Advances in the discovery and analyses of human tandem repeats

Hypotonia in the Neonatal Intensive Care Unit

Hypotonia is a common presenting symptom in the neonatal intensive care unit (NICU). Hypotonia can be a manifestation of an underlying systemic illness, a primary nervous system disease, or a peripheral nervous system disease. Examination and history can suggest specific causes, but rapid and accurate diagnosis remains challenging due to the broad spectrum of causes. Options for disease-targeted therapies have increased the importance of early diagnosis. This article focuses on the evaluation and diagnostic approach of the hypotonic newborn in the NICU, with an emphasis on rapid identification of treatable conditions and updated recommendations on the utilization of genetic testing.

Toward clinical long-read genome sequencing for rare diseases

Genetic diagnostics is driven by technological advances, forming a tight interface between research, clinic and industry, which enables rapid implementation of new technologies. Short-read genome and exome sequencing, the current state of the art in clinical genetics, can detect a broad spectrum of genetic variants across the genome. However, despite these advancements, more than half of individuals with rare diseases remain undiagnosed after genomic investigations. Long-read whole-genome sequencing (LR-WGS) is a promising technology that identifies previously difficult-to-detect variants while also enabling phasing and methylation analysis and has the potential of generating complete personal assemblies. To pave the way for clinical use of LR-WGS, the clinical genomic community must establish standardized protocols and quality parameters while also developing innovative tools for data analysis and interpretation. In this Perspective, we explore the key challenges and benefits in integrating LR-WGS into routine clinical diagnostics.

A Hitchhiker's Guide to long-read genomic analysis

Over the past decade, long-read sequencing has evolved into a pivotal technology for uncovering the hidden and complex regions of the genome. Significant cost efficiency, scalability, and accuracy advancements have driven this evolution. Concurrently, novel analytical methods have emerged to harness the full potential of long reads. These advancements have enabled milestones such as the first fully completed human genome, enhanced identification and understanding of complex genomic variants, and deeper insights into the interplay between epigenetics and genomic variation. This mini-review provides a comprehensive overview of the latest developments in long-read DNA sequencing analysis, encompassing reference-based and de novo assembly approaches. We explore the entire workflow, from initial data processing to variant calling and annotation, focusing on how these methods improve our ability to interpret a wide array of genomic variants. Additionally, we discuss the current challenges, limitations, and future directions in the field, offering a detailed examination of the state-of-the-art bioinformatics methods for long-read sequencing.

STRchive: a dynamic resource detailing population-level and locus-specific insights at tandem repeat disease loci

Approximately 8% of the human genome consists of repetitive elements called tandem repeats (TRs): short tandem repeats (STRs) of 1-6 bp motifs and variable number tandem repeats (VNTRs) of 7 + bp motifs. TR variants contribute to several dozen monogenic diseases but remain understudied and enigmatic. It remains comparatively challenging to interpret the clinical significance of TR variants, particularly relative to single nucleotide variants. We present STRchive ( http://strchive.org/ ), a dynamic resource consolidating information on TR disease loci from the research literature, up-to-date clinical resources, and large-scale genomic databases, streamlining TR variant interpretation at disease-associated loci.

RNA gain-of-function mechanisms in short tandem repeat diseases

As adaptors, catalysts, guides, messengers, scaffolds, and structural components, RNAs perform an impressive array of cellular regulatory functions often by recruiting RNA-binding proteins (RBPs) to form ribonucleoprotein complexes (RNPs). While this RNA-RBP interaction network allows precise RNP assembly and the subsequent structural dynamics required for normal functions, RNA motif mutations may trigger the formation of aberrant RNP structures that lead to cell dysfunction and disease. Here, we provide our perspective on one type of RNA motif mutation, RNA gain-of-function mutations associated with the abnormal expansion of short tandem repeats (STRs) that underlie multiple developmental and degenerative diseases. We first discuss our current understanding of normal polymorphic STR functions in RNA processing and localization followed by an assessment of the pathogenic roles of STR expansions in the neuromuscular disease myotonic dystrophy. We also highlight ongoing questions and controversies focused on STR-based insights into the regulation of nuclear RNA processing and export as well as the relevance of the RNA gain-of-function pathomechanism for other STR expansion disorders in both coding and noncoding genes.

