Sequencing of the large dsDNA genome of Oryctes rhinoceros nudivirus using multiple displacement amplification of nanogram amounts of virus DNA

The genomic sequence analysis of many large dsDNA viruses is hampered by the lack of enough sample materials. Here, we report a whole genome amplification of the Oryctes rhinoceros nudivirus (OrNV) isolate Ma07 starting from as few as about 10 ng of purified viral DNA by application of phi29 DNA polymerase- and exonuclease-resistant random hexamer-based multiple displacement amplification (MDA) method. About 60 microg of high molecular weight DNA with fragment sizes of up to 25 kbp was amplified. A genomic DNA clone library was generated using the product DNA. After 8-fold sequencing coverage, the 127,615 bp of OrNV whole genome was sequenced successfully. The results demonstrate that the MDA-based whole genome amplification enables rapid access to genomic information from exiguous virus samples.     

DNA amplification method tolerant to sample degradation

Despite recent advances in linear whole genome amplification of intact DNA/RNA, amplification of degraded nucleic acids in an unbiased fashion remains a serious challenge for genetic diagnosis. We describe a new whole genome amplification procedure, RCA-RCA (Restriction and Circularization-Aided Rolling Circle Amplification), which retains the allelic differences among degraded amplified genomes while achieving almost complete genome coverage. RCA-RCA utilizes restriction digestion and whole genome circularization to generate genomic sequences amenable to rolling circle amplification. When intact genomic DNA is used, RCA-RCA retains gene-amplification differences (twofold or higher) between complex genomes on a genome-wide scale providing highly improved concordance with unamplified material as compared with other amplification methodologies including multiple displacement amplification. Using RCA-RCA, formalin-fixed samples of modest or substantial DNA degradation were successfully amplified and screened via array-CGH or Taqman PCR that displayed retention of the principal gene amplification features of the original material. Microsatellite analysis revealed that RCA-RCA amplified genomic DNA is representative of the original material at the nucleotide level. Amplification of cDNA is successfully performed via RCA-RCA and results to unbiased gene expression analysis (R(2) = 0.99). The simplicity and universal applicability of RCA-RCA make it a powerful new tool for genome analysis with unique advantages over previous amplification technologies.     

Comparison of Exome and Genome Sequencing Technologies for the Complete Capture of Protein‐Coding Regions

For next‐generation sequencing technologies, sufficient base‐pair coverage is the foremost requirement for the reliable detection of genomic variants. We investigated whether whole‐genome sequencing (WGS) platforms offer improved coverage of coding regions compared with whole‐exome sequencing (WES) platforms, and compared single‐base coverage for a large set of exome and genome samples. We find that WES platforms have improved considerably in the last years, but at comparable sequencing depth, WGS outperforms WES in terms of covered coding regions. At higher sequencing depth (95x–160x), WES successfully captures 95% of the coding regions with a minimal coverage of 20x, compared with 98% for WGS at 87‐fold coverage. Three different assessments of sequence coverage bias showed consistent biases for WES but not for WGS. We found no clear differences for the technologies concerning their ability to achieve complete coverage of 2,759 clinically relevant genes. We show that WES performs comparable to WGS in terms of covered bases if sequenced at two to three times higher coverage. This does, however, go at the cost of substantially more sequencing biases in WES approaches. Our findings will guide laboratories to make an informed decision on which sequencing platform and coverage to choose.     

不同样本测序前处理方案对冠状病毒基因组高通量测序结果的影响

目的以基因组最大RNA病毒(冠状病毒)为代表,研究不同测序前样本处理模式对高通量测序获得病毒全基因组序列信息质量的影响。方法以细胞培养的人冠状病毒HCoV-OC43样本为代表,分为4种测序前样本处理模式,即:未处理组、核酸提取前DNase和RNase处理组、核酸提取后DNase处理组、核酸提取前DNase和RNase处理且核酸提取后DNase处理组。不同模式处理后的核酸分为两份,一份直接RNA测序(未扩增),另一份经序列非依赖的单引物扩增(SISPA)后DNA测序。结果尽管不同处理方式下获得的病毒基因组覆盖率差别不大,但是样本核酸提取后经DNase处理组直接测序获得了最高的基因覆盖度和测序准确性,而SISPA扩增可有效提高病毒测序读长(reads)比例与基因组各位点的测序深度。结论本研究为优化冠状病毒等RNA病毒全基因组测序策略提供了技术参考。     

