High Levels of Constitutive WAF1/Cip1 Protein are Associated with Chemoresistance in Acute Myelogenous Leukemia

The WAFl/Cipl gene product is an important regulator at the G, checkpoint in the cell cycle. WAFl/Cipl expression can be activated through p53-dependent and p53-independent pathways. The WAFl/Cipl protein binds to cyclin- dependent kinase complexes and inhibits the kinase activity that is required for cell cycle progression. In this preliminary study, we analyzed with Western blot assays the steady-state levels of the WAFl/Cipl protein in the leukemia cells of 100 untreated acute myelogenous leukemia (AML) patients. Normal bone marrow cells from six donors were used as a control. The results of these analyses showed that the levels of the WAFl/Cipl protein were very low in normal marrow cells and in the leukemia cells of 83 AML patients. High levels of WAFl/Cipl were detected in 17 patients; these patients with high WAFl/Cipl levels were significantly less likely to achieve complete remission (41% versus 69%, P = 0.03) and were four times as likely to be resistant to therapy (47% versus 12%, P = 0.003) as patients with very low levels of WAFl/Cipl. Median survival was 38 weeks for patients having very low expression levels versus 11 weeks for patients having high expression levels (P = 0.04). The WAFl/Cipl level was an independent predictor for response but not survival in a stepwise multivariate regression analysis. Southern blotting analyses did not detect deletion of the WAFl/Cipl gene in the 12 negative WAFl/Cipl AML samples tested. Also, the level of WAF1/Cipl protein expression was not correlated with overexpression of cyclin Dl, cyclin E, proliferating cell nuclear antigen, cyclin-dependent kinase 4, or p53 in the leukemia cells. However, the levels of cyclin Dl, cyclin E, and cyclin-dependent kinase 4 were elevated in most of the AML samples compared with that in normal marrow. We hypothesize that high-level constitutively expressed WAFl/Cipl in tumor cells may result in an indolent state that is refractory to chemotherapy drugs. We conclude that the WAFl/Cipl expression level may be an important prognostic factor for response to therapy and survival in AML patients.