Modelling Molecular Consequences of Leukemia Initiation by TEL-AML1 Fusion.

We have previously shown that TEL-AML1 (ETV6-RUNX1) fusions in acute lymphoblastic leukemia commonly arise pre-natally. The fusion protein initiates, presumably via transcriptional deregulation, a pre-leukemia condition that infrequently (~1%) converts over a 10 year period to overt leukaemia in the presence of post-natal, secondary genetic changes including TEL deletion.

One challenge is to identify the molecular mechanisms by which TEL-AML1 elicits and sustains stable pre-leukemic clones which are then vulnerable to additional genetic alterations in the context of particular etiological exposures (e.g. infection). We have established cell lines of murine lymphoid progenitor cells that stably express a regulatory, truncated human progesterone receptor ligand-binding domain (pSwitch, Invitrogen) that undergoes a conformational change in the presence of the steroid agonist, mifepristone and conversion to an active form. The active binding domain is then able to bind to and activate transcription from an otherwise silent TEL-AML1 gene that we have also stably introduced into these cells. In the presence of sub-physiological levels of mifepristone, TEL-AML1 becomes abundantly expressed in the nucleus within a few hours. Microarray analysis comparing uninduced (time 0) cDNA with cDNA from both control and TEL-AML1 induced cells grown for 4 days in the presence of mifepristone showed changes in the expression pattern for over 250 genes. The majority of these genes were down regulated in the presence of TEL-AML1, in keeping with a repressive activity for the fusion gene. A number of apoptotic and cell cycle genes were down regulated, while anti-apoptotic genes were up regulated in the presence of the fusion protein. These genes are currently being examined by both in-silico and laboratory-based validation techniques and may provide key insights into the pathways that are deregulated by TEL-AML1 in ALL.