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Auto Tag: Population genetics

June 1, 2021  |  

New discoveries from closing Salmonella genomes using Pacific Biosciences continuous long reads.

The newer hierarchical genome assembly process (HGAP) performs de novo assembly using data from a single PacBio long insert library. To assess the benefits of this method, DNA from several Salmonella enterica serovars was isolated from a pure culture. Genome sequencing was performed using Pacific Biosciences RS sequencing technology. The HGAP process enabled us to close sixteen Salmonella subsp. enterica genomes and their associated mobile elements: The ten serotypes include: Salmonella enterica subsp. enterica serovar Enteritidis (S. Enteritidis) S. Bareilly, S. Heidelberg, S. Cubana, S. Javiana and S. Typhimurium, S. Newport, S. Montevideo, S. Agona, and S. Tennessee. In addition, we were able to detect novel methyltransferases (MTases) by using the Pacific Biosciences kinetic score distributions showing that each serovar appears to have a novel methylation pattern. For example while all Salmonella serovars examined so far have methylase specific activity for 5’-GATC-3’/3’-CTAG-5’ and 5’-CAGAG-3’/3’-GTCTC-5’ (underlined base indicates a modification), S. Heidelberg is uniquely specific for 5’-ACCANCC-3’/3’-TGGTNGG-5’, while S. Typhimurium has uniquely methylase specific for 5′-GATCAG-3’/3′- CTAGTC-5′ sites, for the samples examined so far. We believe that this may be due to the unique environments and phages that these serotypes have been exposed to. Furthermore, our analysis identified and closed a variety of plasmids such as mobilization plasmids, antimicrobial resistance plasmids and IncX plasmids carrying a Type IV secretion system (T4SS). The VirB/D4 T4SS apparatus is important in that it assists with rapid dissemination of antibiotic resistance and virulence determinants. Presently, only limited information exists regarding the genotypic characterization of drug resistance in S. Heidelberg isolates derived from various host species. Here, we characterize two S. Heidelberg outbreak isolates from two different outbreaks. Both isolates contain the IncX plasmid of approximately 35 kb, and carried the genes virB1, virB2, virB3/4, virB5, virB6, virB7, virB8, virB9, virB10, virB11, virD2, and virD4, that are associated with the T4SS. In addition, the outbreak isolate associated with ground turkey carries a 4,473 bp mobilization plasmid and an incompatibility group (Inc) I1 antimicrobial resistance plasmid encoding resistance to gentamicin (aacC2), beta-lactam (bl2b_tem), streptomycin (aadAI) and tetracycline (tetA, tetR) while the outbreak isolate associated with chicken breast carries the IncI1 plasmid encoding resistance to gentamicin (aacC2), streptomycin (aadAI) and sulfisoxazole (sul1). Using this new technology we explored the genetic elements present in resistant pathogens which will achieve a better understanding of the evolution of Salmonella.

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June 1, 2021  |  

Old school/new school genome sequencing: One step backward — a quantum leap forward.

As the costs for genome sequencing have decreased the number of “genome” sequences have increased at a rapid pace. Unfortunately, the quality and completeness of these so–called “genome” sequences have suffered enormously. We prefer to call such genome assemblies as “gene assembly space” (GAS). We believe it is important to distinguish GAS assemblies from reference genome assemblies (RGAs) as all subsequent research that depends on accurate genome assemblies can be highly compromised if the only assembly available is a GAS assembly.

