Molecular characterization of a pericentric inversion in mouse chromosome 8 implicates telomeres as promoters of meiotic recombination

Where to cross? New insights into the location of meiotic crossovers

During meiosis, the repair of induced DNA double-strand breaks (DSBs) produces crossovers (COs). COs are essential for the proper segregation of homologous chromosomes at the first meiotic division. In addition, COs generate new combinations of genetic markers in the progeny. CO localization is tightly controlled, giving rise to patterns that are specific to each species. The underlying mechanisms governing CO location, however, are poorly understood. Recent studies highlight the complexity of the multiple interconnected factors involved in shaping the CO landscape and demonstrate that the mechanisms that control CO distribution can vary from species to species. Here, we provide an overview of the recent findings related to CO distribution and discuss their impact on our understanding of the control of meiotic recombination.

Synapsis in grasshopper bivalents heterozygous for centric shifts

Analysis of surface-spread synaptonemal complexes of zygotene and pachytene spermatocytes was carried out on centric-shift heterozygotes of grasshoppers. These rearrangements affected the M7 chromosome in Chorthippus vagans and the M6 and S8 chromosomes in Chorthippus apricarius. The shifts in the latter two chromosomes were also associated with C-heterochromatin variations between homologous chromosomes. Rearranged chromosomes proceeded directly to heterosynapsis without an apparent intervening homosynaptic phase in M7 bivalents of Ch. vagans and M6 bivalents of Ch. apricarius. In the latter case, axial equalization of the heterochromatin polymorphism was also achieved. On the other hand, asynapsis of the intercentromeric regions throughout pachytene was the rule in the centric shift involving the S8 chromosome of Ch. apricarius. In the three cases analysed, the production of unbalanced gametes in the heterozygotes is precluded either by the lack of chiasma formation in heterosynapsed rearranged segments or by the lack of pairing between such segments. Chiasmata were limited to the homologous regions of the heteromorphic bivalents.

Synapsis and recombination in inversion heterozygotes

Inversion heterozygotes are expected to suffer from reduced fertility and a high incidence of chromosomally unbalanced gametes due to recombination within the inverted region. Non-homologous synapsis of the inverted regions can prevent recombination there and diminish the deleterious effects of inversion heterozygosity. The choice between non-homologous and homologous synapsis depends on the size of inversion, its genetic content, its location in relation to the centromere and telomere, and genetic background. In addition, there is a class of inversions in which homologous synapsis is gradually replaced by non-homologous synapsis during meiotic progression. This process is called synaptic adjustment. The degree of synaptic adjustment depends critically on the presence and location of the COs (crossovers) within the inversion loop. Only bivalents without COs within the loop and those with COs in the middle of the inversion can be completely adjusted and became linear.

The telomere theory of reproductive senescence in women

PURPOSE OF REVIEW

A unifying theory of reproductive aging, based on telomere shortening, is proposed.

RECENT FINDINGS

Telomere shortening may mediate both 'hits' involved in reproductive aging, that is late exit from the fetal production line and long interval to ovulation in the adult.

SUMMARY

As women age egg dysfunction increases, with meiotic nondisjunction, embryonic arrest, apoptosis, and miscarriage. Egg dysfunction results from two 'hits' - reduced formation of chiasmata during fetal oogenesis, and accumulation of reactive oxygen damage during the prolonged interval until ovulation. Late exit from a production line during oogenesis presumably contributes to the first hit. The later insult also involves meiotic spindle abnormalities. Telomeres, repetitive sequences of DNA, cap chromosome ends and dissipate during divisions. Oocytes do not divide, but oogonia do, and telomerase, the enzyme responsible for maintaining telomere length, is inefficient, and remains inactive in oocytes and embryos until blastocyst stage. Reactive oxygen also shortens telomeres, so the prolonged interval between birth and ovulation would further shorten telomeres from chronic exposure to reactive oxygen. In support of this theory, experimental shortening of telomeres in mice produced a phenotype similar to reproductive aging in women, with abnormal chiasmata, spindles, cell cycles, apoptosis, and genomic instability, and telomere length in human eggs correlated with in-vitro fertilization outcome.

