Parental origin of the extra chromosome in prenatally diagnosed fetal trisomy 21

Trisomy 21 (Down syndrome) is one of the most common chromosomal abnormalities. Of cases of free trisomy 21 causing Down syndrome, about 95% result from nondisjunction during meiosis, and about 5% are due to mitotic errors in somatic cells. Previous studies using DNA polymorphisms of chromosome 21 showed that paternal origin of trisomy 21 occurred in only 6.7% of cases. However, these studies were conducted in liveborn trisomy 21-affected infants, and the possible impact of fetal death was not taken into account. Using nine distinct DNA polymorphisms, we tested 110 families with a prenatally diagnosed trisomy 21 fetus. Of the 102 informative cases, parental origin was maternal in 91 cases (89.2%) and paternal in 11 (10.8%). This percentage differs significantly from the 7.0% observed in previous studies (P<0.001). In order to test the influence of genomic parental imprinting, we determined the origin of the extra chromosome 21 in relation to different factors: advanced maternal age, maternal serum human chorionic gonadotropin (hormone of placental origin), severity of the disease, gestational age at diagnosis and fetal gender. We found that the increased frequency of paternal origin of nondisjunction in trisomy 21-affected fetuses cannot obviously be explained by factors leading to selective loss of paternal origin fetuses.     

Non-disjunction of chromosome 21, alphoid DNA variation, and sociogenetic features of Down syndrome

The analysis of non-disjunction of chromosome 21 and alphoid DNA variation by using cytogenetic and molecular cytogenetic techniques (quantitative fluorescence in situ hybridization) in 74 nuclear families was performed. The establishment of possible correlation between alphoid DNA variation, parental age, environmental effects, and non-disjunction of chromosome 21 was made. The efficiency of techniques applied was found to be 92% (68 from 74 cases). Maternal non-disjunction wasfound in 58 cases (86%) and paternal non-disjunction - in 7 cases (10%). Post-zygotic mitotic non-disjunction was determined in 2 cases (3%) and one case was associated with Robertsonian translocation 46,XX,der(21;21)(q10;q10), +21. Maternal meiosis I errors were found in 43 cases (64%) and maternal meiosis II errors--in 15 cases (22%). Paternal meiosis I errors occurred in 2 cases (3%) and paternal meiosis I errors--in 5 cases (7%). The lack of the correlation between alphoid DNA variation and non-disjunction of chromosome 21 was established. Sociogenetic analysis revealed the association of intensive drug therapy of infectious diseases during the periconceptual period and maternal meiotic non-disjunction of chromosome 21. The correlation between non-disjunction of chromosome 21 and increased parental age as well as exposure to irradiation, alcohol, tobacco, mutagenic substances was not found. The possible relevance of data obtained to the subsequent studies of chromosome 21 non-disjunction is discussed.     

Advanced maternal age and the risk of Down syndrome characterized by the meiotic stage of chromosomal error: a population-based study.

The identification of DNA polymorphisms makes it possible to classify trisomy 21 according to the parental origin and stage (meiosis I [MI], meiosis II [MII], or postzygotic mitotic) of the chromosomal error. Studying the effect of parental age on these subgroups could shed light on parental exposures and their timing. From 1989 through 1993, 170 infants with trisomy 21 and 267 randomly selected control infants were ascertained in a population-based, case-control study in metropolitan Atlanta. Blood samples for genetic studies were obtained from case infants and their parents. Using logistic regression, we independently examined the association between maternal and paternal age and subgroups of trisomy 21 defined by parental origin and meiotic stage. The distribution of trisomy 21 by origin was 86% maternal (75% MI and 25% MII), 9% paternal (50% MI and 50% MII), and 5% mitotic. Compared with women <25 years of age, women > or = 40 years old had an odds ratio of 5.2 (95% confidence interval, 1.0-27.4) for maternal MI (MMI) errors and 51.4 (95% confidence interval, 2.3-999.0) for maternal MII (MMII) errors. Birth-prevalence rates for women > or = 40 years old were 4.2/1000 births for MMI errors and 1.9/1000 for MMII errors. These results support an association between advanced maternal age and both MMI and MMII errors. The association with MI does not pinpoint the timing of the error; however, the association with MII implies that there is at least one maternal-age related mechanism acting around the time of conception.     