De novo assembly and characterization of the complete mitochondrial genome of Phellodendron amurense reveals three repeat-mediated recombination

Phellodendron amurense Rupr., a rare herb renowned for its medicinal and ecological significance, has remained genetically unexplored at the mitochondrial level until now. This study presents the first-ever systematic assembly and annotation of the complete mitochondrial genome of P. amurense, achieved through a hybrid strategy combining Illumina and Nanopore sequencing data. The mitochondrial genome spans 566,285 bp with a GC content of 45.51 %, structured into two circular molecules. Our comprehensive analysis identified 32 protein-coding genes (PCGs), 33 tRNA genes, and 3 rRNA genes, alongside 181 simple sequence repeats, 19 tandem repeats, and 310 dispersed repeats. Notably, multiple genome conformations were predicted due to repeat-mediated homologous recombination. Additionally, we assembled the chloroplast genome, identifying 21 mitochondrial plastid sequences that provide insights into organelle genome interactions. A total of 380 RNA-editing sites within the mitochondrial PCGs were predicted, enhancing our understanding of gene regulation and function. Phylogenetic analysis using mitochondrial PCGs from 30 species revealed evolutionary relationships, confirming the homology between P. amurense and Citrus species. This foundational study offers a valuable genetic resource for the Rutaceae family, facilitating further research into genetic evolution and molecular diversity in plant mitochondrial genomes.

Copy Number Variations of Human Ribosomal Genes in Health and Disease: Role and Causes

BACKGROUND

A number of association studies have linked ribosomal DNA gene copy number (rDNA CN) to aging and pathology. Data from these studies are contradictory and depend on the quantitative method.

METHODS

The hybridization technique was used for rDNA quantification in human cells. We determined the rDNA CN from healthy controls (HCs) and patients with schizophrenia (SZ) or cystic fibrosis (CF) (total number of subjects N = 1124). For the first time, rDNA CN was quantified in 105 long livers (90-101 years old). In addition, we conducted a joint analysis of the data obtained in this work and previously published by our group (total, N = 3264).

RESULTS

We found increased rDNA CN in the SZ group (534 ± 108, N = 1489) and CF group (567 ± 100, N = 322) and reduced rDNA CN in patients with mild cognitive impairment (330 ± 60, N = 93) compared with the HC group (422 ± 104, N = 1360). For the SZ, CF, and HC groups, there was a decreased range of rDNA CN variation in older age subgroups compared to child subgroups. For 311 patients with SZ or CF, rDNA CN was determined two or three times, with an interval of months to several years. Only 1.2% of patients demonstrated a decrease in rDNA CN over time. We did not find significant rDNA CN variation in eight different organs of the same patient or in cells of the same fibroblast population.

CONCLUSIONS

The results suggest that rDNA CN is a relatively stable quantitative genetic trait statistically associated with some diseases, which however, can change in rare cases under conditions of chronic oxidative stress. We believe that age- and disease-related differences between the groups in mean rDNA CN and its variance are caused by the biased elimination of carriers of marginal (predominantly low) rDNA CN values.

Enhanced detection and genotyping of disease-associated tandem repeats using HMMSTR and targeted long-read sequencing

Tandem repeat sequences comprise approximately 8% of the human genome and are linked to more than 50 neurodegenerative disorders. Accurate characterization of disease-associated repeat loci remains resource intensive and often lacks high resolution genotype calls. We introduce a multiplexed, targeted nanopore sequencing panel and HMMSTR, a sequence-based tandem repeat copy number caller which outperforms current signal- and sequence-based callers relative to two assemblies and we show it performs with high accuracy in heterozygous regions and at low read coverage. The flexible panel allows us to capture disease associated regions at an average coverage of >150x. Using these tools, we successfully characterize known or suspected repeat expansions in patient derived samples. In these samples, we also identify unexpected expanded alleles at tandem repeat loci not previously associated with the underlying diagnosis. This genotyping approach for tandem repeat expansions is scalable, simple, flexible and accurate, offering significant potential for diagnostic applications and investigation of expansion co-occurrence in neurodegenerative disorders.

Genome-wide investigation of VNTR motif polymorphisms in 8,222 genomes: Implications for biological regulation and human traits

Variable number tandem repeat (VNTR) is a pervasive and highly mutable genetic feature that varies in both length and repeat sequence. Despite the well-studied copy-number variants, the functional impacts of repeat motif polymorphisms remain unknown. Here, we present the largest genome-wide VNTR polymorphism map to date, with over 2.5 million VNTR length polymorphisms (VNTR-LPs) and over 11 million VNTR motif polymorphisms (VNTR-MPs) detected in 8,222 high-coverage genomes. Leveraging the large-scale NyuWa cohort, we identified 2,982,456 (31.8%) NyuWa-specific VNTR-MPs, of which 95.3% were rare. Moreover, we found 1,937 out of 38,685 VNTRs that were associated with gene expression through VNTR-MPs in lymphoblastoid cell lines. Specifically, we clarified that the expansion of a likely causal motif could upregulate gene expression by improving the binding concentration of PU.1. We also explored the potential impacts of VNTR polymorphisms on phenotypic differentiation and disease susceptibility. This study expands our knowledge of VNTR-MPs and their functional implications.

