Origin of nondisjunction in trisomy 21 syndrome: all studies compiled, parental age analysis, and international comparisons

Reliable detection of Trisomy 21 using MALDI-TOF mass spectrometry

PURPOSE

Current diagnostic methods for chromosomal abnormalities rely mainly on karyotyping and occasionally fluorescent in situ hybridization or quantitative polymerase chain reaction. We describe an alternative molecular method for the detection of trisomy 21 involving mass spectrometric analysis of single nucleotide polymorphisms.

METHODS

In collaboration with Sequenom, Inc., 350 blinded amniotic fluid, amniocyte culture, chorionic villus, or amniotic fluid supernatant samples were analyzed for trisomy 21 using SNP analysis and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Peak ratios were calculated for heterozygous genotypes and compared to control values generated from known euploid samples. An analytical algorithm using standard deviations from control values was used to determine the probability of a sample being affected or unaffected.

RESULTS

Seventy-three trisomy 21 samples from among the 350 blinded samples were correctly identified. There were no false-positive or false-negative results among the complete trisomy 21 samples. One sample exhibiting mosaicism for trisomy 21 was identified as being unaffected.

CONCLUSIONS

MALDI-TOF mass spectrometry is a robust and reproducible method for the detection of trisomy 21. Its amenability to high-throughput analysis and high degree of multiplexing make it a potential future diagnostic tool for the detection of other aneuploidies as well.

Adaptive-filtering of trisomy 21: risk of Down syndrome depends on family size and age of previous child

The neonatal incidence rate of Down syndrome (DS) is well-known to accelerate strongly with maternal age. This non-linearity renders mere accumulation of defects at recombination during prolonged first meiotic prophase implausible as an explanation for DS rate increase with maternal age, but might be anticipated from chromosomal drive (CD) for trisomy 21. Alternatively, as there is selection against genetically disadvantaged embryos, the screening system that eliminates embryos with trisomy 21 might decay with maternal age. In this paper, we provide the first evidence for relaxed filtering stringency (RFS) to represent an adaptive maternal response that could explain accelerating DS rates with maternal age. Using historical data, we show that the proportion of aberrant live births decrease with increased family size in older mothers, that inter-birth intervals are longer before affected neonates than before normal ones, and that primiparae exhibit elevated levels of DS incidence at higher age. These findings are predicted by adaptive RFS but cannot be explained by the currently available alternative non-adaptive hypotheses, including CD. The identification of the relaxation control mechanism and therapeutic restoration of a stringent screen may have considerable medical implications.

Effect of paternal age on the frequency of cytogenetic abnormalities in human spermatozoa

Many surveys have been performed to find etiological relationships between pregnancy outcome and specific risk factors, such as exposure to chemicals and radiation or parental age. Advanced maternal age is a strong risk factor for trisomic pregnancies, albeit there are considerable variations among the different chromosomes. The definite incidence of the various structural and numerical chromosome aberrations in spontaneous abortions and liveborns is well known, as well as the rate of maternally and paternally derived rearrangements. Nevertheless studies have failed to assert an age-dependent risk for men fathering chromosomally abnormal children. New techniques using fluorescence in situ hybridization render it possible to analyze spermatozoa directly for numerical and, to some extent, for structural aberrations. This article compiles the findings of studies on human spermatozoa over the last few years.

Trisomy 21: conference report and 1990 update

The most relevant data and stimulating ideas presented and discussed at the symposium are briefly summarized. They centered around four major foci: the genotype, the phenotype, the pathogenesis of Down syndrome (DS), and the Down person. The molecular genetic approaches to the isolation of genes encoded by chromosome 21, the definition of a possible "critical region," and the acquisition of further insights on the origin of trisomy 21 were the main topics of the analysis of the genotype. The study of the phenotype concentrated essentially on three complex traits related to the nervous, immune, and hematologic systems, which show great sensitivity to developmental disturbances, with major effects on DS subjects' health and behavior. The difficulties of investigating the pathogenesis of the syndrome were outlined, but the theoretical bases for devising sound and complete experimental approaches were also delineated. Finally, the special attention that in the last decade the medical and sociopsychological sciences gave to Down persons was also underlined, and future developments indicated. DS still remains a challenge to science and medicine; however, from the symposium emerged a less pessimistic view on actual potentialities for a decisive advancement in its basic knowledge.

Nondisjunction of chromosome 21

Chromosome heteromorphisms and restriction fragment length polymorphisms were used to study the origin of the extra chromosome in 54 trisomy 21 conceptuses. The parental origin was determined in 43 cases, with 39 (91%) being maternally and 4 (9%) parentally derived. Analysis of recombination demonstrated the presence of one or two cross-overs in most cases for which sufficient information was available, suggesting that failure to pair/exchange at meiosis I is relatively unimportant in the genesis of trisomy 21.

Pericentric inversion of chromosome 9: prevalence in 300 Down syndrome families and molecular studies of nondisjunction

The incidence of Down syndrome (DS) families where one of the parents is an heterozygous carrier of pericentric inversion of the heterochromatic region of chromosome 9-inv(9) (qh) - was determined in 3 independent groups of 100 families each. The total number of 17 such families found in the sample is significantly greater than the expected number of 5.73 for a sample of non-DS families of equal size. Consequently, the statistical association of the presence of inv (9) (qh) in one parent with the birth of a DS offspring, and the correlative 3-fold increased risk of a DS child for such families, seem to be demonstrated. A study of the origin of nondisjunction, using restriction fragment length polymorphism (RFLP) segregation analysis with a sufficient number of chromosome 21 specific probes, has provided complete information in 7 of 8 available families. Although the statistical interpretation of the results is not straightforward, due to the small size of the sample, the observed data do not contradict the assumption that the presence of inv (9) (qh) in a parent increases, by a factor of about 3, the chance that the offspring will inherit an extra chromosome 21 from that parent. Nevertheless, gathering further data appears desirable because stronger evidence would have relevance both for clinical implications and for the understanding of the function of heterochromatin, particularly with respect to meiotic and mitotic processes.

