Intragenic DNA inversions expand bacterial coding capacity.

Bacterial populations that originate from a single bacterium are not strictly clonal and often contain subgroups with distinct phenotypes1. Bacteria can generate heterogeneity through phase variation-a preprogrammed, reversible mechanism that alters gene expression levels across a population1. One well-studied type of phase variation involves enzyme-mediated inversion of specific regions of genomic DNA2. Frequently, these DNA inversions flip the orientation of promoters, turning transcription of adjacent coding regions on or off2. Through this mechanism, inversion can affect fitness, survival or group dynamics3,4. Here, we describe the development of PhaVa, a computational tool that identifies DNA inversions using long-read datasets. We also identify 372 'intragenic invertons', a novel class of DNA inversions found entirely within genes, in genomes of bacterial and archaeal isolates. Intragenic invertons allow a gene to encode two or more versions of a protein by flipping a DNA sequence within the coding region, thereby increasing coding capacity without increasing genome size. We validate ten intragenic invertons in the gut commensal Bacteroides thetaiotaomicron, and experimentally characterize an intragenic inverton in the thiamine biosynthesis gene thiC.

The Recurrent Atypical e8a2 BCR::ABL1 Transcript with Insertion of an Inverted 55 Base Pair ABL1 Intron 1b Sequence: A Detailed Molecular Analysis.

Atypical BCR::ABL1 transcripts are found in approximately 2% of cases of chronic myeloid leukemia. It is important to detect them since affected patients also benefit from tyrosine kinase inhibitor therapy. In the rare e8a2 atypical BCR::ABL1 transcript, two out-of-frame exons are fused, thus, interposed nucleotides are usually found at the fusion site to restore the reading frame. In approximately half of previously reported e8a2 BCR::ABL1 cases, an inserted 55 bp sequence homologous to an inverted sequence from ABL1 intron 1b was detected. The generation of this recurrent transcript variant is not obvious. This work describes the molecular analysis of such an e8a2 BCR::ABL1 translocation from a CML patient. The genomic chromosomal breakpoint is identified, and the formation of this transcript is theoretically explained. The clinical course of the patient is reported, and recommendations are provided for the molecular analysis of future e8a2 BCR::ABL1 cases.

Structural basis of direct and inverted DNA sequence repeat recognition by helix-turn-helix transcription factors.

Some transcription factors bind DNA motifs containing direct or inverted sequence repeats. Preference for each of these DNA topologies is dictated by structural constraints. Most prokaryotic regulators form symmetric oligomers, which require operators with a dyad structure. Binding to direct repeats requires breaking the internal symmetry, a property restricted to a few regulators, most of them from the AraC family. The KorA family of transcriptional repressors, involved in plasmid propagation and stability, includes members that form symmetric dimers and recognize inverted repeats. Our structural analyses show that ArdK, a member of this family, can form a symmetric dimer similar to that observed for KorA, yet it binds direct sequence repeats as a non-symmetric dimer. This is possible by the 180° rotation of one of the helix-turn-helix domains. We then probed and confirmed that ArdK shows affinity for an inverted repeat, which, surprisingly, is also recognized by a non-symmetrical dimer. Our results indicate that structural flexibility at different positions in the dimerization interface constrains transcription factors to bind DNA sequences with one of these two alternative DNA topologies.

Characteristics and possible mechanisms of formation of microinversions distinguishing human and chimpanzee genomes.

Genomic inversions come in various sizes. While long inversions are relatively easy to identify by aligning high-quality genome sequences, unambiguous identification of microinversions is more problematic. Here, using a set of extra stringent criteria to distinguish microinversions from other mutational events, we describe microinversions that occurred after the divergence of humans and chimpanzees. In total, we found 59 definite microinversions that range from 17 to 33 nucleotides in length. In majority of them, human genome sequences matched exactly the reverse-complemented chimpanzee genome sequences, implying that the inverted DNA segment was copied precisely. All these microinversions were flanked by perfect or nearly perfect inverted repeats pointing to their key role in their formation. Template switching at inverted repeats during DNA replication was previously discussed as a possible mechanism for the microinversion formation. However, many of definite microinversions found by us cannot be easily explained via template switching owing to the combination of the short length and imperfect nature of their flanking inverted repeats. We propose a novel, alternative mechanism that involves repair of a double-stranded break within the inverting segment via microhomology-mediated break-induced replication, which can consistently explain all definite microinversion events.

