Graphical AbstractThe loosening of chromatin structures gives rise to unrestricted access to DNA and thus transcription factors (TFs) can bind to their otherwise masked target sequences. Regions bound by the same set of TFs tend to be located in close proximity and this might increase the probability of activating illegitimate genomic 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. (shrink)

Recent mutagenesis studies have demonstrated that the chemotherapeutic agent, chlorambucll (CHL), is highly mutagenic in male germ cells of the mouse. Post‐melotic germ cells, and especially early spermatids, are the most sensitive to the cytotoxic and mutagenic effects of this agent. Genetic, cytogenetic and molecular analyses of many induced mutations have shown that, in these germ‐cell stages, CHL induces predominantly chromosomal rearrangements (deletions and translocations), and mutation‐rate studies show that, in terms of tolerated doses, CHL is perhaps five to (...) ten times more efficient in inducing rearrangements than is radiation exposure. Appropriate breeding protocols, along with knowledge of the advantages and limitations associated with the use of CHL, can be used to expand the current resource of chromosomal rearrangements in the mouse and to provide new phenotype‐associated mutations amenable to positional‐cloning techniques. The analysis of CHL‐induced mutations has also contributed to understanding the factors that affect the yield and nature of chemically induced germline mutations in mammals. (shrink)

In vertebrates it is often found that if one considers a group of genes clustered on a certain chromosome, then the homologues of those genes often form another cluster on a different chromosome. There are four explanations, not necessarily mutually exclusive, to explain how such homologous clusters appeared. Homologous clusters are expected at a low probability even if genes are distributed at random. The duplication of a subset of the genome might create homologous clusters, as would a duplication of (...) the entire genome. Alternatively, it may be adaptive for certain combinations of genes to cluster, although clearly the genes must have duplicated prior to rearrangement into clusters. Molecular phylogenetics provides a means to examine the origins of homologous clusters, although it is difficult to discriminate between the different explanations using current data. However, with more extensive sequencing and mapping of vertebrate genomes, especially those of the early diverging chordates, it should soon become possible to resolve the origins of homologous clusters. BioEssays 21:697–703, 1999. © 1999 John Wiley & Sons, Inc. (shrink)

How genomic innovation translates into organismal organization remains largely unanswered. Possessing the largest invertebrate nervous system, in conjunction with many species‐specific organs, coleoid cephalopods (octopuses, squids, cuttlefishes) provide exciting model systems to investigate how organismal novelties evolve. However, dissecting these processes requires novel approaches that enable deeper interrogation of genome evolution. Here, the existence of specific sets of genomic co‐evolutionary signatures between expanded gene families, genome reorganization, and novel genes is posited. It is reasoned that their co‐evolution has (...) contributed to the complex organization of cephalopod nervous systems and the emergence of ecologically unique organs. In the course of reviewing this field, how the first cephalopod genomic studies have begun to shed light on the molecular underpinnings of morphological novelty is illustrated and their impact on directing future research is described. It is argued that the application and evolutionary profiling of evolutionary signatures from these studies will help identify and dissect the organismal principles of cephalopod innovations. By providing specific examples, the implications of this approach both within and beyond cephalopod biology are discussed. (shrink)

Cells mitigate the detrimental consequences of DNA damage on genome stability by attempting high fidelity repair. Homologous recombination templates DNA double‐strand break (DSB) repair on an identical or near identical donor sequence in a process that can in principle access the entire genome. Other physiological processes, such as homolog recognition and pairing during meiosis, also harness the HR machinery using programmed DSBs to physically link homologs and generate crossovers. A consequence of the homology search process by a long (...) nucleoprotein filament is the formation of multi‐invasions (MI), a joint molecule in which the damaged ssDNA has invaded more than one donor molecule. Processing of MI joint molecules can compromise the integrity of both donor sites and lead to their rearrangement. Here, two mechanisms for the generation of rearrangements as a pathological consequence of MI processing are detailed and the potential relevance for non‐allelic homologous recombination discussed. Finally, it is proposed that MI‐induced crossover formation may be a feature of physiological recombination. (shrink)

Ciliated protozoa have separate germline and somatic nuclei, yet unlike larger organisms, both nuclei reside in the same cytoplasm. The micronuclei contain the germline and the macronucleus is the somatic nucleus. Thousands of DNA elements are normally removed from the micronuclear genome as it forms a new macronucleus during each sexual cycle. A recent study directly links the excision of these internal eliminated sequences (IESs) to mating type determination by showing that a pleiotropic mutation affecting mating type also prevents (...) the excision of an IES from a surface protein gene(1). Remarkably, once the IES is present in the old macronucleus it prevents excision of that specific IES during formation of the next macronucleus. (shrink)