Effects of male age on the frequencies of germinal and heritable chromosomal abnormalities in humans and rodents

OBJECTIVE

To review evidence regarding the effects of male age on germinal and heritable chromosomal abnormalities using available human and rodent studies and to evaluate possible underlying mechanisms.

DESIGN

Review of English language-published research using MEDLINE database, excluding case reports and anecdotal data.

RESULT(S)

There was little evidence from offspring or germ cell studies for a generalized male age effect on autosomal aneuploidy, except in rodents. Sex chromosomal nondisjunction increased with age in both human and rodent male germ cells. Both human and rodent data showed age-related increases in the number of sperm with chromosomal breaks and fragments and suggest that postmeiotic cells are particularly vulnerable to the effects of aging. Translocation frequencies increased with age in murine spermatocytes, at rates comparable to mouse and human somatic cells. Age-related mechanisms of induction may include accumulation of environmental damage, reduced efficiency of DNA repair, increased genomic instability, genetic factors, hormonal influences, suppressed apoptosis, or decreased effectiveness of antioxidants and micronutrients.

CONCLUSION(S)

The weight of evidence suggests that the increasing trend toward fathering at older ages may have significant effects on the viability and genetic health of human pregnancies and offspring, primarily as a result of structural chromosomal aberrations in sperm.

Frequency of XY sperm increases with age in fathers of boys with Klinefelter syndrome

With increasing availability of drugs for impotence and advanced reproductive technologies for the treatment of subfertility, more men are fathering children at advanced ages. We conducted a study of the chromosomal content of sperm of healthy men aged 24-57 years to (a) determine whether father's age was associated with increasing frequencies of aneuploid sperm including XY, disomy X, disomy Y, disomy 21, and sperm diploidy, and (b) examine the association between the frequencies of disomy 21 and sex-chromosomal aneuploidies. The study group consisted of 38 fathers of boys with Klinefelter syndrome (47, XXY) recruited nationwide, and sperm aneuploidy was assessed using multicolor X-Y-21 sperm FISH ( approximately 10,000 sperm per donor). Paternal age was significantly correlated with the sex ratio of sperm (Y/X; P=.006) and with the frequency of XY sperm (P=.02), with a clear trend with age by decades (P<.006). Compared with fathers in their 20s (who had an average frequency of 7.5 XY sperm per 10,000), the frequencies of XY sperm were 10% higher among fathers in their 30s, 31% higher among those in their 40s, and 160% higher among those in their 50s (95% CI 69%-300%). However, there was no evidence for age effects on frequencies of sperm carrying nullisomy sex; disomies X, Y, or 21; or meiosis I or II diploidies. The frequencies of disomy 21 sperm were significantly associated with sex-chromosomal aneuploidy (P=.04)-in particular, with disomy X (P=.004), but disomy 21 sperm did not preferentially carry either sex chromosome. These findings suggest that older fathers produce higher frequencies of XY sperm, which may place them at higher risk of fathering boys with Klinefelter syndrome, and that age effects on sperm aneuploidy are chromosome specific.

Molecular analysis of nondisjunction in Down syndrome patients with and without atrioventricular septal defects

BACKGROUND

Congenital heart disease is common in Down syndrome patients, with atrioventricular septal defects accounting for a majority of the abnormalities. The molecular mechanisms of meiotic nondisjunction resulting in Down syndrome were studied for associations with the presence of atrioventricular septal defects.

METHODS AND RESULTS

Twenty highly polymorphic chromosome 21 microsatellite markers were used to genotype two groups of patients (group 1: Down syndrome with atrioventricular septal defects, n = 43; and group 2: Down syndrome without cardiac defects, n = 51) to determine (1) the parental origin of the extra chromosome, (2) the stage of meiotic nondisjunction resulting in the trisomy, (3) the presence or absence of disomic homozygosity or heterozygosity, and (4) the degree of recombination in the nondisjoined chromosomes. The parental origin of the nondisjoined chromosome was maternal in 86.2% of the families, with no significant differences between groups. The most centromeric marker was nonreduced, indicating a meiosis I nondisjunction in 76.5% of maternally derived trisomies, and reduced, indicating a meiosis II nondisjunction in 76.9% of paternally derived trisomies, with no significant differences between groups. There were no significant differences in the proportion of reduced markers at any locus between groups. The distribution of the number of crossovers was significantly different between groups (chi 2 = 14.12, P < .001), with less recombination observed in group 1.

CONCLUSIONS

In Down syndrome patients, no association was found between the presence of an atrioventricular septal defect and the parent of origin, stage of meiotic nondisjunction, or disomic homozygosity or heterozygosity. A significant association was found between the presence of an atrioventricular septal defect and reduced frequency of recombination.

