Thousands of high-quality sequencing samples fail to show meaningful correlation between 5S and 45S ribosomal DNA arrays in humans

The black honey bee genome: insights on specific structural elements and a first step towards pangenomes

BACKGROUND

The honey bee reference genome, HAv3.1, was produced from a commercial line sample that was thought to have a largely dominant Apis mellifera ligustica genetic background. Apis mellifera mellifera, often referred to as the black bee, has a separate evolutionary history and is the original type in western and northern Europe. Growing interest in this subspecies for conservation and non-professional apicultural practices, together with the necessity of deciphering genome backgrounds in hybrids, triggered the necessity for a specific genome assembly. Moreover, having several high-quality genomes is becoming key for taking structural variations into account in pangenome analyses.

RESULTS

Pacific Bioscience technology long reads were produced from a single haploid black bee drone. Scaffolding contigs into chromosomes was done using a high-density genetic map. This allowed for re-estimation of the recombination rate, which was over-estimated in some previous studies due to mis-assemblies, which resulted in spurious inversions in the older reference genomes. The sequence continuity obtained was very high and the only limit towards continuous chromosome-wide sequences seemed to be due to tandem repeat arrays that were usually longer than 10 kb and that belonged to two main families, the 371 and 91 bp repeats, causing problems in the assembly process due to high internal sequence similarity. Our assembly was used together with the reference genome to genotype two structural variants by a pangenome graph approach with Graphtyper2. Genotypes obtained were either correct or missing, when compared to an approach based on sequencing depth analysis, and genotyping rates were 89 and 76% for the two variants.

CONCLUSIONS

Our new assembly for the Apis mellifera mellifera honey bee subspecies demonstrates the utility of multiple high-quality genomes for the genotyping of structural variants, with a test case on two insertions and deletions. It will therefore be an invaluable resource for future studies, for instance by including structural variants in GWAS. Having used a single haploid drone for sequencing allowed a refined analysis of very large tandem repeat arrays, raising the question of their function in the genome. High quality genome assemblies for multiple subspecies such as presented here, are crucial for emerging projects using pangenomes.

Ribosomal DNA copy number is associated with body mass in humans and other mammals

Body mass results from a complex interplay between genetics and environment. Previous studies of the genetic contribution to body mass have excluded repetitive regions due to the technical limitations of platforms used for population scale studies. Here we apply genome-wide approaches, identifying an association between adult body mass and the copy number (CN) of 47S-ribosomal DNA (rDNA). rDNA codes for the 18 S, 5.8 S and 28 S ribosomal RNA (rRNA) components of the ribosome. In mammals, there are hundreds of copies of these genes. Inter-individual variation in the rDNA CN has not previously been associated with a mammalian phenotype. Here, we show that rDNA CN variation associates with post-pubertal growth rate in rats and body mass index in adult humans. rDNA CN is not associated with rRNA transcription rates in adult tissues, suggesting the mechanistic link occurs earlier in development. This aligns with the observation that the association emerges by early adulthood.

Ribosomal DNA copy number variation associates with hematological profiles and renal function in the UK Biobank

The phenotypic impact of genetic variation of repetitive features in the human genome is currently understudied. One such feature is the multi-copy 47S ribosomal DNA (rDNA) that codes for rRNA components of the ribosome. Here, we present an analysis of rDNA copy number (CN) variation in the UK Biobank (UKB). From the first release of UKB whole-genome sequencing (WGS) data, a discovery analysis in White British individuals reveals that rDNA CN associates with altered counts of specific blood cell subtypes, such as neutrophils, and with the estimated glomerular filtration rate, a marker of kidney function. Similar trends are observed in other ancestries. A range of analyses argue against reverse causality or common confounder effects, and all core results replicate in the second UKB WGS release. Our work demonstrates that rDNA CN is a genetic influence on trait variance in humans.

