Parental origin of autosomal trisomies

Evidence for nonhomologous meiotic coorientation in man

Nonhomologous meiotic co-orientation (NMC) was postulated for humans a half of century ago to explain the association between the presence of a rearranged chromosome(s) and the occurrence of aneuploidy for an unrelated chromosome ("interchromosomal effect", ICE). However subsequent studies did not support meiotic nature of ICE phenomenon. At the same time, NMC model can be fruitful for solving a number of problems regarding the etiology of human aneuploidy. Published and own data on the offspring of 322 parental carrier of chromosomal abnormality were analyzed according to the carrier's gender. In families with transmission of der(21;21), among patients with maternally derived trisomy 21 (T21), there is a typical male-biased sex ratio (SR), with 33 males/28 females. Among patients with paternally derived T21, five-fold male prevalence is observed (16 males/3 females), p = 0.0373. In families with maternal balanced non-contributing rearrangement (Rea), SR was male-biased among T21 patients, both those inherited (42 males/30 females) and not inherited the Rea (17 males/11 females). However, in families with paternal balanced Rea, there is an impressive difference between T21 offspring with transmitted paternal Rea and those not inherited paternal Rea, 49 males/21 females vs 4 males/15 females, p = 0.0003. A female predominance is also observed among non-trisomic offspring of paternal carriers of gonadal mosaicism for T21 (2 males/12 females), but not in non-trisomic offspring of maternal carriers (19 males/16 females), p = 0.0253. Unusual sex ratios in offspring of male carriers are considered as the result of NMC of a chromosome abnormality with the X chromosome operating in spermatogenesis.

Analyses stratified by maternal age and recombination further characterize genes associated with maternal nondisjunction of chromosome 21

OBJECTIVE

In our previous work, we performed the first genome-wide association study to find genetic risk factors for maternal nondisjunction of chromosome 21. The objective of the current work was to perform stratified analyses of the same dataset to further elucidate potential mechanisms of genetic risk factors.

METHODS

We focused on loci that were statistically significantly associated with maternal nondisjunction based on this same dataset in our previous study and performed stratified association analyses in seven subgroups defined by age and meiotic recombination profile. In each analysis, we contrasted a different subgroup of mothers with the same set of fathers, the mothers serving as cases (phenotype: meiotic nondisjunction of chromosome 21) and the fathers as controls.

RESULTS

Our stratified analyses identified several genes whose patterns of association are consistent with generalized effects across groups, as well as other genes that are consistent with specific effects in certain groups.

CONCLUSIONS

While our results are epidemiological in nature and cannot conclusively prove mechanisms, we identified a number of patterns that are consistent with specific mechanisms. In many cases those mechanisms are strongly supported by available literature on the associated genes.

A candidate gene analysis and GWAS for genes associated with maternal nondisjunction of chromosome 21

Human nondisjunction errors in oocytes are the leading cause of pregnancy loss, and for pregnancies that continue to term, the leading cause of intellectual disabilities and birth defects. For the first time, we have conducted a candidate gene and genome-wide association study to identify genes associated with maternal nondisjunction of chromosome 21 as a first step to understand predisposing factors. A total of 2,186 study participants were genotyped on the HumanOmniExpressExome-8v1-2 array. These participants included 749 live birth offspring with standard trisomy 21 and 1,437 parents. Genotypes from the parents and child were then used to identify mothers with nondisjunction errors derived in the oocyte and to establish the type of error (meiosis I or meiosis II). We performed a unique set of subgroup comparisons designed to leverage our previous work suggesting that the etiologies of meiosis I and meiosis II nondisjunction differ for trisomy 21. For the candidate gene analysis, we selected genes associated with chromosome dynamics early in meiosis and genes associated with human global recombination counts. Several candidate genes showed strong associations with maternal nondisjunction of chromosome 21, demonstrating that genetic variants associated with normal variation in meiotic processes can be risk factors for nondisjunction. The genome-wide analysis also suggested several new potentially associated loci, although follow-up studies using independent samples are required.

Approximately one of every 700 babies is born with trisomy 21—an extra copy of chromosome 21. Trisomy 21 is caused by the failure of chromosomes to segregate properly during meiosis, generally in the mother. Past studies have defined altered patterns of recombination along nondisjoined chromosomes as risk factors for human nondisjunction and model systems have clearly shown that specific genes involved recombination and other early meiotic processes play a role in the fidelity of chromosome segregation. However, no genome-wide genetic study (GWAS) has ever been conducted using maternal human nondisjunction as the disease phenotype. This study takes the first step to understand predisposing factors. We used chromosome 21 genotypes from the parents and child to identify mothers with nondisjunction errors derived in the oocyte and to establish the type of error (meiosis I or meiosis II). We then conducted a unique set of subgroup comparisons designed to leverage our previous work that shows that the etiologies of meiosis I and meiosis II nondisjunction differ for trisomy 21. Both the candidate gene study and the GWAS provide evidence that meiotic-specific structures and processes are vulnerable to genetic variants that lead to increased risk of human chromosome nondisjunction.