A New Workflow for Whole‐Genome Sequencing of Single Human Cells

Unbiased amplification of the whole‐genome amplification (WGA) of single cells is crucial to study cancer evolution and genetic heterogeneity, but is challenging due to the high complexity of the human genome. Here, we present a new workflow combining an efficient adapter‐linker PCR‐based WGA method with second‐generation sequencing. This approach allows comparison of single cells at base pair resolution. Amplification recovered up to 74% of the human genome. Copy‐number variants and loss of heterozygosity detected in single cell genomes showed concordance of up to 99% to pooled genomic DNA. Allele frequencies of mutations could be determined accurately due to an allele dropout rate of only 2%, clearly demonstrating the low bias of our PCR‐based WGA approach. Sequencing with paired‐end reads allowed genome‐wide analysis of structural variants. By direct comparison to other WGA methods, we further endorse its suitability to analyze genetic heterogeneity.     

SNP frequency estimation using massively parallel sequencing of pooled DNA

Resequencing of genomic regions that have been implicated by linkage and/or association studies to harbor genetic susceptibility loci represents a necessary step to identify causal variants. Massively parallel sequencing (MPS) offers the possibility of SNP discovery and frequency determination among pooled DNA samples. The strategies of pooling DNA samples and pooling PCR amplicons generated from individual DNA samples were evaluated, and both were found to return accurate estimates of SNP frequencies across varying levels of sequence coverage.     

Unbiased whole-genome amplification directly from clinical samples

Preparation of genomic DNA from clinical samples is a bottleneck in genotyping and DNA sequencing analysis and is frequently limited by the amount of specimen available. We use Multiple Displacement Amplification (MDA) to amplify the whole genome 10,000-fold directly from small amounts of whole blood, dried blood, buccal cells, cultured cells, and buffy coats specimens, generating large amounts of DNA for genetic testing. Genomic DNA was evenly amplified with complete coverage and consistent representation of all genes. All 47 loci analyzed from 44 individuals were represented in the amplified DNA at between 0.5- and 3.0-fold of the copy number in the starting genomic DNA template. A high-fidelity DNA polymerase ensures accurate representation of the DNA sequence. The amplified DNA was indistinguishable from the original genomic DNA template in 5 SNP and 10 microsatellite DNA assays on three different clinical sample types for 20 individuals. Amplification of genomic DNA directly from cells is highly reproducible, eliminates the need for DNA template purification, and allows genetic testing from small clinical samples. The low amplification bias of MDA represents a dramatic technical improvement in the ability to amplify a whole genome compared with older, PCR-based methods.     

A Robust Approach for Blind Detection of Balanced Chromosomal Rearrangements with Whole‐Genome Low‐Coverage Sequencing

Balanced chromosomal rearrangement (or balanced chromosome abnormality, BCA) is a common chromosomal structural variation. Next‐generation sequencing has been reported to detect BCA‐associated breakpoints with the aid of karyotyping. However, the complications associated with this approach and the requirement for cytogenetics information has limited its application. Here, we provide a whole‐genome low‐coverage sequencing approach to detect BCA events independent of knowing the affected regions and with low false positives. First, six samples containing BCAs were used to establish a detection protocol and assess the efficacy of different library construction approaches. By clustering anomalous read pairs and filtering out the false‐positive results with a control cohort and the concomitant mapping information, we could directly detect BCA events for each sample. Through optimizing the read depth, BCAs in all samples could be blindly detected with only 120 million read pairs per sample for data from a small‐insert library and 30 million per sample for data from nonsize‐selected mate‐pair library. This approach was further validated using another 13 samples that contained BCAs. Our approach advances the application of high‐throughput whole‐genome low‐coverage analysis for robust BCA detection—especially for clinical samples—without the need for karyotyping.     

