Disruption of regulatory domains and novel transcripts as disease-causing mechanisms

Multi-tool copy number detection highlights common body size-associated variants in miniature pig breeds from different geographical regions

BACKGROUND

Copy number variations (CNVs) represent a common and highly specific type of variation in the genome, potentially influencing genetic diversity and mammalian phenotypic development. Structural variants, such as deletions, duplications, and insertions, have frequently been highlighted as key factors influencing traits in high-production pigs. However, comprehensive CNV analyses in miniature pig breeds are limited despite their value in biomedical research.

RESULTS

This study performed whole-genome sequencing in 36 miniature pigs from nine breeds from America, Asia and Oceania, and Europe. By employing a multi-tool approach (CNVpytor, Delly, GATK gCNV, Smoove), the accuracy of CNV identification was improved. In total, 34 homozygous CNVs overlapped with exonic regions in all samples, suggesting a role in expressing specific phenotypes such as uniform growth patterns, fertility, or metabolic function. In addition, 386 copy number variation regions (CNVRs) shared by all breeds were detected, covering 33.6 Mb (1.48% of the autosomal genome). Further, 132 exclusive CNVRs were identified for American breeds, 47 for Asian and Oceanian breeds, and 114 for European breeds. Functional enrichment analysis revealed genes within the common CNVRs involved in body height determination and other growth-related parameters. Exclusive CNVRs were located in the region of genes enriched for lipid metabolism in American minipigs, reproductive traits in Asian and Oceanian breeds, and cardiovascular features and body height in European breeds. In the selected groups, quantitative trait loci associated with body size, meat quality, reproduction, and disease susceptibility were highlighted.

CONCLUSION

This investigation of the CNV landscape of minipigs underlines the impact of selective breeding on structural variants and its role in the development of specific breed phenotypes across geographical areas. The multi-tool approach provides a valuable resource for future studies on the effects of artificial selection on livestock genomes.

Identifying transgene insertions in Caenorhabditis elegans genomes with Oxford Nanopore sequencing

Genetically modified organisms are commonly used in disease research and agriculture but the precise genomic alterations underlying transgenic mutations are often unknown. The position and characteristics of transgenes, including the number of independent insertions, influences the expression of both transgenic and wild-type sequences. We used long-read, Oxford Nanopore Technologies (ONT) to sequence and assemble two transgenic strains of Caenorhabditis elegans commonly used in the research of neurodegenerative diseases: BY250 (pPdat-1::GFP) and UA44 (GFP and human α-synuclein), a model for Parkinson's research. After scaffolding to the reference, the final assembled sequences were ∼102 Mb with N50s of 17.9 Mb and 18.0 Mb, respectively, and L90s of six contiguous sequences, representing chromosome-level assemblies. Each of the assembled sequences contained more than 99.2% of the Nematoda BUSCO genes found in the C. elegans reference and 99.5% of the annotated C. elegans reference protein-coding genes. We identified the locations of the transgene insertions and confirmed that all transgene sequences were inserted in intergenic regions, leaving the organismal gene content intact. The transgenic C. elegans genomes presented here will be a valuable resource for Parkinson's research as well as other neurodegenerative diseases. Our work demonstrates that long-read sequencing is a fast, cost-effective way to assemble genome sequences and characterize mutant lines and strains.

Enhancers on the edge - how the nuclear envelope controls gene regulatory elements

Precise temporal and sequential control of gene expression during development and in response to environmental stimuli requires tight regulation of the physical contact between gene regulatory elements and promoters. Current models describing how the genome folds in 3D space to establish these interactions often ignore the role of the most stable structural nuclear feature - the nuclear envelope. While contributions of 3D folding within/between topologically associated domains (TADs) have been extensively described, mechanical contributions from the nuclear envelope can impact enhancer-promoter interactions both directly and indirectly through influencing intra/inter-TAD interactions. Importantly, these nuclear envelope contributions clearly link this mechanism to development and, when defective, to human disease. Here, we discuss evidence for nuclear envelope regulation of tissue-specific enhancer-promoter pairings, potential mechanisms for this regulation, exciting recent findings that other regulatory elements such as microRNAs and long noncoding RNAs are under nuclear envelope regulation, the possible involvement of condensates, and how disruption of this regulation can lead to disease.

