introgressions

Transposable elements (TEs) are a major force in the evolution of plant genomes. Differences in the transposition activities and landscapes of TEs can vary substantially, even in closely related species. Interspecific hybridization, a widely employed technique in tomato breeding, results in the creation of novel combinations of TEs from distinct species. The implications of this process for TE transposition activity have not been studied in modern cultivars. In this study, we used nanopore sequencing of extrachromosomal circular DNA (eccDNA) and identified two highly active Ty1/Copia LTR retrotransposon families of tomato (Solanum lycopersicum), called Salsa and Ketchup. Elements of these families produce thousands of eccDNAs under controlled conditions and epigenetic stress. EccDNA sequence analysis revealed that the major parts of eccDNA produced by Ketchup and Salsa exhibited low similarity to the S. lycopersicum genomic sequence. To trace the origin of these TEs, whole-genome nanopore sequencing and de novo genome assembly were performed. We found that these TEs occurred in a tomato breeding line via interspecific introgression from S. peruvianum. Our findings collectively show that interspecific introgressions can contribute to both genetic and phenotypic diversity not only by introducing novel genetic variants, but also by importing active transposable elements from other species.

Identification and characterization of repeat expansions in neurological disorders: Methodologies, tools, and strategies

Tandem repeats are a common, highly polymorphic class of variation in human genomes. Their expansion beyond a pathogenic threshold is a process that contributes to a wide range of neurological and neuromuscular genetic disorders, of which over 60 have been identified to date. The last few years have seen a resurgence in repeat expansion discovery propelled by technological advancements, enabling the identification of over 20 novel repeat expansion disorders. These expansions can occur in coding or non-coding regions of genes, resulting in a range of pathogenic mechanisms. In this article, we review strategies, tools and methods that can be used for efficient detection and characterization of known repeat expansions and identification of new expansion disorders. Features that can be used to prioritize repeat expansions include anticipation, which is characterized by increased severity or earlier onset of symptoms across generations, and founder effects, which contribute to higher prevalence rates in certain populations. Classical technologies such as Southern blotting, repeat-primed polymerase chain reaction (PCR) and long-range PCR can still be used to detect known repeat expansions, although they usually have significant limitations linked to the absence of sequence context. Targeted sequencing of known expansions using either long-range PCR or CRISPR-Cas9 enrichment combined with long-read sequencing or adaptive nanopore sampling are usually better but more expensive alternatives. The development of new bioinformatics tools applied to short-read genome data can now be used to detect repeat expansions either in a targeted manner or at the genome-wide level. In addition, technological advances, particularly long-read technologies such as optical genome mapping (Bionano Genomics), Oxford Nanopore Technologies (ONT) and Pacific Biosciences (PacBio) HiFi sequencing, offer promising avenues for the detection of repeat expansions. Despite challenges in specific DNA extraction requirements, computation resources needed and data interpretation, these technologies have an immense potential to advance our understanding of repeat expansion disorders and improve diagnostic accuracy.

Targeted long-read sequencing for comprehensive detection of CYP21A2 mutations in patients with 21-hydroxylase deficiency

BACKGROUND

21-Hydroxylase deficiency (21-OHD) is caused by pathogenic CYP21A2 variations. CYP21A2 is arranged in tandem with its highly homologous pseudogene CYP21A1P; therefore, it is prone to mismatch and rearrangement, producing different types of complex variations. There were few reports on using only one method to detect different CYP21A2 variants simultaneously.

AIMS

Targeted long-read sequencing method was used to detect all types of CYP21A2 variants in a series of patients with 21-OHD.

METHODS

A total of 59 patients with 21-OHD were enrolled from Peking Union Medical College Hospital. Long-range locus-specific PCR and long-read sequencing (LRS) were performed to detect the pathogenic variants in CYP21A2.

RESULTS

Copy-number variants of CYP21A2 were found in 25.4% of patients, including 5.1% with 3 copies of CYP21A2, 16.9% with 1 copy of CYP21A2, and 3.4% with 0 copy of CYP21A2. The remaining 74.6% of patients had 2 copies of CYP21A2. Pathogenic variants were identified in all 121 alleles of 59 patients. Specifically, single-nucleotide variants and small insertions/deletions (< 50 bp) were detected in 79 alleles, of which conversed from CYP21A1P were detected in 63 alleles, and rare variants were found in the other 16 alleles. Large gene conversions (> 50 bp) from pseudogene were detected in 10 alleles, and different chimeric genes (CYP21A1P/CYP21A2 or TNXA/TNXB) formed by large deletions were detected in 32 alleles. Of all variants, p.I173N was the most common variant (19.0%).

CONCLUSIONS

Our study demonstrated that targeted long-read sequencing is a comprehensive method for detecting CYP21A2 variations, which is helpful for genetic diagnosis in 21-OHD patients.

The genetic landscape and phenotypic spectrum of GAA-FGF14 ataxia in China: a large cohort study

BACKGROUND

An intronic GAA repeat expansion in FGF14 was recently identified as a cause of GAA-FGF14 ataxia. We aimed to characterise the frequency and phenotypic profile of GAA-FGF14 ataxia in a large Chinese ataxia cohort.

METHODS

A total of 1216 patients that included 399 typical late-onset cerebellar ataxia (LOCA), 290 early-onset cerebellar ataxia (EOCA), and 527 multiple system atrophy with predominant cerebellar ataxia (MSA-c) were enrolled. Long-range and repeat-primed PCR were performed to screen for GAA expansions in FGF14. Targeted long-read and whole-genome sequencing were performed to determine repeat size and sequence configuration. A multi-modal study including clinical assessment, MRI, and neurofilament light chain was conducted for disease assessment.

FINDINGS

17 GAA-FGF14 positive patients with a (GAA)≥250 expansion (12 patients with a GAA-pure expansion, five patients with a (GAA)≥250-[(GAA)n (GCA)m]z expansion) and two possible patients with biallelic (GAA)202/222 alleles were identified. The clinical phenotypes of the 19 positive and possible positive cases covered LOCA phenotype, EOCA phenotype and MSA-c phenotype. Five of six patients with EOCA phenotype were found to have another genetic disorder. The NfL levels of patients with EOCA and MSA-c phenotypes were significantly higher than patients with LOCA phenotype and age-matched controls (p < 0.001). NfL levels of pre-ataxic GAA-FGF14 positive individuals were lower than pre-ataxic SCA3 (p < 0.001) and similar to controls.

INTERPRETATION

The frequency of GAA-FGF14 expansion in a large Chinese LOCA cohort was low (1.3%). Biallelic (GAA)202/222 alleles and co-occurrence with other acquired or hereditary diseases may contribute to phenotypic variation and different progression.

FUNDING

This study was funded by the National Key R&D Program of China (2021YFA0805200 to H.J.), the National Natural Science Foundation of China (81974176 and 82171254 to H.J.; 82371272 to Z.C.; 82301628 to L.W.; 82301438 to Z.L.; 82201411 to L.H.), the Innovation Research Group Project of Natural Science Foundation of Hunan Province (2020JJ1008 to H.J.), the Key Research and Development Program of Hunan Province (2020SK2064 to H.J.), the Innovative Research and Development Program of Development and Reform Commission of Hunan Province to H.J., the Natural Science Foundation of Hunan Province (2024JJ3050 to H.J.; 2022JJ20094 and 2021JJ40974 to Z.C.; 2022JJ40783 to L.H.; 2022JJ40703 to Z.L.), the Project Program of National Clinical Research Center for Geriatric Disorders (Xiangya Hospital, 2020LNJJ12 to H.J.), the Central South University Research Programme of Advanced Interdisciplinary Study (2023QYJC010 to H.J.) and the Science and Technology Innovation Program of Hunan Province (2022RC1027 to Z.C.). D.P. holds a Fellowship award from the Canadian Institutes of Health Research (CIHR).

Regulation and function of transposable elements in cancer genomes

Over half of human genomic DNA is composed of repetitive sequences generated throughout evolution by prolific mobile genetic parasites called transposable elements (TEs). Long disregarded as "junk" or "selfish" DNA, TEs are increasingly recognized as formative elements in genome evolution, wired intimately into the structure and function of the human genome. Advances in sequencing technologies and computational methods have ushered in an era of unprecedented insight into how TE activity impacts human biology in health and disease. Here we discuss the current views on how TEs have shaped the regulatory landscape of the human genome, how TE activity is implicated in human cancers, and how recent findings motivate novel strategies to leverage TE activity for improved cancer therapy. Given the crucial role of methodological advances in TE biology, we pair our conceptual discussions with an in-depth review of the inherent technical challenges in studying repeats, specifically related to structural variation, expression analyses, and chromatin regulation. Lastly, we provide a catalog of existing and emerging assays and bioinformatic software that altogether are enabling the most sophisticated and comprehensive investigations yet into the regulation and function of interspersed repeats in cancer genomes.