High-coverage nanopore sequencing of samples from the 1000 Genomes Project to build a comprehensive catalog of human genetic variation

Fewer than half of individuals with a suspected Mendelian or monogenic condition receive a precise molecular diagnosis after comprehensive clinical genetic testing. Improvements in data quality and costs have heightened interest in using long-read sequencing (LRS) to streamline clinical genomic testing, but the absence of control data sets for variant filtering and prioritization has made tertiary analysis of LRS data challenging. To address this, the 1000 Genomes Project (1KGP) Oxford Nanopore Technologies Sequencing Consortium aims to generate LRS data from at least 800 of the 1KGP samples. Our goal is to use LRS to identify a broader spectrum of variation so we may improve our understanding of normal patterns of human variation. Here, we present data from analysis of the first 100 samples, representing all 5 superpopulations and 19 subpopulations. These samples, sequenced to an average depth of coverage of 37× and sequence read N50 of 54 kbp, have high concordance with previous studies for identifying single nucleotide and indel variants outside of homopolymer regions. Using multiple structural variant (SV) callers, we identify an average of 24,543 high-confidence SVs per genome, including shared and private SVs likely to disrupt gene function as well as pathogenic expansions within disease-associated repeats that were not detected using short reads. Evaluation of methylation signatures revealed expected patterns at known imprinted loci, samples with skewed X-inactivation patterns, and novel differentially methylated regions. All raw sequencing data, processed data, and summary statistics are publicly available, providing a valuable resource for the clinical genetics community to discover pathogenic SVs.

Optical Genome Mapping for Applications in Repeat Expansion Disorders

Short tandem repeat (STR) expansions are associated with more than 60 genetic disorders. The size and stability of these expansions correlate with the severity and age of onset of the disease. Therefore, being able to accurately detect the absolute length of STRs is important. Current diagnostic assays include laborious lab experiments, including repeat-primed PCR and Southern blotting, that still cannot precisely determine the exact length of very long repeat expansions. Optical genome mapping (OGM) is a cost-effective and easy-to-use alternative to traditional cytogenetic techniques and allows the comprehensive detection of chromosomal aberrations and structural variants >500 bp in length, including insertions, deletions, duplications, inversions, translocations, and copy number variants. Here, we provide methodological guidance for preparing samples and performing OGM as well as running the analysis pipelines and using the specific repeat expansion workflows to determine the exact repeat length of repeat expansions expanded beyond 500 bp. Together these protocols provide all details needed to analyze the length and stability of any repeat expansion with an expected repeat size difference from the expected wild-type allele of >500 bp. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Genomic ultra-high-molecular-weight DNA isolation, labeling, and staining Basic Protocol 2: Data generation and genome mapping using the Bionano Saphyr® System Basic Protocol 3: Manual De Novo Assembly workflow Basic Protocol 4: Local guided assembly workflow Basic Protocol 5: EnFocus Fragile X workflow Basic Protocol 6: Molecule distance script workflow.

STRchive: a dynamic resource detailing population-level and locus-specific insights at tandem repeat disease loci

Approximately 3% of the human genome consists of repetitive elements called tandem repeats (TRs), which include short tandem repeats (STRs) of 1-6bp motifs and variable number tandem repeats (VNTRs) of 7+bp motifs. TR variants contribute to several dozen mono- and polygenic diseases but remain understudied and "enigmatic," particularly relative to single nucleotide variants. It remains comparatively challenging to interpret the clinical significance of TR variants. Although existing resources provide portions of necessary data for interpretation at disease-associated loci, it is currently difficult or impossible to efficiently invoke the additional details critical to proper interpretation, such as motif pathogenicity, disease penetrance, and age of onset distributions. It is also often unclear how to apply population information to analyses. We present STRchive (S-T-archive, http://strchive.org/ ), a dynamic resource consolidating information on TR disease loci in humans from research literature, up-to-date clinical resources, and large-scale genomic databases, with the goal of streamlining TR variant interpretation at disease-associated loci. We apply STRchive -including pathogenic thresholds, motif classification, and clinical phenotypes-to a gnomAD cohort of ∼18.5k individuals genotyped at 60 disease-associated loci. Through detailed literature curation, we demonstrate that the majority of TR diseases affect children despite being thought of as adult diseases. Additionally, we show that pathogenic genotypes can be found within gnomAD which do not necessarily overlap with known disease prevalence, and leverage STRchive to interpret locus-specific findings therein. We apply a diagnostic blueprint empowered by STRchive to relevant clinical vignettes, highlighting possible pitfalls in TR variant interpretation. As a living resource, STRchive is maintained by experts, takes community contributions, and will evolve as understanding of TR diseases progresses.

