Widespread establishment and regulatory impact of Alu exons in human genes

The Alu element has been a major source of new exons during primate evolution. Thousands of human genes contain spliced exons derived from Alu elements. However, identifying Alu exons that have acquired genuine biological functions remains a major challenge. We investigated the creation and establishment of Alu exons in human genes, using transcriptome profiles of human tissues generated by high-throughput RNA sequencing (RNA-Seq) combined with extensive RT-PCR analysis. More than 25% of Alu exons analyzed by RNA-Seq have estimated transcript inclusion levels of at least 50% in the human cerebellum, indicating widespread establishment of Alu exons in human genes. Genes encoding zinc finger transcription factors have significantly higher levels of Alu exonization. Importantly, Alu exons with high splicing activities are strongly enriched in the 5'-UTR, and two-thirds (10/15) of 5'-UTR Alu exons tested by luciferase reporter assays significantly alter mRNA translational efficiency. Mutational analysis reveals the specific molecular mechanisms by which newly created 5'-UTR Alu exons modulate translational efficiency, such as the creation or elongation of upstream ORFs that repress the translation of the primary ORFs. This study presents genomic evidence that a major functional consequence of Alu exonization is the lineage-specific evolution of translational regulation. Moreover, the preferential creation and establishment of Alu exons in zinc finger genes suggest that Alu exonization may have globally affected the evolution of primate and human transcriptomes by regulating the protein production of master transcriptional regulators in specific lineages.