A combination of cytogenetic and molecular analyses has shown that several different transposable elements are involved in the restructuring of Drosophila chromosomes. Two kinds of elements, P and hobo, are especially prone to induce chromosome rearrangements. The mechanistic details of this process are unclear, but, at least some of the time, it seems to involve ectopic recombination between elements inserted at different chromosomal sites; the available data suggest that these ectopic recombination events are much more likely to occure between (...) elements in the same chromosome than between elements in different chromosomes. Other Drosophila transposons also appear to mediate chromosome restructuring by ectopic recombination; these include the retrotransposons BEL, roo, Docand I and the foldback element FB. In addition, two retrotransposons, HeT‐A and TART, have been found to be associated specifically with the ends of Drosophila chromosomes. Very limited data indicate that transposon‐mediated chromosome restructuring is occurring in natural populations of Drosophila. This suggests that transposable elements may help to shape the structure of the Drosophila genome and implies that they may have a similar role in other organisms. (shrink)

Recent data show that catastrophic events during one cell cycle can cause massive genome damage producing viable clones with unstable genomes. This is in contrast with the traditional view that tumorigenesis requires a long‐term process in which mutations gradually accumulate over decades. These sudden events are likely to result in a large increase in genomic diversity within a relatively short time, providing the opportunity for selective advantages to be gained by a subset of cells within a population. This genetic (...) diversity amplification, arising from a single aberrant cell cycle, may drive a population conversion from benign to malignant. However, there is likely a period of relative genome stability during the clonal expansion of tumors – this may provide an opportunity for therapeutic intervention, especially if mechanisms that limit tolerance of aneuploidy are exploited. (shrink)

In the organelles of plants and mammals, recent evidence suggests that genomic instability stems in large part from template switching events taking place during DNA replication. Although more than one mechanism may be responsible for this, some similarities exist between the different proposed models. These can be separated into two main categories, depending on whether they involve a single‐strand‐switching or a reciprocal‐strand‐switching event. Single‐strand‐switching events lead to intermediates containing Y junctions, whereas reciprocal‐strand‐switching creates Holliday junctions. Common features in all the (...) described models include replication stress, fork stalling and the presence of inverted repeats, but no single element appears to be required in all cases. We review the field, and examine the ideas that several mechanisms may take place in any given genome, and that the presence of palindromes or inverted repeats in certain regions may favor specific rearrangements. (shrink)

Next‐generation sequencing (NGS) technologies have revolutionised the analysis of genomic structural variants (SVs), providing significant insights into SV de novo formation based on analyses of rearrangement breakpoint junctions. The short DNA reads generated by NGS, however, have also created novel obstacles by biasing the ascertainment of SVs, an aspect that we refer to as the ‘short‐read dilemma’. For example, recent studies have found that SVs are often complex, with SV formation generating large numbers of breakpoints in a single event (multi‐breakpoint (...) SVs) or structurally polymorphic loci having multiple allelic states (multi‐allelic SVs). This complexity may be obscured in short reads, unless the data is analysed and interpreted within its wider genomic context. We discuss how novel approaches will help to overcome the short‐read dilemma, and how integration of other sources of information, including the structure of chromatin, may help in the future to deepen the understanding of SV formation processes. (shrink)

Knowledge of eukaryotic life cycles and associated genome dynamics stems largely from research on animals, plants, and a small number of “model” (i.e., easily cultivable) lineages. This skewed sampling results in an underappreciation of the variability among the many microeukaryotic lineages, which represent the bulk of eukaryotic biodiversity. The range of complex nuclear transformations that exists within lineages of microbial eukaryotes challenges the textbook understanding of genome and nuclear cycles. Here, we look in‐depth at Foraminifera, an ancient (∼600 (...) million‐year‐old) lineage widely studied as proxies in paleoceanography and environmental biomonitoring. We demonstrate that Foraminifera challenge the “rules” of life cycles developed largely from studies of plants and animals. To this end, we synthesize data on foraminiferal life cycles, focusing on extensive endoreplication within individuals (i.e., single cells), the unusual nuclear process calledZerfall, and the separation of germline and somatic function into distinct nuclei (i.e., heterokaryosis). These processes highlight complexities within lineages and expand our understanding of the dynamics of eukaryotic genomes. (shrink)

Genome instability has been associated with progression of transformed cells to high tumorigenicity. Although genome instability may result from a variety of factors, some studies suggest that DNA in the region of a chromosome rearrangement can subsequently have much higher rates of DNA deletions or gene amplification. One approach to studying the factors that produce these high rates of DNA rearrangement is by analysis of unstable integration sites for DNA transfected into mammalian cells. Integrated sequences commonly show a (...) temporary instability, and at rare locations this instability is continuous and can be observed even after multiple subclonings. These continuously unstable locations undergo DNA amplification of both the integrated sequences and the surrounding cell DNA, and it can occur either at the original site or on episomes after looping out from the chromosome. Because the adjacent cell DNA plays a role in this instability, and the region can be shown to be stable before integration, the results indicate that these recombinational hotspots can be formed de novo by the process of integration. Current studies are attempting to determine which sequences are responsible for the high rates of recombination and whether similar types of event are involved in the instability associated with endogenous cellular genes in cancer cells. (shrink)

