Methods of Detection and Mechanisms of Origin of Complex Structural Genome Variations

Based on classical karyotyping, structural genome variations (SVs) have generally been considered to be either "simple" (with one or two breakpoints) or "complex" (with more than two breakpoints). Studying the breakpoints of SVs at nucleotide resolution revealed additional, subtle structural variations, such that even "simple" SVs turned out to be "complex." Genome-wide sequencing methods, such as fosmid and paired-end mapping, short-read and long-read whole genome sequencing, and single-molecule optical mapping, also indicated that the number of SVs per individual was considerably larger than expected from karyotyping and high-resolution chromosomal array-based studies. Interestingly, SVs were detected in studies of cohorts of individuals without clinical phenotypes. The common denominator of all SVs appears to be a failure to accurately repair DNA double-strand breaks (DSBs) or to halt cell cycle progression if DSBs persist. This review discusses the various DSB response mechanisms during the mitotic cell cycle and during meiosis and their regulation. Emphasis is given to the molecular mechanisms involved in the formation of translocations, deletions, duplications, and inversions during or shortly after meiosis I. Recently, CRISPR-Cas9 studies have provided unexpected insights into the formation of translocations and chromothripsis by both breakage-fusion-bridge and micronucleus-dependent mechanisms.

Long-read sequence analysis for clustered genomic copy number aberrations revealed architectures of intricately intertwined rearrangements

Most chromosomal aberrations revealed by chromosomal microarray testing (CMA) are simple; however, very complex chromosomal structural rearrangements can also be found. Although the mechanism of structural rearrangements has been gradually revealed, not all mechanisms have been elucidated. We analyzed the breakpoint-junctions (BJs) of two or more clustered copy number variations (CNVs) in the same chromosome arms to understand their conformation and the mechanism of complex structural rearrangements. Combining CMA with long-read whole-genome sequencing (WGS) analysis, we successfully determined all BJs for the clustered CNVs identified in four patients. Multiple CNVs were intricately intertwined with each other, and clustered CNVs in four patients were involved in global complex chromosomal rearrangements. The BJs of two clustered deletions identified in two patients showed microhomologies, and their characteristics were explained by chromothripsis. In contrast, the BJs in the other two patients, who showed clustered deletions and duplications, consisted of blunt-end and nontemplated insertions. These findings could be explained only by alternative nonhomologous end-joining, a mechanism related to polymerase theta. All the patients had at least one inverted segment. Three patients showed cryptic aberrations involving a disruption and a deletion/duplication, which were not detected by CMA but were first identified by WGS. This result suggested that complex rearrangements should be considered if clustered CNVs are observed in the same chromosome arms. Because CMA has potential limitations in genotype-phenotype correlation analysis, a more detailed analysis by whole genome examination is recommended in cases of suspected complex structural aberrations.

Chromoanagenesis, the mechanisms of a genomic chaos

Designated under the name of chromoanagenesis, the phenomena of chromothripsis, chromanasynthesis and chromoplexy constitute new types of complex rearrangements, including many genomic alterations localized on a few chromosomal regions, and whose discovery over the last decade has changed our perception about the formation of chromosomal abnormalities and their etiology. Although exhibiting specific features, these new catastrophic mechanisms generally occur within a single cell cycle and their emergence is closely linked to genomic instability. Various non-exclusive exogenous or cellular mechanisms capable of generating chromoanagenesis have been evoked. However, recent experimental data shed light on 2 major processes, which following a defect in the mitotic segregation of chromosomes, can generate a cascade of cellular events leading to chromoanagenesis. These mechanisms are the formation of micronuclei integrating isolated chromosomal material, and the occurrence of chromatin bridges around chromosomal material resulting from telomeric fusions. In both cases, the cellular and molecular mechanisms of fragmentation, repair and transmission of damaged chromosomal material are consistent with the features of chromoanagenesis-related complex chromosomal rearrangements. In this review, we introduce each type of chromoanagenesis, and describe the experimental models that have allowed to validate the existence of chromoanagenesis events and to better understand their cellular mechanisms of formation and transmission, as well as their impact on the stability and the plasticity of the genome.