Challenges in the discovery of tumor-specific alternative splicing-derived cell-surface antigens in glioma

Despite advancements in cancer immunotherapy, solid tumors remain formidable challenges. In glioma, profound inter- and intra-tumoral heterogeneity of antigen landscape hampers therapeutic development. Therefore, it is critical to consider alternative sources to expand the repertoire of targetable (neo-)antigens and improve therapeutic outcomes. Accumulating evidence suggests that tumor-specific alternative splicing (AS) could be an untapped reservoir of antigens. In this study, we investigated tumor-specific AS events in glioma, focusing on those predicted to generate major histocompatibility complex (MHC)-presentation-independent, cell-surface antigens that could be targeted by antibodies and chimeric antigen receptor-T cells. We systematically analyzed bulk RNA-sequencing datasets comparing 429 tumor samples (from The Cancer Genome Atlas) and 9166 normal tissue samples (from the Genotype-Tissue Expression project), and identified 13 AS events in 7 genes predicted to be expressed in more than 10% of the patients, including PTPRZ1 and BCAN, which were corroborated by an external RNA-sequencing dataset. Subsequently, we validated our predictions and elucidated the complexity of the isoforms using full-length transcript amplicon sequencing on patient-derived glioblastoma cells. However, analyses of the RNA-sequencing datasets of spatially mapped and longitudinally collected clinical tumor samples unveiled remarkable spatiotemporal heterogeneity of the candidate AS events. Furthermore, proteomics analysis did not reveal any peptide spectra matching the putative antigens. Our investigation illustrated the diverse characteristics of the tumor-specific AS events and the challenges of antigen exploration due to their notable spatiotemporal heterogeneity and elusive nature at the protein levels. Redirecting future efforts toward intracellular, MHC-presented antigens could offer a more viable avenue.

Development and proof-of-concept demonstration of a clinical metagenomics method for the rapid detection of bloodstream infection

BACKGROUND

The timely and accurate diagnosis of bloodstream infection (BSI) is critical for patient management. With longstanding challenges for routine blood culture, metagenomics is a promising approach to rapidly provide sequence-based detection and characterisation of bloodborne bacteria. Long-read sequencing technologies have successfully supported the use of clinical metagenomics for syndromes such as respiratory illness, and modified approaches may address two requisite factors for metagenomics to be used as a BSI diagnostic: depletion of the high level of host DNA to then detect the low abundance of microbes in blood.

METHODS

Blood samples from healthy donors were spiked with different concentrations of four prevalent causative species of BSI. All samples were then subjected to a modified saponin-based host DNA depletion protocol and optimised DNA extraction, whole genome amplification and debranching steps in preparation for sequencing, followed by bioinformatical analyses. Two related variants of the protocol are presented: 1mL of blood processed without bacterial enrichment, and 5mL of blood processed following a rapid bacterial enrichment protocol-SepsiPURE.

RESULTS

After first identifying that a large proportion of host mitochondrial DNA remained, the host depletion process was optimised by increasing saponin concentration to 3% and scaling the reaction to allow more sample volume. Compared to non-depleted controls, the 3% saponin-based depletion protocol reduced the presence of host chromosomal and mitochondrial DNA < 106 and < 103 fold respectively. When the modified depletion method was further combined with a rapid bacterial enrichment method (SepsiPURE; with 5mL blood samples) the depletion of mitochondrial DNA improved by a further > 10X while also increasing detectable bacteria by > 10X. Parameters during DNA extraction, whole genome amplification and long-read sequencing were also adjusted, and subsequently amplicons were detected for each input bacterial species at each of the spiked concentrations, ranging from 50-100 colony forming units (CFU)/mL to 1-5 CFU/mL.

CONCLUSION

In this proof-of-concept study, four prevalent BSI causative species were detected in under 12 h to species level (with antimicrobial resistance determinants) at concentrations relevant to clinical blood samples. The use of a rapid and precise metagenomic protocols has the potential to advance the diagnosis of BSI.

Direct detection of canine picornavirus complete coding sequence in wastewater using long-range reverse-transcriptase polymerase chain reaction and long-read sequencing

We describe four complete coding sequence (cCDS) of canine picornavirus from wastewater in Arizona, USA detected by coupling cCDS single-contig (∼7.5 kb) reverse-transcriptase polymerase chain reaction (RT-PCR) and low-cost long-read high-throughput sequencing. For viruses of medical/veterinary importance, this workflow expands possibilities of wastewater based genomic epidemiology for exploring virus evolutionary dynamics especially in low-resource settings.

Genome-wide enhancer-associated tandem repeats are expanded in cardiomyopathy

BACKGROUND

Cardiomyopathy is a clinically and genetically heterogeneous heart condition that can lead to heart failure and sudden cardiac death in childhood. While it has a strong genetic basis, the genetic aetiology for over 50% of cardiomyopathy cases remains unknown.

METHODS

In this study, we analyse the characteristics of tandem repeats from genome sequence data of unrelated individuals diagnosed with cardiomyopathy from Canada and the United Kingdom (n = 1216) and compare them to those found in the general population. We perform burden analysis to identify genomic and epigenomic features that are impacted by rare tandem repeat expansions (TREs), and enrichment analysis to identify functional pathways that are involved in the TRE-associated genes in cardiomyopathy. We use Oxford Nanopore targeted long-read sequencing to validate repeat size and methylation status of one of the most recurrent TREs. We also compare the TRE-associated genes to those that are dysregulated in the heart tissues of individuals with cardiomyopathy.

FINDINGS

We demonstrate that tandem repeats that are rarely expanded in the general population are predominantly expanded in cardiomyopathy. We find that rare TREs are disproportionately present in constrained genes near transcriptional start sites, have high GC content, and frequently overlap active enhancer H3K27ac marks, where expansion-related DNA methylation may reduce gene expression. We demonstrate the gene silencing effect of expanded CGG tandem repeats in DIP2B through promoter hypermethylation. We show that the enhancer-associated loci are found in genes that are highly expressed in human cardiomyocytes and are differentially expressed in the left ventricle of the heart in individuals with cardiomyopathy.

INTERPRETATION

Our findings highlight the underrecognized contribution of rare tandem repeat expansions to the risk of cardiomyopathy and suggest that rare TREs contribute to ∼4% of cardiomyopathy risk.

FUNDING

Government of Ontario (RKCY), The Canadian Institutes of Health Research PJT 175329 (RKCY), The Azrieli Foundation (RKCY), SickKids Catalyst Scholar in Genetics (RKCY), The University of Toronto McLaughlin Centre (RKCY, SM), Ted Rogers Centre for Heart Research (SM), Data Sciences Institute at the University of Toronto (SM), The Canadian Institutes of Health Research PJT 175034 (SM), The Canadian Institutes of Health Research ENP 161429 under the frame of ERA PerMed (SM, RL), Heart and Stroke Foundation of Ontario & Robert M Freedom Chair in Cardiovascular Science (SM), Bitove Family Professorship of Adult Congenital Heart Disease (EO), Canada Foundation for Innovation (SWS, JR), Canada Research Chair (PS), Genome Canada (PS, JR), The Canadian Institutes of Health Research (PS).

Long-read sequencing unveils novel somatic variants and methylation patterns in the genetic information system of early lung cancer

Lung cancer poses a global health challenge, necessitating advanced diagnostics for improved outcomes. Intensive efforts are ongoing to pinpoint early detection biomarkers, such as genomic variations and DNA methylation, to elevate diagnostic precision. We conducted long-read sequencing on cancerous and adjacent non-cancerous tissues from a patient with lung adenocarcinoma. We identified somatic structural variations (SVs) specific to lung cancer by integrating data from various SV calling methods and differentially methylated regions (DMRs) that were distinct between these two tissue samples, revealing a unique methylation pattern associated with lung cancer. This study discovered over 40,000 somatic SVs and over 180,000 DMRs linked to lung cancer. We identified approximately 700 genes of significant relevance through comprehensive analysis, including genes intricately associated with many lung cancers, such as NOTCH1, SMOC2, CSMD2, and others. Furthermore, we observed that somatic SVs and DMRs were substantially enriched in several pathways, such as axon guidance signaling pathways, which suggests a comprehensive multi-omics impact on lung cancer progression across various biological investigation levels. These datasets can potentially serve as biomarkers for early lung cancer detection and may hold significant value in clinical diagnosis and treatment applications.

Annotated genome and transcriptome of the endangered Caribbean mountainous star coral (Orbicella faveolata) using PacBio long-read sequencing

Long-read sequencing is revolutionizing de-novo genome assemblies, with continued advancements making it more readily available for previously understudied, non-model organisms. Stony corals are one such example, with long-read de-novo genome assemblies now starting to be publicly available, opening the door for a wide array of 'omics-based research. Here we present a new de-novo genome assembly for the endangered Caribbean star coral, Orbicella faveolata, using PacBio circular consensus reads. Our genome assembly improved the contiguity (51 versus 1,933 contigs) and complete and single copy BUSCO orthologs (93.6% versus 85.3%, database metazoa_odb10), compared to the currently available reference genome generated using short-read methodologies. Our new de-novo assembled genome also showed comparable quality metrics to other coral long-read genomes. Telomeric repeat analysis identified putative chromosomes in our scaffolded assembly, with these repeats at either one, or both ends, of scaffolded contigs. We identified 32,172 protein coding genes in our assembly through use of long-read RNA sequencing (ISO-seq) of additional O. faveolata fragments exposed to a range of abiotic and biotic treatments, and publicly available short-read RNA-seq data. With anthropogenic influences heavily affecting O. faveolata, as well as its increasing incorporation into reef restoration activities, this updated genome resource can be used for population genomics and other 'omics analyses to aid in the conservation of this species.

