Identification of EMS-Induced Mutations in Drosophila melanogaster by Whole-Genome Sequencing

Next-generation methods for rapid whole-genome sequencing enable the identification of single-base-pair mutations in Drosophila by comparing a chromosome bearing a new mutation to the unmutagenized sequence. To validate this approach, we sought to identify the molecular lesion responsible for a recessive EMS-induced mutation affecting egg shell morphology by using Illumina next-generation sequencing. After obtaining sufficient sequence from larvae that were homozygous for either wild-type or mutant chromosomes, we obtained high-quality reads for base pairs composing ~70% of the third chromosome of both DNA samples. We verified 103 single-base-pair changes between the two chromosomes. Nine changes were nonsynonymous mutations and two were nonsense mutations. One nonsense mutation was in a gene, encore, whose mutations produce an egg shell phenotype also observed in progeny of homozygous mutant mothers. Complementation analysis revealed that the chromosome carried a new functional allele of encore, demonstrating that one round of next-generation sequencing can identify the causative lesion for a phenotype of interest. This new method of whole-genome sequencing represents great promise for mutant mapping in flies, potentially replacing conventional methods.     

美科学家首次完成一家四口全基因组测序

2010年3月10日,杂志在线发表了一篇关于一家四口(父母及其孩子)的全基因组测序的文章.通过测序,研究人员发现了影响人类自发性基因突变平均速度,以及一些与影响兄弟姐妹的疾病有关的基因.     

A Strategy for Direct Mapping and Identification of Mutations by Whole-Genome Sequencing

