ΔNp73 /TAp73 Expression Ratio Is Associated with Ara-C-Induced Apoptosis Resistance in Acute Myeloid Leukemia.

The p73, a homologue to the tumor suppressor gene p53, is involved in oncogenesis, although its specific role remains unclear. This gene has two distinct promoters which encode for two protein isoforms with opposite transcriptional activities: full-length TAp73 and the ΔNp73, the latter differing by the lack of the N-terminal transactivating domain and, has a dominant-negative function towards p53 and TAp73 activity. In the present study, we have investigated, by Real Time PCR, the gene expression pattern of TAp73 and ΔNp73 in 147 samples from patients with de novo AML and 22 samples of CD34+ hematopoietic progenitor cells isolated from bone marrow of healthy donors. We detected a significantly higher expression of TAp73 and ΔNp73 genes in AML samples compared to normal hematopoietic progenitors. AML samples harboring the chromosomal translocations: PML-RARa, AML1-ETO and CBFβ-MYH11 presented significantly higher levels of the TAp73 than those negative for these rearrangements. In contrast, samples without these gene rearrangements expressed higher levels of the truncated form ΔNp73. No difference was observed among patients with PML-RARa, AML1-ETO and CBFβ-MYH11. In order to determine if the ratio between the relative expression of TAp73 and ΔNp73 genes was associated with differential susceptibility to pro-apoptotic stimuli, we performed an in vitro cytotoxicity assay using Ara-C. Blasts from 20 AML patients were incubated for 24 hours in a medium containing 100μg-ml of Ara-C and evaluated for apoptosis status using Anexin V and Propidium Iodite. The leukemic cells with a higher ΔNp73 / TAp73 ratio were significantly more apoptosis-resistant (Pearson’s R²: −0.607). We also assessed the protein expression by flow cytometry using saturating amounts of specific antibodies for each isoform and analyzing the mean fluorescence channel. Estimated protein expression corroborated the gene expression results. Our results suggest an association between the presence of PML-RARα, AML1-ETO or CBFβ-MYH11 chromosomal translocation and a higher TAp73 gene expression compared to ΔNp73 isoforms expression in AML samples. Moreover, the higher resistence to apoptosis detected in blasts with higher ΔNp73 / TAp73 values suggest that this pathway may contribute to prognosis.