Interstitial (TTAGGG)(n) sequences are not hot spots of recombination in the chicken lampbrush macrochromosomes 1-3

To study the role of telomere (TTAGGG)(n) sequences in promoting of crossing over in chicken female meiosis, we have localized telomere repeats by FISH and studied the distribution of chiasmata in the giant diplotene bivalents, the chicken lampbrush macrochromosomes 1--3. We show that all interstitial clusters of the (TTAGGG)(n) repeat in these chromosomes do not coincide with hot spots of genetic recombination (crossing over) in the chicken female. Moreover, terminal TTAGGG-positive chromomeres also are not chiasma hot spots. We conclude that, at least in chicken macrochromosomes in female meiosis, a role for canonical telomere sequences in promoting of crossing over is not confirmed.

Meiotic exchange and segregation in female mice heterozygous for paracentric inversions

Inversion heterozygosity has long been noted for its ability to suppress the transmission of recombinant chromosomes, as well as for altering the frequency and location of recombination events. In our search for meiotic situations with enrichment for nonexchange and/or single distal-exchange chromosome pairs, exchange configurations that are at higher risk for nondisjunction in humans and other organisms, we examined both exchange and segregation patterns in 2728 oocytes from mice heterozygous for paracentric inversions, as well as controls. We found dramatic alterations in exchange position in the heterozygotes, including an increased frequency of distal exchanges for two of the inversions studied. However, nondisjunction was not significantly increased in oocytes heterozygous for any inversion. When data from all inversion heterozygotes were pooled, meiotic nondisjunction was slightly but significantly higher in inversion heterozygotes (1.2%) than in controls (0%), although the frequency was still too low to justify the use of inversion heterozygotes as a model of human nondisjunction.

Irregular telomeres impair meiotic synapsis and recombination in mice

Telomere shortening can lead to chromosome instability, replicative senescence, and apoptosis in both somatic and male germ cells. To study roles for mammalian telomeres in homologous pairing and recombination, we characterized effects of telomere shortening on spermatogenesis and oogenesis in late-generation telomerase-deficient mice. We show that shortened telomeres of late-generation telomerase-deficient mice impair meiotic synapsis and decrease recombination, in particular, in females. In response to telomere shortening, male germ cells mostly undergo apoptosis, whereas female germ cells preferentially arrest in early meiosis, suggesting sexually dimorphic surveillance mechanisms for telomere dysfunction during meiosis in mice. Further, meiocytes of late-generation telomerase-deficient females with shortened telomeres, bred with early-generation males harboring relatively long telomeres, exhibit severely impaired chromosome pairing and synapsis and reduced meiotic recombination. These findings imply that functional telomeres are important in mammalian meiotic synapsis and recombination.

Telomere-driven genomic instability in cancer cells

Telomeres, the ends of linear chromosomes, play a major role in the maintenance of genome integrity. Telomerase or alternative lengthening of telomeres (ALT) mechanisms exist in most cancer cells in order to stabilize telomere length by the addition of telomeric repeats. Telomere loss can be dramatically mutagenic. Chromosomes lacking one telomere remain unstable until they are capped, generating chromosomal instability, gene amplification via breakage/fusion/bridge (B/F/B) cycles and resulting in chromosome imbalances. The chronology of the occurrence of gene amplification and chromosome imbalances detected in human tumors is still unknown. All of the aberrations that occur prior to, during or after activation of a telomere maintenance mechanism promote the development of cancer.