DNA satellite and chromatin organization at mouse centromeres and pericentromeres

BACKGROUND

Centromeres are essential for faithful chromosome segregation during mitosis and meiosis. However, the organization of satellite DNA and chromatin at mouse centromeres and pericentromeres is poorly understood due to the challenges of assembling repetitive genomic regions.

RESULTS

Using recently available PacBio long-read sequencing data from the C57BL/6 strain, we find that contrary to the previous reports of their homogeneous nature, both centromeric minor satellites and pericentromeric major satellites exhibit a high degree of variation in sequence and organization within and between arrays. While most arrays are continuous, a significant fraction is interspersed with non-satellite sequences, including transposable elements. Using chromatin immunoprecipitation sequencing (ChIP-seq), we find that the occupancy of CENP-A and H3K9me3 chromatin at centromeric and pericentric regions, respectively, is associated with increased sequence enrichment and homogeneity at these regions. The transposable elements at centromeric regions are not part of functional centromeres as they lack significant CENP-A enrichment. Furthermore, both CENP-A and H3K9me3 nucleosomes occupy minor and major satellites spanning centromeric-pericentric junctions and a low yet significant amount of CENP-A spreads locally at centromere junctions on both pericentric and telocentric sides. Finally, while H3K9me3 nucleosomes display a well-phased organization on major satellite arrays, CENP-A nucleosomes on minor satellite arrays are poorly phased. Interestingly, the homogeneous class of major satellites also phase CENP-A and H3K27me3 nucleosomes, indicating that the nucleosome phasing is an inherent property of homogeneous major satellites.

CONCLUSIONS

Our findings reveal that mouse centromeres and pericentromeres display a high diversity in satellite sequence, organization, and chromatin structure.

Identification of knowledge gaps in whole-genome sequence analysis of multi-resistant thermotolerant Campylobacter spp

BACKGROUND

Campylobacter spp. is the most frequent cause of bacterial food-borne gastroenteritis and a high priority antibiotic resistant bacterium according to the World Health Organization (WHO). European monitoring of thermotolerant Campylobacter spp. does not reflect the global burden of resistances already circulating within the bacterial population worldwide.

METHODS

We systematically compared whole genome sequencing with comprehensive phenotypic antimicrobial susceptibility, analyzing 494 thermotolerant Campylobacter poultry isolates from Vietnam and Germany. Any discrepancy was checked by repeating the wet lab and improving the dry lab part. Selected isolates were additionally analyzed via long-read Oxford Nanopore technology, leading to closed chromosomes and plasmids.

RESULTS

Overall, 22 different resistance genes and gene variants (e. g. erm(B), aph(3')-IIIa, aph(2'')-If, catA, lnu(C), blaOXA, sat4) and point mutations in three distinct genes (gyrA, 23S rRNA, rpsL) associated with AMR were present in the Campylobacter isolates. Two AMR genes were missing in the database and one falsely associated with resistance. Bioinformatic analysis based on short-read data partly failed to identify tet(O) and aadE, when the genes were present as duplicate or homologous gene variants. Intriguingly, isolates also contained different determinants, redundantly conferring resistance to chloramphenicol, gentamicin, kanamycin, lincomycin and streptomycin. We found a novel tet(W) in tetracycline sensitive strains, harboring point mutations. Furthermore, analysis based on assemblies from short-read data was impaired to identify full length phase variable aad9, due to variations of the poly-C tract within the gene. The genetic determinant responsible for gentamicin resistance of one isolate from Germany could not be identified. GyrT86I, presenting the main determinant for (fluoro-)quinolone resistance led to a rare atypical phenotype of ciprofloxacin resistance but nalidixic acid sensitivity. Long-read sequencing predicted AMR genes were mainly located on the chromosome, and rarely on plasmids. Predictions from long- and short-read sequencing, respectively, often differed. AMR genes were often organized in multidrug resistance islands (MDRI) and partially located in proximity to transposase genes, suggesting main mobilization of resistance determinants is via natural transformation and transposition in Campylobacter.

CONCLUSIONS

The results of this study suggest that there is frequent resistance gene duplication, mosaicism, and mutation leading to gene variation and truncation in Campylobacter strains that have not been reported in previous studies and are missing from databases. Furthermore, there is a need for deciphering yet unknown resistance mechanisms and resistance spread in thermotolerant Campylobacter spp. that may pose a challenge to global food safety.

Targeted long-read sequencing identified a causal structural variant in X-linked nephrogenic diabetes insipidus

BACKGROUND

X-linked nephrogenic diabetes insipidus (NDI) is a rare genetic renal disease caused by pathogenic variants in the AVPR2 gene. Single nucleotide variants and small insertions/deletions in AVPR2 are reliably detected by routine clinical sequencing. Nevertheless, structural variants involving AVPR2 are challenging to identify accurately by conventional genetic testing. Here, we report a novel deletion of AVPR2 in a Czech family identified for the first time by targeted long-read sequencing (T-LRS).

METHODS

A male proband with X-linked NDI underwent clinical sequencing of the AVPR2 gene that failed and thus indicated possible whole-gene deletion. Therefore, PCR mapping and subsequent targeted long-read sequencing (T-LRS) using a Pacific Biosciences sequencer were applied to search for the suspected deletion. To validate the deletion breakpoints and prove variant segregation in the family with X-linked NDI, Sanger sequencing of the deletion junction was performed. Quantitative real-time PCR was further carried out to confirm the carrier status of heterozygous females.

RESULTS

By T-LRS, a novel 7.5 kb deletion of AVPR2 causing X-linked NDI in the proband was precisely identified. Sanger sequencing of the deletion junction confirmed the variant breakpoints and detected the deletion in the probands´ mother, maternal aunt, and maternal cousin with X-linked NDI. The carrier status in heterozygous females was further validated by quantitative real-time PCR.

CONCLUSIONS

Identifying the 7.5 kb deletion gave a precise molecular diagnosis for the proband, enabled genetic counselling and genetic testing for the family, and further expanded the spectrum of structural variants causing X-linked NDI. Our results also show that T-LRS has significant potential for accurately identifying putative structural variants.

Nanopore-Mediated Sequencing of Circular RNA

Circular RNAs (circRNAs) constitute a group of RNAs defined by a covalent bond between the 5' and 3' end formed by a unique back-splicing event. Most circRNAs are composed of more than one exon, which are spliced together in a linear fashion. This protocol describes methods to sequence full-length circRNA across the back-splicing junction, allowing unambiguous characterization of circRNA-specific exon-intron structures by long-read sequencing (LRS). Two different sequencing approaches are provided: (1) Global circRNA sequencing (the circNick-LRS strategy) relying on circRNA enrichment from total RNA followed by total circRNA long-read sequencing, and (2) targeted circRNA sequencing (the circPanel-LRS strategy) where a preselected panel of circRNA are sequenced without prior circRNA enrichment. Both methods were originally described in Karim et al. (Rahimi et al., Nat Commun 12: 4825, 2021) where they were applied to characterize the exon-intron structure of >10.000 circRNAs in mouse and human brains.

Detecting and Validating MAPT Mutations in Neurodegeneration Patients and Analysis of Exon Splicing Consequences

Mutation of MAPT has been observed in patients with parkinsonism, progressive supranuclear palsy, and corticobasal degeneration and is a significant cause of frontotemporal dementia. In this chapter, we discuss considerations for next-generation sequencing analysis to identify MAPT mutations in patient genomic DNA and describe the validation of these mutations by Sanger sequencing. One of the most common effects of MAPT mutations is differential splicing of exon 10, which leads to an imbalance in the proportion of 3-repeat and 4-repeat tau isoforms. We describe how to investigate the effect of novel DNA variants on the splicing efficiency of this exon in vitro using the exon-trapping technique, also known as the splicing reporter minigene assay.

Bulk RNA-seq Assessment of Murine Spleen Using a Portable MinION Sequencing Device

Comprehensive profiling of the transcriptome reflects the dynamic changes in complex regulator interactions between genes and proteins in the pathological processes of autoimmune disease. To enable real-time adaptive sampling and run rapid sequencing with maximal expected readings of 50 Gb of data with accessibility to clinicians, basic researchers, and even students, a portable and affordable sequencing device, MinION, was employed in our laboratory for both basic and clinical studies. Here, the workflow of bulk RNA-seq in murine spleen using MinION is introduced. The methodology of both laboratory library preparation and the establishment of a bioinformatic pipeline are included in this chapter.