Two Patients with Complex Rearrangements Suggestive of Germline Chromoanagenesis

Chromoanagenesis, a phenomenon characterized by complex chromosomal rearrangement and reorganization events localized to a limited number of genomic regions, includes the subcategories chromothripsis, chromoanasynthesis, and chromoplexy. Although definitions of these terms are evolving, constitutional chromoanagenesis events have been reported in a limited number of patients with variable phenotypes. We report on 2 cases with complex genomic events characterized by multiple copy number gains and losses confined to a single chromosome region, which are suggestive of constitutional chromoanagenesis. Case 1 is a 43-year-old male with intellectual disability and recently developed generalized tonic-clonic seizures. Chromosomal microarray analysis identified a complex rearrangement involving chromosome region 14q31.1q32.2, consisting of 16 breakpoints ranging in size from 0.2 to 6.2 Mb, with 5 segments of normal copy number present between these alterations. Interestingly, this case represents the oldest known patient with a complex rearrangement indicative of constitutional chromoanagenesis. Case 2 is a 2-year-old female with developmental delay, speech delay, low muscle tone, and seizures. Chromosomal microarray analysis identified a complex rearrangement consisting of 28 breakpoints localized to 18q21.32q23. The size of the copy number alterations ranged from 0.042 to 5.1 Mb, flanked by 12 small segments of normal copy number. These cases add to a growing body of literature demonstrating complex chromosomal rearrangements as a disease mechanism for congenital anomalies.

Copy number variations and constitutional chromothripsis (Review)

Both copy number variations (CNVs) and chromothripsis are phenomena that involve complex genomic rearrangements. Chromothripsis results in CNVs and other structural changes. CNVs are frequently observed in the human genome. Studies on CNVs have been increasing exponentially; the Database of Genomic Variants shows an increase in the number of data published on structural variations added to the database in the last 15 years. CNVs may be a result of replicative and non-replicative mechanisms, and are hypothesized to serve important roles in human health and disease. Chromothripsis is a phenomena of chromosomal rearrangement following chromosomal breaks at multiple locations and involves impaired DNA repair. In 2011, Stephens et al coined the term chromothripsis for this type of fragmenting event. Several proposed mechanisms have been suggested to underlie chromothripsis, such as p53 inactivation, micronuclei formation, abortive apoptosis and telomere fusions in telomere crisis. Chromothripsis gives rise to normal or abnormal phenotypes. In this review, constitutional chromothripsis, which may coexist with multiple de novo CNVs are described and discussed. This reviews aims to summarize recent advances in our understanding of CNVs and chromothripsis, and describe the effects of these phenomena on human health and birth defects.

De Novo Small Supernumerary Marker Chromosomes Arising From Partial Trisomy Rescue

Small supernumerary marker chromosomes (SMCs) are rare cytogenetic abnormalities. De novo small SMCs, particularly those combined with uniparental disomy (UPD), are assumed to result from incomplete trisomy rescue. Recently, a one-off cellular event designated as chromothripsis was reported as a mechanism for trisomy rescue in micronuclei. This Perspective article aims to highlight a possible association among trisomy rescue, chromothripsis, and SMCs. We propose that chromothripsis-mediated incomplete trisomy rescue in micronuclei underlies various chromosomal rearrangements including SMCs, although other mechanisms such as U-type exchange may also yield SMCs. These assumptions are primarily based on observations of previously reported patients with complex rearrangements and our patient with a small SMC. Given the high frequency of trisomic cells in human preimplantation embryos, chromothripsis-mediated trisomy rescue may be a physiologically important phenomenon. Nevertheless, trisomy rescue has a potential to produce UPD, SMCs, and other chromosomal rearrangements. The concepts of trisomy rescue, chromothripsis, and micronuclei provide novel insights into the mechanism for the maintenance and modification of human chromosomes.

