ZAP-70 Expression Is More Predictive Than V_H Gene Mutational Status of BCR-Mediated Tyrosine Phosphorylation, NF-κB Activation and CLL Cell Survival.

We analysed tyrosine phosphorylation, NF-κB activation and survival in a cohort of 64 chronic lymphocytic leukemia patients. By graphing V_H gene mutational status, CD38 expression and ZAP-70 expression as continuous variables, we investigated correlations between these prognostic markers and cell signaling. There was a strong correlation between CD38 expression and basal levels of tyrosine phosphorylation (r² = 0.68). V_H gene mutational status and ZAP-70 expression correlated with the induction of tyrosine phosphorylation by anti-IgM (r² = 0.31 and 0.46 respectively). These findings suggest that tyrosine phosphorylation integrates these three prognostic phenotypes. In the context of NF-κB signaling, the percentage change in tyrosine phosphorylation (post anti-IgM) and ZAP-70 expression correlated with NF-κB activation (r² = 0.2 and 0.19 respectively) and the ability to activate NF-κB was strongly correlated with cell survival (r² = 0.67). The therapeutic rationale for the inhibition of NF-κB was strengthened by our observations that the NF-κB inhibitor, BAY 11–7082, caused a concentration-dependent increase in apoptosis in CLL samples. Taken together, our data suggest that CD38 is a surrogate marker of basal cellular activation in CLL that is independent of the B-cell receptor. Furthermore, ZAP-70 expression is the most important predictor of tyrosine phosphorylation, downstream NF-κB activation and CLL cell survival following B-cell receptor cross-linking. These findings provide a mechanism-based rationale for the poor prognosis of CD38⁺ and ZAP-70⁺ CLL patients.