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      Inference of the human polyadenylation code

      research-article
      1 , 2 , 1 , 2 , 1 , 2 , 3
      Bioinformatics
      Oxford University Press

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          Abstract

          Motivation

          Processing of transcripts at the 3′-end involves cleavage at a polyadenylation site followed by the addition of a poly(A)-tail. By selecting which site is cleaved, the process of alternative polyadenylation enables genes to produce transcript isoforms with different 3′-ends. To facilitate the identification and treatment of disease-causing mutations that affect polyadenylation and to understand the sequence determinants underlying this regulatory process, a computational model that can accurately predict polyadenylation patterns from genomic features is desirable.

          Results

          Previous works have focused on identifying candidate polyadenylation sites and classifying tissue-specific sites. By training on how multiple sites in genes are competitively selected for polyadenylation from 3′-end sequencing data, we developed a deep learning model that can predict the tissue-specific strength of a polyadenylation site in the 3′ untranslated region of the human genome given only its genomic sequence. We demonstrate the model’s broad utility on multiple tasks, without any application-specific training. The model can be used to predict which polyadenylation site is more likely to be selected in genes with multiple sites. It can be used to scan the 3′ untranslated region to find candidate polyadenylation sites. It can be used to classify the pathogenicity of variants near annotated polyadenylation sites in ClinVar. It can also be used to anticipate the effect of antisense oligonucleotide experiments to redirect polyadenylation. We provide analysis on how different features affect the model’s predictive performance and a method to identify sensitive regions of the genome at the single-based resolution that can affect polyadenylation regulation.

          Supplementary information

          Supplementary data are available at Bioinformatics online.

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          Most cited references29

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          A quantitative atlas of polyadenylation in five mammals

          We developed PolyA-seq, a strand-specific and quantitative method for high-throughput sequencing of 3′ ends of polyadenylated transcripts, and used it to globally map polyadenylation (polyA) sites in 24 matched tissues in human, rhesus, dog, mouse, and rat. We show that PolyA-seq is as accurate as existing RNA sequencing (RNA-seq) approaches for digital gene expression (DGE), enabling simultaneous mapping of polyA sites and quantitative measurement of their usage. In human, we confirmed 158,533 known sites and discovered 280,857 novel sites (FDR < 2.5%). On average 10% of novel human sites were also detected in matched tissues in other species. Most novel sites represent uncharacterized alternative polyA events and extensions of known transcripts in human and mouse, but primarily delineate novel transcripts in the other three species. A total of 69.1% of known human genes that we detected have multiple polyA sites in their 3′UTRs, with 49.3% having three or more. We also detected polyadenylation of noncoding and antisense transcripts, including constitutive and tissue-specific primary microRNAs. The canonical polyA signal was strongly enriched and positionally conserved in all species. In general, usage of polyA sites is more similar within the same tissues across different species than within a species. These quantitative maps of polyA usage in evolutionarily and functionally related samples constitute a resource for understanding the regulatory mechanisms underlying alternative polyadenylation.
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            Ending the message: poly(A) signals then and now.

            Polyadenylation [poly(A)] signals (PAS) are a defining feature of eukaryotic protein-coding genes. The central sequence motif AAUAAA was identified in the mid-1970s and subsequently shown to require flanking, auxiliary elements for both 3'-end cleavage and polyadenylation of premessenger RNA (pre-mRNA) as well as to promote downstream transcriptional termination. More recent genomic analysis has established the generality of the PAS for eukaryotic mRNA. Evidence for the mechanism of mRNA 3'-end formation is outlined, as is the way this RNA processing reaction communicates with RNA polymerase II to terminate transcription. The widespread phenomenon of alternative poly(A) site usage and how this interrelates with pre-mRNA splicing is then reviewed. This shows that gene expression can be drastically affected by how the message is ended. A central theme of this review is that while genomic analysis provides generality for the importance of PAS selection, detailed mechanistic understanding still requires the direct analysis of specific genes by genetic and biochemical approaches.
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              Transcript length bias in RNA-seq data confounds systems biology

              Background Several recent studies have demonstrated the effectiveness of deep sequencing for transcriptome analysis (RNA-seq) in mammals. As RNA-seq becomes more affordable, whole genome transcriptional profiling is likely to become the platform of choice for species with good genomic sequences. As yet, a rigorous analysis methodology has not been developed and we are still in the stages of exploring the features of the data. Results We investigated the effect of transcript length bias in RNA-seq data using three different published data sets. For standard analyses using aggregated tag counts for each gene, the ability to call differentially expressed genes between samples is strongly associated with the length of the transcript. Conclusion Transcript length bias for calling differentially expressed genes is a general feature of current protocols for RNA-seq technology. This has implications for the ranking of differentially expressed genes, and in particular may introduce bias in gene set testing for pathway analysis and other multi-gene systems biology analyses. Reviewers This article was reviewed by Rohan Williams (nominated by Gavin Huttley), Nicole Cloonan (nominated by Mark Ragan) and James Bullard (nominated by Sandrine Dudoit).
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                Author and article information

                Contributors
                Role: Associate Editor
                Journal
                Bioinformatics
                Bioinformatics
                bioinformatics
                Bioinformatics
                Oxford University Press
                1367-4803
                1367-4811
                01 September 2018
                10 April 2018
                10 April 2018
                : 34
                : 17
                : 2889-2898
                Affiliations
                [1 ]Department of Electrical and Computer Engineering, University of Toronto, Toronto, Canada
                [2 ]Deep Genomics, MaRS Centre, Toronto, Canada
                [3 ]Banting and Best Department of Medical Research, University of Toronto, Toronto, Canada
                Author notes
                To whom correspondence should be addressed. E-mail: frey@ 123456psi.toronto.edu
                Article
                bty211
                10.1093/bioinformatics/bty211
                6129302
                29648582
                3131c47d-adf5-434b-b890-f451a6e5aa79
                © The Author(s) 2018. Published by Oxford University Press.

                This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals.permissions@oup.com

                History
                : 27 June 2017
                : 15 October 2017
                : 09 April 2018
                Page count
                Pages: 10
                Funding
                Funded by: Natural Science and Engineering Research Council of Canada and Deep Genomics
                Categories
                Original Papers
                Genome Analysis

                Bioinformatics & Computational biology
                Bioinformatics & Computational biology

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