Expression of p14ARF in De Novo AML with Normal Karyotype. Implication on Drug Resistance and Survival.

The p53 protein exerts a significant role in growth control of cells and impaired function of p53 by mutation or otherwise is regarded to be important for development of many human cancers. The p53 protein is strongly regulated by the E3-ligase HDM-2 that specifically binds to p53 and causes proteosomal degradation. P14^ARF encoded from the INK 4a/ARF gene locus of chromosome 9p, activates the p53 pathway by binding to and inhibiting HDM-2. The objective was to study if the level of mRNA for p14^ARF in leukemic cells from patients with AML was related to in vitro drug resistance and clinical outcome. Leukemic cells (>90% pure) isolated from 46 adult patients with normal karyotype de novo AML were incubated with antileukemic drugs (daunorubicin, mitoxantrone, etoposide and Ara-C) cultured for 4 days. The effect was determined by bioluminiscence measuring of intracellular ATP compared to an untreated control. We also tested the activity against PRIMA (p53-dependent reactivation and induction of massive apoptosis), a novel, small molecule shown to activate down regulated p53. mRNA for p14^ARF was determined by real time PCR.

Results: Patients whose leukemic cells expressed high level of p14^ARF mRNA in the leukemic cells (≥0.2 compared to the housekeeping gene) had a significantly better survival compared to those with low level (<0.02). Median survival not obtained compared to 9 months. The mean activity of all tested conventional antileukemic drugs was higher on leukemic cell samples expressing p14^ARF mRNA ≥0.2 compared to those with low levels. In contrast, PRIMA exerted significantly higher in vitro effect on leukemic cell samples with low levels of p14^ARF mRNA. We conclude that low levels of p14^ARF in leukemic cells from patients with normal karyotype AML is a marker for poor prognosis, which may depend on impaired p53 activity causing resistance against conventional antileukemic drugs. Treatment with p53 targeting drugs as PRIMA may be a future possibility to improve the outcome for these patients.