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Asset Tag: SMRT Technology,

June 1, 2021

Sequencing of expanded CGG repeats in the FMR1 gene.

Alleles of the FMR1 gene with more than 200 CGG repeats generally undergo methylation-coupled gene silencing, resulting in fragile X syndrome, the leading heritable form of cognitive impairment. Smaller expansions (55-200 CGG repeats) result in elevated levels of FMR1 mRNA, which is directly responsible for the late-onset neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS). For mechanistic studies and genetic counseling, it is important to know with precision the number of CGG repeats; however, no existing DNA sequencing method is capable of sequencing through more than ~100 CGG repeats, thus limiting the ability to precisely characterize the disease-causing alleles. The recent development of single molecule, real-time sequencing represents a novel approach to DNA sequencing that couples the intrinsic processivity of DNA polymerase with the ability to read polymerase activity on a single-molecule basis. Further, the accuracy of the method is improved through the use of circular templates, such that each molecule can be read multiple times to produce a circular consensus sequence (CCS). We have succeeded in generating CCS reads representing multiple passes through both strands of repeat tracts exceeding 700 CGGs (>2 kb of 100 percent CG) flanked by native FMR1 sequence, with single-molecule readlengths exceeding 12 kb. This sequencing approach thus enables us to fully characterize the previously intractable CGG-repeat sequence, leading to a better understanding of the distinct associated molecular pathologies. Real-time kinetic data also provides insight into the activity of DNA polymerase inside this unique sequence. The methodology should be widely applicable for studies of the molecular pathogenesis of an increasing number of repeat expansion-associated neurodegenerative and neurodevelopmental disorders, and for the efficient identification of such disorders in the clinical setting.

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June 1, 2021

Highly sensitive, non-invasive detection of colorectal cancer mutations using single molecule, third generation sequencing.

Colorectal cancer (CRC) represents one of the most prevalent and lethal malignant neoplasms and every individual of age 50 and above should undergo regular CRC screening. Currently, the most effective procedure to detect adenomas, the precursors to CRC, is colonoscopy, which reduces CRC incidence by 80%. However, it is an invasive approach that is unpleasant for the patient, expensive, and poses some risk of complications such as colon perforation. A non-invasive screening approach with detection rates comparable to those of colonoscopy has not yet been established. The current study applies Pacific Biosciences third generation, single molecule sequencing to the inspection of CRC-driving mutations. Our approach combines the screening power and the extremely high accuracy of circular consensus (CCS) third generation sequencing with the non-invasiveness of using stool DNA to detect CRC-associated mutations present at extremely low frequencies and establishes a foundation for a non-invasive, highly sensitive assay to screen the population for CRC and early stage adenomas. We performed a series of experiments using a pool of fifteen amplicons covering the genes most frequently mutated in CRC (APC, Beta Catenin, KRAS, BRAF, and TP53), ensuring a theoretical screening coverage of over 97% for both CRC and adenomas. The assay was able to detect mutations in DNA isolated from stool samples from patients diagnosed with CRC at frequencies below 0.5 % with no false positives. The mutations were then confirmed by sequencing DNA isolated from the excised tumor samples. Our assay should be sensitive enough to allow the early identification of adenomatous polyps using stool DNA as analyte. In conclusion, we have developed an assay to detect mutations in the genes associated with CRC and adenomas using Pacific Biosciences RS Single Molecule, Real Time Circular Consensus Sequencing (SMRT-CCS). With no systematic bias and a much higher raw base-calling quality (CCS) compared to other sequencing methods, the assay was able to detect mutations in stool DNA at frequencies below 0.5 % with no false positives. This level of sensitivity should be sufficient to allow the detection of most adenomatous polyps using stool DNA as analyte, a feature that would make our approach the first non-invasive assay with a sensitivity comparable to that of colonoscopy and a strong candidate for the non-invasive preventive CRC screening of the general population.

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June 1, 2021

Single Molecule, Real-Time sequencing of full-length cDNA transcripts uncovers novel alternatively spliced isoforms.

