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June 1, 2021  |  

Advances in sequence consensus and clustering algorithms for effective de novo assembly and haplotyping applications.

One of the major applications of DNA sequencing technology is to bring together information that is distant in sequence space so that understanding genome structure and function becomes easier on a large scale. The Single Molecule Real Time (SMRT) Sequencing platform provides direct sequencing data that can span several thousand bases to tens of thousands of bases in a high-throughput fashion. In contrast to solving genomic puzzles by patching together smaller piece of information, long sequence reads can decrease potential computation complexity by reducing combinatorial factors significantly. We demonstrate algorithmic approaches to construct accurate consensus when the differences between reads are dominated by insertions and deletions. High-performance implementations of such algorithms allow more efficient de novo assembly with a pre-assembly step that generates highly accurate, consensus-based reads which can be used as input for existing genome assemblers. In contrast to recent hybrid assembly approach, only a single ~10 kb or longer SMRTbell library is necessary for the hierarchical genome assembly process (HGAP). Meanwhile, with a sensitive read-clustering algorithm with the consensus algorithms, one is able to discern haplotypes that differ by less than 1% different from each other over a large region. One of the related applications is to generate accurate haplotype sequences for HLA loci. Long sequence reads that can cover the whole 3 kb to 4 kb diploid genomic regions will simplify the haplotyping process. These algorithms can also be applied to resolve individual populations within mixed pools of DNA molecules that are similar to each, e.g., by sequencing viral quasi-species samples.

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June 1, 2021  |  

Automated, non-hybrid de novo genome assemblies and epigenomes of bacterial pathogens.

Understanding the genetic basis of infectious diseases is critical to enacting effective treatments, and several large-scale sequencing initiatives are underway to collect this information. Sequencing bacterial samples is typically performed by mapping sequence reads against genomes of known reference strains. While such resequencing informs on the spectrum of single-nucleotide differences relative to the chosen reference, it can miss numerous other forms of variation known to influence pathogenicity: structural variations (duplications, inversions), acquisition of mobile elements (phages, plasmids), homonucleotide length variation causing phase variation, and epigenetic marks (methylation, phosphorothioation) that influence gene expression to switch bacteria from non- pathogenic to pathogenic states. Therefore, sequencing methods which provide complete, de novo genome assemblies and epigenomes are necessary to fully characterize infectious disease agents in an unbiased, hypothesis-free manner. Hybrid assembly methods have been described that combine long sequence reads from SMRT DNA Sequencing with short reads (SMRT CCS (circular consensus) or second-generation reads), wherein the short reads are used to error-correct the long reads which are then used for assembly. We have developed a new paradigm for microbial de novo assemblies in which SMRT sequencing reads from a single long insert library are used exclusively to close the genome through a hierarchical genome assembly process, thereby obviating the need for a second sample preparation, sequencing run, and data set. We have applied this method to achieve closed de novo genomes with accuracies exceeding QV50 (>99.999%) for numerous disease outbreak samples, including E. coli, Salmonella, Campylobacter, Listeria, Neisseria, and H. pylori. The kinetic information from the same SMRT Sequencing reads is utilized to determine epigenomes. Approximately 70% of all methyltransferase specificities we have determined to date represent previously unknown bacterial epigenetic signatures. With relatively short sequencing run times and automated analysis pipelines, it is possible to go from an unknown DNA sample to its complete de novo genome and epigenome in about a day.

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June 1, 2021  |  

Automated, non-hybrid de novo genome assemblies and epigenomes of bacterial pathogens

Understanding the genetic basis of infectious diseases is critical to enacting effective treatments, and several large-scale sequencing initiatives are underway to collect this information. Sequencing bacterial samples is typically performed by mapping sequence reads against genomes of known reference strains. While such resequencing informs on the spectrum of single nucleotide differences relative to the chosen reference, it can miss numerous other forms of variation known to influence pathogenicity: structural variations (duplications, inversions), acquisition of mobile elements (phages, plasmids), homonucleotide length variation causing phase variation, and epigenetic marks (methylation, phosphorothioation) that influence gene expression to switch bacteria from non-pathogenic to pathogenic states. Therefore, sequencing methods which provide complete, de novo genome assemblies and epigenomes are necessary to fully characterize infectious disease agents in an unbiased, hypothesis-free manner. Hybrid assembly methods have been described that combine long sequence reads from SMRT DNA sequencing with short, high-accuracy reads (SMRT (circular consensus sequencing) CCS or second-generation reads) to generate long, highly accurate reads that are then used for assembly. We have developed a new paradigm for microbial de novo assemblies in which long SMRT sequencing reads (average readlengths >5,000 bases) are used exclusively to close the genome through a hierarchical genome assembly process, thereby obviating the need for a second sample preparation, sequencing run and data set. We have applied this method to achieve closed de novo genomes with accuracies exceeding QV50 (>99.999%) to numerous disease outbreak samples, including E. coli, Salmonella, Campylobacter, Listeria, Neisseria, and H. pylori. The kinetic information from the same SMRT sequencing reads is utilized to determine epigenomes. Approximately 70% of all methyltransferase specificities we have determined to date represent previously unknown bacterial epigenetic signatures. The process has been automated and requires less than 1 day from an unknown DNA sample to its complete de novo genome and epigenome.