Mutant screens have proven powerful for genetic dissection of a myriad of biological processes, but subsequent identification and isolation of the causative mutations are usually complex and time consuming. We have made the process easier by establishing a novel strategy that employs whole-genome sequencing to simultaneously map and identify mutations without the need for any prior genetic mapping.THE challenges posed by the identification of a causal mutation in a mutant of interest have in effect restricted the use of forward genetics to those organisms benefiting from a solid genetic toolbox. Whole-genome sequencing (WGS) is promising to revolutionize the way phenotypic traits are assigned to genes. However, current strategies to identify causal mutations using WGS require first the identification of an approximate genomic location containing the mutation of interest (Sarin et al. 2008; Smith et al. 2008; Srivatsan et al. 2008; Blumenstiel et al. 2009; Irvine et al. 2009). This is because genomes contain many natural sequence variations (Denver et al. 2004; Hillier et al. 2008; Sarin et al. 2010), which, along with mutagen-induced ones, complicate the identification of the causal mutation when an approximate genomic location has not been previously identified. Mapping has previously been achieved with time-consuming and laborious techniques that, in addition, rely on an organism''s single-nucleotide polymorphism (SNP) map and established variant strains. For example, traditional SNP-based mapping (Wicks et al. 2001; Davis et al. 2005) has previously been used in Caenorhabditis elegans to narrow down the genomic region containing the mutation of interest, prior to conducting WGS (Sarin et al. 2008). In Arabidopsis, simultaneous SNP mapping and mutation identification has been achieved with WGS, but this requires the generation of a mapping population of up to 500 F₂ progeny to identify only one allele (Schneeberger et al. 2009). This is a challenging prospect for many model systems. Indeed, if the mutant phenotype is subtle, the isolation of such numbers of recombinants is very tedious. Furthermore, it is not applicable in those organisms where a mapping population cannot be generated, simply because of a lack of intercrossable variants or because of life cycles (parasitic organisms, for example) that would make it extremely difficult to follow and isolate many recombinant individuals.Here, we describe a strategy to simultaneously and rapidly locate and identify multiple mutations from a mutagenesis screen with WGS that circumvents these limitations. This powerful and straightforward method directly uses mutagen-induced nucleotide changes that are linked to the causal mutation to identify its specific genomic location, thus negating the construction of genetic mapping populations and subsequent mapping.Treatment of organisms with a chemical mutagen induces nucleotide changes throughout the genome. Following mutagenesis, backcrossing or outcrossing of the mutagenized organism to unmutagenized counterparts is performed to eliminate mutagen-induced mutations (Figure 1A; supporting information, File S2). The phenotype-causing mutation remains as only backcrossed individuals showing the phenotype of interest are retained. In addition, mutagen-induced nucleotide changes that are genetically linked to the causal mutation and physically surround it on the chromosome will remain, in contrast to unlinked nucleotide changes (Figure 1A). As a result of this genetic linkage, a high-density cluster of typical mutagen-induced variants is visualized from sequence data obtained by WGS, which is positioned around the causal mutation. By locating such high-density regions, one maps the approximate genomic location of the causal mutation and subsequently identifies the affected gene within this region.Open in a separate windowFigure 1.—Mapping mutations on the basis of density of mutagen-induced DNA damage across the genome. (A) Visual representation of our WGS cloning strategy. Mutagen treatment induces point mutations throughout the genome (red asterisks). Backcrossing to the original unmutated parent strain removes much of the mutagen-induced nucleotide changes except for the causal mutation (green asterisk) and those genetically linked to it. WGS sequencing can be used to detect canonical mutagen-induced point mutations, thus revealing a physical position for the causal mutation. Shared background variants (yellow crosses) are filtered out from WGS data by comparing the sequences of mutants sequenced side-by-side, revealing a high-density variant cluster in only one genomic region. Importantly, genomic sequences of mutants derived from the same starting strain must be compared, to allow subtraction of nucleotide variants that are common to this particular strain, through sequence comparison. (B) Physical map of total nucleotide variations per megabase across the genome compared to the wild-type reference genome for each mutant (fp6, fp9, and fp12) after WGS. (C) After sequence quality filtering, subtraction of common variants between the 3 mutants, and filtering out noncanonical EMS nucleotide changes, high-density variant peaks are obtained in one genomic location for each mutant (red boxes). Steps 1 and 3 are essential for clear visualization of the high-density peaks whereas step 2 improves visualization. (D) Close-up of variants on chromosome III for fp6. Within this peak we identified only 6 candidate mutations that could potentially affect a protein sequence. We confirmed that the missense mutation in egl-5 was the causal mutation (Figure S2). For fp9 and fp12 we identified only 10 (9 missense and 1 3′-UTR) and 4 (2 premature stop and 2 missense) candidate mutations, respectively, within each mutant''s EMS-based mapped region. Thus, our method consistently allowed precise mapping in 3 different mutants to a region small enough to contain only a handful of candidate mutations.As a proof-of-principle, we simultaneously mapped and sequenced the causal mutations of multiple C. elegans mutants isolated from an EMS mutagenesis screen using this strategy. The mutagenesis screen itself was undertaken to identify genes that controlled the reprogramming of a single cell called Y into another cell called PDA during C. elegans development (Jarriault et al. 2008). After EMS treatment, three distinct mutant alleles (fp6, fp9, and fp12) were backcrossed to the original unmutagenized strain 4-6X. It is important to note that a backcrossing or outcrossing step is necessary for the analysis of mutants obtained from all mutagenesis screens, irrespective of the type of mutant identification strategy used or the type of mutagen or organism used (and, as such, does not represent an extra step introduced by our method). The mutants then underwent WGS side-by-side (Table S1, Table S2, Figure S1, and File S2). After alignment to the wild-type N2 reference genome using MAQgene software (Bigelow et al. 2009), the sequencing data obtained for each mutant were compared, and we subtracted common nucleotide variants that were shared between at least two of our three mutants (File S1). These shared variants, which are very unlikely to be either the causal mutation or EMS-induced mutations from the screen itself, represent strain differences between the N2 used to generate the reference genome and the PS3662 strain used here for mutagenesis. Note that this step eliminated ∼2000 point mutations as potential candidates for our causal mutation. This result strongly emphasizes the advantage of conducting WGS on two or more mutants side-by-side, as reference genomes may contain many nucleotide variations when compared to organisms sequenced from the laboratory (Denver et al. 2004; Hillier et al. 2008; Sarin et al. 2010; this study) and as such would confound mutation identification.To identify EMS-induced changes linked to the causal mutation and expose its location, we looked only at variants that matched the canonical EMS-induced G/C > A/T transitions (Drake and Baltz 1976), revealing localized peaks of high-density variation on a single chromosome for each mutant (Figure 1, B and C). These peaks correspond to regions of high mutagen-induced damage that were not removed during backcrossing and therefore are most likely genetically linked to the causal mutation. We therefore focused our attention on these physical regions to identify candidate mutations within them. We localized fp6 to a 4.29-Mb region on chromosome III, fp9 to a 7.11-Mb region on chromosome X, and fp12 to a 1.28-Mb region on a different part of chromosome X (Figure 1C).As a proof of principle, we further examined the nucleotide changes present in the interval to which fp6 was linked. Taking into consideration all variant types (point mutations and indels), we identified only six candidate mutations that potentially affected a gene''s function (Figure 1D and Table S3). One of these, affecting the egl-5/hox gene, lies almost perfectly in the middle of the predicted EMS-based mapped region. We confirmed the existence of the mutation in egl-5 by manual resequencing. Both egl-5 targeted RNAi and noncomplementation with the egl-5(n945) null allele confirmed that fp6 affected egl-5 and caused the Y-to-PDA reprogramming defect (Figure S2). fp9 and fp12 each map to distinct regions on chromosome X that also contain only a handful of candidate mutations (10 and 4, respectively) (Figure 1C). Thus, our method consistently allowed precise mapping in 3 different mutants to a region small enough to contain only a handful of candidate mutations and subsequent identification of the causal mutation.We calculated that comparison of WGS data for only two mutants of the same mutagenesis screen is sufficient to localize and sequence the causal mutation (Table S4). Thirteen times sequence coverage has been found to be sufficient to identify a mutation in a pre-SNP mapped C. elegans mutant (Shen et al. 2008). Here, we tested the sequence coverage necessary to perform simultaneous mapping and mutant identification using our strategy and found that 13× was more than enough (Table S4). In addition, by performing longer reads and/or paired-end sequencing, our method can be scaled up to bigger genomes or allow multiple mutant sequencing on each flow cell lane [for, e.g., using multiplex WGS (Cronn et al. 2008)]. Furthermore, because direct sequence comparison is ultimately made between two mutants sequenced side-by-side, the quality of an organism''s reference genome (which is used only for alignment purposes) does not have a bearing on the mapping or mutant identification outcome. Moreover, recent advances in de novo alignment of short reads generated from next generation sequencing platforms (Li et al. 2010; Nowrousian et al. 2010; Webb and Rosenthal 2010; Young et al. 2010) suggest that a reference genome may not even be required to perform mutagen-based mapping and mutant identification with WGS. We predict that technical advances in these areas will make it possible to perform mutagenesis screens on any nonsequenced and genetically uncharacterized organism and use our strategy to quickly identify the causal mutation of an interesting mutant.