In higher eukaryotic organisms, the majority of multi-exon genes are alternatively spliced. Different mRNA isoforms from the same gene can produce proteins that have distinct properties such as structure, function, or subcellular localization. Thus, the importance of understanding the full complement of transcript isoforms with potential phenotypic impact cannot be underscored. 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. The Iso-Seq protocol developed at PacBio offers the only solution for direct sequencing of full-length, single-molecule cDNA sequences to survey transcriptome isoform diversity useful for gene discovery and annotation. 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 (avg. read length: 10-15 kb) without requiring fragmentation or post-sequencing assembly. Our open-source computational pipeline delivers high-quality, non-redundant sequences for unambiguous identification of alternative splicing events, alternative transcriptional start sites, polyA tail, and gene fusion events. The standard Iso-Seq protocol workflow available for all researchers is presented using a deep dataset of full- length cDNA sequences from the MCF-7 cancer cell line, and multiple tissues (brain, heart, and liver). Detected novel transcripts approaching 10 kb and alternative splicing events are highlighted. Even in extensively profiled samples, the method uncovered large numbers of novel alternatively spliced isoforms and previously unannotated genes.

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June 1, 2021

The resurgence of reference quality genome sequence.

Since the advent of Next-Generation Sequencing (NGS), the cost of de novo genome sequencing and assembly have dropped precipitately, which has spurred interest in genome sequencing overall. Unfortunately the contiguity of the NGS assembled sequences, as well as the accuracy of these assemblies have suffered. Additionally, most NGS de novo assemblies leave large portions of genomes unresolved, and repetitive regions are often collapsed. When compared to the reference quality genome sequences produced before the NGS era, the new sequences are highly fragmented and often prove to be difficult to properly annotate. In some cases the contiguous portions are smaller than the average gene size making the sequence not nearly as useful for biologists as the earlier reference quality genomes including of Human, Mouse, C. elegans, or Drosophila. Recently, new 3rd generation sequencing technologies, long-range molecular techniques, and new informatics tools have facilitated a return to high quality assembly. We will discuss the capabilities of the technologies and assess their impact on assembly projects across the tree of life from small microbial and fungal genomes through large plant and animal genomes. Beyond improvements to contiguity, we will focus on the additional biological insights that can be made with better assemblies, including more complete analysis genes in their flanking regulatory context, in-depth studies of transposable elements and other complex gene families, and long-range synteny analysis of entire chromosomes. We will also discuss the need for new algorithms for representing and analyzing collections of many complete genomes at once.

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June 1, 2021

High-accuracy, single-base resolution of near-full-length HIV genomes.

Background: The HIV-1 proviral reservoir is incredibly stable, even while undergoing antiretroviral therapy, and is seen as the major barrier to HIV-1 eradication. Identifying and comprehensively characterizing this reservoir will be critical to achieving an HIV cure. Historically, this has been a tedious and labor intensive process, requiring high-replicate single-genome amplification reactions, or overlapping amplicons that are then reconstructed into full-length genomes by algorithmic imputation. Here, we present a deep sequencing and analysis method able to determine the exact identity and relative abundances of near-full-length HIV genomes from samples containing mixtures of genomes without shearing or complex bioinformatic reconstruction. Methods: We generated clonal near-full-length (~9 kb) amplicons derived from single genome amplification (SGA) of primary proviral isolates or PCR of well-documented control strains. These clonal products were mixed at various abundances and sequenced as near-full-length (~9 kb) amplicons without shearing. Each mixture yielded many near-full-length HIV-1 reads. Mathematical analysis techniques resolved the complex mixture of reads into estimates of distinct near-full-length viral genomes with their relative abundances. Results: Single Molecule, Real-Time (SMRT) Sequencing data contained near-full-length (~9 kb) continuous reads for each sample including some runs with greater than 10,000 near-full-length-genome reads in a three-hour sequencing run. Our methods correctly recapitulated exactly the originating genomes at a single-base resolution and their relative abundances in both mixtures of clonal controls and SGAs, and these results were validated using independent sequencing methods. Correct resolution was achieved even when genomes differed only by a single base. Minor abundances of 5% were reliably detected. Conclusions: SMRT Sequencing yields long-read sequencing results from individual DNA molecules, a rapid time-to-result. The single-molecule, full-length nature of this sequencing method allows us to estimate variant subspecies and relative abundances with single-nucleotide resolution. This method allows for reference-agnostic and cost-effective full-genome sequencing of HIV-1, which could both further our understanding of latent infection and develop novel and improved tools for quantifying HIV provirus, which will be critical to cure HIV.

