Sequencing genomes: from individuals to populations

Evolutionary and ecological functional genomics, from lab to the wild

Plant phenotypes are the result of both genetic and environmental forces that act to modulate trait expression. Over the last few years, numerous approaches in functional genomics and systems biology have led to a greater understanding of plant phenotypic variation and plant responses to the environment. These approaches, and the questions that they can address, have been loosely termed evolutionary and ecological functional genomics (EEFG), and have been providing key insights on how plants adapt and evolve. In particular, by bringing these studies from the laboratory to the field, EEFG studies allow us to gain greater knowledge of how plants function in their natural contexts.

New technologies for DNA analysis--a review of the READNA Project

The REvolutionary Approaches and Devices for Nucleic Acid analysis (READNA) project received funding from the European Commission for 41/2 years. The objectives of the project revolved around technological developments in nucleic acid analysis. The project partners have discovered, created and developed a huge body of insights into nucleic acid analysis, ranging from improvements and implementation of current technologies to the most promising sequencing technologies that constitute a 3(rd) and 4(th) generation of sequencing methods with nanopores and in situ sequencing, respectively.

Overview of high throughput sequencing technologies to elucidate molecular pathways in cardiovascular diseases

High throughput sequencing technologies have become essential in studies on genomics, epigenomics, and transcriptomics. Although sequencing information has traditionally been elucidated using a low throughput technique called Sanger sequencing, high throughput sequencing technologies are capable of sequencing multiple DNA molecules in parallel, enabling hundreds of millions of DNA molecules to be sequenced at a time. This advantage allows high throughput sequencing to be used to create large data sets, generating more comprehensive insights into the cellular genomic and transcriptomic signatures of various diseases and developmental stages. Within high throughput sequencing technologies, whole exome sequencing can be used to identify novel variants and other mutations that may underlie many genetic cardiac disorders, whereas RNA sequencing can be used to analyze how the transcriptome changes. Chromatin immunoprecipitation sequencing and methylation sequencing can be used to identify epigenetic changes, whereas ribosome sequencing can be used to determine which mRNA transcripts are actively being translated. In this review, we will outline the differences in various sequencing modalities and examine the main sequencing platforms on the market in terms of their relative read depths, speeds, and costs. Finally, we will discuss the development of future sequencing platforms and how these new technologies may improve on current sequencing platforms. Ultimately, these sequencing technologies will be instrumental in further delineating how the cardiovascular system develops and how perturbations in DNA and RNA can lead to cardiovascular disease.

Integrated view of genome structure and sequence of a single DNA molecule in a nanofluidic device

We show how a bird's-eye view of genomic structure can be obtained at ∼1-kb resolution from long (∼2 Mb) DNA molecules extracted from whole chromosomes in a nanofluidic laboratory-on-a-chip. We use an improved single-molecule denaturation mapping approach to detect repetitive elements and known as well as unique structural variation. Following its mapping, a molecule of interest was rescued from the chip; amplified and localized to a chromosome by FISH; and interrogated down to 1-bp resolution with a commercial sequencer, thereby reconciling haplotype-phased chromosome substructure with sequence.

Improving mapping and SNP-calling performance in multiplexed targeted next-generation sequencing

Background

Compared to classical genotyping, targeted next-generation sequencing (tNGS) can be custom-designed to interrogate entire genomic regions of interest, in order to detect novel as well as known variants. To bring down the per-sample cost, one approach is to pool barcoded NGS libraries before sample enrichment. Still, we lack a complete understanding of how this multiplexed tNGS approach and the varying performance of the ever-evolving analytical tools can affect the quality of variant discovery. Therefore, we evaluated the impact of different software tools and analytical approaches on the discovery of single nucleotide polymorphisms (SNPs) in multiplexed tNGS data. To generate our own test model, we combined a sequence capture method with NGS in three experimental stages of increasing complexity (E. coli genes, multiplexed E. coli, and multiplexed HapMap BRCA1/2 regions).

Results

We successfully enriched barcoded NGS libraries instead of genomic DNA, achieving reproducible coverage profiles (Pearson correlation coefficients of up to 0.99) across multiplexed samples, with <10% strand bias. However, the SNP calling quality was substantially affected by the choice of tools and mapping strategy. With the aim of reducing computational requirements, we compared conventional whole-genome mapping and SNP-calling with a new faster approach: target-region mapping with subsequent ‘read-backmapping’ to the whole genome to reduce the false detection rate. Consequently, we developed a combined mapping pipeline, which includes standard tools (BWA, SAMtools, etc.), and tested it on public HiSeq2000 exome data from the 1000 Genomes Project. Our pipeline saved 12 hours of run time per Hiseq2000 exome sample and detected ~5% more SNPs than the conventional whole genome approach. This suggests that more potential novel SNPs may be discovered using both approaches than with just the conventional approach.

Conclusions

We recommend applying our general ‘two-step’ mapping approach for more efficient SNP discovery in tNGS. Our study has also shown the benefit of computing inter-sample SNP-concordances and inspecting read alignments in order to attain more confident results.