Successful transmission and transcriptional deployment of a human chromosome via mouse male meiosis

Most human aneuploidies originate maternally, due in part to the presence of highly stringent checkpoints during male meiosis. Indeed, male sterility is common among aneuploid mice used to study chromosomal abnormalities, and male germline transmission of exogenous DNA has been rarely reported. Here we show that, despite aberrant testis architecture, males of the aneuploid Tc1 mouse strain produce viable sperm and transmit human chromosome 21 to create aneuploid offspring. In these offspring, we mapped transcription, transcriptional initiation, enhancer activity, non-methylated DNA, and transcription factor binding in adult tissues. Remarkably, when compared with mice derived from female passage of human chromosome 21, the chromatin condensation during spermatogenesis and the extensive epigenetic reprogramming specific to male germline transmission resulted in almost indistinguishable patterns of transcriptional deployment. Our results reveal an unexpected tolerance of aneuploidy during mammalian spermatogenesis, and the surprisingly robust ability of mouse developmental machinery to accurately deploy an exogenous chromosome, regardless of germline transmission.

1(st) trimester miscarriage: four decades of study

Miscarriage is a very common occurrence in humans. This paper sets out to present published data on research that has provided increased understanding of pregnancy failure. Clarification of definitions, exploring the range of failures from preclinical to later pregnancy losses, and the scientific tools employed to find information on the losses have been documented. What is now understood, which tools work best, and the associated limitations are all discussed. Early studies used cytogenetic methods and tissue culture to obtain results. Improvements in laboratory tools such as better tissue culture incubators, inverted microscopes, laminar flow hoods, improvements in culture media, all contributed to obtaining more results for patients. These studies demonstrated the significant contribution of unbalanced chromosomal karyotypes to pregnancy failure. Maternal age as a contributing factor in trisomy was clearly demonstrated. First trimester miscarriage exhibits very high cytogenetic abnormality; in contrast to very low rates in later losses. Combining data across all time periods of pregnancy will affect the significance of chromosomal error in the early pregnancy failures. Cytogenetic methods investigate whole genomes, and are considered to represent the standard against which new methods must be validated. New molecular genetic methods provide the opportunity to examine samples without the necessity of tissue culture. Techniques may be site-specific or whole genome. Fluorescent in situ hybridisation (FISH), comparative genomic hybridisation (CGH), array-based CGH, single nucleotide polymorphism (SNP) detection, quantitative polymerase chain reaction (qPCR), and quantitative fluorescent PCR (QF-PCR), have all been utilised. In comparison studies with classical/conventional cytogenetics, each newer method offers advantages and limitations. At the present time, a combined approach using conventional and molecular methods will elucidate the cause of miscarriage for almost all samples. In a clinical setting this would be optimum.

Reprogenetics: Preimplantational genetics diagnosis

Preimplantational Genetics Diagnosis (PGD) is requested by geneticists and reproductive specialists. Usually geneticists ask for PGD because one or both members of the couple have an increased genetic risk for having an affected offspring. On the other hand, reproductive specialists ask for embryo aneuploidy screening (PGS) to assures an euploid embryo transfer, with the purpose to achieve an ongoing pregnancy, although the couple have normal karyotypes. As embryonic aneuploidies are responsible for pre and post implantation abortions, it is logical to considerer that the screening of the embryonic aneuploidies prior to embryo transfer could improve the efficiency of the in vitro fertilization procedures. Nevertheless, it is still premature to affirm this until well-designed clinical trials were done, especially in women of advanced age where the rate of embryos with aneuploidies is much greater. Although the indications of PGD are similar to conventional prenatal diagnosis (PND), PGD has less ethical objections than the PND. As with the PGD/PGS results only unaffected embryos are transferred, both methods can avoid the decision to interrupt the pregnancy due to a genetic problem; this makes an important difference when compared to conventional prenatal diagnosis.

Intrauterine growth retardation fetus with trisomy 16 mosaicism

Fetal trisomy 16 is considered uniformly lethal early in gestation. It has been reported to be associated with the variability of clinical features and outcomes. Mosaic trisomy 16 leads to a high risk of abnormality in prenatal cases. Intrauterine growth retardation (IUGR) is a common outcome of mosaic trisomy 16. Herein, we report on the case of Thai male IUGR fetus with trisomy 16 mosaicism. The fetal body was too small. Postmortem investigation of placenta revealed the abnormality including small placenta with furcated cord insertion and single umbilical cord artery. Cytogenetic study demonstrated trisomy 16 that was found 100% in placenta and only 16% in the fetal heart while other organs had normal karyotype. In addition, cardiac and other internal organs examination revealed normal morphology.

Validation of microarray comparative genomic hybridization for comprehensive chromosome analysis of embryos

OBJECTIVE

To validate and determine the best array-comparative genomic hybridization (aCGH; array-CGH) protocols for preimplantation genetic screening (PGS).

DESIGN

Embryos had one cell removed as a biopsy specimen and analyzed by one of two array-CGH protocols. Abnormal embryos were reanalyzed by fluorescence in situ hybridization (FISH).

SETTING

Reference laboratory.

PATIENT(S)

Patients donating embryos or undergoing PGS.

INTERVENTION(S)

Embryo biopsy, array-CGH, FISH reanalysis.

