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September 22, 2019

Single-cell mRNA isoform diversity in the mouse brain.

Alternative mRNA isoform usage is an important source of protein diversity in mammalian cells. This phenomenon has been extensively studied in bulk tissues, however, it remains unclear how this diversity is reflected in single cells.Here we use long-read sequencing technology combined with unique molecular identifiers (UMIs) to reveal patterns of alternative full-length isoform expression in single cells from the mouse brain. We found a surprising amount of isoform diversity, even after applying a conservative definition of what constitutes an isoform. Genes tend to have one or a few isoforms highly expressed and a larger number of isoforms expressed at a low level. However, for many genes, nearly every sequenced mRNA molecule was unique, and many events affected coding regions suggesting previously unknown protein diversity in single cells. Exon junctions in coding regions were less prone to splicing errors than those in non-coding regions, indicating purifying selection on splice donor and acceptor efficiency.Our findings indicate that mRNA isoform diversity is an important source of biological variability also in single cells.

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September 22, 2019

Genome re-annotation of the wild strawberry Fragaria vesca using extensive Illumina-and SMRT-based RNA-seq datasets

The genome of the wild diploid strawberry species Fragaria vesca, an ideal model system of cultivated strawberry (Fragaria × ananassa, octoploid) and other Rosaceae family crops, was first published in 2011 and followed by a new assembly (Fvb). However, the annotation for Fvb mainly relied on ab initio predictions and included only predicted coding sequences, therefore an improved annotation is highly desirable. Here, a new annotation version named v2.0.a2 was created for the Fvb genome by a pipeline utilizing one PacBio library, 90 Illumina RNA-seq libraries, and 9 small RNA-seq libraries. Altogether, 18,641 genes (55.6% out of 33,538 genes) were augmented with information on the 5′ and/or 3′ UTRs, 13,168 (39.3%) protein-coding genes were modified or newly identified, and 7,370 genes were found to possess alternative isoforms. In addition, 1,938 long non-coding RNAs, 171 miRNAs, and 51,714 small RNA clusters were integrated into the annotation. This new annotation of F. vesca is substantially improved in both accuracy and integrity of gene predictions, beneficial to the gene functional studies in strawberry and to the comparative genomic analysis of other horticultural crops in Rosaceae family.

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September 22, 2019

Universal alternative splicing of noncoding exons.

The human transcriptome is so large, diverse, and dynamic that, even after a decade of investigation by RNA sequencing (RNA-seq), we have yet to resolve its true dimensions. RNA-seq suffers from an expression-dependent bias that impedes characterization of low-abundance transcripts. We performed targeted single-molecule and short-read RNA-seq to survey the transcriptional landscape of a single human chromosome (Hsa21) at unprecedented resolution. Our analysis reaches the lower limits of the transcriptome, identifying a fundamental distinction between protein-coding and noncoding gene content: almost every noncoding exon undergoes alternative splicing, producing a seemingly limitless variety of isoforms. Analysis of syntenic regions of the mouse genome shows that few noncoding exons are shared between human and mouse, yet human splicing profiles are recapitulated on Hsa21 in mouse cells, indicative of regulation by a deeply conserved splicing code. We propose that noncoding exons are functionally modular, with alternative splicing generating an enormous repertoire of potential regulatory RNAs and a rich transcriptional reservoir for gene evolution. Crown Copyright © 2017. Published by Elsevier Inc. All rights reserved.

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September 22, 2019

Transcriptome analysis of distinct cold tolerance strategies in the rubber tree (Hevea brasiliensis)

Natural rubber is an indispensable commodity used in approximately 40,000 products and is fundamental to the tire industry. Among the species that produce latex, the rubber tree [Hevea brasiliensis (Willd. ex Adr. de Juss.) Muell-Arg.], a species native to the Amazon rainforest, is the major producer of latex used worldwide. The Amazon Basin presents optimal conditions for rubber tree growth, but the occurrence of South American leaf blight, which is caused by the fungus Microcyclus ulei (P. Henn) v. Arx, limits rubber tree production. Currently, rubber tree plantations are located in scape regions that exhibit suboptimal conditions such as high winds and cold temperatures. Rubber tree breeding programs aim to identify clones that are adapted to these stress conditions. However, rubber tree breeding is time-consuming, taking more than 20 years to develop a new variety. It is also expensive and requires large field areas. Thus, genetic studies could optimize field evaluations, thereby reducing the time and area required for these experiments. Transcriptome sequencing using next-generation sequencing (RNA-seq) is a powerful tool to identify a full set of transcripts and for evaluating gene expression in model and non-model species. In this study, we constructed a comprehensive transcriptome to evaluate the cold response strategies of the RRIM600 (cold-resistant) and GT1 (cold-tolerant) genotypes. Furthermore, we identified putative microsatellite (SSR) and single-nucleotide polymorphism (SNP) markers. Alternative splicing, which is an important mechanism for plant adaptation under abiotic stress, was further identified, providing an important database for further studies of cold tolerance.