New telomere to telomere assembly of human chromosome 8 reveals a previous underestimation of G-quadruplex forming sequences and inverted repeats.

Taking advantage of evolving and improving sequencing methods, human chromosome 8 is now available as a gapless, end-to-end assembly. Thanks to advances in long-read sequencing technologies, its centromere, telomeres, duplicated gene families and repeat-rich regions are now fully sequenced. We were interested to assess if the new assembly altered our understanding of the potential impact of non-B DNA structures within this completed chromosome sequence. It has been shown that non-B secondary structures, such as G-quadruplexes, hairpins and cruciforms, have important regulatory functions and potential as targeted therapeutics. Therefore, we analysed the presence of putative G-quadruplex forming sequences and inverted repeats in the current human reference genome (GRCh38) and in the new end-to-end assembly of chromosome 8. The comparison revealed that the new assembly contains significantly more inverted repeats and G-quadruplex forming sequences compared to the current reference sequence. This observation can be explained by improved accuracy of the new sequencing methods, particularly in regions that contain extensive repeats of bases, as is preferred by many non-B DNA structures. These results show a significant underestimation of the prevalence of non-B DNA secondary structure in previous assembly versions of the human genome and point to their importance being not fully appreciated. We anticipate that similar observations will occur as the improved sequencing technologies fill in gaps across the genomes of humans and other organisms.

Lineage-Specific Variation in IR Boundary Shift Events, Inversions, and Substitution Rates among Caprifoliaceae s.l. (Dipsacales) Plastomes.

Caprifoliaceae s.l. plastid genomes (plastomes) show that one inversion and two inverted repeat boundary shifts occurred in the common ancestor of this family, after which the plastomes are generally conserved. This study reports plastome sequences of five additional species, Fedia cornucopiae, Valeriana fauriei, and Valerianella locusta from the subfamily Valerianoideae, as well as Dipsacus japonicus and Scabiosa comosa from the subfamily Dipsacoideae. Combined with the published plastomes, these plastomes provide new insights into the structural evolution of plastomes within the family. Moreover, the three plastomes from the subfamily Valerianoideae exhibited accelerated nucleotide substitution rates, particularly at synonymous sites, across the family. The patterns of accD sequence divergence in the family are dynamic with structural changes, including interruption of the conserved domain and increases in nonsynonymous substitution rates. In particular, the Valeriana accD gene harbors a large insertion of amino acid repeat (AAR) motifs, and intraspecific polymorphism with a variable number of AARs in the Valeriana accD gene was detected. We found a correlation between intron losses and increased ratios of nonsynonymous to synonymous substitution rates in the clpP gene with intensified positive selection. In addition, two Dipsacoideae plastomes revealed the loss of the plastid-encoded rps15, and a potential functional gene transfer to the nucleus was confirmed.

Machine-learning predicts genomic determinants of meiosis-driven structural variation in a eukaryotic pathogen.

Species harbor extensive structural variation underpinning recent adaptive evolution. However, the causality between genomic features and the induction of new rearrangements is poorly established. Here, we analyze a global set of telomere-to-telomere genome assemblies of a fungal pathogen of wheat to establish a nucleotide-level map of structural variation. We show that the recent emergence of pesticide resistance has been disproportionally driven by rearrangements. We use machine learning to train a model on structural variation events based on 30 chromosomal sequence features. We show that base composition and gene density are the major determinants of structural variation. Retrotransposons explain most inversion, indel and duplication events. We apply our model to Arabidopsis thaliana and show that our approach extends to more complex genomes. Finally, we analyze complete genomes of haploid offspring in a four-generation pedigree. Meiotic crossover locations are enriched for new rearrangements consistent with crossovers being mutational hotspots. The model trained on species-wide structural variation accurately predicts the position of >74% of newly generated variants along the pedigree. The predictive power highlights causality between specific sequence features and the induction of chromosomal rearrangements. Our work demonstrates that training sequence-derived models can accurately identify regions of intrinsic DNA instability in eukaryotic genomes.

