Transcription influences the types of deletion and expansion products in an orientation-dependent manner from GAC*GTC repeats

A B-Z junction induced by an A … A mismatch in GAC repeats in the gene for cartilage oligomeric matrix protein promotes binding with the hZαADAR1 protein

GAC repeat expansion from five to seven in the exonic region of the gene for cartilage oligomeric matrix protein (COMP) leads to pseudoachondroplasia, a skeletal abnormality. However, the molecular mechanism by which GAC expansions in the COMP gene lead to skeletal dysplasias is poorly understood. Here we used molecular dynamics simulations, which indicate that an A … A mismatch in a d(GAC)6·d(GAC)6 duplex induces negative supercoiling, leading to a local B-to-Z DNA transition. This transition facilitates the binding of d(GAC)7·d(GAC)7 with the Zα-binding domain of human adenosine deaminase acting on RNA 1 (ADAR1, hZαADAR1), as confirmed by CD, NMR, and microscale thermophoresis studies. The CD results indicated that hZαADAR1 recognizes the zigzag backbone of d(GAC)7·d(GAC)7 at the B-Z junction and subsequently converts it into Z-DNA via the so-called passive mechanism. Molecular dynamics simulations carried out for the modeled hZαADAR1-d(GAC)6d(GAC)6 complex confirmed the retention of previously reported important interactions between the two molecules. These findings suggest that hZαADAR1 binding with the GAC hairpin stem in COMP can lead to a non-genetic, RNA editing-mediated substitution in COMP that may then play a crucial role in the development of pseudoachondroplasia.

Microsatellite evolutionary rate and pattern in Schistocerca gregaria inferred from direct observation of germline mutations

Unravelling variation among taxonomic orders regarding the rate of evolution in microsatellites is crucial for evolutionary biology and population genetics research. The mean mutation rate of microsatellites tends to be lower in arthropods than in vertebrates, but data are scarce and mostly concern accumulation of mutations in model species. Based on parent-offspring segregations and a hierarchical Bayesian model, the mean rate of mutation in the orthopteran insect Schistocerca gregaria was estimated at 2.1e(-4) per generation per untranscribed dinucleotide locus. This is close to vertebrate estimates and one order of magnitude higher than estimates from species of other arthropod orders, such as Drosophila melanogaster and Daphnia pulex. We also found evidence of a directional bias towards expansions even for long alleles and exceptionally large ranges of allele sizes. Finally, at transcribed microsatellites, the mean rate of mutation was half the rate found at untranscribed loci and the mutational model deviated from that usually considered, with most mutations involving multistep changes that avoid disrupting the reading frame. Our direct estimates of mutation rate were discussed in the light of peculiar biological and genomic features of S. gregaria, including specificities in mismatch repair and the dependence of its activity to allele length. Shedding new light on the mutational dynamics of grasshopper microsatellites is of critical importance for a number of research fields. As an illustration, we showed how our findings improve microsatellite application in population genetics, by obtaining a more precise estimation of S. gregaria effective population size from a published data set based on the same microsatellites.

The mismatch repair protein MSH2 is rate limiting for repeat expansion in a fragile X premutation mouse model

Fragile X-associated tremor and ataxia syndrome, Fragile X-associated primary ovarian insufficiency, and Fragile X syndrome are Repeat Expansion Diseases caused by expansion of a CGG•CCG-repeat microsatellite in the 5 UTR of the FMR1 gene. To help understand the expansion mechanism responsible for these disorders, we have crossed mice containing∼147 CGG•CCG repeats in the endogenous murine Fmr1 gene with mice containing a null mutation in the gene encoding the mismatch repair protein MSH2. MSH2 mutations are associated with elevated levels of generalized microsatellite instability. However, we show here for the first time that in the FX mouse model, all maternally and paternally transmitted expansions require Msh2. Even the loss of one Msh2 allele reduced the intergenerational expansion frequency significantly. Msh2 is also required for all somatic expansions and loss of even one functional Msh2 allele reduced the extent of somatic expansion in some organs. Tissues with lower levels of MSH2 were more sensitive to the loss of a single Msh2 allele. This suggests that MSH2 is rate limiting for expansion in this mouse model and that MSH2 levels may be a key factor that accounts for tissue-specific differences in expansion risk.

