Effect of meiotic recombination on the production of aneuploid gametes in humans

Within the last decade, aberrant meiotic recombination has been confirmed as a molecular risk factor for chromosome nondisjunction in humans. Recombination tethers homologous chromosomes, linking and guiding them through proper segregation at meiosis I. In model organisms, mutations that disturb the recombination pathway increase the frequency of chromosome malsegregation and alterations in both the amount and placement of meiotic recombination are associated with nondisjunction. This association has been established for humans as well. Significant alterations in recombination have been found for all meiosis I-derived trisomies studied to date and a subset of so called "meiosis II" trisomy. Often exchange levels are reduced in a subset of cases where the nondisjoining chromosome fails to undergo recombination. For other trisomies, the placement of meiotic recombination has been altered. It appears that recombination too near the centromere or too far from the centromere imparts an increased risk for nondisjunction. Recent evidence from trisomy 21 also suggests an association may exist between recombination and maternal age, the most widely identified risk factor for aneuploidy. Among cases of maternal meiosis I-derived trisomy 21, increasing maternal age is associated with a decreasing frequency of recombination in the susceptible pericentromeric and telomeric regions. It is likely that multiple risk factors lead to nondisjunction, some age dependent and others age independent, some that act globally and others that are chromosome specific. Future studies are expected to shed new light on the timing and placement of recombination, providing additional clues to the link between altered recombination and chromosome nondisjunction.     

Association between maternal age and meiotic recombination for trisomy 21

Altered genetic recombination has been identified as the first molecular correlate of chromosome nondisjunction in both humans and model organisms. Little evidence has emerged to link maternal age--long recognized as the primary risk factor for nondisjunction--with altered recombination, although some studies have provided hints of such a relationship. To determine whether an association does exist, chromosome 21 recombination patterns were examined in 400 trisomy 21 cases of maternal meiosis I origin, grouped by maternal age. These recombination patterns were used to predict the chromosome 21 exchange patterns established during meiosis I. There was no statistically significant association between age and overall rate of exchange. The placement of meiotic exchange, however, differed significantly among the age groups. Susceptible patterns (pericentromeric and telomeric exchanges) accounted for 34% of all exchanges among the youngest class of women but only 10% of those among the oldest class. The pattern of exchanges among the oldest age group mimicked the pattern observed among normally disjoining chromosomes 21. These results suggest that the greatest risk factor for nondisjunction among younger women is the presence of a susceptible exchange pattern. We hypothesize that environmental and age-related insults accumulate in the ovary as a woman ages, leading to malsegregation of oocytes with stable exchange patterns. It is this risk, due to recombination-independent factors, that would be most influenced by increasing age, leading to the observed maternal age effect.     

Nondisjunction in trisomy 21: origin and mechanisms

Chromosomal aneuploidy is a fundamental characteristic of the human species. In this review we summarize the knowledge about the origin and mechanisms of nondisjunction in human trisomy 21 that has accumulated during the last decade by using DNA polymorphism analysis. The first molecular correlate of nondisjunction in humans is altered recombination, meiosis I errors being associated with reduced recombination and maternal meiosis II errors with increased recombination between the nondisjoined chromosomes. Thus, virtually all maternal meiotic errors of chromosome 21 seem to be initiated in meiosis I. Advanced maternal age remains the only well documented risk factor for maternal meiotic nondisjunction, but there is, however, still a surprising lack of understanding of the basic mechanisms behind the maternal age effect.     

Altered patterns of multiple recombinant events are associated with nondisjunction of chromosome 21

