RNA decay defines the therapeutic response to transcriptional perturbation in leukemia [SLAMseq K562 RNAPII targeting]
Therapeutic targeting of dysregulated transcriptional programs has arisen as a promising strategy for the treatment of leukemias. The therapeutic response to small molecule inhibitors of Bromodomain-Containing Proteins (BRD), such as BRD2 and BRD4, P300/cAMP-response element binding protein (CBP) and Cyclin Dependent Kinases (CDKs), is generally attributed to the selective disruption of oncogenic gene expression networks driven by enhancers, super-enhancers (SEs) and lineage-specific transcription factors (TFs), including the c-MYC oncogene. Using technologies such as thiol (SH)-linked alkylation for the metabolic sequencing of RNA sequencing (SLAM-seq) to profile messenger RNA (mRNA) decay and production rates, we demonstrate that gene intrinsic properties largely govern the selectivity associated with transcriptional inhibition, where total mRNA response signatures are dominated with genes that have short transcript half-lives, including those regulated by SEs and oncogenic TFs. Further highlighting that gene sensitivities only occur in the context of short transcript half-lives, stabilisation of the c-MYC transcript through changes in the 3' UTR rendered it insensitive to transcriptional targeting. However, this was not sufficient to rescue c-MYC target gene transcription and anti-leukemia effects following transcriptional inhibition. Importantly, long-lived mRNAs encoding essential genes that evade transcriptional targeting can be rendered sensitive via modulation of mRNA decay kinetics through inhibition of the RNA Binding Protein (RBP), ELAV Like RNA binding protein 1 (ELAVL1)/ Human Antigen R (HuR). Taken together, these data demonstrate that mRNA decay shapes the therapeutic response to transcriptional perturbation and can be modulated for novel therapeutic outcomes using transcriptional agents in leukemia. Overall design: K562 cells (1 x 10^6 per treatment) were first pre-treated with small molecule inhibitors for a total time of 2 hours to pre-establish protein-target inhibition. Newly synthesized RNA in K562 cells was then labelled using 100 µM 4-sU in the final 1 hour of treatment at 37°C and 5% carbon dioxide.