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June 1, 2021  |  

Structural variant detection with low-coverage Pacbio sequencing

Despite amazing progress over the past quarter century in the technology to detect genetic variants, intermediate-sized structural variants (50 bp to 50 kb) have remained difficult to identify. Such variants are too small to detect with array comparative genomic hybridization, but too large to reliably discover with short-read DNA sequencing. Recent de novo assemblies of human genomes have demonstrated the power of PacBio Single Molecule, Real-Time (SMRT) Sequencing to fill this technology gap and sensitively identify structural variants in the human genome. While de novo assembly is the ideal method to identify variants in a genome, it requires high depth of coverage. A structural variant discovery approach that utilizes lower coverage would facilitate evaluation of large patient and population cohorts. Here we introduce such an approach and apply it to 10-fold coverage of several human genomes generated on the PacBio Sequel System. To identify structural variants in low-fold coverage whole genome sequencing data, we apply a reference-based, re-sequencing workflow. First, reads are mapped to the human reference genome with a local aligner. The local alignments often end at structural variant loci. To connect co-linear local alignments across structural variants, we apply a novel algorithm that merges alignments into “chains” and refines the alignment edges. Then, the chained alignments are scanned for windows with an excess of insertions or deletions to identify candidate structural variant loci. Finally, the read support at each putative variant locus is evaluated to produce a variant call. Single nucleotide information is incorporated to phase and evaluate the zygosity of each structural variant. In 10-fold coverage human genome sequence, we identify the vast majority of the structural variants found by de novo assembly, thus demonstrating the power of low-fold coverage SMRT Sequencing to affordably and effectively detect structural variants.


June 1, 2021  |  

SMRT Sequencing of full-length androgen receptor isoforms in prostate cancer reveals previously hidden drug resistant variants

Prostate cancer is the most frequently diagnosed male cancer. For prostate cancer that has progressed to an advanced or metastatic stage, androgen deprivation therapy (ADT) is the standard of care. ADT inhibits activity of the androgen receptor (AR), a master regulator transcription factor in normal and cancerous prostate cells. The major limitation of ADT is the development of castration-resistant prostate cancer (CRPC), which is almost invariably due to transcriptional re-activation of the AR. One mechanism of AR transcriptional re-activation is expression of AR-V7, a truncated, constitutively active AR variant (AR-V) arising from alternative AR pre-mRNA splicing. Noteworthy, AR-V7 is being developed as a predictive biomarker of primary resistance to androgen receptor (AR)-targeted therapies in CRPC. Multiple additional AR-V species are expressed in clinical CRPC, but the extent to which these may be co-expressed with AR-V7 or predict resistance is not known.


June 1, 2021  |  

Simplified sequencing of full-length isoforms in cancer on the PacBio Sequel platform

Tremendous flexibility is maintained in the human proteome via alternative splicing, and cancer genomes often subvert this flexibility to promote survival. Identification and annotation of cancer-specific mRNA isoforms is critical to understanding how mutations in the genome affect the biology of cancer cells. While microarrays and other NGS-based methods have become useful for studying transcriptomes, these technologies yield short, fragmented transcripts that remain a challenge for accurate, complete reconstruction of splice variants. In cancer proteomics studies, the identification of biomarkers from mass spectroscopy data is often limited by incomplete gene isoform expression information to support protein to transcript mapping. The Iso-Seq protocol developed at PacBio offers the only solution for direct sequencing of full-length, single-molecule cDNA sequences needed to discover biomarkers for early detection and cancer stratification, to fully characterize gene fusion events, and to elucidate drug resistance mechanisms. Knowledge of the complete isoform repertoire is also key for accurate quantification of isoform abundance. As most transcripts range from 1 – 10 kb, fully intact RNA molecules can be sequenced using SMRT® Sequencing without requiring fragmentation or post-sequencing assembly. However, some cancer research applications have presented a challenge for the Iso-Seq protocol, due to the combination of limited sample input and the need to deeply sequence heterogenous samples. Here we report the optimization of the Iso-Seq library preparation protocol for the PacBio Sequel platform and its application to cancer cell lines and tumor samples. We demonstrate how loading enhancements on the higher-throughput Sequel instrument have decreased the need for size fractionation steps, reducing sample input requirements while simultaneously simplifying the sample preparation workflow and increasing the number of full-length transcripts per SMRT Cell.