We have previously examined characteristics of maternal chromosomes 21 that exhibited a single recombination on 21q and proposed that certain recombination configurations are risk factors for either meiosis I (MI) or meiosis II (MII) nondisjunction. The primary goal of this analysis was to examine characteristics of maternal chromosomes 21 that exhibited multiple recombinant events on 21q to determine whether additional risk factors or mechanisms are suggested. In order to identify the origin (maternal or paternal) and stage (MI or MII) of the meiotic errors, as well as placement of recombination, we genotyped over 1,500 SNPs on 21q. Our analyses included 785 maternal MI errors, 87 of which exhibited two recombinations on 21q, and 283 maternal MII errors, 81 of which exhibited two recombinations on 21q. Among MI cases, the average location of the distal recombination was proximal to that of normally segregating chromosomes 21 (35.28 vs. 38.86 Mb), a different pattern than that seen for single events and one that suggests an association with genomic features. For MII errors, the most proximal recombination was closer to the centromere than that on normally segregating chromosomes 21 and this proximity was associated with increasing maternal age. This pattern is same as that seen among MII errors that exhibit only one recombination. These findings are important as they help us better understand mechanisms that may underlie both age-related and nonage-related meiotic chromosome mal-segregation.     

The meiotic stage of nondisjunction in trisomy 21: Determination by using DNA polymorphisms

We have studied DNA polymorphisms at loci in the pericentromeric region on the long arm of chromosome 21 in 200 families with trisomy 21, in order to determine the meiotic origin of nondisjunction. Maintenance of heterozygosity for parental markers in the individual with trisomy 21 was interpreted as resulting from a meiosis I error, while reduction to homozygosity was attributed to a meiosis II error. Nondisjunction was paternal in 9 cases and was maternal in 188 cases, as reported earlier. Among the 188 maternal cases, nondisjunction occurred in meiosis I in 128 cases and in meiosis II in 38 cases; in 22 cases the DNA markers used were uninformative. Therefore meiosis I was responsible for 77.1% and meiosis II for 22.9% of maternal nondisjunction. Among the 9 paternal nondisjunction cases the error occurred in meiosis I in 2 cases (22.2%) and in meiosis II in 7 (77.8%) cases. Since there was no significant difference in the distribution of maternal ages between maternal I error versus maternal II error, it is unlikely that an error at a particular of maternal ages between maternal I error versus maternal II error, it is unlikely that an error at a particular meiotic stage contributes significantly to the increasing incidence of Down syndrome with advancing maternal age. Although the DNA polymorphisms used were at loci which map close to the centromere, it is likely that rare errors in meiotic-origin assignments may have occurred because of a small number of crossovers between the markers and the centromere.(ABSTRACT TRUNCATED AT 250 WORDS)     

Centromere mapping functions for aneuploid meiotic products: Analysis of rec8, rec10 and rec11 mutants of the fission yeast Schizosaccharomyces pombe.

Recent evidence suggests that the position of reciprocal recombination events (crossovers) is important for the segregation of homologous chromosomes during meiosis I and sister chromatids during meiosis II. We developed genetic mapping functions that permit the simultaneous analysis of centromere-proximal crossover recombination and the type of segregation error leading to aneuploidy. The mapping functions were tested in a study of the rec8, rec10, and rec11 mutants of fission yeast. In each mutant we monitored each of the three chromosome pairs. Between 38 and 100% of the chromosome segregation errors in the rec8 mutants were due to meiosis I nondisjunction of homologous chromosomes. The remaining segregation errors were likely the result of precocious separation of sister chromatids, a previously described defect in the rec8 mutants. Between 47 and 100% of segregation errors in the rec10 and rec11 mutants were due to nondisjunction of sister chromatids during meiosis II. In addition, centromere-proximal recombination was reduced as much as 14-fold or more on chromosomes that had experienced nondisjunction. These results demonstrate the utility of the new mapping functions and support models in which sister chromatid cohesion and crossover position are important determinants for proper chromosome segregation in each meiotic division.     

Reduced recombination and paternal age effect in Klinefelter syndrome

Summary The parental origin of the additional sex chromosome was studied in 47 cases with an XXY sex chromosome consitution. In 23 cases (49%), the error occurred during the first paternal meiotic division. Maternal origin of the additional chromosome was found in the remaining 24 cases (51%). Centromeric homo- versus heterozygosity could be determined in 18 out of the 24 maternally derived cases. According to the centromeric status and recombination rate, the nondisjunction was attributable in 9 cases (50%) to an error at the first maternal meiotic division, in 7 cases (39%) to an error at the second maternal meiotic division and in 2 cases (11%) to a nullo-chiasmata nondisjunction at meiosis II or to postzygotic mitotic error. No recombination, and in particular none in the pericentromeric region, was found in any of the 9 cases due to nondisjunction at the first maternal meiotic division. Significantly increased paternal age was found in the paternally derived cases. Maternal age was significantly higher in the maternally derived cases due to a meiotic I error compared with those due to a meiotic II error. There were no significant clinical differences between patients with respect to the origin of the additional X chromosome.     