Modelling human genetic disorders in Xenopus tropicalis

Recent progress in human disease genetics is leading to rapid advances in understanding pathobiological mechanisms. However, the sheer number of risk-conveying genetic variants being identified demands in vivo model systems that are amenable to functional analyses at scale. Here we provide a practical guide for using the diploid frog species Xenopus tropicalis to study many genes and variants to uncover conserved mechanisms of pathobiology relevant to human disease. We discuss key considerations in modelling human genetic disorders: genetic architecture, conservation, phenotyping strategy and rigour, as well as more complex topics, such as penetrance, expressivity, sex differences and current challenges in the field. As the patient-driven gene discovery field expands significantly, the cost-effective, rapid and higher throughput nature of Xenopus make it an essential member of the model organism armamentarium for understanding gene function in development and in relation to disease.

Evolutionary analysis of gene ages across TADs associates chromatin topology with whole-genome duplications

Topologically associated domains (TADs) are interaction subnetworks of chromosomal regions in 3D genomes. TAD boundaries frequently coincide with genome breaks while boundary deletion is under negative selection, suggesting that TADs may facilitate genome rearrangements and evolution. We show that genes co-localize by evolutionary age in humans and mice, resulting in TADs having different proportions of younger and older genes. We observe a major transition in the age co-localization patterns between the genes born during vertebrate whole-genome duplications (WGDs) or before and those born afterward. We also find that genes recently duplicated in primates and rodents are more frequently essential when they are located in old-enriched TADs and interact with genes that last duplicated during the WGD. Therefore, the evolutionary relevance of recent genes may increase when located in TADs with established regulatory networks. Our data suggest that TADs could play a role in organizing ancestral functions and evolutionary novelty.

Stochasticity in genetics and gene regulation

Development from fertilized egg to functioning multi-cellular organism requires precision. There is no precision, and often no survival, without plasticity. Plasticity is conferred partly by stochastic variation, present inherently in all biological systems. Gene expression levels fluctuate ubiquitously through transcription, alternative splicing, translation and turnover. Small differences in gene expression are exploited to trigger early differentiation, conferring distinct function on selected individual cells and setting in motion regulatory interactions. Non-selected cells then acquire new functions along the spatio-temporal developmental trajectory. The differentiation process has many stochastic components. Meiotic segregation, mitochondrial partitioning, X-inactivation and the dynamic DNA binding of transcription factor assemblies-all exhibit randomness. Non-random X-inactivation generally signals deleterious X-linked mutations. Correct neural wiring, such as retina to brain, arises through repeated confirmatory activity of connections made randomly. In immune system development, both B-cell antibody generation and the emergence of balanced T-cell categories begin through stochastic trial and error followed by functional selection. Aberrant selection processes lead to immune dysfunction. DNA sequence variants also arise through stochastic events: some involving environmental fluctuation (radiation or presence of pollutants), or genetic repair system malfunction. The phenotypic outcome of mutations is also fluid. Mutations may be advantageous in some circumstances, deleterious in others. This article is part of a discussion meeting issue 'Causes and consequences of stochastic processes in development and disease'.

Disruption of OVOL2 Distal Regulatory Elements as a Possible Mechanism Implicated in Corneal Endothelial Dystrophy

The genetic architecture of corneal endothelial dystrophies remains unknown in a substantial number of affected individuals. The proband investigated in the current study was diagnosed in the neonatal period with bilateral corneal opacification due to primary endothelial cell dysfunction. Neither his parents nor his sister had signs of corneal disease. Conventional karyotyping revealed a de novo translocation involving chromosomes 3 and 20, t(3;20)(q25;p11-12). Following genome and targeted Sanger sequencing analysis, the breakpoints were mapped at the nucleotide level. Notably, the breakpoint on chromosome 20 was identified to lie within the same topologically associated domain (TAD) as corneal endothelial dystrophy-associated gene OVOL2, and it is predicted to disrupt distal enhancers. The breakpoint at chromosome 3 is located within intron 2 of PFN2, which is currently not associated with any human disease. Further interrogation of the proband's genome failed to identify any additional potentially pathogenic variants in corneal endothelial dystrophy-associated genes. Disruption of a candidate cis-regulatory element and/or positional effects induced by translocation of OVOL2 to a novel genomic context may lead to an aberrant OVOL2 expression, a previously characterized disease mechanism of corneal endothelial dystrophy. Further research is necessary to explore how disruption of regulatory elements may elucidate genetically unsolved corneal endothelial dystrophies.