Recurrent duplication-driven transposition of DNA during hominoid evolution

Independent expansion, selection and hypervariability of the TBC1D3 gene family in humans

TBC1D3 is a primate-specific gene family that has expanded in the human lineage and has been implicated in neuronal progenitor proliferation and expansion of the frontal cortex. The gene family and its expression have been challenging to investigate because it is embedded in high-identity and highly variable segmental duplications. We sequenced and assembled the gene family using long-read sequencing data from 34 humans and 11 nonhuman primate species. Our analysis shows that this particular gene family has independently duplicated in at least five primate lineages, and the duplicated loci are enriched at sites of large-scale chromosomal rearrangements on chromosome 17. We find that most humans vary along two TBC1D3 clusters where human haplotypes are highly variable in copy number, differing by as many as 20 copies, and structure (structural heterozygosity 90%). We also show evidence of positive selection, as well as a significant change in the predicted human TBC1D3 protein sequence. Lastly, we find that, despite multiple duplications, human TBC1D3 expression is limited to a subset of copies and, most notably, from a single paralog group: TBC1D3-CDKL. These observations may help explain why a gene potentially important in cortical development can be so variable in the human population.

Genomic structural variation: A complex but important driver of human evolution

Structural variants (SVs)-including duplications, deletions, and inversions of DNA-can have significant genomic and functional impacts but are technically difficult to identify and assay compared with single-nucleotide variants. With the aid of new genomic technologies, it has become clear that SVs account for significant differences across and within species. This phenomenon is particularly well-documented for humans and other primates due to the wealth of sequence data available. In great apes, SVs affect a larger number of nucleotides than single-nucleotide variants, with many identified SVs exhibiting population and species specificity. In this review, we highlight the importance of SVs in human evolution by (1) how they have shaped great ape genomes resulting in sensitized regions associated with traits and diseases, (2) their impact on gene functions and regulation, which subsequently has played a role in natural selection, and (3) the role of gene duplications in human brain evolution. We further discuss how to incorporate SVs in research, including the strengths and limitations of various genomic approaches. Finally, we propose future considerations in integrating existing data and biospecimens with the ever-expanding SV compendium propelled by biotechnology advancements.

SVA retrotransposons and a low copy repeat in humans and great apes: a mobile connection

Segmental duplications (SDs) constitute a considerable fraction of primate genomes. They contribute to genetic variation and provide raw material for evolution. Groups of SDs are characterized by the presence of shared core duplicons. One of these core duplicons, low copy repeat (lcr)16a, has been shown to be particularly active in the propagation of interspersed SDs in primates. The underlying mechanisms are, however, only partially understood. Alu short interspersed elements (SINEs) are frequently found at breakpoints and have been implicated in the expansion of SDs.

Detailed analysis of lcr16a-containing SDs shows that the hominid-specific SVA (SINE-R-VNTR-Alu) retrotransposon is an integral component of the core duplicon in Asian and African great apes. In orang-utan, it provides breakpoints and contributes to both interchromosomal and intrachromosomal lcr16a mobility by inter-element recombination. Furthermore, the data suggest that in hominines (human, chimpanzee, gorilla) SVA recombination-mediated integration of a circular intermediate is the founding event of a lineage-specific lcr16a expansion. One of the hominine lcr16a copies displays large flanking direct repeats, a structural feature shared by other SDs in the human genome.

Taken together, the results obtained extend the range of SVAs’ contribution to genome evolution from RNA-mediated transduction to DNA-based recombination. In addition, they provide further support for a role of circular intermediates in SD mobilization.

Alpha Satellite Insertion Close to an Ancestral Centromeric Region

Human centromeres are mainly composed of alpha satellite DNA hierarchically organized as higher-order repeats (HORs). Alpha satellite dynamics is shown by sequence homogenization in centromeric arrays and by its transfer to other centromeric locations, for example, during the maturation of new centromeres. We identified during prenatal aneuploidy diagnosis by fluorescent in situ hybridization a de novo insertion of alpha satellite DNA from the centromere of chromosome 18 (D18Z1) into cytoband 15q26. Although bound by CENP-B, this locus did not acquire centromeric functionality as demonstrated by the lack of constriction and the absence of CENP-A binding. The insertion was associated with a 2.8-kbp deletion and likely occurred in the paternal germline. The site was enriched in long terminal repeats and located ∼10 Mbp from the location where a centromere was ancestrally seeded and became inactive in the common ancestor of humans and apes 20-25 million years ago. Long-read mapping to the T2T-CHM13 human genome assembly revealed that the insertion derives from a specific region of chromosome 18 centromeric 12-mer HOR array in which the monomer size follows a regular pattern. The rearrangement did not directly disrupt any gene or predicted regulatory element and did not alter the methylation status of the surrounding region, consistent with the absence of phenotypic consequences in the carrier. This case demonstrates a likely rare but new class of structural variation that we name "alpha satellite insertion." It also expands our knowledge on alphoid DNA dynamics and conveys the possibility that alphoid arrays can relocate near vestigial centromeric sites.

22q11.2 Low Copy Repeats Expanded in the Human Lineage

Segmental duplications or low copy repeats (LCRs) constitute duplicated regions interspersed in the human genome, currently neglected in standard analyses due to their extreme complexity. Recent functional studies have indicated the potential of genes within LCRs in synaptogenesis, neuronal migration, and neocortical expansion in the human lineage. One of the regions with the highest proportion of duplicated sequence is the 22q11.2 locus, carrying eight LCRs (LCR22-A until LCR22-H), and rearrangements between them cause the 22q11.2 deletion syndrome. The LCR22-A block was recently reported to be hypervariable in the human population. It remains unknown whether this variability also exists in non-human primates, since research is strongly hampered by the presence of sequence gaps in the human and non-human primate reference genomes. To chart the LCR22 haplotypes and the associated inter- and intra-species variability, we de novo assembled the region in non-human primates by a combination of optical mapping techniques. A minimal and likely ancient haplotype is present in the chimpanzee, bonobo, and rhesus monkey without intra-species variation. In addition, the optical maps identified assembly errors and closed gaps in the orthologous chromosome 22 reference sequences. These findings indicate the LCR22 expansion to be unique to the human population, which might indicate involvement of the region in human evolution and adaptation. Those maps will enable LCR22-specific functional studies and investigate potential associations with the phenotypic variability in the 22q11.2 deletion syndrome.

Predicting the Evolution of Syntenies—An Algorithmic Review

Syntenies are genomic segments of consecutive genes identified by a certain conservation in gene content and order. The notion of conservation may vary from one definition to another, the more constrained requiring identical gene contents and gene orders, while more relaxed definitions just require a certain similarity in gene content, and not necessarily in the same order. Regardless of the way they are identified, the goal is to characterize homologous genomic regions, i.e., regions deriving from a common ancestral region, reflecting a certain gene co-evolution that can enlighten important functional properties. In addition of being able to identify them, it is also necessary to infer the evolutionary history that has led from the ancestral segment to the extant ones. In this field, most algorithmic studies address the problem of inferring rearrangement scenarios explaining the disruption in gene order between segments with the same gene content, some of them extending the evolutionary model to gene insertion and deletion. However, syntenies also evolve through other events modifying their content in genes, such as duplications, losses or horizontal gene transfers, i.e., the movement of genes from one species to another. Although the reconciliation approach between a gene tree and a species tree addresses the problem of inferring such events for single-gene families, little effort has been dedicated to the generalization to segmental events and to syntenies. This paper reviews some of the main algorithmic methods for inferring ancestral syntenies and focus on those integrating both gene orders and gene trees.