Chromosomal rearrangements frequently occur at specific places (“hot spots”) in the genome. These recombination hot spots are usually separated by 50–100 kb regions of DNA that are rarely involved in rearrangements. It is quite likely that there is a correlation between the above‐mentioned distances and the average size of DNA loops fixed at the nuclear matrix. Recent studies have demonstrated that DNA loop anchorage regions can be fairly long and can harbor DNA recombination hot spots. We previously (...) proposed that chromosomal DNA loops may constitute the basic units of genome organization in higher eukaryotes. In this review, we consider recombination between DNA loop anchorage regions as a possible source of genome evolution. (shrink)

Replication protein A (RPA) is the main eukaryotic single‐stranded DNA (ssDNA) binding protein, having essential roles in all DNA metabolic reactions involving ssDNA. RPA binds ssDNA with high affinity, thereby preventing the formation of secondary structures and protecting ssDNA from the action of nucleases, and directly interacts with other DNA processing proteins. Here, we discuss recent results supporting the idea that one function of RPA is to prevent annealing between short repeats that can lead to chromosome rearrangements by microhomology‐mediated (...) end joining or the formation of hairpin structures that are substrates for structure‐selective nucleases. We suggest that replication fork catastrophe caused by depletion of RPA could result from cleavage of secondary structures by nucleases, and that failure to cleave hairpin structures formed at DNA ends could lead to gene amplification. These studies highlight the important role RPA plays in maintaining genome integrity. (shrink)

Tumour‐associated genetic changes frequently involve DNA translocation or deletion. Many of these events will have arisen from initial genomic damage, induced by either the activity of endogenous metabolic processes or from exposure to environmental genotoxic agents. Although initial genomic damage will have been widely distributed, tumorigenic events are confined to certain DNA target sites. Furthermore, within these target sites there appear to be regions of preferential DNA rearrangement, and examination of these sites implies that the location and extent of such (...) rearrangement may be influenced by DNA primary and secondary structure rather than simply by the point of damage. We selectively review evidence relating to DNA structures that may predispose certain regions of the genome to damage‐induced rearrangement, and discuss the possible role of interstitial, inverted telomere‐like sequence arrays in promoting chromosomal events of a type known to be associated with some human and animal tumours. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Kennedy Institute of Ethics Journal 14.1 (2004) 47-54 [Access article in PDF] Ethical Guidelines for Human Embryonic Stem Cell Research*(A Recommended Manuscript) Adopted on 16 October 2001Revised on 20 August 2002 Ethics Committee of the Chinese National Human Genome Center at Shanghai, Shanghai 201203 Human embryonic stem cell (ES) research is a great project in the frontier of biomedical science for the twenty-first century. Be- cause the research (...) involves the use of human embryos, it triggers serious debate on ethical issues. Opponents consider the embryo to be an early form of human life that should be respected and not destroyed. However, the majority of scientists support embryonic stem cell research, believing that it offers good prospects for the treatment of diseases that have remained incurable until now and so will benefit humankind. The Ethics Committee of the Department of Ethical, Legal, and Social Implications of the Chinese National Human Genome Center at Shanghai seriously discussed the ethical debate initiated by embryonic stem cell research. We concluded that we should support the scientists of our country in actively carrying out human embryonic stem cell research for the noble cause of "medicine being a beneficent art." For the healthy and orderly development of human embryonic stem cell research in our country, we put forward the following recommended Ethical Guidelines for Human Embryonic Stem Cell Research as a reference for leaders, administrative departments, and related scientists. Preface Article 1. Human embryonic stem cells are the primitive cells that play the main role in the growth and development of a human body. These [End Page 47] primitive cells have the potential for infinite proliferation, self-renewal, and multi-directional differentiation. If scientists can discover the mechanism of differentiation of human embryonic stem cells, it will be possible to induce differentiation of human embryonic stem cells to form various types of human cells for clinical cell therapy. If human embryonic stem cell research can be integrated with modern biomedical engineering techniques, it also will be possible to make repair and replacement of human tissues and organs a reality. Article 2. There are two ways to classify human embryonic stem cells. One way is to classify them according to their potential for differentiation. There could be three kinds of stem cells: totipotent stem cells, pluripotent stem cells, and unipotent stem cells.A totipotent stem cell has the potential to develop into a whole individual. It can differentiate into the more than 200 cell types in the whole body, construct any tissue or organ of the body, and finally develop into a whole individual. The fertilized egg and the cleavage cells at the very early stage of embryonic development are totipotent stem cells.A pluripotent stem cell has the potential to differentiate into many cell types derived from the three embryonic layers. However, it has lost the capacity to develop into a complete organism.A unipotent stem cell is derived from the further differentiation of the pluripotent stem cell. It can differentiate only into one cell type such as a hematopoietic stem cell, neural stem cell, and others.The other way is to classify human stem cells according to their source. There could be two kinds of human stem cells: embryonic stem cells and tissue stem cells (also called adult stem cells). The former involves experimentation with embryos, which has serious ethical implications. Ethical issues associated with the latter are mainly expressed in the various opinions regarding the allocation of health resources. Article 3. Human embryonic stem cells are the group of cells called the blastocyst inner cell mass during the early stage of embryonic development. They are the main source of totipotent stem cells, and hence the focal and hot point in stem cell research. Studies on the clinical application of embryonic stem cells probably will involve use of the somatic cell nucleus transfer (SCNT) technique, which destroys the early human embryo. At present, the ethical and moral debate is very serious in human embryonic stem cell research regarding whether the research will develop... (shrink)

RNA's role in genome rearrangement. Ciliated protists, such as Oxytricha, shown, have two types of nuclei that differentiate from each other. Development in these microbial eukaryotes provides a paragon for studies of genome remodeling, with extensive use of non‐coding RNAs. On pages 84–87 of this issue, Wenwen Fang and Laura Landweber discuss how RNA‐guided processes of genome rearrangement or repair could influence other eukaryotes. Cover by Wenyi Fang. SEM images courtesy of Robert Hammersmith.