Paternal age as a risk factor for Down syndrome

Although the effect of maternal age as a risk factor for Down syndrome (DS) is well known, the role of paternal age in the cause of DS has not been clearly established. To investigate this phenomenon we conducted a case-control study between July 1989 and February 1990. The cases were 318 children and teenagers with DS studied at the Specialized Educational Institutions of Lima City, Perú. They were paired with 1,196 control individuals that were selected from the birth records of 2 general hospitals of the city. For each case we tried to obtain 4 controls, paired by their date of birth, sex, and maternal age. The means of paternal age in the 2 groups were compared, first globally and then by groups of maternal age (< 21 years, 21-29 years, 30-34 years, 35-39 years and > 39 years). None of the comparisons gave a statistically significant difference between the 2 groups, using either the Student t-test or the Mann-Whitney U-test. The results obtained in this study give no evidence that paternal age can be considered a risk factor for the conception of a child with DS.

Maternal age effect: the enigma of Down syndrome and other trisomic conditions

Aneuploidy is the most frequently observed chromosome abnormality in human liveborn, abortuses and oocytes. The only etiological factor that has been established is advanced maternal age for the occurrence of trisomies, particularly trisomy 21 which causes Down syndrome. The maternal age effect remains an enigma. Recent molecular data bearing on this question are reviewed as are the hypotheses that have been proposed linking nondisjunction and maternal age. Rationale is presented for a compromised microcirculation hypothesis that explains the cause of nondisjunction and why its occurrence changes with maternal age from menarche to menopause. It takes into account two facts: (1) 95% of Down syndrome children receive their extra chromosome from their mother, and in 80% or more of these the nondisjunction occurred in the first meiotic division, which is completed in the ovary. (2) The ovarian follicle containing the primary oocyte has no internal circulation. The hypothesis proposes that aneuploid oocytes arise from a concatenation of events. It begins with hormonal imbalance that causes a less-than-optimal microvasculature to develop around the maturing and mature follicles. The resulting decrease in the size of the perifollicular capillary bed reduces the volume of blood flow through the area, leading to an oxygen deficit and a concomitant increase inside the follicle of carbon dioxide and anaerobic products, such as lactic acid. This in turn causes a decrease in the intracellular pH of the oocyte that diminishes the size of the spindle, with consequent displacement and nondisjunction of a chromosome. The compromised microcirculation hypothesis explains the occurrence of aneuploidy in primary and secondary oocytes, sperm precursor cells, tumor and embryonic cells. It also explains why women of all reproductive ages may have a Down syndrome child.

Parental origin determination in thirty de novo Robertsonian translocations

Cytogenetic heteromorphisms and restriction fragment length polymorphisms were used to assign the parental origins of 30 de novo non-homologous Robertsonian translocations. The balanced and unbalanced translocations studied included 20 rob(14q21q) four rob(13q14q)four rob(15q21q) one rob(13q15q), and one rob(13q21q). Significantly more maternally (26/30) than paternally (4/30) derived de novo translocations were noted and all rob(14q21q) ascertained through unbalanced probands (20/20) were maternal in origin. Interestingly, 12/13 probands who were trisomic and informative for proximal chromosome 21q loci were homozygous for the markers tested. Segregation (2:1) of the Robertsonian translocation into one daughter cell in meiosis I and subsequent failure of the chromosome 21 chromatids to separate in meiosis II may account for our observation of homozygosity for proximal chromosome 21 loci in the majority of de novo rearrangements tested.

Trisomy and age at menopause: predicted associations given a link with rate of oocyte atresia

The association of trisomy with advancing maternal chronological age suggests that some aspect of physiological aging is accelerated in women with trisomic pregnancies. This paper develops a quantitative theoretical model based on the hypothesis that trisomy risk is primarily a function of the size of the oocyte pool and, in particular, that risk is increased in women with accelerated rates of oocyte atresia and hence smaller pools at given chronological ages. Since the rate of oocyte atresia is a determinant of age at menopause, this hypothesis leads to the prediction that women who have had trisomic pregnancies reach menopause earlier than women who have not. We used data relating chronological age to oocyte number, trisomy and menopause to deduce the distribution of oocyte atresia rates in all women and in women with trisomic pregnancies. Given certain simplifying assumptions, we predict that associations between trisomy and age at menopause will vary with a woman's age at the time of trisomy such that trisomies at 34-43 years will be associated with a 1-3.4 year earlier onset of menopause, while trisomies at younger or older ages will have no or little association with age at menopause. This model, while vulnerable to the uncertainties that attend its assumptions, provides a testable prediction that permits separation of one aspect of physiological age from chronological age.

Distribution of aneuploidy in human gametes: comparison between human sperm and oocytes

The frequency and distribution of aneuploidy was compared in 11,615 karyotyped human sperm and 772 karyotyped human oocytes to determine if all chromosomes are equally likely to be involved in aneuploid events or if some chromosomes are particularly susceptible to nondisjunction. The frequency of hypohaploidy and hyperhaploidy was compared among different chromosome groups and individual chromosomes for human sperm and oocytes. In general, hypohaploid chromosome complements were more frequent than hyperhaploid complements, in sperm and oocytes. The distribution of chromosome loss in the hypohaploid complements indicated that significantly fewer of the large chromosomes and significantly more of the small chromosomes were lost, suggesting that technical loss predominantly affects small chromosomes. A conservative estimate of aneuploidy (2 X hyperhaploidy) was approximately 3-4% in the human sperm and 18-19% in human oocytes. All chromosome groups were represented among hyperhaploid human sperm and oocytes. For human sperm, the observed frequency of hyperhaploidy equaled the expected frequency based on the assumption that the frequency of nondisjunction is equal for all chromosome groups, with two exceptions: group G and the sex chromosomes. Among individual chromosomes in human sperm, chromosomes 1 and 21 and the sex chromosomes had a significant excess of hyperhaploidy. For human oocytes, there were fewer hyperhaploid oocytes than expected for chromosome groups C and F and more than expected for chromosome groups D and G. Among individual chromosomes there was a significant excess for chromosome 21. These results indicate that all chromosomes are susceptible to nondisjunction but that chromosome 21 is particularly prone to aneuploidy in both human sperm and oocytes. They also demonstrate that sex chromosome aneuploidy is common in human sperm but not in human oocytes.