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June 1, 2021  |  

HLA variant identification techniques

The Human Leukocyte Antigen (HLA) genes located on chromosome 6 are responsible for regulating immune function via antigen presentation and are one of the determining factors for stem cell and organ transplantation compatibility. Additionally various alleles within this region have been implicated in autoimmune disorders, cancer, vaccine response and both non-infectious and infectious disease risk. The HLA region is highly variable; containing repetitive regions; and co-dominantly expressed genes. This complicates short read mapping and means that assessing the effect of variation within a gene requires full phase information to resolve haplotypes.One solution to the problem of HLA identification is the use of statistical inference to suggest the most likely diploid alleles given the genotypes observed. The assumption of this approach is the availability of an extensive reference panel. Whilst there exists good population genetics data for imputing European populations, there remains a paucity of information about variation in African populations. Filling this gap is one of the aims of the Genome Diversity in Africa Project and as a first step we are performing a pilot study to identify the optimal method for determining HLA type information for large numbers of samples from African populations.To that end we have obtained samples from 125 consented African participants selected from 5 populations across Africa (Morrocan, Ashanti, Igbo, Kalenjin, and Zulu). The methods included in our pilot study are Sanger sequencing (ABI), NGS on HiSeqX Ten platform (Illumina); long-range PCR combined with single molecule real-time (SMRT) sequencing (PacBio); and for a subset of samples library preparation on GemCode Platform (10x Genomics), which delivers valuable long range contextual information, combined with Illumina NGS sequencing.Results from capillary sequencing suggests the presence of a minimum of two novel alleles. Long Range PCR have been performed initially on a subset of samples using both primers sourced from GenDX and designed as described in Shiina et al (2012). Initial results from both primer sets were promising on Promega DNA test samples but only the GenDX primers proved effective on the African samples, producing consistently PCR products of the expected size in the Igbo, Ashanti, Morrocan and Zulu samples. We will present early results from our evaluation of the different sequencing technologies

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June 1, 2021  |  

Characterizing haplotype diversity at the immunoglobulin heavy chain locus across human populations using novel long-read sequencing and assembly approaches

The human immunoglobulin heavy chain locus (IGH) remains among the most understudied regions of the human genome. Recent efforts have shown that haplotype diversity within IGH is elevated and exhibits population specific patterns; for example, our re-sequencing of the locus from only a single chromosome uncovered >100 Kb of novel sequence, including descriptions of six novel alleles, and four previously unmapped genes. Historically, this complex locus architecture has hindered the characterization of IGH germline single nucleotide, copy number, and structural variants (SNVs; CNVs; SVs), and as a result, there remains little known about the role of IGH polymorphisms in inter-individual antibody repertoire variability and disease. To remedy this, we are taking a multi-faceted approach to improving existing genomic resources in the human IGH region. First, from whole-genome and fosmid-based datasets, we are building the largest and most ethnically diverse set of IGH reference assemblies to date, by employing PacBio long-read sequencing combined with novel algorithms for phased haplotype assembly. In total, our effort will result in the characterization of >15 phased haplotypes from individuals of Asian, African, and European descent, to be used as a representative reference set by the genomics and immunogenetics community. Second, we are utilizing this more comprehensive sequence catalogue to inform the design and analysis of novel targeted IGH genotyping assays. Standard targeted DNA enrichment methods (e.g., exome capture) are currently optimized for the capture of only very short (100’s of bp) DNA segments. Our platform uses a modified bench protocol to pair existing capture-array technologies with the enrichment of longer fragments of DNA, enabling the use of PacBio sequencing of DNA segments up to 7 Kb. This substantial increase in contiguity disambiguates many of the complex repeated structures inherent to the locus, while yielding the base pair fidelity required to call SNVs. Together these resources will establish a stronger framework for further characterizing IGH genetic diversity and facilitate IGH genomic profiling in the clinical and research settings, which will be key to fully understanding the role of IGH germline variation in antibody repertoire development and disease.

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June 1, 2021  |  

Structural variant combining Illumina and low-coverage PacBio

Structural variant calling combining Illumina and low-coverage Pacbio Detection of large genomic variation (structural variants) has proven challenging using short-read methods. Long-read approaches which can span these large events have promise to dramatically expand the ability to accurately call structural variants. Although sequencing with Pacific Biosciences (Pacbio) long-read technology has become increasingly high throughput, generating high coverage with the technology can still be limiting and investigators often would like to know what pacbio coverages are adequate to call structural variants. Here, we present a method to identify a substantially higher fraction of structural variants in the human genome using low-coverage pacbio data by multiple strategies for ensembling data types and algorithms. Algorithmically, we combine three structural variant callers: PBHoney by Adam English, Sniffles by Fritz Sedlazeck, and Parliament by Adam English (which we have modified to improve for speed). Parliament itself uses a combination of Pacbio and Illumina data with a number of short-read callers (Breakdancer, Pindel, Crest, CNVnator, Delly, and Lumpy). We show that the outputs of these three programs are largely complementary to each other, with each able to uniquely access different sets of structural variants at different coverages. Combining them together can more than double the recall of true structural variants from a truth set relative to sequencing with Illumina alone, with substantial improvements even at low pacbio coverages (3x – 7x). This allows us to present for the first time cost-benefit tradeoffs to investigators about how much pacbio sequencing will yield what improvements in SV-calling. This work also builds upon the foundational work of Genome in a Bottle led by Justin Zook in establishing a truth set for structural variants in the Ashkenazim-Jewish trio data recently released. This work demonstrates the power of this benchmark set – one of the first of its kind for structural variation data – to help understand and refine the accuracies of calling structural variants with a number of approaches.