Ribosomal DNA Copy Number Variation is Coupled with DNA Methylation Changes at the 45S rDNA Locus

The human ribosomal DNA (rDNA) copy number (CN) has been challenging to analyse, and its sequence has been excluded from reference genomes due to its highly repetitive nature. The 45S rDNA locus encodes essential components of the cell, nevertheless rDNA displays high inter-individual CN variation that could influence human health and disease. CN alterations in rDNA have been hypothesized as a possible factor in autism spectrum disorders (ASD) and were shown to be altered in Schizophrenia patients. We tested whether whole-genome bisulphite sequencing can be used to simultaneously quantify rDNA CN and measure DNA methylation at the 45S rDNA locus. Using this approach, we observed high inter-individual variation in rDNA CN, and limited intra-individual copy differences in several post-mortem tissues. Furthermore, we did not observe any significant alterations in rDNA CN or DNA methylation in Autism Spectrum Disorder (ASD) brains in 16 ASD vs 11 control samples. Similarly, no difference was detected when comparing neurons form 28 Schizophrenia (Scz) patients vs 25 controls or oligodendrocytes from 22 Scz samples vs 20 controls. However, our analysis revealed a strong positive correlation between CN and DNA methylation at the 45S rDNA locus in multiple tissues. This was observed in brain and confirmed in small intestine, adipose tissue, and gastric tissue. This should shed light on a possible dosage compensation mechanism that silences additional rDNA copies to ensure homoeostatic regulation of ribosome biogenesis.

Developing population identification tool based on polymorphism of rDNA for traditional Chinese medicine: Artemisia annua L

BACKGROUND

Artemisia annua, a well-known traditional Chinese medicine, is the main source for production of artemisinin, an anti-malaria drug. A. annua is distributed globally, with great diversity of morphological characteristics and artemisinin contents. Diverse traits among A. annua populations impeded the stable production of artemisinin, which needs an efficient tool to identify strains and assess population genetic homogeneity.

PURPOSE

In this study, ribosomal DNA (rDNA), were characterized for A. annua for strains identification and population genetic homogeneity assessment.

METHODS

The ribosomal RNA (rRNA) genes were identified using cmscan and assembled using rDNA unit of LQ-9 as a reference. rDNA among Asteraceae species were compared performing with 45S rDNA. The rDNA copy number was calculated based on sequencing depth. The polymorphisms of rDNA sequences were identified with bam-readcount, and confirmed by Sanger sequencing and restriction enzyme experiment. The ITS2 amplicon sequencing was used to verify the stability of ITS2 haplotype analysis.

RESULTS

Different from other Asteraceae species, 45S and 5S linked-type rDNA was only found in Artemisia genus. Rich polymorphisms of copy number and sequence of rDNA were identified in A. annua population. The haplotype composition of internal transcribed spacer 2 (ITS2) region which had moderate sequence polymorphism and relative short size was significantly different among A. annua strains. A population discrimination method was developed based on ITS2 haplotype analysis with high-throughput sequencing.

CONCLUSION

This study provides comprehensive characteristics of rDNA and suggests that ITS2 haplotype analysis is ideal tool for A. annua strain identification and population genetic homogeneity assessment.

Substantial rDNA copy number reductions alter timing of development and produce variable tissue-specific phenotypes in C. elegans

The genes that encode ribosomal RNAs are present in several hundred copies in most eukaryotes. These vast arrays of repetitive ribosomal DNA (rDNA) have been implicated not just in ribosome biogenesis, but also aging, cancer, genome stability, and global gene expression. rDNA copy number is highly variable among and within species; this variability is thought to associate with traits relevant to human health and disease. Here we investigate the phenotypic consequences of multicellular life at the lower bounds of rDNA copy number. We use the model Caenorhabditis elegans, which has previously been found to complete embryogenesis using only maternally provided ribosomes. We find that individuals with rDNA copy number reduced to ∼5% of wild type are capable of further development with variable penetrance. Such individuals are sterile and exhibit severe morphological defects, particularly in post-embryonically dividing tissues such as germline and vulva. Developmental completion and fertility are supported by an rDNA copy number ∼10% of wild type, with substantially delayed development. Worms with rDNA copy number reduced to ∼33% of wild type display a subtle developmental timing defect that was absent in worms with higher copy numbers. Our results support the hypothesis that rDNA requirements vary across tissues and indicate that the minimum rDNA copy number for fertile adulthood is substantially less than the lowest naturally observed total copy number. The phenotype of individuals with severely reduced rDNA copy number is highly variable in penetrance and presentation, highlighting the need for continued investigation into the biological consequences of rDNA copy number variation.