Sample and Library Preparation for PacBio Long-Read Sequencing in Grapevine

PacBio long-read sequencing is a third-generation technology that generates long reads up to 20 kilobases (kb), unlike short-read sequencing instruments that produce up to 600 bases. Long-read sequencing is particularly advantageous in higher organisms, such as humans and plants, where repetitive regions in the genome are more abundant. The PacBio long-read sequencing uses a single molecule, real-time approach where the SMRT cells contain several zero-mode waveguides (ZMWs). Each ZMW contains a single DNA molecule bound by a DNA polymerase. All ZMWs are flushed with deoxy nucleotides with a fluorophore specific to each nucleotide. As the sequencing proceeds, the detector detects the wavelength of the fluorescence and the nucleotides are read in real-time. This chapter describes the sample and library preparation for PacBio long-read sequencing for grapevine.

A targeted long-read sequencing approach questions the association of OXTR methylation with high-functioning autism

BACKGROUND

DNA sequence variation and altered epigenetic regulation of the oxytocin receptor gene (OXTR) have been implicated in autism and autistic-like behaviors. While previous studies have examined subsegments of OXTR, nanopore Cas9-targeted sequencing (nCATS) allows deep characterization of entire genes with simultaneous assessment of epigenetic 5-methylcytosine (5mC) modification and without the need for prior DNA amplification or bisulfite conversion. This pilot study uses an nCATS approach to sequence the entire OXTR gene and its regulatory construct and screen for 5mC modification to compare results between individuals with high-functioning autism (HFA) and neurotypical controls (NC).

METHODS

Using DNA extracted from peripheral blood, OXTR (Hg38, chr3: 8750381-8770434, 20,054 base pairs) was analyzed by nCATS. 5mC modification probabilities were calculated and visualized across the gene and differential methylation analysis was performed.

RESULTS

Twenty adults with HFA (10 males, 10 females) and 20 age- and sex-matched NC (± 5 years) were included. There were no apparent group differences in the entire OXTR gene sequence, except for the intron variant rs918316, which was clustered in the HFA group. However, differential methylation analysis did not reveal a single significant group-dependent differentially methylated site among the 412 CpG sites captured.

LIMITATIONS

Limitations of this study include the small number of samples due to the pilot nature of the study, which particularly limits the relevance of the sequence variants found. It should also be noted that the use of peripheral blood material limits the ability to draw conclusions about central processes.

CONCLUSIONS

Previous findings of autism-associated OXTR epigenetic alterations were not reproducible with our method. In our opinion, this may lead to a reconsideration of the relevance of altered methylation at individual OXTR CpG positions in autism research. However, given the pilot nature of the study, these results need to be replicated in independent cohorts and with larger sample sizes.

Beyond the exome: utility of long-read whole genome sequencing in exome-negative autosomal recessive diseases

BACKGROUND

Long-read whole genome sequencing (lrWGS) has the potential to address the technical limitations of exome sequencing in ways not possible by short-read WGS. However, its utility in autosomal recessive Mendelian diseases is largely unknown.

METHODS

In a cohort of 34 families in which the suspected autosomal recessive diseases remained undiagnosed by exome sequencing, lrWGS was performed on the Pacific Bioscience Sequel IIe platform.

RESULTS

Likely causal variants were identified in 13 (38%) of the cohort. These include (1) a homozygous splicing SV in TYMS as a novel candidate gene for lethal neonatal lactic acidosis, (2) a homozygous non-coding SV that we propose impacts STK25 expression and causes a novel neurodevelopmental disorder, (3) a compound heterozygous SV in RP1L1 with complex inheritance pattern in a family with inherited retinal disease, (4) homozygous deep intronic variants in LEMD2 and SNAP91 as novel candidate genes for neurodevelopmental disorders in two families, and (5) a promoter SNV in SLC4A4 causing non-syndromic band keratopathy. Surprisingly, we also encountered causal variants that could have been identified by short-read exome sequencing in 7 families. The latter highlight scenarios that are especially challenging at the interpretation level.

CONCLUSIONS

Our data highlight the continued need to address the interpretation challenges in parallel with efforts to improve the sequencing technology itself. We propose a path forward for the implementation of lrWGS sequencing in the setting of autosomal recessive diseases in a way that maximizes its utility.

CLN2 disease resulting from a novel homozygous deep intronic splice variant in TPP1 discovered using long-read sequencing

Neuronal ceroid lipofuscinosis type 2 (CLN2) is an autosomal recessive neurodegenerative disorder with enzyme replacement therapy available. We present two siblings with a clinical diagnosis of CLN2 disease, but no identifiable TPP1 variants after standard clinical testing. Long-read sequencing identified a homozygous deep intronic variant predicted to affect splicing, confirmed by clinical DNA and RNA sequencing. This case demonstrates how traditional laboratory assays can complement emerging molecular technologies to provide a precise molecular diagnosis.

North and East African mitochondrial genetic variation needs further characterization towards precision medicine

INTRODUCTION

Mitochondria are maternally inherited cell organelles with their own genome, and perform various functions in eukaryotic cells such as energy production and cellular homeostasis. Due to their inheritance and manifold biological roles in health and disease, mitochondrial genetics serves a dual purpose of tracing the history as well as disease susceptibility of human populations across the globe. This work requires a comprehensive catalogue of commonly observed genetic variations in the mitochondrial DNAs for all regions throughout the world. So far, however, certain regions, such as North and East Africa have been understudied.

OBJECTIVES

To address this shortcoming, we have created the most comprehensive quality-controlled North and East African mitochondrial data set to date and use it for characterizing mitochondrial genetic variation in this region.

METHODS

We compiled 11 published cohorts with novel data for mitochondrial genomes from 159 Sudanese individuals. We combined these 641 mitochondrial sequences with sequences from the 1000 Genomes (n = 2504) and the Human Genome Diversity Project (n = 828) and used the tool haplocheck for extensive quality control and detection of in-sample contamination, as well as Nanopore long read sequencing for haplogroup validation of 18 samples.

RESULTS

Using a subset of high-coverage mitochondrial sequences, we predict 15 potentially novel haplogroups in North and East African subjects and observe likely phylogenetic deviations from the established PhyloTree reference for haplogroups L0a1 and L2a1.

CONCLUSION

Our findings demonstrate common hitherto unexplored variants in mitochondrial genomes of North and East Africa that lead to novel phylogenetic relationships between haplogroups present in these regions. These observations call for further in-depth population genetic studies in that region to enable the prospective use of mitochondrial genetic variation for precision medicine.

Evaluating the clinical utility of a long-read sequencing-based approach in genetic testing of fragile-X syndrome

BACKGROUND

Fragile X syndrome (FXS) arises from the FMR1 CGG expansion. Comprehensive genetic testing for FMR1 CGG expansions, AGG interruptions, and microdeletions is essential to provide genetic counseling for females carrying premutation alleles. However, conventional PCR-based FMR1 assays mainly focus on CGG repeats, and could detect AGG interruption only in males.

METHODS

The clinical utility of a long-read sequencing-based assay termed comprehensive analysis of FXS (CAFXS) was evaluated in 238 high-risk samples by comparing to conventional PCR assays.

RESULTS

PCR assays identified five premuation and three full mutation categories alleles in all the samples, and CAFXS successfully called all the FMR1 CGG expansion. CAFXS identified 24-bp microdeletions upstream to the trinucleotide region with 30 CGG repeats, which was miscalled by the length-based PCR methods. CAFXS also identified a 187-bp deletion in about 1/7 of the sequencing reads in a male patient with mosaic full mutation alleles. CAFXS allowed for precise constructing the FMR1 CGG repeat and AGG interruption pattern in all the samples, and identified a novel and alternative CGA interruption in one normal female sample.

CONCLUSIONS

CAFXS represents a more comprehensive and accurate approach for FXS genetic testing that potentially enables more informed genetic counseling compared to PCR-based methods.

Molecular characterization of a novel 83.9-kb deletion of the α-globin upstream regulatory elements by long-read sequencing

Inherited deletions of upstream regulatory elements of α-globin genes give rise to α-thalassemia, which is an autosomal recessive monogenic disease. However, conventional thalassemia target diagnosis often fails to identify these rare deletions. Here we reported a family with two previous pregnancies of Hb Bart's hydrops fetalis and was seeking for prenatal diagnosis during the third pregnancy. Both parents had low level of Hemoglobin A2 indicating α-thalassemia. Conventional Gap-PCR and PCR-reverse dot blot showed the father carried -SEA deletion but did not identify any variants in the mother. Multiplex ligation-dependent probe amplification identified a deletion containing two HS-40 probes but could not determine the exact region. Finally, a long-read sequencing (LRS)-based approach directly identified that the exact deletion region was chr16: 48,642-132,584, which was located in the α-globin upstream regulatory elements and named (αα)JM after the Jiangmen city. Gap-PCR and Sanger sequencing confirmed the breakpoint. Both the mother and fetus from the third pregnancy carried heterozygous (αα)JM, and the fetus was normally delivered at gestational age of 39 weeks. This study demonstrated that LRS technology had great advantages over conventional target diagnosis methods for identifying rare thalassemia variants and assisted better carrier screening and prenatal diagnosis of thalassemia.

