Silencing of TEL/AML1 In Definitive Leukemic Cells Does Not Impair Cell Survival

Abstract 3229

The most frequent structural chromosomal aberration in childhood acute lymphoblastic leukemia t(12;21) generates TEL/AML1 fusion gene. Resulting TEL/AML1 protein probably acts as an aberrant transcription factor that deregulates AML1-dependent transcription but its target genes and thus also the exact role in leukemic cells remain unknown. In vivo studies showed that TEL/AML1 itself is not sufficient to cause leukemia but may induce a preleukemic state characterized by the increased numbers of multipotent or B-cell progenitors with an incomplete block of differentiation. Despite its role for leukemia establishment the relevance of TEL/AML1 fusion gene for leukemia persistance has not been studied enough.To address this question and to explore the possibility of TEL/AML1-targeted therapy, we studied the effects of RNAi-mediated TEL/AML1 silencing on leukemic cells. As the only siRNA used for TEL/AML1 silencing published so far (Diakos et al, Blood, 2007) targets also the wild type AML1 (46% transcript reduction, our data), our first goal was to identify efficient and TEL/AML1-specific siRNA. We designed eleven different siRNAs spanning the fusion point of TEL/AML1 lacking the total sequence homology to wild type TEL and AML1 alleles to avoid their silencing. These 11 siRNAs were tested in HeLa cells transgenic for TEL/AML1-ires2-EGFP reporter. After lipofection into HeLa cells the efficiency of individual siRNAs was measured as a decrease of EGFP reporter fluorescence by flow cytometry. The best five siRNAs, that induced 50–58% silencing of the EGFP reporter, were tested at the mRNA level in TEL/AML-positive leukemic cell line. 24h after electroporation of siRNAs, when the silencing reached its maximum, two most efficient siRNAs induced 58% and 57% TEL/AML1 transcript reduction, respectively. We achieved 61% TEL/AML1 transcript reduction with the pool of both siRNAs while there was only slight reduction (14%) of wild type AML1 transcript. We used this efficient and specific siRNA pool to silence TEL/AML1 in REH and UOC-B6 TEL-AML1 positive cell lines and studied its effect on cell viability, proliferation and global gene expression. Applying two rounds of siRNA electroporation within 48 hours interval we achieved 74% and 86% TEL/AML1 protein knockdown in REH and UOC-B6 cells, respectively. We used trypan blue staining followed by optical microscopy to monitor cell viability and staining with annexin V and propidium idode to assess apoptosis rate by flow cytometry. Analysis of DNA content using staining with propidium iodide was performed to assess cell-cycle distribution. Incorporation of nucleoside analog was measured by flow cytometry to analyse de novo DNA synthesis as an indicator of proliferation rate. Despite the common expectation derived from studies on other fusion oncogenes (BCR/ABL, AML1/ETO, E2A/PBX), TEL/AML1 silencing neither decreased cell viability, nor induced apoptosis. On the contrary, TEL/AML1 depletion was accompanied by the slight but significant increase in the fraction of S-phase cells and corresponding rise in proliferation rate. Opposite effects on cell cycle distribution and proliferation were induced when we silenced wild type AML1. These findings support our hypothesis that TEL/AML1 may block previously established AML1 function in G1/S progression through the cell cycle. In line with the lack of effect on cell viability and discreet effect on cell-cycle distribution and proliferation we found no significant changes in global gene expression pattern upon TEL/AML1 depletion. Our data indicate, that TEL/AML1 is dispensable for the survival of definitive leukemic cells.

This work was supported by grants MSM0021620813 and MZOFNM2005.