Chromosomal and cytoplasmic context determines predisposition to maternal age-related aneuploidy: brief overview and update on MCAK in mammalian oocytes

It has been known for more than half a century that the risk of conceiving a child with trisomy increases with advanced maternal age. However, the origin of the high susceptibility to nondisjunction of whole chromosomes and precocious separation of sister chromatids, leading to aneuploidy in aged oocytes and embryos derived from them, cannot be traced back to a single disturbance and mechanism. Instead, analysis of recombination patterns of meiotic chromosomes of spread oocytes from embryonal ovary, and of origins and exchange patterns of extra chromosomes in trisomies, as well as morphological and molecular studies of oocytes and somatic cells from young and aged females, show chromosome-specific risk patterns and cellular aberrations related to the chronological age of the female. In addition, analysis of the function of meiotic- and cell-cycle-regulating genes in oogenesis, and the study of the spindle and chromosomal status of maturing oocytes, suggest that several events contribute synergistically to errors in chromosome segregation in aged oocytes in a chromosome-specific fashion. For instance, loss of cohesion may differentially predispose chromosomes with distal or pericentromeric chiasmata to nondisjunction. Studies on expression in young and aged oocytes from human or model organisms, like the mouse, indicate that the presence and functionality/activity of gene products involved in cell-cycle regulation, spindle formation and organelle integrity may be altered in aged oocytes, thus contributing to a high risk of error in chromosome segregation in meiosis I and II. Genes that are often altered in aged mouse oocytes include MCAK (mitotic-centromere-associated protein), a microtubule depolymerase, and AURKB (Aurora kinase B), a protein of the chromosomal passenger complex that has many targets and can also phosphorylate and regulate MCAK localization and activity. Therefore we explored the role of MCAK in maturing mouse oocytes by immunofluorescence, overexpression of a MCAK-EGFP (enhanced green fluorescent protein) fusion protein, knockdown of MCAK by RNAi (RNA interference) and inhibition of AURKB. The observations suggest that MCAK is involved in spindle regulation, chromosome congression and cell-cycle control, and that reductions in mRNA and protein in a context of permissive SAC (spindle assembly checkpoint) predispose to aneuploidy. Failure to recruit MCAK to centromeres and low expression patterns, as well as disturbances in regulation of enzyme localization and activity, e.g. due to alterations in activity of AURKB, may therefore contribute to maternal age-related rises in aneuploidy in mammalian oocytes.     

Trisomy 21 (Down syndrome): studying nondisjunction and meiotic recombination by using cytogenetic and molecular polymorphisms that span chromosome 21.

By combining molecular and cytogenetic techniques, we demonstrated the feasibility and desirability of a comprehensive approach to analysis of nondisjunction for chromosome 21. We analyzed the parental origin and stage of meiotic errors resulting in trisomy 21 in each of five families by successfully using cytogenetic heteromorphisms and DNA polymorphisms. The 16 DNA fragments used to detect polymorphisms spanned the length of the long arm and detected recombinational events on nondisjoined chromosomes in both maternal meiosis I and maternal meiosis II errors. The meiotic stage at which errors occurred was determined by sandwiching the centromere between cytogenetic heteromorphisms on 21p and an informative haplotype constructed using two polymorphic DNA probes that map to 21q just below the centromere. This study illustrates the necessity of combining cytogenetic polymorphisms on 21p with DNA polymorphisms spanning 21q to determine (1) the source and stage of meiotic errors that lead to trisomy 21 and (2) whether an association exists between nondisjunction and meiotic recombination.     