Long-read sequencing reveals the landscape of aberrant alternative splicing and novel therapeutic target in colorectal cancer

BACKGROUND

Alternative splicing complexity plays a vital role in carcinogenesis and cancer progression. Improved understanding of novel splicing events and the underlying regulatory mechanisms may contribute new insights into developing new therapeutic strategies for colorectal cancer (CRC).

METHODS

Here, we combined long-read sequencing technology with short-read RNA-seq methods to investigate the transcriptome complexity in CRC. By using experiment assays, we explored the function of newly identified splicing isoform TIMP1 Δ4-5. Moreover, a CRISPR/dCasRx-based strategy to induce the TIMP1 exon 4-5 exclusion was introduced to inhibit neoplasm growth.

RESULTS

A total of 90,703 transcripts were identified, of which > 62% were novel compared with current transcriptome annotations. These novel transcripts were more likely to be sample specific, expressed at relatively lower levels with more exons, and oncogenes displayed a characteristic to generate more transcripts in CRC. Clinical outcome data analysis showed that 1472 differentially expressed alternative splicing events (DEAS) were tightly associated with CRC patients' prognosis, and many novel isoforms were likely to be important determinants for patient survival. Among these, newly identified splicing isoform TIMP1 Δ4-5 was significantly downregulated in CRC. Further in vitro and in vivo assays demonstrated that ectopic expression of TIMP1 Δ4-5 significantly suppresses tumor cell growth and metastasis. Serine/arginine-rich splicing factor 1 (SRSF1) acts as a onco-splicing regulator through sustaining the inclusion of TIMP1 exon 4-5. Furthermore, CRISPR/dCasRx-based strategies designed to induce TIMP1 exon 4-5 exclusion have the potential to restrain the CRC growth.

CONCLUSIONS

This data provides a rich resource for deeper studies of gastrointestinal malignancies. Newly identified splicing isoform TIMP1 Δ4-5 plays an important role in mediating CRC progression and may be a potential therapy target in CRC.

in "Long-read sequencing settings for efficient structural variation detection based on comprehensive evaluation"

A recent paper by Jiang et al. in BMC Bioinformatics presented guidelines on long-read sequencing settings for structural variation (SV) calling, and benchmarked the performance of various SV calling tools, including NanoVar. In their simulation-based benchmarking, NanoVar was shown to perform poorly compared to other tools, mostly due to low SV recall rates. To investigate the causes for NanoVar's poor performance, we regenerated the simulation datasets (3× to 20×) as specified by Jiang et al. and performed benchmarking for NanoVar and Sniffles. Our results did not reflect the findings described by Jiang et al. In our analysis, NanoVar displayed more than three times the F1 scores and recall rates as reported in Jiang et al. across all sequencing coverages, indicating a previous underestimation of its performance. We also observed that NanoVar outperformed Sniffles in calling SVs with genotype concordance by more than 0.13 in F1 scores, which is contrary to the trend reported by Jiang et al. Besides, we identified multiple detrimental errors encountered during the analysis which were not addressed by Jiang et al. We hope that this commentary clarifies NanoVar's validity as a long-read SV caller and provides assurance to its users and the scientific community.

Structural variation and introgression from wild populations in East Asian cattle genomes confer adaptation to local environment

BACKGROUND

Structural variations (SVs) in individual genomes are major determinants of complex traits, including adaptability to environmental variables. The Mongolian and Hainan cattle breeds in East Asia are of taurine and indicine origins that have evolved to adapt to cold and hot environments, respectively. However, few studies have investigated SVs in East Asian cattle genomes and their roles in environmental adaptation, and little is known about adaptively introgressed SVs in East Asian cattle.

RESULTS

In this study, we examine the roles of SVs in the climate adaptation of these two cattle lineages by generating highly contiguous chromosome-scale genome assemblies. Comparison of the two assemblies along with 18 Mongolian and Hainan cattle genomes obtained by long-read sequencing data provides a catalog of 123,898 nonredundant SVs. Several SVs detected from long reads are in exons of genes associated with epidermal differentiation, skin barrier, and bovine tuberculosis resistance. Functional investigations show that a 108-bp exonic insertion in SPN may affect the uptake of Mycobacterium tuberculosis by macrophages, which might contribute to the low susceptibility of Hainan cattle to bovine tuberculosis. Genotyping of 373 whole genomes from 39 breeds identifies 2610 SVs that are differentiated along a "north-south" gradient in China and overlap with 862 related genes that are enriched in pathways related to environmental adaptation. We identify 1457 Chinese indicine-stratified SVs that possibly originate from banteng and are frequent in Chinese indicine cattle.

CONCLUSIONS

Our findings highlight the unique contribution of SVs in East Asian cattle to environmental adaptation and disease resistance.

A universal method for generating knockout mice in multiple genetic backgrounds using zygote electroporation

Genetically engineered mouse models are essential tools for understanding mammalian gene functions and disease pathogenesis. Genome editing allows the generation of these models in multiple inbred strains of mice without backcrossing. Zygote electroporation dramatically removed the barrier for introducing the CRISPR-Cas9 complex in terms of cost and labour. Here, we demonstrate that the generalised zygote electroporation method is also effective for generating knockout mice in multiple inbred strains. By combining in vitro fertilisation and electroporation, we obtained founders for knockout alleles in eight common inbred strains. Long-read sequencing analysis detected not only intended mutant alleles but also differences in read frequency of intended and unintended alleles among strains. Successful germline transmission of knockout alleles demonstrated that our approach can establish mutant mice targeting the same locus in multiple inbred strains for phenotyping analysis, contributing to reverse genetics and human disease research.

Long-read sequencing identifies a common transposition haplotype predisposing for CLCNKB deletions

BACKGROUND

Long-read sequencing is increasingly used to uncover structural variants in the human genome, both functionally neutral and deleterious. Structural variants occur more frequently in regions with a high homology or repetitive segments, and one rearrangement may predispose to additional events. Bartter syndrome type 3 (BS 3) is a monogenic tubulopathy caused by deleterious variants in the chloride channel gene CLCNKB, a high proportion of these being large gene deletions. Multiplex ligation-dependent probe amplification, the current diagnostic gold standard for this type of mutation, will indicate a simple homozygous gene deletion in biallelic deletion carriers. However, since the phenotypic spectrum of BS 3 is broad even among biallelic deletion carriers, we undertook a more detailed analysis of precise breakpoint regions and genomic structure.

METHODS

Structural variants in 32 BS 3 patients from 29 families and one BS4b patient with CLCNKB deletions were investigated using long-read and synthetic long-read sequencing, as well as targeted long-read sequencing approaches.

RESULTS

We report a ~3 kb duplication of 3'-UTR CLCNKB material transposed to the corresponding locus of the neighbouring CLCNKA gene, also found on ~50 % of alleles in healthy control individuals. This previously unknown common haplotype is significantly enriched in our cohort of patients with CLCNKB deletions (45 of 51 alleles with haplotype information, 2.2 kb and 3.0 kb transposition taken together, p=9.16×10-9). Breakpoint coordinates for the CLCNKB deletion were identifiable in 28 patients, with three being compound heterozygous. In total, eight different alleles were found, one of them a complex rearrangement with three breakpoint regions. Two patients had different CLCNKA/CLCNKB hybrid genes encoding a predicted CLCNKA/CLCNKB hybrid protein with likely residual function.

CONCLUSIONS

The presence of multiple different deletion alleles in our cohort suggests that large CLCNKB gene deletions originated from many independently recurring genomic events clustered in a few hot spots. The uncovered associated sequence transposition haplotype apparently predisposes to these additional events. The spectrum of CLCNKB deletion alleles is broader than expected and likely still incomplete, but represents an obvious candidate for future genotype/phenotype association studies. We suggest a sensitive and cost-efficient approach, consisting of indirect sequence capture and long-read sequencing, to analyse disease-relevant structural variant hotspots in general.

vamos: variable-number tandem repeats annotation using efficient motif sets

Roughly 3% of the human genome is composed of variable-number tandem repeats (VNTRs): arrays of motifs at least six bases. These loci are highly polymorphic, yet current approaches that define and merge variants based on alignment breakpoints do not capture their full diversity. Here we present a method vamos: VNTR Annotation using efficient Motif Sets that instead annotates VNTR using repeat composition under different levels of motif diversity. Using vamos we estimate 7.4-16.7 alleles per locus when applied to 74 haplotype-resolved human assemblies, compared to breakpoint-based approaches that estimate 4.0-5.5 alleles per locus.

Applications of advanced technologies for detecting genomic structural variation

Chromosomal structural variation (SV) encompasses a heterogenous class of genetic variants that exerts strong influences on human health and disease. Despite their importance, many structural variants (SVs) have remained poorly characterized at even a basic level, a discrepancy predicated upon the technical limitations of prior genomic assays. However, recent advances in genomic technology can identify and localize SVs accurately, opening new questions regarding SV risk factors and their impacts in humans. Here, we first define and classify human SVs and their generative mechanisms, highlighting characteristics leveraged by various SV assays. We next examine the first-ever gapless assembly of the human genome and the technical process of assembling it, which required third-generation sequencing technologies to resolve structurally complex loci. The new portions of that "telomere-to-telomere" and subsequent pangenome assemblies highlight aspects of SV biology likely to develop in the near-term. We consider the strengths and limitations of the most promising new SV technologies and when they or longstanding approaches are best suited to meeting salient goals in the study of human SV in population-scale genomics research, clinical, and public health contexts. It is a watershed time in our understanding of human SV when new approaches are expected to fundamentally change genomic applications.

