On the origin of the maternal age effect in trisomy 21 Down syndrome: the Oocyte Mosaicism Selection model

DNA damage in preimplantation embryos and gametes: specification, clinical relevance and repair strategies

BACKGROUND

DNA damage is a hazard that affects all cells of the body. DNA-damage repair (DDR) mechanisms are in place to repair damage and restore cellular function, as are other damage-induced processes such as apoptosis, autophagy and senescence. The resilience of germ cells and embryos in response to DNA damage is less well studied compared with other cell types. Given that recent studies have described links between embryonic handling techniques and an increased likelihood of disease in post-natal life, an update is needed to summarize the sources of DNA damage in embryos and their capacity to repair it. In addition, numerous recent publications have detailed novel techniques for detecting and repairing DNA damage in embryos. This information is of interest to medical or scientific personnel who wish to obtain undamaged embryos for use in offspring generation by ART.

OBJECTIVE AND RATIONALE

This review aims to thoroughly discuss sources of DNA damage in male and female gametes and preimplantation embryos. Special consideration is given to current knowledge and limits in DNA damage detection and screening strategies. Finally, obstacles and future perspectives in clinical diagnosis and treatment (repair) of DNA damaged embryos are discussed.

SEARCH METHODS

Using PubMed and Google Scholar until May 2021, a comprehensive search for peer-reviewed original English-language articles was carried out using keywords relevant to the topic with no limits placed on time. Keywords included ‘DNA damage repair’, ‘gametes’, ‘sperm’, ‘oocyte’, ‘zygote’, ‘blastocyst’ and ‘embryo’. References from retrieved articles were also used to obtain additional articles. Literature on the sources and consequences of DNA damage on germ cells and embryos was also searched. Additional papers cited by primary references were included. Results from our own studies were included where relevant.

OUTCOMES

DNA damage in gametes and embryos can differ greatly based on the source and severity. This damage affects the development of the embryo and can lead to long-term health effects on offspring. DDR mechanisms can repair damage to a certain extent, but the factors that play a role in this process are numerous and altogether not well characterized. In this review, we describe the multifactorial origin of DNA damage in male and female gametes and in the embryo, and suggest screening strategies for the selection of healthy gametes and embryos. Furthermore, possible therapeutic solutions to decrease the frequency of DNA damaged gametes and embryos and eventually to repair DNA and increase mitochondrial quality in embryos before their implantation is discussed.

WIDER IMPLICATIONS

Understanding DNA damage in gametes and embryos is essential for the improvement of techniques that could enhance embryo implantation and pregnancy success. While our knowledge about DNA damage factors and regulatory mechanisms in cells has advanced greatly, the number of feasible practical techniques to avoid or repair damaged embryos remains scarce. Our intention is therefore to focus on strategies to obtain embryos with as little DNA damage as possible, which will impact reproductive biology research with particular significance for reproductive clinicians and embryologists.

Understanding etiology of chromosome 21 nondisjunction from gene × environment models

Maternal risk factors and their interactions with each other that associate chromosome 21 nondisjunction are intriguing and need incisive study to be resolved. We determined recombination profile of nondisjoined chromosome 21 and maternal genotypes for four selected polymorphic variants from the folate regulators genes stratifying the women according to the origin of segregation error and age at conception. We conducted association study for genotype and maternal addiction to smokeless chewing tobacco, usually chopped tobacco leaves or paste of tobacco leaves with the incidence of Down syndrome birth. Additionally, we designed various logistic regression models to explore the effects of maternal genotype, maternal habit of smokeless chewing tobacco, maternal age at conception and all possible interactions among them on chromosome 21 nondisjunction. We found folate regulator gene mutations are associated with maternal meiosis II error. Regression models revealed smokeless chewing tobacco and folate polymorphic/mutant risk genotype interact with each other to increase the risk of reduced and single peri-centromeric recombination events on chromosome 21 that nondisjoined at meiosis II in the oocytes and the effect is maternal age independent. We inferred maternal folate polymorphic/mutant risk genotypes and habit of smokeless chewing tobacco interact with each other and increase the risk of meiosis II error in oocytes in maternal age-independent manner.