Association of structural variation with cardiometabolic traits in Finns

The contribution of genome structural variation (SV) to quantitative traits associated with cardiometabolic diseases remains largely unknown. Here, we present the results of a study examining genetic association between SVs and cardiometabolic traits in the Finnish population. We used sensitive methods to identify and genotype 129,166 high-confidence SVs from deep whole-genome sequencing (WGS) data of 4,848 individuals. We tested the 64,572 common and low-frequency SVs for association with 116 quantitative traits and tested candidate associations using exome sequencing and array genotype data from an additional 15,205 individuals. We discovered 31 genome-wide significant associations at 15 loci, including 2 loci at which SVs have strong phenotypic effects: (1) a deletion of the ALB promoter that is greatly enriched in the Finnish population and causes decreased serum albumin level in carriers (p = 1.47 × 10-54) and is also associated with increased levels of total cholesterol (p = 1.22 × 10-28) and 14 additional cholesterol-related traits, and (2) a multi-allelic copy number variant (CNV) at PDPR that is strongly associated with pyruvate (p = 4.81 × 10-21) and alanine (p = 6.14 × 10-12) levels and resides within a structurally complex genomic region that has accumulated many rearrangements over evolutionary time. We also confirmed six previously reported associations, including five led by stronger signals in single nucleotide variants (SNVs) and one linking recurrent HP gene deletion and cholesterol levels (p = 6.24 × 10-10), which was also found to be strongly associated with increased glycoprotein level (p = 3.53 × 10-35). Our study confirms that integrating SVs in trait-mapping studies will expand our knowledge of genetic factors underlying disease risk.

Genomic Chaos Begets Psychiatric Disorder

The processes that created the primordial genome are inextricably linked to current day vulnerability to developing a psychiatric disorder as summarized in this review article. Chaos and dynamic forces including duplication, transposition, and recombination generated the protogenome. To survive early stages of genome evolution, self-organization emerged to curb chaos. Eventually, the human genome evolved through a delicate balance of chaos/instability and organization/stability. However, recombination coldspots, silencing of transposable elements, and other measures to limit chaos also led to retention of variants that increase risk for disease. Moreover, ongoing dynamics in the genome creates various new mutations that determine liability for psychiatric disorders. Homologous recombination, long-range gene regulation, and gene interactions were all guided by spooky action-at-a-distance, which increased variability in the system. A probabilistic system of life was required to deal with a changing environment. This ensured the generation of outliers in the population, which enhanced the probability that some members would survive unfavorable environmental impacts. Some of the outliers produced through this process in man are ill suited to cope with the complex demands of modern life. Genomic chaos and mental distress from the psychological challenges of modern living will inevitably converge to produce psychiatric disorders in man.

An evolutionary driver of interspersed segmental duplications in primates

BACKGROUND

The complex interspersed pattern of segmental duplications in humans is responsible for rearrangements associated with neurodevelopmental disease, including the emergence of novel genes important in human brain evolution. We investigate the evolution of LCR16a, a putative driver of this phenomenon that encodes one of the most rapidly evolving human-ape gene families, nuclear pore interacting protein (NPIP).

RESULTS

Comparative analysis shows that LCR16a has independently expanded in five primate lineages over the last 35 million years of primate evolution. The expansions are associated with independent lineage-specific segmental duplications flanking LCR16a leading to the emergence of large interspersed duplication blocks at non-orthologous chromosomal locations in each primate lineage. The intron-exon structure of the NPIP gene family has changed dramatically throughout primate evolution with different branches showing characteristic gene models yet maintaining an open reading frame. In the African ape lineage, we detect signatures of positive selection that occurred after a transition to more ubiquitous expression among great ape tissues when compared to Old World and New World monkeys. Mouse transgenic experiments from baboon and human genomic loci confirm these expression differences and suggest that the broader ape expression pattern arose due to mutational changes that emerged in cis.

CONCLUSIONS

LCR16a promotes serial interspersed duplications and creates hotspots of genomic instability that appear to be an ancient property of primate genomes. Dramatic changes to NPIP gene structure and altered tissue expression preceded major bouts of positive selection in the African ape lineage, suggestive of a gene undergoing strong adaptive evolution.

Refining the Phenotype of Recurrent Rearrangements of Chromosome 16

Chromosome 16 is one of the most gene-rich chromosomes of our genome, and 10% of its sequence consists of segmental duplications, which give instability and predisposition to rearrangement by the recurrent mechanism of non-allelic homologous recombination. Microarray technologies have allowed for the analysis of copy number variations (CNVs) that can contribute to the risk of developing complex diseases. By array comparative genomic hybridization (CGH) screening of 1476 patients, we detected 27 cases with CNVs on chromosome 16. We identified four smallest regions of overlapping (SROs): one at 16p13.11 was found in seven patients; one at 16p12.2 was found in four patients; two close SROs at 16p11.2 were found in twelve patients; finally, six patients were found with atypical rearrangements. Although phenotypic variability was observed, we identified a male bias for Childhood Apraxia of Speech associated to 16p11.2 microdeletions. We also reported an elevated frequency of second-site genomic alterations, supporting the model of the second hit to explain the clinical variability associated with CNV syndromes. Our goal was to contribute to the building of a chromosome 16 disease-map based on disease susceptibility regions. The role of the CNVs of chromosome 16 was increasingly made clear in the determination of developmental delay. We also found that in some cases a second-site CNV could explain the phenotypic heterogeneity by a simple additive effect or a pejorative synergistic effect.

Continuous-Trait Probabilistic Model for Comparing Multi-species Functional Genomic Data

A large amount of multi-species functional genomic data from high-throughput assays are becoming available to help understand the molecular mechanisms for phenotypic diversity across species. However, continuous-trait probabilistic models, which are key to such comparative analysis, remain under-explored. Here we develop a new model, called phylogenetic hidden Markov Gaussian processes (Phylo-HMGP), to simultaneously infer heterogeneous evolutionary states of functional genomic features in a genome-wide manner. Both simulation studies and real data application demonstrate the effectiveness of Phylo-HMGP. Importantly, we applied Phylo-HMGP to analyze a new cross-species DNA replication timing (RT) dataset from the same cell type in five primate species (human, chimpanzee, orangutan, gibbon, and green monkey). We demonstrate that our Phylo-HMGP model enables discovery of genomic regions with distinct evolutionary patterns of RT. Our method provides a generic framework for comparative analysis of multi-species continuous functional genomic signals to help reveal regions with conserved or lineage-specific regulatory roles.

Cross-Regulation between Transposable Elements and Host DNA Replication

Transposable elements subvert host cellular functions to ensure their survival. Their interaction with the host DNA replication machinery indicates that selective pressures lead them to develop ancestral and convergent evolutionary adaptations aimed at conserved features of this fundamental process. These interactions can shape the co-evolution of the transposons and their hosts.