TABLE 1

Summary of WGS cloning strategy

Comprehensive Characterization of Human Genome Variation by High Coverage Whole-Genome Sequencing of Forty Four Caucasians

Whole genome sequencing studies are essential to obtain a comprehensive understanding of the vast pattern of human genomic variations. Here we report the results of a high-coverage whole genome sequencing study for 44 unrelated healthy Caucasian adults, each sequenced to over 50-fold coverage (averaging 65.8×). We identified approximately 11 million single nucleotide polymorphisms (SNPs), 2.8 million short insertions and deletions, and over 500,000 block substitutions. We showed that, although previous studies, including the 1000 Genomes Project Phase 1 study, have catalogued the vast majority of common SNPs, many of the low-frequency and rare variants remain undiscovered. For instance, approximately 1.4 million SNPs and 1.3 million short indels that we found were novel to both the dbSNP and the 1000 Genomes Project Phase 1 data sets, and the majority of which (∼96%) have a minor allele frequency less than 5%. On average, each individual genome carried ∼3.3 million SNPs and ∼492,000 indels/block substitutions, including approximately 179 variants that were predicted to cause loss of function of the gene products. Moreover, each individual genome carried an average of 44 such loss-of-function variants in a homozygous state, which would completely “knock out” the corresponding genes. Across all the 44 genomes, a total of 182 genes were “knocked-out” in at least one individual genome, among which 46 genes were “knocked out” in over 30% of our samples, suggesting that a number of genes are commonly “knocked-out” in general populations. Gene ontology analysis suggested that these commonly “knocked-out” genes are enriched in biological process related to antigen processing and immune response. Our results contribute towards a comprehensive characterization of human genomic variation, especially for less-common and rare variants, and provide an invaluable resource for future genetic studies of human variation and diseases.     