MAIN OUTCOME MEASURE(S)

Diagnosis, no result rate and error rate.

RESULT(S)

Method one produced 11.2% of embryos with no results and a 9.1% error rate compared with 3% and 1.9% for method two, respectively. Thereafter, only method two was used clinically. The aneuploidy rate for cleavage-stage embryos was 63.2%, significantly increasing with maternal age. The chromosomes most involved in aneuploidy were 16, 22, 21, and 15. We report the first live births after array-CGH combined with single blastomere biopsy.

CONCLUSION(S)

Array-CGH is proved to be highly robust (2.9% no results) and specific (1.9% error rate) when applied to rapid (24-hour) analysis of single cells biopsied from cleavage-stage embryos. This comprehensive chromosome analysis technique is the first to be validated by reanalyzing the same embryos with another technique (e.g., FISH). Unlike some alternative techniques for comprehensive chromosome screening, array-CGH does not require prior testing of parental DNA and thus advance planning and careful scheduling are unnecessary.

Epidemiology and Etiology of Mental Retardation

Mental retardation (MR) is a manifestation of a heterogeneous set of impairments and conditions that result in cognitive limitation. It is a condition of medical, educational, and social importance. Physicians identify profound, severe, and moderate MR but rarely diagnose mild MR unless it is associated with a genetic or medical syndrome. From a medical perspective, the quest for etiology and the possibility of medical or surgical intervention to minimize deterioration are paramount. Educators, on the other hand are less concerned with causation than with academic achievement and school success. The majority of cases of mild MR is identified in school settings. Finally, the public uses the label to describe poor adaptive skills. Adults with MR who hold jobs, live independently, and participate in society are not always described as having MR. Thus some individuals characterized in childhood or adolescence as having mild MR become indistinguishable from the general population in adulthood.

CONSERVATION SCIENCE IN A TERRORIST AGE: THE IMPACT OF AIRPORT SECURITY SCREENING ON THE VIABILITY AND DNA INTEGRITY OF FROZEN FELID SPERMATOZOA

In response to growing terrorism concerns, the Transportation Security Administration now requires that all checked baggage at U.S. airports be scanned through a cabinet x-ray system, which may increase risk of radiation damage to transported biologic samples and other sensitive genetic material. The objective of this study was to investigate the effect of these new airport security regulations on the viability and DNA integrity of frozen felid spermatozoa. Semen was collected from two domestic cats (Felis silvestris catus) and one fishing cat (Prionailurus viverrinus), cryopreserved in plastic freezing straws, and transferred into liquid nitrogen dry shippers for security screening. Treatment groups included frozen samples from each male scanned once or three times using a Transportation Security Administration-operated cabinet x-ray system, in addition to non-scanned samples (i.e., negative control) and samples previously scanned three times and exposed to five additional high-intensity x-ray bursts (i.e., positive control). Dosimeters placed in empty dry shippers were used to quantify radiation exposure. Following treatment, straws were thawed and spermatozoa analyzed for post-thaw motility (percentage motile and rate of progressive movement), acrosome status, and DNA integrity using single-cell gel electrophoresis (i.e., the comet assay). Dosimeter measurements determined that each airport screening procedure produced approximately 16 mrem of radiation exposure. Our results indicated that all levels of radiation exposure adversely affected (P < 0.05) post-thaw sperm motility, but the percentage of acrosome-intact spermatozoa did not differ (P > 0.05) among treatment groups. Results also showed that the amount of double-stranded DNA damage was greater (P < 0.05) in sperm samples from both cat species scanned three times compared to samples scanned once or negative controls. Findings suggest that new airport security measures may cause radiation-induced damage to frozen spermatozoa and other valuable biologic samples transported on passenger aircraft and that alternative modes of sample transportation should be used whenever possible.

Complete cytogenetic investigation of oocytes from a young cancer patient with the use of comparative genomic hybridisation reveals meiotic errors

OBJECTIVES

The complete cytogenetic investigation of human oocytes and the corresponding first polar bodies (PBs) derived from an 18-year old female cancer patient.

METHODS

A whole-genome amplification method combined with comparative genomic hybridisation (CGH) was employed for the analysis of 14 oocytes and their corresponding first PBs.

RESULTS

Chromosome abnormalities were detected in two oocyte-PB complexes. One oocyte had lost X-chromosome material (23,X,-Xcht), while its corresponding first PB showed the reciprocal gain (23,X,+Xcht). Double aneuploidy involving loss of chromatids for chromosomes X and 21 was identified in another first PB (23,X,-21cht,-Xcht). Aneuploidy was attributed to unbalanced pre-division of chromatids at meiosis I.

CONCLUSIONS

Meiotic errors in chromosome segregation can occur even in oocytes derived from young women, confirming the existence of age-independent factors contributing to aneuploidy. Such factors are of relevance to fertility, miscarriage and preimplantation aneuploidy screening for the purposes of increasing IVF success rates. The reliability of CGH in examining the whole chromosome complement of a single cell and of being able to detect chromatid anomalies is confirmed by this study.

Clinical, cytogenetic, and molecular findings of prenatally diagnosed mosaic trisomy 4

OBJECTIVES

To present the clinical, cytogenetic, and molecular findings of prenatally diagnosed mosaic trisomy 4.

