New insights into human nondisjunction of chromosome 21 in oocytes

Nondisjunction of chromosome 21 is the leading cause of Down syndrome. Two risk factors for maternal nondisjunction of chromosome 21 are increased maternal age and altered recombination. In order to provide further insight on mechanisms underlying nondisjunction, we examined the association between these two well established risk factors for chromosome 21 nondisjunction. In our approach, short tandem repeat markers along chromosome 21 were genotyped in DNA collected from individuals with free trisomy 21 and their parents. This information was used to determine the origin of the nondisjunction error and the maternal recombination profile. We analyzed 615 maternal meiosis I and 253 maternal meiosis II cases stratified by maternal age. The examination of meiosis II errors, the first of its type, suggests that the presence of a single exchange within the pericentromeric region of 21q interacts with maternal age-related risk factors. This observation could be explained in two general ways: 1) a pericentromeric exchange initiates or exacerbates the susceptibility to maternal age risk factors or 2) a pericentromeric exchange protects the bivalent against age-related risk factors allowing proper segregation of homologues at meiosis I, but not segregation of sisters at meiosis II. In contrast, analysis of maternal meiosis I errors indicates that a single telomeric exchange imposes the same risk for nondisjunction, irrespective of the age of the oocyte. Our results emphasize the fact that human nondisjunction is a multifactorial trait that must be dissected into its component parts to identify specific associated risk factors.     

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.     

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.     

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.     

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.     

The parental origin of the extra X chromosome in 47,XXX females.

We used X-linked DNA polymorphisms to study the parental origin of X chromosome nondisjunction in 28 47,XXX live-born females. Errors in oogenesis accounted for 26 of the cases, with the majority of these being attributable to an error at meiosis I. We observed an association between advanced parental age and meiosis I nondisjunction--but not meiosis II nondisjunction--in the maternally derived cases. In studies of recombination we found little evidence for an association between pairing failure and X chromosome nondisjunction, but our results suggest that increased recombination near the centromere may play a role in the etiology of the 47,XXX condition.     

Nondisjunction of human acrocentric chromosomes: studies of 432 trisomic fetuses and liveborns

The present report summarizes molecular studies on the parent and meiotic stage of origin of the additional chromosome in 432 fetuses or liveborns with an additional chromosome 13, 14, 15, 21, or 22. Our studies suggest that there is little variation in the origin of nondisjunction among the five acrocentric trisomies and that there is no association between the origin of nondisjunction and the likelihood of survival to term of the trisomic conceptus. The proportion of cases of paternal origin was similar among the five trisomies: 12% for trisomy 13, 17% for trisomy 14, 12% for trisomy 15, 9% for trisomy 21, and 11% for trisomy 22. The stage of nondisjunction was also similar among the five trisomies, with the majority of cases of maternal origin being due to nondisjunction at meiosis I, whereas for paternally derived cases, nondisjuction occurred primarily at meiosis II.     

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.     

Recombination and maternal age-dependent nondisjunction: molecular studies of trisomy 16.

Trisomy 16 is the most common human trisomy, occurring in > or = 1% of all clinically recognized pregnancies. It is thought to be completely dependent on maternal age and thus provides a useful model for studying the association of increasing maternal age and nondisjunction. We have been conducting a study to determine the parent and meiotic stage of origin of trisomy 16 and the possible association of nondisjunction and aberrant recombination. In the present report, we summarize our observations on 62 spontaneous abortions with trisomy 16. All trisomies were maternally derived, and in virtually all the error occurred at meiosis I. In studies of genetic recombination, we observed a highly significant reduction in recombination in the trisomy-generating meioses by comparison with normal female meioses. However, most cases of trisomy 16 had at least one detectable crossover between the nondisjoined chromosomes, indicating that it is reduced--and not absent--recombination that is the important predisposing factor. Additionally, our data indicate an altered distribution of crossing-over in trisomy 16, as we rarely observed crossovers in the proximal long and short arms. Thus, it may be that, at least for trisomy 16, the association between maternal age and trisomy is due to diminished recombination, particularly in the proximal regions of the chromosome.     

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.     

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)     

Altered LINE-1 Methylation in Mothers of Children with Down Syndrome

Down syndrome (DS, also known as trisomy 21) most often results from chromosomal nondisjunction during oogenesis. Numerous studies sustain a causal link between global DNA hypomethylation and genetic instability. It has been suggested that DNA hypomethylation might affect the structure and dynamics of chromatin regions that are critical for chromosome stability and segregation, thus favouring chromosomal nondisjunction during meiosis. Maternal global DNA hypomethylation has not yet been analyzed as a potential risk factor for chromosome 21 nondisjunction. This study aimed to asses the risk for DS in association with maternal global DNA methylation and the impact of endogenous and exogenous factors that reportedly influence DNA methylation status. Global DNA methylation was analyzed in peripheral blood lymphocytes by quantifying LINE-1 methylation using the MethyLight method. Levels of global DNA methylation were significantly lower among mothers of children with maternally derived trisomy 21 than among control mothers (P = 0.000). The combination of MTHFR C677T genotype and diet significantly influenced global DNA methylation (R² = 4.5%, P = 0.046). The lowest values of global DNA methylation were observed in mothers with MTHFR 677 CT+TT genotype and low dietary folate. Although our findings revealed an association between maternal global DNA hypomethylation and trisomy 21 of maternal origin, further progress and final conclusions regarding the role of global DNA methylation and the occurrence of trisomy 21 are facing major challenges.     

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.     

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.     

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.     

A Meiotic Mutant Affecting Recombination in Female DROSOPHILA MELANOGASTER

mei-S282 is a female meiotic mutant isolated from a natural population of Drosophila melanogaster. It is a recessive mutation located at approximately map position 5 on the third chromosome which has two major effects. It causes a nonuniform decrease in recombination which is most drastic in distal chromosome regions and nondisjunction of all chromosome pairs is elevated at the first meiotic division. Nondisjunctional events are positively correlated; furthermore, nondisjoining chromosomes, themselves nonrecombinant, are preferentially recovered from cells in which nonhomologs are preferentially recovered from cells in which nonhomologs are also non-recombinant.-It is concluded that mei-S282 is a defect which occurs early in meiosis I prior to the time of exchange. In the mutant, the frequency of no-exchange tetrads for each of the major chromosomes is increased-and in cells which contain two or more no-exchange tetrads, an interaction between these chromosomes leads to correlated nondisjunction. mei-S282(+) then, is an exchange precondition necessary for the normal frequency and distribution of exchanges.     

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.