Telomeres in mammalian male germline cells

Telomeres are terminal chromosomal domains that protect chromosome ends from degradation and fusion and promote complete replication of DNA. Telomeres are involved in the regulation of cellular replicative lifespan and tumorigenesis. These important functions of the telomeres have evoked high interest: numerous studies have resulted in a detailed description of telomere composition and structure in somatic cells. Much less is known about telomeres in germline cells. Emerging novel features and unique behavior of telomeres in the process of gamete differentiation suggest that they may have additional germline-specific function(s). This review describes recent studies revealing changes in the telomere organization in the course of differentiation from the germline stem cells to mature sperm in mammals. Similarities and differences between somatic and spermatogenic cells in telomere nuclear localization, protein composition, DNA length, telomerase activity, and chromatin structure are discussed. The exceptional features of the germline telomeres may be important for regulation of telomerase activity during spermatogenesis, homologous chromosome pairing during recombination, as well as for male pronucleus development and ordered chromosome withdrawal post-fertilization.

Chromosomal rearrangements and evolution of recombination: comparison of chiasma distribution patterns in standard and robertsonian populations of the house mouse

The effects of chromosomal rearrangements on recombination rates were tested by the analysis of chiasma distribution patterns in wild house mice. Males and females of two chromosomal races from Tunisia differing by nine pairs of Robertsonian (Rb) fusions (standard all-acrocentric, 2N = 40 and 2N = 22) were studied. A significant decrease in chiasma number (CN) was observed in Rb mice compared to standard ones for both sexes. The difference in CN was due to a reduction in the number of proximal chiasmata and was associated with an overall more distal redistribution. These features were related to distance of chiasmata to the centromere, suggesting that the centromere effect was more pronounced in Rb fusions than in acrocentric chromosomes. These modifications were interpreted in terms of structural meiotic constraints, although genic factors were likely involved in patterning the observed differences between sexes within races. Thus, the change in chromosomal structure in Rb mice was associated with a generalized decrease in recombination due to a reduction in diploid number, a lower CN, and a decrease in the efficiency of recombination. The effects of such modifications on patterns of genic diversity are discussed in the light of models of evolution of recombination.

Radiation and germline mutation at repeat sequences: Are we in the middle of a paradigm shift?

Two assumptions are commonly made in the estimation of genetic risk: (1) that the seven specific loci in the mouse constitute a suitable basis for extrapolation to genetic disease in humans, and (2) that mutations are induced by radiation damage (energy-loss events leading to double-stranded damage) occurring within the gene and are induced linearly with dose, at least at low doses. Recent evidence on the mutability of repeat sequences is reviewed that suggests that neither of these assumptions is as well founded as we like to think. Repeat sequences are common in the human genome, and alterations in them may have health consequences. Many of them are unstable, both spontaneously and after irradiation. The fact that changes in DNA repeat sequences can clearly arise as a result of radiation damage outside the sequence concerned and the likely involvement of some sort of signal transduction process mean that the nature of the radiation dose response cannot be assumed. While the time has not come to abandon the current paradigms, it would seem sensible to invest more effort in exploring the induction of changes in repeat sequences after irradiation and the consequences of such changes for health.

Mitotic recombination produces the majority of recessive fibroblast variants in heterozygous mice

Mice heterozygous at Aprt (adenine phosphoribosyltransferase) were used as a model to study in vivo loss of heterozygosity (LOH) in normal fibroblasts. Somatic cell variants that exhibited functional loss of the wild-type Aprt in vivo were recovered as APRT-deficient cell colonies after culturing in selection medium containing 2, 6-diaminopurine (DAP), an adenine analog that is toxic only to cells with APRT enzyme activity. DAP-resistant (DAP(r)) fibroblast variants were recovered at a median frequency of 12 x 10(-5) from individual ears from progeny of crosses between mouse strains 129/Sv and C3H/HeJ. The frequency of DAP(r) variants varied greatly among individual ears, suggesting that they preexisted in vivo and arose at various times during development. Polymorphic molecular markers and a cytological marker on the centromere of chromosome 8 made it possible to discriminate between each of six possible mechanistic pathways of LOH. The majority (about 80%) of the DAP(r) variants were a consequence of mitotic recombination. The prevalence of mitotic recombination in regions proximal to Aprt did not correlate with meiotic map distances. In particular, there was a higher than expected frequency of crossovers within the interval 59 cM to 67 cM. The high spontaneous frequency of Aprt LOH, mediated primarily by mitotic recombination, is fully consistent with our previous results with human peripheral T cells from individuals known to be heterozygous at APRT. Thus, this Aprt heterozygote mouse is a valid model for studying somatic mutagenesis and mitotic recombination in vivo.