Soluble Components of Adenovirus Type 8

Soluble components of type 8 adenovirus have been studied. Four different components were isolated by anion-exchange chromatography and purified by further chromatographic procedures, by zonal centrifugation, and by erythrocyte absorption and elution. The four components exhibited the following characteristics. (i) Fiber antigen was trypsin-resistant and functioned as incomplete hemagglutinin (agglutinated rat and human erythrocytes only in the presence of certain types of adenovirus antisera). (ii) The penton was trypsin-sensitive, exerted a cytotoxic effect, and also showed incomplete hemagglutination, being active in the presence of a majority of heterotypic adenovirus antisera studied. (iii) The group-specific hexon antigen reacted in complement fixation reaction and gel precipitation with sera prepared against other types of adenoviruses, besides showing characteristics indicating the presence of a type-specific antigen component. (iv) The soluble complete hemagglutinin was trypsin-sensitive, displayed cytotoxic effect, adsorbed easily to human and rat erythrocytes, and could be eluted from them by means of receptor-destroying enzyme. The three hemagglutinins of adenovirus type 8 proved to be highly unstable, and their demonstration was only successful by using a large quantity of freshly prepared concentrated virus material. Considering these conditions, a method was developed for their concentration and purification.     

Two Novel Toxin Variants Revealed by Whole-Genome Sequencing of 175 Clostridium botulinum Type E Strains

We sequenced 175 Clostridium botulinum type E strains isolated from food, clinical, and environmental sources from northern Canada and analyzed their botulinum neurotoxin (bont) coding sequences (CDSs). In addition to bont/E1 and bont/E3 variant types, neurotoxin sequence analysis identified two novel BoNT type E variants termed E10 and E11. Strains producing type E10 were found along the eastern coastlines of Hudson Bay and the shores of Ungava Bay, while strains producing type E11 were only found in the Koksoak River region of Nunavik. Strains producing BoNT/E3 were widespread throughout northern Canada, with the exception of the coast of eastern Hudson Bay.     

Defective Virions in Human Adenovirus Type 12

Purified preparations of human adenovirus type 12 showed two bands when subjected to isopycnic centrifugation in a density gradient of cesium chloride. Their density difference was about 0.003 g/ml, suggesting a small difference in their deoxyribonucleic acid to protein ratio. Virions with a lighter density can kill human KB cells and induce T antigen as efficiently as the heavy virions. However, they appeared incapable to form plaques. Two passages of the heavy infectious virions at low multiplicity of infection did not produce significant amounts of light virions; however, when it was passed at high multiplicity of infection, the light band became visible in a cesium chloride density gradient.     

解磷菌PSB-R全基因组测序鉴定及其解磷特性分析

采用生理生化指标结合分子生物学技术对解磷菌PSB-R进行分类学鉴定,确定为粘质沙雷氏菌（Serratia marcescens）,通过二代测序平台Illumina NovaSeq PE150对PSB-R进行全基因组测序,分析预测了与解磷能力相关基因及其他植物促生基因组成情况。通过响应面优化试验检测了PSB-R最大解磷能力为805.199 mg/L,连续培养10代后解磷能力稳定且对多种难溶性磷酸盐均具有溶解能力。本研究为解磷菌解磷机制的进一步研究提供了基因组数据基础,同时证实PSB-R具有应用于菌肥的潜力,为后续解磷菌肥的研制提供了研究基础。     

Whole-Genome Shotgun Sequencing of a Colonizing Multilocus Sequence Type 17 Streptococcus agalactiae Strain

This report highlights the whole-genome shotgun draft sequence for a Streptococcus agalactiae strain representing multilocus sequence type (ST) 17, isolated from a colonized woman at 8 weeks postpartum. This sequence represents an important addition to the published genomes and will promote comparative genomic studies of S. agalactiae recovered from diverse sources.     