The Role of HSA21 Encoded Mirna in Down Syndrome Pathophysiology:Opportunities in miRNA-Targeted Pharmacotherapy and Diagnosis of the Down Syndrome

Trisomy 21 is the most prevalent aneuploidy disorder among live-born children worldwide. Itresults from the presence of an extra copy of chromosome 21 which leads to a wide spectrum ofpathophysiological abnormalities and intellectual disabilities. Nevertheless human chromosome21 (HSA21) possess protein non-coding regions where HAS-21 derived-microRNA genes aretranscribed from. In turn, these HSA21-derived miRNAs curb protein translation of severalgenes which are essential to meet memory and cognitive abilities. From the genetics andmolecular biology standpoints, dissecting the mechanistic relationship between DS pathology/symptoms and five chromosome 21-encoded miRNAs including miR-99a, let-7c, miR-125b-2,miR-155 and miR-802 seems pivotal for unraveling novel therapeutic targets. Recently,several studies have successfully carried out small molecule inhibition of miRNAs function,maturation, and biogenesis. One might assume in the case of DS trisomy, the pharmacologicalinhibition of these five overexpressed miRNAs might open new avenues for amelioration of theDS symptoms and complications. In this review, we primarily elucidated the role of HSA21-encoded miRNAs in the DS pathology which in turn introduced and addressed importanttherapeutic targets. Moreover, we reviewed relevant pharmaceutical efforts that based theirgoals on inhibition of these pathological miRNAs at their different biogenesis steps. We havealso discussed the challenges that undermine and question the reliability of miRNAs as noneinvasivebiomarkers in prenatal diagnosis.

Evidence for the Oocyte Mosaicism Selection model on the origin of Patau syndrome (trisomy 13)

INTRODUCTION

In 2008, Hultén et al hypothesized that maternal ovarian trisomy 21 mosaicism might be the primary causative factor for fetal Down syndrome. We hypothesize that this theory can be extended to trisomy 13.

MATERIAL AND METHODS

We collected fetal ovarian tissue from seven female fetuses between 16 and 23 gestational weeks, following the termination of the pregnancy for non-genetic reasons. All procedures were performed with informed consent and ethical approval from the local ethics committee. We used touch preparation techniques from fetal ovarian tissues and an anti-stromal antigen 3 antibody against the meiosis-specific stromal antigen 3 protein to differentiate between germ cells, ovarian stromal cells and the cells entering their first meiotic prophase. We used fluorescence in situ hybridization analysis to determine chromosome 13 numbers in each cell.

RESULTS

We were able to detect a proportion of trisomy 13 cells in all cases. The average incidence of trisomy 13 cells was 2.04% in stromal antigen 3-positive and 0.91% in the stromal antigen 3-negative cells. The number of the trisomic cells increased significantly with gestational age (for stromal antigen 3-positive cells r = 0.93, P = 0.0038, for stromal antigen 3-negative cells r = 0.85, P = 0.0071).

CONCLUSIONS

This study indicates that besides trisomy 21, the Oocyte Mosaicism Selection model could be extended to trisomy 13 as well. The crucial factor for trisomy 13 seems to be the pre-meiotic/mitotic trisomy 13 mosaicism, leading to a so-called secondary meiotic nondisjunction of those oocytes having three copies of chromosome 13.

Down syndrome and microRNAs

In recent years numerous studies have indicated the importance of microRNAs (miRNA/miRs) in human pathology. Down syndrome (DS) is the most prevalent survivable chromosomal disorder and is attributed to trisomy 21 and the subsequent alteration of the dosage of genes located on this chromosome. A number of miRNAs are overexpressed in down syndrome, including miR-155, miR-802, miR- 125b-2, let-7c and miR-99a. This overexpression may contribute to the neuropathology, congenital heart defects, leukemia and low rate of solid tumor development observed in patients with DS. MiRNAs located on other chromosomes and with associated target genes on or off chromosome 21 may also be involved in the DS phenotype. In the present review, an overview of miRNAs and the haploinsufficiency and protein translation of specific miRNA targets in DS are discussed. This aimed to aid understanding of the pathogenesis of DS, and may contribute to the development of novel strategies for the prevention and treatment of the pathologies of DS.

Disomy 21 in spermatozoa and the paternal origin of trisomy 21 Down syndrome

Background

Trisomy 21 Down syndrome is the most common genetic cause for congenital malformations and intellectual disability. It is well known that in the outstanding majority of cases the extra chromosome 21 originates from the mother but only in less than 10 % from the father. The mechanism underlying this striking difference in parental origin of Trisomy 21 Down syndrome is still unknown. However, it seems likely that the main reason is a much higher stringency in the elimination of any trisomy 21 cells during fetal testicular than ovarian development. We have here focussed attention on the paternal gametic output, i.e. the incidence of disomy 21 in spermatozoa.