June 1, 2021  |  

Screening for causative structural variants in neurological disorders using long-read sequencing

Over the past decades neurological disorders have been extensively studied producing a large number of candidate genomic regions and candidate genes. The SNPs identified in these studies rarely represent the true disease-related functional variants. However, more recently a shift in focus from SNPs to larger structural variants has yielded breakthroughs in our understanding of neurological disorders.Here we have developed candidate gene screening methods that combine enrichment of long DNA fragments with long-read sequencing that is optimized for structural variation discovery. We have also developed a novel, amplification-free enrichment technique using the CRISPR/Cas9 system to target genomic regions.We sequenced gDNA and full-length cDNA extracted from the temporal lobe for two Alzheimer’s patients for 35 GWAS candidate genes. The multi-kilobase long reads allowed for phasing across the genes and detection of a broad range of genomic variants including SNPs to multi-kilobase insertions, deletions and inversions. In the full-length cDNA data we detected differential allelic isoform complexity, novel exons as well as transcript isoforms. By combining the gDNA data with full-length isoform characterization allows to build a more comprehensive view of the underlying biological disease mechanisms in Alzheimer’s disease. Using the novel PCR-free CRISPR-Cas9 enrichment method we screened several genes including the hexanucleotide repeat expansion C9ORF72 that is associated with 40% of familiar ALS cases. This method excludes any PCR bias or errors from an otherwise hard to amplify region as well as preserves the basemodication in a single molecule fashion which allows you to capture mosaicism present in the sample.


June 1, 2021  |  

Detecting pathogenic structural variants with low-coverage PacBio sequencing.

Though a role for structural variants in human disease has long been recognized, it has remained difficult to identify intermediate-sized variants (50 bp to 5 kb), which are too small to detect with array comparative genomic hybridization, but too large to reliably discover with short-read DNA sequencing. Recent studies have demonstrated that PacBio Single Molecule, Real-Time (SMRT) sequencing fills this technology gap. SMRT sequencing detects tens of thousands of structural variants in the human genome, approximately five times the sensitivity of short-read DNA sequencing.


June 1, 2021  |  

Multiplexing strategies for microbial whole genome sequencing using the Sequel System

For microbial sequencing on the PacBio Sequel System, the current yield per SMRT Cell is in excess relative to project requirements. Multiplexing offers a viable solution; greatly increasing throughput, efficiency, and reducing costs per genome. This approach is achieved by incorporating a unique barcode for each microbial sample into the SMRTbell adapters and using a streamlined library preparation process. To demonstrate performance,12 unique barcodes assigned to B. subtilis and sequenced on a single SMRT Cell. To further demonstrate the applicability of this method, we multiplexed the genomes of 16 strains of H. pylori. Each DNA was sheared to 10 kb, end-repaired and ligated with a barcoded adapter in a single-tube reaction. The barcoded samples were pooled in equimolar quantities and a single SMRTbell library was prepared. Successful de novo microbial assemblies were achieved from all multiplexes tested (12-, and 16-plex) using data generated from a single SMRTbell library, run on a single SMRT Cell 1M with the PacBio Sequel System, and analyzed with standard SMRT Analysis assembly methods. Here, we describe a protocol that facilitated the multiplexing up to 12-plex of microbial genomes in one SMRT Cell 1M on the Sequel System that produced near-complete microbial de novo assemblies of <10 contigs for genomes <5 Mb in size.


June 1, 2021  |  

Structural variant detection with low-coverage PacBio sequencing

Structural variants (genomic differences =50 base pairs) contribute to the evolution of organisms traits and human disease. Most structural variants (SVs) are too small to detect with array comparative genomic hybridization but too large to reliably discover with short-read DNA sequencing. Recent studies in human genomes show that PacBio SMRT Sequencing sensitively detects structural variants.