Comparative genome analysis revealed gene inversions, boundary expansions and contractions, and gene loss in the Stemona sessilifolia (Miq.) Miq. chloroplast genome.

Stemona sessilifolia (Miq.) Miq., commonly known as Baibu, is one of the most popular herbal medicines in Asia. In the Chinese Pharmacopoeia, Baibu has multiple authentic sources and there are many similar herbs sold as Baibu in herbal medicine markets. The existence of counterfeits of Baibu brings challenges to its identification. To assist in its accurate identification, we sequenced and analyzed the complete chloroplast genome of S. sessilifolia using next-generation sequencing technology. The genome was found to be 154,037 bp in length, possessing a typical quadripartite structure consisting of a pair of inverted repeats (IRs: 27,090 bp) separated by a large single copy (LSC: 81,949 bp) and a small single copy (SSC: 17,908 bp). A total of 112 unique genes were identified, including 80 protein-coding, 28 transfer RNA and four ribosomal RNA genes. In addition, 45 tandem, 27 forward, 23 palindromic and 104 simple sequence repeats were detected in the genome by repeated analysis. Compared with its counterfeits (Asparagus officinalis and Carludovica palmata) we found that IR expansion and SSC contraction events of S. sessilifolia resulted in two copies of the rpl22 gene in the IR regions and a partial duplication of the ndhF gene in the SSC region. An approximately 3-kb-long inversion was also identified in the LSC region, leading to the petA and cemA genes being presented in the complementary strand of the chloroplast DNA molecule. Comparative analysis revealed some highly variable regions, including trnF-GAA_ndhJ, atpB_rbcL, rps15_ycf1, trnG-UCC_trnR-UCU, ndhF_rpl32, accD_psaI, rps2_rpoC2, trnS-GCU_trnG-UCC, trnT-UGU_trnL-UAA and rps16_trnQ-UUG. Finally, gene loss events were investigated in the context of phylogenetic relationships. In summary, the complete plastome of S. sessilifolia will provide valuable information for the distinction between Baibu and its counterfeits and assist in elucidating the evolution of S. sessilifolia.

Detection of a mosaic CDKL5 deletion and inversion by optical genome mapping ends an exhaustive diagnostic odyssey.

BACKGROUND: Currently available structural variant (SV) detection methods do not span the complete spectrum of disease-causing SVs. Optical genome mapping (OGM), an emerging technology with the potential to resolve diagnostic dilemmas, was performed to investigate clinically-relevant SVs in a 4-year-old male with an epileptic encephalopathy of undiagnosed molecular origin. METHODS: OGM was utilized to image long, megabase-size DNA molecules, fluorescently labeled at specific sequence motifs throughout the genome with high sensitivity for detection of SVs greater than 500 bp in size. OGM results were confirmed in a CLIA-certified laboratory via mate-pair sequencing. RESULTS: OGM identified a mosaic, de novo 90 kb deletion and inversion on the X chromosome disrupting the CDKL5 gene. Detection of the mosaic deletion, which had been previously undetected by chromosomal microarray, an infantile epilepsy panel including exon-level microarray for CDKL5, exome sequencing as well as genome sequencing, resulted in a diagnosis of X-linked dominant early infantile epileptic encephalopathy-2. CONCLUSION: OGM affords an effective technology for the detection of SVs, especially those that are mosaic, since these remain difficult to detect with current NGS technologies and with conventional chromosomal microarrays. Further research in undiagnosed populations with OGM is warranted.

Multiple Immunoglobulin κ Gene Rearrangements within a Single Clone Unraveled by Next-Generation Sequencing-Based Clonality Assessment.