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September 22, 2019

Clonal distribution of BCR-ABL1 mutations and splice isoforms by single-molecule long-read RNA sequencing.

The evolution of mutations in the BCR-ABL1 fusion gene transcript renders CML patients resistant to tyrosine kinase inhibitor (TKI) based therapy. Thus screening for BCR-ABL1 mutations is recommended particularly in patients experiencing poor response to treatment. Herein we describe a novel approach for the detection and surveillance of BCR-ABL1 mutations in CML patients.To detect mutations in the BCR-ABL1 transcript we developed an assay based on the Pacific Biosciences (PacBio) sequencing technology, which allows for single-molecule long-read sequencing of BCR-ABL1 fusion transcript molecules. Samples from six patients with poor response to therapy were analyzed both at diagnosis and follow-up. cDNA was generated from total RNA and a 1,6 kb fragment encompassing the BCR-ABL1 transcript was amplified using long range PCR. To estimate the sensitivity of the assay, a serial dilution experiment was performed.Over 10,000 full-length BCR-ABL1 sequences were obtained for all samples studied. Through the serial dilution analysis, mutations in CML patient samples could be detected down to a level of at least 1%. Notably, the assay was determined to be sufficiently sensitive even in patients harboring a low abundance of BCR-ABL1 levels. The PacBio sequencing successfully identified all mutations seen by standard methods. Importantly, we identified several mutations that escaped detection by the clinical routine analysis. Resistance mutations were found in all but one of the patients. Due to the long reads afforded by PacBio sequencing, compound mutations present in the same molecule were readily distinguished from independent alterations arising in different molecules. Moreover, several transcript isoforms of the BCR-ABL1 transcript were identified in two of the CML patients. Finally, our assay allowed for a quick turn around time allowing samples to be reported upon within 2 days.In summary the PacBio sequencing assay can be applied to detect BCR-ABL1 resistance mutations in both diagnostic and follow-up CML patient samples using a simple protocol applicable to routine diagnosis. The method besides its sensitivity, gives a complete view of the clonal distribution of mutations, which is of importance when making therapy decisions.

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September 22, 2019

Circular RNA architecture and differentiation during leaf bud to young leaf development in tea (Camellia sinensis).

Circular RNA (circRNA) discovery, expression patterns and experimental validation in developing tea leaves indicates its correlation with circRNA-parental genes and potential roles in ceRNA interaction network. Circular RNAs (circRNAs) have recently emerged as a novel class of abundant endogenous stable RNAs produced by circularization with regulatory potential. However, identification of circRNAs in plants, especially in non-model plants with large genomes, is challenging. In this study, we undertook a systematic identification of circRNAs from different stage tissues of tea plant (Camellia sinensis) leaf development using rRNA-depleted circular RNA-seq. By combining two state-of-the-art detecting tools, we characterized 3174 circRNAs, of which 342 were shared by each approach, and thus considered high-confidence circRNAs. A few predicted circRNAs were randomly chosen, and 20 out of 24 were experimental confirmed by PCR and Sanger sequencing. Similar in other plants, tissue-specific expression was also observed for many C. sinensis circRNAs. In addition, we found that circRNA abundances were positively correlated with the mRNA transcript abundances of their parental genes. qRT-PCR validated the differential expression patterns of circRNAs between leaf bud and young leaf, which also indicated the low expression abundance of circRNAs compared to the standard mRNAs from the parental genes. We predicted the circRNA-microRNA interaction networks, and 54 of the differentially expressed circRNAs were found to have potential tea plant miRNA binding sites. The gene sets encoding circRNAs were significantly enriched in chloroplasts related GO terms and photosynthesis/metabolites biosynthesis related KEGG pathways, suggesting the candidate roles of circRNAs in photosynthetic machinery and metabolites biosynthesis during leaf development.

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September 22, 2019

Transcriptome comparative analysis of salt stress responsiveness in chrysanthemum (Dendranthema grandiflorum) roots by Illumina- and Single-Molecule Real-Time-based RNA sequencing.