Human adaptation and evolution by segmental duplication

Duplications are the primary force by which new gene functions arise and provide a substrate for large-scale structural variation. Analysis of thousands of genomes shows that humans and great apes have more genetic differences in content and structure over recent segmental duplications than any other euchromatic region. Novel human-specific duplicated genes, ARHGAP11B and SRGAP2C, have recently been described with a potential role in neocortical expansion and increased neuronal spine density. Large segmental duplications and the structural variants they promote are also frequently stratified between human populations with a subset being subjected to positive selection. The impact of recent duplications on human evolution and adaptation is only beginning to be realized as new technologies enhance their discovery and accurate genotyping.

Interchromosomal core duplicons drive both evolutionary instability and disease susceptibility of the Chromosome 8p23.1 region

Recurrent rearrangements of Chromosome 8p23.1 are associated with congenital heart defects and developmental delay. The complexity of this region has led to inconsistencies in the current reference assembly, confounding studies of genetic variation. Using comparative sequence-based approaches, we generated a high-quality 6.3-Mbp alternate reference assembly of an inverted Chromosome 8p23.1 haplotype. Comparison with nonhuman primates reveals a 746-kbp duplicative transposition and two separate inversion events that arose in the last million years of human evolution. The breakpoints associated with these rearrangements map to an ape-specific interchromosomal core duplicon that clusters at sites of evolutionary inversion (P = 7.8 × 10⁻⁵). Refinement of microdeletion breakpoints identifies a subgroup of patients that map to the same interchromosomal core involved in the evolutionary formation of the duplication blocks. Our results define a higher-order genomic instability element that has shaped the structure of specific chromosomes during primate evolution contributing to rearrangements associated with inversion and disease.

Signals of historical interlocus gene conversion in human segmental duplications

Standard methods of DNA sequence analysis assume that sequences evolve independently, yet this assumption may not be appropriate for segmental duplications that exchange variants via interlocus gene conversion (IGC). Here, we use high quality multiple sequence alignments from well-annotated segmental duplications to systematically identify IGC signals in the human reference genome. Our analysis combines two complementary methods: (i) a paralog quartet method that uses DNA sequence simulations to identify a statistical excess of sites consistent with inter-paralog exchange, and (ii) the alignment-based method implemented in the GENECONV program. One-quarter (25.4%) of the paralog families in our analysis harbor clear IGC signals by the quartet approach. Using GENECONV, we identify 1477 gene conversion tracks that cumulatively span 1.54 Mb of the genome. Our analyses confirm the previously reported high rates of IGC in subtelomeric regions and Y-chromosome palindromes, and identify multiple novel IGC hotspots, including the pregnancy specific glycoproteins and the neuroblastoma breakpoint gene families. Although the duplication history of a paralog family is described by a single tree, we show that IGC has introduced incredible site-to-site variation in the evolutionary relationships among paralogs in the human genome. Our findings indicate that IGC has left significant footprints in patterns of sequence diversity across segmental duplications in the human genome, out-pacing the contributions of single base mutation by orders of magnitude. Collectively, the IGC signals we report comprise a catalog that will provide a critical reference for interpreting observed patterns of DNA sequence variation across duplicated genomic regions, including targets of recent adaptive evolution in humans.

The role of gene conversion in preserving rearrangement hotspots in the human genome

Hotspots of non-allelic homologous recombination (NAHR) have a crucial role in creating genetic diversity and are also associated with dozens of genomic disorders. Recent studies suggest that many human NAHR hotspots have been preserved throughout the evolution of primates. NAHR hotspots are likely to remain active as long as the segmental duplications (SDs) promoting NAHR retain sufficient similarity. Here, we propose an evolutionary model of SDs that incorporates the effect of gene conversion and compare it with a null model that assumes SDs evolve independently without gene conversion. The gene conversion model predicts a much longer lifespan of NAHR hotspots compared with the null model. We show that the literature on copy number variants (CNVs) and genomic disorders, and also the results of additional analysis of CNVs, are all more consistent with the gene conversion model.

Male-specific region of the bovine Y chromosome is gene rich with a high transcriptomic activity in testis development

The male-specific region of the mammalian Y chromosome (MSY) contains clusters of genes essential for male reproduction. The highly repetitive and degenerative nature of the Y chromosome impedes genomic and transcriptomic characterization. Although the Y chromosome sequence is available for the human, chimpanzee, and macaque, little is known about the annotation and transcriptome of nonprimate MSY. Here, we investigated the transcriptome of the MSY in cattle by direct testis cDNA selection and RNA-seq approaches. The bovine MSY differs radically from the primate Y chromosomes with respect to its structure, gene content, and density. Among the 28 protein-coding genes/families identified on the bovine MSY (12 single- and 16 multicopy genes), 16 are bovid specific. The 1,274 genes identified in this study made the bovine MSY gene density the highest in the genome; in comparison, primate MSYs have only 31-78 genes. Our results, along with the highly transcriptional activities observed from these Y-chromosome genes and 375 additional noncoding RNAs, challenge the widely accepted hypothesis that the MSY is gene poor and transcriptionally inert. The bovine MSY genes are predominantly expressed and are differentially regulated during the testicular development. Synonymous substitution rate analyses of the multicopy MSY genes indicated that two major periods of expansion occurred during the Miocene and Pliocene, contributing to the adaptive radiation of bovids. The massive amplification and vigorous transcription suggest that the MSY serves as a genomic niche regulating male reproduction during bovid expansion.

Mechanisms for Structural Variation in the Human Genome

It has been known for several decades that genetic variation involving changes to chromosomal structure (i.e., structural variants) can contribute to disease; however this relationship has been brought into acute focus in recent years largely based on innovative new genomics approaches and technology. Structural variants (SVs) arise from improperly repaired DNA double-strand breaks (DSB). DSBs are a frequent occurrence in all cells and two major pathways are involved in their repair: homologous recombination and non-homologous end joining. Errors during these repair mechanisms can result in SVs that involve losses, gains and rearrangements ranging from a few nucleotides to entire chromosomal arms. Factors such as rearrangements, hotspots and induced DSBs are implicated in the formation of SVs. While de novo SVs are often associated with disease, some SVs are conserved within human subpopulations and may have had a meaningful influence on primate evolution. As the ability to sequence the whole human genome rapidly evolves, the diversity of SVs is illuminated, including very complex rearrangements involving multiple DSBs in a process recently designated as "chromothripsis". Elucidating mechanisms involved in the etiology of SVs informs disease pathogenesis as well as the dynamic function associated with the biology and evolution of human genomes.

A common copy-number breakpoint of ERBB2 amplification in breast cancer colocalizes with a complex block of segmental duplications

Introduction

Segmental duplications (low-copy repeats) are the recently duplicated genomic segments in the human genome that display nearly identical (> 90%) sequences and account for about 5% of euchromatic regions. In germline, duplicated segments mediate nonallelic homologous recombination and thus cause both non-disease-causing copy-number variants and genomic disorders. To what extent duplicated segments play a role in somatic DNA rearrangements in cancer remains elusive. Duplicated segments often cluster and form genomic blocks enriched with both direct and inverted repeats (complex genomic regions). Such complex regions could be fragile and play a mechanistic role in the amplification of the ERBB2 gene in breast tumors, because repeated sequences are known to initiate gene amplification in model systems.