Parental origin and mechanism of formation of polysomy X: an XXXXX case and four XXXXY cases determined with RFLPs

The parental origin and mechanism of formation of polysomy X were studied in five cases (one case of 49,XXXXX; four cases of 49,XXXXY), using various X-linked restriction fragment length polymorphisms as genetic markers. Segregation and densitometric analyses on the polymorphic DNA fragments revealed that, in all five cases, the additional X chromosomes are of maternal origin and the mechanism of formation is most probably a result of three non-disjunctions during maternal meiotic divisions: once at the first meiosis and simultaneously twice at the second meiosis. The identical origin and the identical mechanism of formation among the five cases are unlikely to be coincidental and suggest a common cause in the mothers of the five cases.

Frequency and distribution of aneuploidy in human female gametes

During the past 6 years, 14 cytogenetic studies on human oocytes recovered during in vitro fertilization procedures have been published; they report contradictory results. The present survey has pooled the more than 1500 oocyte chromosome complements examined to date, in order to determine generalized trends in chromosomal abnormalities of female gametes. The overall frequency of abnormalities in mature oocytes is 24.0% with a large majority of aneuploidies (22.8%) over structural aberrations (1.2%), which could be explained by the difficulty in the detection of structural abnormalities in oocyte chromosome sets. An analysis of the distribution of non-disjunction among all chromosomes was also performed. In the A, C, D, and especially in the G groups, there is a significant difference between the observed non-disjunction and the frequencies expected from an equal partitioning of non-disjunction among all chromosomes. These data are discussed with reference to the differences obtained from cytogenetic studies on human sperm and from investigations on spontaneous abortion.

Cytogenetic analysis of mouse oocytes and one-cell zygotes as a potential assay for heritable germ cell aneuploidy

Assays are needed for detecting chemically-induced aneuploidy, for investigating the mechanisms of aneuploidy production, and for obtaining heritable germ cell data that can be used to formulate human risk estimates. In this report, we describe the results of experiments designed to study aneuploidy in metaphase II (MII) oocytes induced by intraperitoneal (i.p.) or oral dosages of colchicine, and to investigate the proportion of aneuploid oocytes transmitted to one-cell (1C) zygotes following oral administration of colchicine immediately following HCG. The proportions (and percentages) of hyperploid MII oocytes were: 1/606 (0.2), 37/504 (7.3), 152/731 (20.8) and 75/319 (23.5) for control, 0.2, 0.3 and 0.4 mg/kg, respectively for i.p. administration of colchicine; and 3/216 (1.4), 8/539 (1.5), 81/511 (15.9), 71/398 (17.8) and 98/391 (25.1) for control, 1.0, 2.0, 3.0 and 4.0 mg/kg, respectively for oral administration of colchicine. The proportions of hyperploid 1C zygotes were 2/327 (0.6), 21/389 (5.4), 62/435 (14.3) and 69/438 (15.8) for control, 2.0, 3.0 and 4.0 mg/kg, respectively for oral colchicine. The proportions of hyperploid MII oocytes and 1C zygotes were significantly higher (Chi-square, P less than 0.01) at each i.p. or oral dose (except 1.0 mg/kg oral) than in the controls. The frequencies of hyperploidy induced by oral doses of colchicine were greater in MII oocytes than in 1C zygotes. We also found that the frequency of developmentally delayed and polyploid 1C zygotes increased with the dose of oral colchicine. Developmentally delayed zygotes contained male-derived chromosomes and female-derived fragmented pronuclei and pronuclei with decondensed chromosomes. These results indicate that higher doses of oral colchicine are needed to induce comparable levels of aneuploidy found after i.p. administration, and that aneuploid oocytes are fertilized and reach first cleavage metaphase. In addition, colchicine induces a spectrum of events including aneuploidy, polyploidy and developmentally delayed oocytes and zygotes.

Parental origin and mechanism of formation of X chromosome structural abnormalities: four cases determined with RFLPs

Parental origin and mechanism of formation of X chromosome structural abnormalities were studied in one each case of dup(X)(pter----p11.4::p22.1----qter), del(X)(qter----p11:), i(X)(qter----cen----qter), and inv dup(X) (pter----q22::q22----pter) using various X-linked RFLPs as genetic markers. Segregation and densitometric analyses on polymorphic DNAs revealed that the dup(Xp) and the del(Xp) are both of paternal origin and the i(Xq) and i dic(X) are of maternal origin. The dup(Xp) had arisen by an unequal sister chromatid exchange and the del(Xp) had occurred through an intrachromosomal breakage-reunion mechanism, both in the paternal X chromosome. The i(Xq) had arisen either through centromere fission of a maternal X chromosome, followed by duplication of its long-arm, or through a translocation between two maternal X chromosomes after meiotic crossing-over. The inv dup(X) arose through sister chromatid breakage and reunion in a maternal X chromosome. These results, together with those of previous studies, suggest that the de novo abnormalities due to events involving centromere disruption arise predominantly during oogenesis, while those due to simple breakage-reunion events occur preferentially during spermatogenesis.