Investigation of somatic CNVs in brains of synucleinopathy cases using targeted SNCA analysis and single cell sequencing

Synucleinopathies are mostly sporadic neurodegenerative disorders of partly unexplained aetiology, and include Parkinson's disease (PD) and multiple system atrophy (MSA). We have further investigated our recent finding of somatic SNCA (α-synuclein) copy number variants (CNVs, specifically gains) in synucleinopathies, using Fluorescent in-situ Hybridisation for SNCA, and single-cell whole genome sequencing for the first time in a synucleinopathy. In the cingulate cortex, mosaicism levels for SNCA gains were higher in MSA and PD than controls in neurons (> 2% in both diseases), and for MSA also in non-neurons. In MSA substantia nigra (SN), we noted SNCA gains in > 3% of dopaminergic (DA) neurons (identified by neuromelanin) and neuromelanin-negative cells, including olig2-positive oligodendroglia. Cells with CNVs were more likely to have α-synuclein inclusions, in a pattern corresponding to cell categories mostly relevant to the disease: DA neurons in Lewy-body cases, and other cells in the striatonigral degeneration-dominant MSA variant (MSA-SND). Higher mosaicism levels in SN neuromelanin-negative cells may correlate with younger onset in typical MSA-SND, and in cingulate neurons with younger death in PD. Larger sample sizes will, however, be required to confirm these putative findings. We obtained genome-wide somatic CNV profiles from 169 cells from the substantia nigra of two MSA cases, and pons and putamen of one. These showed somatic CNVs in ~ 30% of cells, with clonality and origins in segmental duplications for some. CNVs had distinct profiles based on cell type, with neurons having a mix of gains and losses, and other cells having almost exclusively gains, although control data sets will be required to determine possible disease relevance. We propose that somatic SNCA CNVs may contribute to the aetiology and pathogenesis of synucleinopathies, and that genome-wide somatic CNVs in MSA brain merit further study.

A 14q distal chromoanagenesis elucidated by whole genome sequencing

Chromoanagenesis represents an extreme form of genomic rearrangements involving multiple breaks occurring on a single or multiple chromosomes. It has been recently described in both acquired and rare constitutional genetic disorders. Constitutional chromoanagenesis events could lead to abnormal phenotypes including developmental delay and congenital anomalies, and have also been implicated in some specific syndromic disorders. We report the case of a girl presenting with growth retardation, hypotonia, microcephaly, dysmorphic features, coloboma, and hypoplastic corpus callosum. Karyotype showed a de novo structurally abnormal chromosome 14q31qter region. Molecular characterization using SNP-array revealed a complex unbalanced rearrangement in 14q31.1-q32.2, on the paternal chromosome 14, including thirteen interstitial deletions ranging from 33 kb to 1.56 Mb in size, with a total of 4.1 Mb in size, thus suggesting that a single event like chromoanagenesis occurred. To our knowledge, this is one of the first case of 14q distal deletion due to a germline chromoanagenesis. Genome sequencing allowed the characterization of 50 breakpoints, leading to interruption of 10 genes including YY1 which fit with the patient's phenotype. This precise genotyping of breaking junction allowed better definition of genotype-phenotype correlations.

Single molecule real-time (SMRT) sequencing comes of age: applications and utilities for medical diagnostics

Short read massive parallel sequencing has emerged as a standard diagnostic tool in the medical setting. However, short read technologies have inherent limitations such as GC bias, difficulties mapping to repetitive elements, trouble discriminating paralogous sequences, and difficulties in phasing alleles. Long read single molecule sequencers resolve these obstacles. Moreover, they offer higher consensus accuracies and can detect epigenetic modifications from native DNA. The first commercially available long read single molecule platform was the RS system based on PacBio's single molecule real-time (SMRT) sequencing technology, which has since evolved into their RSII and Sequel systems. Here we capsulize how SMRT sequencing is revolutionizing constitutional, reproductive, cancer, microbial and viral genetic testing.