Methods

We conducted polymerase chain reaction (PCR)-based assays for primary breast tumors and analyzed publically available array-comparative genomic hybridization data to map a common copy-number breakpoint in ERBB2-amplified primary breast tumors. We further used molecular, bioinformatics, and population-genetics approaches to define duplication contents, structural variants, and haplotypes within the common breakpoint.

Results

We found a large (> 300-kb) block of duplicated segments that was colocalized with a common-copy number breakpoint for ERBB2 amplification. The breakpoint that potentially initiated ERBB2 amplification localized in a region 1.5 megabases (Mb) on the telomeric side of ERBB2. The region is very complex, with extensive duplications of KRTAP genes, structural variants, and, as a result, a paucity of single-nucleotide polymorphism (SNP) markers. Duplicated segments are varied in size and degree of sequence homology, indicating that duplications have occurred recurrently during genome evolution.

Conclusions

Amplification of the ERBB2 gene in breast tumors is potentially initiated by a complex region that has unusual genomic features and thus requires rigorous, labor-intensive investigation. The haplotypes we provide could be useful to identify the potential association between the complex region and ERBB2 amplification.

Origin of a novel protein-coding gene family with similar signal sequence in Schistosoma japonicum

BACKGROUND

Evolution of novel protein-coding genes is the bedrock of adaptive evolution. Recently, we identified six protein-coding genes with similar signal sequence from Schistosoma japonicum egg stage mRNA using signal sequence trap (SST). To find the mechanism underlying the origination of these genes with similar core promoter regions and signal sequence, we adopted an integrated approach utilizing whole genome, transcriptome and proteome database BLAST queries, other bioinformatics tools, and molecular analyses.

RESULTS

Our data, in combination with database analyses showed evidences of expression of these genes both at the mRNA and protein levels exclusively in all developmental stages of S. japonicum. The signal sequence motif was identified in 27 distinct S. japonicum UniGene entries with multiple mRNA transcripts, and in 34 genome contigs distributed within 18 scaffolds with evidence of genome-wide dispersion. No homolog of these genes or similar domain was found in deposited data from any other organism. We observed preponderance of flanking repetitive elements (REs), albeit partial copies, especially of the RTE-like and Perere class at either side of the duplication source locus. The role of REs as major mediators of DNA-level recombination leading to dispersive duplication is discussed with evidence from our analyses. We also identified a stepwise pathway towards functional selection in evolving genes by alternative splicing. Equally, the possible transcription models of some protein-coding representatives of the duplicons are presented with evidence of expression in vitro.

CONCLUSION

Our findings contribute to the accumulating evidence of the role of REs in the generation of evolutionary novelties in organisms' genomes.

The challenges of studying complex and dynamic regions of the human genome

Recent work has emphasised that the human genome is not simple and static, but complex and dynamic. This review focuses on the regions that are particularly hard to dissect and analyse, yet hold clues to how the genome changes during evolution and disease. I begin by summarising recent key advances in the understanding of the variable structure of our genome, and then I discuss a medley of methods that may allow us to analyse this structure in fine detail. In the final part, I describe potential future developments in this field, and make an argument that, just as we routinely genotype single-nucleotide polymorphisms now and will routinely re-sequence genomes in the near future, we should be aiming to physically re-map the individual human genome for each individual we study.

The origin, global distribution, and functional impact of the human 8p23 inversion polymorphism

Genomic inversions are an increasingly recognized source of genetic variation. However, a lack of reliable high-throughput genotyping assays for these structures has precluded a full understanding of an inversion's phylogenetic, phenotypic, and population genetic properties. We characterize these properties for one of the largest polymorphic inversions in man (the ∼4.5-Mb 8p23.1 inversion), a structure that encompasses numerous signals of natural selection and disease association. We developed and validated a flexible bioinformatics tool that utilizes SNP data to enable accurate, high-throughput genotyping of the 8p23.1 inversion. This tool was applied retrospectively to diverse genome-wide data sets, revealing significant population stratification that largely follows a clinal “serial founder effect” distribution model. Phylogenetic analyses establish the inversion's ancestral origin within the Homo lineage, indicating that 8p23.1 inversion has occurred independently in the Pan lineage. The human inversion breakpoint was localized to an inverted pair of human endogenous retrovirus elements within the large, flanking low-copy repeats; experimental validation of this breakpoint confirmed these elements as the likely intermediary substrates that sponsored inversion formation. In five data sets, mRNA levels of disease-associated genes were robustly associated with inversion genotype. Moreover, a haplotype associated with systemic lupus erythematosus was restricted to the derived inversion state. We conclude that the 8p23.1 inversion is an evolutionarily dynamic structure that can now be accommodated into the understanding of human genetic and phenotypic diversity.

Recurrent chromosome 16p13.1 duplications are a risk factor for aortic dissections

Chromosomal deletions or reciprocal duplications of the 16p13.1 region have been implicated in a variety of neuropsychiatric disorders such as autism, schizophrenia, epilepsies, and attention-deficit hyperactivity disorder (ADHD). In this study, we investigated the association of recurrent genomic copy number variants (CNVs) with thoracic aortic aneurysms and dissections (TAAD). By using SNP arrays to screen and comparative genomic hybridization microarrays to validate, we identified 16p13.1 duplications in 8 out of 765 patients of European descent with adult-onset TAAD compared with 4 of 4,569 controls matched for ethnicity (P = 5.0×10⁻⁵, OR = 12.2). The findings were replicated in an independent cohort of 467 patients of European descent with TAAD (P = 0.005, OR = 14.7). Patients with 16p13.1 duplications were more likely to harbor a second rare CNV (P = 0.012) and to present with aortic dissections (P = 0.010) than patients without duplications. Duplications of 16p13.1 were identified in 2 of 130 patients with familial TAAD, but the duplications did not segregate with TAAD in the families. MYH11, a gene known to predispose to TAAD, lies in the duplicated region of 16p13.1, and increased MYH11 expression was found in aortic tissues from TAAD patients with 16p13.1 duplications compared with control aortas. These data suggest chromosome 16p13.1 duplications confer a risk for TAAD in addition to the established risk for neuropsychiatric disorders. It also indicates that recurrent CNVs may predispose to disorders involving more than one organ system, an observation critical to the understanding of the role of recurrent CNVs in human disease and a finding that may be common to other recurrent CNVs involving multiple genes.

Thoracic aortic aneurysms and acute aortic dissections (TAAD) have ranked as high as the fifteenth leading cause of death in the United States. TAAD can be inherited in families in an autosomal dominant manner, and mutations in ACTA2 and MYH11, genes encoding two major components of the smooth muscle contractile unit, are responsible for approximately 15% of familial TAAD. However, the majority of patients with TAAD do not have an identified syndrome or family history of aortic disease, and genetic factors predisposing to these sporadic cases have not been identified. To determine whether recurrent genomic copy number variants (CNVs) contribute to TAAD pathogenesis, we screened 765 patients with adult-onset TAAD for CNVs and identified recurrent 16p13.1 duplications in 1% of TAAD cases compared with 0.09% of controls. The 16p13.1 duplication involves 9 genes, including MYH11. This recurrent duplication of 16p13.1 has also been determined to be associated with neuropsychiatric conditions, specifically schizophrenia and attention-deficit hyperactivity disorder. Our study suggests that recurrent duplications of 16p13.1 confer a risk for both neuropsychiatric diseases and TAAD, a finding that may be common to other recurrent CNVs involving multiple genes.