Results

We have used fluorescence in situ hybridisation (FISH) to determine the copy number of chromosome 21 in spermatozoa from 11 men with normal spermiograms. Due to the well-known risk of false positive and false negative signals using a single FISH probe, we have applied two chromosome 21q probes, and we have added a chromosome 18-specific probe to allow differentiation between disomy 21 and diploidy. Analysing a total number of 2000 spermatozoa per case, we documented an average incidence of disomy 21 at 0.13 %, with a range of 0.00-0.25 % and a SD of 0.08. There was no indication of diploidy in this cohort of 22,000 sperm.

Conclusion

Numerous previous studies on the incidence of disomy 21 in sperm have been published, using FISH. As far as we are aware, none of these have applied more than a single chromosome 21-specific probe. Accepting our mean of 0.13 % of disomy 21, and providing there is no selective fertilisation capability of disomy 21 sperm in relation to the normal, we conclude that around 1 in 800 conceptions is expected to be trisomic for chromosome 21 of paternal origin. Bearing in mind that the maternal origin likely is at least 10 times more common, we tentatively propose that around 1 in 80 oocytes in the maternal ovarian reserve may be disomy 21. One reason for this discrepancy may be a more stringent selection against aberrant chromosome numbers during spermatogenesis than oogenesis. Further work is required to determine the relevant stages of spermatogenesis at which such a selection may take place.

Genome-wide maps of recombination and chromosome segregation in human oocytes and embryos show selection for maternal recombination rates

Crossover recombination reshuffles genes and prevents errors in segregation that lead to extra or missing chromosomes (aneuploidy) in human eggs, a major cause of pregnancy failure and congenital disorders. Here, we generate genome-wide maps of crossovers and chromosome segregation patterns by recovering all three products of single female meioses. Genotyping > 4 million informative single-nucleotide polymorphisms (SNPs) from 23 complete meioses allowed us to map 2,032 maternal and 1,342 paternal crossovers and to infer the segregation patterns of 529 chromosome pairs. We uncover a novel reverse chromosome segregation pattern in which both homologs separate their sister chromatids at meiosis I; detect selection for higher recombination rates in the female germline by the elimination of aneuploid embryos; and report chromosomal drive against non-recombinant chromatids at meiosis II. Collectively, our findings reveal that recombination not only affects homolog segregation at meiosis I but also the fate of sister chromatids at meiosis II.

Inheritance of a Chromosome 3 and 21 Translocation in the Fetuses, with One also Having Trisomy 21, in Three Pregnancies in One Family

The majority of chromosome rearrangements are balanced reciprocal and Robertsonian translocations. It is now known that such abnormalities cause no phenotypic effect on the carrier but lead to increased risk of producing unbalanced gametes. Here, we report the inheritance of a translocation between chromosomes 3 and 21 in a family with one of two fetuses with Down Syndrome carrying the same translocation and the other also carrying the same translocation without the additional chromosome 21. Chromosomal analysis from fetal amniotic fluid and peripheral blood lymphocytes from the family were performed at the Çukurova University Hospital at Adana, Turkey. We assessed a family in which the translocation between chromosomes 3 and 21 segregates: one of the three progenies carried the 47,XX,+21,t(3;21)(q21;q22) karyotype and presented with Down Syndrome; another of the three progenies carried the 46,XX,t(3;21) (q21;q22) karyotype and the third had the 46,XY karyotype. Their mother is phenotypically normal. Apparently this rearrangement occurred due to an unbalanced chromosome segregation of the mother [t(3;21)(q21;q22)mat]. This family will enable us to explain the behavior of segregation patterns and the mechanism for each type of translocation from carrier to carrier and their effects on reproduction and numerical aberrations. These findings can be used in clinical genetics and may be used as an effective tool for reproductive guidance and genetic counseling.

The origin, mechanisms, incidence and clinical consequences of chromosomal mosaicism in humans

BACKGROUND

Chromosomal mosaicism, the presence of two or more distinct cell lines, is prevalent throughout human pre- and post-implantation development and can lead to genetic abnormalities, miscarriages, stillbirths or live births. Due to the prevalence and significance of mosaicism in the human species, it is important to understand the origins, mechanisms and incidence of mosaicism throughout development.