Deciphering the sex bias in housekeeping gene expression in adipose tissue: a comprehensive meta-analysis of transcriptomic studies

BACKGROUND

As the housekeeping genes (HKG) generally involved in maintaining essential cell functions are typically assumed to exhibit constant expression levels across cell types, they are commonly employed as internal controls in gene expression studies. Nevertheless, HKG may vary gene expression profile according to different variables introducing systematic errors into experimental results. Sex bias can indeed affect expression display, however, up to date, sex has not been typically considered as a biological variable.

METHODS

In this study, we evaluate the expression profiles of six classical housekeeping genes (four metabolic: GAPDH, HPRT, PPIA, and UBC, and two ribosomal: 18S and RPL19) to determine expression stability in adipose tissues (AT) of Homo sapiens and Mus musculus and check sex bias and their overall suitability as internal controls. We also assess the expression stability of all genes included in distinct whole-transcriptome microarrays available from the Gene Expression Omnibus database to identify sex-unbiased housekeeping genes (suHKG) suitable for use as internal controls. We perform a novel computational strategy based on meta-analysis techniques to identify any sexual dimorphisms in mRNA expression stability in AT and to properly validate potential candidates.

RESULTS

Just above half of the considered studies informed properly about the sex of the human samples, however, not enough female mouse samples were found to be included in this analysis. We found differences in the HKG expression stability in humans between female and male samples, with females presenting greater instability. We propose a suHKG signature including experimentally validated classical HKG like PPIA and RPL19 and novel potential markers for human AT and discarding others like the extensively used 18S gene due to a sex-based variability display in adipose tissue. Orthologs have also been assayed and proposed for mouse WAT suHKG signature. All results generated during this study are readily available by accessing an open web resource ( https://bioinfo.cipf.es/metafun-HKG ) for consultation and reuse in further studies.

CONCLUSIONS

This sex-based research proves that certain classical housekeeping genes fail to function adequately as controls when analyzing human adipose tissue considering sex as a variable. We confirm RPL19 and PPIA suitability as sex-unbiased human and mouse housekeeping genes derived from sex-specific expression profiles, and propose new ones such as RPS8 and UBB.

Under the magnifying glass: The ups and downs of rDNA copy number

The ribosomal DNA (rDNA) in Drosophila is found as two additive clusters of individual 35 S cistrons. The multiplicity of rDNA is essential to assure proper translational demands, but the nature of the tandem arrays expose them to copy number variation within and between populations. Here, we discuss means by which a cell responds to insufficient rDNA copy number, including a historical view of rDNA magnification whose mechanism was inferred some 35 years ago. Recent work has revealed that multiple conditions may also result in rDNA loss, in response to which rDNA magnification may have evolved. We discuss potential models for the mechanism of magnification, and evaluate possible consequences of rDNA copy number variation.

A new method for determining ribosomal DNA copy number shows differences between Saccharomyces cerevisiae populations

Ribosomal DNA genes (rDNA) encode the major ribosomal RNAs and in eukaryotes typically form tandem repeat arrays. Species have characteristic rDNA copy numbers, but there is substantial intra-species variation in copy number that results from frequent rDNA recombination. Copy number differences can have phenotypic consequences, however difficulties in quantifying copy number mean we lack a comprehensive understanding of how copy number evolves and the consequences. Here we present a genomic sequence read approach to estimate rDNA copy number based on modal coverage to help overcome limitations with existing mean coverage-based approaches. We validated our method using Saccharomyces cerevisiae strains with known rDNA copy numbers. Application of our pipeline to a global sample of S. cerevisiae isolates showed that different populations have different rDNA copy numbers. Our results demonstrate the utility of the modal coverage method, and highlight the high level of rDNA copy number variation within and between populations.