Coincident maternal meiotic nondisjunction of chromosomes X and 21 without evidence of autosomal aysnapsis

Summary A family in which the proband showed phenotypic signs of both the Turner and Down syndromes was studied cytogenetically and with restriction fragment length polymorphisms. The proband's karyotype was 46,X,+21, showing double aneuploidy without any signs of mosaicism. The single X and one chromosome 21 were of paternal origin while two chromosomes 21 were of maternal origin. The nondisjunction of chromosome 21 took place in maternal meiosis II. If it is assumed that the absence of mosaicism renders postzygotic mitotic loss of the X chromosome unlikely, then the X chromosome would have been lost in maternal meiosis I or II. Recombination had occurred between the nondisjoined chromosomes 21. We conclude that double nondisjunction took place in one parent and that asynapsis was not a prerequisite for the autosomal nondisjunction.     

Trisomy of human chromosome 18: Molecular studies on parental origin and cell stage of nondisjunction

We investigated the parent and cell division of origin of the extra chromosome 18 in 62 aneuploids with a free trisomy 18 by using chromosome-18-specific pericentromeric short-sequence repeats. In 46 cases, DNA of patients was recovered from archival specimens, such as paraffin-embedded tissues and fixed chromosomal spreads. In 56 families, the supernumerary chromosome was maternal in origin; in six families, it was paternal. Among the 56 maternally derived aneuploids, we could exclude a postzygotic mitotic error in 52 cases. Among those in which the nondisjunction was attributable to an error at meiosis, 11 were the result of a meiosis I nondisjunction and 17 were caused by a meiosis II error. This result differs markedly from findings in acrocentric chromosomes where nondisjunction at maternal meiosis I predominates. Among the six paternally derived cases, two originated from a meiotic error, indicating that a nondisjunction in paternal meiosis is not as rare as previously suggested.Dedicated to Professor Dr. W. Gottschalk on the occasion of his 75th birthday     

Maternal age and risk for trisomy 21 assessed by the origin of chromosome nondisjunction: a report from the Atlanta and National Down Syndrome Projects

We examined the association between maternal age and chromosome 21 nondisjunction by origin of the meiotic error. We analyzed data from two population-based, case–control studies: Atlanta Down Syndrome Project (1989–1999) and National Down Syndrome Project (2001–2004). Cases were live born infants with trisomy 21 and controls were infants without trisomy 21 delivered in the same geographical regions. We enrolled 1,215 of 1,881 eligible case families and 1,375 of 2,293 controls. We report four primary findings. First, the significant association between advanced maternal age and chromosome 21 nondisjunction was restricted to meiotic errors in the egg; the association was not observed in sperm or in post-zygotic mitotic errors. Second, advanced maternal age was significantly associated with both meiosis I (MI) and meiosis II (MII). For example, compared to mothers of controls, mothers of infants with trisomy 21 due to MI nondisjunction were 8.5 times more likely to be ≥40 years old than 20–24 years old at the birth of the index case (95% CI = 5.6–12.9). Where nondisjunction occurred in MII, mothers were 15.1 times more likely to be ≥40 years (95% CI = 8.4–27.3). Third, the ratio of MI to MII errors differed by maternal age. The ratio was lower among women <19 years of age and those ≥40 years (2.1, 2.3, respectively) and higher in the middle age group (3.6). Lastly, we found no effect of grand-maternal age on the risk for maternal nondisjunction. This study emphasizes the complex association between advanced maternal age and nondisjunction of chromosome 21 during oogenesis. The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.     

Risk factors for nondisjunction of trisomy 21

The leading cause of Down syndrome (DS) is nondisjunction of chromosome 21 occurring during the formation of gametes. In this review, we discuss the progress made to identify risk factors associated with this type of chromosome error occurring in oogenesis and spermatogenesis. For errors occurring in oocytes, the primary risk factors are maternal age and altered recombination. We review the current progress made with respect to these factors and briefly outline the potential environmental and genetic influences that may play a role. Although the studies of paternal nondisjunction are limited due to the relatively small proportion of errors of this type, we review the potential influence of paternal age, recombination and other environmental and genetic factors on susceptibility. Although progress has been made to understand the mechanisms and risk factors that underlie nondisjunction, considerably more research needs to be conducted to dissect this multifactorial trait, one that has a considerable impact on our species.     