Salt response has long been considered a polygenic-controlled character in plants. Under salt stress conditions, plants respond by activating a great amount of proteins and enzymes. To develop a better understanding of the molecular mechanism and screen salt responsive genes in chrysanthemum under salt stress, we performed the RNA sequencing (RNA-seq) on both salt-processed chrysanthemum seedling roots and the control group, and gathered six cDNA databases eventually. Moreover, to overcome the Illumina HiSeq technology’s limitation on sufficient length of reads and improve the quality and accuracy of the result, we combined Illumina HiSeq with single-molecule real-time sequencing (SMRT-seq) to decode the full-length transcripts. As a result, we successfully collected 550,823 unigenes, and from which we selected 48,396 differentially expressed genes (DEGs). Many of these DEGs were associated with the signal transduction, biofilm system, antioxidant system, and osmotic regulation system, such as mitogen-activated protein kinase (MAPK), Acyl-CoA thioesterase (ACOT), superoxide (SOD), catalase (CAT), peroxisomal membrane protein (PMP), and pyrroline-5-carboxylate reductase (P5CR). The quantitative real-time polymerase chain reaction (qRT-PCR) analysis of 15 unigenes was performed to test the data validity. The results were highly consistent with the RNA-seq results. In all, these findings could facilitate further detection of the responsive molecular mechanism under salt stress. They also provided more accurate candidate genes for genetic engineering on salt-tolerant chrysanthemums.

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September 22, 2019

Novel exons and splice variants in the human antibody heavy chain identified by single cell and single molecule sequencing.

Antibody heavy chains contain a variable and a constant region. The constant region of the antibody heavy chain is encoded by multiple groups of exons which define the isotype and therefore many functional characteristics of the antibody. We performed both single B cell RNAseq and long read single molecule sequencing of antibody heavy chain transcripts and were able to identify novel exons for IGHA1 and IGHA2 as well as novel isoforms for IGHM antibody heavy chain.

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September 22, 2019

Isoform sequencing provides insight into natural genetic diversity in maize.

W64A, as a member of non-stiff stalk maize, has been used to develop current corn in plant breeding, and serving as one of broadest parent line for the commercial hybrid seed production (Huffman, 1984). The inbred had the characteristics of early flowering, average plant and ear height at its maturity, very strong roots and good stalks (Runge, 2004). In addition, W64A serves as an invaluable germplasm to study gene functions especially in the field of corn nutrition and endosperm texture given its nearly complete vitreousness and hardness (Figure 1a). However, little is known about the background of genetic and genomic information for W64A. With the advent of the revolutionary technology of PacBio long-read sequencing, we can simultaneously obtain a large amount of full-length cDNA up to 20 kb (An et al., 2018). This article is protected by copyright. All rights reserved.This article is protected by copyright. All rights reserved.

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September 22, 2019

Alternative isoform analysis of Ttc8 expression in the rat pineal gland using a multi-platform sequencing approach reveals neural regulation.

Alternative isoform regulation (AIR) vastly increases transcriptome diversity and plays an important role in numerous biological processes and pathologies. However, the detection and analysis of isoform-level differential regulation is difficult, particularly in the face of complex and incompletely-annotated transcriptomes. Here we have used Illumina short-read/high-throughput RNA-Seq to identify 55 genes that exhibit neurally-regulated AIR in the pineal gland, and then used two other complementary experimental platforms to further study and characterize the Ttc8 gene, which is involved in Bardet-Biedl syndrome and non-syndromic retinitis pigmentosa. Use of the JunctionSeq analysis tool led to the detection of several novel exons and splice junctions in this gene, including two novel alternative transcription start sites which were found to display disproportionately strong neurally-regulated differential expression in several independent experiments. These high-throughput sequencing results were validated and augmented via targeted qPCR and long-read Pacific Biosciences SMRT sequencing. We confirmed the existence of numerous novel splice junctions and the selective upregulation of the two novel start sites. In addition, we identified more than 20 novel isoforms of the Ttc8 gene that are co-expressed in this tissue. By using information from multiple independent platforms we not only greatly reduce the risk of errors, biases, and artifacts influencing our results, we also are able to characterize the regulation and splicing of the Ttc8 gene more deeply and more precisely than would be possible via any single platform. The hybrid method outlined here represents a powerful strategy in the study of the transcriptome.

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September 22, 2019

Single-cell isoform RNA sequencing characterizes isoforms in thousands of cerebellar cells.

Full-length RNA sequencing (RNA-Seq) has been applied to bulk tissue, cell lines and sorted cells to characterize transcriptomes, but applying this technology to single cells has proven to be difficult, with less than ten single-cell transcriptomes having been analyzed thus far. Although single splicing events have been described for =200 single cells with statistical confidence, full-length mRNA analyses for hundreds of cells have not been reported. Single-cell short-read 3′ sequencing enables the identification of cellular subtypes, but full-length mRNA isoforms for these cell types cannot be profiled. We developed a method that starts with bulk tissue and identifies single-cell types and their full-length RNA isoforms without fluorescence-activated cell sorting. Using single-cell isoform RNA-Seq (ScISOr-Seq), we identified RNA isoforms in neurons, astrocytes, microglia, and cell subtypes such as Purkinje and Granule cells, and cell-type-specific combination patterns of distant splice sites. We used ScISOr-Seq to improve genome annotation in mouse Gencode version 10 by determining the cell-type-specific expression of 18,173 known and 16,872 novel isoforms.