Clonality assessment of the Ig heavy- and light-chain genes (IGH and IGK) using GeneScan analysis is an important supplemental assay in diagnostic testing for lymphoma. Occasionally cases with an IGK rearrangement pattern that cannot readily be assigned to a monoclonal lymphoma are encountered, whereas the occurrence of biclonal lymphomas is rare, and the result of the IGH locus of these cases is in line with monoclonality. Three such ambiguous cases were assessed for clonality using next-generation sequencing. Information on the sequences of the rearrangements, combined with knowledge of the complex organization of the IGK locus, pointed to two explanations that can attribute seemingly biclonal IGK rearrangements to a single clone. In two cases, this explanation involved inversion rearrangements on the IGK locus, whereas in the third case, the cross-reactivity of primers generated an additional clonal product. In conclusion, next-generation sequencing-based clonality assessment allows for the detection of both inversion rearrangements and the cross-reactivity of primers, and can therefore facilitate the interpretation of cases of lymphoma with complex IGK rearrangement patterns.

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

Long-read and strand-specific sequencing technologies together facilitate the de novo assembly of high-quality haplotype-resolved human genomes without parent-child trio data. We present 64 assembled haplotypes from 32 diverse human genomes. These highly contiguous haplotype assemblies (average minimum contig length needed to cover 50% of the genome: 26 million base pairs) integrate all forms of genetic variation, even across complex loci. We identified 107,590 structural variants (SVs), of which 68% were not discovered with short-read sequencing, and 278 SV hotspots (spanning megabases of gene-rich sequence). We characterized 130 of the most active mobile element source elements and found that 63% of all SVs arise through homology-mediated mechanisms. This resource enables reliable graph-based genotyping from short reads of up to 50,340 SVs, resulting in the identification of 1526 expression quantitative trait loci as well as SV candidates for adaptive selection within the human population.

Functional differences in the hypothalamic-pituitary-gonadal axis are associated with alternative reproductive tactics based on an inversion polymorphism.

The evolution of social behavior depends on genetic changes, yet, how genomic variation manifests itself in behavioral diversity is still largely unresolved. Chromosomal inversions can play a pivotal role in producing distinct behavioral phenotypes, in particular, when inversion genes are functionally associated with hormone synthesis and signaling. Male ruffs exhibit alternative reproductive tactics (ARTs) with an autosomal inversion determining two alternative morphs with clear behavioral and hormonal differences from the ancestral morph. We investigated hormonal and transcriptomic differences in the pituitary and gonads. Using a GnRH challenge, we found that the ability to synthesize testosterone in inversion carriers is severely constrained, whereas the synthesis of androstenedione, a testosterone precursor, is not. Inversion morphs were able to produce a transient increase in androstenedione following the GnRH injection, supporting the view that pituitary sensitivity to GnRH is comparable to that of the ancestral morph. We then performed gene expression analyses in a second set of untreated birds and found no evidence of alterations to pituitary sensitivity, gonadotropin production or gonad sensitivity to luteinizing hormone or follicle-stimulating hormone across morphs. Inversion morphs also showed reduced progesterone receptor expression in the pituitary. Strikingly, in the gonads, inversion morphs over-expressed STAR, a gene that is located outside of the inversion and responsible for providing the cholesterol substrate required for the synthesis of sex hormones. In conclusion, our results suggest that the gonads determine morph-specific differences in hormonal regulation.

Alternative DNA Structures In Vivo: Molecular Evidence and Remaining Questions.

Duplex DNA naturally folds into a right-handed double helix in physiological conditions. Some sequences of unusual base composition may nevertheless form alternative structures, as was shown for many repeated sequences in vitro However, evidence for the formation of noncanonical structures in living cells is difficult to gather. It mainly relies on genetic assays demonstrating their function in vivo or through genetic instability reflecting particular properties of such structures. Efforts were made to reveal their existence directly in a living cell, mainly by generating antibodies specific to secondary structures or using chemical ligands selected for their affinity to these structures. Among secondary structure-forming DNAs are G-quadruplexes, human fragile sites containing minisatellites, AT-rich regions, inverted repeats able to form cruciform structures, hairpin-forming CAG/CTG triplet repeats, and triple helices formed by homopurine-homopyrimidine GAA/TTC trinucleotide repeats. Many of these alternative structures are involved in human pathologies, such as neurological or developmental disorders, as in the case of trinucleotide repeats, or cancers triggered by translocations linked to fragile sites. This review will discuss and highlight evidence supporting the formation of alternative DNA structures in vivo and will emphasize the role of the mismatch repair machinery in binding mispaired DNA duplexes, triggering genetic instability.