Cis-Acting Determinants Affecting Centromere Function, Sister-Chromatid Cohesion and Reciprocal Recombination during Meiosis in Saccharomyces Cerevisiae

We have employed a system that utilizes homologous pairs of human DNA-derived yeast artificial chromosomes (YACs) as marker chromosomes to assess the specific role (s) of conserved centromere DNA elements (CDEI, CDEII and CDEIII) in meiotic chromosome disjunction fidelity. Thirteen different centromere (CEN) mutations were tested for their effects on meiotic centromere function. YACs containing a wild-type CEN DNA sequence segregate with high fidelity in meiosis I (99% normal segregation) and in meiosis II (96% normal segregation). YACs containing a 31-bp deletion mutation in centromere DNA element II (CDEIIδ31) in either a heterocentric (mutant/wild type), homocentric (mutant/mutant) or monosomic (mutant/--) YAC pair configuration exhibited high levels (16-28%) of precocious sister-chromatid segregation (PSS) and increased levels (1-6%) of nondisjunction meiosis I (NDI). YACs containing this mutation also exhibit high levels (21%) of meiosis II nondisjunction. Interestingly, significant alterations in homolog recombination frequency were observed in the exceptional PSS class of tetrads, suggesting unusual interactions between prematurely separated sister chromatids and their homologous nonsister chromatids. We also have assessed the meiotic segregation effects of rare gene conversion events occurring at sites located immediately adjacent to or distantly from the centromere region. Proximal gene conversion events were associated with extremely high levels (60%) of meiosis I segregation errors (including both PSS and NDI), whereas distal events had no apparent effect. Taken together, our results indicate a critical role for CDEII in meiosis and underscore the importance of maintaining sister-chromatid cohesion for proper recombination in meiotic prophase and for proper disjunction in meiosis I.     

The impact of imprinting: Prader-Willi syndrome resulting from chromosome translocation, recombination, and nondisjunction.

Prader-Willi syndrome (PWS) is most often the result of a deletion of bands q11.2-q13 of the paternally derived chromosome 15, but it also occurs either because of maternal uniparental disomy (UPD) of this region or, rarely, from a methylation imprinting defect. A significant number of cases are due to structural rearrangements of the pericentromeric region of chromosome 15. We report two cases of PWS with UPD in which there was a meiosis I nondisjunction error involving an altered chromosome 15 produced by both a translocation event between the heteromorphic satellite regions of chromosomes 14 and 15 and recombination. In both cases, high-resolution banding of the long arm was normal, and FISH of probes D15S11, SNRPN, D15S10, and GABRB3 indicated no loss of this material. Chromosome heteromorphism analysis showed that each patient had maternal heterodisomy of the chromosome 15 short arm, whereas PCR of microsatellites demonstrated allele-specific maternal isodisomy and heterodisomy of the long arm. SNRPN gene methylation analysis revealed only a maternal imprint in both patients. We suggest that the chromosome structural rearrangements, combined with recombination in these patients, disrupted normal segregation of an imprinted region, resulting in uniparental disomy and PWS.     

Obligate short-arm exchange in de novo Robertsonian translocation formation influences placement of crossovers in chromosome 21 nondisjunction

Robertsonian translocations (ROBs) involving chromosome 21 are found in approximately 5% of patients with Down syndrome (DS). The most common nonhomologous ROB in DS is rob(14q21q). Aberrant recombination is associated with nondisjunction (NDJ) leading to trisomy 21. Haplotype analysis of 23 patients with DS and de novo rob(14q21q) showed that all translocations and all nondisjoined chromosomes 21 were maternally derived. Meiosis II NDJ occurred in 21 of 23 families. For these, a ROB DS chromosome 21 genetic map was constructed and compared to a normal female map and a published trisomy 21 map derived from meiosis II NDJ. The location of exchanges differed significantly from both maps, with a significant shift to a more distal interval in the ROB DS map. The shift may perturb segregation, leading to the meiosis II NDJ in this study, and is further evidence for crossover interference. More importantly, because the event in the short arms that forms the de novo ROB influences the placement of chiasmata in the long arm, it is most likely that the translocation formation occurs through a recombination pathway in meiosis. Additionally, we have demonstrated that events that occur in meiosis I can influence events, such as chromatid segregation in meiosis II, many decades later.     

