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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.


June 1, 2021  |  

Highly accurate read mapping of third generation sequencing reads for improved structural variation analysis

Characterizing genomic structural variations (SV) is vital for understanding how genomes evolve. Furthermore, SVs are known for playing a role in a wide range of diseases including cancer, autism, and schizophrenia. Nevertheless, due to their complexity they remain harder to detect and less understood than single nucleotide variations. Recently, third-generation sequencing has proven to be an invaluable tool for detecting SVs. The markedly higher read length not only allows single reads to span a SV, it also enables reliable mapping to repetitive regions of the genome. These regions often contain SVs and are inaccessible to short-read mapping. However, current sequencing technologies like PacBio show a raw read error rate of 10% or more consisting mostly of insertions and deletions. Especially in repetitive regions the high error rate causes current mapping methods to fail finding exact borders for SVs, to split up large deletions and insertions into several small ones, or in some cases, like inversions, to fail reporting them at all. Furthermore, for complex SVs it is not possible to find one end-to-end alignment for a given read. The decision of when to split a read into two or more separate alignments without knowledge of the underlying SV poses an even bigger challenge to current read mappers. Here we present NextGenMap-LR for long single molecule PacBio reads which addresses these issues. NextGenMap-LR uses a fast k-mer search to quickly find anchor regions between parts of a read and the reference and evaluates them using a vectorized implementation of the Smith-Waterman (SW) algorithm. The resulting high-quality anchors are then used to determine whether a read spans an SV and has to be split or can be aligned contiguously. Finally, NextGenMap-LR uses a banded SW algorithm to compute the final alignment(s). In this last step, to account for both the sequencing error and real genomic variations, we employ a non-affine gap model that penalizes gap extensions for longer gaps less than for shorter ones. Based on simulated as well as verified human breast cancer SV data we show how our approach significantly improves mapping of long reads around SVs. The non-affine gap model is especially effective at more precisely identifying the position of the breakpoint, and the enhanced scoring scheme enables subsequent variation callers to identify SVs that would have been missed otherwise.


June 1, 2021  |  

The value of long read amplicon sequencing for clinical applications

NGS is commonly used for amplicon sequencing in clinical applications to study genetic disorders and detect disease-causing mutations. This approach can be plagued by limited ability to phase sequence variants and makes interpretation of sequence data difficult when pseudogenes are present. Long-read highly accurate amplicon sequencing can provide very accurate, efficient, high throughput (through multiplexing) sequences from single molecules, with read lengths largely limited by PCR. Data is easy to interpret; phased variants and breakpoints are present within high fidelity individual reads. Here we show SMRT Sequencing of the PMS2 and OPN1 (MW and LW) genes using the Sequel System. Homologous regions make NGS and MLPA results very difficult to interpret.


June 1, 2021  |  

Comprehensive structural and copy-number variant detection with long reads

To comprehensively detect large variants in human genomes, we have extended pbsv – a structural variant caller for long reads – to call copy-number variants (CNVs) from read-clipping and read-depth signatures. In human germline benchmark samples, we detect more than 300 CNVs spanning around 10 Mb, and we call hundreds of additional events in re-arranged cancer samples. Long-read sequencing of diverse humans has revealed more than 20,000 insertion, deletion, and inversion structural variants spanning more than 12 Mb in a typical human genome. Most of these variants are too large to detect with short reads and too small for array comparative genome hybridization (aCGH). While the standard approaches to calling structural variants with long reads thrive in the 50 bp to 10 kb size range, they tend to miss exactly the large (>50 kb) copy-number variants that are called more readily with aCGH and short reads. Standard algorithms rely on reference-based mapping of reads that fully span a variant or on de novo assembly; and copy-number variants are often too large to be spanned by a single read and frequently involve segmentally duplicated sequence that is not yet included in most de novo assemblies.