METHODS

Literature searches were conducted utilizing Pubmed, with emphasis on human pre- and post-implantation mosaicism.

RESULTS

Mosaicism persists in two separate forms: general and confined. General mosaicism is routine during human embryonic growth as detected by preimplantation genetic screening at either the cleavage or blastocyst stage, leading to mosaicism within both the placenta and fetus proper. Confined mosaicism has been reported in the brain, gonads and placenta, amongst other places. Mosaicism is derived from a variety of mechanisms including chromosome non-disjunction, anaphase lagging or endoreplication. Anaphase lagging has been implicated as the main process by which mosaicism arises in the preimplantation embryo. Furthermore, mosaicism can be caused by any one of numerous factors from paternal, maternal or exogenous factors such as culture media or possibly controlled ovarian hyperstimulation during in vitro fertilization (IVF). Mosaicism has been reported in as high as 70 and 90% of cleavage- and blastocyst-stage embryos derived from IVF, respectively.

CONCLUSIONS

The clinical consequences of mosaicism depend on which chromosome is involved, and when and where an error occurs. Mitotic rescue of a meiotic error or a very early mitotic error will typically lead to general mosaicism while a mitotic error at a specific cell lineage point typically leads to confined mosaicism. The clinical consequences of mosaicism are dependent on numerous aspects, with the consequences being unique for each event.

Maternal Germinal Trisomy 21 in Down Syndrome

It has now been over 50 years since it was discovered that Down syndrome is caused by an extra chromosome 21, i.e., trisomy 21. In the interim, it has become clear that in the majority of cases, the extra chromosome is inherited from the mother, and there is, in this respect, a strong maternal age effect. Numerous investigations have been devoted to clarifying the underlying mechanism, most recently suggesting that this situation is exceedingly complex, involving both biological and environmental factors. On the other hand, it has also been proposed that germinal trisomy 21 mosaicism, arising during the very early stages of maternal oogenesis with accumulation of trisomy 21 germ cells during subsequent development, may be the main predisposing factor. We present data here on the incidence of trisomy 21 mosaicism in a cohort of normal fetal ovarian samples, indicating that an accumulation of trisomy 21 germ cells does indeed take place during fetal oogenesis, i.e., from the first to the second trimester of pregnancy. We presume that this accumulation of trisomy 21 (T21) cells is caused by their delay in maturation and lagging behind the normal cells. We further presume that this trend continues during the third trimester of pregnancy and postnatally, up until ovulation, thereby explaining the maternal age effect in Down syndrome.

Reduced quality and accelerated follicle loss with female reproductive aging - does decline in theca dehydroepiandrosterone (DHEA) underlie the problem?

Infertility, spontaneous abortion and conception of trisomic offspring increase exponentially with age in mammals but in women there is an apparent acceleration in the rate from about age 37. The problems mostly commonly occur when the ovarian pool of follicles is depleted to a critical level with age but are also found in low follicular reserve of other etiologies. Since recent clinical studies have indicated that dehydroepiandrosterone (DHEA) supplementation may reverse the problem of oocyte quality, this review of the literature was undertaken in an attempt to find an explanation of why this is effective? In affected ovaries, oxygenation of follicular fluid is low, ultrastructural disturbances especially of mitochondria, occur in granulosa cells and oocytes, and considerable disturbances of meiosis occur. There is, however, no evidence to date that primordial follicles are compromised. In females with normal fertility, pre-antral ovarian theca cells respond to stimulation by inhibin B to provide androgen-based support for the developing follicle. With depletion of follicle numbers, inhibin B is reduced with consequent reduction in theca DHEA. Theca cells are the sole ovarian site of synthesis of DHEA, which is both a precursor of androstenedione and an essential ligand for peroxisome proliferator-activated receptor alpha (PPARα), the key promoter of genes affecting fatty acid metabolism and fat transport and genes critical to mitochondrial function. As well as inducing a plethora of deleterious changes in follicular cytoplasmic structure and function, the omega 9 palmitate/oleate ratio is increased by lowered activity of PPARα. This provides conditions for increased ceramide synthesis and follicular loss through ceramide-induced apoptosis is accelerated. In humans critical theca DHEA synthesis occurs at about 70 days prior to ovulation thus effective supplementation needs to be undertaken about four months prior to intended conception; timing which is also suggested by successful interventions to date. In humans and primates that undergo adrenarche, the adrenal zona reticularis (ZR) is the major site of DHEA production, however this is also reduced with age. Concomitant loss in function of the ZR might account for the acceleration in the rate of aging seen in humans in the late thirties' age group.

