Using Acute Promyelocytic Leukemia to study the H3.3 Histone chaperone system and its role in pediatric malignancies

Cancer genome projects have discovered frequent mutations in genes for Chromatin-remodeling-factors in pediatric tumors. These include genes encoding Daxx and ATRX, two proteins involved in deposition of histone variant H3.3, indicating a role for H3.3 deposition in carcinogenesis. The macromolecular complex harboring Daxx/ATRX is also the target of the Acute Promyelocytic Leukemia oncoprotein PML-RARα. PML-RARα leads to a disruption of the localization of Daxx/ATRX, physiologically found at PML nuclear bodies, which can be reversed by treatment with All-Trans-Retinoic Acid (ATRA). We used this to search for novel components of the PML-associated Daxx/ATRX complex using Proximity-mediated Biotin Identification (BioID). We have used a BioID-Daxx bait protein to identify proteins interacting with Daxx in APL cells in an ATRA-dependent manner by Mass Spectrometry. Using this method, we have identified the Swi/Snf-ATPase subunit SMARCA4 as a protein interacting with Daxx.. This provides an unexpected link between two seemingly distinct Chromatin-modulating mechanisms, H3.3 incorporation (Daxx/ATRX) and nucleosome remodeling (Swi/Snf), which might open up novel therapeutic possibilities