June 1, 2021  |  

From RNA to full-length transcripts: The PacBio Iso-Seq method for transcriptome analysis and genome annotation

A single gene may encode a surprising number of proteins, each with a distinct biological function. This is especially true in complex eukaryotes. 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. The PacBio Isoform Sequencing (Iso-Seq™) method employs long reads to sequence transcript isoforms from the 5’ end to their poly-A tails, eliminating the need for transcript reconstruction and inference. These long reads result in complete, unambiguous information about alternatively spliced exons, transcriptional start sites, and poly- adenylation sites. This allows for the characterization of the full complement of isoforms within targeted genes, or across an entire transcriptome. Here we present improved genome annotations for two avian models of vocal learning, Anna’s hummingbird (Calypte anna) and zebra finch (Taeniopygia guttata), using the Iso-Seq method. We present graphical user interface and command line analysis workflows for the data sets. From brain total RNA, we characterize more than 15,000 isoforms in each species, 9% and 5% of which were previously unannotated in hummingbird and zebra finch, respectively. We highlight one example where capturing full-length transcripts identifies additional exons and UTRs.


June 1, 2021  |  

Targeted enrichment without amplification and SMRT Sequencing of repeat-expansion disease causative genomic regions

Targeted sequencing has proven to be an economical means of obtaining sequence information for one or more defined regions of a larger genome. However, most target enrichment methods are reliant upon some form of amplification. Amplification removes the epigenetic marks present in native DNA, and some genomic regions, such as those with extreme GC content and repetitive sequences, are recalcitrant to faithful amplification. Yet, a large number of genetic disorders are caused by expansions of repeat sequences. Furthermore, for some disorders, methylation status has been shown to be a key factor in the mechanism of disease. We have developed a novel, amplification-free enrichment technique that employs the CRISPR/Cas9 system for specific targeting of individual human genes. This method, in conjunction with SMRT Sequencing’s long reads, high consensus accuracy, and uniform coverage, allows the sequencing of complex genomic regions that cannot be investigated with other technologies. Using human genomic DNA samples and this strategy, we have successfully targeted the loci of a number of repeat expansion disorders (HTT, FMR1, ATXN10, C9orf72). With this data, we demonstrate the ability to isolate hundreds of individual on-target molecules and accurately sequence through long repeat stretches, regardless of the extreme GC-content, followed by accurate sequencing on a single PacBio RS II SMRT Cell or Sequel SMRT Cell 1M. The method is compatible with multiplexing of multiple targets and multiple samples in a single reaction. Furthermore, this technique also preserves native DNA molecules for sequencing, allowing for the possibility of direct detection and characterization of epigenetic signatures. We demonstrate detection of 5-mC in human promoter sequences and CpG islands.


June 1, 2021  |  

Detecting pathogenic structural variants with long-read PacBio SMRT Sequencing

Most of the base pairs that differ between two human genomes are in intermediate-sized structural variants (50 bp to 5 kb), which are too small to detect with array comparative genomic hybridization or optical mapping but too large to reliably discover with short-read DNA sequencing. Long-read sequencing with PacBio Single Molecule, Real-Time (SMRT) Sequencing platforms fills this technology gap. PacBio SMRT Sequencing detects tens of thousands of structural variants in a human genome with approximately five times the sensitivity of short-read DNA sequencing. Effective application of PacBio SMRT Sequencing to detect structural variants requires quality bioinformatics tools that account for the characteristics of PacBio reads. To provide such a solution, we developed pbsv, a structural variant caller for PacBio reads that works as a chain of simple stages: 1) map reads to the reference genome, 2) identify reads with signatures of structural variation, 3) cluster nearby reads with similar signatures, 4) summarize each cluster into a consensus variant, and 5) filter for variants with sufficient read support. To evaluate the baseline performance of pbsv, we generated high coverage of a diploid human genome on the PacBio Sequel System, established a target set of structural variants, and then titrated to lower coverage levels. The false discovery rate for pbsv is low at all coverage levels. Sensitivity is high even at modest coverage: above 85% at 10-fold coverage and above 95% at 20-fold coverage. To assess the potential for PacBio SMRT Sequencing to identify pathogenic variants, we evaluated an individual with clinical symptoms suggestive of Carney complex for whom short-read whole genome sequencing was uninformative. The individual was sequenced to 9-fold coverage on the PacBio Sequel System, and structural variants were called with pbsv. Filtering for rare, genic structural variants left six candidates, including a heterozygous 2,184 bp deletion that removes the first coding exon of PRKAR1A. Null mutations in PRKAR1Acause autosomal dominant Carney complex, type 1. The variant was determined to be de novo, and it was classified as likely pathogenic based on ACMG standards and guidelines for variant interpretation. These case studies demonstrate the ability of pbsv to detect structural variants in low-coverage PacBio SMRT Sequencing and suggest the importance of considering structural variants in any study of human genetic variation.


