Haplotype-resolved diverse human genomes and integrated analysis of structural variation

Mining the gaps of chromosome 8

The first gapless, telomere-to-telomere sequence of a human autosome, chromosome 8, is complete. Sequencing and assembly of the corresponding centromere in the chimpanzee, orangutan and macaque reveals details of its rapid evolution over the past 25 million years.

Expectations and blind spots for structural variation detection from long-read assemblies and short-read genome sequencing technologies

Virtually all genome sequencing efforts in national biobanks, complex and Mendelian disease programs, and medical genetic initiatives are reliant upon short-read whole-genome sequencing (srWGS), which presents challenges for the detection of structural variants (SVs) relative to emerging long-read WGS (lrWGS) technologies. Given this ubiquity of srWGS in large-scale genomics initiatives, we sought to establish expectations for routine SV detection from this data type by comparison with lrWGS assembly, as well as to quantify the genomic properties and added value of SVs uniquely accessible to each technology. Analyses from the Human Genome Structural Variation Consortium (HGSVC) of three families captured ~11,000 SVs per genome from srWGS and ~25,000 SVs per genome from lrWGS assembly. Detection power and precision for SV discovery varied dramatically by genomic context and variant class: 9.7% of the current GRCh38 reference is defined by segmental duplication (SD) and simple repeat (SR), yet 91.4% of deletions that were specifically discovered by lrWGS localized to these regions. Across the remaining 90.3% of reference sequence, we observed extremely high (93.8%) concordance between technologies for deletions in these datasets. In contrast, lrWGS was superior for detection of insertions across all genomic contexts. Given that non-SD/SR sequences encompass 95.9% of currently annotated disease-associated exons, improved sensitivity from lrWGS to discover novel pathogenic deletions in these currently interpretable genomic regions is likely to be incremental. However, these analyses highlight the considerable added value of assembly-based lrWGS to create new catalogs of insertions and transposable elements, as well as disease-associated repeat expansions in genomic sequences that were previously recalcitrant to routine assessment.

Genomic Tackling of Human Satellite DNA: Breaking Barriers through Time

(Peri)centromeric repetitive sequences and, more specifically, satellite DNA (satDNA) sequences, constitute a major human genomic component. SatDNA sequences can vary on a large number of features, including nucleotide composition, complexity, and abundance. Several satDNA families have been identified and characterized in the human genome through time, albeit at different speeds. Human satDNA families present a high degree of sub-variability, leading to the definition of various subfamilies with different organization and clustered localization. Evolution of satDNA analysis has enabled the progressive characterization of satDNA features. Despite recent advances in the sequencing of centromeric arrays, comprehensive genomic studies to assess their variability are still required to provide accurate and proportional representation of satDNA (peri)centromeric/acrocentric short arm sequences. Approaches combining multiple techniques have been successfully applied and seem to be the path to follow for generating integrated knowledge in the promising field of human satDNA biology.

Single-molecule optical genome mapping in nanochannels: multidisciplinarity at the nanoscale

The human genome contains multiple layers of information that extend beyond the genetic sequence. In fact, identical genetics do not necessarily yield identical phenotypes as evident for the case of two different cell types in the human body. The great variation in structure and function displayed by cells with identical genetic background is attributed to additional genomic information content. This includes large-scale genetic aberrations, as well as diverse epigenetic patterns that are crucial for regulating specific cell functions. These genetic and epigenetic patterns operate in concert in order to maintain specific cellular functions in health and disease. Single-molecule optical genome mapping is a high-throughput genome analysis method that is based on imaging long chromosomal fragments stretched in nanochannel arrays. The access to long DNA molecules coupled with fluorescent tagging of various genomic information presents a unique opportunity to study genetic and epigenetic patterns in the genome at a single-molecule level over large genomic distances. Optical mapping entwines synergistically chemical, physical, and computational advancements, to uncover invaluable biological insights, inaccessible by sequencing technologies. Here we describe the method's basic principles of operation, and review the various available mechanisms to fluorescently tag genomic information. We present some of the recent biological and clinical impact enabled by optical mapping and present recent approaches for increasing the method's resolution and accuracy. Finally, we discuss how multiple layers of genomic information may be mapped simultaneously on the same DNA molecule, thus paving the way for characterizing multiple genomic observables on individual DNA molecules.

ASHLEYS: automated quality control for single-cell Strand-seq data

Summary

Single-cell DNA template strand sequencing (Strand-seq) enables chromosome length haplotype phasing, construction of phased assemblies, mapping sister-chromatid exchange events and structural variant discovery. The initial quality control of potentially thousands of single-cell libraries is still done manually by domain experts. ASHLEYS automates this tedious task, delivers near-expert performance and labels even large datasets in seconds.

Availability and implementation

github.com/friendsofstrandseq/ashleys-qc, MIT license.

Supplementary information

Supplementary data are available at Bioinformatics online.

Opportunities and challenges for high-quality biodiversity tissue archives in the age of long-read sequencing

The technological ability to characterize genetic variation at a genome-wide scale provides an unprecedented opportunity to study the genetic underpinnings and evolutionary mechanisms that promote and sustain biodiversity. The transition from short- to long-read sequencing is particularly promising and allows a more holistic view on any changes in genetic diversity across time and space. Long-read sequencing has tremendous potential but sequencing success strongly depends on the long-range integrity of DNA molecules and therefore on the availability of high-quality tissue samples. With the scope of genomic experiments expanding and wild populations simultaneously disappearing at an unprecedented rate, access to high-quality samples may soon be a major concern for many projects. The need for high-quality biodiversity tissue archives is therefore urgent but sampling and preserving high-quality samples is not a trivial exercise. In this review, I will briefly outline how long-read sequencing can benefit the study of molecular ecology, how this will substantially increase the demand for high-quality tissues and why it is challenging to preserve DNA integrity. I will then provide an overview of preservation approaches and end with a call for support to acknowledge the efforts needed to assemble high-quality tissue archives. In doing so, I hope to simultaneously motivate field biologists to expand sampling practices and molecular biologists to develop (cost) efficient guidelines for the sampling and long-term storage of tissues. A concerted, interdisciplinary, effort is needed to catalogue the genetic variation underlying contemporary biodiversity and will eventually provide a critical resource for future studies.

The structure, function and evolution of a complete human chromosome 8

The complete assembly of each human chromosome is essential for understanding human biology and evolution^1,2. Here we use complementary long-read sequencing technologies to complete the linear assembly of human chromosome 8. Our assembly resolves the sequence of five previously long-standing gaps, including a 2.08-Mb centromeric α-satellite array, a 644-kb copy number polymorphism in the β-defensin gene cluster that is important for disease risk, and an 863-kb variable number tandem repeat at chromosome 8q21.2 that can function as a neocentromere. We show that the centromeric α-satellite array is generally methylated except for a 73-kb hypomethylated region of diverse higher-order α-satellites enriched with CENP-A nucleosomes, consistent with the location of the kinetochore. In addition, we confirm the overall organization and methylation pattern of the centromere in a diploid human genome. Using a dual long-read sequencing approach, we complete high-quality draft assemblies of the orthologous centromere from chromosome 8 in chimpanzee, orangutan and macaque to reconstruct its evolutionary history. Comparative and phylogenetic analyses show that the higher-order α-satellite structure evolved in the great ape ancestor with a layered symmetry, in which more ancient higher-order repeats locate peripherally to monomeric α-satellites. We estimate that the mutation rate of centromeric satellite DNA is accelerated by more than 2.2-fold compared to the unique portions of the genome, and this acceleration extends into the flanking sequence.