Epigenetic mechanisms of PADI4 in hematopoiesis and evaluation as a therapeutic target

Tal1 (Scl1, stem cell leukemia 1) belongs to a class of transcription factors, which are required for normal hematopoietic stem cell development. The aberrant expression of Tal1 in T-cells leads to pediatric T-cell acute lymphoblastic leukemia (T-ALL) (1). Tal1 is a member of the basic-helix-loop-helix class (bHLH) of transcription factors. It binds to recognition sites on target sequences with the E-box motif CANNTG as a heterodimer with ubiquitous bHLH-factors, such as the E2A-proteins E12 and E47 (2). To understand the function of Tal1 during development and in disease the components of Tal1 dependent complexes need to be known, however the cell type specific composition of Tal1 complexes is largely elusive. Therefore, we are identifying novel interaction partners of Tal1 using single-step affinity purification, followed by mass spectrometry analysis of the copurified proteins. With this approach we identified the peptidyl arginine deiminase type IV (PADI4), an histone modifying enzyme, which post-translationally converts peptidylarginine to citrulline (3). We could demonstrate that Tal1 directly interacts with PADI4, using endogenous immunoprecipitation and GST-pulldown assays. Additional, chromatin immunoprecipition (ChIP) analysis revealed the occupancy of PADI4 on Tal1 target genes during megakaryopoiesis.

Recent studies showed an increased PADI4 expression in a variety of malignant tumours and that PADI4 participates in the process of tumourigenesis (4). Currently we are evaluating the influences of PADI4 on proliferation and differentation in Tal1 dependent leukemia cell lines. Additional, the influence of an small molecule inhibitor of PADI4 in these cell lines will be examined, which could offer a novel therapeutical approach in the treatment of leukemia.