Enhanced Detection and Genotyping of Disease-Associated Tandem Repeats Using HMMSTR and Targeted Long-Read Sequencing

Tandem repeat sequences comprise approximately 8% of the human genome and are linked to more than 50 neurodegenerative disorders. Accurate characterization of disease-associated repeat loci remains resource intensive and often lacks high resolution genotype calls. We introduce a multiplexed, targeted nanopore sequencing panel and HMMSTR, a sequence-based tandem repeat copy number caller. HMMSTR outperforms current signal- and sequence-based callers relative to two assemblies and we show it performs with high accuracy in heterozygous regions and at low read coverage. The flexible panel allows us to capture disease associated regions at an average coverage of >150x. Using these tools, we successfully characterize known or suspected repeat expansions in patient derived samples. In these samples we also identify unexpected expanded alleles at tandem repeat loci not previously associated with the underlying diagnosis. This genotyping approach for tandem repeat expansions is scalable, simple, flexible, and accurate, offering significant potential for diagnostic applications and investigation of expansion co-occurrence in neurodegenerative disorders.

Abstract Figure

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.

Nanopore sequencing of 1000 Genomes Project samples to build a comprehensive catalog of human genetic variation

Less than half of individuals with a suspected Mendelian condition receive a precise molecular diagnosis after comprehensive clinical genetic testing. Improvements in data quality and costs have heightened interest in using long-read sequencing (LRS) to streamline clinical genomic testing, but the absence of control datasets for variant filtering and prioritization has made tertiary analysis of LRS data challenging. To address this, the 1000 Genomes Project ONT Sequencing Consortium aims to generate LRS data from at least 800 of the 1000 Genomes Project samples. Our goal is to use LRS to identify a broader spectrum of variation so we may improve our understanding of normal patterns of human variation. Here, we present data from analysis of the first 100 samples, representing all 5 superpopulations and 19 subpopulations. These samples, sequenced to an average depth of coverage of 37x and sequence read N50 of 54 kbp, have high concordance with previous studies for identifying single nucleotide and indel variants outside of homopolymer regions. Using multiple structural variant (SV) callers, we identify an average of 24,543 high-confidence SVs per genome, including shared and private SVs likely to disrupt gene function as well as pathogenic expansions within disease-associated repeats that were not detected using short reads. Evaluation of methylation signatures revealed expected patterns at known imprinted loci, samples with skewed X-inactivation patterns, and novel differentially methylated regions. All raw sequencing data, processed data, and summary statistics are publicly available, providing a valuable resource for the clinical genetics community to discover pathogenic SVs.

Expanding horizons of tandem repeats in biology and medicine: Why 'genomic dark matter' matters

Approximately half of the human genome includes repetitive sequences, and these DNA sequences (as well as their transcribed repetitive RNA and translated amino-acid repeat sequences) are known as the repeatome. Within this repeatome there are a couple of million tandem repeats, dispersed throughout the genome. These tandem repeats have been estimated to constitute ∼8% of the entire human genome. These tandem repeats can be located throughout exons, introns and intergenic regions, thus potentially affecting the structure and function of tandemly repetitive DNA, RNA and protein sequences. Over more than three decades, more than 60 monogenic human disorders have been found to be caused by tandem-repeat mutations. These monogenic tandem-repeat disorders include Huntington's disease, a variety of ataxias, amyotrophic lateral sclerosis and frontotemporal dementia, as well as many other neurodegenerative diseases. Furthermore, tandem-repeat disorders can include fragile X syndrome, related fragile X disorders, as well as other neurological and psychiatric disorders. However, these monogenic tandem-repeat disorders, which were discovered via their dominant or recessive modes of inheritance, may represent the 'tip of the iceberg' with respect to tandem-repeat contributions to human disorders. A previous proposal that tandem repeats may contribute to the 'missing heritability' of various common polygenic human disorders has recently been supported by a variety of new evidence. This includes genome-wide studies that associate tandem-repeat mutations with autism, schizophrenia, Parkinson's disease and various types of cancers. In this article, I will discuss how tandem-repeat mutations and polymorphisms could contribute to a wide range of common disorders, along with some of the many major challenges of tandem-repeat biology and medicine. Finally, I will discuss the potential of tandem repeats to be therapeutically targeted, so as to prevent and treat an expanding range of human disorders.