Short arms of human acrocentric chromosomes and the completion of the human genome sequence

The complete, ungapped sequence of the short arms of human acrocentric chromosomes (SAACs) is still unknown almost 20 years after the near completion of the Human Genome Project. Yet these short arms of Chromosomes 13, 14, 15, 21, and 22 contain the ribosomal DNA (rDNA) genes, which are of paramount importance for human biology. The sequences of SAACs show an extensive variation in the copy number of the various repetitive elements, the full extent of which is currently unknown. In addition, the full spectrum of repeated sequences, their organization, and the low copy number functional elements are also unknown. The Telomere-to-Telomere (T2T) Project using mainly long-read sequence technology has recently completed the assembly of the genome from a hydatidiform mole, CHM13, and has thus established a baseline reference for further studies on the organization, variation, functional annotation, and impact in human disorders of all the previously unknown genomic segments, including the SAACs. The publication of the initial results of the T2T Project will update and improve the reference genome for a better understanding of the evolution and function of the human genome.

Genomic architecture of 5S rDNA cluster and its variations within and between species

BACKGROUND

Ribosomal DNAs (rDNAs) are arranged in purely tandem repeats, preventing them from being reliably assembled onto chromosomes during generation of genome assembly. The uncertainty of rDNA genomic structure presents a significant barrier for studying their function and evolution.

RESULTS

Here we generate ultra-long Oxford Nanopore Technologies (ONT) and short NGS reads to delineate the architecture and variation of the 5S rDNA cluster in the different strains of C. elegans and C. briggsae. We classify the individual rDNA's repeating units into 25 types based on the unique sequence variations in each unit of C. elegans (N2). We next perform assembly of the cluster by taking advantage of the long reads that carry these units, which led to an assembly of 5S rDNA cluster consisting of up to 167 consecutive 5S rDNA units in the N2 strain. The ordering and copy number of various rDNA units are consistent with the separation time between strains. Surprisingly, we observed a drastically reduced level of variation in the unit composition in the 5S rDNA cluster in the C. elegans CB4856 and C. briggsae AF16 strains than in the C. elegans N2 strain, suggesting that N2, a widely used reference strain, is likely to be defective in maintaining the 5S rDNA cluster stability compared with other wild isolates of C. elegans or C. briggsae.

CONCLUSIONS

The results demonstrate that Nanopore DNA sequencing reads are capable of generating assembly of highly repetitive sequences, and rDNA units are highly dynamic both within and between population(s) of the same species in terms of sequence and copy number. The detailed structure and variation of the 5S rDNA units within the rDNA cluster pave the way for functional and evolutionary studies.

First discovered, long out of sight, finally visible: ribosomal DNA

With the advent of long-read sequencing, previously unresolvable genomic elements are being revisited in an effort to generate fully complete reference genomes. One such element is ribosomal DNA (rDNA), the highly conserved genomic region that encodes rRNAs. Genomic structure and content of the rDNA are variable in both prokarya and eukarya, posing interesting questions about the biology of rDNA. Here, we consider the types of variation observed in rDNA - including locus structure and number, copy number, and sequence variation - and their known phenotypic consequences. With recent advances in long-read sequencing technology, incorporating the full rDNA sequence into reference genomes is within reach. This knowledge will have important implications for understanding rDNA biology within the context of cell physiology and whole-organism phenotypes.

Genetic variation at mouse and human ribosomal DNA influences associated epigenetic states

BACKGROUND

Ribosomal DNA (rDNA) displays substantial inter-individual genetic variation in human and mouse. A systematic analysis of how this variation impacts epigenetic states and expression of the rDNA has thus far not been performed.

