Low Expression of p14ARF in De Novo AML with Normal Karyotype Is Associated with Short Survival but Increased in Vitro Sensitivity to p53 Modulating Drugs

Background: The p53 protein is strongly regulated by the E3 ligase HDM-2, which specifically binds to p53 and causes proteosomal degradation. The p14^ARF protein activates p53 by binding to and inhibiting HDM-2. Design and methods: To further study the prognostic impact of p14^ARF in AML, leukemic cells from 57 adult patients with normal karyotype de novo AML were analysed for p14^ARF mRNA expression level using real-time PCR. We also measured the cytotoxicity against conventional cytostatics and PRIMA-1, a novel small molecule shown to activate mutated p53 (Nature Medicine 2002; 8: 282-288) using an ATP-assay. Intracellular p53 protein was measured by FACS after incubation with PRIMA-1 in combination with the HDM-2 blocking agent RITA (Nature Medicine 2004; 10: 1321-1328)
Results: Patients whose cells expressed more p14^ARF mRNA than the 75^th percentile (0.26) showed significantly better survival compared with those with lower levels, 61% vs. 30% 3-year survival (p=0.033). The difference remained significant also when NPM1/FLT3 status was considered. The mean effects of all tested conventional antileukemic drugs were greater in leukemic cell samples expressing p14^ARF mRNA ≥0.26, but the differences did not reach statistical significance. In contrast, PRIMA-1 had a significantly greater effect on leukemic cell samples with low levels of p14^ARF mRNA and PRIMA-1 and RITA significantly increased intracellular p53 levels. Conclusions: Low level of p14^ARF mRNA in leukemic cells from patients with normal karyotype AML is a strong marker for poor prognosis and decreased sensitivity to conventional cytostatics. Treatment with drugs targeting p53 can be a future possibility to improve the outcome for these patients.