CASE

An amniocentesis was performed at 21 weeks' gestation because of maternal anxiety. Cytogenetic analysis revealed mosaicism for trisomy 4, 47,XX,+4[4]/46,XX[16]. Level II ultrasound demonstrated tetralogy of Fallot. Repeated amniocentesis at 23 weeks' gestation revealed 47,XX,+4[4]/46,XX[19]. The pregnancy was terminated. Phenotypic findings included tetralogy of Fallot, hypertelorism, micrognathia, abnormal ears, duplicated phalanges of the left thumb, clinodactyly, and overlapping of the toes. The karyotype of the cord blood was 46,XX. Cytogenetic analyses of the multiple tissue samplings showed a karyotype of 47,XX,+4 in 40/40 cells of the amniotic membrane (amnion), and 47,XX,+4/46,XX with various levels of trisomy 4 in the cells of the liver, lungs, placenta, skin, and umbilical cord. The levels of trisomy 4 were 11/40 in the liver, 8/40 in the lungs, 31/40 in the placenta, 9/40 in the skin, and 8/40 in the umbilical cord.

METHOD

The parental origin and meiotic origin of trisomy 4 were determined by examining the amniotic membrane using quantitative fluorescent polymerase chain reaction assays with polymorphic markers specific for chromosome 4. The result was consistent with a paternal meiosis I nondisjunction error. The cord blood showed a biparental inheritance. An extra paternal heterozygous allele with partial dosage increase was noted in other fetal and extraembryonic tissues studied.

CONCLUSION

A diagnosis of trisomy 4 mosaicism in amniocytes indicates an increased risk for fetal abnormalities. Associated abnormal findings include congenital heart defects and anomalies of the digits and thumb. A confirmatory placental sampling may be helpful, whereas a fetal blood sampling is of a very limited value. A postnatal amnion sampling may provide additional clues to the fetal involvement of trisomy 4.

DNA damage in human and mouse spermatozoa after in vitro-irradiation assessed by the comet assay

The comet assay is widely employed as a method to measure DNA damage in a wide variety of cell types following genotoxic insult. We have used this method in order to characterise DNA damage in spermatozoa following in vitro irradiation with 137Cs gamma rays. In contrast to somatic cells, the DNA of mammalian spermatozoa is bound by protamine molecules allowing a sixfold more highly compact structure and thus rendering conventional cell lysis protocols ineffective. Therefore, this new method uses an extensive lysis step to ensure effective removal of DNA-associated proteins allowing DNA damage to be scored reproducibly in both murine and human spermatozoa. Mouse spermatozoa collected from the vas deferens at post-mortem or human spermatozoa provided by donors were irradiated with doses of gamma-rays from 0-100 Gy using a 137Cs source and then processed for both alkaline and neutral comet assays. Under neutral electrophoresis conditions, which permits the measurement of double-stranded DNA breaks, a linear increase in the amount of DNA damage measured was observed with increasing radiation dose for both murine and human spermatozoa. Similarly, using alkaline electrophoresis conditions to examine DNA single-strand breaks and alkali-labile sites, a linear relationship was also observed for murine sperm but in contrast no such relationship was apparent for human spermatozoa subjected to the same radiation treatments. Interestingly, unirradiated sperm (both human and mouse) showed extensive DNA migration from the nucleus after alkaline assay. Since it is unlikely that the DNA of normal spermatozoa contains high numbers of single-strand breaks and damage was not detected for unirradiated sperm in the neutral assay, it is more likely that this DNA migration is due to the presence of high numbers of alkali labile sites within sperm DNA and that these may be related to the highly condensed structure of spermatozoal DNA. The large radiation doses used in these experiments to produce measurable amounts of DNA damage reflects the high radioresistance of spermatozoa compared to somatic cells and this may also be related to the differences in DNA packaging and conformation. In conclusion, this work shows that the comet assay represents a new method for examining DNA damage in spermatozoa and should be evaluated for use in reproductive toxicity testing.

Increased paternal age in CHARGE association

The acronym CHARGE refers to a non-random clustering of congenital malformations whose cause remains unknown. Here, we report on a series of 41 patients and find a significant increase in mean paternal age of birth of CHARGE patients (33.7 +/- 8 years) compared with the control population (30.8 +/- 5 years). In contrast, maternal age was not statistically different in patients and controls. These data suggest the possible role of a dominant mutation or, less likely, a subtle chromosomal abnormality in CHARGE association.