RESULTS

Using a combination of long- and short-read sequencing, we establish that 45S rDNA units in the C57BL/6J mouse strain exist as distinct genetic haplotypes that influence the epigenetic state and transcriptional output of any given unit. DNA methylation dynamics at these haplotypes are dichotomous and life-stage specific: at one haplotype, the DNA methylation state is sensitive to the in utero environment, but refractory to post-weaning influences, whereas other haplotypes entropically gain DNA methylation during aging only. On the other hand, individual rDNA units in human show limited evidence of genetic haplotypes, and hence little discernible correlation between genetic and epigenetic states. However, in both species, adjacent units show similar epigenetic profiles, and the overall epigenetic state at rDNA is strongly positively correlated with the total rDNA copy number. Analysis of different mouse inbred strains reveals that in some strains, such as 129S1/SvImJ, the rDNA copy number is only approximately 150 copies per diploid genome and DNA methylation levels are < 5%.

CONCLUSIONS

Our work demonstrates that rDNA-associated genetic variation has a considerable influence on rDNA epigenetic state and consequently rRNA expression outcomes. In the future, it will be important to consider the impact of inter-individual rDNA (epi)genetic variation on mammalian phenotypes and diseases.

Nucleolar Organizer Regions as Transcription-Based Scaffolds of Nucleolar Structure and Function

Eukaryotic genomes maintain multiple copies of ribosomal DNA gene repeats in tandem arrays to provide sufficient ribosomal RNAs to make ribosomes. These DNA repeats are the most highly transcribed regions of the genome, with dedicated transcriptional machinery to manage the enormous task of producing more than 50% of the total RNA in a proliferating cell. The arrays are called nucleolar organizer regions (NORs) and constitute the scaffold of the nucleolar compartment, where ribosome biogenesis occurs. Advances in molecular and cellular biology have brought great insights into how these arrays are transcribed and organized within genomes. Much of their biology is driven by their high transcription level, which has also driven the development of unique methods to understand rDNA gene activity, beginning with classic techniques such as silver staining and Miller spreads. However, the application of modern methodologies such as CRISPR gene editing, super-resolution microscopy, and long-read sequencing has enabled recent advances described herein, with many more discoveries possible soon. This chapter highlights what is known about NOR transcription and organization and the techniques applied historically and currently. Given the potential for NORs to impact organismal health and disease, as highlighted at the end of the chapter, the field must continue to develop and apply innovative analysis to understand genetic, epigenetic, and organizer properties of the ribosomal DNA repeats.

Analyses of the Updated "Animal rDNA Loci Database" with an Emphasis on Its New Features

We report on a major update to the animal rDNA loci database, which now contains cytogenetic information for 45S and 5S rDNA loci in more than 2600 and 1000 species, respectively. The data analyses show the following: (i) A high variability in 5S and 45S loci numbers, with both showing 50-fold or higher variability. However, karyotypes with an extremely high number of loci were rare, and medians generally converged to two 5S sites and two 45S rDNA sites per diploid genome. No relationship was observed between the number of 5S and 45S loci. (ii) The position of 45S rDNA on sex chromosomes was relatively frequent in some groups, particularly in arthropods (14% of karyotypes). Furthermore, 45S rDNA was almost exclusively located in microchromosomes when these were present (in birds and reptiles). (iii) The proportion of active NORs (positively stained with silver staining methods) progressively decreased with an increasing number of 45S rDNA loci, and karyotypes with more than 12 loci showed, on average, less than 40% of active loci. In conclusion, the updated version of the database provides some new insights into the organization of rRNA genes in chromosomes. We expect that its updated content will be useful for taxonomists, comparative cytogeneticists, and evolutionary biologists.

Expanding studies of chromosome structure and function in the era of T2T genomics

The recent accomplishment of a truly complete human genome has afforded a new view of chromosome structure and function that was limited 30 years ago. Here, we discuss the expansion of knowledge from the early cytological studies of the genome to the current high-resolution genomic, epigenetic and functional maps that have been achieved by recent technology and computational advances. These studies have revealed unexpected complexities of genome organization and function and uncovered new views of fundamental chromosomal elements. Comprehensive genomic maps will enable accurate diagnosis of human diseases caused by altered chromosome structure and function, facilitate development of chromosome-based therapies and shape the future of preventative medicine and healthcare.