Site- and allele-specific de-silencing of polycomb repressive activity by insertional oncogenesis: a new recurrent mechanism of TAL1 activation in T-ALL

T-cell acute lymphoblastic leukemia’s (T-ALL) are malignant proliferations of thymocytes occurring through a large diversity of genomic and epigenetic alteration. TAL1 is amongst the most frequently deregulated oncogenes. Known TAL1 dysregulation mechanisms consist of t(1;14) translocations and SIL-TAL deletions. Yet, over half of TAL1+ cases lack TAL1 lesions, pointing to unrecognized (epi)genetic deregulation mechanisms. In such “unresolved cases”, TAL1 expression can be mono-allelic, compatible with a direct alteration in cis within or around the TAL1 gene, or bi-allelic, likely reflecting indirect deregulation in trans. Using ChIP-seq, we show that TAL1 is normally silenced in the T-cell lineage, and that the polycomb H3K27me3 repressive mark is focally diminished in TAL1+ T-ALLs. Sequencing revealed that >20% of mono-allelic TAL1+ patients without previously known alterations display micro-insertions or RAG1/2-mediated episomal reintegration in a single site 5’ to TAL1. Using “allelic-ChIP” and CrispR assays, we demonstrate that such insertions induce selective switch from H3K27me3 to H3K27ac at the inserted but not the germline allele. We also show that, despite a considerable mechanistic diversity, the mode of oncogenic TAL1 activation, rather than expression levels, impact on clinical outcome. Altogether, these studies reveal a new adverse mechanism of mono-allelic oncogenic activation through site-specific epigenetic de-silencing. ChIP-seq experiments were designed to asses the dynamic enrichment of H3K27me3 and H3K27ac marks on the genome and understand the mechanisms underlying the TAL1 expression cys-regulation.