Efficient detection of chromosome imbalances and single nucleotide variants using targeted sequencing in the clinical setting

We evaluated an approach to detect copy number variants (CNVs) and single nucleotide changes (SNVs), using a clinically focused exome panel complemented with a backbone and SNP probes that allows for genome-wide copy number changes and copy-neutral absence of heterozygosity (AOH) calls; this approach potentially substitutes the use of chromosomal microarray testing and sequencing into a single test. A panel of 16 DNA samples with known alterations ranging from megabase-scale CNVs to single base modifications were used as positive controls for sequencing data analysis. The DNA panel included CNVs (n = 13) of variable sizes (23 Kb to 27 Mb), uniparental disomy (UPD; n = 1), and single point mutations (n = 2). All DNA sequence changes were identified by the current platform, showing that CNVs of at least 23 Kb can be properly detected. The estimated size of genomic imbalances detected by microarrays and next generation sequencing are virtually the same, indicating that the resolution and sensitivity of this approach are at least similar to those provided by DNA microarrays. Accordingly, our data show that the combination of a sequencing platform comprising focused exome and whole genome backbone, with appropriate algorithms, enables a cost-effective and efficient solution for the simultaneous detection of CNVs and SNVs.     

唐氏综合征患儿线粒体基因组突变的分析

目的 研究唐氏综合征中线粒体DNA突变情况.方法 采用高通量测序和焦磷酸测序检测7个唐氏综合征(Down's syndrome,DS)家系中的患儿和母亲的线粒体基因组序列,分析线粒体基因组序列的变化情况.结果 ①DS患儿中检测到36个与其母亲中不同的线粒体DNA突变,其中14个位点是首次在唐氏综合征样本中发现;②36个线粒体DNA突变主要发生于D-Loop区和线粒体复合物Ⅰ中;③ 线粒体基因组13个编码基因中,有11个基因检测到线粒体DNA的突变;④ 焦磷酸测序对线粒体基因组杂合突变频率的检测结果和高通量测序结果吻合.结论 DS患儿中广泛存在线粒体DNA的突变,这些突变可能与唐氏综合征的线粒体功能异常相关.     

SCOMP is superior to degenerated oligonucleotide primed-polymerase chain reaction for global amplification of minute amounts of DNA from microdissected archival tissue samples

Global genome amplification from formalin-fixed tissues is still problematic when performed with low cell numbers. Here, we tested a recently developed method for whole genome amplification termed "SCOMP" (single cell comparative genomic hybridization) on archival tissues of different ages. We show that the method is very well suited for formalin-fixed paraffin-embedded samples obtained by nuclei extraction or laser microdissection. The polymerase chain reaction (PCR) products can be used for subsequent comparative genomic hybridization, loss of heterozygosity studies, and DNA sequencing. To control for PCR-induced artifacts we amplified genomic DNA isolated from 20 nuclei of archival formalin-fixed, paraffin-embedded nonpathological lymph nodes. Subsequent comparative genomic hybridization revealed the expected balanced profiles. For loss of heterozygosity analysis by microsatellite PCR 60 to 160 cells were sufficient. In comparative experiments the approach turned out to be superior to published degenerated oligonucleotide-primed-PCR protocols. The method provides a robust and valuable tool to study very small cell samples, such as the genomes of dysplastic cells or the clonal evolution within heterogeneous tumors.     

引物延伸预扩增结合简并引物PCR在单细胞比较基因组杂交分析染色体异常中的应用

目的 建立一种可信的单细胞全基因组扩增(whole genome amplification.WGA)技术,结合比较基因组杂交(comparative genomic hybridization,CGH)分析单细胞的染色体拷贝数变化,探讨其在胚胎植入前遗传学诊断(preimplantation genetic diagnosis,PGD)中的应用前景.方法 采用引物延伸预扩增结合简并核苷酸引物PCR(primer extension preamplification with degenerate oligonucleotide primed-PCR,PEP-DOP-PCR)的方法,扩增12个已知核型的单细胞标本(包括5个绒毛标本、4个干细胞标本和3个淋巴细胞标本)和4个经PGD检测发现染色体异常的单卵裂球标本,将扩增产物标记红色荧光染料后,与标记绿色荧光染料的正常DNA等量混匀,与正常中期分裂相进行比较基因组杂交分析.同时,应用单纯的简并寡核苷酸引物-PCR(DOP-PCR)扩增10个单细胞DNA,标记后进行CGH分析.比较两种单细胞全基因组扩增方法的扩增效率及随后用于CGH分析染色体拷贝数时的准确性.结果 所有的单细胞采用PEP-DOP-PCR扩增时,均能获得稳定均匀的PCR产物,片段大小范围在100～1000 bp之间,集中分布于400 bp左右的区域,CGH分析结果显示染色体拷贝数变化与其它技术检测的结果一致.而10个单纯的DOP-PCR扩增只有6个标本成功,扩增产物进行CGH分析时,杂交信号不均匀,有2个显示与其它技术分析的结果不一致.结论 PEP-DOP-PCR技术能有效地扩增单细胞的全基因组DNA,其扩增产物可应用CGH技术成功检测单个细胞的染色体拷贝数变化,而单纯的DOP-PCR技术易于出现扩增失败、扩增产物杂交后信号不均一的缺点.PEP-DOP-PCR全基因组扩增结合CGH技术在胚胎植入前遗传学诊断中有良好的应用前景.     