Germline mosaicism does not explain the maternal age effect on trisomy

A variety of hypotheses have been proposed to explain the association between trisomy and increasing maternal age in humans, virtually all of which assume that the underlying mechanisms involve meiotic errors. However, recently Hultén and colleagues [Hulten et al., 2010b] proposed a provocative model-the Oocyte Mosaicism Selection Model (OMSM)-that links age-dependent trisomy 21 to pre-meiotic errors in the ovary. Specifically, they propose that nondisjunctional events occur in a proportion of germ cells as they mitotically proliferate, resulting in mosaicism for trisomy 21. Assuming that the presence of an additional chromosome 21 delays meiotic progression, these cells would be ovulated later in reproductive life, resulting in an age-dependent increase in aneuploid eggs. Because this model has important clinical implications, we initiated studies to test it. We first analyzed oocytes from two trisomy 21 fetuses, combining immunostaining with FISH to determine the likelihood of detecting the additional chromosome 21 at different stages of meiosis. The detection of trisomy was enhanced during the earliest stage of prophase (leptotene), before homologs synapsed. Accordingly, in subsequent studies we examined the chromosome content of leptotene oocytes in seven second trimester female fetuses, analyzing three chromosomes commonly associated with human trisomies (i.e., 13, 16, and 21). In contrast to the prediction of the OMSM, we found no evidence of trisomy mosaicism for any chromosome. We conclude that errors in pre-meiotic germ cells are not a major contributor to human aneuploidy and do not provide an explanation for the age-related increase in trisomic conceptions.

Biomarkers of ovarian reserve

Although it is widely appreciated that age is the primary determinant of the number of primordial follicles remaining within the ovary, it is now also recognized that for any given age two women can have a 100-fold difference in their ovarian reserve. Consequently, age alone has relatively poor accuracy in determining the reproductive potential for young women, and this has led to the development of additional biomarkers that more accurately reflect the ovarian reserve. In this review we discuss the strengths and limitations of the classical and novel biomarkers and provide a rationale for the adoption of biomarkers to facilitate the individualization of reproductive health.

Non-meiotic chromosome instability in human immature oocytes

Aneuploidy has been a major issue in human gametes and is closely related to fertility problems, as it is known to be present in cleavage stage embryos and gestational losses. Pre-meiotic chromosome abnormalities in women have been previously described. The aim of this study is to assess the whole-chromosome complement in immature oocytes to find those abnormalities caused by mitotic instability. For this purpose, a total of 157 oocytes at the germinal vesicle or metaphase I stage, and discarded from IVF cycles, were analysed by CGH. Fifty-six women, between 18 and 45 years old (mean 32.5 years), including 32 IVF patients (25-45 years of age) and 24 IVF oocyte donors (18-33 years of age), were included in the study. A total of 25/157 (15.9%) of the oocytes analysed, obtained from three IVF clinics, contained chromosome abnormalities, including both aneuploidy (24/157) and structural aberrations (9/157). Independently of the maternal age, the incidence of abnormal oocytes which originated before meiosis is 15.9%, and these imbalances were found in 33.9% of the females studied. This work sheds light on the relevance of mitotic instability responsible for the generation of the abnormalities present in human oocytes.

Studying recombination in mouse oocytes

Meiosis is the specialized cell division in sexually reproducing organisms in which haploid gametes are produced. Meiotic prophase I is the defining stage of meiosis, when pairing and synapsis occur between homologous chromosomes, concurrent with reciprocal recombination (or crossing over) events that arise between them. Any disruption of these events during prophase I can lead to improper segregation of homologous chromosomes which can cause severe birth defects in the resulting progeny, and this occurs with alarming frequency in human oocytes. Thus, while the pathways that regulate these events in prophase I are highly conserved in both males and females, the stringency with which these events are monitored and/or controlled appears to be dramatically lower in females. These observations underscore the need to examine and compare meiotic mechanisms across the sexes. However, the study of female meiosis is impeded by the early start of meiosis during fetal development and the very limited amount of ovarian tissue available for meiotic analyses. Here we describe three different techniques which are useful for meiotic prophase I analysis in mouse/human oocytes, ranging from early prophase I events through until the resolution of crossing over at the first and second meiotic divisions.