On the Complexity of Mechanisms and Consequences of Chromothripsis: An Update

In the present review, we focus on the phenomenon of chromothripsis, a new type of complex chromosomal rearrangements. We discuss the challenges of chromothripsis detection and its distinction from other chromoanagenesis events. Along with already known causes and mechanisms, we introduce aberrant epigenetic regulation as a possible pathway to chromothripsis. We address the issue of chromothripsis characteristics in cancers and benign tumours, as well as chromothripsis inheritance in cases of its occurrence in germ cells, zygotes and early embryos. Summarising the presented data on different phenotypic effect of chromothripsis, we assume that its consequences are most likely determined not by the chromosome shattering and reassembly themselves, but by the genome regions involved in the rearrangement.

The Iceberg under Water: Unexplored Complexity of Chromoanagenesis in Congenital Disorders

Structural variation, composed of balanced and unbalanced genomic rearrangements, is an important contributor to human genetic diversity with prominent roles in somatic and congenital disease. At the nucleotide level, structural variants (SVs) have been shown to frequently harbor additional breakpoints and copy-number imbalances, a complexity predicted to emerge wholly as a single-cell division event. Chromothripsis, chromoplexy, and chromoanasynthesis, collectively referred to as chromoanagenesis, are three major mechanisms that explain the occurrence of complex germline and somatic SVs. While chromothripsis and chromoplexy have been shown to be key signatures of cancer, chromoanagenesis has been detected in numerous cases of developmental disease and phenotypically normal individuals. Such observations advocate for a deeper study of the polymorphic and pathogenic properties of complex germline SVs, many of which go undetected by traditional clinical molecular and cytogenetic methods. This review focuses on congenital chromoanagenesis, mechanisms leading to occurrence of these complex rearrangements, and their impact on chromosome organization and genome function. We highlight future applications of routine screening of complex and balanced SVs in the clinic, as these represent a potential and often neglected genetic disease source, a true "iceberg under water."

Chromoanagenesis: cataclysms behind complex chromosomal rearrangements

Background

During the last decade, genome sequencing projects in cancer genomes as well as in patients with congenital diseases and healthy individuals have led to the identification of new types of massive chromosomal rearrangements arising during single chaotic cellular events. These unanticipated catastrophic phenomenon are termed chromothripsis, chromoanasynthesis and chromoplexis., and are grouped under the name of “chromoanagenesis”.

Results

For each process, several specific features have been described, allowing each phenomenon to be distinguished from each other and to understand its mechanism of formation and to better understand its aetiology. Thus, chromothripsis derives from chromosome shattering followed by the random restitching of chromosomal fragments with low copy-number change whereas chromoanasynthesis results from erroneous DNA replication of a chromosome through serial fork stalling and template switching with variable copy-number gains, and chromoplexy refers to the occurrence of multiple inter-and intra-chromosomal translocations and deletions with little or no copy-number alterations in prostate cancer. Cumulating data and experimental models have shown that chromothripsis and chromoanasynthesis may essentially result from lagging chromosome encapsulated in micronuclei or telomere attrition and end-to-end telomere fusion.

Conclusion

The concept of chromanagenesis has provided new insight into the aetiology of complex structural rearrangements, the connection between defective cell cycle progression and genomic instability, and the complexity of cancer evolution. Increasing reported chromoanagenesis events suggest that these chaotic mechanisms are probably much more frequent than anticipated.

Exonic duplication of the OTC gene by a complex rearrangement that likely occurred via a replication-based mechanism: a case report

BACKGROUND

Ornithine transcarbamylase deficiency (OTCD) is an X-linked recessive disorder involving a defect in the urea cycle caused by OTC gene mutations. Although a total of 417 disease-causing mutations in OTC have been reported, structural abnormalities in this gene are rare. We here describe a female OTCD case caused by an exonic duplication of the OTC gene (exons 1-6).