RNA's role in genome rearrangement. Ciliated protists, such as Oxytricha, shown, have two types of nuclei that differentiate from each other. Development in these microbial eukaryotes provides a paragon for studies of genome remodeling, with extensive use of non‐coding RNAs. On pages 84–87 of this issue, Wenwen Fang and Laura Landweber discuss how RNA‐guided processes of genome rearrangement or repair could influence other eukaryotes. Cover by Wenyi Fang. SEM images courtesy of Robert Hammersmith.

All species continuously evolve to adapt to changing environments. The genetic variation that fosters such adaptation is caused by a plethora of mechanisms, including meiotic recombination that generates novel allelic combinations in the progeny of two parental lineages. However, a considerable number of eukaryotic species, including many fungi, do not have an apparent sexual cycle and are consequently thought to be limited in their evolutionary potential. As such organisms are expected to have reduced capability to eliminate deleterious mutations, they are (...) often considered as evolutionary dead ends. However, inspired by recent reports we argue that such organisms can be as persistent as organisms with conventional sexual cycles through the use of other mechanisms, such as genomic rearrangements, to foster adaptation. (shrink)

Mutations in enhancer‐associated chromatin‐modifying components and genomic alterations in non‐coding regions of the genome occur frequently in cancer, and other diseases pointing to the importance of enhancer fidelity to ensure proper tissue homeostasis. In this review, I will use specific examples to discuss how mutations in chromatin‐modifying factors might affect enhancer activity of disease‐relevant genes. I will then consider direct evidence from single nucleotide polymorphisms, small insertions, or deletions but also larger genomic rearrangements such as duplications, deletions, translocations, (...) and inversions of specific enhancers to demonstrate how they have the ability to impact enhancer activity of disease genes including oncogenes and tumor suppressor genes. Considering that the scientific community only fairly recently has begun to focus its attention on “enhancer malfunction” in disease, I propose that multiple new enhancer‐regulated and disease‐relevant processes will be uncovered in the near future that will constitute the mechanistic basis for novel therapeutic avenues. (shrink)

Mutations in enhancer‐associated chromatin‐modifying components and genomic alterations in non‐coding regions of the genome occur frequently in cancer, and other diseases pointing to the importance of enhancer fidelity to ensure proper tissue homeostasis. In this review, I will use specific examples to discuss how mutations in chromatin‐modifying factors might affect enhancer activity of disease‐relevant genes. I will then consider direct evidence from single nucleotide polymorphisms, small insertions, or deletions but also larger genomic rearrangements such as duplications, deletions, translocations, (...) and inversions of specific enhancers to demonstrate how they have the ability to impact enhancer activity of disease genes including oncogenes and tumor suppressor genes. Considering that the scientific community only fairly recently has begun to focus its attention on “enhancer malfunction” in disease, I propose that multiple new enhancer‐regulated and disease‐relevant processes will be uncovered in the near future that will constitute the mechanistic basis for novel therapeutic avenues. (shrink)

Methylation of cytosine DNA residues is a well‐studied epigenetic modification with important roles in formation of heterochromatic regions of the genome, and also in tissue‐specific repression of transcription. However, we recently found that the ciliate Oxytricha uses methylcytosine in a novel DNA elimination pathway important for programmed genome restructuring. Remarkably, mounting evidence suggests that methylcytosine can play a dual role in ciliates, repressing gene expression during some life‐stages and directing DNA elimination in others. In this essay, I describe (...) these recent advances in the DNA methylation field and discuss whether this unexpected novel role for methylcytosine in DNA elimination might be more widely conserved in eukaryotic biology, particularly in apoptotic pathways. (shrink)

The use of a reporter gene in transgenic mice indicates that there are many local mutations and large genomic rearrangements per somatic cell that accumulate with age at different rates per organ and without visible effects. Dissociation of the cells for monolayer culture brings out great heterogeneity of size and loss of function among cells that presumably reflect genetic and epigenetic differences among the cells, but are masked in organized tissue. The regulatory power of a mass of contiguous normal (...) cells is expressed in its capacity to normalize the appearance and growth behavior of solitary homophilic neoplastic cells, and to redirect differentiation of solitary heterophilic stem‐like cells. Intimate contact between the interacting cells is required to induce these changes. The normalization of the neoplastic phenotype does not require gap junctional communication between cells, though transdifferentiation might. These varied relationships are manifestations of the unifying biological principle of “order in the large over heterogeneity in the small”. BioEssays 28: 515–524, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Graphical AbstractCephalopods provide a unique model system to investigate how organismal novelties evolve. In article number 1900073, Elena A. Ritschard et al. discuss how co-evolutionary signatures among various genomic characters have contributed to cephalopod organismal novelties and can be used to dissect their functional organization. Cover illustration by Hannah Schmidbaur.