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June 1, 2021  |  

Effect of coverage depth and haplotype phasing on structural variant detection with PacBio long reads

Each human genome has thousands of structural variants compared to the reference assembly, up to 85% of which are difficult or impossible to detect with Illumina short reads and are only visible with long, multi-kilobase reads. The PacBio RS II and Sequel single molecule, real-time (SMRT) sequencing platforms have made it practical to generate long reads at high throughput. These platforms enable the discovery of structural variants just as short-read platforms did for single nucleotide variants. Numerous software algorithms call structural variants effectively from PacBio long reads, but algorithm sensitivity is lower for insertion variants and all heterozygous variants. Furthermore, the impact of coverage depth and read lengths on sensitivity is not fully characterized. To quantify how zygosity, coverage depth, and read lengths impact the sensitivity of structural variant detection, we obtained high coverage PacBio sequences for three human samples: haploid CHM1, diploid NA12878, and diploid SK-BR-3. For each dataset, reads were randomly subsampled to titrate coverage from 0.5- to 50-fold. The structural variants detected at each coverage were compared to the set at “full” 50-fold coverage. For the diploid samples, additional titrations were performed with reads first partitioned by phase using single nucleotide variants for essentially haploid structural variant discovery. Even at low coverages (1- to 5-fold), PacBio long reads reveal hundreds of structural variants that are not seen in deep 50-fold Illumina whole genome sequences. At moderate 10-fold PacBio coverage, a majority of structural variants are detected. Sensitivity begins to level off at around 40-fold coverage, though it does not fully saturate before 50-fold. Phasing improves sensitivity for all variant types, especially at moderate 10- to 20-fold coverage. Long reads are an effective tool to identify and phase structural variants in the human genome. The majority of variants are detected at moderate 10-fold coverage, and even extremely low long-read coverage (1- to 5-fold) reveals variants that are invisible to short-read sequencing. Performance will continue to improve with better software and longer reads, which will empower studies to connect structural variants to healthy and disease traits in the human population.

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June 1, 2021  |  

The MHC Diversity in Africa Project (MDAP) pilot – 125 African high resolution HLA types from 5 populations

The major histocompatibility complex (MHC), or human leukocyte antigen (HLA) in humans, is a highly diverse gene family with a key role in immune response to disease; and has been implicated in auto-immune disease, cancer, infectious disease susceptibility, and vaccine response. It has clinical importance in the field of solid organ and bone marrow transplantation, where donors and recipient matching of HLA types is key to transplanted organ outcomes. The Sanger based typing (SBT) methods currently used in clinical practice do not capture the full diversity across this region, and require specific reference sequences to deconvolute ambiguity in HLA types. However, reference databases are based largely on European populations, and the full extent of diversity in Africa remains poorly understood. Here, we present the first systematic characterisation of HLA diversity within Africa in the pilot phase of the MHC Diversity in Africa Project, together with an evaluation of methods to carry out scalable cost-effective, as well as reliable, typing of this region in African populations.To sample a geographically representative panel of African populations we obtained 125 samples, 25 each from the Zulu (South Africa), Igbo (Nigeria), Kalenjin (Kenya), Moroccan and Ashanti (Ghana) groups. For methods validation we included two controls from the International Histocompatibility Working Group (IHWG) collection with known typing information. Sanger typing and Illumina HiSeq X sequencing of these samples indicated potentially novel Class I and Class II alleles; however, we found poor correlation between HiSeq X sequencing and SBT for both classes. Long Range PCR and high resolution PacBio RS-II typing of 4 of these samples identified 7 novel Class II alleles, highlighting the high levels of diversity in these populations, and the need for long read sequencing approaches to characterise this comprehensively. We have now expanded this approach to the entire pilot set of 125 samples. We present these confirmed types and discuss a workflow for scaling this to 5000 individuals across Africa.The large number of new alleles identified in our pilot suggests the high level of African HLA diversity and the utility of high resolution methods. The MDAP project will provide a framework for accurate HLA typing, in addition to providing an invaluable resource for imputation in GWAS, boosting power to identify and resolve HLA disease associations.