Paternal age and trisomy among spontaneous abortions

The relationship of paternal age to specific types of trisomy and to chromosomally normal loss was investigated in data drawn from a case-control study of spontaneous abortions. Differences in paternal age between karyotype groups and controls delivering after 28 weeks gestation were tested using an urn model analysis which adjusted, by regression, for maternal age and, by stratification, for the effects of design variables (payment status, phase of study) and demographic factors (language, ethnicity). The magnitude of paternal age differences was estimated using least squares regression analysis. For chromosomally normal cases there was no association with paternal age. Among the fourteen trisomy categories examined, four (7, 9, 18, 21) showed increased paternal age (greater than or equal to 1 year above expectation), three (13, 20, 22) showed decreased paternal age and the rest, including the most common, trisomy 16, showed negligible differences. Only the association with trisomy 22 was statistically significant (P = 0.012), with a predicted reduction in paternal age of 2.1 years (95% CI -4.9, -0.5 years). This association did not vary with maternal age, payment status, phase of study, language or ethnicity. Because previous observations are extensive, the relation of paternal age to trisomy 21 was examined further. The overall association was not significant (beta = 0.8 years; 95% CI -0.8, 2.4 years). Moreover, there was evidence that the magnitude and direction of paternal age associations vary significantly within the sample, although not between subgroups defined on the basis of payment, phase of study, language or ethnicity. With respect to maternal age, the trend is towards a greater paternal age difference for trisomy 21 losses in younger women (P = 0.058). Given the number of tests performed, the finding for trisomy 22 and reduced paternal age could be due to chance. Among trisomy types, the direction of paternal age associations was not consistent for chromosomes grouped according to characteristics that might relate to the probability of nondisjunction, such as size, arm ratio, or nucleolar organizer region content, or to the potential viability of the trisomy. Thus, neither on statistical nor biological grounds do the data provide compelling evidence of paternal age effects on the trisomies found among spontaneous abortions, or on chromosomally normal losses.

Down syndrome: nursing interventions newborn through preschool age years

Down syndrome is the most common cause of mental retardation in children. The children need opportunities to become important members of their families. The preschool years are an excellent time for the children and family to learn about the condition and to establish behaviors that will optimize opportunities for the children to reach their maximum potential. Examples of nursing interventions based on developmental periods are provided.

Distribution of meiotic recombination along nondisjunction chromosomes 21 in Down syndrome determined using cytogenetics and RFLP haplotyping

Ten families (Down syndrome children and their parents) showing evidence of meiotic recombination between intraparental chromosomes transmitted after nondisjunction were studied. Cytogenetic polymorphisms and a cassette of RFLP markers distributed along chromosome 21 were used to analyze these families to localize the regions of meiotic recombination. Results indicated that only one crossover occurred per meiotic division and that nine of ten nondisjunctions appeared to be of maternal origin. In one family the crossover had taken place in the pericentromeric region, proximal to marker D21S13, which is quite exceptional. A chance of meiotic recombination within region 21q21, flanked by marker D21S72 and the amyloid gene, could be demonstrated in seven of the ten families. Most strikingly, this chance significantly decreased distal to q21, with frequencies of 0.3 and 0.1 in regions q22.2 and q22.3-qter, respectively. It is hypothesized that decreased chiasmata formation in the most distal part of chromosome 21q might promote nondisjunction. Furthermore, data from the ten crossovers made it possible to map provisionally two previously undefined markers, D21S24 and D21S82, to regions q21-qter and q22.1-qter, respectively.

Isolation of chromosome-21-specific DNA probes and their use in the analysis of nondisjunction in Down syndrome

Thirteen single-copy, chromosome-21-specific DNA probes were isolated from a recombinant library made from flow-sorted chromosome 21 DNA and regionally mapped using a panel of somatic cell hybrids. Five probes mapped in the 21q21-q22.1 region, six to the 21q22.1-qter region, and one to each of the regions 21q22.1-q22.2 and 21q22.3. Two of these probes, one of which maps in the critical region for Down syndrome, have recently been shown to be expressed at high levels in Down syndrome brain tissue (Stefani et al. 1988). Following preliminary screening for restriction fragment length polymorphisms (RFLPs), five polymorphisms were discovered with four of the chromosome 21 DNA probes. A frequent MspI polymorphism detected by one of the probes was used in conjunction with four previously described polymorphic chromosome 21 probes to analyse the origin of nondisjunction in 33 families with a child or fetus with trisomy 21. The parental origin of the additional chromosome 21 was determined in 12 cases: in 9 (75%) of these it was derived from the mother and in the other 3 cases (25%) it was of paternal origin. Cytogenetic analysis of Q-banding heteromorphisms was informative in three of five families tested, and in each case the RFLP results were confirmed. The meiotic stage of nondisjunction was defined with confidence in five families, the results being obtained with pericentromeric RFLP or cytogenetic markers. Recombination between two nondisjoined chromosomes was demonstrated in one family and is consistent with the view that a lack of recombination between chromosome 21 homologues or failure of their conjunction is not the invariable cause of trisomy 21.

The microtubular cytoskeleton and chromosomes of unfertilized human oocytes aged in vitro

To detect structural alterations in human oocytes that may give rise to predisposition to aneuploidy, unfertilized human oocytes from an IVF programme were processed for indirect anti-tubulin immunofluorescence. The spindle of oocytes aged for 2 days is rather small, and bi- or multipolar. Chromosomes are no longer aligned at the spindle equator but are scattered all over the degenerating spindle. This implies that human oocytes aged for 2 days may no longer be able to develop into a chromosomally balanced, normal embryo. In oocytes aged for 3-4 days the chromosomes become more decondensed and form a restitution nucleus. Microtubules radiate out from the latter towards the cell periphery and form a network of fibres in the cytoplasm. A similar alignment of tubules is found in unfertilized, activated oocytes. Oocytes with an aberrant cytoskeleton and chromosomal array were predominantly obtained from aged females. They include two binucleated oocytes with two sets of chromosomes and two oocytes with displaced chromosomes one of which had a tripolar spindle.