Sex ratios in fetuses and liveborn infants with autosomal aneuploidy

Ten data sources were used substantially to increase the available data for estimating fetal and livebirth sex ratios for Patau (trisomy 13), Edwards (trisomy 18), and Down (trisomy 21) syndromes and controls. The fetal sex ratio estimate was 0.88 (N = 584) for trisomy 13, 0.90 (N = 1702) for trisomy 18, and 1.16 (N = 3154) for trisomy 21. All were significantly different from prenatal controls (1.07). The estimated ratios in prenatal controls were 1.28 (N = 1409) for CVSs and 1.06 (N = 49427) for amniocenteses, indicating a clear differential selection against males, mostly during the first half of fetal development. By contrast, there were no sex ratio differences for any of the trisomies when comparing gestational ages < 16 and > 16 weeks. The livebirth sex ratio estimate was 0.90 (N = 293) for trisomy 13, 0.63 (N = 497) for trisomy 18, and 1.15 (N = 6424) for trisomy 21, the latter two being statistically different than controls (1.05) (N = 3660707). These ratios for trisomies 13 and 18 were also statistically different than the ratio for trisomy 21. Only in trisomy 18 did the sex ratios in fetuses and livebirths differ, indicating a prenatal selection against males > 16 weeks. No effects of maternal age or race were found on these estimates for any of the fetal or livebirth trisomies. Sex ratios for translocations and mosaics were also estimated for these aneuploids. Compared to previous estimates, these results are less extreme, most likely because of larger sample sizes and less sample bias. They support the hypothesis that these trisomy sex ratios are skewed at conception, or become so during embryonic development through differential intrauterine selection. The estimate for Down syndrome livebirths is also consistent with the hypothesis that its higher sex ratio is associated with paternal nondisjunction.

Trisomy 15 mosaic derived from trisomic conceptus: report of a case and a review

We report on a fetus with 47,XX,+15 chromosome abnormality detected on chorionic villus sampling (CVS). The pregnancy was terminated at 15.5 weeks of gestation and chromosome analysis done on amniocytes and fetal tissues showed a karyotype 46,XX/47,XX,+15. Autopsy showed multiple abnormalities. Short-arm polymorphisms of the three number 15 chromosomes were highly informative in the delineation of parental origin and the stage of meiotic error. Using fluorescent in situ hybridization (FISH) with D15Z1 and a chromosome 15 painting probe, in addition to DA/DAPI and G-banding, we were able to show that the trisomic conceptus was derived through maternal meiosis I error. The trisomic state was then partially corrected by the loss of one of the two maternal 15s resulting in mosaicism without uniparental disomy (UPD). Striking differences in the proportion of trisomic cells in kidneys, blood, intestine, and skin, and lower proportions of trisomic cells in transformed and frozen than in fresh tissues, illustrate the continuing cell selection in this fetus in favour of the normal cell line. Trisomy 15 conceptions are usually aborted spontaneously in the first trimester of pregnancy. The longer survival of this fetus is most probably the result of a chromosome 15 loss from the trisomic zygote. To the best of our knowledge, the presence of this lethal trisomy has been reported in only five live-born infants, and in five fetuses including the present case, it was detected prenatally and the pregnancies were terminated.

Clinical and molecular studies in full trisomy 22: further delineation of the phenotype and review of the literature

Trisomy 22 is commonly found among spontaneous abortions, second in frequency of occurrence only to trisomy 16. Most earlier reports of surviving trisomy 22 cases in the literature are thought to represent the product of unbalanced 11;22 translocations or the result of undetected mosaicism, since this condition is thought to manifest early embryonic or fetal lethality. We present two strikingly similar cases of non-mosaic trisomy 22 surviving to late gestation. In this paper we emphasize the unique phenotype of this trisomy which included intrauterine growth retardation, microcephaly, broad flat nasal bridge with epicanthal folds and ocular hypertelorism, microtia, variable cleft palate, webbed neck, congenital heart defects involving anomalous great vessels, anorectal and renal anomalies, and hypoplastic distal digits with thumb anomalies. We also explore why some cases survive to late gestation. Confined placental mosaicism, a frequent finding in other lethal trisomies, has been ruled out in one of the cases. Molecular studies done to assess the parental origin of the extra chromosome in the other case showed that the non-disjunction originated during maternal meiosis II. Parental origin of the extra chromosome does not seem to play a role in late survival for trisomy 22.

Increased risk of Alzheimer's disease in mothers of adults with Down's syndrome

Most adults with Down's syndrome (DS) develop neuropathology characteristic of Alzheimer's disease (AD) by the age of 40. Most of the non-dysjunction events in DS are of maternal origin. We postulated therefore that a shared genetic susceptibility to DS and AD would be associated with an increased frequency of AD among mothers, but not fathers, of individuals with DS. We further hypothesised that the shared susceptibility could involve an accelerated ageing process, leading to the birth of a child with DS to a relatively young mother and to an increased risk of dementia in the mother and her relatives. Families of 96 adults with DS and of 80 adults with other forms of mental retardation were ascertained through the New York State Developmental Disabilities services network. A semi-structured interview was used to obtain information on the presence or absence of non-stroke-related dementia and other disorders in parents. There was an increase in risk of dementia among mothers of DS probands compared with control mothers (risk ratio 2.6 [95% CI 0.9-7.3]). The risk of dementia among mothers who were 35 or younger when their DS children were born was 5 times that of control mothers (4.9 [1.6-15.4]). There was no increase in risk of dementia among mothers who were older (> 35 years) at the proband's birth (0.8 [0.2-3.4]). There was no difference in risk of dementia between fathers of DS cases and fathers of controls (1.2 [0.4-3.9]) and no discernible influence of age on this risk. Familial aggregation of dementia among mothers of adults with DS supports the hypothesis of a shared genetic susceptibility to DS and AD.

