June 1, 2021  |  

Full-length transcript profiling with the Iso-Seq method for improved genome annotations

Incomplete annotation of genomes represents a major impediment to understanding biological processes, functional differences between species, and evolutionary mechanisms. Often, genes that are large, embedded within duplicated genomic regions, or associated with repeats are difficult to study by short-read expression profiling and assembly. In addition, most genes in eukaryotic organisms produce alternatively spliced isoforms, broadening the diversity of proteins encoded by the genome, which are difficult to resolve with short-read methods. Short-read RNA sequencing (RNA-seq) works by physically shearing transcript isoforms into smaller pieces and bioinformatically reassembling them, leaving opportunity for misassembly or incomplete capture of the full diversity of isoforms from genes of interest. In contrast, Single Molecule, Real-Time (SMRT) Sequencing directly sequences full-length transcripts without the need for assembly and imputation. Here we apply the Iso-Seq method (long-read RNA sequencing) to detect full-length isoforms and the new IsoPhase algorithm to retrieve allele-specific isoform information for two avian models of vocal learning, Anna’s hummingbird (Calypte anna) and zebra finch (Taeniopygia guttata).

April 21, 2020  |  

deSALT: fast and accurate long transcriptomic read alignment with de Bruijn graph-based index

Long-read RNA sequencing (RNA-seq) is promising to transcriptomics studies, however, the alignment of the reads is still a fundamental but non-trivial task due to the sequencing errors and complicated gene structures. We propose deSALT, a tailored two-pass long RNA-seq read alignment approach, which constructs graph-based alignment skeletons to sensitively infer exons, and use them to generate spliced reference sequence to produce refined alignments. deSALT addresses several difficult issues, such as small exons, serious sequencing errors and consensus spliced alignment. Benchmarks demonstrate that this approach has a better ability to produce high-quality full-length alignments, which has enormous potentials to transcriptomics studies.

April 21, 2020  |  

Long-Read RNA Sequencing Identifies Alternative Splice Variants in Hepatocellular Carcinoma and Tumor-Specific Isoforms.

Alternative splicing (AS) allows generation of cell type-specific mRNA transcripts and contributes to hallmarks of cancer. Genome-wide analysis for AS in human hepatocellular carcinoma (HCC), however, is limited. We sought to obtain a comprehensive AS landscape in HCC and define tumor-associated variants. Single-molecule real-time long-read RNA sequencing was performed on patient-derived HCC cells, and presence of splice junctions was defined by SpliceMap-LSC-IDP algorithm. We obtained an all-inclusive map of annotated AS variants and further discovered 362 alternative spliced variants that are not previously reported in any database (neither RefSeq nor GENCODE). They were mostly derived from intron retention and early termination codon with an in-frame open reading frame in 81.5%. We corroborated many of these predicted unannotated and annotated variants to be tumor specific in an independent cohort of primary HCC tumors and matching nontumoral liver. Using the combined Sanger sequencing and TaqMan junction assays, unique and common expressions of spliced variants including enzyme regulators (ARHGEF2, SERPINH1), chromatin modifiers (DEK, CDK9, RBBP7), RNA-binding proteins (SRSF3, RBM27, MATR3, YBX1), and receptors (ADRM1, CD44v8-10, vitamin D receptor, ROR1) were determined in HCC tumors. We further focused functional investigations on ARHGEF2 variants (v1 and v3) that arise from the common amplified site chr.1q22 of HCC. Their biological significance underscores two major cancer hallmarks, namely cancer stemness and epithelial-to-mesenchymal transition-mediated cell invasion and migration, although v3 is consistently more potent than v1. Conclusion: Alternative isoforms and tumor-specific isoforms that arise from aberrant splicing are common during the liver tumorigenesis. Our results highlight insights gained from the analysis of AS in HCC. © 2019 The Authors. Hepatology published by Wiley Periodicals, Inc., on behalf of American Association for the Study of Liver Diseases.

April 21, 2020  |  

Targeted Long-Read RNA Sequencing Demonstrates Transcriptional Diversity Driven by Splice-Site Variation in MYBPC3.

To date, clinical sequencing has focused on genomic DNA using targeted panels and exome sequencing. Sequencing of a large hypertrophic cardiomyopathy (HCM) cohort revealed that positive identification of a disease-associated variant was returned in only 32% of patients, with an additional 15% receiving inconclusive results. When genome sequencing fails to reveal causative variants, the transcriptome may provide additional diagnostic clarity. A recent study examining patients with genetically undiagnosed muscle disorders found that RNA sequencing, when used as a complement to exome and whole genome sequencing, had an overall diagnosis rate of 35%.

April 21, 2020  |  

Single-Molecule Sequencing: Towards Clinical Applications.

In the past several years, single-molecule sequencing platforms, such as those by Pacific Biosciences and Oxford Nanopore Technologies, have become available to researchers and are currently being tested for clinical applications. They offer exceptionally long reads that permit direct sequencing through regions of the genome inaccessible or difficult to analyze by short-read platforms. This includes disease-causing long repetitive elements, extreme GC content regions, and complex gene loci. Similarly, these platforms enable structural variation characterization at previously unparalleled resolution and direct detection of epigenetic marks in native DNA. Here, we review how these technologies are opening up new clinical avenues that are being applied to pathogenic microorganisms and viruses, constitutional disorders, pharmacogenomics, cancer, and more.Copyright © 2018 Elsevier Ltd. All rights reserved.

April 21, 2020  |  

Unveiling novel targets of paclitaxel resistance by single molecule long-read RNA sequencing in breast cancer.