Trisomy: Chromosome competition or maternal strategy?: Increase of trisomy incidence with increasing maternal age does not result from competition between chromosomes

Axelrod and Hamilton (Science 211:1390, 1981) suggested that trisomies may result from an end-game strategy between chromosomes competing to get on the gamete as the mother approaches menopause. We tested this hypothesis by reviewing studies of the parental origin of the extra chromosome in trisomy 21 births. These data show that there is no significant rise in trisomy 21 conceptions as the mother ages. The increase in trisomies with maternal age results not from an increase in nondisjunctions, but from a decrease in rejection of trisomy zygotes, which may be adaptive for the mother towards the end of her reproductive life. This decreasing rate of rejection may result from the changing inclusive benefits of two maternal strategies as menopause approaches.     

Origin of trisomy 21 in Down syndrome cases from a Spanish population registry

We have carried out a population-based study on the origin of the extra chromosome 21 in 38 families with Down syndrome (DS) offspring in El Vallès (Spain). From 1991 to 1994, a higher prevalence of DS (22.7/10000 live births, stillbirths and induced abortions) was found compared to the majority of EUROCAT registries. The distribution of trisomy 21 by origin was 88% maternal (90.6% meiosis I, 6.2% meiosis II, 3.1% maternal mosaicism), 5.6% paternal (50% meiosis I, 50% meiosis II) and 5.6% mitotic. The percentage of parental mosaicism was 2.7%. These percentages are similar to those previously reported. Recombination study revealed a maternal meiosis I genetic map of 32.68 cM (approximately one-half the length of the normal female map). Mean maternal age among non-recombinant cases involving MI errors was significantly lower (31.1 years) than among those cases showing one observable crossover (36.1 years) (P<0.05); this could support the hypothesis that 'achiasmate' chromosomes may be subject to aberrant segregation regardless of maternal age.     

Parental origin and meiotic stage of non-disjunction in 139 cases of trisomy 21

The parental origin and the meiotic stage of non-disjunction have been determined in 139 Down syndrome patients with regular trisomy 21 and in their parents through the analysis of DNA polymorphism. The meiotic error is maternal in 91.60% cases and paternal in 8.39% of cases. Of the maternal cases, 72.41% were due to meiosis I errors (MMI) and 27.58% were due to meiosis II errors (MMII). Of the paternal cases, 45.45% were due to meiosis I (PMI) and 54.54% were due to meiosis II (PMII). The mean maternal ages were 31.6 +/- 5.3 (+/- SD) years in errors from MMI, 32.3 +/- 6.4 years in errors from MMII, 31.4 +/- 4.6 years in errors from PMI and 29.5 +/- 2.7 years in errors from PMII. No significant statistical differences were observed between maternal and paternal errors, further supporting the presence of a constant chromosome 21 non-disjunction error type.     

Origin of the extra chromosome in trisomy 18

Summary The parental origin of an extra chromosome in five patients with trisomy 18 was traced using a restriction fragment length polymorphism (RFLP) of the human prealbumin (PA) gene, localized to 18p11.1–q12.1, as a genetic marker. MspI digests of the genomic DNAs of the five patients, their parents and normal controls were hybridized with the PAcDNA. Densitometric analysis on the gene dose of the polymorphic fragments of these patients revealed that three had originated from a maternal meiotic error. The other two patients were uninformative for the parental origin of trisomy 18. Our results indicate that nondisjunctional errors leading to trisomy 18 may occur predominantly at the maternal meiosis, consistent with the results of previous studies on the parental origin of trisomies 21 and 13.     

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.     

Evidence of Selection against Complex Mitotic-Origin Aneuploidy during Preimplantation Development

Whole-chromosome imbalances affect over half of early human embryos and are the leading cause of pregnancy loss. While these errors frequently arise in oocyte meiosis, many such whole-chromosome abnormalities affecting cleavage-stage embryos are the result of chromosome missegregation occurring during the initial mitotic cell divisions. The first wave of zygotic genome activation at the 4–8 cell stage results in the arrest of a large proportion of embryos, the vast majority of which contain whole-chromosome abnormalities. Thus, the full spectrum of meiotic and mitotic errors can only be detected by sampling after the initial cell divisions, but prior to this selective filter. Here, we apply 24-chromosome preimplantation genetic screening (PGS) to 28,052 single-cell day-3 blastomere biopsies and 18,387 multi-cell day-5 trophectoderm biopsies from 6,366 in vitro fertilization (IVF) cycles. We precisely characterize the rates and patterns of whole-chromosome abnormalities at each developmental stage and distinguish errors of meiotic and mitotic origin without embryo disaggregation, based on informative chromosomal signatures. We show that mitotic errors frequently involve multiple chromosome losses that are not biased toward maternal or paternal homologs. This outcome is characteristic of spindle abnormalities and chaotic cell division detected in previous studies. In contrast to meiotic errors, our data also show that mitotic errors are not significantly associated with maternal age. PGS patients referred due to previous IVF failure had elevated rates of mitotic error, while patients referred due to recurrent pregnancy loss had elevated rates of meiotic error, controlling for maternal age. These results support the conclusion that mitotic error is the predominant mechanism contributing to pregnancy losses occurring prior to blastocyst formation. This high-resolution view of the full spectrum of whole-chromosome abnormalities affecting early embryos provides insight into the cytogenetic mechanisms underlying their formation and the consequences for human fertility.     