The process of chromatin diminution in Parascaris and Ascaris is a developmentally controlled genome rearrangement, which results in quantitative and qualitative differences in DNA content between germ line and somatic cells. Chromatin diminution involves chromosomal breakage, new telomere formation and DNA degradation. The programmed elimination of chromatin in presomatic cells might serve as an alternative way of gene regulation. We put forward a new hypothesis of how an ancient partial genome duplication and chromatin diminution may have served to (...) maintain the genetic balance in somatic cells and simultaneously endowed the germ line cells with a selective advantage. (shrink)

It has become a truism that we humans are genetically about 99% identical to chimpanzees. The origins of this assertion are clear: among early studies of DNA sequences, nucleotide identity between humans and chimpanzees was found to average around 98.9%.1 However, this figure is correct only with respect to regions of the genome that are shared between humans and chimpanzees. Often ignored are the many parts of their genomes that are not shared. Genomic rearrangements, including insertions, deletions, translocations (...) and duplications, have long been recognized as potentially important sources of novel genomic material2,3 and are known to account for major genomic differences between humans and chimpanzees.4 Further, such changes have been implicated in a number of genetic disorders, such as DiGeorge, Angelman/Prader‐Willi and Charcot‐Marie‐Tooth syndromes5. BioEssays 28: 335–338, 2006. © 2006 Wiley Periodicals, Inc. (shrink)

Homologous recombination (HR) is essential for the accurate repair of DNA double‐strand breaks and damaged replication forks. However, inappropriate or aberrant HR can also result in genome rearrangements. The maintenance of cell viability is, therefore, a careful balancing act between the benefits of HR (the error‐free repair of DNA strand breaks) and the potential detrimental outcomes of HR (chromosomal rearrangements). Two papers have recently provided a mechanistic insight into how HR may be tempered by RecQ helicases to (...) prevent genome instability and diseases that are a consequence of this, such as cancer.1,2 BioEssays 30:291–295, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

Polyploid cells contain multiple copies of all chromosomes. Polyploidization can be developmentally programmed to sustain tissue barrier function or to increase metabolic potential and cell size. Programmed polyploidy is normally associated with terminal differentiation and poor proliferation capacity. Conversely, non‐programmed polyploidy can give rise to cells that retain the ability to proliferate. This can fuel rapid genome rearrangements and lead to diseases like cancer. Here, the mechanisms that generate polyploidy are reviewed and the possible challenges upon polyploid cell (...) division are discussed. The discussion is framed around a recent study showing that asynchronous cell cycle progression (an event that is named “chronocrisis”) of different nuclei from a polyploid cell can generate DNA damage at mitotic entry. The potential mechanisms explaining how mitosis in non‐programmed polyploid cells can generate abnormal karyotypes and genetic instability are highlighted. (shrink)

In eukaryotic cells intron sequences are usually spliced out with a high degree of precision from heterogenous nuclear RNA (hnRNA) to give functional mRNA with exons in their right order. Provided with the right substrates, cell extracts can achieve the same. With exotic substrates, on the other hand, the same extracts can cut exons from one RNA and join them to exons from another RNA, a process termed trans‐splicing. In vivo, RNA trans‐splicing could lead to faulty, but also to novel (...) proteins and, through reverse transcription, to genomic rearrangements. So far, in living cells trans‐splicing has only been invoked to occur in trypanosomes. In these unicellular organisms most if not all mRNAs are made discontinuously, so that the mature mRNAs carry, at their 5′ end, a common 35‐nucleotide sequence, called a miniexon, encoded separately in the genome. The miniexon is most likely joined to the body of the mRNA by trans‐splicing. (shrink)

DNA damage repair within telomeres are suppressed to maintain the integrity of linear chromosomes, but the accidental activation of repairs can lead to genome instability. This review develops the concept that mechanisms to repair DNA damage in telomeres contribute to genetic variability and karyotype evolution, rather than catastrophe. Spontaneous breaks in telomeres can be repaired by telomerase, but in some cases DNA repair pathways are activated, and can cause chromosomal rearrangements or fusions. The resultant changes can also affect (...) subtelomeric regions that are adjacent to telomeres. Subtelomeres are actively involved in such chromosomal changes, and are therefore the most variable regions in the genome. The case of Caenorhabditis elegans in the context of changes of subtelomeric structures revealed by long‐read sequencing is also discussed. Theoretical and methodological issues covered in this review will help to explore the mechanism of chromosome evolution by reconstruction of chromosomal ends in nature. (shrink)

