Cytochrome C Related Caspase Activation Determines Treatment Response in Childhood B- Precursor ALL.

Drug resistance and treatment failure in acute leukemia may be attributed to apoptosis resistance in leukemia cells since defects in apoptosis signal transduction are commonly acquired during malignant transformation. Expression analysis of single molecules with regard to clinical outcome could so far not identify common apoptosis defects with prognostic value in primary acute leukemias. We addressed the role of apoptosis signaling for the initial response to induction treatment in pediatric B precursor lymphoblastic leukemia by functional assays. Apoptosis signaling in response to survival factor withdrawal was assessed in 82 primary leukemia samples by measurement of the key apoptotic events, caspase-3 activation and mitochondrial cytochrome c release.

Caspase-3 activation directly correlated to the initial response to treatment assessed by the percentage of leukemia cells in bone marrow on day 15 (p = 0.015). Intact apoptosis signaling was indicated by a significant correlation of caspase activation and cytochrome c release was found especially in the groups of good responding patients (rs 0.375 – 0.502; p = 0.001). Interestingly, this correlation was completely absent in all groups of poor responding patients defined by insufficient leukemia cell reduction on day 8, 15 or 33.

The efficacy of apoptosis signal induction in the individual leukemia is expressed by the parameter CRAC (Cytochrome c Related Activation of Caspase-3), relating the extent of caspase activation to cytochrome c release in a single patient sample. Ten of twelve patients with prednisone poor response (chi square, p= 0.031) and all patients not achieving remission on day 33 (4 of 4) had negative CRAC values, indicating deficient cytochrome c related caspase activation. Furthermore, the CRAC negative group revealed significant higher percentages of leukemia cells in bone marrow on day 15 (n=37, mean 22.8) than the CRAC positive patients (n=30, mean 6.2, p = 0.004). In addition all four relapse patients showed negative CRAC values indicating a prognostic value beyond initial treatment response.

The data show that the intact relation of cytochrome c release and caspase activation indicating an intact apoptosome function is important for efficient remission induction treatment in childhood B precursor ALL. Assessment and quantification of this relation for individual patients by the parameter CRAC may serve as potential factor for treatment stratification.