3023 – CHARACTERIZATION OF THE ONCOGENIC ACTIVITY OF TLX1 AND TLX3 TRANSCRIPTION COMPLEXES IN T-ALL

T-cell acute lymphoblastic leukaemia (T-ALL) is an aggressive haematological disease characterised by genetic defects, including kinase mutations causing alterations of signalling pathways, and ectopic expression of transcription factors (TLX1, TLX3, etc) of which the functional activity remains elusive. Thus, we aim to elucidate the role of TLX1/3 and their transcriptional complexes in driving T-ALL development. Proteomics results demonstrate that TLX3 directly interacts with a variety of transcriptional regulators including RUNX factors and RNA binding proteins. In addition, ChIP-seq analysis showed enrichment of RUNX binding sites at the regulatory regions where TLX3 binding was observed. Together these data illustrate co-binding of TLX3 with RUNX factors. Combination of RNA-seq data of induced TLX3 expression and ChIP-seq data further indicated that TLX3 is a direct repressor of TLE4, a transcriptional inhibitor. Overexpression of TLE4 suppressed the proliferation of TLX3 transformed cells, showing that TLE4 downregulation is beneficial for leukemia proliferation. Accordingly, T-ALL patients data indicated that TLX3 subgroup expresses very low levels of TLE4 and high content of RUNX1 mutations. Expression of TLX1/3 in primary mouse T- cells led to impactful genetic deregulation, marked by histone binding downregulation, upregulation of the JAK/STAT pathway, and deregulation of ETS factors, yet with subtle different genetic effects between TLX1 and TLX3. In parallel, we observed that TLX1 knock-down in T-ALL cell lines caused changes in chromatin accessibility and gene expression with slow kinetics which, also here the ETS transcription factor family was enriched in the data. Overall, we provide insights on transcriptional deregulation in T-ALL associated to TLX1/3 that could lead to the identification of potential therapeutic targets against T-ALL.