Transition to Next Generation Analysis of the Whole Mitochondrial Genome: A Summary of Molecular Defects

The diagnosis of mitochondrial disorders is challenging because of the clinical variability and genetic heterogeneity. Conventional analysis of the mitochondrial genome often starts with a screening panel for common mitochondrial DNA (mtDNA) point mutations and large deletions (mtScreen). If negative, it has been traditionally followed by Sanger sequencing of the entire mitochondrial genome (mtWGS). The recently developed “Next‐Generation Sequencing” (NGS) technology offers a robust high‐throughput platform for comprehensive mtDNA analysis. Here, we summarize the results of the past 6 years of clinical practice using the mtScreen and mtWGS tests on 9,261 and 2,851 unrelated patients, respectively. A total of 344 patients (3.7%) had mutations identified by mtScreen and 99 (3.5%) had mtDNA mutations identified by mtWGS. The combinatorial analyses of mtDNA and POLG revealed a diagnostic yield of 6.7% in patients with suspected mitochondrial disorders but no recognizable syndromes. From the initial mtWGS–NGS cohort of 391 patients, 21 mutation‐positive cases (5.4%) have been identified. The mtWGS–NGS provides a one‐step approach to detect common and uncommon point mutations, as well as deletions. Additionally, NGS provides accurate, sensitive heteroplasmy measurement, and the ability to map deletion breakpoints. A new era of more efficient molecular diagnosis of mtDNA mutations has arrived.     

A Random Survey of the Cryptosporidium parvum Genome

Cryptosporidium parvum is an obligate intracellular pathogen responsible for widespread infections in humans and animals. The inability to obtain purified samples of this organism's various developmental stages has limited the understanding of the biochemical mechanisms important for C. parvum development or host-parasite interaction. To identify C. parvum genes independent of their developmental expression, a random sequence analysis of the 10.4-megabase genome of C. parvum was undertaken. Total genomic DNA was sheared by nebulization, and fragments between 800 and 1,500 bp were gel purified and cloned into a plasmid vector. A total of 442 clones were randomly selected and subjected to automated sequencing by using one or two primers flanking the cloning site. In this way, 654 genomic survey sequences (GSSs) were generated, corresponding to >320 kb of genomic sequence. These sequences were assembled into 408 contigs containing >250 kb of unique sequence, representing approximately 2.5% of the C. parvum genome. Comparison of the GSSs with sequences in the public DNA and protein databases revealed that 107 contigs (26%) displayed similarity to previously identified proteins and rRNA and tRNA genes. These included putative genes involved in the glycolytic pathway, DNA, RNA, and protein metabolism, and signal transduction pathways. The repetitive sequence elements identified included a telomere-like sequence containing hexamer repeats, 57 microsatellite-like elements composed of dinucleotide or trinucleotide repeats, and a direct repeat sequence. This study demonstrates that large-scale genomic sequencing is an efficient approach to analyze the organizational characteristics and information content of the C. parvum genome.     

Phylogenetically diverse TT virus viremia among pregnant women

Infections during pregnancy have been suggested to be involved in childhood leukemias. We used high-throughput sequencing to describe the viruses most readily detectable in serum samples of pregnant women. Serum DNA of 112 mothers to leukemic children was amplified using whole genome amplification. Sequencing identified one TT virus (TTV) isolate belonging to a known type and two putatively new TTVs. For 22 mothers, we also performed TTV amplification by general primer PCR before sequencing. This detected 39 TTVs, two of which were identical to the TTVs found after whole genome amplification.Altogether, we found 40 TTV isolates, 29 of which were putatively new types (similarities ranging from 89% to 69%). In conclusion, high throughput sequencing is useful to describe the known or unknown viruses that are present in serum samples of pregnant women.     