Positional information resolves structural variations and uncovers an evolutionarily divergent genetic locus in accessions of Arabidopsis thaliana

Genome sequencing of closely related individuals has yielded valuable insights that link genome evolution to phenotypic variations. However, advancement in sequencing technology has also led to an escalation in the number of poor quality–drafted genomes assembled based on reference genomes that can have highly divergent or haplotypic regions. The self-fertilizing nature of Arabidopsis thaliana poses an advantage to sequencing projects because its genome is mostly homozygous. To determine the accuracy of an Arabidopsis drafted genome in less conserved regions, we performed a resequencing experiment on a ∼371-kb genomic interval in the Landsberg erecta (Ler-0) accession. We identified novel structural variations (SVs) between Ler-0 and the reference accession Col-0 using a long-range polymerase chain reaction approach to generate an Illumina data set that has positional information, that is, a data set with reads that map to a known location. Positional information is important for accurate genome assembly and the resolution of SVs particularly in highly duplicated or repetitive regions. Sixty-one regions with misassembly signatures were identified from the Ler-0 draft, suggesting the presence of novel SVs that are not represented in the draft sequence. Sixty of those were resolved by iterative mapping using our data set. Fifteen large indels (>100 bp) identified from this study were found to be located either within protein-coding regions or upstream regulatory regions, suggesting the formation of novel alleles or altered regulation of existing genes in Ler-0. We propose future genome-sequencing experiments to follow a clone-based approach that incorporates positional information to ultimately reveal haplotype-specific differences between accessions.

Characterization of the past and current duplication activities in the human 22q11.2 region

Background

Segmental duplications (SDs) on 22q11.2 (LCR22), serve as substrates for meiotic non-allelic homologous recombination (NAHR) events resulting in several clinically significant genomic disorders.

Results

To understand the duplication activity leading to the complicated SD structure of this region, we have applied the A-Bruijn graph algorithm to decompose the 22q11.2 SDs to 523 fundamental duplication sequences, termed subunits. Cross-species syntenic analysis of primate genomes demonstrates that many of these LCR22 subunits emerged very recently, especially those implicated in human genomic disorders. Some subunits have expanded more actively than others, and young Alu SINEs, are associated much more frequently with duplicated sequences that have undergone active expansion, confirming their role in mediating recombination events. Many copy number variations (CNVs) exist on 22q11.2, some flanked by SDs. Interestingly, two chromosome breakpoints for 13 CNVs (mean length 65 kb) are located in paralogous subunits, providing direct evidence that SD subunits could contribute to CNV formation. Sequence analysis of PACs or BACs identified extra CNVs, specifically, 10 insertions and 18 deletions within 22q11.2; four were more than 10 kb in size and most contained young AluYs at their breakpoints.

Conclusions

Our study indicates that AluYs are implicated in the past and current duplication events, and moreover suggests that DNA rearrangements in 22q11.2 genomic disorders perhaps do not occur randomly but involve both actively expanded duplication subunits and Alu elements.

A novel third type of recurrent NF1 microdeletion mediated by nonallelic homologous recombination between LRRC37B-containing low-copy repeats in 17q11.2

Large microdeletions encompassing the neurofibromatosis type-1 (NF1) gene and its flanking regions at 17q11.2 belong to the group of genomic disorders caused by aberrant recombination between segmental duplications. The most common NF1 microdeletions (type-1) span 1.4-Mb and have breakpoints located within NF1-REPs A and C, low-copy repeats (LCRs) containing LRRC37-core duplicons. We have identified a novel type of recurrent NF1 deletion mediated by nonallelic homologous recombination (NAHR) between the highly homologous NF1-REPs B and C. The breakpoints of these approximately 1.0-Mb ("type-3") NF1 deletions were characterized at the DNA sequence level in three unrelated patients. Recombination regions, spanning 275, 180, and 109-bp, respectively, were identified within the LRRC37B-P paralogues of NF1-REPs B and C, and were found to contain sequences capable of non-B DNA formation. Both LCRs contain LRRC37-core duplicons, abundant and highly dynamic sequences in the human genome. NAHR between LRRC37-containing LCRs at 17q21.31 is known to have mediated the 970-kb polymorphic inversions of the MAPT-locus that occurred independently in different primate species, but also underlies the syndromes associated with recurrent 17q21.31 microdeletions and reciprocal microduplications. The novel NF1 microdeletions reported here provide further evidence for the unusually high recombinogenic potential of LRRC37-containing LCRs in the human genome.

A recurrent 16p12.1 microdeletion supports a two-hit model for severe developmental delay

We report the identification of a recurrent 520-kbp 16p12.1 microdeletion significantly associated with childhood developmental delay. The microdeletion was detected in 20/11,873 cases vs. 2/8,540 controls (p=0.0009, OR=7.2) and replicated in a second series of 22/9,254 cases vs. 6/6,299 controls (p=0.028, OR=2.5). Most deletions were inherited with carrier parents likely to manifest neuropsychiatric phenotypes (p=0.037, OR=6). Probands were more likely to carry an additional large CNV when compared to matched controls (10/42 cases, p=5.7×10^-5, OR=6.65). Clinical features of cases with two mutations were distinct from and/or more severe than clinical features of patients carrying only the co-occurring mutation. Our data suggest a two-hit model in which the 16p12.1 microdeletion both predisposes to neuropsychiatric phenotypes as a single event and exacerbates neurodevelopmental phenotypes in association with other large deletions or duplications. Analysis of other microdeletions with variable expressivity suggests that this two-hit model may be more generally applicable to neuropsychiatric disease.

Efficient algorithms for analyzing segmental duplications with deletions and inversions in genomes

Background

Segmental duplications, or low-copy repeats, are common in mammalian genomes. In the human genome, most segmental duplications are mosaics comprised of multiple duplicated fragments. This complex genomic organization complicates analysis of the evolutionary history of these sequences. One model proposed to explain this mosaic patterns is a model of repeated aggregation and subsequent duplication of genomic sequences.

Results

We describe a polynomial-time exact algorithm to compute duplication distance, a genomic distance defined as the most parsimonious way to build a target string by repeatedly copying substrings of a fixed source string. This distance models the process of repeated aggregation and duplication. We also describe extensions of this distance to include certain types of substring deletions and inversions. Finally, we provide a description of a sequence of duplication events as a context-free grammar (CFG).

Conclusion

These new genomic distances will permit more biologically realistic analyses of segmental duplications in genomes.

Tandem repeats modify the structure of human genes hosted in segmental duplications

BACKGROUND

Recently duplicated genes are often subject to genomic rearrangements that can lead to the development of novel gene structures. Here we specifically investigated the effect of variations in internal tandem repeats (ITRs) on the gene structure of human paralogs located in segmental duplications.

