Comparative analysis of transposed element insertion within human and mouse genomes reveals Alu's unique role in shaping the human transcriptome

Background: Transposed elements (TEs) have a substantial impact on mammalian evolution and are involved in numerous genetic diseases. We compared the impact of TEs on the human transcriptome and the mouse transcriptome. Results: We compiled a dataset of all TEs in the human and mouse genomes, identifying 3,932,058 and 3,122,416 TEs, respectively. We than extracted TEs located within human and mouse genes and, surprisingly, we found that 60% of TEs in both human and mouse are located in intronic sequences, even though introns comprise only 24% of the human genome. All TE families in both human and mouse can exonize. TE families that are shared between human and mouse exhibit the same percentage of TE exonization in the two species, but the exonization level of Alu, a primatespecific retroelement, is significantly greater than that of other TEs within the human genome, leading to a higher level of TE exonization in human than in mouse (1,824 exons compared with 506 exons, respectively). We detected a primate-specific mechanism for intron gain, in which Alu insertion into an exon creates a new intron located in the 3' untranslated region (termed 'intronization'). Finally, the insertion of TEs into the first and last exons of a gene is more frequent in human than in mouse, leading to longer exons in human. Conclusion: Our findings reveal many effects of TEs on these two transcriptomes. These effects are substantially greater in human than in mouse, which is due to the presence of Alu elements in human

Alu Exonization Events Reveal Features Required for Precise Recognition of Exons by the Splicing Machinery

Despite decades of research, the question of how the mRNA splicing machinery precisely identifies short exonic islands within the vast intronic oceans remains to a large extent obscure. In this study, we analyzed Alu exonization events, aiming to understand the requirements for correct selection of exons. Comparison of exonizing Alus to their non-exonizing counterparts is informative because Alus in these two groups have retained high sequence similarity but are perceived differently by the splicing machinery. We identified and characterized numerous features used by the splicing machinery to discriminate between Alu exons and their non-exonizing counterparts. Of these, the most novel is secondary structure: Alu exons in general and their 5′ splice sites (5′ss) in particular are characterized by decreased stability of local secondary structures with respect to their non-exonizing counterparts. We detected numerous further differences between Alu exons and their non-exonizing counterparts, among others in terms of exon–intron architecture and strength of splicing signals, enhancers, and silencers. Support vector machine analysis revealed that these features allow a high level of discrimination (AUC = 0.91) between exonizing and non-exonizing Alus. Moreover, the computationally derived probabilities of exonization significantly correlated with the biological inclusion level of the Alu exons, and the model could also be extended to general datasets of constitutive and alternative exons. This indicates that the features detected and explored in this study provide the basis not only for precise exon selection but also for the fine-tuned regulation thereof, manifested in cases of alternative splicing

The pivotal roles of TIA proteins in 5' splice-site selection of alu exons and across evolution

More than 5% of alternatively spliced internal exons in the human genome are derived from Alu elements in a process termed exonization. Alus are comprised of two homologous arms separated by an internal polypyrimidine tract (PPT). In most exonizations, splice sites are selected from within the same arm. We hypothesized that the internal PPT may prevent selection of a splice site further downstream. Here, we demonstrate that this PPT enhanced the selection of an upstream 5' splice site (5'ss), even in the presence of a stronger 5'ss downstream. Deletion of this PPT shifted selection to the stronger downstream 5'ss. This enhancing effect depended on the strength of the downstream 5'ss, on the efficiency of base-pairing to U1 snRNA, and on the length of the PPT. This effect of the PPT was mediated by the binding of TIA proteins and was dependent on the distance between the PPT and the upstream 5'ss. A wide-scale evolutionary analysis of introns across 22 eukaryotes revealed an enrichment in PPTs within approximately 20 nt downstream of the 5'ss. For most metazoans, the strength of the 5'ss inversely correlated with the presence of a downstream PPT, indicative of the functional role of the PPT. Finally, we found that the proteins that mediate this effect, TIA and U1C, and in particular their functional domains, are highly conserved across evolution. Overall, these findings expand our understanding of the role of TIA1/TIAR proteins in enhancing recognition of exons, in general, and Alu exons, in particular.GA is supported by a grant from the Israel Science Foundation (1449/04), MOP Germany-Israel, GIF, and DIP. SS is a fellow of the Edmond J. Safra Bioinformatic Program at Tel Aviv University. OR is supported by EURASNET. EE is supported by the Catalan Institute for Research and Advanced Studies (ICREA) and by the BIO2008-01091 grant from the Spanish Ministry of Science

The pivotal roles of TIA proteins in 5' splice-site selection of alu exons and across evolution

More than 5% of alternatively spliced internal exons in the human genome are derived from Alu elements in a process termed exonization. Alus are comprised of two homologous arms separated by an internal polypyrimidine tract (PPT). In most exonizations, splice sites are selected from within the same arm. We hypothesized that the internal PPT may prevent selection of a splice site further downstream. Here, we demonstrate that this PPT enhanced the selection of an upstream 5' splice site (5'ss), even in the presence of a stronger 5'ss downstream. Deletion of this PPT shifted selection to the stronger downstream 5'ss. This enhancing effect depended on the strength of the downstream 5'ss, on the efficiency of base-pairing to U1 snRNA, and on the length of the PPT. This effect of the PPT was mediated by the binding of TIA proteins and was dependent on the distance between the PPT and the upstream 5'ss. A wide-scale evolutionary analysis of introns across 22 eukaryotes revealed an enrichment in PPTs within approximately 20 nt downstream of the 5'ss. For most metazoans, the strength of the 5'ss inversely correlated with the presence of a downstream PPT, indicative of the functional role of the PPT. Finally, we found that the proteins that mediate this effect, TIA and U1C, and in particular their functional domains, are highly conserved across evolution. Overall, these findings expand our understanding of the role of TIA1/TIAR proteins in enhancing recognition of exons, in general, and Alu exons, in particular.GA is supported by a grant from the Israel Science Foundation (1449/04), MOP Germany-Israel, GIF, and DIP. SS is a fellow of the Edmond J. Safra Bioinformatic Program at Tel Aviv University. OR is supported by EURASNET. EE is supported by the Catalan Institute for Research and Advanced Studies (ICREA) and by the BIO2008-01091 grant from the Spanish Ministry of Science