Expanding the transcriptomic sequence space in leukaemia by RUNX1/RUNX1T1-mediated alternative splicing

The fusion oncogene RUNX1/RUNX1T1 encodes an aberrant transcription factor, which plays a key role in the initiation and maintenance of acute myeloid leukaemia. In this study we comprehensively identify and characterize splicing events associated with RUNX1/RUNX1T1. We show that this oncogene is a regulator of the alternative RNA splicing in leukaemic cells. We found two principal mechanisms underlying changes in the production of RNA isoforms: (i) RUNX1/RUNX1T1-mediated regulation of alternative transcription start sites selection, and (ii) direct or indirect control of the expression of the genes encoding splicing factors. The first mechanism leads to the expression of RNA isoforms with alternative structure of the 5’-UTR regions. The second mechanism generates alternative transcripts with new junctions between internal cassettes and constitutive exons. We also show that RUNX1/RUNX1T1-mediated differential splicing affects several functional groups of genes and produces proteins with unique conserved domain structures. In summary, this study reveals a novel layer of transcriptome re-organization in leukaemia by an aberrant transcriptional regulator. Nascent RNA-sequencing of t(8;21)-positive AML, Kasumi-1 cell line, with siRNA targeting the fusion site of the RUNX1/RUNX1T1 mRNA (siRR) and a mismatch siRNA (siMM) served as a control