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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.

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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.

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June 1, 2021  |  

Joint calling and PacBio SMRT Sequencing for indel and structural variant detection in populations

Fast and effective variant calling algorithms have been crucial to the successful application of DNA sequencing in human genetics. In particular, joint calling – in which reads from multiple individuals are pooled to increase power for shared variants – is an important tool for population surveys of variation. Joint calling was applied by the 1000 Genomes Project to identify variants across many individuals each sequenced to low coverage (about 5-fold). This approach successfully found common small variants, but broadly missed structural variants and large indels for which short-read sequencing has limited sensitivity. To support use of large variants in rare disease and common trait association studies, it is necessary to perform population-scale surveys with a technology effective at detecting indels and structural variants, such as PacBio SMRT Sequencing. For these studies, it is important to have a joint calling workflow that works with PacBio reads. We have developed pbsv, an indel and structural variant caller for PacBio reads, that provides a two-step joint calling workflow similar to that used to build the ExAC database. The first stage, discovery, is performed separately for each sample and consolidates whole genome alignments into a sparse representation of potentially variant loci. The second stage, calling, is performed on all samples together and considers only the signatures identified in the discovery stage. We applied the pbsv joint calling workflow to PacBio reads from twenty human genomes, with coverage ranging from 5-fold to 80-fold per sample for a total of 460-fold. The analysis required only 102 CPU hours, and identified over 800,000 indels and structural variants, including hundreds of inversions and translocations, many times more than discovered with short-read sequencing. The workflow is scalable to thousands of samples. The ongoing application of this workflow to thousands of samples will provide insight into the evolution and functional importance of large variants in human evolution and disease.

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June 1, 2021  |  

FALCON-Phase integrates PacBio and HiC data for de novo assembly, scaffolding and phasing of a diploid Puerto Rican genome (HG00733)

Haplotype-resolved genomes are important for understanding how combinations of variants impact phenotypes. The study of disease, quantitative traits, forensics, and organ donor matching are aided by phased genomes. Phase is commonly resolved using familial data, population-based imputation, or by isolating and sequencing single haplotypes using fosmids, BACs, or haploid tissues. Because these methods can be prohibitively expensive, or samples may not be available, alternative approaches are required. de novo genome assembly with PacBio Single Molecule, Real-Time (SMRT) data produces highly contiguous, accurate assemblies. For non-inbred samples, including humans, the separate resolution of haplotypes results in higher base accuracy and more contiguous assembled sequences. Two primary methods exist for phased diploid genome assembly. The first, TrioCanu requires Illumina data from parents and PacBio data from the offspring. The long reads from the child are partitioned into maternal and paternal bins using parent-specific sequences; the separate PacBio read bins are then assembled, generating two fully phased genomes. An alternative approach (FALCON-Unzip) does not require parental information and separates PacBio reads, during genome assembly, using heterozygous SNPs. The length of haplotype phase blocks in FALCON-Unzip is limited by the magnitude and distribution of heterozygosity, the length of sequence reads, and read coverage. Because of this, FALCON-Unzip contigs typically contain haplotype-switch errors between phase blocks, resulting in primary contig of mixed parental origin. We developed FALCON-Phase, which integrates Hi-C data downstream of FALCON-Unzip to resolve phase switches along contigs. We applied the method to a human (Puerto Rican, HG00733) and non-human genome assemblies and evaluated accuracy using samples with trio data. In a cattle genome, we observe >96% accuracy in phasing when compared to TrioCanu assemblies as well as parental SNPs. For a high-quality PacBio assembly (>90-fold Sequel coverage) of a Puerto Rican individual we scaffolded the FALCON-Phase contigs, and re-phased the contigs creating a de novo scaffolded, phased diploid assembly with chromosome-scale contiguity.

