Massive Genomic Rearrangement Acquired in a Single Catastrophic Event during Cancer Development

Tumors frequently arise as a result of progressive acquisition of genetic anomalies causing altered regulation of critical genes important for aspects of cell survival.Identification of genetic abnormalities associated with cancer development has been facilitated by DNA sequencing technology, with next-generation sequencing readily identifying and cataloging abnormalities.In the report of their study at the Wellcome Trust Sanger Institute (Cambridge, UK), Stephens and colleagues describe deep sequencing data from a broad range of cancers and cancer cell lines revealing data consistent with shattering and/or restitching of chromosomes, leading to repeated and varied copy number alterations occurring across single chromosomes (sparing the partner chromosome), and resulting in hundreds of breakpoints.This phenomenon, dubbed chromothripsis (chromosome shattering) occurs in up to 25% of bone cancer (osteosarcomas and chordomas), and in 2-3% of other tumors, including melanoma, lung cancer, glioma, synovial sarcoma, and esophageal, colorectal, renal, thyroid, and hematologic malignancies.Chromosome shattering may result from inappropriate cues directing condensation of an incompletely replicated chromosome.The resulting fragments are likely to be joined back together by nonhomologous end joining.Although detailed mechanistic insights into this apparently common process remain to be clarified, it appears that cells can survive this colossal chromosomal insult and rearrangement, resulting in multiple rearrangements that confer growth advantages, thus leading to cancer.(Image

Lessons from the Cancer Genome

Next-generation sequencing of DNA from human tumors or individuals with developmental abnormalities has led to the discovery of a process we term chromoanagenesis, in which large numbers of complex rearrangements occur at one or a few chromosomal loci in a single catastrophic event. Two mechanisms underlie these rearrangements, both of which can be facilitated by a mitotic chromosome segregation error to produce a micronucleus containing the chromosome to undergo rearrangement. In the first, chromosome shattering (chromothripsis) is produced by mitotic entry before completion of DNA replication within the micronucleus, with a failure to disassemble the micronuclear envelope encapsulating the chromosomal fragments for random reassembly in the subsequent interphase. Alternatively, locally defective DNA replication initiates serial, microhomology-mediated template switching (chromoanasynthesis) that produces local rearrangements with altered gene copy numbers. Complex rearrangements are present in a broad spectrum of tumors and in individuals with congenital or developmental defects, highlighting the impact of chromoanagenesis on human disease.

Stress induced by premature chromatin condensation triggers chromosome shattering and chromothripsis at DNA sites still replicating in micronuclei or multinucleate cells when primary nuclei enter mitosis

Chromosome Catastrophes Involve Replication Mechanisms Generating Complex Genomic Rearrangements

Chromo Me Thriptic-chaos amongst the chromosomes!

The classical genetic model of colorectal cancer presents somatic APC mutations as the earliest genomic alterations, followed by KRAS and TP53 mutations. However, the timing and relative order of clonal expansion and other types of somatic genome alterations such as genomic rearrangements are still unclear. Here, we performed detailed analysis of somatic genetic alterations, including point mutations, copy number alterations and genomic rearrangements, in 63 whole-genome sequenced colorectal cancers from the Cancer Genome Atlas Research Network. The relative order of these alterations occurring during tumorigenesis was inferred from variant allele fractions. We found that driver point mutations, gene fusions, and arm level copy losses typically arise early. Copy-neutral loss of heterozygosity is often a two-step event: a deletion followed by a duplication. Different mechanisms act in different genomic regions to optimize the DNA dosage. Chromothripsis, clustered genomic rearrangements previously thought to occur as a single catastrophic event, is frequent and may occur multiple times independently in the same tumor through different mechanisms. In contrast to recent studies reporting neutral growth of tumors, selection is often present on subclones. Our results suggest that different evolutionary models can operate in a single tumor at different stages. Combining these results, we present a refined tumor progression model for human colorectal cancer. Our enhanced genetic model significantly expands our understanding of tumorigenesis process. Citation Format: Lixing Yang. An enhanced tumor evolution model for colorectal cancer evolution [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-210.

The use of new technologies in the routine diagnosis of constitutional abnormalities, such as high-resolution chromosomal microarray and next-generation sequencing, has unmasked new mechanisms for generating structural variation of the human genome. For example, complex chromosome rearrangements can originate by a chromosome catastrophe phenomenon in which numerous genomic rearrangements are apparently acquired in a single catastrophic event. This phenomenon is named chromoanagenesis (from the Greek “chromo” for chromosome and “anagenesis” for rebirth). Herein, we report 2 cases of genomic chaos detected at prenatal diagnosis. The terms “chromothripsis” and “chromoanasynthesis” and the challenge of genetic counseling are discussed.