Comparative mapping and sequencing show that turnover of sex determining genes and chromosomes, and sex chromosome rearrangements, accompany speciation in many vertebrates. Here I review the evidence and propose that the evolution of therian mammals was precipitated by evolution of the male‐determining SRY gene, defining a novel XY sex chromosome pair, and interposing a reproductive barrier with the ancestral population of synapsid reptiles 190 million years ago (MYA). Divergence was reinforced by multiple translocations in monotreme sex chromosomes, the first (...) of which supplied a novel sex determining gene. A sex chromosome‐autosome fusion may have separated eutherians (placental mammals) from marsupials 160 MYA. Another burst of sex chromosome change and speciation is occurring in rodents, precipitated by the degradation of the Y. And although primates have a more stable Y chromosome, it may be just a matter of time before the same fate overtakes our own lineage. (shrink)

Studies on Neurospora chromosome segment duplications (Dps) performed since the publication of Perkins's comprehensive review in 1997 form the focus of this article. We present a brief summary of Perkins's seminal work on chromosome rearrangements, specifically, the identification of insertional and quasiterminal translocations that can segregate Dp progeny when crossed with normal sequence strains (i.e., T × N). We describe the genome defense process called meiotic silencing by unpaired DNA that renders Dp‐heterozygous crosses (i.e., Dp × N) barren, (...) which provides a basis for identifying Dps, and discuss whether other processes also might contribute to the barren phenotype of Dp × N and Dp × Dp crosses. We then turn to studies suggesting that large Dps (i.e., >300 kbp) can allow smaller gene‐sized duplications to escape another genome defense process called repeat‐induced point mutation (RIP), possibly by titration of the RIP machinery. Finally, we assess whether in natural populations dominant RIP suppressor Dps provide an “RIP‐free” niche for evolution of new genes following the duplication of existing genes. (shrink)

A team of US researchers recently reported the design, assembly and in vivo functionality of a synthetic chromosome III (SynIII) for the yeast Saccharomyces cerevisiae. The synthetic chromosome was assembled bottom‐up from DNA oligomers by teams of students working over several years with researchers as the first part of an international synthetic yeast genome project. Embedded into the sequence of the synthetic chromosome are multiple design changes that include a novel in‐built recombination scheme that can be induced to catalyse (...) intra‐chromosomal rearrangements in a variety of different conditions. This system, along with the other synthetic sequence changes, is intended to aid researchers develop a deeper understanding of how genomes function and find new ways to exploit yeast in future biotechnologies. The landmark of the first synthesised designer eukaryote chromosome, and the power of its massively parallel recombination system, provide new perspectives on the future of synthetic biology and genome research. (shrink)

The eukaryotic genome is packaged into a periodic nucleoprotein structure termed chromatin. The repeating unit of chromatin, the nucleosome, consists of DNA that is wound nearly two times around an octamer of histone proteins. To facilitate DNA‐directed processes in chromatin, it is often necessary to rearrange or to mobilize the nucleosomes. This remodeling of the nucleosomes is achieved by the action of chromatin‐remodeling complexes, which are a family of ATP‐dependent molecular machines. Chromatin‐remodeling factors share a related ATPase subunit and (...) participate in transcriptional regulation, DNA repair, homologous recombination and chromatin assembly. In this review, we provide an overview of chromatin‐remodeling enzymes and discuss two possible mechanisms by which these factors might act to reorganize nucleosome structure. BioEssays 25:1192–1200, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Mammalian cells frequently depend on homologous recombination (HR) to repair DNA damage accurately and to help rescue stalled or collapsed replication forks. The essence of HR is an exchange of nucleotides between identical or nearly identical sequences. Although HR fulfills important biological roles, recombination between inappropriate sequence partners can lead to translocations or other deleterious rearrangements and such events must be avoided. For example, the recombination machinery must follow stringent rules to preclude recombination between the many repetitive elements in (...) a mammalian genome that share significant but imperfect homology. This paper takes a conceptual approach in addressing the homology requirements for recombination in mammalian genomes as well as the general strategy used by cells to reject recombination between similar but imperfectly matched sequences. A mechanism of heteroduplex rejection that involves the unwinding of recombination intermediates that may form between mismatched sequences is discussed. BioEssays 30:1163–1171, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

A hypothesis for the control of eukaryotic DNA replication at the chromosomal level is proposed. The specific regulatory problem arises from the subdivision of the genome into thousands of individually replicating units, each of which must be duplicated a single time during S‐phase. The hypothesis is based on the finding of direct repeats at replication origins. Such repeats can adopt, beyond the full‐length double helical structure, another configuration exposing two single‐stranded loops that provide suitable templates for the initiation of (...) DNA replication. Any further initiation at the same origin is excluded as the single strandedness is eliminated by the replication process. Restoration of the initiable loop structure is proposed to occur by DNA‐protein rearrangements involved in chromosome condensation and duplication of the chromosomal protein backbone during mitosis. A possible role of the maturation promoting factor (MPF) is suggested. (shrink)