RESULTS

We found that around 7% of the primate-specific genes located within duplicated regions of the genome contain variable tandem repeats. These genes are members of large groups of recently duplicated paralogs that are often polymorphic in the human population. Half of the identified ITRs occur within coding exons and may be either kept or spliced out from the mature transcript. When ITRs reside within exons, they encode variable amino acid repeats. When located at exon-intron boundaries, ITRs can generate alternative splicing patterns through the formation of novel introns.

CONCLUSIONS

Our study shows that variation in the number of ITRs impacts on recently duplicated genes by modifying their coding sequence, splicing pattern, and tissue expression. The resulting effect is the production of a variety of primate-specific proteins, which mostly differ in number and sequence of amino acid repeats.

Sequencing primate genomes: what have we learned?

We summarize the progress in whole-genome sequencing and analyses of primate genomes. These emerging genome datasets have broadened our understanding of primate genome evolution revealing unexpected and complex patterns of evolutionary change. This includes the characterization of genome structural variation, episodic changes in the repeat landscape, differences in gene expression, new models regarding speciation, and the ephemeral nature of the recombination landscape. The functional characterization of genomic differences important in primate speciation and adaptation remains a significant challenge. Limited access to biological materials, the lack of detailed phenotypic data and the endangered status of many critical primate species have significantly attenuated research into the genetic basis of primate evolution. Next-generation sequencing technologies promise to greatly expand the number of available primate genome sequences; however, such draft genome sequences will likely miss critical genetic differences within complex genomic regions unless dedicated efforts are put forward to understand the full spectrum of genetic variation.

The origins and impact of primate segmental duplications

Duplicated sequences are substrates for the emergence of new genes and are an important source of genetic instability associated with rare and common diseases. Analyses of primate genomes have shown an increase in the proportion of interspersed segmental duplications (SDs) within the genomes of humans and great apes. This contrasts with other mammalian genomes that seem to have their recently duplicated sequences organized in a tandem configuration. In this review, we focus on the mechanistic origin and impact of this difference with respect to evolution, genetic diversity and primate phenotype. Although many genomes will be sequenced in the future, resolution of this aspect of genomic architecture still requires high quality sequences and detailed analyses.

The evolution of human segmental duplications and the core duplicon hypothesis

Duplicated sequences are important sources of genetic instability and in the evolution of new gene function within species. Hominids have a preponderance of intrachromosomal duplications organized in an interspersed fashion, as opposed to tandem duplications, which are common in other mammalian genomes such as mouse, dog, and cow. Multiple lines of evidence, including sequence divergence, comparative primate genomes, and fluorescence in situ hybridization (FISH) analyses, point to an excess of segmental duplications in the common ancestor of humans and African great apes. We find that much of the interspersed human duplication architecture within chromosomes is focused around common sequence elements referred to as "core duplicons." These cores correspond to the expansion of gene families, some of which show signatures of positive selection and lack orthologs present in other mammalian species. This genomic architecture predisposes apes and humans not only to extensive genetic diversity, but also to large-scale structural diversity mediated by nonallelic homologous recombination. In humans, many de novo large-scale genomic changes mediated by these duplications are associated with neuropsychiatric and neurodevelopmental disease. We propose that the disadvantage of a high rate of new mutations is offset by the selective advantage of newly minted genes within the cores.

Evolutionary toggling of the MAPT 17q21.31 inversion region

Using comparative sequencing approaches, we investigated the evolutionary history of the European-enriched 17q21.31 MAPT inversion polymorphism. We present a detailed, BAC-based sequence assembly of the inverted human H2 haplotype and compare it to the sequence structure and genetic variation of the corresponding 1.5-Mb region for the noninverted H1 human haplotype and that of chimpanzee and orangutan. We found that inversion of the MAPT region is similarly polymorphic in other great ape species, and we present evidence that the inversions occurred independently in chimpanzees and humans. In humans, the inversion breakpoints correspond to core duplications with the LRRC37 gene family. Our analysis favors the H2 configuration and sequence haplotype as the likely great ape and human ancestral state, with inversion recurrences during primate evolution. We show that the H2 architecture has evolved more extensive sequence homology, perhaps explaining its tendency to undergo microdeletion associated with mental retardation in European populations.

A role for DNA polymerase mu in the emerging DJH rearrangements of the postgastrulation mouse embryo

The molecular complexes involved in the nonhomologous end-joining process that resolves recombination-activating gene (RAG)-induced double-strand breaks and results in V(D)J gene rearrangements vary during mammalian ontogeny. In the mouse, the first immunoglobulin gene rearrangements emerge during midgestation periods, but their repertoires have not been analyzed in detail. We decided to study the postgastrulation DJ(H) joints and compare them with those present in later life. The embryo DJ(H) joints differed from those observed in perinatal life by the presence of short stretches of nontemplated (N) nucleotides. Whereas most adult N nucleotides are introduced by terminal deoxynucleotidyl transferase (TdT), the embryo N nucleotides were due to the activity of the homologous DNA polymerase mu (Polmu), which was widely expressed in the early ontogeny, as shown by analysis of Polmu(-/-) embryos. Based on its DNA-dependent polymerization ability, which TdT lacks, Polmu also filled in small sequence gaps at the coding ends and contributed to the ligation of highly processed ends, frequently found in the embryo, by pairing to internal microhomology sites. These findings show that Polmu participates in the repair of early-embryo, RAG-induced double-strand breaks and subsequently may contribute to preserve the genomic stability and cellular homeostasis of lymphohematopoietic precursors during development.

Primate-specific spliced PMCHL RNAs are non-protein coding in human and macaque tissues

Background

Brain-expressed genes that were created in primate lineage represent obvious candidates to investigate molecular mechanisms that contributed to neural reorganization and emergence of new behavioural functions in Homo sapiens. PMCHL1 arose from retroposition of a pro-melanin-concentrating hormone (PMCH) antisense mRNA on the ancestral human chromosome 5p14 when platyrrhines and catarrhines diverged. Mutations before divergence of hylobatidae led to creation of new exons and finally PMCHL1 duplicated in an ancestor of hominids to generate PMCHL2 at the human chromosome 5q13. A complex pattern of spliced and unspliced PMCHL RNAs were found in human brain and testis.

Results

Several novel spliced PMCHL transcripts have been characterized in human testis and fetal brain, identifying an additional exon and novel splice sites. Sequencing of PMCHL genes in several non-human primates allowed to carry out phylogenetic analyses revealing that the initial retroposition event took place within an intron of the brain cadherin (CDH12) gene, soon after platyrrhine/catarrhine divergence, i.e. 30–35 Mya, and was concomitant with the insertion of an AluSg element. Sequence analysis of the spliced PMCHL transcripts identified only short ORFs of less than 300 bp, with low (VMCH-p8 and protein variants) or no evolutionary conservation. Western blot analyses of human and macaque tissues expressing PMCHL RNA failed to reveal any protein corresponding to VMCH-p8 and protein variants encoded by spliced transcripts.

Conclusion

Our present results improve our knowledge of the gene structure and the evolutionary history of the primate-specific chimeric PMCHL genes. These genes produce multiple spliced transcripts, bearing short, non-conserved and apparently non-translated ORFs that may function as mRNA-like non-coding RNAs.