Global assessment of genomic variation in cattle by genome resequencing and high-throughput genotyping

Background

Integration of genomic variation with phenotypic information is an effective approach for uncovering genotype-phenotype associations. This requires an accurate identification of the different types of variation in individual genomes.

Results

We report the integration of the whole genome sequence of a single Holstein Friesian bull with data from single nucleotide polymorphism (SNP) and comparative genomic hybridization (CGH) array technologies to determine a comprehensive spectrum of genomic variation. The performance of resequencing SNP detection was assessed by combining SNPs that were identified to be either in identity by descent (IBD) or in copy number variation (CNV) with results from SNP array genotyping. Coding insertions and deletions (indels) were found to be enriched for size in multiples of 3 and were located near the N- and C-termini of proteins. For larger indels, a combination of split-read and read-pair approaches proved to be complementary in finding different signatures. CNVs were identified on the basis of the depth of sequenced reads, and by using SNP and CGH arrays.

Conclusions

Our results provide high resolution mapping of diverse classes of genomic variation in an individual bovine genome and demonstrate that structural variation surpasses sequence variation as the main component of genomic variability. Better accuracy of SNP detection was achieved with little loss of sensitivity when algorithms that implemented mapping quality were used. IBD regions were found to be instrumental for calculating resequencing SNP accuracy, while SNP detection within CNVs tended to be less reliable. CNV discovery was affected dramatically by platform resolution and coverage biases. The combined data for this study showed that at a moderate level of sequencing coverage, an ensemble of platforms and tools can be applied together to maximize the accurate detection of sequence and structural variants.

A device for extraction, manipulation and stretching of DNA from single human chromosomes

We describe the structure and operation of a micro/nanofluidic device in which individual metaphase chromosomes can be isolated and processed without being displaced during exchange of reagents. The change in chromosome morphology as a result of introducing protease into the device was observed by time-lapse imaging; pressure-driven flow was then used to shunt the chromosomal DNA package into a nanoslit. A long linear DNA strand (>1.3 Mbp) was seen to stretch out from the DNA package and along the length of the nanoslit. Delivery of DNA in its native metaphase chromosome package as well as the microfluidic environment prevented DNA from shearing and will be important for preparing ultra-long lengths of DNA for nanofluidic analysis.

Contemporary human genetic strategies in aging research

Human aging is a complex, multifactorial process influenced by a number of genetic and non-genetic factors. This article first reviews genetic strategies for human aging research and considers the advantages and disadvantages of each. We then discuss the issue of phenotypic definition for genetic studies of aging, including longevity/life span, as well as disease-free survival and other endophenotypes. Finally, we argue that extensions of this area of research, including incorporation of gene×environment interactions, multivariate phenotypes, integration of functional genomic annotations, and exploitation of orthology - many of which are already initiated and ongoing - are critical to advancing this field.

Combining target enrichment with barcode multiplexing for high throughput SNP discovery

Background

The primary goal of genetic linkage analysis is to identify genes affecting a phenotypic trait. After localisation of the linkage region, efficient genetic dissection of the disease linked loci requires that functional variants are identified across the loci. These functional variations are difficult to detect due to extent of genetic diversity and, to date, incomplete cataloguing of the large number of variants present both within and between populations. Massively parallel sequencing platforms offer unprecedented capacity for variant discovery, however the number of samples analysed are still limited by cost per sample. Some progress has been made in reducing the cost of resequencing using either multiplexing methodologies or through the utilisation of targeted enrichment technologies which provide the ability to resequence genomic areas of interest rather that full genome sequencing.

Results

We developed a method that combines current multiplexing methodologies with a solution-based target enrichment method to further reduce the cost of resequencing where region-specific sequencing is required. Our multiplex/enrichment strategy produced high quality data with nominal reduction of sequencing depth. We undertook a genotyping study and were successful in the discovery of novel SNP alleles in all samples at uniplex, duplex and pentaplex levels.

Conclusion

Our work describes the successful combination of a targeted enrichment method and index barcode multiplexing to reduce costs, time and labour associated with processing large sample sets. Furthermore, we have shown that the sequencing depth obtained is adequate for credible SNP genotyping analysis at uniplex, duplex and pentaplex levels.

Only connect: personal genomics and the future of American medicine

Access to one's own complete genome was unheard of just a few years ago. At present we have a smattering of identifiable complete human genomes, but the coming months and years will undoubtedly bring thousands more. What will this mean for the practice of medicine in the US? No one knows, but given the remarkable drop in the cost of DNA sequencing over the last few years, it seems a safe bet that within the next decade, primary care physicians will order patients' whole genome sequences with no more fanfare than they would a complete blood count. But the challenges of transforming that easily accessible information into cost savings and better health outcomes will be daunting. Obviously, we lack interpretive abilities and phenotypic information commensurate with our skill in amassing DNA sequences. Worse, we have exacerbated these problems by failing to embrace the increasing ubiquity of genomic information, the populace's interest in it, and its relevance to virtually every medical specialty. The success of personal genomics will require a profound cultural shift by every entity with a stake in human health.