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June 1, 2021

Assembly of complete KIR haplotypes from a diploid individual by the direct sequencing of full-length fosmids.

We show that linearizing and directly sequencing full-length fosmids simplifies the assembly problem such that it is possible to unambiguously assemble individual haplotypes for the highly repetitive 100-200 kb killer Ig-like receptor (KIR) gene loci of chromosome 19. A tiling of targeted fosmids can be used to clone extended lengths of genomic DNA, 100s of kb in length, but repeat complexity in regions of particular interest, such as the KIR locus, means that sequence assembly of pooled samples into complete haplotypes is difficult and in many cases impossible. The current maximum read length generated by SMRT Sequencing exceeds the length of a 40 kb fosmid; it is therefore possible to span an entire fosmid in one sequencing read. Shearing, sequencing and assembling fosmids in a shotgun approach is prone to errors when the underlying sequence is highly repetitive. We show that it is possible to directly sequence linearized fosmids and generate a high-quality consensus by simple alignment, removing the need for an error-prone assembly step. The high-quality sequence of complete fosmids can then be tiled into full haplotypes. We demonstrate the method on DNA samples from a number of individuals and fully recover the sequence of both haplotypes from a pool of KIR fosmids. The ability to haplotype and sequence complex immunogenetic regions will bring exciting opportunities to explore the evolution of disease associations of the immune sub-genome. This simple and robust approach can be scaled-up allowing a complex genomic region to be sequenced at a population level. We expect such sequencing to be valuable in disease association research.

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June 1, 2021

Profiling metagenomic communities using circular consensus and Single Molecule, Real-Time Sequencing.

There are many sequencing-based approaches to understanding complex metagenomic communities spanning targeted amplification to whole-sample shotgun sequencing. While targeted approaches provide valuable data at low sequencing depth, they are limited by primer design and PCR amplification. Whole-sample shotgun experiments generally use short-read, second-generation sequencing, which results in data processing difficulties. For example, reads less than 1 kb in length will likely not cover a complete gene or region of interest, and will require assembly. This not only introduces the possibility of incorrectly combining sequence from different community members, it requires a high depth of coverage. As such, rare community members may not be represented in the resulting assembly. Circular-consensus, single molecule, real-time (SMRT) Sequencing reads in the 1-2 kb range, with >99% accuracy can be efficiently generated for low amounts of input DNA. 10 ng of input DNA sequenced in 4 SMRT Cells would generate >100,000 such reads. While throughput is low compared to second-generation sequencing, the reads are a true random sampling of the underlying community, since SMRT Sequencing has been shown to have no sequence-context bias. Long read lengths mean that that it would be reasonable to expect a high number of the reads to include gene fragments useful for analysis.

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June 1, 2021

Analysis of full-length metagenomic 16S genes by Single Molecule, Real-Time Sequencing

High-throughput sequencing of the complete 16S rRNA gene has become a valuable tool for characterizing microbial communities. However, the short reads produced by second-generation sequencing cannot provide taxonomic classification below the genus level. In this study, we demonstrate the capability of PacBio’s Single Molecule, Real-Time (SMRT) Sequencing to generate community profiles using mock microbial community samples from BEI Resources. We also evaluate multiplexing capabilities using PacBio barcodes on pooled samples comprising heterogeneous 16S amplicon populations representing soil, fecal, and mock communities.