Most molecular biological concepts derive from physical chemical assumptions about the genetic code that are basically more than 40 years old. Additionally, systems biology, another quantitative approach, investigates the sum of interrelations to obtain a more holistic picture of nucleotide sequence order. Recent empirical data on genetic code compositions and rearrangements by mobile genetic elements and non-coding RNAs, together with results of virus research and their role in evolution, does not really fit into these concepts and compel a re-examination. (...) In this review we try to find an alternate hypothesis. It seems plausible now that if we look at the abundance of regulatory RNAs and persistent viruses in host genomes, we will find more and more evidence that the key players that edit the genetic codes of host genomes are consortia of RNA agents and viruses that drive evolutionary novelty and regulation of cellular processes in all steps of development. This agent-based approach may lead to a qualitative RNA sociology that investigates and identifies relevant behavioural motifs of cooperative RNA consortia. In addition to molecular biological perspectives this may lead to a better understanding of genetic code evolution and dynamics. (shrink)

Two reports have shown that mammalian artificial chromosomes (MAC) can be constructed from cloned human centromere DNA and telomere repeats, proving the principle that chromosomes can form from naked DNA molecules transfected into human cells. The MACs were mitotically stable, low copy number and bound antibodies associated with active centromeres. As a step toward second-generation MACs, yeast and bacterial cloning systems will have to be adapted to achieve large MAC constructs having a centromere, two telomeres, and genomic copies of mammalian (...) genes. Available construction techniques are discussed along with a new P1 artificial chromosome (PAC)-derived telomere vector (pTAT) that can be joined to other PACs in vitro, avoiding a cloning step during which large repetitive arrays often rearrange. The PAC system can be used as a route to further define the optimal DNA elements required for efficient MAC formation, to investigate the expression of genes on MACs, and possibly to develop efficient MAC-delivery protocols. BioEssays 1999;21:76–83. © 1999 John Wiley & Sons, Inc. (shrink)

The presence of small polydisperse extrachromosomal circular (spc) DNAs composed entirely of chromosomal sequences seems to reflect the plasticity of eukaryotic genomes. The size distribution and number of spc DNAs is found to vary with development, growth state and mitotic capacity. In particular, spc DNAs are observed to be several fold smaller in established immortal cell lines than in diploid cells with a limited life span. Analysis of cloned spc DNA fragments revealed that: (1) most of the spc DNAs thus (...) far investigated contain repetitive sequences; (2) some of the repetitive sequences are enriched in spc DNA clones relative to their representation in the chromosome; (3) various repetitive sequences of different reiteration frequency were found in spc DNA from immortal cell lines while a particular sequence of retroposon was predominant in primary lymphoid tissue cells and stem cells; (4) most of these spc DNAs appear to represent the extra copies generated by replicon‐misfiring in immortal cell lines or by retrotranscription in stem cells, and intermediates or by‐products produced by abortive rearrangement in diploid cells with a finite replicative capacity. (shrink)

Monoallelic gene expression has played a significant role in the evolution of mammals enabling the expansion of a vast repertoire of olfactory receptor types and providing increased sensitivity and diversity. Monoallelic expression of immune receptor genes has also increased diversity for antigen recognition, while the same mechanism that marks a single allele for preferential rearrangement also provides a distinguishing feature for directing hypermutations. Random monoallelic expression of the X chromosome is necessary to balance gene dosage across sexes. In marsupials only (...) the maternal X chromosome is expressed, while in eutherian mammals the paternal X genes are silenced in the developing placenta and early blastocyst. These examples of epigenetic gene regulation commonly employ asynchrony of replication, the binding of polycomb proteins and antisense RNA, and histone modifications to chromatin structure. The same is true for genomic imprinting which among vertebrates is unique to mammals and represents a special kind of epigenetic modification that is heritable according to parent of origin. Genomic imprinting pervades many aspects of mammalian growth and evolution but in particular has played a significant role in the co‐adaptive evolution of the mother and foetus. (shrink)

Mitochondria contain a molecular genetic system to express the 13 protein components of the electron transport system encoded in the mitochondrial genome (mtDNA). Defects in the function of this system result in some diaseases, many of which are multisystem disorders, prominently involving highly aerobic, postmitotic tissues. These defects can be caused by large‐scale rearrangements of mtDNA, by point mutations, or by nuclear gene mutations resulting in abnormalities in mtDNA. Although any of these mutations would be expected to produce (...) a similar clinical phenotype by compromising oxidative phosphorylation, the surprising and puzzling result is that different clinical phenotypes are generally associated with specific mtDNA mutations. Moreover, the same mutation can produce a distinct clinical phenotype in different individuals or pedigrees. MtDNA rearrangements are also found in aged individuals, but at a subclinical level, suggesting that normal and pathological processes can differ by the effect of genetic or environmental factors on the error rate of mtDNA replication. (shrink)

The use of Drosophila chromosomal rearrangements and transposon constructs involving the white gene reveals the existence of repressive chromatin domains that can spread over considerable genomic distances. One such type of domain is found in heterochromatin and is responsible for classical position‐effect variegation. Another type of repressive domain is established, beginning at specific sequences, by complexes of Polycomb Group proteins. Such complexes, which normally regulate the expression of many genes, including the homeotic loci, are responsible for silencing, white gene (...) variegation, pairing‐dependent effects and insertional targeting. (shrink)