Influence of postovulatory aging on chromosome segregation during the second meiotic division in mouse oocytes: a parthenogenetic analysis

A high incidence of parthenogenetic activation was observed when postovulatory aged mouse oocytes were exposed briefly to hyaluronidase in culture medium at 18-26 h after the human chorionic gonadotropin injection for inducing superovulation. The majority of the activated oocytes extruded a second polar body and developed a single haploid pronucleus. Cytogenetic analysis of this class of parthenogenone at metaphase of the first-cleavage mitosis has clearly demonstrated that the completion of the second meiotic division in activated aged oocytes is not associated with a significant increase in the incidence of chromosome segregation errors. The increasing postovulatory age of oocytes prior to activation was observed to significantly decrease the capacity of activated oocytes to extrude the second polar body.

Origin of the extra chromosome in trisomy 18. A study on five patients using a restriction fragment length polymorphism

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 PA-cDNA. 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.

Second meiotic nondisjunction is not increased in postovulatory aged murine oocytes fertilized in vitro

We used an in vitro fertilization system to examine the effects of postovulatory oocyte age on nondisjunction at the second meiotic division. After ovulatory-inducing injections of hormone, we recovered mouse oocytes either at the estimated time of ovulation (controls) or 2, 4, 5, 10, or 14 h later. Oocytes were subjected to an in vitro fertilization procedure, and chromosomal preparations were made from first cleavage metaphase eggs. The first cleavage assay reveals morphologically distinguishable paternal and maternal chromosomes. Many of the aged oocytes were activated rather than fertilized by the in vitro procedure, but could still be analyzed for nondisjunction. We found a tendency toward retention of the second polar body after 10 and 14 h aging. A total of 488 maternal genomes, 290 of which were in the control group, were analyzable for nondisjunction. Seven hyperhaploid genomes (2.4%) were observed in the controls and 6 (3.0%) in the combined aged group. The difference between these two frequencies is not significant (Gadj = 0.164, P greater than 0.50). In the aged group, one hyperhaploid genome was in the 2-h population, three in the 5-h population, and two in the 10-h population. We were unable to find any significant increase in the frequency of nondisjunction after postovulatory oocyte aging.

The use of DNA probes to establish parental origin in Down syndrome

Molecular analysis was performed to determine the parental origin of the extra number 21 chromosome in 20 couples following the birth of a child with standard trisomy 21. The parent of origin was successfully identified in 9 of 20 (45%) using five chromosome-21-specific DNA probes and eight restriction endonucleases by restriction fragment length polymorphism and dosage analysis; seven were of maternal and two of paternal origin. Utilizing the observed allele frequencies, the expected frequencies of informative homozygous matings [2(p2q2)] approximate 10% for seven of eight enzyme/probe combinations; the eighth, TaqI/pPW231F (D21S3), is 3%. The observed phenotype frequencies for all enzyme/probe combinations tested conform closely to predictions by the Hardy-Weinberg law. Strong linkage disequilibrium was observed between the DNA markers of EcoRI and TaqI with probe pPW236B; identical results were obtained with G-95 alpha 1-11a. We were able to demonstrate that although these two probes are of different size, and hence not identical, they detect the same TaqI and EcoRI polymorphisms; therefore both should be assigned to a single locus, D21S11.

On the origin of recurrent trisomy 21: determination using chromosomal and DNA polymorphisms

A family is described in which two cases of trisomy 21 occurred in, respectively, a newborn infant and a prenatally diagnosed fetus. Using fluorescent chromosomal polymorphisms, it was established that in both cases the extra chromosome resulted from a first meiotic division error in the mother and that the father contributed the same centromeric region to both children. RFLP-associated probes were used to examine the genetic content of the chromosomes. It was noted that the polymorphism patterns of the chromosomes 21 which both children inherited from their parents were identical for three, but not identical for one of the probes studied. This difference must be the result of recombination. This result is discussed in relation to the suggestion that the increased recurrence rate in mothers with a trisomic child could be due to a reduced recombination rate.

Sexual behavior and Down syndrome: the biological mechanism

Sufficient evidence of a correlation between infrequent coitus and Down syndrome (DS) has now accumulated to warrant reevaluation of the suggested biological mechanisms. The evidence provides no support for delayed fertilization as the mechanism responsible for this correlation, as was originally proposed by German [Nature 217:516-519, 1968]. A better explanation of this association is provided by the sperm aging hypothesis, which gains its support from both animal studies and chromosomal findings of a paternal contribution to DS. The animal studies supporting this hypothesis show an increased incidence of sperm-derived trisomies resulting from sperm stored for prolonged periods in the male tract. The chromosomal findings show a paternal origin in 20% of DS infants; the sperm aging hypothesis concerns the biological mechanism in this 20%. In addition to explaining the excess of DS for older mothers, the sperm aging hypothesis explains the excess for teenage unwed mothers and indicates that sperm aging from decreased ejaculatory frequency may be a cause of DS in all age groups. Testable directly in humans against German's delayed fertilization hypothesis, the sperm aging hypothesis has immediate clinical implications. It suggests 1) an approach to reduce the incidence of DS and miscarriages and 2) clinical research that will increase knowledge of the circumstances indicating a need for antenatal monitoring.