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June 1, 2021

Long-read assembly of the Aedes aegypti Aag2 cell line genome resolves ancient endogenous viral elements

Transmission of arboviruses such as Dengue Virus by Aedes aegypti causes debilitating disease across the globe. Disease in humans can include severe acute symptoms such as hemorrhagic fever and organ failure, but mosquitoes tolerate high titers of virus in a persistent infection. The mechanisms responsible for this viral tolerance are unclear. Recent publications highlighted the integration of genetic material from non-retroviral RNA viruses into the genome of the host during infection that relies upon endogenous retro-transcriptase activity from transposons. These endogenous viral elements (EVEs) found in the genome are predicted to be ancient, and at least some EVEs are under purifying selection, suggesting they are beneficial to the host. To characterize EVE biogenesis in a tractable system, we sequenced the Ae. aegypti cell line, Aag2, to 58-fold coverage and present a de novo assembly of the genome. The assembly contains 1.7 Gb of genomic and 255 Mb of alternative haplotype specific sequence, consisting of contigs with a N50 of 1.4 Mb; a value that, when compared with other assemblies of the Aedes genus, is from 1-3 orders of magnitude longer. The Aag2 genome is highly repetitive (70%), most of which is classified as transposable elements (60%). We identify EVEs in the genome homologous to a range of extant viruses, many of which cluster in these regions of repetitive DNA. The contiguous assembly allows for more comprehensive identification of the transposable elements and EVEs that are most likely to be lost in assemblies lacking the read length of SMRT Sequencing.

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June 1, 2021

An improved circular consensus algorithm with an application to detection of HIV-1 Drug-Resistance Associated Mutations (DRAMs)

Scientists who require confident resolution of heterogeneous populations across complex regions have been unable to transition to short-read sequencing methods. They continue to depend on Sanger Sequencing despite its cost and time inefficiencies. Here we present a new redesigned algorithm that allows the generation of circular consensus sequences (CCS) from individual SMRT Sequencing reads. With this new algorithm, dubbed CCS2, it is possible to reach arbitrarily high quality across longer insert lengths at a lower cost and higher throughput than Sanger Sequencing. We apply this new algorithm, dubbed CCS2, to the characterization of the HIV-1 K103N drug-resistance associated mutation, which is both important clinically, and represents a challenge due to regional sequence context. A mutation was introduced into the 3rd position of amino acid position 103 (A>C substitution) of the RT gene on a pNL4-3 backbone by site-directed mutagenesis. Regions spanning ~1,300 bp were PCR amplified from both the non-mutated and mutant (K103N) plasmids, and were sequenced individually and as a 50:50 mixture. Sequencing data were analyzed using the new CCS2 algorithm, which uses a fully-generative probabilistic model of our SMRT Sequencing process to polish consensus sequences to arbitrarily high accuracy. This result, previously demonstrated for multi-molecule consensus sequences with the Quiver algorithm, is made possible by incorporating per-Zero Mode Waveguide (ZMW) characteristics, thus accounting for the intrinsic changes in the sequencing process that are unique to each ZMW. With CCS2, we are able to achieve a per-read empirical quality of QV30 with 19X coverage. This yields ~5000 1.3 kb consensus sequences with a collective empirical quality of ~QV40. Additionally, we demonstrate a 0% miscall rate in both unmixed samples, and estimate a 48:52% frequency for the K103N mutation in the mixed sample, consistent with data produced by orthogonal platforms.

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June 1, 2021

Highly sensitive and cost-effective detection of somatic cancer variants using single-molecule, real-time sequencing