CASE PRESENTATION

A 23-year-old woman with late-onset OTCD diagnosed by biochemical testing was subjected to subsequent genetic testing. Sanger sequencing revealed no pathogenic mutation throughout the coding exons of the OTC gene, but multiplex ligation-dependent probe amplification (MLPA) revealed duplication of exons 1-6. Further genetic analyses revealed an inversion of duplicated exon 1 and a tandem duplication of exons 2-6. Each of the junctions of the inversion harbored a microhomology and non-templated microinsertion, respectively, suggesting a replication-based mechanism. The duplication was also of de novo origin but segregation analysis indicated that it took place in the paternal chromosome.

CONCLUSION

We report the first OTCD case harboring an exonic duplication in the OTC gene. The functional defects caused by this anomaly were determined via structural analysis of its complex rearrangements.

Replicative and non-replicative mechanisms in the formation of clustered CNVs are indicated by whole genome characterization

Clustered copy number variants (CNVs) as detected by chromosomal microarray analysis (CMA) are often reported as germline chromothripsis. However, such cases might need further investigations by massive parallel whole genome sequencing (WGS) in order to accurately define the underlying complex rearrangement, predict the occurrence mechanisms and identify additional complexities. Here, we utilized WGS to delineate the rearrangement structure of 21 clustered CNV carriers first investigated by CMA and identified a total of 83 breakpoint junctions (BPJs). The rearrangements were further sub-classified depending on the patterns observed: I) Cases with only deletions (n = 8) often had additional structural rearrangements, such as insertions and inversions typical to chromothripsis; II) cases with only duplications (n = 7) or III) combinations of deletions and duplications (n = 6) demonstrated mostly interspersed duplications and BPJs enriched with microhomology. In two cases the rearrangement mutational signatures indicated both a breakage-fusion-bridge cycle process and haltered formation of a ring chromosome. Finally, we observed two cases with Alu- and LINE-mediated rearrangements as well as two unrelated individuals with seemingly identical clustered CNVs on 2p25.3, possibly a rare European founder rearrangement.

In conclusion, through detailed characterization of the derivative chromosomes we show that multiple mechanisms are likely involved in the formation of clustered CNVs and add further evidence for chromoanagenesis mechanisms in both “simple” and highly complex chromosomal rearrangements. Finally, WGS characterization adds positional information, important for a correct clinical interpretation and deciphering mechanisms involved in the formation of these rearrangements.

Clustered copy number variants (CNVs) as detected by chromosomal microarray are often reported as germline chromoanagenesis. However, such cases might need further investigation by whole genome sequencing (WGS) to accurately resolve the complexity of the structural rearrangement and predict underlying mutational mechanisms. Here, we used WGS to characterize 83 breakpoint-junctions (BPJs) from 21 clustered CNVs, and outlined the rearrangement connectivity pictures. Cases with only deletions often had additional structural rearrangements, such as insertions and inversions, which could be a result of multiple double-strand DNA breaks followed by non-homologous repair, typical to chromothripsis. In contrast, cases with only duplications or combinations of deletions and duplications, demonstrated mostly interspersed duplications and BPJs enriched with microhomology, consistent with serial template switching during DNA replication (chromoanasynthesis). Only two rearrangements were repeat mediated. In aggregate, our results suggest that multiple CNVs clustered on a single chromosome may arise through either chromothripsis or chromoanasynthesis.

Flanking complex copy number variants in the same family formed through unequal crossing-over during meiosis

Two phenomena that have been described in germline complex genomic rearrangements (CGRs) formation are chromothripsis and chromoanasynthesis, characterized by distinct features such as the orientation and copy number of the involved fragments. Herein we present different CGRs on chromosome 5p in a mother and her daughter that through unequal crossing-over during meiosis has evolved from a chromothriptic rearrangement in the mother into another complex rearrangement in her daughter involving both deletions and duplications. Initially, both rearrangements were classified as simple copy number variants, but follow-up studies using whole-genome sequencing revealed a much more complex nature of both rearrangements and enabled us to decipher the biological process involved in the formation of the rearrangement found in the daughter. In conclusion, these two cases highlight the need of analyzing the inheritance patterns of CGRs, and provide an example of a disease-causing CGR formed through multiple genetic events.