AluI-resistant chromatin of chromosome 18: classification, frequencies and implications

The pericentric chromatin of chromosome 18 was found to be far more heterogeneous for restriction endonuclease AluI (5'...AG decrease CT...3') than previously thought. The extent of such heterogeneity was characterized using 50 normal Caucasians and 5 cases of trisomy 18 or Edwards' Syndrome. The AluI-resistant chromatin can arbitrarily be classified into at least five sizes by comparison with the length of the short arm (p) of chromosome 18. They are: negative (1), small (2), medium (3), large (4) and very large (5) with incidences of 11.30%, 19.13%, 29.57%, 29.57% and 10.43%, respectively. In addition the location of the chromatin can be classified into four types depending upon the position relative to the primary constriction. For example: Type I (absent); Type II (present on p arm only); Type III (present on q arm only); Type IV (present on centromere and extending into both p and q arms). The incidences of types I, II, III, and IV were 11.30%, 62.61%, 0.87%, and 25.22%, respectively. Based on limited data, AluI-resistant chromatin was found to be predominantly "large" and "very large" in Edwards' Syndrome samples. In addition, no case with negative Alu-resistant chromatin was noted. Therefore, it is tempting to speculate that the amount of chromatin present on the centromere might play a role in non-disjunction in Edwards' Syndrome cases. Although the variation observed in the present study is continuous, the proposed classification has some important implications for future investigations.

Molecular genetics of human chromosome 21

Chromosome 21 is the smallest autosome, comprising only about 1.9% of human DNA, but represents one of the most intensively studied regions of the genome. Much of the interest in chromosome 21 can be attributed to its association with Down's syndrome, a genetic disorder that afflicts one in every 700 to 1000 newborns. Although only 17 genes have been assigned to chromosome 21, a very large number of cloned DNA segments of unknown function have been isolated and regionally mapped. The majority of these segments detect restriction fragment length polymorphisms (RFLPs) and therefore represent useful genetic markers. Continued molecular genetic investigation of chromosome 21 will be central to elucidating molecular events leading to meiotic non-disjunction and consequent trisomy, the contribution of specific genes to the pathology of Down's syndrome, and the possible role of chromosome 21 in Alzheimer's disease and other as yet unmapped genetic defects.

BrDU-Giemsa labeling studies of satellite associations in parents of children with trisomy 21 or 13

Studies on satellite association (SA) in parents of trisomy 21 offspring have not provided meaningful comparisons of SA frequencies since the latter was not expressed as a function of cell division number. We have used BrDU-labeling to compare SA frequencies in first and second division metaphases from lymphocytes of parents with either a trisomy 21 or trisomy 13 child and a control group. Parental origin of nondisjunction was determined in three of six families using quinacrine heteromorphisms. In the two cases of trisomy 13 determined, the errors occurred in maternal meiosis. BrDU-labeled metaphases were analyzed for SA frequency in four groups: A) parents contributing the extra chromosome; B) spouses of the parents in A; C) parents (nine) in whom the origin of a trisomy 13 or 21 was unknown; and D) healthy controls (five). The mean numbers of SAs/cell and of chromosomes/SA were not significantly different among the four groups for both first and second division cells. Sex and age showed no effect on SA frequency. There were significant decreases in mean numbers of SA/cell and chromosomes/SA in second-division cells (chromatids differentially stained) compared with first-division cells (chromatids undifferentiated). In second-division cells, two-chromosome SAs of all types showed random concordant and discordant alignment in each subject. The results from this BrDU-labeling approach provide no evidence that either quantitative or qualitative parameters of SA are directly related to a tendency of nondisjunction. They also show that acrocentric nondisjunction occurs in the presence of random chromatid alignment in SAs.

Some trends in medical populations genetics

Five topics concerning medical population genetics have been selected for discussion: in the field of population cytogenetics, the frequency of chromosomal aberrations and the roles of mutation and selection in the maintenance of balanced rearrangements are studied; the long term genetic effects of treatment and prevention of genetic diseases are reviewed; the relationships between malaria and the sickle-cell trait are discussed; some recent works concerning human DNA polymorphisms in the field of population genetics are presented, and finally, some methods of genetic epidemiology are described.

A further improved method for identifying heteromorphism of acrocentric chromosomes

An improved method for identifying an individual heteromorphic acrocentric chromosome, a high-resolution dual Q-R banding, is described. The acrocentrics of 550-850 band-stage prometaphase showed more distinct fluorescent heteromorphic patterns at their paracentromeric regions than those of 400 band-stage metaphase. In the two families studied, all four (two pairs of) parental prometaphase homologues in every kind of acrocentric except chromosome 22 were clearly distinguished from each other, leading to accurate determination of the parental origin of all the children's acrocentric chromosomes. The dual Q-R banding on metaphase plates could distinguish at most three of four such homologues of the parents in both families. Several possible applications of this method are discussed.

Attrition of trisomies as a maternal screening device. An explanation of the association of trisomy 21 with maternal age

Analysis of published data indicates that the increased rate of trisomy 21 pregnancies with advancing maternal age is due to failure of a postulated maternal screening process between fertilisation and recognition. This mechanism may also operate in other trisomies.

