Jan 2015 Nature communications

Site- and allele-specific polycomb dysregulation in T-cell leukaemia.

Auteurs

Navarro JM, Touzart A, Pradel LC, Loosveld M, Koubi M, Fenouil R, Le Noir S, Maqbool MA, Morgado E, Gregoire C, Jaeger S, Mamessier E, Pignon C, Hacein-Bey-Abina S, Malissen B, Gut M, Gut IG, Dombret H, Macintyre EA, Howe SJ, Gaspar HB, Thrasher AJ, Ifrah N, Payet-Bornet D, Duprez E, Andrau JC, Asnafi V, Nadel B

Résumé

T-cell acute lymphoblastic leukaemias (T-ALL) are aggressive malignant proliferations characterized by high relapse rates and great genetic heterogeneity. TAL1 is amongst the most frequently deregulated oncogenes. Yet, over half of the TAL1(+) cases lack TAL1 lesions, suggesting unrecognized (epi)genetic deregulation mechanisms. Here 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 reveals that >20% of monoallelic TAL1(+) patients without previously known alterations display microinsertions 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 a 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 establish site-specific epigenetic desilencing as a mechanism of oncogenic activation.

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