of apoptosis signaling pathways may be responsible for treatment failure in acute leukemia. Here, we address the impact of intact apoptosis signaling in 78 patients with pediatric precursor B-cell acute lymphoblastic leukemia (ALL) by analysis of 2 key apoptogenic events: caspase-3 activation and cytochrome c release in leukemia cells cultured in vitro. Both events correlated only in the group of patients who had a good response and patients in continuous remission, suggesting that intact apoptosis signaling is a characteristic for favorable outcome. By combining both parameters, we identified a novel indicator, cytochrome c-related activation of caspase-3 (CRAC). CRAC directly connects the extent of caspase-3 activation to cytochrome c release in single cells in an individual patient sample. In CRAC-positive patients, indicating proficient apoptosis signaling, the number of persisting leukemia cells on day 15 was significantly lower than in the CRAC-negative patient group (n = 27, mean 6.0% versus n = 36, mean 22.6%; P = .003). At a median follow-up of 31 months, disease-free survival was 84 months (95% CI = 76 to 91 months) and 66 months (95% CI = 52 to 80 months) for patients with positive and negative CRAC, respectively (P = .019). CRAC may serve as a functionally defined risk factor for treatment stratification.