Somatic expansion in mouse and human carriers of fragile X premutation alleles

Repeat expansion diseases result from expansion of a specific tandem repeat. The three fragile X-related disorders (FXDs) arise from germline expansions of a CGG•CCG repeat tract in the 5' UTR (untranslated region) of the fragile X mental retardation 1 (FMR1) gene. We show here that in addition to germline expansion, expansion also occurs in the somatic cells of both mice and humans carriers of premutation alleles. Expansion in mice primarily affects brain, testis, and liver with very little expansion in heart or blood. Our data would be consistent with a simple two-factor model for the organ specificity. Somatic expansion in humans may contribute to the mosaicism often seen in individuals with one of the FXDs. Because expansion risk and disease severity are related to repeat number, somatic expansion may exacerbate disease severity and contribute to the age-related increased risk of expansion seen on paternal transmission in humans. As little somatic expansion occurs in murine lymphocytes, our data also raise the possibility that there may be discordance in humans between repeat numbers measured in blood and that present in brain. This could explain, at least in part, the variable penetrance seen in some of these disorders.

Xpa deficiency reduces CAG trinucleotide repeat instability in neuronal tissues in a mouse model of SCA1

Expansion of trinucleotide repeats (TNRs) is responsible for a number of human neurodegenerative disorders. The molecular mechanisms that underlie TNR instability in humans are not clear. Based on results from model systems, several mechanisms for instability have been proposed, all of which focus on the ability of TNRs to form alternative structures during normal DNA transactions, including replication, DNA repair and transcription. These abnormal structures are thought to trigger changes in TNR length. We have previously shown that transcription-induced TNR instability in cultured human cells depends on several genes known to be involved in transcription-coupled nucleotide excision repair (NER). We hypothesized that NER normally functions to destabilize expanded TNRs. To test this hypothesis, we bred an Xpa null allele, which eliminates NER, into the TNR mouse model for spinocerebellar ataxia type 1 (SCA1), which carries an expanded CAG repeat tract at the endogenous mouse Sca1 locus. We find that Xpa deficiency does not substantially affect TNR instability in either the male or female germline; however, it dramatically reduces CAG repeat instability in neuronal tissues-striatum, hippocampus and cerebral cortex-but does not alter CAG instability in kidney or liver. The tissue-specific effect of Xpa deficiency represents a novel finding; it suggests that tissue-to-tissue variation in CAG repeat instability arises, in part, by different underlying mechanisms. These results validate our original findings in cultured human cells and suggest that transcription may induce NER-dependent TNR instability in neuronal tissues in humans.

New insights into repeat instability: role of RNA•DNA hybrids

Expansion of tandem repeat sequences is responsible for more than 20 human diseases. Several cis elements and trans factors involved in repeat instability (expansion and contraction) have been identified. However no comprehensive model explaining large intergenerational or somatic changes of the length of the repeating sequences exists. Several lines of evidence, accumulated from different model studies, indicate that transcription through repeat sequences is an important factor promoting their instability. The persistent interaction between transcription template DNA and nascent RNA (RNA•DNA hybrids, R loops) was shown to stimulate genomic instability. Recently, we demonstrated that cotranscriptional RNA•DNA hybrids are preferentially formed at GC-rich trinucleotide and tetranucleotide repeat sequences in vitro as well as in human genomic DNA. Additionally, we showed that cotranscriptional formation of RNA•DNA hybrids at CTG•CAG and GAA•TTC repeats stimulate instability of these sequences in both E. coli and human cells. Our results suggest that persistent RNA•DNA hybrids may also be responsible for other downstream effects of expanded trinucleotide repeats, including gene silencing. Considering the extent of transcription through the human genome as well as the abundance of GC-rich and/or non-canonical DNA structure forming tandem repeats, RNA•DNA hybrids may represent a common mutagenic conformation. Hence, R loops are potentially attractive therapeutic target in diseases associated with genomic instability.

