A genome-wide functional analysis of conserved intronic regions reveals essential roles for speckle-associated detained introns [RNAseq_candidates.HAP1]

Thousands of human introns harbour extended regions of high evolutionarily conservation that have not been previously characterized. A survey of these sequences reveals that they are associated with intron retention and enriched in genes that function in RNA processing, chromatin remodelling and neuronal biology. Using a dual CRISPR-Cas editing approach, we targeted 2,600 of these regions for deletion and observe that a subset of these perturbations affectaffects cell growth. Many of these ‘fitness’ sequences affect intron retention and or expression levels of their host genes. Deletions in nuclear speckle-associated retained introns in the FNBP4 and DDX5 genes are further linked to downstream effects on cell growth-related genes and intron retention, respectively. The DDX5 deletion additionally results in the accumulation of R-loops overlapping first introns in speckle-proximal genes. Overall, the results highlight multifactetedmultifaceted, critical roles of highly conserved intronic sequences in the control of gene regulation, R-loop resolution and cell growth.