Hybrid sequencing discloses unique aspects of the transcriptomic architecture in equid alphaherpesvirus 1

This study employed both short-read sequencing (SRS, Illumina) and long-read sequencing (LRS Oxford Nanopore Technologies) platforms to conduct a comprehensive analysis of the equid alphaherpesvirus 1 (EHV-1) transcriptome. The study involved the annotation of canonical mRNAs and their transcript variants, encompassing transcription start site (TSS) and transcription end site (TES) isoforms, in addition to alternative splicing forms. Furthermore, the study revealed the presence of numerous non-coding RNA (ncRNA) molecules, including intergenic and antisense transcripts, produced by EHV-1. An intriguing finding was the abundant production of chimeric transcripts, some of which potentially encode fusion polypeptides. Moreover, EHV-1 exhibited a greater incidence of transcriptional overlaps and splicing compared to related viruses. It is noteworthy that many genes have their unique TESs along with the co-terminal transcription ends, a characteristic scarcely seen in other alphaherpesviruses. The study also identified transcripts that overlap the replication origins of the virus. Moreover, a novel ncRNA, referred to as NOIR, was found to intersect with the 5'-ends of longer transcript isoform specified by the major transactivator genes ORF64 and ORF65, surrounding the OriL. These findings together imply the existence of a key regulatory mechanism that governs both transcription and replication through, among others, a process that involves interference between the DNA and RNA synthesis machineries.

High functional allelic diversity and copy number in both MHC classes in the common buzzard

BACKGROUND

The major histocompatibility complex (MHC), which encodes molecules that recognize various pathogens and parasites and initiates the adaptive immune response in vertebrates, is renowned for its exceptional polymorphism and is a model of adaptive gene evolution. In birds, the number of MHC genes and sequence diversity varies greatly among taxa, believed due to evolutionary history and differential selection pressures. Earlier characterization studies and recent comparative studies suggest that non-passerine species have relatively few MHC gene copies compared to passerines. Additionally, comparative studies that have looked at partial MHC sequences have speculated that non-passerines have opposite patterns of selection on MHC class I (MHC-I) and class II (MHC-II) loci than passerines: namely, greater sequence diversity and signals of selection on MHC-II than MHC-I. However, new sequencing technology is revealing much greater MHC variation than previously expected while also facilitating full sequence variant detection directly from genomic data. Our study aims to take advantage of high-throughput sequencing methods to fully characterize both classes and domains of MHC of a non-passerine bird of prey, the common buzzard (Buteo buteo), to test predictions of MHC variation and differential selection on MHC classes.

RESULTS

Using genetic, genomic, and transcriptomic high-throughput sequencing data, we established common buzzards have at least three loci that produce functional alleles at both MHC classes. In total, we characterize 91 alleles from 113 common buzzard chicks for MHC-I exon 3 and 41 alleles from 125 chicks for MHC-IIB exon 2. Among these alleles, we found greater sequence polymorphism and stronger diversifying selection at MHC-IIB exon 2 than MHC-I exon 3, suggesting differential selection pressures on MHC classes. However, upon further investigation of the entire peptide-binding groove by including genomic data from MHC-I exon 2 and MHC-IIA exon 2, this turned out to be false. MHC-I exon 2 was as polymorphic as MHC-IIB exon 2 and MHC-IIA exon 2 was essentially invariant. Thus, comparisons between MHC-I and MHC-II that included both domains of the peptide-binding groove showed no differences in polymorphism nor diversifying selection between the classes. Nevertheless, selection analysis indicates balancing selection has been acting on common buzzard MHC and phylogenetic inference revealed that trans-species polymorphism is present between common buzzards and species separated for over 33 million years for class I and class II.

CONCLUSIONS

We characterize and confirm the functionality of unexpectedly high copy number and allelic diversity in both MHC classes of a bird of prey. While balancing selection is acting on both classes, there is no evidence of differential selection pressure on MHC classes in common buzzards and this result may hold more generally once more data for understudied MHC exons becomes available.

Applications of long-read sequencing to Mendelian genetics

Advances in clinical genetic testing, including the introduction of exome sequencing, have uncovered the molecular etiology for many rare and previously unsolved genetic disorders, yet more than half of individuals with a suspected genetic disorder remain unsolved after complete clinical evaluation. A precise genetic diagnosis may guide clinical treatment plans, allow families to make informed care decisions, and permit individuals to participate in N-of-1 trials; thus, there is high interest in developing new tools and techniques to increase the solve rate. Long-read sequencing (LRS) is a promising technology for both increasing the solve rate and decreasing the amount of time required to make a precise genetic diagnosis. Here, we summarize current LRS technologies, give examples of how they have been used to evaluate complex genetic variation and identify missing variants, and discuss future clinical applications of LRS. As costs continue to decrease, LRS will find additional utility in the clinical space fundamentally changing how pathological variants are discovered and eventually acting as a single-data source that can be interrogated multiple times for clinical service.

Implementation of Nanopore sequencing as a pragmatic workflow for copy number variant confirmation in the clinic

BACKGROUND

Diagnosis of rare genetic diseases can be a long, expensive and complex process, involving an array of tests in the hope of obtaining an actionable result. Long-read sequencing platforms offer the opportunity to make definitive molecular diagnoses using a single assay capable of detecting variants, characterizing methylation patterns, resolving complex rearrangements, and assigning findings to long-range haplotypes. Here, we demonstrate the clinical utility of Nanopore long-read sequencing by validating a confirmatory test for copy number variants (CNVs) in neurodevelopmental disorders and illustrate the broader applications of this platform to assess genomic features with significant clinical implications.

METHODS

We used adaptive sampling on the Oxford Nanopore platform to sequence 25 genomic DNA samples and 5 blood samples collected from patients with known or false-positive copy number changes originally detected using short-read sequencing. Across the 30 samples (a total of 50 with replicates), we assayed 35 known unique CNVs (a total of 55 with replicates) and one false-positive CNV, ranging in size from 40 kb to 155 Mb, and assessed the presence or absence of suspected CNVs using normalized read depth.

RESULTS

Across 50 samples (including replicates) sequenced on individual MinION flow cells, we achieved an average on-target mean depth of 9.5X and an average on-target read length of 4805 bp. Using a custom read depth-based analysis, we successfully confirmed the presence of all 55 known CNVs (including replicates) and the absence of one false-positive CNV. Using the same CNV-targeted data, we compared genotypes of single nucleotide variant loci to verify that no sample mix-ups occurred between assays. For one case, we also used methylation detection and phasing to investigate the parental origin of a 15q11.2-q13 duplication with implications for clinical prognosis.

CONCLUSIONS

We present an assay that efficiently targets genomic regions to confirm clinically relevant CNVs with a concordance rate of 100%. Furthermore, we demonstrate how integration of genotype, methylation, and phasing data from the Nanopore sequencing platform can potentially simplify and shorten the diagnostic odyssey.

de novo structural variations of Escherichia coli detected by Nanopore long-read sequencing

Spontaneous mutations power evolution, while large-scale structural variations (SVs) remain poorly studied, primarily because of the lack of long-read sequencing techniques and powerful analytical tools. Here, we explore the SVs of Escherichia coli by running 67 wild-type (WT) and 37 MMR-deficient (ΔmutS) mutation accumulation lines, each experiencing more than 4000 cell divisions, by applying Nanopore long-read sequencing and Illumina PE150 sequencing, and verifying the results by Sanger sequencing. In addition to precisely repeating previous mutation rates of base-pair substitutions and indels mutation rates, we do find significant improvement in insertion and deletion detection using long-read sequencing. The long-read sequencing and corresponding softwares can particularly detect bacterial SVs in both simulated and real datasets with high accuracy. These lead to SV rates of 2.77 × 10-4 (WT) and 5.26 × 10-4 (MMR-deficient) per cell division per genome, which is comparable with previous reports. This study provides the SV rates of E. coli by applying long-read sequencing and SV-detection programs, revealing a broader and more accurate picture of spontaneous mutations in bacteria.

Long-read sequencing: An effective method for genetic analysis of CYP21A2 variation in congenital adrenal hyperplasia

BACKGROUND

The sequence similarity between CYP21A2 gene and its inactive pseudogene CYP21A1P, and copy number variation (CNV) caused by unequal crossover, make it challenging to characterize the CYP21A2 gene through traditional methods. This study aimed to evaluate the clinical utility of the long-read sequencing (LRS) method in carrier screening and genetic diagnosis of congenital adrenal hyperplasia (CAH) by comparing the efficiency of the LRS method with the conventional multiplex ligation-dependent probe amplification (MLPA) plus Sanger sequencing approaches in CYP21A2 analysis.