During the eukaryotic cell cycle, chromosomes undergo large structural transitions and spatial rearrangements that are associated with the major cell functions of genome replication, transcription and chromosome condensation to metaphase chromosomes. Eukaryotic cells have evolved cell cycle dependent processes that modulate histone:DNA interactions in chromosomes. These are; (i) acetylations of lysines; (ii) phosphorylations of serines and threonines and (iii) ubiquitinations of lysines. All of these reversible modifications are contained in the well‐defined very basic N‐ and C‐ terminal domains (...) of histones. Acetylations and phosphorylations markedly affect the charge densities of these domains whereas ubiquitination adds a bulky globular protein, ubiquitin, to lysines in the C‐terminal tails of H2A and H2B. Histone acetylations are strictly associated with genome replication and transcription; histone H1 and H3 phosphorylations correlate with the process of chromosome condensation. The subunits of histone H1 kinase have now been shown to be cyclins and the p34CDC2 kinase product of the cell cycle control gene CDC2. It is probable that all of the processes that control chromosome structure:function relationships are also involved in the control of the cell cycle. (shrink)

Fusion genes formed by chromosomal rearrangements are common drivers of cancer. Recent innovations in the field of next‐generation sequencing (NGS) have seen a dynamic shift from traditional fusion detection approaches, such as visual characterization by fluorescence, to more precise multiplexed methods. There are many different NGS‐based approaches to fusion gene detection and deciding on the most appropriate method can be difficult. Beyond the experimental approach, consideration needs to be given to factors such as the ease of implementation, processing time, (...) associated costs, and the level of expertise required for data analysis. Here, the different NGS‐based methods for fusion gene detection, the basic principles underlying the techniques, and the benefits and limitations of each approach are reviewed. This article concludes with a discussion of how NGS will impact fusion gene detection in a clinical context and from where the next innovations are evolving. (shrink)

Enzymatic misrepair of ionizing‐radiation‐induced DNA damage can produce large‐scale rearrangements of the genome, such as translocations and dicentrics. These and other chromosome exchange aberrations can cause major phenotypic alterations, including cell death, mutation and neoplasia. Exchange formation requires that two (or more) genomic loci come together spatially. Consequently, the surprisingly rich aberration spectra uncovered by recently developed techniques, when combined with biophysically based computer modeling, help characterize large‐scale chromatin architecture in the interphase nucleus. Most results are consistent with (...) a picture whereby chromosomes are mainly confined to territories, chromatin motion is limited, and interchromosomal interactions involve mainly territory surfaces. Aberration spectra and modeling also help characterize DNA repair/misrepair mechanisms. Quantitative results for mammalian cells are best described by a breakage‐and‐reunion model, suggesting that the dominant recombinational mechanism during the G0/G1 phase of the cell cycle is non‐homologous end‐joining of radiogenic DNA double strand breaks. In turn, better mechanistic and quantitative understanding of aberration formation gives new insights into health‐related applications. BioEssays 24:714–723, 2002. © 2002 Wiley Periodicals, Inc. (shrink)

Two reports have shown that mammalian artificial chromosomes (MAC) can be constructed from cloned human centromere DNA and telomere repeats, proving the principle that chromosomes can form from naked DNA molecules transfected into human cells. The MACs were mitotically stable, low copy number and bound antibodies associated with active centromeres. As a step toward second-generation MACs, yeast and bacterial cloning systems will have to be adapted to achieve large MAC constructs having a centromere, two telomeres, and genomic copies of mammalian (...) genes. Available construction techniques are discussed along with a new P1 artificial chromosome (PAC)-derived telomere vector (pTAT) that can be joined to other PACs in vitro, avoiding a cloning step during which large repetitive arrays often rearrange. The PAC system can be used as a route to further define the optimal DNA elements required for efficient MAC formation, to investigate the expression of genes on MACs, and possibly to develop efficient MAC-delivery protocols. BioEssays 1999;21:76–83. © 1999 John Wiley & Sons, Inc. (shrink)

Transposons are ubiquitous genetic elements discovered so far in all investigated prokaryotes and eukaryotes. In remarkable contrast to all other genes, transposable elements are able to move to new locations within their host genomes. Transposition of transposons into coding sequences and their initiation of chromosome rearrangements have tremendous impact on gene expression and genome evolution. While transposons have long been known in bacteria, plants, and animals, only in recent years has there been a significant increase in the number (...) of transposable elements discovered in filamentous fungi. Like those of other eukaryotes, each fungal transposable element is either of class I or of class II. While class I elements transpose by a RNA intermediate and employ reverse transcriptases, class II elements transpose directly at the DNA level. We present structural and functional features for such transposons that have been identified so far in filamentous fungi. Emphasis is given to specific advantages or unique features when fungal systems are used to study transposable elements, e.g., the evolutionary impact of transposons in coenocytic organisms and possible experimental approaches toward horizontal gene transfer. Finally, we focus on the potential of transposons for tagging and identifying fungal genes. BioEssays 20:652–659, 1998.© 1998 John Wiley & Sons Inc. (shrink)