Analysis of Multiple Ethyl Methanesulfonate-Mutagenized Caenorhabditis elegans Strains by Whole-Genome Sequencing

Whole-genome sequencing (WGS) of organisms displaying a specific mutant phenotype is a powerful approach to identify the genetic determinants of a plethora of biological processes. We have previously validated the feasibility of this approach by identifying a point-mutated locus responsible for a specific phenotype, observed in an ethyl methanesulfonate (EMS)-mutagenized Caenorhabditis elegans strain. Here we describe the genome-wide mutational profile of 17 EMS-mutagenized genomes as assessed with a bioinformatic pipeline, called MAQGene. Surprisingly, we find that while outcrossing mutagenized strains does reduce the total number of mutations, a striking mutational load is still observed even in outcrossed strains. Such genetic complexity has to be taken into account when establishing a causative relationship between genotype and phenotype. Even though unintentional, the 17 sequenced strains described here provide a resource of allelic variants in almost 1000 genes, including 62 premature stop codons, which represent candidate knockout alleles that will be of further use for the C. elegans community to study gene function.INDUCING molecular lesions in a genome is an effective approach to interrogate the genome for its functional elements. Molecular lesions can be induced using a variety of methods. Because of their efficiency and their ability to generate alleles with various different alterations in gene activity (e.g., amorphic, antimorphic, hypomorphic, and hypermorphic), chemical mutagens, such as ethyl methanesulfonate (EMS), are frequently used in genetic mutant screens (Anderson 1995). However, due to mutagen efficiency, a mutant animal selected for a single-locus phenotype invariably contains EMS-induced “background mutations” in its genome. Experimenters try to minimize the potential impact of background mutations through outcrossing to animals with a wild-type genome. Yet no full snapshots of genome sequences right after EMS mutagenesis and after outcrossing have so far been provided to illustrate the extent of background mutations and the extent to which they can indeed be eliminated.Another caveat of using base-changing chemical mutagens is the relative difficulty associated with identifying the phenotype-causing molecular lesion. In multicellular genetic model organisms, mutant identification involves time-consuming positional cloning approaches, usually involving breeding with genetically marked strains that allow pinpointing of the location of a molecular lesion. Even with rapid, SNP-based mapping approaches in animals with short generation times, such as Caenorhabditis elegans, substantial time hurdles, particularly in the final, fine-mapping stages, still exist. Conceptually similar problems in defining the location of a molecular lesion are encountered by human geneticists who attempt to identify disease-causing genetic lesions.Whole-genome sequencing (WGS) is beginning to emerge as an efficient and cost-effective tool to shortcut time-consuming mapping and positional cloning efforts (Hobert 2010). The sequencing of an entire genome and its ensuing comparison to a wild-type reference genome can potentially directly pinpoint the molecular lesion that results in the mutant phenotype the animal has been selected for. Proof-of-concept studies in bacteria, yeast, plants, worms, and flies have validated the applicability of this approach (Sarin et al. 2008; Smith et al. 2008; Srivatsan et al. 2008; Blumenstiel et al. 2009; Irvine et al. 2009; Flowers et al. 2010).Present-day deep sequencing platforms used for WGS generate relatively short sequence reads, thereby posing the bioinformatic challenge to align those reads to a reference genome. We previously described a software pipeline, MAQGene, which is based on the standard alignment program MAQ (Li et al. 2008) and facilitates this bioinformatic step by providing the end user with an extensively curated list of sequence variants from a WGS run of a mutated genome compared to a reference genome (Bigelow et al. 2009). This pipeline can be used for well-annotated, assembled genomes, such as C. elegans or Drosophila. In this article, we describe that this pipeline can identify not only point mutations but also deletions. We then use this pipeline to analyze a total of 17 EMS-mutagenized genomes. We find that EMS-mutagenized genomes carry a significant mutational load including presumptive loss-of-function alleles in several protein-coding genes that can lead to synthetic genetic interactions, one of which we describe here in more detail. We show that outcrossing to wild-type animals can lighten the mutational load; however, a substantial number of sequence variants are also introduced during outcrossing. Even though background mutations uncovered by WGS may complicate the interpretation of mutant phenotypes, they do provide a potentially useful source for functional studies of the affected genes.