RNA sequencing has become one of the most common technology to study transcriptomes in cancer, whereas its length limits its application on alternative splicing (AS) events and novel isoforms. Firstly, we applied single molecule long-read RNA sequencing (Iso-seq) and de novo assembly with short-read RNA sequencing (RNA-seq) in both wild type (231-WT) and paclitaxel resistant type (231-PTX) of human breast cancer cell MDA-MBA-231. The two sequencing technology provide both the accurate transcript sequences and the deep transcript coverage. Then we combined shor-read and long-read RNA-seq to analyze alternative events and novel isoforms. Last but not the least, we selected BAK1 as our candidate target to verify our analysis. Our results implied that improved characterization of cancer genomic function may require the application of the single molecule long-read RNA sequencing to get the deeper and more precise view to transcriptional level. Our results imply that improved characterization of cancer genomic function may require the application of the single molecule long-read RNA sequencing to get the deeper and more precise view to transcriptional level.

April 21, 2020  |  

Long-read assembly of the Chinese rhesus macaque genome and identification of ape-specific structural variants.

We present a high-quality de novo genome assembly (rheMacS) of the Chinese rhesus macaque (Macaca mulatta) using long-read sequencing and multiplatform scaffolding approaches. Compared to the current Indian rhesus macaque reference genome (rheMac8), rheMacS increases sequence contiguity 75-fold, closing 21,940 of the remaining assembly gaps (60.8 Mbp). We improve gene annotation by generating more than two million full-length transcripts from ten different tissues by long-read RNA sequencing. We sequence resolve 53,916 structural variants (96% novel) and identify 17,000 ape-specific structural variants (ASSVs) based on comparison to ape genomes. Many ASSVs map within ChIP-seq predicted enhancer regions where apes and macaque show diverged enhancer activity and gene expression. We further characterize a subset that may contribute to ape- or great-ape-specific phenotypic traits, including taillessness, brain volume expansion, improved manual dexterity, and large body size. The rheMacS genome assembly serves as an ideal reference for future biomedical and evolutionary studies.

April 21, 2020  |  

Sequence properties of certain GC rich avian genes, their origins and absence from genome assemblies: case studies.

More and more eukaryotic genomes are sequenced and assembled, most of them presented as a complete model in which missing chromosomal regions are filled by Ns and where a few chromosomes may be lacking. Avian genomes often contain sequences with high GC content, which has been hypothesized to be at the origin of many missing sequences in these genomes. We investigated features of these missing sequences to discover why some may not have been integrated into genomic libraries and/or sequenced.The sequences of five red jungle fowl cDNA models with high GC content were used as queries to search publicly available datasets of Illumina and Pacbio sequencing reads. These were used to reconstruct the leptin, TNFa, MRPL52, PCP2 and PET100 genes, all of which are absent from the red jungle fowl genome model. These gene sequences displayed elevated GC contents, had intron sizes that were sometimes larger than non-avian orthologues, and had non-coding regions that contained numerous tandem and inverted repeat sequences with motifs able to assemble into stable G-quadruplexes and intrastrand dyadic structures. Our results suggest that Illumina technology was unable to sequence the non-coding regions of these genes. On the other hand, PacBio technology was able to sequence these regions, but with dramatically lower efficiency than would typically be expected.High GC content was not the principal reason why numerous GC-rich regions of avian genomes are missing from genome assembly models. Instead, it is the presence of tandem repeats containing motifs capable of assembling into very stable secondary structures that is likely responsible.

April 21, 2020  |  

Getting the Entire Message: Progress in Isoform Sequencing

The advent of second-generation sequencing and its application to RNA sequencing has revolutionized the field of genomics by allowing the quantification of expression of entire genes as well as single TSS, exons and splice sites, RNA-editing sites as well as polyA-sites. However, due to the sequencing of fragments of cDNAs these methods have not given a reliable picture of complete RNA isoforms. Third-generation sequencing has filled this gap and allows end-to-end sequencing of entire RNA/cDNA molecules. This approach to transcriptomics has been a ‘niche’ technology for a couple of years but now is becoming mainstream with many different applications. Here, we review the background and progress made to date in this rapidly growing field. We start by reviewing the progressive realization that alternative splicing is omnipresent. We then focus on long-non-coding RNA isoforms and the distinct combination patterns of exons in non-coding and coding genes. We consider the implications of the recent technologies of direct RNA sequencing and single-cell isoform RNA sequencing. Finally, we discuss the parameters that define the success of long-read RNA sequencing experiments and strategies commonly used to make the most of such data.

April 21, 2020  |  

Long-Read Sequencing Emerging in Medical Genetics

The wide implementation of next-generation sequencing (NGS) technologies has revolutionized the field of medical genetics. However, the short read lengths of currently used sequencing approaches pose a limitation for identification of structural variants, sequencing repetitive regions, phasing alleles and distinguishing highly homologous genomic regions. These limitations may significantly contribute to the diagnostic gap in patients with genetic disorders who have undergone standard NGS, like whole exome or even genome sequencing. Now, the emerging long-read sequencing (LRS) technologies may offer improvements in the characterization of genetic variation and regions that are difficult to assess with the currently prevailing NGS approaches. LRS has so far mainly been used to investigate genetic disorders with previously known or strongly suspected disease loci. While these targeted approaches already show the potential of LRS, it remains to be seen whether LRS technologies can soon enable true whole genome sequencing routinely. Ultimately, this could allow the de novo assembly of individual whole genomes used as a generic test for genetic disorders. In this article, we summarize the current LRS-based research on human genetic disorders and discuss the potential of these technologies to facilitate the next major advancements in medical genetics.

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