Xdrop: Targeted sequencing of long DNA molecules from low input samples using droplet sorting

Long‐read sequencing can resolve regions of the genome that are inaccessible to short reads, and therefore are ideal for genome‐gap closure, solving structural rearrangements and sequencing through repetitive elements. Here we introduce the Xdrop technology: a novel microfluidic‐based system that allows for targeted enrichment of long DNA molecules starting from only a few nanograms of DNA. Xdrop is based on the isolation of long DNA fragments in millions of droplets, where the droplets containing a target sequence of interest are fluorescently labeled and sorted using flow cytometry. The final product from the Xdrop procedure is an enriched population of long DNA molecules that can be investigated by sequencing. To demonstrate the capability of Xdrop, we performed enrichment of the human papilloma virus 18 integrated into the genome of human HeLa cells. Analysis of the sequencing reads resolved three HPV18‐chr8 integrations at base‐pair resolution, and the captured fragments extended up to 30 kb into the human genome at the integration sites. Further, we enriched the complete TP53 locus in a leukemia cell line and could successfully phase coexisting mutations using PacBio sequencing. In summary, our results show that Xdrop is an efficient enrichment technology for studying complex genomic regions.     

Sequencing the complete mitochondrial genome of Eimeria mitis strain USDA 50 (Apicomplexa: Eimeriidae) suggests conserved start positions for mtCOI- and mtCOIII-coding regions

Four complete mitochondrial (mt) sequences from a single-oocyst-derived line of Eimeria mitis USDA 50 were obtained (three from cloned whole-genome PCR products, one from directly sequenced whole-genome PCR product). The mt genome is 6,408 bp long with three genes (CytB, cytochrome c oxidase subunit I (COI) and cytochrome c oxidase subunit III (COIII)) and many rDNA fragments (large subunit rDNA 13, small subunit rDNA 10); organisation was identical to other Eimeria sp. mt genomes. Conserved start codon positions for both COI and COIII are suggested for all Eimeria mt genomes; these start codon positions exist and may also be conserved, in related apicomplexan parasites. Within the three separate cloned PCR products of near-complete mt genomes, there were 26 nucleotide differences (collectively) compared to the directly sequenced mt genome. These changes appear to be base misincorporations during PCR. Direct sequencing of long PCR amplification products may be more likely to generate accurate mt genomic sequences than cloning and subsequent sequencing.     

Loss of Heterozygosity Analysis Using Whole Genome Amplification, Cell Sorting, and Fluorescence-Based PCR

Loss of heterozygosity (LOH) is a common genetic lesion found in many human neoplasms. Extending investigation of LOH to large-scale clinical and public health science studies has proven difficult because of the small size and cellular and genetic heterogeneity of human neoplasms, in addition to the challenges associated with increasing throughput. Our approach to LOH analysis was developed using clinical biopsy samples from patients with Barrett's esophagus (BE) and uses flow cytometric cell sorting to increase sample purity, whole genome amplification to increase sample amount, and automated fluorescent genotyping to increase sample throughput. This approach allows LOH assessment at 20 loci in DNA extracted from 1000 flow-purified cells while maintaining accurate and reproducible allele ratios compared with the standard method of using genomic DNA. This method of analysis should allow accurate, reproducible determination of allele ratios in a variety of human tumors and premalignant conditions.     

三代测序技术在染色体变异检测中的应用初探

目的应用纳米孔三代测序技术检测人类染色体非整倍体样本,并探讨其性能及应用前景。方法使用MinION三代测序仪对从分别携带X染色体单体和7q11.23-q21.3区22.5 Mb缺失的两种人类细胞系样本中提取的DNA进行检测,并对测序结果进行数据统计和序列分析。结果两例样本在24小时内分别获得了555872和2679882条Reads,基因组覆盖度分别为53.75%和88.63%。在测序深度分别为0.81×和2.40×的条件下,通过Minimap2比对分析可以检出染色体异常区域。结论在低深度全基因组测序的条件下,使用纳米孔三代测序技术有望在24小时内完成对染色体整倍性异常样本的快速检测与分析,但进一步应用推广尚需克服成本的限制。