Aberrant Spliceosome Activity via Elevated Intron Retention and Upregulation and Phosphorylation of SF3B1 in Chronic Lymphocytic Leukemia

Splicing factor 3b subunit 1 (SF3B1) is the largest subunit and core component of the spliceosome. Inhibition of SF3B1 was associated with an increase in broad intron retention (IR) on most transcripts, suggesting that IR can be used as a marker of spliceosome inhibition in CLL cells. Furthermore, we separately analyzed exonic and intronic mapped reads on annotated RNA-Seq transcripts obtained from B cells (n=98 CLL patients) and healthy volunteers (n=09). We measured intron/exon ration to use that as a surrogate for ARS and found that 66% of CLL B-cells transcripts had significant IR elevation compared to NBC and that correlated with mRNA downregulation and low expression levels. Transcripts with highest IR levels belonged to biological pathways associated with gene expression and RNA splicing. A >2-fold increase of active pSF3B1 in CLL B-cells compared to NBC. Additionally, when the CLL-B cells were treated with macrolides (pladienolide-B), a significant decrease in pSF3B1, but not total SF3B1 protein was observed. These findings suggest that IR/ARS is increased in CLL, which is associated with SF3B1 phosphorylation and susceptibility to SF3B1 inhibitors. These data provide additional support to the relevance of ARS in carcinogenesis and evidence of pSF3B1 participation in this process.