Human maternal uniparental disomy for chromosome 16 and fetal development

Two severely growth-retarded fetuses found to have maternal uniparental disomy (UPD) for chromosome 16 and trisomy 16 placental mosaicism both had an unfavourable outcome. Antenatally, the first case was complicated by an unexplained raised maternal serum alpha-fetoprotein concentration, preterm premature rupture of the membranes, and growth retardation detectable at 21 weeks' gestation, whilst the other had an unexplained raised maternal serum human chorionic gonadotrophin level, a two-vessel cord on ultrasound, and cessation of growth at 25 weeks. At post-mortem, both babies had an imperforate anus. Fetal maternal UPD may explain the poor outcome that occurs in some cases of confined placental mosaicism for chromosome 16 and is also associated with specific fetal abnormalities.

Confined placental mosaicism and stillbirth

The cause of stillbirth can usually be determined in only 20% of cases. An increased frequency of adverse pregnancy outcome, including pregnancy loss, intrauterine growth restriction, and premature labor, has been observed in association with confined placental mosaicism (CPM), which is characterized by a discrepancy between the karyotype of the fetus and placenta. Specific chromosomal trisomies have been observed in CPM more frequently than others, with trisomy of chromosomes 7, 16, and 18 being the most prevalent. In pregnancies with CPM it has been shown that the zygote is often trisomic, and postzygotic loss of the additional chromosome occurred in the embryonic progenitor cells leading to a dichotomy between the placenta and the embryo/fetus. In one third of such cases fetal uniparental disomy (UPD), which is the presence of both homologues of a chromosome derived from one parent, can be expected. The specific role of the trisomic placenta and the presence of fetal UPD in cases of altered intrauterine fetal development has not been fully established for various chromosomes. Therefore, to enhance our understanding of the pathogenesis of stillbirth it is imperative that cytogenetic analysis of both fetal and placental tissues be performed in all cases of unexplained stillbirth.

Future of germ cell cytogenetics

The celebration of the 25th Anniversary of the Environmental Mutagen Society provides an excellent opportunity to assess the status of research in a broad range of areas, with an emphasis on the directions in which they are going. This chapter concentrates on the analysis of chromosomal alterations in mammalian germ cells. The future developments in germ cell cytogenetics research will build heavily upon techniques developed over the past 25 years. With these it is possible to assess numerical and structural alterations in the male in differentiating spermatogonia, spermatocytes, and post-meiotic cells (at the first cleavage division) and for the female in oocytes and the zygote. The most predictable advances will be in the identification of specific alterations through FISH of interphase spermatozoa in humans and further improvements with the human sperm/hamster egg in vitro fertilization technique. Of particular importance is the fact that this will allow for the study of effects in human germ cells. From a more speculative viewpoint it might be possible to assess the role of particular genomic organization on genetic outcomes by direct observation; these might include genomic imprinting and the visual separation of male and female genomes. The overall aim of germ cell cytogenetic studies will remain as improving our ability to identify and estimate the true genetic risk in humans.

Methods and concepts in detecting abnormal reproductive outcomes of paternal origin

There is conclusive evidence that exposures of human males to ionizing radiation or certain chemicals can diminish sperm production and reduce fertility. Of approximately 100 chemical agents and mixtures that have been evaluated in men by semen analysis, about half (mostly drugs and a few occupational exposures) reduced sperm quantity and quality; several of these agents also affected the fertility of exposed men. It is now well recognized that the importance of the father in reproduction goes beyond fertilization. Abnormalities in paternal chromosomes (structural and numeric) have been found in various abnormal reproductive outcomes, including chromosomal abnormality syndromes among newborns. In rodent systems, exposure of males to mutagens before mating induces transmissible cytogenetic and genetic abnormalities as well as morphologic defects and cancer among offspring. Consistent with animal findings, there is growing epidemiologic evidence of associations between male exposures to exogenous agents and abnormal reproductive outcomes (fetal loss, birth defects, childhood cancer, etc.). However, no clear links have been established between exposure, mechanism of transmission, and abnormal reproductive outcomes. It is not known to what extent male-mediated birth defects and childhood cancer are due to genetic, epigenetic, or nongenetic causes. Viewed in a multigenerational context, the role of the father in abnormal reproductive outcomes is dependent on his exposure history and susceptibilities as well as those of his mate. Relevant exposures may occur any time between conception of the parents and production of their fertilizing gametes, including their development in utero, childhood, and adolescence. Efficient measurements (including biomarkers) of relevant exposure, early biologic effects, and susceptibility in human males are under development. An integrated approach is recommended for assessing male reproductive and genetic toxicity that utilizes biomarkers in (a) epidemiologic studies of exposed human populations, (b) risk characterization in sensitive laboratory species, and (c) in vivo and in vitro studies of the molecular mechanisms of action of toxicants. A special category of "bridging" biomarkers is needed for evaluating animal data for risk assessment and for discriminating among genetic, epigenetic, and nongenetic mechanisms of abnormal reproductive outcomes of paternal origin.