Pathogenic 12-kb copy-neutral inversion in syndromic intellectual disability identified by high-fidelity long-read sequencing.

We report monozygotic twin girls with syndromic intellectual disability who underwent exome sequencing but with negative pathogenic variants. To search for variants that are unrecognized by exome sequencing, high-fidelity long-read genome sequencing (HiFi LR-GS) was applied. A 12-kb copy-neutral inversion was precisely identified by HiFi LR-GS after trio-based variant filtering. This inversion directly disrupted two genes, CPNE9 and BRPF1, the latter of which attracted our attention because pathogenic BRPF1 variants have been identified in autosomal dominant intellectual developmental disorder with dysmorphic facies and ptosis (IDDDFP), which later turned out to be clinically found in the twins. Trio-based HiFi LR-GS together with haplotype phasing revealed that the 12-kb inversion occurred de novo on the maternally transmitted chromosome. This study clearly indicates that submicroscopic copy-neutral inversions are important but often uncharacterized culprits in monogenic disorders and that long-read sequencing is highly advantageous for detecting such inversions involved in genetic diseases.

CRISPR-Cas9-mediated 75.5-Mb inversion in maize.

CRISPR-Cas is a powerful double-strand-break technology with wide-ranging applications from gene discovery to commercial product development. Thus far, this tool has been almost exclusively used for gene knockouts and deletions, with a few examples of gene edits and targeted gene insertions. Here, we demonstrate the application of CRISPR-Cas9 technology to mediate targeted 75.5-Mb pericentric inversion in chromosome 2 in one of the elite maize inbred lines from Corteva Agriscience. This inversion unlocks a large chromosomal region containing substantial genetic variance for recombination, thus providing opportunities for the development of new maize varieties with improved phenotypes.

The barley pan-genome reveals the hidden legacy of mutation breeding.

Genetic diversity is key to crop improvement. Owing to pervasive genomic structural variation, a single reference genome assembly cannot capture the full complement of sequence diversity of a crop species (known as the 'pan-genome'1). Multiple high-quality sequence assemblies are an indispensable component of a pan-genome infrastructure. Barley (Hordeum vulgare L.) is an important cereal crop with a long history of cultivation that is adapted to a wide range of agro-climatic conditions2. Here we report the construction of chromosome-scale sequence assemblies for the genotypes of 20 varieties of barley-comprising landraces, cultivars and a wild barley-that were selected as representatives of global barley diversity. We catalogued genomic presence/absence variants and explored the use of structural variants for quantitative genetic analysis through whole-genome shotgun sequencing of 300 gene bank accessions. We discovered abundant large inversion polymorphisms and analysed in detail two inversions that are frequently found in current elite barley germplasm; one is probably the product of mutation breeding and the other is tightly linked to a locus that is involved in the expansion of geographical range. This first-generation barley pan-genome makes previously hidden genetic variation accessible to genetic studies and breeding.

First submicroscopic inversion of the OPA1 gene identified in dominant optic atrophy - a case report.

BACKGROUND: Dominant optic atrophy (DOA) is an inherited optic neuropathy that mainly affects visual acuity, central visual fields and color vision due to a progressive loss of retinal ganglion cells and their axons that form the optic nerve. Approximately 45-90% of affected individuals with DOA harbor pathogenic variants in the OPA1 gene. The mutation spectrum of OPA1 comprises nonsense, canonical and non-canonical splice site, frameshift and missense as well as copy number variants, but intragenic inversions have not been reported so far. CASE PRESENTATION: We report a 33-year-old male with characteristic clinical features of DOA. Whole-genome sequencing identified a structural variant of 2.4 kb comprising an inversion of 937 bp at the OPA1 locus. Fine mapping of the breakpoints to single nucleotide level revealed that the structural variation was an inversion flanked by two deletions. As this rearrangement inverts the entire first exon of OPA1, it was classified as likely pathogenic. CONCLUSIONS: We report the first DOA case harboring an inversion in the OPA1 gene. Our study demonstrates that copy-neutral genomic rearrangements have to be considered as a possible cause of disease in DOA cases.