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September 22, 2019

Cartography of neurexin alternative splicing mapped by single-molecule long-read mRNA sequencing.

Neurexins are evolutionarily conserved presynaptic cell-adhesion molecules that are essential for normal synapse formation and synaptic transmission. Indirect evidence has indicated that extensive alternative splicing of neurexin mRNAs may produce hundreds if not thousands of neurexin isoforms, but no direct evidence for such diversity has been available. Here we use unbiased long-read sequencing of full-length neurexin (Nrxn)1a, Nrxn1ß, Nrxn2ß, Nrxn3a, and Nrxn3ß mRNAs to systematically assess how many sites of alternative splicing are used in neurexins with a significant frequency, and whether alternative splicing events at these sites are independent of each other. In sequencing more than 25,000 full-length mRNAs, we identified a novel, abundantly used alternatively spliced exon of Nrxn1a and Nrxn3a (referred to as alternatively spliced sequence 6) that encodes a 9-residue insertion in the flexible hinge region between the fifth LNS (laminin-a, neurexin, sex hormone-binding globulin) domain and the third EGF-like sequence. In addition, we observed several larger-scale events of alternative splicing that deleted multiple domains and were much less frequent than the canonical six sites of alternative splicing in neurexins. All of the six canonical events of alternative splicing appear to be independent of each other, suggesting that neurexins may exhibit an even larger isoform diversity than previously envisioned and comprise thousands of variants. Our data are consistent with the notion that a-neurexins represent extracellular protein-interaction scaffolds in which different LNS and EGF domains mediate distinct interactions that affect diverse functions and are independently regulated by independent events of alternative splicing.

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September 22, 2019

Current developments in molecular monitoring in chronic myeloid leukemia.

Molecular monitoring plays an essential role in the clinical management of chronic myeloid leukemia (CML) patients, and now guides clinical decision making. Quantitative reverse-transcriptase-polymerase-chain-reaction (qRT-PCR) assessment of BCR-ABL1 transcript levels has become the standard of care protocol in CML. However, further developments are required to assess leukemic burden more efficiently, monitor minimal residual disease (MRD), detect mutations that drive resistance to tyrosine kinase inhibitor (TKI) therapy and identify predictors of response to TKI therapy. Cartridge-based BCR-ABL1 quantitation, digital PCR and next generation sequencing are examples of technologies which are currently being explored, evaluated and translated into the clinic. Here we review the emerging molecular methods/technologies currently being developed to advance molecular monitoring in CML.

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September 22, 2019

Shannon: an information-optimal de novo RNA-Seq assembler

De novo assembly of short RNA-Seq reads into transcripts is challenging due to sequence similarities in transcriptomes arising from gene duplications and alternative splicing of transcripts. We present Shannon, an RNA-Seq assembler with an optimality guarantee derived from principles of information theory: Shannon reconstructs nearly all information-theoretically reconstructable transcripts. Shannon is based on a theory we develop for de novo RNA-Seq assembly that reveals differing abundances among transcripts to be the key, rather than the barrier, to effective assembly. The assembly problem is formulated as a sparsest-flow problem on a transcript graph, and the heart of Shannon is a novel iterative flow-decomposition algorithm. This algorithm provably solves the information-theoretically reconstructable instances in linear-time even though the general sparsest-flow problem is NP-hard. Shannon also incorporates several additional new algorithmic advances: a new error-correction algorithm based on successive cancelation, a multi-bridging algorithm that carefully utilizes read information in the k-mer de Bruijn graph, and an approximate graph partitioning algorithm to split the transcriptome de Bruijn graph into smaller components. In tests on large RNA-Seq datasets, Shannon obtains significant increases in sensitivity along with improvements in specificity in comparison to state-of-the-art assemblers.

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September 22, 2019

High-quality reference transcript datasets hold the key to transcript-specific RNA-sequencing analysis in plants.

Re-programming of the transcriptome involves both transcription and alternative splicing (AS). Some genes are regulated only at the AS level with no change in expression at the gene level. AS data must be incorporated as an essential aspect of the regulation of gene expression. RNA-sequencing (RNA-seq) can deliver both transcriptional and AS information, but accurate methods to analyse the added complexity in RNA-seq data are needed. The construction of a comprehensive reference transcript dataset (RTD) for a specific plant species, variety or accession, from all available sequence data, will immediately allow more robust analysis of RNA-seq data. RTDs will continually evolve and improve, a process that will be more efficient if resources across a community are shared and pooled.© 2016 The Authors. New Phytologist © 2016 New Phytologist Trust.

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