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June 1, 2021  |  

Improving the reference with a diversity panel of sequence-resolved structural variation

Although the accuracy of the human reference genome is critical for basic and clinical research, structural variants (SVs) have been difficult to assess because data capable of resolving them have been limited. To address potential bias, we sequenced a diversity panel of nine human genomes to high depth using long-read, single-molecule, real-time sequencing data. Systematically identifying and merging SVs =50 bp in length for these nine and one public genome yielded 83,909 sequence-resolved insertions, deletions, and inversions. Among these, 2,839 (2.0 Mbp) are shared among all discovery genomes with an additional 13,349 (6.9 Mbp) present in the majority of humans, indicating minor alleles or errors in the reference, which is partially explained by an enrichment for GC-content and repetitive DNA. Genotyping 83% of these in 290 additional genomes confirms that at least one allele of the most common SVs in unique euchromatin are now sequence-resolved. We observe a 9-fold increase within 5 Mbp of chromosome telomeric ends and correlation with de novo single-nucleotide variant mutations showing that such variation is nonrandomly distributed defining potential hotspots of mutation. We identify SVs affecting coding and noncoding regulatory loci improving annotation and interpretation of functional variation. To illustrate the utility of sequence-resolved SVs in resequencing experiments, we mapped 30 diverse high-coverage Illumina-sequenced samples to GRCh38 with and without contigs containing SV insertions as alternate sequences, and we found these additional sequences recover 6.4% of unmapped reads. For reads mapped within the SV insertion, 25.7% have a better mapping quality, and 18.7% improved by 1,000-fold or more. We reveal 72,964 occurrences of 15,814 unique variants that were not discoverable with the reference sequence alone, and we note that 7% of the insertions contain an SV in at least one sample indicating that there are additional alleles in the population that remain to be discovered. These data provide the framework to construct a canonical human reference and a resource for developing advanced representations capable of capturing allelic diversity. We present a summary of our findings and discuss ideas for revealing variation that was once difficult to ascertain.

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June 1, 2021  |  

New advances in SMRT Sequencing facilitate multiplexing for de novo and structural variant studies

The latest advancements in Sequel II SMRT Sequencing have increased average read lengths up to 50% compared to Sequel II chemistry 1.0 which allows multiplexing of 2-3 small organisms (<500 Mb) such as insects and worms for producing reference quality assemblies, calling structural variants for up to 2 samples with ~3 Gb genomes, analysis of 48 microbial genomes, and up to 8 communities for metagenomic profiling in a single SMRT Cell 8M. With the improved processivity of the new Sequel II sequencing polymerase, more SMRTbell molecules reach rolling circle mode resulting in longer overall read lengths, thus allowing efficient detection of barcodes (up to 80%) in the SMRTbell templates. Multiplexing of genomes larger than microbial organisms is now achievable. In collaboration with the Wellcome Sanger Institute, we have developed a workflow for multiplexing two individual Anopheles coluzzii using as low as 150 ng genomic DNA per individual. The resulting assemblies had high contiguity (contig N50s over 3 Mb) and completeness (>98% of conserved genes) for both individuals. For microbial multiplexing, we multiplexed 48 microbes with varying complexities and sizes ranging 1.6-8.0 Mb in single SMRT Cell 8M. Using a new end-to-end analysis (Microbial Assembly Analysis, SMRT Link 8.0), assemblies resulted in complete circularized genomes (>200-fold coverage) and efficient detection of >3-200 kb plasmids. Finally, the long read lengths (>90 kb) allows detection of barcodes in large insert SMRTbell templates (>15 kb) thus facilitating multiplex of two human samples in 1 SMRT Cell 8M for detecting SVs, Indels and CNVs. Here, we present results and describe workflows for multiplexing samples for specific applications for SMRT Sequencing.

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