Transcription destabilizes triplet repeats

Triplet repeat expansion is the molecular basis for several human diseases. Intensive studies using systems in bacteria, yeast, flies, mammalian cells, and mice have provided important insights into the molecular processes that are responsible for mediating repeat instability. The age-dependent, ongoing repeat instability in somatic tissues, especially in terminally differentiated neurons, strongly suggests a robust role for pathways that are independent of DNA replication. Several genetic studies have indicated that transcription can play a critical role in repeat instability, potentially providing a basis for the instability observed in neurons. Transcription-induced repeat instability can be modulated by several DNA repair proteins, including those involved in mismatch repair (MMR) and transcription-coupled nucleotide excision repair (TC-NER). Though the mechanism is unclear, it is likely that transcription facilitates the formation of repeat-specific secondary structures, which act as intermediates to trigger DNA repair, eventually leading to changes in the length of the repeat tract. In addition, other processes associated with transcription can also modulate repeat instability, as shown in a variety of different systems. Overall, the mechanisms underlying repeat instability in humans are unexpectedly complicated. Because repeat-disease genes are widely expressed, transcription undoubtedly contributes to the repeat instability observed in many diseases, but it may be especially important in nondividing cells. Transcription-induced instability is likely to involve an extensive interplay not only of the core transcription machinery and DNA repair proteins, but also of proteins involved in chromatin remodeling, regulation of supercoiling, and removal of stalled RNA polymerases, as well as local DNA sequence effects.

Long intronic GAA*TTC repeats induce epigenetic changes and reporter gene silencing in a molecular model of Friedreich ataxia

Friedreich ataxia (FRDA) is caused by hyperexpansion of GAA*TTC repeats located in the first intron of the FXN gene, which inhibits transcription leading to the deficiency of frataxin. The FXN gene is an excellent target for therapeutic intervention since (i) 98% of patients carry the same type of mutation, (ii) the mutation is intronic, thus leaving the FXN coding sequence unaffected and (iii) heterozygous GAA*TTC expansion carriers with approximately 50% decrease of the frataxin are asymptomatic. The discovery of therapeutic strategies for FRDA is hampered by a lack of appropriate molecular models of the disease. Herein, we present the development of a new cell line as a molecular model of FRDA by inserting 560 GAA*TTC repeats into an intron of a GFP reporter minigene. The GFP_(GAA*TTC)(560) minigene recapitulates the molecular hallmarks of the mutated FXN gene, i.e. inhibition of transcription of the reporter gene, decreased levels of the reporter protein and hypoacetylation and hypermethylation of histones in the vicinity of the repeats. Additionally, selected histone deacetylase inhibitors, known to stimulate the FXN gene expression, increase the expression of the GFP_(GAA*TTC)(560) reporter. This FRDA model can be adapted to high-throughput analyses in a search for new therapeutics for the disease.

Mutagenic roles of DNA "repair" proteins in antibody diversity and disease-associated trinucleotide repeat instability

While DNA repair proteins are generally thought to maintain the integrity of the whole genome by correctly repairing mutagenic DNA intermediates, there are cases where DNA "repair" proteins are involved in causing mutations instead. For instance, somatic hypermutation (SHM) and class switch recombination (CSR) require the contribution of various DNA repair proteins, including UNG, MSH2 and MSH6 to mutate certain regions of immunoglobulin genes in order to generate antibodies of increased antigen affinity and altered effector functions. Another instance where "repair" proteins drive mutations is the instability of gene-specific trinucleotide repeats (TNR), the causative mutations of numerous diseases including Fragile X mental retardation syndrome (FRAXA), Huntington's disease (HD), myotonic dystrophy (DM1) and several spinocerebellar ataxias (SCAs) all of which arise via various modes of pathogenesis. These healthy and deleterious mutations that are induced by repair proteins are distinct from the genome-wide mutations that arise in the absence of repair proteins: they occur at specific loci, are sensitive to cis-elements (sequence context and/or epigenetic marks) and transcription, occur in specific tissues during distinct developmental windows, and are age-dependent. Here we review and compare the mutagenic role of DNA "repair" proteins in the processes of SHM, CSR and TNR instability.

Fragile X repeats are potent inducers of complex, multiple site rearrangements in flanking sequences in Escherichia coli

(CGG.CCG)n repeats induce the formation of complex, multiple site rearrangements and/or gross deletions in flanking DNA sequences in Escherichia coli plasmids. DNA sequence analyses of mutant clones revealed the influence of (a) the length (24, 44 or 73 repeats), (b) the orientation of the CGG.CCG region relative to the unidirectional origin, and (c) its transcription status. Complex rearrangements had occurred in the mutant clones since some products contained deletions, inversions and insertions and some products had only gross deletions. Furthermore, the CGG.CCG repeats repeatedly induced, up to 22 times, the formation of identical (to the bp) mutagenic products indicating the powerful nature of the complex processes involved. Also, the mutations were bidirectional from the CGG.CCG tract. The healed junctions had CG-rich microhomologies of 1-6bp, CG-rich regions and putative cruciforms and slipped structures. Hence, the fragile X syndrome mutagenic spectrum has been found, at least in part, in our model system.