Is next generation sequencing (NGS) capable of detecting smaller sub-chromosomal rearrangements in human embryos than the manufacturer's quoted resolution suggests? NGS was able to detect unbalanced chromosome segments smaller than the manufacturer's resolution. Array Comparative Genomic Hybridization (array-CGH) has been the gold standard platform used for PGD of chromosome rearrangements. NGS is a viable alternative to array-CGH for PGD of chromosome arrangements given that the manufacturer's guidelines quote a resolution of ≥20 Mb. However, as many patients carry a chromosome rearrangement <20 Mb, the detection limits of NGS warrant further investigation. This study involved a retrospective assessment of stored DNA samples from embryos that had previously been diagnosed as unbalanced by array-CGH as part of routine PGD in two separate IVF clinics between November 2013 and April 2017. SurePlex whole genome amplification (WGA) products derived from DNA extracted from an embryo biopsy sample known to carry an unbalanced form of a chromosome rearrangement were subjected to a specific NGS workflow (VeriSeq PGS). The results from the two technologies were compared for each sample. WGA products from 200 embryos known to carry unbalanced rearrangements were sequenced and analysed. These embryos had been created by 75 patients known to carry a chromosome rearrangement (68 reciprocal translocations, 3 pericentric inversions, 1 paracentric inversion, 2 insertions and 1 dual reciprocal and inversion). Each sample was assessed for the size of the segmental gain/loss (Mb), copy number for each segment and chromosome, segregation pattern, the number of bins in the analysis software used and concordance with array-CGH results. A total of 294 unbalanced chromosome segments were assessed. NGS was capable of detecting 285/294 (97%) unbalanced segments previously identified using array-CGH. The final PGD diagnosis was concordant for 200/200 (100%) embryos. In total, 44/75 (59%) patients contained an unbalanced chromosome segment below the quoted 20 Mb manufacturer's stated resolution. Of these, 35/44 (80%) patients had segments that were able to be detected using NGS, whilst maintaining clinical outcome concordance. Our study subset did not include any rearrangements involving the Y chromosome. NGS has less available bins per chromosome compared to the array-CGH platform used, thus it remains possible that chromosome rearrangements predicted to be small but still detectable by array-CGH may not be feasible for testing using NGS. This should be considered when undertaking a theoretical feasibility assessment for detecting the chromosome rearrangement in question. Only one specific workflow for WGA and NGS was investigated in this study. This study has shown that NGS is available for the detection of unbalanced chromosome rearrangements ≥10 Mb. Part sponsorship of the VeriSeq PGS kits used was provided by Illumina. The remainder of the kits were provided by two commercial IVF clinics. None of the authors has any conflicting interests to declare. N/A.

Many tumors have highly rearranged genomes, but a major unknown is the relative importance and timing of genome rearrangements compared to sequence-level mutation. Chromosome instability might arise early, be a late event contributing little to cancer development, or happen as a single catastrophic event. Another unknown is which of the point mutations and rearrangements are selected. To address these questions we show, using the breast cancer cell line HCC1187 as a model, that we can reconstruct the likely history of a breast cancer genome. We assembled probably the most complete map to date of a cancer genome, by combining molecular cytogenetic analysis with sequence data. In particular, we assigned most sequence-level mutations to individual chromosomes by sequencing of flow sorted chromosomes. The parent of origin of each chromosome was assigned from SNP arrays. We were then able to classify most of the mutations as earlier or later according to whether they occurred before or after a landmark event in the evolution of the genome, endoreduplication (duplication of its entire genome). Genome rearrangements and sequence-level mutations were fairly evenly divided earlier and later, suggesting that genetic instability was relatively constant throughout the life of this tumor, and chromosome instability was not a late event. Mutations that caused chromosome instability would be in the earlier set. Strikingly, the great majority of inactivating mutations and in-frame gene fusions happened earlier. The non-random timing of some of the mutations may be evidence that they were selected.

Chromothripsis-Mediated Structural Variations and Clonal Evolution In Recurrent Childhood High Hyperdiploid Acute Lymphoblastic Leukemia

Chromothripsis is a single catastrophic event that can lead to massive genomic rearrangements confined to one or a few chromosomes. It provides an alternative paradigm in cancer development and changes the conventional view that cancer develops in a stepwise progression. The mechanisms underlying chromothripsis and their specific impact on tumorigenesis are still poorly understood, and further examination of a large number of identified chromothripsis samples is needed. Unfortunately, this data are difficult to access, as they are scattered across multiple publications, come in different formats and descriptions, or are hidden in figures and supplementary materials. To improve access to this data and promote meta-analysis, we developed ChromothripsisDB, a manually curated database containing a unified description of all published chromothripsis cases and relevant genomic aberrations. Currently, 423 chromothripsis samples representing 107 research articles are included in our database. ChromothripsisDB represents an extraordinary resource for mining the existing knowledge of chromothripsis, and will facilitate the identification of mechanisms involved in this phenomenon. ChromothripsisDB is freely available at http://cgma.scu.edu.cn/ChromothripsisDB CONTACT: haoyang.cai@scu.edu.cn Supplementary data are available at Bioinformatics online.

BackgroundThe classical genetic model of colorectal cancer presents APC mutations as the earliest genomic alterations, followed by KRAS and TP53 mutations. However, the timing and relative order of clonal expansion and other types of genomic alterations, such as genomic rearrangements, are still unclear.ResultsHere, we perform comprehensive bioinformatic analysis to dissect the relative timing of somatic genetic alterations in 63 colorectal cancers with whole-genome sequencing data. Utilizing allele fractions of somatic single nucleotide variants as molecular clocks while accounting for the presence of copy number changes and structural alterations, we identify key events in the evolution of colorectal tumors. We find that driver point mutations, gene fusions, and arm-level copy losses typically arise early in tumorigenesis; different mechanisms act on distinct genomic regions to drive DNA copy changes; and chromothripsis—clustered rearrangements previously thought to occur as a single catastrophic event—is frequent and may occur multiple times independently in the same tumor through different mechanisms. Furthermore, our computational approach reveals that, in contrast to recent studies, selection is often present on subclones and that multiple evolutionary models can operate in a single tumor at different stages.ConclusionCombining these results, we present a refined tumor progression model which significantly expands our understanding of the tumorigenic process of human colorectal cancer.

Transient hypermutability, chromothripsis and replication-based mechanisms in the generation of concurrent clustered mutations

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