Stable transmission of an unbalanced chromosome 21 derived from chromoanasynthesis in a patient with a SYNGAP1 likely pathogenic variant

Background

Complex genomic structural variations, involving chromoanagenesis, have been implicated in multiple congenital anomalies and abnormal neurodevelopment. Familial inheritance of complex chromosomal structural alteration resulting from germline chromoanagenesis-type mechanisms are limited.

Case presentation

We report a two-year eleven-month old male presenting with epilepsy, ataxia and dysmorphic features of unknown etiology. Chromosomal microarray identified a complex unbalanced rearrangement involving chromosome 21. G-banding and FISH for targeted regions of chromosome 21 revealed that the copy number imbalances were limited to gains dispersed throughout the long arm of chromosome 21, characteristic of a chromosome derived from chromoanagenesis. Family studies showed that the unbalanced chromosome had been stably inherited, as it was present in both his healthy mother and maternal grandfather. Further molecular testing for non-syndromic intellectual disability genes found a likely pathogenic mutation in SYNGAP1 (NM_006772.2:c.3722_3723del).

Conclusions

This study indicates that complex rearrangements involving an unbalanced chromosome derived from chromoanasynthesis can be familial and should be not be presumed pathogenic.

Genes, Proteins, and Biological Pathways Preventing Chromothripsis

The highly complex structural genome variations chromothripsis, chromoanasynthesis, and chromoplexy are subsumed under the term chromoanagenesis, which means chromosome rebirth. Precipitated by numerous DNA double-strand breaks, they differ in number of and distances between breakpoints, associated copy number variations, order and orientation of segments, and flanking sequences at joining points. Results from patients with the autosomal dominant cancer susceptibility disorder Li-Fraumeni syndrome implicated somatic TP53 mutations in chromothripsis. TP53 participates in the G2/M phase checkpoint, halting cell cycling after premature chromosome compaction during the second half of the S phase, thus preventing chromosome shattering. By experimental TP53 ablation and micronucleus induction, one or a few isolated chromosomes underwent desynchronized replication and chromothripsis. Secondly, chromothripsis occurred after experimental induction of telomere crisis after which dicentric chromosomes sustained TREX1-mediated resolution of chromosome bridges and kataegis. Third, DNA polymerase Polθ-dependent chromothripsis has been documented. Finally, a family with chromothripsis after L1 element-dependent retrotransposition and Alu/Alu homologous recombination has been reported. Human chromosomal instability syndromes share defects in responses to DNA double-strand breaks, characteristic cell cycle perturbations, elevated rates of micronucleus formation, premature chromosome compaction, and apoptosis. They are also associated with elevated susceptibility to malignant disease, such as medulloblastomas and gliomas in ataxia-telangiectasia, leukemia and lymphoma in Bloom syndrome, and osteosarcoma and soft tissue sarcoma in Werner syndrome. The latter syndrome is characterized by a premature aging-like progressive decline of mesenchymal tissues. In all thus far studied cases, constitutional chromothripsis occurred in the male germline and male patients with defects in the double-strand break response genes ATM, MRE11, BLM, LIG4, WRN, and Ku70 show impaired fertility. Conceivably, chromothripsis may, in a stochastic rather than deterministic way, be implicated in germline structural variation, malignant disease, premature aging, genome mosaicism in somatic tissues, and male infertility.

Clinical Consequences of Chromothripsis and Other Catastrophic Cellular Events

Chromothripsis was initially described as a novel cause of chromosomal rearrangements in cancer cells and was subsequently implicated in the development of gross chromosomal rearrangements in the germline. Other catastrophic cellular events such as chromoanasynthesis and chromoplexy have also been observed in human cells. Such events have been associated with various phenotypes including mental retardation and congenital malformations. Here, we introduce representative cases of human disorders arising from somatic or germline chromothripsis or similar catastrophic events. In this chapter, we use the term "chromoanagenesis" to indicate all catastrophic events including chromothripsis.