The genome-wide determinants of human and chimpanzee microsatellite evolution

Mutation rates of microsatellites vary greatly among loci. The causes of this heterogeneity remain largely enigmatic yet are crucial for understanding numerous human neurological diseases and genetic instability in cancer. In this first genome-wide study, the relative contributions of intrinsic features and regional genomic factors to the variation in mutability among orthologous human-chimpanzee microsatellites are investigated with resampling and regression techniques. As a result, we uncover the intricacies of microsatellite mutagenesis as follows. First, intrinsic features (repeat number, length, and motif size), which all influence the probability and rate of slippage, are the strongest predictors of mutability. Second, mutability increases nonuniformly with length, suggesting that processes additional to slippage, such as faulty repair, contribute to mutations. Third, mutability varies among microsatellites with different motif composition likely due to dissimilarities in secondary DNA structure formed by their slippage intermediates. Fourth, mutability of mononucleotide microsatellites is impacted by their location on sex chromosomes vs. autosomes and inside vs. outside of Alu repeats, the former confirming the importance of replication and the latter suggesting a role for gene conversion. Fifth, transcription status and location in a particular isochore do not influence microsatellite mutability. Sixth, compared with intrinsic features, regional genomic factors have only minor effects. Finally, our regression models explain approximately 90% of variation in microsatellite mutability and can generate useful predictions for the studies of human diseases, forensics, and conservation genetics.

Transcription-induced CAG repeat contraction in human cells is mediated in part by transcription-coupled nucleotide excision repair

Expansions of CAG repeat tracts in the germ line underlie several neurological diseases. In human patients and mouse models, CAG repeat tracts display an ongoing instability in neurons, which may exacerbate disease symptoms. It is unclear how repeats are destabilized in nondividing cells, but it cannot involve DNA replication. We showed previously that transcription through CAG repeats induces their instability (Y. Lin, V. Dion, and J. H. Wilson, Nat. Struct. Mol. Biol. 13:179-180). Here, we present a genetic analysis of the link between transcription-induced repeat instability and nucleotide excision repair (NER) in human cells. We show that short interfering RNA-mediated knockdown of CSB, a component specifically required for transcription-coupled NER (TC-NER), and knockdowns of ERCC1 and XPG, which incise DNA adjacent to damage, stabilize CAG repeat tracts. These results suggest that TC-NER is involved in the pathway for transcription-induced CAG repeat instability. In contrast, knockdowns of OGG1 and APEX1, key components involved in base excision repair, did not affect repeat instability. In addition, repeats are stabilized by knockdown of transcription factor IIS, consistent with a requirement for RNA polymerase II (RNAPII) to backtrack from a transcription block. Repeats also are stabilized by knockdown of either BRCA1 or BARD1, which together function as an E3 ligase that can ubiquitinate arrested RNAPII. Treatment with the proteasome inhibitor MG132, which stabilizes repeats, confirms proteasome involvement. We integrate these observations into a tentative pathway for transcription-induced CAG repeat instability that can account for the contractions observed here and potentially for the contractions and expansions seen with human diseases.

Proofreading and secondary structure processing determine the orientation dependence of CAG x CTG trinucleotide repeat instability in Escherichia coli

Expanded CAG x CTG trinucleotide repeat tracts are associated with several human inherited diseases, including Huntington's disease, myotonic dystrophy, and spinocerebellar ataxias. Here we describe a new model system to investigate repeat instability in the Escherichia coli chromosome. Using this system, we reveal patterns of deletion instability consistent with secondary structure formation in vivo and address the molecular basis of orientation-dependent instability. We demonstrate that the orientation dependence of CAG x CTG trinucleotide repeat deletion is determined by the proofreading subunit of DNA polymerase III (DnaQ) in the presence of the hairpin nuclease SbcCD (Rad50/Mre11). Our results suggest that, although initiation of slippage can occur independently of CAG x CTG orientation, the folding of the intermediate affects its processing and this results in orientation dependence. We propose that proofreading is inefficient on the CTG-containing strand because of its ability to misfold and that SbcCD contributes to processing in a manner that is dependent on proofreading and repeat tract orientation. Furthermore, we demonstrate that transcription and recombination do not influence instability in this system.