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June 1, 2021  |  

Getting the most out of your PacBio libraries with size selection.

PacBio RS II sequencing chemistries provide read lengths beyond 20 kb with high consensus accuracy. The long read lengths of P4-C2 chemistry and demonstrated consensus accuracy of 99.999% are ideal for applications such as de novo assembly, targeted sequencing and isoform sequencing. The recently launched P5-C3 chemistry generates even longer reads with N50 often >10,000 bp, making it the best choice for scaffolding and spanning structural rearrangements. With these chemistry advances, PacBio’s read length performance is now primarily determined by the SMRTbell library itself. Size selection of a high-quality, sheared 20 kb library using the BluePippin™ System has been demonstrated to increase the N50 read length by as much as 5 kb with C3 chemistry. BluePippin size selection or a more stringent AMPure® PB selection cutoff can be used to recover long fragments from degraded genomic material. The selection of chemistries, P4-C2 versus P5-C3, is highly dependent on the final size distribution of the SMRTbell library and experimental goals. PacBio’s long read lengths also allow for the sequencing of full-length cDNA libraries at single-molecule resolution. However, longer transcripts are difficult to detect due to lower abundance, amplification bias, and preferential loading of smaller SMRTbell constructs. Without size selection, most sequenced transcripts are 1-1.5 kb. Size selection dramatically increases the number of transcripts >1.5 kb, and is essential for >3 kb transcripts.

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June 1, 2021  |  

Using whole exome sequencing and bacterial pathogen sequencing to investigate the genetic basis of pulmonary non-tuberculous mycobacterial infections.

Pulmonary non-tuberculous mycobacterial (PNTM) infections occur in patients with chronic lung disease, but also in a distinct group of elderly women without lung defects who share a common body morphology: tall and lean with scoliosis, pectus excavatum, and mitral valve prolapse. In order to characterize the human host susceptibility to PNTM, we performed whole exome sequencing (WES) of 44 individuals in extended families of patients with active PNTM as well as 55 additional unrelated individuals with PNTM. This unique collection of familial cohorts in PNTM represents an important opportunity for a high yield search for genes that regulate mucosal immunity. An average of 58 million 100bp paired-end Illumina reads per exome were generated and mapped to the hg19 reference genome. Following variant detection and classification, we identified 58,422 potentially high-impact SNPs, 97.3% of which were missense mutations. Segregating variants using the family pedigrees as well as comparisons to the unrelated individuals identified multiple potential variants associated with PNTM. Validations of these candidate variants in a larger PNTM cohort are underway. In addition to WES, we sequenced the genomes of 52 mycobacterial isolates, including 9 from these PNTM patients, to integrate host PNTM susceptibility with mycobacterial genotypes and gain insights into the key factors involved in this devastating disease. These genomes were sequenced using a combination of 454, Illumina, and PacBio platforms and assembled using multiple genome assemblers. The resulting genome sequences were used to identify mycobacterial genotypes associated with virulence, invasion, and drug resistance.

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June 1, 2021  |  

Integrative biology of a fungus: Using PacBio SMRT Sequencing to interrogate the genome, epigenome, and transcriptome of Neurospora crassa.

PacBio SMRT Sequencing has the unique ability to directly detect base modifications in addition to the nucleotide sequence of DNA. Because eukaryotes use base modifications to regulate gene expression, the absence or presence of epigenetic events relative to the location of genes is critical to elucidate the function of the modification. Therefore an integrated approach that combines multiple omic-scale assays is necessary to study complex organisms. Here, we present an integrated analysis of three sequencing experiments: 1) DNA sequencing, 2) base-modification detection, and 3) Iso-seq analysis, in Neurospora crassa, a filamentous fungus that has been used to make many landmark discoveries in biochemistry and genetics. We show that de novo assembly of a new strain yields complete assemblies of entire chromosomes, and additionally contains entire centromeric sequences. Base-modification analyses reveal candidate sites of increased interpulse duration (IPD) ratio, that may signify regions of 5mC, 5hmC, or 6mA base modifications. Iso-seq method provides full-length transcript evidence for comprehensive gene annotation, as well as context to the base-modifications in the newly assembled genome. Projects that integrate multiple genome-wide assays could become common practice for identifying genomic elements and understanding their function in new strains and organisms.