The leptotene-zygotene transition of meiosis

The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.

Germ cell-specific enhancer activity of a repeated element in a variable region of the mouse genome

We recently described a complex genetic structure on mouse chromosome 8, a region of the murine genome in which genetic rearrangements frequently occur. A large repeated element specific to this chromosome was found to overlap with one of the cadherin genes (Cad11). An additional degree of complexity became apparent with the identification, in a subset of laboratory strains of mice, of a retrogene integrated into one of the repeated units. Designated Sycp1-ps2, it originated from the early meiotic gene encoding Synaptonemal Complex Protein 1. We now report that, among wild Mus species in which the retrogene is not present, this region of Chr 8 shows a high degree of variability. Sequence analysis showed that integration of Sycp1-ps2 created a 5' transcription initiator element. Transcription of the pseudogene in the testis was directly demonstrated. A germ cell-specific enhancer activity was localized within a 1117 bp region of the repeat, which was sufficient to direct the expression of reporter genes in transgenic mice to late meiotic and post-meiotic spermatogenic cells.

The preference for GT-rich DNA by the yeast Rad51 protein defines a set of universal pairing sequences

The Rad51 protein of Saccharomyces cerevisiae is a eukaryotic homolog of the RecA protein, the prototypic DNA strand-exchange protein of Escherichia coli. RAD51 gene function is required for efficient genetic recombination and for DNA double-strand break repair. Recently, we demonstrated that RecA protein has a preferential affinity for GT-rich DNA sequences-several of which exhibit enhanced RecA protein-promoted homologous pairing activity. The fundamental similarity between the RecA and Rad51 proteins suggests that Rad51 might display an analogous bias. Using in vitro selection, here we show that the yeast Rad51 protein shares the same preference for GT-rich sequences as its prokaryotic counterpart. This bias is also manifest as an increased ability of Rad51 protein to promote the invasion of supercoiled DNA by homologous GT-rich single-stranded DNA, an activity not previously described for the eukaryotic pairing protein. We propose that the preferred utilization of GT-rich sequences is a conserved feature among all homologs of RecA protein, and that GT-rich regions are loci for increased genetic exchange in both prokaryotes and eukaryotes.

Emerin deletion reveals a common X-chromosome inversion mediated by inverted repeats

Emery-Dreifuss muscular dystrophy (EMD) is an X-linked disorder characterized by contractures, progressive muscle weakness and cardiomyopathy. The emerin gene, located in human Xq28, is approximately 2 kb in length, is composed of 6 exons and falls within a 219-kb region that has been completely sequenced. Immediately centromeric to emerin is the 26-kb filamin gene (FLN1), composed of 48 exons and encoding the actin-binding protein 280 (refs 7,8). Flanking this 48-kb FLN1/emerin region are two large inverted repeats, each 11.3 kb, that exhibit > 99% sequence identity. The high level of genomic detail in this region allowed us to characterize the first complete emerin gene deletion mutation that also involved a partial duplication of the nearby FLN1 gene. This rearrangement could be explained by mispairing of the large inverted repeats, followed by double recombination among one set of mispaired repeats and internal sequences. Furthermore, our characterization of this rare DNA rearrangement revealed a more common result of the mispairing of these large inverted repeats--recombination contained within the inverted repeats leading to the maintenance of repeat sequence homogeneity and inversion of the 48-kb FLN1/emerin region. The presence of this frequent inversion, found in the heterozygous state in 33% of females, helps to explain the discrepancies observed between the genetic and physical map distances in this region of the X chromosome. It also illustrates the biological insights which can be gleaned by sequencing the human genome.