Cytogenetic and molecular analysis of sex-chromosome monosomy.

X chromosome- and Y chromosome-specific DNA probes were used to study different aspects of the genesis of sex-chromosome monosomy. Using X-linked RFLPs, we studied the parental origin of the single X chromosome in 35 spontaneously aborted and five live-born 45,X conceptions. We determined the origin in 35 cases; 28 had a maternal X (Xm) and seven had a paternal X (Xp). There was a correlation between parental origin and parental age, with the Xp category having a significantly reduced mean maternal age by comparison with the Xm group. Studies aimed at detecting mosaicism demonstrated the presence of a Y chromosome or a second X chromosome in three of 33 spontaneous abortions, a level of mosaicism much lower than that reported for live-born Turner syndrome individuals.     

Down syndrome rates and relaxed selection at older maternal ages.

Preferential survival in older mothers of fetuses with Down syndrome has been proposed as contributing to the maternal-age effect of this condition. If correct, this provocative hypothesis, which may be termed "relaxed selection," has major implications for approaches to prevention of Down syndrome live births in older women. Several predictions of this hypothesis are examined here by comparisons of parental ages among various populations. These revealed that: (1) mean maternal age of Down syndrome live births is slightly lower than that of Down syndrome spontaneous fetal deaths; (2) mean maternal age of those with mutant D/21 translocation Down syndrome is about the same as that of controls; (3) the ages of Down syndrome mothers who have Down syndrome live births is slightly lower than ages of Down syndrome mothers who have unaffected live births; and (4) in recent data on 47, +21 cases in which the extra chromosome 21 is of paternal origin, the mean maternal ages are 4-5 years lower than the maternal ages of cases of maternal origin (in contrast to earlier reports). All of these observations are contrary to the hypothesis that relaxed selection contributes significantly to the maternal-age association of Down syndrome. If there is any effect of relaxed selection, it is likely to be very weak and/or act primarily upon abortions that occur before recognition of pregnancy.     

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.     

Evidence implicating heterozygous deletion of chromosome 7 in the pathogenesis of familial leukemia associated with monosomy 7.

Complete or partial monosomy 7 is a recurring cytogenetic abnormality in acute myelogenous leukemia (AML) and myeloproliferative syndromes (MPS) and is particularly common in patients with Fanconi's anemia and in secondary AML. A familial form of monosomy 7 has been recognized in which two or more siblings develop MPS or AML before age 20. We tested the hypothesis that a recessive cancer susceptibility locus on chromosome 7 was important in the pathogenesis of leukemia in familial monosomy 7 by determining the parental origins of the chromosome 7 retained in the bone marrows of three pairs of affected siblings. We found no overlapping region where all three pairs retained DNA derived from the same paternal or maternal chromosome. These data suggest that inactivation of a single allele of a putative tumor-suppressor gene may be sufficient to contribute to leukemic transformation in familial monosomy 7.     

Unusual supernumerary chromosomes: types encountered in a referred population,and high incidence of associated maternal chromosome abnormalities

Summary In a 6-year period 128 patients with supernumerary autosomes were identified in our laboratory. The majority had primary trisomy, but 19 (15%) had extra, unusual chromosomes, not just a normal chromosome present in an extra copy. Of these, 18 were complex and did not resemble any one part of the standard chromosome complement. There was a preponderance of females among the 19 cases. Chromosome analysis of the parents in the 14 most recent cases revealed maternal chromosome abnormalities in 11 (79%). Of these 11, eight mothers had balanced reciprocal translocations; nondisjunction led to the smaller of their translocation chromosomes being passed on as the supernumerary chromosome in their offspring. Thus, nondisjunction of maternal translocations accounts for a major proportion of the unusual supernumerary chromosomes found by our laboratory. Advanced maternal age was noted in this group of mothers. Three mothers had supernumerary chromosomes themselves. We conclude that unusual supernumerary chromosomes (1) are not rare among patients referred for chromosome studies; (2) are generally not simple products of breakage; (3) are very frequently the result of malsegregation of a balanced maternal reciprocal translocation; and (4) are very difficult to characterize unless a balanced parental translocation is identified. Parental karyotypes should be obtained whenever a patient has an extra, unusual chromosome.     