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June 1, 2021  |  

SMRT Sequencing solutions for large genomes and transcriptomes.

Single Molecule, Real-Time (SMRT) Sequencing holds promise for addressing new frontiers in large genome complexities, such as long, highly repetitive, low-complexity regions and duplication events, and differentiating between transcript isoforms that are difficult to resolve with short-read technologies. We present solutions available for both reference genome improvement (>100 MB) and transcriptome research to best leverage long reads that have exceeded 20 Kb in length. Benefits for these applications are further realized with consistent use of size-selection of input sample using the BluePippin™ device from Sage Science. Highlights from our genome assembly projects using the latest P5-C3 chemistry on model organisms will be shared. Assembly contig N50 have exceeded 6 Mb and we observed longest contig exceeding 12.5 Mb with an average base quality of QV50. Additionally, the value of long, intact reads to provide a no-assembly approach to investigate transcript isoforms using our Iso-Seq Application will be presented.

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June 1, 2021  |  

A comparison of assemblers and strategies for complex, large-genome sequencing with PacBio long reads.

PacBio sequencing holds promise for addressing large-genome complexities, such as long, highly repetitive, low-complexity regions and duplication events that are difficult to resolve with short-read technologies. Several strategies, with varying outcomes, are available for de novo sequencing and assembling of larger genomes. Using a diploid fungal genome, estimated to be ~80 Mb in size, as the basis dataset for comparison, we highlight assembly options when using only PacBio sequencing or a combined strategy leveraging data sets from multiple sequencing technologies. Data generated from SMRT Sequencing was subjected to assembly using different large-genome assemblers, and comparisons of the results will be shown. These include results generated with HGAP, Celera Assembler, MIRA, PBJelly, and other assembly tools currently in development. Improvements observed include a near 50% reduction in the number of contigs coupled with at least a doubling of contig N50 size in genome assemblies incorporating SMRT Sequencing data. We further show how incorporating long reads also highlights new challenges and missed insights of short-read assemblies arising from heterozygosity inherent in multiploid genomes.

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June 1, 2021  |  

Near perfect de novo assemblies of eukaryotic genomes using PacBio long read sequencing.

Third generation single molecule sequencing technology from Pacific Biosciences, Moleculo, Oxford Nanopore, and other companies are revolutionizing genomics by enabling the sequencing of long, individual molecules of DNA and RNA. One major advantage of these technologies over current short read sequencing is the ability to sequence much longer molecules, thousands or tens of thousands of nucleotides instead of mere hundreds. This capacity gives researchers substantially greater power to probe into microbial, plant, and animal genomes, but it remains unknown on how to best use these data. To answer this, we systematically evaluated the human genome and 25 other important genomes across the tree of life ranging in size from 1Mbp to 3Gbp in an attempt to answer how long the reads need to be and how much coverage is necessary to completely assemble their chromosomes with single molecule sequencing. We also present a novel error correction and assembly algorithm using a combination of PacBio and pre-assembled Illumina sequencing. This new algorithm greatly outperforms other published hybrid algorithms.

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June 1, 2021  |  

An interactive workflow for the analysis of contigs from the metagenomic shotgun assembly of SMRT Sequencing data.

The data throughput of next-generation sequencing allows whole microbial communities to be analyzed using a shotgun sequencing approach. Because a key task in taking advantage of these data is the ability to cluster reads that belong to the same member in a community, single-molecule long reads of up to 30 kb from SMRT Sequencing provide a unique capability in identifying those relationships and pave the way towards finished assemblies of community members. Long reads become even more valuable as samples get more complex with lower intra-species variation, a larger number of closely related species, or high intra-species variation. Here we present a collection of tools tailored for PacBio data for the analysis of these fragmented metagenomic assembles, allowing improvements in the assembly results, and greater insight into the communities themselves. Supervised classification is applied to a large set of sequence characteristics, e.g., GC content, raw-read coverage, k-mer frequency, and gene prediction information, allowing the clustering of contigs from single or highly related species. A unique feature of SMRT Sequencing data is the availability of base modification / methylation information, which can be used to further analyze clustered contigs expected to be comprised of single or very closely related species. Here we show base modification information can be used to further study variation, based on differences in the methylated DNA motifs involved in the restriction modification system. Application of these techniques is demonstrated on a monkey intestinal microbiome sample and an in silico mix of real sequencing data from distinct bacterial samples.

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June 1, 2021  |  

Developments in PacBio metagenome sequencing: Shotgun whole genomes and full-length 16S.

