RNAi Silencing of TEL-AML1 Reveals a Heat Shock Protein Dependent Mechanism in the Development of Leukemia.

The t(12;21) chromosomal translocation generating the TEL-AML1 fusion product is the most frequent genetic aberration of childhood acute lymphoblastic leukemia. This translocation has been shown to occur in utero and thus represents an early or even initiating event in leukemia development. However, additional molecular changes are necessary for the clinical manifestation of the disease. Despite increasing knowledge on TEL and AML1, the role of the TEL-AML1 fusion gene and its contribution to the malignant transformation is largely unknown. In particular, the mechanisms by which TEL-AML1 influences the biology of the affected cell, and thus survival and expansion, are yet to be defined. The aim of this study was to investigate the functional contribution of TEL-AML1 to the leukemic cell phenotype. We particularly sought to search for TEL-AML1 dependent genes as well as for pathways that function in a fusion gene dependent manner. The t(12;21) positive B cell precursor leukemia cell line REH was used as model system and RNA interference technology, a new and highly specific approach, was employed to block the expression of the TEL-AML1 fusion gene. The successful silencing of TEL-AML1 was demonstrated by Western blot analysis. In order to gain a broad overview of the impact of TEL-AML1 silencing upon the leukemic cell, RNA was isolated and subjected to Affymetrix gene chip analysis. In addition, we studied the proteome by high-resolution 2D electrophoresis. Expression profiling resulted in a specific pattern of differentially regulated genes after TEL-AML1 suppression with a particular emphasis on genes involved in the control of apoptosis. Genes with anti-apoptotic properties were down regulated after TEL-AML1 silencing while those that contribute to apoptosis were largely unaffected. Among the affected anti-apoptotic genes were the heat shock proteins, the most prominent representatives of which were HSP90 and HSP70. Their changes were observed at the mRNA but also at the protein level. This is of special interest, since both heat shock proteins interfere with the intrinsic as well as the extrinsic pathway of apoptosis and hence provide a powerful anti-apoptotic signal. These first data support a model in which TEL-AML1, as an early event, is functionally linked to an anti-apoptotic network and especially to the members of the heat shock protein family. Engagement of these mechanisms might rescue a cell from apoptosis and in consequence give rise to the development of a pre-leukemic clone, a reservoir for the acquisition of further mutations that might eventually give rise to leukemia. Thus, decreasing the apoptosis defence by silencing TEL-AML1 or by inhibiting the heat shock proteins might sensitize TEL-AML1 positive cells to apoptosis. This would not only be an option for future therapy of leukemias but also for the eradication of the pre-leukemic clone as a proposed source of relapse.