Variation in alphoid DNA size and trisomy 21: a possible cause of nondisjunction

The cause of nondisjunction of chromosome 21 remains largely unknown. In the present report, we investigate the hypothesis that variation in alphoid DNA size has a role in trisomy formation. Pulsed-field gel electrophoresis was used to examine the chromosome 21 alphoid DNA array lengths in 23 families (all of Northern European ancestry) with an affected child with trisomy 21 in whom the parental and meiotic origin of nondisjunction had been determined as maternal meiosis I, and in 38 controls. Initially, the combined alphoid size of both chromosome 21 homologues was assessed. This indicated an association between small combined alphoid size and maternal meiosis I nondisjunction. Moreover, in a subset of the families under study (n=12), it was possible to study the alpha21-I size of individual chromosome 21 homologues (simple alphoid size); this provided further evidence that the risk for nondisjunction is related to the size of the alphoid array of one of the two chromosome 21 homologues being small.     

Telomere length is associated with types of chromosome 21 nondisjunction: a new insight into the maternal age effect on Down syndrome birth

Advanced maternal age is a well-documented risk factor of chromosome 21 nondisjunction in humans, but understanding of this association at the genetic level is still limited. In particular, the state of maternal genetic age is unclear. In the present study, we estimated maternal genetic age by measuring telomere length of peripheral blood lymphocytes among age-matched mothers of children with Down syndrome (cases: N = 75) and mothers of euploid children (controls: N = 75) in an age range of 18–42 years. All blood samples were taken within 1 week of the birth of the child in both cases and controls. The telomere length estimation was performed by restriction digestion—Southern blot hybridization method. We stratified the cases on the basis of centromeric STR genotyping into maternal meiosis I (N = 48) and maternal meiosis II (N = 27) nondisjunction groups and used linear regression to compare telomere length as a function of age in the euploid, meiosis I and meiosis II groups. Our results show that all three groups have similar telomere length on average for younger mothers. As age increases, all groups show telomere loss, but that loss is largest in the meiosis II mother group and smallest in the euploid mother group with the meiosis I mother group in the middle. The regression lines for all three were statistically significantly different from each other (p < 0.001). Our results do not support the theory that younger women who have babies with Down syndrome do so because are ‘genetically older’ than their chronological age, but we provide the first evidence that older mothers who have babies with Down syndrome are “genetically older” than controls, who have euploid babies at the same age. We also show for the first time that telomere length attrition may be associated in some way with meiosis I and meiosis II nondisjunction of chromosome 21 and subsequent Down syndrome births at advanced maternal age.     

Genetics and biology of human ovarian teratomas. II. Molecular analysis of origin of nondisjunction and gene-centromere mapping of chromosome I markers.

Chromosomal heteromorphisms and DNA polymorphisms have been utilized to identify the mechanisms that lead to formation of human ovarian teratomas and to construct a gene-centromere map of chromosome 1 by using those teratomas that arise by meiotic nondisjunction. Of 61 genetically informative ovarian teratomas, 21.3% arose by nondisjunction at meiosis I, and 39.3% arose by meiosis II nondisjunction. Eight polymorphic marker loci on chromosome 1p and one marker on 1q were used to estimate a gene-centromere map. The results show clear linkage of the most proximal 1p marker (NRAS) and the most proximal 1q marker (D1S61) to the centromere at a distance of 14 cM and 20 cM, respectively. Estimated gene-centromere distances suggest that, while recombination occurs normally in ovarian teratomas arising by meiosis II errors, ovarian teratomas arising by meiosis I nondisjunction have altered patterns of recombination. Furthermore, the estimated map demonstrates clear evidence of chiasma interference. Our results suggest that ovarian teratomas can provide a rapid method for mapping genes relative to the centromere.