METHODS

In a retrospective study, full sequence analysis of the CYP21A2 and CYP21A1P was performed for three pedigrees through long-range locus-specific PCR followed by LRS based on the Pacific Biosciences (PacBio, California, USA) single-molecule real-time (SMRT) platform, and the results were compared with those obtained from next-generation sequencing (NGS)-based whole exome sequencing (WES) and the traditional methods of MLPA plus Sanger sequencing.

RESULTS

The LRS method successfully identified seven CYP21A2 variants , including three single nucleotide variants (NM_000500.9:c.1451G>C p.(Arg484Pro), c.293-13A/C>G (IVS2-13A/C>G), c.518T>A p.(Ile173Asn)), one 111-bp polynucleotide insertion, one set of 3'URT variants (NM_000500.9:c.*368T>C, c.*390A>G, c.*440C>T, c.*443T>C) and two types of chimeric genes and straightforwardly depicted the inheritance patterns of these variants within families. Moreover, the LRS method enabled us to determine the cis-trans configuration of multiple variants in one assay, without the need to analyze additional family samples. Compared with traditional methods, this LRS method can achieve a precise, comprehensive and intuitive result in the genetic diagnosis of 21-hydroxylase deficiency (21-OHD).

CONCLUSION

The LRS method is comprehensive in CYP21A2 analysis and intuitive in result presentation, which holds substantial promise in clinical application as a crucial tool for carrier screening and genetic diagnosis of CAH.

Best Practices in Microbial Experimental Evolution: Using Reporters and Long-Read Sequencing to Identify Copy Number Variation in Experimental Evolution

Copy number variants (CNVs), comprising gene amplifications and deletions, are a pervasive class of heritable variation. CNVs play a key role in rapid adaptation in both natural, and experimental, evolution. However, despite the advent of new DNA sequencing technologies, detection and quantification of CNVs in heterogeneous populations has remained challenging. Here, we summarize recent advances in the use of CNV reporters that provide a facile means of quantifying de novo CNVs at a specific locus in the genome, and nanopore sequencing, for resolving the often complex structures of CNVs. We provide guidance for the engineering and analysis of CNV reporters and practical guidelines for single-cell analysis of CNVs using flow cytometry. We summarize recent advances in nanopore sequencing, discuss the utility of this technology, and provide guidance for the bioinformatic analysis of these data to define the molecular structure of CNVs. The combination of reporter systems for tracking and isolating CNV lineages and long-read DNA sequencing for characterizing CNV structures enables unprecedented resolution of the mechanisms by which CNVs are generated and their evolutionary dynamics.

Comprehensive de novo mutation discovery with HiFi long-read sequencing

BACKGROUND

Long-read sequencing (LRS) techniques have been very successful in identifying structural variants (SVs). However, the high error rate of LRS made the detection of small variants (substitutions and short indels < 20 bp) more challenging. The introduction of PacBio HiFi sequencing makes LRS also suited for detecting small variation. Here we evaluate the ability of HiFi reads to detect de novo mutations (DNMs) of all types, which are technically challenging variant types and a major cause of sporadic, severe, early-onset disease.

METHODS

We sequenced the genomes of eight parent-child trios using high coverage PacBio HiFi LRS (~ 30-fold coverage) and Illumina short-read sequencing (SRS) (~ 50-fold coverage). De novo substitutions, small indels, short tandem repeats (STRs) and SVs were called in both datasets and compared to each other to assess the accuracy of HiFi LRS. In addition, we determined the parent-of-origin of the small DNMs using phasing.

RESULTS

We identified a total of 672 and 859 de novo substitutions/indels, 28 and 126 de novo STRs, and 24 and 1 de novo SVs in LRS and SRS respectively. For the small variants, there was a 92 and 85% concordance between the platforms. For the STRs and SVs, the concordance was 3.6 and 0.8%, and 4 and 100% respectively. We successfully validated 27/54 LRS-unique small variants, of which 11 (41%) were confirmed as true de novo events. For the SRS-unique small variants, we validated 42/133 DNMs and 8 (19%) were confirmed as true de novo event. Validation of 18 LRS-unique de novo STR calls confirmed none of the repeat expansions as true DNM. Confirmation of the 23 LRS-unique SVs was possible for 19 candidate SVs of which 10 (52.6%) were true de novo events. Furthermore, we were able to assign 96% of DNMs to their parental allele with LRS data, as opposed to just 20% with SRS data.

CONCLUSIONS

HiFi LRS can now produce the most comprehensive variant dataset obtainable by a single technology in a single laboratory, allowing accurate calling of substitutions, indels, STRs and SVs. The accuracy even allows sensitive calling of DNMs on all variant levels, and also allows for phasing, which helps to distinguish true positive from false positive DNMs.

A high quality, high molecular weight DNA extraction method for PacBio HiFi genome sequencing of recalcitrant plants

BACKGROUND

PacBio HiFi sequencing provides highly accurate long-read sequencing datasets which are of great advantage for whole genome sequencing projects. One limitation of the method is the requirement for high quality, high molecular weight input DNA. This can be particularly challenging for plants that frequently contain common and species-specific secondary metabolites, which often interfere with downstream processes. Cape Primroses (genus Streptocarpus), are some of these recalcitrant plants and are selected here as material to develop a high quality, high molecular weight DNA extraction protocol for long read genome sequencing.

RESULTS

We developed a DNA extraction method for PacBio HiFi sequencing for Streptocarpus grandis and Streptocarpus kentaniensis. A CTAB lysis buffer was employed to avoid guanidine, and the traditional chloroform and phenol purification steps were replaced with pre-lysis sample washes. Best cells/nucleus lysis was achieved with 4 h at 58 °C. The obtained high quality and high molecular weight DNAs were tested in PacBio SMRTBell™ library preparations, which resulted in circular consensus sequencing (CCS) reads from 17 to 27 Gb per cell, and a read length N50 from 14 to 17 kbp. To evaluate the quality of the reads for whole genome sequencing, they were assembled with HiFiasm into draft genomes, with N50 = 49 Mb and 23 Mb, and L50 = 10 and 11. The longest contigs were 95 Mb and 57 Mb respectively, showing good contiguity as these are longer than the theoretical chromosome length (genome size/chromosome number) of 78 Mb and 55 Mb, for S. grandis and S. kentaniensis respectively.

CONCLUSIONS

DNA extraction is a critical step towards obtaining a complete genome assembly. Our DNA extraction method here provided the required high quality, high molecular weight DNA for successful standard-input PacBio HiFi library preparation. The contigs from those reads showed a high contiguity, providing a good starting draft assembly towards obtaining a complete genome. The results obtained here were highly promising, and demonstrated that the DNA extraction method developed here is compatible with PacBio HiFi sequencing and suitable for de novo whole genome sequencing projects of plants.

Benchmarking datasets for assembly-based variant calling using high-fidelity long reads

BACKGROUND

Recent advances in long-read sequencing technologies have enabled accurate identification of all genetic variants in individuals or cells; this procedure is known as variant calling. However, benchmarking studies on variant calling using different long-read sequencing technologies are still lacking.

RESULTS

We used two Caenorhabditis elegans strains to measure several variant calling metrics. These two strains shared true-positive genetic variants that were introduced during strain generation. In addition, both strains contained common and distinguishable variants induced by DNA damage, possibly leading to false-positive estimation. We obtained accurate and noisy long reads from both strains using high-fidelity (HiFi) and continuous long-read (CLR) sequencing platforms, and compared the variant calling performance of the two platforms. HiFi identified a 1.65-fold higher number of true-positive variants on average, with 60% fewer false-positive variants, than CLR did. We also compared read-based and assembly-based variant calling methods in combination with subsampling of various sequencing depths and demonstrated that variant calling after genome assembly was particularly effective for detection of large insertions, even with 10 × sequencing depth of accurate long-read sequencing data.

CONCLUSIONS

By directly comparing the two long-read sequencing technologies, we demonstrated that variant calling after genome assembly with 10 × or more depth of accurate long-read sequencing data allowed reliable detection of true-positive variants. Considering the high cost of HiFi sequencing, we herein propose appropriate methodologies for performing cost-effective and high-quality variant calling: 10 × assembly-based variant calling. The results of the present study may facilitate the development of methods for identifying all genetic variants at the population level.

Complete sequence verification of plasmid DNA using the Oxford Nanopore Technologies' MinION device

BACKGROUND

Sequence verification is essential for plasmids used as critical reagents or therapeutic products. Typically, high-quality plasmid sequence is achieved through capillary-based Sanger sequencing, requiring customized sets of primers for each plasmid. This process can become expensive, particularly for applications where the validated sequence needs to be produced within a regulated and quality-controlled environment for downstream clinical research applications.