Next-Generation Sequencing (NGS) technologies allow for molecular profiling of cancer samples with high sensitivity and speed at reduced cost. For efficient profiling of cancer samples, it is important that the NGS methods used are not only robust, but capable of accurately detecting low-frequency somatic mutations. Single Molecule, Real-Time (SMRT) Sequencing offers several advantages, including the ability to sequence single molecules with very high accuracy (>QV40) using the circular consensus sequencing (CCS) approach. The availability of genetically defined, human genomic reference standards provides an industry standard for the development and quality control of molecular assays for studying cancer variants. Here we characterize SMRT Sequencing for the detection of low-frequency somatic variants using the Quantitative Multiplex DNA Reference Standards from Horizon Discovery, combined with amplification of the variants using the Multiplicom Tumor Hotspot MASTR Plus assay. First, we sequenced a reference standard containing precise allelic frequencies from 1% to 24.5% for major oncology targets verified using digital PCR. This reference material recapitulates the complexity of tumor composition and serves as a well-characterized control. The control sample was amplified using the Multiplicom Tumor Hotspot MASTR Plus assay that targets 252 amplicons (121-254 bp) from 26 relevant cancer genes, which includes all 11 variants in the control sample. Next, we sequenced control samples prepared by SeraCare Life Sciences, which contained a defined mutation at allelic frequencies from 10% down to 0.1%. The wild type and mutant amplicons were serially diluted, sequenced and analyzed using SMRT Sequencing to identify the variants and determine the observed frequency. The random error profile and high-accuracy CCS reads make it possible to accurately detect low-frequency somatic variants.

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June 1, 2021

An improved circular consensus algorithm with an application to detect HIV-1 Drug Resistance Associated Mutations (DRAMs)

Scientists who require confident resolution of heterogeneous populations across complex regions have been unable to transition to short-read sequencing methods. They continue to depend on Sanger sequencing despite its cost and time inefficiencies. Here we present a new redesigned algorithm that allows the generation of circular consensus sequences (CCS) from individual SMRT Sequencing reads. With this new algorithm, dubbed CCS2, it is possible to reach high quality across longer insert lengths at a lower cost and higher throughput than Sanger sequencing. We applied CCS2 to the characterization of the HIV-1 K103N drug-resistance associated mutation in both clonal and patient samples. This particular DRAM has previously proved to be clinically relevant, but challenging to characterize due to regional sequence context. First, a mutation was introduced into the 3rd position of amino acid position 103 (A>C substitution) of the RT gene on a pNL4-3 backbone by site-directed mutagenesis. Regions spanning ~1.3 kb were PCR amplified from both the non-mutated and mutant (K103N) plasmids, and were sequenced individually and as a 50:50 mixture. Additionally, the proviral reservoir of a subject with known dates of virologic failure of an Efavirenz-based regimen and with documented emergence of drug resistant (K103N) viremia was sequenced at several time points as a proof-of-concept study to determine the kinetics of retention and decay of K103N.Sequencing data were analyzed using the new CCS2 algorithm, which uses a fully-generative probabilistic model of our SMRT Sequencing process to polish consensus sequences to high accuracy. With CCS2, we are able to achieve a per-read empirical quality of QV30 (99.9% accuracy) at 19X coverage. A total of ~5000 1.3 kb consensus sequences with a collective empirical quality of ~QV40 (99.99%) were obtained for each sample. We demonstrate a 0% miscall rate in both unmixed control samples, and estimate a 48:52 frequency for the K103N mutation in the mixed (50:50) plasmid sample, consistent with data produced by orthogonal platforms. Additionally, the K103N escape variant was only detected in proviral samples from time points subsequent (19%) to the emergence of drug resistant viremia. This tool might be used to monitor the HIV reservoir for stable evolutionary changes throughout infection.

