Sequence-dependent and -independent effects of intron-mediated enhancement learned from thousands of random introns.

Spliceosomal introns are a ubiquitous feature of eukaryotic genes, whose presence often boosts the expression of their host gene, a phenomenon known as intron-mediated enhancement (IME). IME has been noted across diverse genes and organisms but remains mysterious in many respects. For example, how does intron sequence affect the magnitude of IME? In this study, we performed a massively parallel reporter assay (MPRA) to assess the effect of varying intron sequence on gene expression in a high-throughput manner, in human cells, using tens of thousands of synthetic introns with natural splice sites and randomized internal sequence. We observe that most random introns splice efficiently and enhance gene expression as well as or better than fully natural introns. Nearly all introns stimulate gene expression ∼eight-fold above an intronless control, at both mRNA and protein levels, suggesting that the primary mechanism acts to increase mRNA levels. IME strength is positively associated with splicing efficiency and with the intronic content of poly-uridine stretches, which we confirm using reporter experiments. In sum, this work assesses the IME of a diverse library of introns and uncovers sequence-dependent aspects, but suggests that enhancement of gene expression is a general property of splicing, largely independent of intron sequence.