Explaining human uniqueness: genome interactions with environment, behaviour and culture

What makes us human? Specialists in each discipline respond through the lens of their own expertise. In fact, 'anthropogeny' (explaining the origin of humans) requires a transdisciplinary approach that eschews such barriers. Here we take a genomic and genetic perspective towards molecular variation, explore systems analysis of gene expression and discuss an organ-systems approach. Rejecting any 'genes versus environment' dichotomy, we then consider genome interactions with environment, behaviour and culture, finally speculating that aspects of human uniqueness arose because of a primate evolutionary trend towards increasing and irreversible dependence on learned behaviours and culture - perhaps relaxing allowable thresholds for large-scale genomic diversity.

How segmental duplications shape our genome: recent evolution of ABCC6 and PKD1 Mendelian disease genes

The completion of the Human Genome Project has brought the understanding that our genome contains an unexpectedly large proportion of segmental duplications. This poses the challenge of elucidating the consequences of recent duplications on physiology. We have conducted an in-depth study of a subset of segmental duplications on chromosome 16. We focused on PKD1 and ABCC6 duplications because mutations affecting these genes are responsible for the Mendelian disorders autosomal dominant polycystic kidney disease and pseudoxanthoma elasticum, respectively. We establish that duplications of PKD1 and ABCC6 are associated to low-copy repeat 16a and show that such duplications have occurred several times independently in different primate species. We demonstrate that partial duplication of PKD1 and ABCC6 has numerous consequences: the pseudogenes give rise to new transcripts and mediate gene conversion, which not only results in disease-causing mutations but also serves as a reservoir for sequence variation. The duplicated segments are also involved in submicroscopic and microscopic genomic rearrangements, contributing to structural variation in human and chromosomal break points in the gibbon. In conclusion, our data shed light on the recent and ongoing evolution of chromosome 16 mediated by segmental duplication and deepen our understanding of the history of two Mendelian disorder genes.

Copy number variation and evolution in humans and chimpanzees

Copy number variants (CNVs) underlie many aspects of human phenotypic diversity and provide the raw material for gene duplication and gene family expansion. However, our understanding of their evolutionary significance remains limited. We performed comparative genomic hybridization on a single human microarray platform to identify CNVs among the genomes of 30 humans and 30 chimpanzees as well as fixed copy number differences between species. We found that human and chimpanzee CNVs occur in orthologous genomic regions far more often than expected by chance and are strongly associated with the presence of highly homologous intrachromosomal segmental duplications. By adapting population genetic analyses for use with copy number data, we identified functional categories of genes that have likely evolved under purifying or positive selection for copy number changes. In particular, duplications and deletions of genes with inflammatory response and cell proliferation functions may have been fixed by positive selection and involved in the adaptive phenotypic differentiation of humans and chimpanzees.

The genomic distribution of intraspecific and interspecific sequence divergence of human segmental duplications relative to human/chimpanzee chromosomal rearrangements

Background

It has been suggested that chromosomal rearrangements harbor the molecular footprint of the biological phenomena which they induce, in the form, for instance, of changes in the sequence divergence rates of linked genes. So far, all the studies of these potential associations have focused on the relationship between structural changes and the rates of evolution of single-copy DNA and have tried to exclude segmental duplications (SDs). This is paradoxical, since SDs are one of the primary forces driving the evolution of structure and function in our genomes and have been linked not only with novel genes acquiring new functions, but also with overall higher DNA sequence divergence and major chromosomal rearrangements.

Results

Here we take the opposite view and focus on SDs. We analyze several of the features of SDs, including the rates of intraspecific divergence between paralogous copies of human SDs and of interspecific divergence between human SDs and chimpanzee DNA. We study how divergence measures relate to chromosomal rearrangements, while considering other factors that affect evolutionary rates in single copy DNA.

Conclusion

We find that interspecific SD divergence behaves similarly to divergence of single-copy DNA. In contrast, old and recent paralogous copies of SDs do present different patterns of intraspecific divergence. Also, we show that some relatively recent SDs accumulate in regions that carry inversions in sister lineages.

Recurrent reciprocal deletions and duplications of 16p13.11: the deletion is a risk factor for MR/MCA while the duplication may be a rare benign variant

BACKGROUND

Genomic disorders are often caused by non-allelic homologous recombination between segmental duplications. Chromosome 16 is especially rich in a chromosome-specific low copy repeat, termed LCR16.

METHODS AND RESULTS

A bacterial artificial chromosome (BAC) array comparative genome hybridisation (CGH) screen of 1027 patients with mental retardation and/or multiple congenital anomalies (MR/MCA) was performed. The BAC array CGH screen identified five patients with deletions and five with apparently reciprocal duplications of 16p13 covering 1.65 Mb, including 15 RefSeq genes. In addition, three atypical rearrangements overlapping or flanking this region were found. Fine mapping by high-resolution oligonucleotide arrays suggests that these deletions and duplications result from non-allelic homologous recombination (NAHR) between distinct LCR16 subunits with >99% sequence identity. Deletions and duplications were either de novo or inherited from unaffected parents. To determine whether these imbalances are associated with the MR/MCA phenotype or whether they might be benign variants, a population of 2014 normal controls was screened. The absence of deletions in the control population showed that 16p13.11 deletions are significantly associated with MR/MCA (p = 0.0048). Despite phenotypic variability, common features were identified: three patients with deletions presented with MR, microcephaly and epilepsy (two of these had also short stature), and two other deletion carriers ascertained prenatally presented with cleft lip and midline defects. In contrast to its previous association with autism, the duplication seems to be a common variant in the population (5/1682, 0.29%).

CONCLUSION

These findings indicate that deletions inherited from clinically normal parents are likely to be causal for the patients' phenotype whereas the role of duplications (de novo or inherited) in the phenotype remains uncertain. This difference in knowledge regarding the clinical relevance of the deletion and the duplication causes a paradigm shift in (cyto)genetic counselling.

DupMasker: a tool for annotating primate segmental duplications

Segmental duplications (SDs) play an important role in genome rearrangement, evolution, and the copy-number variation (CNV) of primate genomes. Such sequences are difficult to detect, a priori, because they share no defining sequence features that distinguish them from unique portions of the genome. Current sequence annotation of segmental duplications requires computationally intensive, genome-wide self-comparisons that cannot be easily implemented on new data sets. Based on the successful implementation of RepeatMasker, we developed a new genome annotation tool, DupMasker. The program uses a library of nonredundant consensus sequences of human segmental duplications, wherein a majority of the ancestral origins have been determined based on comparisons to mammalian outgroup genomes. Using DupMasker, new human and nonhuman primate (NHP) sequences may be readily queried to provide details on the origin and degree of sequence identity of each duplicon. This program can be applied to delineate the order and orientation of duplicons within complex duplication blocks and used to characterize structural variation differences between sequenced human haplotypes. We predict this tool will be valuable in the annotation of large-insert sequence clones, allowing putative unique and duplicated regions of the genomes to be annotated prior to whole genome assembly comparisons.