A possible cause of non-disjunction of additional chromosome 21 in Down syndrome

A possible cause of non-disjunction of chromosome 21 in Down Syndromes has been cytogenetically evaluated by examining the parents by Ag-staining technique. In all the cases studied so far, the contributing parents have active ribosomal cistrons on both chromosomes 21 i.e. both chromosomes are stained positively by silver staining. These results show that the active NORs might play an essential role in meiotic non-disjunction. Furthermore, the preliminary results demonstrate that the acrocentric associations of homologous and non-homologous nature involving chromosome 21 are the most frequent in the contributing parent which may further indicate the role of multiple cellular factors affecting the associations in promoting the nondisjunction in addition to active NORs. The possible mechanisms regarding the non-disjunction of chromosome 21 have been described.

Mothers of children with Down syndrome have higher herpes simplex virus type 2 (HSV-2) antibody levels

The antibody response to herpes simplex virus (HSV) was studied in 53 mothers of children with Down syndrome (Ds) and compared with that in 154 controls, using sera sampled during pregnancy or at delivery. Conventional analysis of HSV complement fixing antibodies showed the same frequency of positivity for the two groups (70%). When the levels of IgG antibodies to an HSV-1 and an HSV-2 antigen preparation were determined by an enzyme-linked immunosorbent assay (ELISA) technique, it was found that the Ds and control mothers had similar levels of IgG antibodies to HSV-1, whereas the level of IgG antibodies to HSV-2 was significantly (P less than 0.001) higher in Ds mothers. The ratio of HSV-2 to HSV-1 ELISA IgG was calculated for each mother and the distribution of these ratios also differed significantly between the control and Ds mothers. The differences found were not due to differences in age distribution in the control and Ds groups. For comparison a third procedure, measurement of thymidine kinase blocking antibody (TK ab), was used. With this procedure the mothers were divided into groups estimated to be positive for HSV-1, HSV-2, or both. Statistical analyses showed a good correlation between the type found in TK ab analyses and the ratio found in the ELISA HSV test. The results clearly demonstrated an overrepresentation of HSV-2 antibody positivity among Ds mothers, though not of sufficient magnitude to imply that HSV-2 can be the major cause of Ds. It is discussed whether HSV-2 might be related to the recently increased birthrate of children with Ds among young mothers in Sweden or to localized geographical clustering of Ds births, or whether the increased HSV-2 antibody positivity merely indicates that factors following the same epidemiological pattern are involved in the aetiology of Ds.

Mammalian in vivo assays for aneuploidy in female germ cells

This paper presents an evaluation of and offers recommendations for assays to detect chemically induced aneuploidy in mammalian female germ cells. 72 papers on female germ cell aneuploidy, published from 1970 to 1984, were reviewed. 28 papers were selected for critical evaluation; the other 44 papers were rejected according to pre-established criteria. Salient points emerging from the information reviewed allow an assessment of the current status of mammalian female germ cell tests for aneuploidy. The majority of data have been obtained by analyzing metaphase II mouse oocyte chromosomes following superovulation. Various classes of chemicals were administered usually around the time of ovulation. Dose-response relationships have not been obtained for the majority of chemicals evaluated. The method of data reporting and analysis usually was not conducive to comparisons among different studies. Few of the 16 chemicals studied can be regarded as negative for their ability to induce aneuploidy, whereas an even smaller number should be considered as positive. Certainly, a need exists to identify the chemicals and the dosages that could increase the incidence of aneuploidy in mammalian female germ cells. Obtaining such data definitely is feasible in cytogenetic laboratories. However, the mammalian female germ cell aneuploid assay should not be perceived as a rapid, inexpensive, routine procedure. The assay is capable of detecting aneuploidy following anaphase I when metaphase II oocytes are studied and following anaphases I and II when first-cleavage zygotes are studied.

Further studies on bivalent chiasma frequency in human males with normal karyotypes

Previously unpublished data on the chiasma frequency of individual bivalents identified by a triple staining technique are presented for four males. The total autosomal cell chiasma frequency and sex chromosome univalence frequency are also given for these males and for three others. All seven males had apparently normal 46,XY karyotypes and normal spermatogenesis. The extent of inter-individual variation in cell and bivalent chiasma frequency and the gross relationship between chromosome length and chiasma frequency are discussed.

A direct cytogenetic technique for assessing the rate of first meiotic non-disjunction in the human male by the analysis of cells at metaphase II

The successful application of a triple staining technique incorporating quinacrine mustard fluorescence, lacto-propionic orcein staining and C-banding to metaphase II cells in the human male is described. This procedure overcomes the major technical difficulties associated with the analysis of these cells, enables unambiguous chromosome counts to be made and allows the majority of cells to be karyotyped. Preliminary results on two hundred cells from six men with apparently normal karyotypes are presented.

An aetiological study of 290 XXY males, with special reference to the role of paternal age

Data on 290 non-mosaic 47,XXY males have been analysed for possible associations with parental ages at birth, season of birth, sex ratio among sibs, and twinning. Comparison with matched population controls revealed a highly significant association with parental age, which was fully explained by dependence on maternal age and maternal age alone. The maternal age effect was determined with greater precision than in an earlier study of the same material, in which siblings were used as controls, and was estimated to result in an increased risk of between 5% and 10% per annum (p.a.). The estimated independent effect of paternal age, after fitting maternal age, was marginally (but not significantly) negative, and excluded an increased risk in excess of 3% p.a. Paternal age therefore appears to have little if any independent significance in the aetiology of 47,XXY. After correcting for seasonal variations in the population birth rate and smoothing, there was a peak of XXY births in March and a trough in November. Though not statistically significant, the pattern resembled that reported in previous studies, and was similar for both younger and older mothers. The twinning rate for both the XXYs and their sibs, and the sex ratio among the latter, were close to the corresponding population values.