Detection of the heteromorphic spectrum of heterochromatin in the human genome by in situ digestion using restriction endonuclease AluI

A battery of selective banding techniques has been utilized to identify the heteromorphic markers in the human genome. The recent addition of the AluI/Giemsa (G)-technique has helped not only in identifying the variable sites, but in characterizing their heteromorphic spectra. In the present investigation, we classified the pericentromeric heterochromatin by the AluI/G-technique by its size and position using 50 normal individuals and suggested the potential uses of this banding technique over earlier methods.

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.

The appropriate upper age limit for semen donors: a review of the genetic effects of paternal age

There is little evidence to support a correlation between increased paternal age and the incidence of chromosome anomalies. Though a few early studies demonstrated an increased risk of Down syndrome with advanced paternal age, especially after the age of 55, numerous later studies failed to confirm any paternal age effect. Among structural chromosome anomalies, only the inherited reciprocal translocations were found to be more common among children of older fathers. Chromosome analysis of semen donors would rule out this problem. There is evidence, however, that the incidence of serious nonchromosomal birth defects, especially those arising from new autosomal mutations, increases with paternal age. Risk estimates have been established for increased paternal age and contribution to new dominant mutations. The established association between increased paternal age and new autosomal mutations and the fact that most of the disorders associated with such mutations cannot be prenatally diagnosed may be important in establishing the upper age limit for semen donors. Recommendations from the literature are interpreted to advise men to have their children before age 40. This suggestion would obviously benefit immediate offspring and perhaps successive generations as well. The limit of 35 years set by AATB standards, therefore, is in harmony with such benefit, but may be too stringent in eliminating potential donors. Accordingly, the Reproductive Council of the AATB has initiated steps to modify its standards to the age limit of 40.

Triple autosomal trisomy in a pregnancy at risk for Bloom's syndrome

Cytogenetic analysis of the products of conception in a pregnancy at risk for Bloom's syndrome (BS) documented the karyotype 49,XX, +2, +8, +11. Autosomal triple trisomy has previously been reported in abortuses but is exceedingly rare. Other interesting but previously unreported observations made during the present study were the following: BS in a Brazilian individual, the first instance of BS diagnosed in South America; transmission of the BS mutation in Jews that are non-Ashkenazi; a medulloblastoma in the propositus, the first malignant brain tumor reported in BS; and, as in all previously examined pregnancies at risk for BS, non-homozygosity for the BS mutation.

Trisomy 14 mosaicism in a 5-year-old boy

We report on a 5-year-old boy with failure to thrive, mental retardation, a broad nose, hypertelorism, slight antimongoloid slant palpebral fissures, mild ptosis, microphthalmia, short and wide neck, apparently acyanotic tetralogy of Fallot, dislocation of the left hip, generalized linear and patchy hyperpigmentation, micropenis, and undescended testes. He had mosaicism of 46,XY/47,XY, + 14 in a ratio of 3:1. Comparisons are made with the other reports of trisomy 14 mosaicism and relationship to incontinentia pigmenti.

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.

On the parental origin of de novo mutation in man

Studies tracing parental origins of human mutations by means of cytogenetic polymorphisms and RFLPs show that most trisomics arise out of maternal errors of segregation at the first meiotic division in oocytes. Temporal disturbance of meiotic progression seems likely to underly aneuploidy production in the female mouse, and this could equally be true in women, most especially as they approach the menopause when irregular cyclicity sets in. For human monosomy X, a high proportion of cases show loss of the paternal sex chromosome, and from experimental data giving similar findings in the mouse, it seems likely that the error could arise at the pronuclear stage after sperm entry into the egg, rather than at meiosis in the male. For human point mutations and structural rearrangements, a bias exists towards paternal origins. Errors arising during spermatogonial proliferation in men could contribute point mutations, these accumulating over a lifetime to give paternal age effects. For structural rearrangements, the hypersensitive stage is likely to be the post-meiotic differentiating spermatid, a stage not subject to germinal selection, and one which in Drosophila has been shown to combine high breakability with enhanced repair. Lack of a comparable cell type to the condensing spermatid of the male might be a reason why balanced structural rearrangements are produced rather rarely in females, at least in the mouse.

Pre- and postnatal diagnosis of trisomy 4 mosaicism

A liveborn girl with 46,XX/47,XX+4 mosaicism is reported for the first time. The diagnosis of true mosaicism was established initially in the assay of cultured amniotic fluid cells, although fetal blood obtained by percutaneous umbilical blood sampling showed a 46,XX chromosome constitution. The liveborn infant had manifestations previously reported in dup(4p) and dup(4q) syndromes. Cells in cord and arterial blood samples also were 46,XX, but cultures of placenta and bilateral forearm skin biopsies showed 46,XX/47,XX,+4 mosaicism. This case illustrates the disadvantage of chromosome analysis from blood alone when tissue-specific mosaicism is present.

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.

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

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 chromosome 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 patient and that asynapsis was not a prerequisite for the autosomal nondisjunction.