Identification of Genomic Alterations in Sporadic Pancreatic Neurogenic Tumors.

Pancreatic neurogenic tumors, including schwannoma and neurofibroma, are rare, and their genetic aberrances have not been defined. The present study aimed at investigating the genomic alterations of pancreatic schwannoma and neurofibroma. Two patients with pancreatic schwannoma and 1 patient with neurofibroma, who underwent surgical resection at the First Affiliated Hospital, Sun Yat-sen University between June 2016 and April 2019, were recruited into the study. Their tumor tissues were analyzed by exome sequencing and genome sequencing. Exome sequencing revealed a MUTYH likely pathogenic germline variant in 1 schwannoma with somatic NF2del and NOTCH1 amplification. Pathway enrichment analysis on the other schwannoma case showed that the main abnormal function involved DNA damage repair, mitosis, and cell cycle. In addition, genome sequencing showed the inversion (INV) variant of SPIRE gene and multiple mitochondrial INV variants in both schwannoma cases. Furthermore, exome sequencing revealed NF1del, single nucleotide variation, TP53, and ERBB3 amplification in neurofibroma, whereas genomic duplication/deletion variants and mitochondrial abnormalities were much less than that in schwannoma. In conclusion, variants in NF1 and NF2 genes, amplification of key driver genes, and somatic and mitochondrial INV variants may play important roles in the development of pancreatic schwannoma and neurofibroma.

Large-scale chromosome flip-flop reversible inversion mediates phenotypic switching of expression of antibiotic resistance in lactococci.

Bacteria can gain resistance to antimicrobials by acquiring and expressing genetic elements that encode resistance determinants such as efflux pumps and drug-modifying enzymes, thus hampering treatment of infection. Previously we showed that acquisition of spectinomycin resistance in a lactococcal strain was correlated with a reversible genomic inversion, but the precise location and the genes affected were unknown. Here we use long-read whole-genome sequencing to precisely define the genomic inversion and we use quantitative PCR to identify associated changes in gene expression levels. The boundaries of the inversion fall within two identical copies of a prophage-like sequence, located on the left and right replichores; this suggests possible mechanisms for inversion through homologous recombination or prophage activity. The inversion is asymmetrical in respect of the axis between the origin and terminus of the replication and modulates the expression of a SAM-dependent methyltransferase, whose heterologous expression confers resistance to spectinomycin in lactococci and that is up-regulated on exposure to spectinomycin. This study provides one of the first examples of phase variation via large-scale chromosomal inversions that confers a switch in antimicrobial resistance in bacteria and the first outside of Staphylococcus aureus.

Evolution of tRNA gene rearrangement in the mitochondrial genome of ichneumonoid wasps (Hymenoptera: Ichneumonoidea).

Gene rearrangements in the mitochondrial genome (mt genome) are common in certain insect groups and can be an informative character for phylogenetic reconstruction. However, knowledge of the mechanism and biases of gene rearrangement in insect mt genomes is still limited. With an accelerated rate of gene rearrangements, Hymenoptera is an important group for mt genome rearrangements diversity and for understanding the gene rearrangement evolution in mt genomes. Here, we sequenced the complete mt genome of Aphidius gifuensis and analyzed the evolution of tRNA gene rearrangements in the mt genomes of ichneumonoid wasps. Two control regions were detected in A. gifuensis and most of the tRNA rearrangement events occurred around these control regions. tRNA gene rearrangements occurred in almost all of the sequenced mt genomes of Ichneumonoidea and the gene block CR-trnI-trnQ-trnM-ND2-trnW-trnC-trnY was the main hot spot of gene rearrangement. Mapped over the backbone phylogeny of Ichneumonoidea, we found that the inversion and translocation of both trnI and trnM is likely a synapomorphic rearrangement in Braconidae. Our study also demonstrated that the gene block CR-trnI-trnQ-trnM-ND2-trnW-trnC-trnY was important for inferring the gene rearrangement dynamics in Ichneumonoidea.