Double-strand breaks in the myotonic dystrophy type 1 and the fragile X syndrome triplet repeat sequences induce different types of mutations in DNA flanking sequences in Escherichia coli

The putative role of double-strand breaks (DSBs) created in vitro by restriction enzyme cleavage in or near CGG*CCG or CTG*CAG repeat tracts on their genetic instabilities, both within the repeats and in their flanking sequences, was investigated in an Escherichia coli plasmid system. DSBs at TRS junctions with the vector generated a large number of mutagenic events in flanking sequences whereas DSBs within the repeats elicited no similar products. A substantial enhancement in the number of mutants was caused by transcription of the repeats and by the absence of recombination functions (recA-, recBC-). Surprisingly, DNA sequence analyses on mutant clones revealed the presence of only single deletions of 0.4-1.6 kb including the TRS and the flanking sequence from plasmids originally containing (CGG*CCG)43 but single, double and multiple deletions as well as insertions were found for plasmids originally containing (CTG*CAG)n (where n = 43 or 70). Non-B DNA structures (slipped structures with loops, cruciforms, triplexes and tetraplexes) as well as microhomologies are postulated to participate in the recombination and/or repair processes.

Non-B DNA conformations formed by long repeating tracts of myotonic dystrophy type 1, myotonic dystrophy type 2, and Friedreich's ataxia genes, not the sequences per se, promote mutagenesis in flanking regions

The expansions of long repeating tracts of CTG.CAG, CCTG.CAGG, and GAA.TTC are integral to the etiology of myotonic dystrophy type 1 (DM1), myotonic dystrophy type 2 (DM2), and Friedreich's ataxia (FRDA). Essentially all studies on the molecular mechanisms of this expansion process invoke an important role for non-B DNA conformations which may be adopted by these repeat sequences. We have directly evaluated the role(s) of the repeating sequences per se, or of the non-B DNA conformations formed by these sequences, in the mutagenic process. Studies in Escherichia coli and three types of mammalian (COS-7, CV-1, and HEK-293) fibroblast-like cells revealed that conditions which promoted the formation of the non-B DNA structures enhanced the genetic instabilities, both within the repeat sequences and in the flanking sequences of up to approximately 4 kbp. The three strategies utilized included: the in vivo modulation of global negative supercoil density using topA and gyrB mutant E. coli strains; the in vivo cleavage of hairpin loops, which are an obligate consequence of slipped-strand structures, cruciforms, and intramolecular triplexes, by inactivation of the SbcC protein; and by genetic instability studies with plasmids containing long repeating sequence inserts that do, and do not, adopt non-B DNA structures in vitro. Hence, non-B DNA conformations are critical for these mutagenesis mechanisms.

The involvement of non-B DNA structures in gross chromosomal rearrangements

Non-B DNA conformations adopted by certain types of DNA sequences promote genetic instabilities, especially gross rearrangements including translocations. We conclude the following: (a) slipped (hairpin) structures, cruciforms, triplexes, tetraplexes and i-motifs, and left-handed Z-DNA are formed in chromosomes and elicit profound genetic consequences via recombination-repair, (b) repeating sequences, probably in their non-B conformations, cause gross genomic rearrangements (translocations, deletions, insertions, inversions, and duplications), and (c) these rearrangements are the genetic basis for numerous human diseases including polycystic kidney disease, adrenoleukodystrophy, follicular lymphomas, and spermatogenic failure.

Mechanisms of tandem repeat instability in bacteria

Hypermutable tandem repeat sequences (TRSs) are present in the genomes of both prokaryotic and eukaryotic organisms. Numerous studies have been conducted in several laboratories over the past decade to investigate the mechanisms responsible for expansions and contractions of microsatellites (a subset of TRSs with a repeat length of 1-6 nucleotides) in the model prokaryotic organism Escherichia coli. Both the frequency of tandem repeat instability (TRI), and the types of mutational events that arise, are markedly influenced by the DNA sequence of the repeat, the number of unit repeats, and the types of cellular pathways that process the TRS. DNA strand slippage is a general mechanism invoked to explain instability in TRSs. Misaligned DNA sequences are stabilized both by favorable base pairing of complementary sequences and by the propensity of TRSs to form relatively stable secondary structures. Several cellular processes, including replication, recombination and a variety of DNA repair pathways, have been shown to interact with such structures and influence TRI in bacteria. This paper provides an overview of our current understanding of mechanisms responsible for TRI in bacteria, with an emphasis on studies that have been carried out in E. coli. In addition, new experimental data are presented, suggesting that TLS polymerases (PolII, PolIV and PolV) do not contribute significantly to TRI in E. coli.