FISH technology in chromosome and genome research

Fluorescent in situ hybridization technology is one of the most exciting and versatile research tools to be developed in recent years. It has enabled research to progress at a phenomenal rate in diverse areas of basic research as well as in clinical medicine. Fluorescent in situ hybridization has applications in physical mapping, the study of nuclear architecture and chromatin packaging, and the investigation of fundamental principles of biology such as DNA replication, RNA processing, gene amplification, gene integration and chromatin elimination. This review highlights some of these areas and provides source material for the reader who seeks more information on a specific field.

The involvement of telomeric sequences in chromosomal aberrations

Three functional elements are required for the stable transmission of eukaryotic chromosomes: replication origins, centromeres and telomeres. In the yeast Saccharomyces cerivisiae the DNA sequences defining each of these elements are known. The simplest and most widely conserved of these sequences is that of the telomere. As the name implies, the telomere is the end of a linear eukaryotic chromosome. Two of the main functions of the telomere are to prevent DNA loss as a consequence of replication and to prevent interactions with other chromosomal ends. Thus, telomeres play a major role in maintaining chromosome stability and consequently they have been considered as likely to be involved in some aspects of chromosomal aberration formation. The involvement of telomeric DNA sequences in stabilizing normal and broken chromosome ends, in "hot spots' for aberration formation and in delayed chromosomal instability will be reviewed here drawing on material presented at the Workshop and the published literature.

Telomeric repeats (TTAGGC)n are sufficient for chromosome capping function in Caenorhabditis elegans

Telomeres are specialized structures located at the ends of linear eukaryotic chromosomes that ensure their complete replication and protect them from fusion and degradation. We report here the characterization of the telomeres of the nematode Caenorhabditis elegans. We show that the chromosomes terminate in 4-9 kb of tandem repeats of the sequence TTAGGC. Furthermore, we have isolated clones corresponding to 11 of the 12 C. elegans telomeres. Their subtelomeric sequences are all different from each other, demonstrating that the terminal TTAGGC repeats are sufficient for general chromosomal capping functions. Finally, we demonstrate that the me8 meiotic mutant, which is defective in X chromosome crossing over and segregation, bears a terminal deficiency, that was healed by the addition of telomeric repeats, presumably by the activity of a telomerase enzyme. The 11 cloned telomeres represent an important advance for the completion of the physical map and for the determination of the entire sequence of the C. elegans genome.

The influence of a (GT)29 microsatellite sequence on homologous recombination in the hamster adenine phosphoribosyltransferase gene

Several DNA sequence elements are thought to stimulate homologous recombination, illegitimate recombination, or both in mammalian cells. Some are implicated by their recurrence around rearrangement breakpoints, others by their effects on recombination of extrachromosomal plasmids. None of these sequences, however, has been tested on the chromosome in a defined context. In this paper we show how the adenine phosphoribosyltransferase locus in CHO cells can be used to study the recombinogenic potential of defined DNA sequences. As an example we have measured the effect on homologous recombination of a dinucleotide repeat, (GT)29, which has been shown to stimulate homologous recombination in extrachromosomal vectors 3-20 fold. On the chromosome at the adenine phosphoribosyltransferase locus, however, this sequence shows no capacity to stimulate recombination or to influence the distribution of recombination events.

