Epigenetic silencing of the proapoptotic gene bim in anaplastic large cell lymphoma

BIM is a proapoptotic member of the Bcl-2 family. The BIM promoter can be the target of epigenetic silencing in various types of cancer. Here, we investigated the epigenetic status of BIM locus in NPM-ALK+ Anaplastic Large Cell Lymophoma (ALCL) cell lines and in lymph node biopsies form NPM-ALK+ ALCL patients. In all the cell lines tested, the BIM 5’UTR was densely methylated. Conversely, only very limited evidence of methylated lymphocytes from healthy donors. Treatment with the demethylating agent 5-azacytidine led to 5’UTR demethylation and BIM upregulation. By Chromatin Immunoprecipitation experiments, we also showed that BIM silencing occurs through recruitment of MeCP2 and of the SIN3a/Histone Deacetylase (HDAC) 1/2 co-repressor complex. BIM downregulation is associated with protection from apoptosis. Treatment with the deacetylase inhibitor Trichostatin-A restores the acetylation, upregulates BIM expression and induces massive cell death. Finally, the recruitment of the MeCP2/SIN3/HDAC1-2 silencing complex relies on BIM CpG methylation. Demethylation of BIM CpG island with 5-azacytidine leads to the detachment of the MeCP2 corepressor complex and to the reacetylation of the histone tails. Crizotinib is a selective ATP-competitive small-molecule inhibitor of both ALK tyrosine kinases and their oncogenic variants (e.g. mutations, fusion proteins) and c-MET/ HGF receptor which are implicated in the progression of several cancers. In this study we also demonstrated that NPM-ALK however, is not directly involved in determining the epigenetic status of Bim locus in ALCL NPM-ALK+. This study shows that, the epigenetic therapy such as demethylating agents and HDACi, in association with ALK inhibitor, Crizotinib, treatment could act synergistically, inducing massive apoptosis in ALCL by reactivating BIM expression and by NPM/ALK inactivation respectively.