Correlation between ZAP-70, phospho-ZAP-70, and phospho-Syk expression in leukemic cells from patients with CLL

For patients with chronic lymphocytic leukemia (CLL), expression of ZAP-70 in the leukemic cells is an indicator of poor prognosis. However, the mechanism that accounts for this effect is not known. ZAP-70 expression has previously been associated with increased B cell antigen receptor signaling upon surface immunoglobulin ligation in vitro as shown by ZAP-70 and Syk phosphorylation. This finding has led to the suggestion that a more aggressive clinical course is correlated with B cell antigen receptor signaling. Using high resolution immunophenotyping to analyze CLL cells ex vivo (without stimulation in vitro), we have demonstrated CLL cells from all patients express some ZAP-70 and that increased expression of ZAP-70 is correlated with decreased levels of phosphorylated ZAP-70 and phosphorylated Syk measured directly ex vivo. Conversely, high levels of phosphorylated ZAP-70 and phosphorylated Syk are found only in samples with low levels of ZAP-70 expression, and Syk and ZAP-70 phosphorylation appear to be mostly independent of each other. Additionally, Syk phosphorylation is directly correlated with levels of p21(cip1), a cell cycle inhibitor and a p53 target. Together these findings suggest that lower levels of p21(cip1) and/or a defect in p53 activity may account in part for the more aggressive disease course in patients with high levels of ZAP-70 rather than enhanced B cell antigen receptor signaling as has been previously hypothesized.