Chiasma-based genetic map of the mouse X chromosome

The X chromosome pair was identified in diakinesis/metaphase I stage mouse oocytes using a repeat sequence DNA probe and fluorescence in situ hybridisation. Chiasma positions along the X bivalent were measured in 57 oocytes from 4 females. Overall, our observations showed that while there were no obvious "hotspots" for chiasma formation along the X chromosome, there was a tendency to favour the distal end. Minimum inter-chiasma distances were substantial indicating the occurrence of strong genetic interference. Estimates of both genetic distances and recombination fractions for any interval along the chromosome can be calculated from the chiasma data. The average chiasma frequency for the X bivalent was 1.37 giving an estimated total genetic map length of 68.5 cM. In general, the pattern of chiasma distribution along the X chromosome resembled that anticipated from recombination distances in published consensus linkage maps. There were, however, some intriguing differences between the two approaches. The reason for these discrepancies are unknown but may be related to lack of precision in cytogenetic mapping of loci, inter-strain and/or interspecies differences in the genetic controls over the distribution of crossover events. One advantage of the chiasma analysis approach is its suitability for investigating these problems.

RAP1 stimulates single- to double-strand association of yeast telomeric DNA: implications for telomere-telomere interactions

Repressor Activator Protein 1 (RAP1) of Saccharomyces cerevisiae is an abundant nuclear protein implicated in telomere length maintenance, transactivation, and in the establishment of silent chromatin domains. The RAP1 binding site 5' of the yeast HIS4 gene is also a region of hyperrecombination in meiosis. We report here that as RAP1 binds its recognition consensus, it appears to untwist double-stranded DNA, which we detect as the introduction of a negative supercoil in circularization assays. Coincident with the RAP1-dependent untwisting, we observe stimulation of the association of a single-stranded yeast telomeric sequence with its homologous double-stranded sequence in a supercoiled plasmid. This unusual distortion of the DNA double helix by RAP1 may contribute to the RAP1-dependent enhancement of recombination rates and promote non-duplex strand interactions at telomeres.

Telomeric repeat sequences

Chromosomes not only carry transcribed genes and their regulatory DNA sequences, but also contain regions that are required for the stability and maintenance of the chromosome as a unit. These include centromeres, telomeres and origins of replication. It is clear for replication origins and centromeres that the positions of these chromosomal organelles are determined by sites of the appropriate DNA sequences, but also that functional performance requires one or more contributing proteins. Telomeres are also structurally complex, with one or more DNA components, including simple telomeric repeats and more complex telomere-associated sequences, as well as one or more specific proteins that recognize these sequences. Accumulating evidence suggests that the simple telomeric repeats are required in most, but not all species, although they are not sufficient to determine the chromosomal position of a telomere.

Role of mouse germ-cell mutagenesis in understanding genetic risk and in generating mutations that are prime tools for studies in modern biology

Highlights are presented on (1) the role mouse germ-cell mutagenesis has played in assessing the genetic harm from radiations and chemicals, and (2) the contributions to the field of modern biology that are being made by the products of this research--the propagated mutations. Among the numerous findings in radiation mutagenesis were the humped dose-effect curve for spermatogonial stem cells, the major differences between the sexes and between germ-cell stages of each sex in both yield and nature of mutations, the dose-rate effect, which provided the first evidence for repair of mutational (or premutational) damage, the augmenting effect of certain regimes of dose fractionation, and many others. Chemical mutagenesis studies that followed revealed at least three patterns of mutation yield and demonstrated that germ-cell stage--much more than the nature of the chemical--governs the nature of the DNA lesions induced. Two "supermutagens," one for intragenic mutations and one for deletions and other rearrangements, have become very useful in the manufacture of mutations for specific purposes. The mutations propagated from radiation- and chemical-mutagenesis experiments are providing prime resources for basic studies in genome organization, gene structure, and function. DNA lesions that involve specific loci have made possible increasingly detailed characterization of extensive deletion complexes that facilitate high-intensity physical and functional mapping within them. Numerous loci associated with interesting developmental anomalies have been identified and have become accessible to positional cloning. Several of the genes accessed with the aid of induced mutations (deletions, other rearrangements, and point mutations) are furnishing prime reagents for elucidating human disease conditions.