Molecular mechanisms of chromosomal rearrangement during primate evolution

Breakpoint analysis of the large chromosomal rearrangements which have occurred during primate evolution promises to yield new insights into the underlying mechanisms of mutagenesis. Comparison of these evolutionary breakpoints with those that are disease-associated in humans, and which occur during either meiotic or mitotic cell division, should help to identify basic mechanistic similarities as well as differences. It has recently become clear that segmental duplications (SDs) have had a very significant impact on genome plasticity during primate evolution. In comparisons of the human and chimpanzee genomes, SDs have been found in flanking regions of 70-80% of inversions and approximately 40% of deletions/duplications. A strong spatial association between primate-specific breakpoints and SDs has also become evident from comparisons of human with other mammalian genomes. The lineage-specific hyperexpansion of certain SDs observed in the genomes of human, chimpanzee, gorilla and gibbon is indicative of the intrinsic instability of some SDs in primates. However, since many primate-specific breakpoints map to regions lacking SDs, but containing interspersed high-copy repetitive sequence elements such as SINEs, LINEs, LTRs, alpha-satellites and (AT)( n ) repeats, we may infer that a range of different molecular mechanisms have probably been involved in promoting chromosomal breakage during the evolution of primate genomes.

DA and Xiao-two giant and composite LTR-retrotransposon-like elements identified in the human genome

We discovered two new complex elements while studying large genomic rearrangements and segmental duplications in the human genome. Both resemble bacterial composite DNA transposon Tn9, consisting of a core flanked by mobile elements, except that the flanking element is not a DNA transposon but instead is long terminal repeat retrotransposon-like with human endogenous retrovirus and satellite sequences. Based on the core size, we named them Xiao ( approximately 30 kb) and DA ( approximately 280 kb), meaning small and big, respectively, in Chinese. Xiao originated from a 19p region encoding olfactory receptor 7E members after the human/ape divergence from Old World monkeys, while DA likely evolved from a Xiao by inserting approximately 200 kb of chimeric sequence from 16p and 21q into the Xiao core, resulting in a target site duplication of 3.4 kb. DA/Xiao was identified in 30 loci on 12 chromosomes, and only DAs mediated intrachromosomal rearrangements, based on our reconstructed human-mouse-rat ancestral genome and the rhesus macaque genome.

Repetitive element-mediated recombination as a mechanism for new gene origination in Drosophila

Previous studies of repetitive elements (REs) have implicated a mechanistic role in generating new chimerical genes. Such examples are consistent with the classic model for exon shuffling, which relies on non-homologous recombination. However, recent data for chromosomal aberrations in model organisms suggest that ectopic homology-dependent recombination may also be important. Lack of a dataset comprising experimentally verified young duplicates has hampered an effective examination of these models as well as an investigation of sequence features that mediate the rearrangements. Here we use ∼7,000 cDNA probes (∼112,000 primary images) to screen eight species within the Drosophila melanogaster subgroup and identify 17 duplicates that were generated through ectopic recombination within the last 12 mys. Most of these are functional and have evolved divergent expression patterns and novel chimeric structures. Examination of their flanking sequences revealed an excess of repetitive sequences, with the majority belonging to the transposable element DNAREP1 family, associated with the new genes. Our dataset strongly suggests an important role for REs in the generation of chimeric genes within these species.

In numerous organisms, many new genes have been found to originate through dispersed gene duplication and exon/domain shuffling. What recombination mechanisms were involved in the duplication and the shuffling processes? Lack of the intermediate products of recombination that share adequate sequence identity between homologous sequences, or the parental sequences from which the new genes were derived, often makes answering these questions difficult. We identified a number of young genes that originated in recently diverged branches in the evolutionary tree of the eight Drosophila melanogaster subgroup species, by using fluorescence in situ hybridization with polytene chromosomes. We analyzed the genomic regions surrounding 17 new dispersed duplicate genes and observed that most of these genes are flanked by repetitive elements (REs), including a large and diverged transposable element family, DNAREP1. Several copies of these REs are kept in both new and parental gene regions, and their degeneration is correlated with the increasing ages of the identified new genes. These data suggest that REs mediate the recombination responsible for the new gene origination.

From microscopes to microarrays: dissecting recurrent chromosomal rearrangements

Submicroscopic chromosomal rearrangements that lead to copy-number changes have been shown to underlie distinctive and recognizable clinical phenotypes. The sensitivity to detect copy-number variation has escalated with the advent of array comparative genomic hybridization (CGH), including BAC and oligonucleotide-based platforms. Coupled with improved assemblies and annotation of genome sequence data, these technologies are facilitating the identification of new syndromes that are associated with submicroscopic genomic changes. Their characterization reveals the role of genome architecture in the aetiology of many clinical disorders. We review a group of genomic disorders that are mediated by segmental duplications, emphasizing the impact that high-throughput detection methods and the availability of the human genome sequence have had on their dissection and diagnosis.

Ancestral reconstruction of segmental duplications reveals punctuated cores of human genome evolution

Human segmental duplications are hotspots for nonallelic homologous recombination leading to genomic disorders, copy-number polymorphisms and gene and transcript innovations. The complex structure and history of these regions have precluded a global evolutionary analysis. Combining a modified A-Bruijn graph algorithm with comparative genome sequence data, we identify the origin of 4,692 ancestral duplication loci and use these to cluster 437 complex duplication blocks into 24 distinct groups. The sequence-divergence data between ancestral-derivative pairs and a comparison with the chimpanzee and macaque genome support a 'punctuated' model of evolution. Our analysis reveals that human segmental duplications are frequently organized around 'core' duplicons, which are enriched for transcripts and, in some cases, encode primate-specific genes undergoing positive selection. We hypothesize that the rapid expansion and fixation of some intrachromosomal segmental duplications during great-ape evolution has been due to the selective advantage conferred by these genes and transcripts embedded within these core duplications.

Hominoid lineage specific amplification of low-copy repeats on 22q11.2 (LCR22s) associated with velo-cardio-facial/digeorge syndrome

Segmental duplications or low-copy repeats (LCRs) constitute approximately 5% of the sequenced portion of the human genome and are associated with many human congenital anomaly disorders. The low-copy repeats on chromosome 22q11.2 (LCR22s) mediate chromosomal rearrangements resulting in deletions, duplications and translocations. The evolutionary mechanisms leading to LCR22 formation is unknown. Four genes, USP18, BCR, GGTLA and GGT, map adjacent to the LCR22s and pseudogene copies are located within them. It has been hypothesized that gene duplication occurred during primate evolution, followed by recombination events, forming pseudogene copies. We investigated whether gene duplication could be detected in non-human hominoid species. FISH mapping was performed using probes to the four functional gene loci. There was evidence for a single copy in humans but additional copies in hominoid species. We then compared LCR22 copy number using LCR22 FISH probes. Lineage specific LCR22 variation was detected in the hominoid species supporting the hypothesis. To independently validate initial findings, real time PCR, and screening of gorilla BAC library filters were performed. This was compared to array comparative genome hybridization data available. The most striking finding was a dramatic amplification of LCR22s in the gorilla. The LCR22s localized to the telomeric or subtelomeric bands of gorilla chromosomes. The most parsimonious explanation is that the LCR22s became amplified by inter-chromosomal recombination between telomeric bands. In summary, our results are consistent with a lineage specific coupling between gene and LCR22 duplication events. The LCR22s thus serve as an important model for evolution of genome variation.