June 1, 2021  |  

Targeted sequencing using a long-read sequencing technology

Targeted sequencing employing PCR amplification is a fundamental approach to studying human genetic disease. PacBio’s Sequel System and supporting products provide an end-to-end solution for amplicon sequencing, offering better performance to Sanger technology in accuracy, read length, throughput, and breadth of informative data. Sample multiplexing is supported with three barcoding options providing the flexibility to incorporate unique sample identifiers during target amplification or library preparation. Multiplexing is key to realizing the full capacity of the 1 million individual reactions per Sequel SMRT Cell. Two analysis workflows that can generate high-accuracy results support a wide range of amplicon sizes in two ranges from 250 bp to 3 kb and from 3 kb to >10 kb. The Circular Consensus Sequencing workflow results in high accuracy through intra-molecular consensus generation, while high accuracy for the Long Amplicon Analysis workflow is achieved by clustering of individual long reads from multiple reactions. Here we present workflows and results for single- molecule sequencing of amplicons for human genetic analysis.


June 1, 2021  |  

De novo assembly and preliminary annotation of the Schizocardium californicum genome

Animals in the phylum Hemichordata have provided key understanding of the origins and development of body patterning and nervous system organization. However, efforts to sequence and assemble the genomes of highly heterozygous non-model organisms have proven to be difficult with traditional short read approaches. Long repetitive DNA structures, extensive structural variation between haplotypes in polyploid species, and large genome sizes are limiting factors to achieving highly contiguous genome assemblies. Here we present the highly contiguous de novo assembly and preliminary annotation of an indirect developing hemichordate genome, Schizocardium californicum, using SMRT Sequening long reads.


June 1, 2021  |  

Best practices for diploid assembly of complex genomes using PacBio: A case study of Cascade Hops

A high quality reference genome is an essential resource for plant and animal breeding and functional and evolutionary studies. The common hop (Humulus lupulus, Cannabaceae) is an economically important crop plant used to flavor and preserve beer. Its genome is large (flow cytometrybased estimates of diploid length >5.4Gb1), highly repetitive, and individual plants display high levels of heterozygosity, which make assembly of an accurate and contiguous reference genome challenging with conventional short-read methods. We present a contig assembly of Cascade Hops using PacBio long reads and the diploid genome assembler, FALCON-Unzip2. The assembly has dramatically improved contiguity and completeness over earlier short-read assemblies. The genome is primarily assembled as haplotypes due to the outbred nature of the organism. We explore patterns of haplotype divergence across the assembly and present strategies to deduplicate haplotypes prior to scaffolding


June 1, 2021  |  

Haplotyping of full-length transcript reads from long-read sequencing can reveal allelic imbalances in isoform expression

The Pacific Biosciences Iso-Seq method, which can produce high-quality isoform sequences of 10 kb and longer, has been used to annotate many important plant and animal genomes. Here, we develop an algorithm called IsoPhase that postprocesses Iso-Seq data to retrieve allele specific isoform information. Using simulated data, we show that for both diploid and tetraploid genomes, IsoPhase results in good SNP recovery with low FDR at error rates consistent with CCS reads. We apply IsoPhase to a haplotyperesolved genome assembly and multiple fetal tissue Iso-Seq dataset from a F1 cross of Angus x Brahman cattle subspecies. IsoPhase-called haplotypes were validated by the phased assembly and demonstrate the potential for revealing allelic imbalances in isoform expression.


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