Break-induced replication links microsatellite expansion to complex genome rearrangements

The instability of microsatellite DNA repeats is responsible for at least 40 neurodegenerative diseases. Recently, Mirkin and co-workers presented a novel mechanism for microsatellite expansions based on break-induced replication (BIR) at sites of microsatellite-induced replication stalling and fork collapse. The BIR model aims to explain single-step, large expansions of CAG/CTG trinucleotide repeats in dividing cells. BIR has been characterized extensively in Saccharomyces cerevisiae as a mechanism to repair broken DNA replication forks (single-ended DSBs) and degraded telomeric DNA. However, the structural footprints of BIR-like DSB repair have been recognized in human genomic instability and tied to the etiology of diverse developmental diseases; thus, the implications of the paper by Kim et al. (Kim JC, Harris ST, Dinter T, Shah KA, et al., Nat Struct Mol Biol 24: 55-60) extend beyond trinucleotide repeat expansion in yeast and microsatellite instability in human neurological disorders. Significantly, insight into BIR-like repair can explain certain pathways of complex genome rearrangements (CGRs) initiated at non-B form microsatellite DNA in human cancers.

Defining the diverse spectrum of inversions, complex structural variation, and chromothripsis in the morbid human genome

Background

Structural variation (SV) influences genome organization and contributes to human disease. However, the complete mutational spectrum of SV has not been routinely captured in disease association studies.

Results

We sequenced 689 participants with autism spectrum disorder (ASD) and other developmental abnormalities to construct a genome-wide map of large SV. Using long-insert jumping libraries at 105X mean physical coverage and linked-read whole-genome sequencing from 10X Genomics, we document seven major SV classes at ~5 kb SV resolution. Our results encompass 11,735 distinct large SV sites, 38.1% of which are novel and 16.8% of which are balanced or complex. We characterize 16 recurrent subclasses of complex SV (cxSV), revealing that: (1) cxSV are larger and rarer than canonical SV; (2) each genome harbors 14 large cxSV on average; (3) 84.4% of large cxSVs involve inversion; and (4) most large cxSV (93.8%) have not been delineated in previous studies. Rare SVs are more likely to disrupt coding and regulatory non-coding loci, particularly when truncating constrained and disease-associated genes. We also identify multiple cases of catastrophic chromosomal rearrangements known as chromoanagenesis, including somatic chromoanasynthesis, and extreme balanced germline chromothripsis events involving up to 65 breakpoints and 60.6 Mb across four chromosomes, further defining rare categories of extreme cxSV.

Conclusions

These data provide a foundational map of large SV in the morbid human genome and demonstrate a previously underappreciated abundance and diversity of cxSV that should be considered in genomic studies of human disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1158-6) contains supplementary material, which is available to authorized users.

Catastrophic cellular events leading to complex chromosomal rearrangements in the germline

Although complex chromosomal rearrangements were thought to reflect the accumulation of DNA damage over time, recent studies have shown that such rearrangements frequently arise from 'all-at-once' catastrophic cellular events. These events, designated chromothripsis, chromoanasynthesis, and chromoanagenesis, were first documented in the cancer genome and subsequently observed in the germline. These events likely result from micronucleus-mediated chromosomal shattering and subsequent random reassembly of DNA fragments, although several other mechanisms have also been proposed. Typically, only one or a few chromosomes of paternal origin are affected per event. These events can produce intrachromosomal deletions, duplications, inversions, and translocations, as well as interchromosomal translocations. Germline complex rearrangements of autosomes often result in developmental delay and dysmorphic features, whereas X chromosomal rearrangements are usually associated with relatively mild clinical manifestations. The concept of these catastrophic events provides novel insights into the etiology of human genomic disorders. This review introduces the molecular characteristics and phenotypic outcomes of catastrophic cellular events in the germline.