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.     

Genomic imprinting of the human serotonin-receptor (HTR2) gene involved in development of retinoblastoma.

Epidemiological and genetic studies of retinoblastoma (RB) suggested that imprinted genes might be genetically linked to the RB gene. In this study, we found that the human serotonin-receptor, HTR2, gene, which had been mapped nearby the RB gene on chromosome 13, was expressed only in human fibroblasts with a maternal allele and not in cells without a maternal allele. The 5' genomic region of the human HTR2 gene was cloned by PCR-mediated method. Only the 5' region of the gene was methylated in cells with the maternal gene, and it was not methylated in cells without the maternal gene. A polymorphism of PvuII site of the gene was also found and useful for the segregation analysis in a family of a RB patient and for analysis of loss of heterozygosity on chromosome 13 in tumor and its parental origin. These results suggest that the human HTR2 gene might be affected by genomic imprinting and that exclusive expression of the maternal HTR2 gene may be associated with the delayed occurrence of RB, which had lost the maternal chromosome 13.     

Chromosomal radiosensitivity of Down syndrome lymphocytes at different stages of the cell cycle

Summary Data on 151 non-mosaic 47,XXY males from Sardinia, previously reported by Filippi (1986), were analysed for associations with parental ages at birth, sib order and sex ratio among siblings. The results confirm those of earlier Scottishbased studies in that: (1) there was a significant increase in risk of 47,XXY livebirths at advanced parental ages; (2) maternal age, and maternal age alone, was sufficient to explain the effect; (3) there were no independent effects of paternal age or sib order once maternal age had been taken into account; (4) there was no evidence of any distortion of the sex ratio among siblings. Estimates of relative risk at different maternal ages were compatible with those from the Scottish studies, and pooled estimates are therefore derived. They suggest, for example, that the risk at maternal age 40 years is 2–3 times that at age 30 years. In 33 cases, the parental origin of the supernumerary X chromosome was determined by analysing the segregation of genetic markers. The mean parental ages of 19 maternal cases were significantly raised above those of controls, whereas those of 14 paternal cases were slightly, and marginally significantly, reduced. The conclusions were essentially unaffected by whether the Sardinian population, the siblings of cases or a group of 94 unrelated Sardinian males were used as controls.     

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.     

Trisomy 18: studies of the parent and cell division of origin and the effect of aberrant recombination on nondisjunction.

We have studied the mechanism of origin of 63 cases of trisomy 18. In 2 the additional chromosome was paternal in origin, and in the remaining 61 it was maternal in origin. Both paternal cases were attributable to a postzygotic mitotic (PZM) error. Among the 54 maternal cases for which the cell division of error was established, only 16 were attributable to an error at the first meiotic division (mat MI), whereas no fewer than 35 were due to an error at the second meiotic division (mat MII), the remaining 3 being the result of a PZM error involving the maternal chromosome 18. A standard map of chromosome 18 was constructed and compared with the nondisjunctional map. Approximately one-third of the mat MI errors were associated with complete absence of recombination, whereas in the remaining two-thirds and in all the mat MII errors recombination in the nondisjoined chromosomes appeared to be normal. All the maternal errors were associated with an increased maternal age, although this reached significance only for the mat MII category of nondisjunction. Our observations on chromosome 18 are compared with those on other chromosomes for which there are comparable data.     

Chromosome 15 uniparental disomy is not frequent in Angelman syndrome.

Genetic imprinting has been implicated in the etiology of two clinically distinct but cytogenetically indistinguishable disorders--Angelman syndrome (AS) and Prader-Willi syndrome (PWS). This hypothesis is derived from two lines of evidence. First, while the molecular extents of de novo cytogenetic deletions of chromosome 15q11q13 in AS and PWS patients are the same, the deletions originate from different parental chromosomes. In AS, the deletion occurs in the maternally inherited chromosome 15, while in PWS the deletion is found in the paternally inherited chromosome 15. The second line of evidence comes from the deletion of an abnormal parental contribution of 15q11q13 in PWS patients without a cytogenetic and molecular deletion. These patients have two maternal copies and no paternal copy of 15q11q13 (maternal uniparental disomy) instead of one copy from each parent. By qualitative hybridization with chromosome 15q11q13 specific DNA markers, we have now examined DNA samples from 10 AS patients (at least seven of which are familial cases) with no cytogenetic or molecular deletion of chromosome 15q11q13. Inheritance of one maternal copy and one paternal copy of 15q11q13 was observed in each family, suggesting that paternal uniparental disomy of 15q11q13 is not responsible for expression of the AS phenotype in these patients.