The assembly of metagenomes is dramatically improved by the long read lengths of SMRT Sequencing. This is demonstrated in an experimental design to sequence a mock community from the Human Microbiome Project, and assemble the data using the hierarchical genome assembly process (HGAP) at Pacific Biosciences. Results of this analysis are promising, and display much improved contiguity in the assembly of the mock community as compared to publicly available short-read data sets and assemblies. Additionally, the use of base modification information to make further associations between contigs provides additional data to improve assemblies, and to distinguish between members within a microbial community. The epigenetic approach is a novel validation method unique to SMRT Sequencing. In addition to whole-genome shotgun sequencing, SMRT Sequencing also offers improved classification resolution and reliability of metagenomic and microbiome samples by the full-length sequencing of 16S rRNA (~1500 bases long). Microbial communities can be detected at the species level in some cases, rather than being limited to the genus taxonomic classification as constrained by short-read technologies. The performance of SMRT Sequencing for these metagenomic samples achieved >99% predicted concordance to reference sequences in cecum, soil, water, and mock control investigations for bacterial 16S. Community samples are estimated to contain from 2.3 and up to 15 times as many species with abundance levels as low as 0.05% compared to the identification of phyla groups.

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June 1, 2021  |  

SMRT Sequencing solutions for investigative studies to understand evolutionary processes.

Single Molecule, Real-Time (SMRT) Sequencing holds promise for addressing new frontiers to understand molecular mechanisms in evolution and gain insight into adaptive strategies. With read lengths exceeding 10 kb, we are able to sequence high-quality, closed microbial genomes with associated plasmids, and investigate large genome complexities, such as long, highly repetitive, low-complexity regions and multiple tandem-duplication events. Improved genome quality, observed at 99.9999% (QV60) consensus accuracy, and significant reduction of gap regions in reference genomes (up to and beyond 50%) allow researchers to better understand coding sequences with high confidence, investigate potential regulatory mechanisms in noncoding regions, and make inferences about evolutionary strategies that are otherwise missed by the coverage biases associated with short- read sequencing technologies. Additional benefits afforded by SMRT Sequencing include the simultaneous capability to detect epigenomic modifications and obtain full-length cDNA transcripts that obsolete the need for assembly. With direct sequencing of DNA in real-time, this has resulted in the identification of numerous base modifications and motifs, which genome-wide profiles have linked to specific methyltransferase activities. Our new offering, the Iso-Seq Application, allows for the accurate differentiation between transcript isoforms that are difficult to resolve with short-read technologies. PacBio reads easily span transcripts such that both 5’/3’ primers for cDNA library generation and the poly-A tail are observed. As such, exon configuration and intron retention events can be analyzed without ambiguity. This technological advance is useful for characterizing transcript diversity and improving gene structure annotations in reference genomes. We review solutions available with SMRT Sequencing, from targeted sequencing efforts to obtaining reference genomes (>100 Mb). This includes strategies for identifying microsatellites and conducting phylogenetic comparisons with targeted gene families. We highlight how to best leverage our long reads that have exceeded 20 kb in length for research investigations, as well as currently available bioinformatics strategies for analysis. Benefits for these applications are further realized with consistent use of size selection of input sample using the BluePippin™ device from Sage Science as demonstrated in our genome improvement projects. Using the latest P5-C3 chemistry on model organisms, these efforts have yielded an observed contig N50 of ~6 Mb, with the longest contig exceeding 12.5 Mb and an average base quality of QV50.

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June 1, 2021  |  

SMRT Sequencing solutions for plant genomes and transcriptomes

Single Molecule, Real-Time (SMRT) Sequencing provides efficient, streamlined solutions to address new frontiers in plant genomes and transcriptomes. Inherent challenges presented by highly repetitive, low-complexity regions and duplication events are directly addressed with multi- kilobase read lengths exceeding 8.5 kb on average, with many exceeding 20 kb. Differentiating between transcript isoforms that are difficult to resolve with short-read technologies is also now possible. We present solutions available for both reference genome and transcriptome research that best leverage long reads in several plant projects including algae, Arabidopsis, rice, and spinach using only the PacBio platform. Benefits for these applications are further realized with consistent use of size-selection of input sample using the BluePippin™ device from Sage Science. We will share highlights from our genome projects using the latest P5- C3 chemistry to generate high-quality reference genomes with the highest contiguity, contig N50 exceeding 1 Mb, and average base quality of QV50. Additionally, the value of long, intact reads to provide a no-assembly approach to investigate transcript isoforms using our Iso-Seq protocol will be presented for full transcriptome characterization and targeted surveys of genes with complex structures. PacBio provides the most comprehensive assembly with annotation when combining offerings for both genome and transcriptome research efforts. For more focused investigation, PacBio also offers researchers opportunities to easily investigate and survey genes with complex structures.

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