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June 1, 2021

WGS SMRT Sequencing of patient samples from a fecal microbiota transplant trial

Fecal samples were obtained from human subjects in the first blinded, placebo-controlled trial to evaluate the efficacy and safety of fecal microbiota transplant (FMT) for treatment of recurrent C. difficile infection. Samples included pre-and post-FMT transplant, post-placebo transplant, and the donor control; samples were taken at 2 and 8 week post-FMT. Sequencing was done on the PacBio Sequel System, with the goal of obtaining high quality sequences covering whole genes or gene clusters, which will be used to better understand the relationship between the composition and functional capabilities of intestinal microbiomes and patient health. Methods: Samples were randomly sheared to 2-3 kb fragments, a sufficient length to cover most genes, and SMRTbell libraries were prepared using standard protocols. Libraries were run on the Sequel System, which has a throughput of hundreds of thousands of reads per SMRT Cell, adequate yield to sample the complex microbiomes of post-transplant and donor samples.Results: Here we characterize samples, describe library prep methods and detail Sequel System operation, including run conditions. Descriptive statistics of data output (primary analysis) are presented, along with SMRT Analysis reports on circular consensus sequence (CCS) reads generated using an updated algorithm (CCS2). Final sequencing yields are filtered at various levels of predicted accuracy from 90% to 99.9%. Previous studies done using the PacBio RS II System demonstrated the ability to profile at the species level, and in some cases the strain level, and provided functional insight. Conclusions: These results demonstrate that the Sequel System is well-suited for characterization of complex microbial communities, with the ability for high-throughput generation of extremely accurate single-molecule sequences, each several kilobases in length. The entire process from shearing and library prep through sequencing and CCS analysis can be completed in less than 48 hours.

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June 1, 2021

Enrichment of unamplified DNA and long-read SMRT Sequencing to unlock repeat expansion disorders

Nucleotide repeat expansions are a major cause of neurological and neuromuscular disease in humans, however, the nature of these genomic regions makes characterizing them extremely challenging. Accurate DNA sequencing of repeat expansions using short-read sequencing technologies is difficult, as short-read technologies often cannot read through regions of low sequence complexity. Additionally, these short reads do not span the entire region of interest and therefore sequence assembly is required. Lastly, most target enrichment methods are reliant upon amplification which adds the additional caveat of PCR bias. 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 PacBio’s long reads and uniform coverage, enables 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 Huntington’s Disease (HTT; CAG repeat), Fragile X (FMR1; CGG repeat), ALS (C9orf72; GGGGCC repeat), and Spinocerebellar ataxia type 10 (SCA10; variable ATTCT repeat) for examination. With this data, we demonstrate the ability to isolate hundreds of individual on-target molecules in a single SMRT Cell and accurately sequence through long repeat stretches, regardless of the extreme GC-content. The method is compatible with multiplexing of multiple targets and multiple samples in a single reaction. This technique also captures native DNA molecules for sequencing, allowing for the possibility of direct detection and characterization of epigenetic signatures.

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June 1, 2021

“SMRTer Confirmation”: Scalable clinical read-through variant confirmation using the Pacific Biosciences SMRT Sequencing platform

Next-generation sequencing (NGS) has significantly improved the cost and turnaround time for diagnostic genetic tests. ACMG recommends variant confirmation by an orthogonal method, unless sufficiently high sensitivity and specificity can be demonstrated using NGS alone. Most NGS laboratories make extensive use of Sanger sequencing for secondary confirmation of single nucleotide variants (SNVs) and indels, representing a large fraction of the cost and time required to deliver high quality genetic testing data to clinicians and patients. Despite its established data quality, Sanger is not a high-throughput method by today’s standards from either an assay or analysis standpoint as it can involve manual review of Sanger traces and is not amenable to multiplexing. Toward a scalable solution for confirmation, Invitae has developed a fully automated and LIMS-tracked assay and informatics pipeline that utilizes the Pacific Biosciences SMRT sequencing platform. Invitae’s pipeline generates PCR amplicons that encompass the variant(s) of interest, which are converted to closed DNA structures (SMRTbells) and sequenced in pools of 96 per SMRTcell. Each amplicon is appended with a 16nt barcode that encodes the patient and variant IDs. Per-sample de-multiplexing, alignment, variant calling, and confirmation resolution are handled via an automated pipeline. The confirmation process was validated by analyzing 243 clinical SNVs and indels in parallel with the gold standard Sanger sequencing method. Amplicons were sequenced and analyzed in technical replicates to demonstrate reproducibility. In this study, the PacBio-based confirmation pipeline demonstrated high reproducibility (97.5%), and outperformed Sanger in the fraction of primary NGS variants confirmed (PacBio = 93.4% and 94.7% confirmed across two replicates, Sanger = 84.8%) while having 100% concordance of confirmation status among overlapping confirmation calls.

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