Use of restriction fragment length polymorphic probes in the analysis of Down's syndrome trisomy

Restriction fragment length polymorphic probes are being used more frequently in the molecular analysis of Down's syndrome and in the origin of nondisjunction in the syndrome. The type of information gained from RFLPs overlaps but differs from the information from cytogenetic heteromorphisms. From the allele frequencies of commonly available probes we have derived the expected frequencies of all matings in the population. Each mating has been defined and partitioned to show the genotypes and phenotypes expected, with numerical values based on studies with heteromorphisms. From this we show how the various phenotypes can be used to calculate the origin of nondisjunctions and their expected frequencies. Further, an alternative method is outlined for mapping the distance between a probe and its centromere based on the distortion, caused by crossing-over, of the expected 1st to 2nd division nondisjunction ratio. Finally, we discuss prospects for various uses of probes in the analysis of Down's syndrome.

Ocular findings in partial trisomy 3q. A case report and review of the literature

The authors report the ocular findings in a patient with partial trisomy 3 [46, XY-15, der(15), t(3;15) (q26;128) mat] and review the literature on this chromosomal anomaly. The patient's main ocular finding was infantile glaucoma. The patient's mother was phenotypically normal and carried a balanced chromosome translocation. Awareness of the ocular abnormalities associated with partial trisomy syndromes is essential since more children with these syndromes survive today than in the past.

Maternal age in trisomy

By comparison with a more general theory, data on trisomy in live births, amniocenteses, and spontaneous abortions by year of maternal age are shown to fit a logistic augmented by a proportion independent of maternal age. The frequency of trisomy increases monotonically, with no discrepancy at extremely low or high maternal age. Trisomy 16 is exceptional in that all cases appear to be age-dependent. For groups A, B, and C most trisomies arise by a process independent of maternal age. A small proportion of these trisomies and about half of trisomies for smaller chromosomes (excluding trisomies 16 and perhaps 22) originate by some process dependent on maternal (but not paternal) age and therefore presumably independent of crossingover, which in the female takes place before birth.

Klinefelter's syndrome: an analysis of the origin of the additional sex chromosome using molecular probes

The results of our study of the origin of the additional X chromosome in 39 males with a 47,XXY chromosome constitution are reported. We used a total of 20 X-linked RFLPs and successfully determined the origin of all 32 patients in whom DNA from both parents was available, and a further 3 in whom DNA was available from the patient and mother only. Males whose additional X chromosome was maternal in origin were further investigated using an X-linked centromere specific probe to determine the cell division at which the error occurred. Our results showed 53% of the non-disjunction to be attributable to pat mei I errors, 34% to mat mei I errors, 9% to mat mei II errors and 3% to a post-zygotic mitotic error. In the great majority of patients resulting from an error of maternal meiosis there was clear evidence of recombination involving the non-disjoined chromosomes, suggesting that absence of recombination is not an important aetiological factor in non-disjunction of the X chromosome in female meiosis. There was no alteration of parental age associated with the paternally derived 47,XXY males but a marked increase in maternal age among the maternally derived 47,XXY males, the increase being associated with mat mei I but not mat mei II errors. The proportion of paternally and maternally derived cases was similar among different ascertainment classes, suggesting that there is no dramatic effect of parental origin of the additional X chromosome on the phenotype of 47,XXY males.

Cytogenetic studies of couples with repeated spontaneous abortions of known karyotype

Several cytogenetic studies have reported an increased frequency of aneuploidy in peripheral blood cultures of couples with multiple spontaneous abortions. However, in none of the studies have the chromosome constitutions of the fetuses been known, making it difficult to interpret these observations. In the present study, we summarize our cytogenetic observations on 23 couples with multiple miscarriages, 12 of whom had repeated chromosomally normal spontaneous abortions, and 11 repeated trisomic spontaneous abortions. We were unable to demonstrate a significant difference in the level of aneuploidy between the two groups. Therefore, it seems unlikely that aneuploidy detectable in peripheral blood preparations is an important indicator of fetal aneuploidy.

Cytogenetic and molecular studies of trisomy 13

Chromosome heteromorphisms, restriction fragment length polymorphisms, or both were used to study the parental origin of 33 cases of simple trisomy 13 and eight cases of translocation trisomy 13. The most common origin for the simple trisomies was non-disjunction at maternal meiosis I, while for the translocations an equal number of paternally and maternally derived cases was observed. In seven of the simple trisomies, information was obtained from both the cytogenetic and molecular markers, making it possible to study recombination between the two non-disjoined chromosomes. Five of the seven cases involved errors at meiosis I, with crossing over being detected in two of three cases of maternal origin and in one of two cases of paternal origin. This indicates that absence of recombination because of pairing failure is unlikely to be of major importance in the genesis of trisomy 13.

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.

Regional localization and characterization of a DNA segment on the long arm of chromosome 21

A human genomic DNA fragment, pAM37 (HGM8; D21S22), was mapped to chromosome 21q2.1-q2.21 by in situ hybridization. This segment is therefore situated on the boundary of the "pathological region" of Down syndrome. A genomic restriction map encompassing 35 kb of chromosome 21 was derived and two restriction fragment length polymorphisms (RFLPs) were mapped and characterized. A homologous sequence was detected in the mouse genome but no homologous RNA was detected in a range of human tissues. This DNA segment will contribute to the linkage mapping of chromosome 21 and will facilitate delineation of the pathological region of Down syndrome.