June 1, 2021  |  

Copy-number variant detection with PacBio long reads

Long-read sequencing of diverse humans has revealed more than 20,000 insertion, deletion, and inversion structural variants spanning more than 12 Mb in a healthy human genome. Most of these variants are too large to detect with short reads and too small for array comparative genome hybridization (aCGH). While the standard approaches to calling structural variants with long reads thrive in the 50 bp to 10 kb size range, they tend to miss exactly the large (>50 kb) copy-number variants that are called more readily with aCGH. Standard algorithms rely on reference-based mapping of reads that fully span a variant or on de novo assembly; and copy-number variants are often too large to be spanned by a single read and frequently involve segmentally duplicated sequence that is not yet included in most de novo assemblies. To comprehensively detect large variants in human genomes, we extended pbsv – a structural variant caller for long reads – to call copy-number variants (CNVs) from read-clipping and read-depth signatures. In human germline benchmark samples, we detect more than 300 CNVs spanning around 10 Mb, and we call hundreds of additional events in re-arranged cancer samples. Together with insertion, deletion, inversion, duplication, and translocation calling from spanning reads, this allows pbsv to comprehensively detect large variants from a single data type.


April 21, 2020  |  

Long-read sequencing for rare human genetic diseases.

During the past decade, the search for pathogenic mutations in rare human genetic diseases has involved huge efforts to sequence coding regions, or the entire genome, using massively parallel short-read sequencers. However, the approximate current diagnostic rate is <50% using these approaches, and there remain many rare genetic diseases with unknown cause. There may be many reasons for this, but one plausible explanation is that the responsible mutations are in regions of the genome that are difficult to sequence using conventional technologies (e.g., tandem-repeat expansion or complex chromosomal structural aberrations). Despite the drawbacks of high cost and a shortage of standard analytical methods, several studies have analyzed pathogenic changes in the genome using long-read sequencers. The results of these studies provide hope that further application of long-read sequencers to identify the causative mutations in unsolved genetic diseases may expand our understanding of the human genome and diseases. Such approaches may also be applied to molecular diagnosis and therapeutic strategies for patients with genetic diseases in the future.


April 21, 2020  |  

Tracking short-term changes in the genetic diversity and antimicrobial resistance of OXA-232-producing Klebsiella pneumoniae ST14 in clinical settings.

To track stepwise changes in genetic diversity and antimicrobial resistance in rapidly evolving OXA-232-producing Klebsiella pneumoniae ST14, an emerging carbapenem-resistant high-risk clone, in clinical settings.Twenty-six K. pneumoniae ST14 isolates were collected by the Korean Nationwide Surveillance of Antimicrobial Resistance system over the course of 1 year. Isolates were subjected to whole-genome sequencing and MIC determinations using 33 antibiotics from 14 classes.Single-nucleotide polymorphism (SNP) typing identified 72 unique SNP sites spanning the chromosomes of the isolates, dividing them into three clusters (I, II and III). The initial isolate possessed two plasmids with 18 antibiotic-resistance genes, including blaOXA-232, and exhibited resistance to 11 antibiotic classes. Four other plasmids containing 12 different resistance genes, including blaCTX-M-15 and strA/B, were introduced over time, providing additional resistance to aztreonam and streptomycin. Moreover, chromosomal integration of insertion sequence Ecp1-blaCTX-M-15 mediated the inactivation of mgrB responsible for colistin resistance in four isolates from cluster III. To the best of our knowledge, this is the first description of K. pneumoniae ST14 resistant to both carbapenem and colistin in South Korea. Furthermore, although some acquired genes were lost over time, the retention of 12 resistance genes and inactivation of mgrB provided resistance to 13 classes of antibiotics.We describe stepwise changes in OXA-232-producing K. pneumoniae ST14 in vivo over time in terms of antimicrobial resistance. Our findings contribute to our understanding of the evolution of emerging high-risk K. pneumoniae clones and provide reference data for future outbreaks.Copyright © 2019 European Society of Clinical Microbiology and Infectious Diseases. Published by Elsevier Ltd. All rights reserved.


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