RESULTS

Here, we describe a cost-effective and accurate plasmid sequencing and consensus generation procedure using the Oxford Nanopore Technologies' MinION device as an alternative to capillary-based plasmid sequencing options. This procedure can verify the identity of a pure population of plasmid, either confirming it matches the known and expected sequence, or identifying mutations present in the plasmid if any exist. We use a full MinION flow cell per plasmid, maximizing available data and allowing for stringent quality filters. Pseudopairing reads for consensus base calling reduces read error rates from 5.3 to 0.53%, and our pileup consensus approach provides per-base counts and confidence scores, allowing for interpretation of the certainty of the resulting consensus sequences. For pure plasmid samples, we demonstrate 100% accuracy in the resulting consensus sequence, and the sensitivity to detect small mutations such as insertions, deletions, and single nucleotide variants. In test cases where the sequenced pool of plasmids contains subclonal templates, detection sensitivity is similar to that of traditional capillary sequencing.

CONCLUSIONS

Our pipeline can provide significant cost savings compared to outsourcing clinical-grade sequencing of plasmids, making generation of high-quality plasmid sequence for clinical sequence verification more accessible. While other long-read-based methods offer higher-throughput and less cost, our pipeline produces complete and accurate sequence verification for cases where absolute sequence accuracy is required.

A case of epilepsy with myoclonic atonic seizures caused by SLC6A1 gene mutation due to balanced chromosomal translocation

INTRODUCTION

Epilepsy with myoclonic atonic seizures (EMAtS) was previously thought to occur in normally developing children. We report a female case of EMAtS and mild developmental delay before onset. Importantly, a de novo balanced chromosomal translocation was recognized in the patient.

CASE PRESENTATION

The patient was a 4-year-old girl. Mild developmental delay was observed during infancy. At the age of one and a half years, she developed atonic seizures once a month. At 4 years of age, her seizures increased to more than 10 times per hour. An ictal electroencephalogram (EEG) showed a 3-4-Hz spike-and-wave complex, which was consistent with atonic and myoclonic seizures of the trunk, eyelids, and lips. Therefore, EMAtS was diagnosed based on the symptoms and EEG findings. After administration of valproic acid (VPA), the epileptic seizures disappeared immediately. At the age of 5 years and 2 months, the seizures recurred but disappeared again when the dose of VPA was increased. Subsequently, no recurrence was observed until 6 years and 3 months of age on VPA and lamotrigine. Chromosome analysis of the patient disclosed 46,XX,t(3;11)(p25;q13.1)dn. Long-read sequencing of the the patient's genomic DNA revealed that the 3p25.3 translocation breakpoint disrupted the intron 7 of the SLC6A1 gene.

CONCLUSION

The SLC6A1 disruption by chromosome translocation well explains the clinical features of this patient. Long-read sequencing is a powerful technique to determine genomic abnormality at the nucleotide level for disease-associated chromosomal abnormality.

Evaluation of sequencing and PCR-based methods for the quantification of the viral genome formula

Viruses show great diversity in their genome organization. Multipartite viruses package their genome segments into separate particles, most or all of which are required to initiate infection in the host cell. The benefits of such seemingly inefficient genome organization are not well understood. One hypothesised benefit of multipartition is that it allows for flexible changes in gene expression by altering the frequency of each genome segment in different environments, such as encountering different host species. The ratio of the frequency of segments is termed the genome formula (GF). Thus far, formal studies quantifying the GF have been performed for well-characterised virus-host systems in experimental settings using RT-qPCR. However, to understand GF variation in natural populations or novel virus-host systems, a comparison of several methods for GF estimation including high-throughput sequencing (HTS) based methods is needed. Currently, it is unclear how HTS-methods compare a golden standard, such as RT-qPCR. Here we show a comparison of multiple GF quantification methods (RT-qPCR, RT-digital PCR, Illumina RNAseq and Nanopore direct RNA sequencing) using three host plants (Nicotiana tabacum, Nicotiana benthamiana, and Chenopodium quinoa) infected with cucumber mosaic virus (CMV), a tripartite RNA virus. Our results show that all methods give roughly similar results, though there is a significant method effect on genome formula estimates. While the RT-qPCR and RT-dPCR GF estimates are congruent, the GF estimates from HTS methods deviate from those found with PCR. Our findings emphasize the need to tailor the GF quantification method to the experimental aim, and highlight that it may not be possible to compare HTS and PCR-based methods directly. The difference in results between PCR-based methods and HTS highlights that the choice of quantification technique is not trivial.

Hybrid Genome Assembly of Short and Long Reads in Galaxy

Galaxy is a web browser-based data analysis platform that is widely used in biology. Public Galaxy instances allow the analysis of data and interpretation of results without requiring software installation. NanoGalaxy is a public Galaxy instance with tools and workflows for nanopore data analysis. This chapter describes the steps involved in performing genome assembly using short and long reads in NanoGalaxy.

De Novo Genome Assembly of Japanese Black Cattle as Model of an Economically Relevant Animal

A genetic analysis of Japanese Black cattle using short reads and guided by the reference genome from Western breeds would miss the structural variation and/or other unique characteristics of Japanese Black cattle. To overcome this difficulty, a de novo genome assembly independent from the reference genome is required. This chapter describes the technical developments, with respect to both experimental and bioinformatics procedures, including the use of short and long reads, required for de novo genome assembly of Japanese Black cattle.

Unfolding the Bacterial Transcriptome Landscape Using Oxford Nanopore Technology Direct RNA Sequencing

Current genome annotation ignores important features of the transcriptome, such as untranslated regions and operon maps. RNA sequencing (RNA-seq) helps in identifying such features; however, the fragmentation step of classical RNA-seq makes this task challenging. Long-read sequencing methods, such as that of Oxford Nanopore Technologies (ONT), enable the sequencing of intact RNA molecules. Here, we present a method to annotate the full features of bacterial transcriptomes by combining a modified ONT direct RNA-seq method with our computational pipeline, UNAGI bacteria. The method reveals the full complexity of the bacterial transcriptome landscape, including transcription start sites, transcription termination sites, operon maps, and novel genes.

Long-Read Whole-Genome Sequencing Using a Nanopore Sequencer and Detection of Structural Variants in Cancer Genomes

Long-read sequencing technologies enable us to precisely identify structural variants (SVs), which would be occasionally associated with various types of diseases, including cancers. In this section, we introduce experimental and computational procedures for conducting long-read whole-genome sequencing (WGS) of cancer genomes from fresh frozen tissues/cells. We also demonstrate the analysis of SVs in cancer genomes using long-read WGS data from lung cancer cell lines by several representative computational tools, such as cuteSV and Sniffles2, as examples.

Diagnosis of Prader-Willi syndrome and Angelman syndrome by targeted nanopore long-read sequencing

The CpG island flanking the promoter region of SNRPN on chromosome 15q11.2 contains CpG sites that are completely methylated in the maternally derived allele and unmethylated in the paternally derived allele. Both unmethylated and methylated alleles are observed in normal individuals. Only the methylated allele is observed in patients with Prader-Willi syndrome, whereas only the unmethylated allele is observed in those with Angelman syndrome. Hence, detection of aberrant methylation at the differentially methylated region is fundamental to the molecular diagnosis of Prader-Willi syndrome and Angelman syndromes. Traditionally, bisulfite treatment and methylation-sensitive restriction enzyme treatment or methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) have been used. We here developed a long-read sequencing assay that can distinguish methylated and unmethylated CpG sites at 15q11.2 by the difference in current intensity generated from nanopore reads. We successfully diagnosed 4 Prader-Willi syndrome patients and 3 Angelman syndrome patients by targeting differentially methylated regions. Concurrent copy number analysis, homozygosity analysis, and structural variant analysis also allowed us to precisely delineate the underlying pathogenic mechanisms, including gross deletion, uniparental heterodisomy, uniparental isodisomy, or imprinting defect. Furthermore, we showed allele-specific methylation in imprinting-related differentially methylated regions on chromosomes 6, 7, 11, 14, and 20 in a normal individual together with 4 Prader-Willi patients and 3 Angelman syndrome patients. Hence, presently reported method is likely to be applicable to the diagnosis of imprinting disorders other than Prader-Willi syndrome and Angelman syndrome as well.

Advances in sequencing technologies for amyotrophic lateral sclerosis research

Amyotrophic lateral sclerosis (ALS) is caused by upper and lower motor neuron loss and has a fairly rapid disease progression, leading to fatality in an average of 2-5 years after symptom onset. Numerous genes have been implicated in this disease; however, many cases remain unexplained. Several technologies are being used to identify regions of interest and investigate candidate genes. Initial approaches to detect ALS genes include, among others, linkage analysis, Sanger sequencing, and genome-wide association studies. More recently, next-generation sequencing methods, such as whole-exome and whole-genome sequencing, have been introduced. While those methods have been particularly useful in discovering new ALS-linked genes, methodological advances are becoming increasingly important, especially given the complex genetics of ALS. Novel sequencing technologies, like long-read sequencing, are beginning to be used to uncover the contribution of repeat expansions and other types of structural variation, which may help explain missing heritability in ALS. In this review, we discuss how popular and/or upcoming methods are being used to discover ALS genes, highlighting emerging long-read sequencing platforms and their role in aiding our understanding of this challenging disease.