Roles of double-strand breaks, nicks, and gaps in stimulating deletions of CTG.CAG repeats by intramolecular DNA repair

A series of plasmids harboring CTG.CAG repeats with double-strand breaks (DSB), single-strand nicks, or single-strand gaps (15 or 30 nucleotides) within the repeat regions were used to determine their capacity to induce genetic instabilities. These plasmids were introduced into Escherichia coli in the presence of a second plasmid containing a sequence that could support homologous recombination repair between the two plasmids. The transfer of a point mutation from the second to the first plasmid was used to monitor homologous recombination (gene conversion). Only DSBs increased the overall genetic instability. This instability took place by intramolecular repair, which was not dependent on RuvA. Double-strand break-induced instabilities were partially stabilized by a mutation in recF. Gaps of 30 nt formed a distinct 30 nt deletion product, whereas single strand nicks and gaps of 15 nt did not induce expansions or deletions. Formation of this deletion product required the CTG.CAG repeats to be present in the single-stranded region and was stimulated by E.coli DNA ligase, but was not dependent upon the RecFOR pathway. Models are presented to explain the intramolecular repair-induced instabilities and the formation of the 30 nt deletion product.

Increased negative superhelical density in vivo enhances the genetic instability of triplet repeat sequences

The influence of negative superhelical density on the genetic instabilities of long GAA.TTC, CGG.CCG, and CTG.CAG repeat sequences was studied in vivo in topologically constrained plasmids in Escherichia coli. These repeat tracts are involved in the etiologies of Friedreich ataxia, fragile X syndrome, and myotonic dystrophy type 1, respectively. The capacity of these DNA tracts to undergo deletions-expansions was explored with three genetic-biochemical approaches including first, the utilization of topoisomerase I and/or DNA gyrase mutants, second, the specific inhibition of DNA gyrase by novobiocin, and third, the genetic removal of the HU protein, thus lowering the negative supercoil density (-sigma). All three strategies revealed that higher -sigma in vivo enhanced the formation of deleted repeat sequences. The effects were most pronounced for the Friedreich ataxia and the fragile X triplet repeat sequences. Higher levels of -sigma stabilize non-B DNA conformations (i.e. triplexes, sticky DNA, flexible and writhed DNA, slipped structures) at appropriate repeat tracts; also, numerous prior genetic instability investigations invoke a role for these structures in promoting the slippage of the DNA complementary strands. Thus, we propose that the in vivo modulation of the DNA structure, localized to the repeat tracts, is responsible for these behaviors. Presuming that these interrelationships are also found in humans, dynamic alterations in the chromosomal nuclear matrix may modulate the -sigma of certain DNA regions and, thus, stabilize/destabilize certain non-B conformations which regulate the genetic expansions-deletions responsible for the diseases.

Advances in mechanisms of genetic instability related to hereditary neurological diseases

Substantial progress has been realized in the past several years in our understanding of the molecular mechanisms responsible for the expansions and deletions (genetic instabilities) of repeating tri-, tetra- and pentanucleotide repeating sequences associated with a number of hereditary neurological diseases. These instabilities occur by replication, recombination and repair processes, probably acting in concert, due to slippage of the DNA complementary strands relative to each other. The biophysical properties of the folded-back repeating sequence strands play a critical role in these instabilities. Non-B DNA structural elements (hairpins and slipped structures, DNA unwinding elements, tetraplexes, triplexes and sticky DNA) are described. The replication mechanisms are influenced by pausing of the replication fork, orientation of the repeat strands, location of the repeat sequences relative to replication origins and the flap endonuclease. Methyl-directed mismatch repair, nucleotide excision repair, and repair of damage caused by mutagens are discussed. Genetic recombination and double-strand break repair advances in Escherichia coli, yeast and mammalian models are reviewed. Furthermore, the newly discovered capacities of certain triplet repeat sequences to cause gross chromosomal rearrangements are discussed.