Non-invasive prenatal diagnostics using next generation sequencing: technical, legal and social challenges

INTRODUCTION

Newly developed non-invasive prenatal diagnostic techniques, using maternal blood samples, have the potential to reduce or obviate the need for invasive prenatal diagnostic practices such as amniocentesis or chorionic villous sampling. This will lead to a change in how obstetric care is extended by health care providers to pregnant women at-risk of bearing an aneuploid child.

AREAS COVERED

The process leading to the development of fetal aneuploidy detection via the analysis of cell-free DNA in maternal plasma by massive parallel sequencing. Optimization of these strategies and approaches used in the recent or up-coming commercial launches. In addition, this review provides insight into legal implications, potential patent disputes, ethical and societal concerns raised by this development, such as whole genome data storage, retrieval and access.

EXPERT OPINION

There is a need for engagement by professional societies, to ensure correct usage of these newly emerging technologies and their restriction to high-risk pregnancies. National agencies need to ensure the necessary degree of high quality required for prenatal diagnosis.

Effect of aging on superovulation efficiency, aneuploidy rates, and sister chromatid cohesion in mice aged up to 15 months

Human eggs are highly aneuploid, with female age being the only known risk factor. Here this aging phenomenon was further studied in Swiss CD1 mice aged between 1 and 15 mo. The mean number of eggs ± SEM recovered from mice following superovulation peaked at 22.5 ± 3.8 eggs/oviduct in 3-mo-old females, decreasing markedly between 6 and 9 mo old, and was only 2.1 ± 0.2 eggs/oviduct by 15 mo. Measurement of aneuploidy in these eggs revealed a low rate, ∼3-4%, in mice aged 1 and 3 mo, rising to 12.5% by 9 mo old and to 37.5% at 12 mo. Fifteen-month-old mice had the highest rate of aneuploidy, peaking at 60%. The in situ chromosome counting technique used here allowed us to measure with accuracy the distance between the kinetochores in the sister chromatids of the eggs analyzed for aneuploidy. We observed that this distance increased in eggs from older females, from 0.38 ± 0.01 μm at 1 mo old to 0.82 ± 0.03 μm by 15 mo. Furthermore, in 3- to 12-mo-old females, aneuploid eggs had significantly larger interkinetochore distances than euploid eggs from the same age, and measurements were similar to eggs from the oldest mice. However, the association between aneuploidy and interkinetochore distance was not observed at the oldest, 15-mo age, despite such measurements being maximal. We conclude that in aging CD1 mice, a reduction in the ovulated egg number precedes a rise in aneuploidy and, furthermore, except at very advanced ages, increased interkinetochore distance is associated with aneuploidy.

Multiple meiotic errors caused by predivision of chromatids in women of advanced maternal age undergoing in vitro fertilisation

Chromosome aneuploidy is a major cause of pregnancy loss, abnormal pregnancy and live births following both natural conception and in vitro fertilisation (IVF) and increases exponentially with maternal age in the decade preceding the menopause. Molecular genetic analysis following natural conception and spontaneous miscarriage demonstrates that trisomies arise mainly in female meiosis and particularly in the first meiotic division. Here, we studied copy number gains and losses for all chromosomes in the two by-products of female meiosis, the first and second polar bodies, and the corresponding zygotes in women of advanced maternal age undergoing IVF, using microarray comparative genomic hybridisation (array CGH). Analysis of the segregation patterns underlying the copy number changes reveals that premature predivision of chromatids rather than non-disjunction of whole chromosomes causes almost all errors in the first meiotic division and unlike natural conception, over half of aneuploidies result from errors in the second meiotic division. Furthermore, most abnormal zygotes had multiple aneuploidies. These differences in the aetiology of aneuploidy in IVF compared with natural conception may indicate a role for ovarian stimulation in perturbing meiosis in ageing oocytes.

Meiotic origins of maternal age-related aneuploidy

Chromosome segregation errors in female meiosis lead to aneuploidy in the resulting egg and embryo, making them one of the leading genetic causes of spontaneous abortions and developmental disabilities in humans. It is known that aneuploidy of meiotic origin increases dramatically as women age, and current evidence suggests that most errors occur in meiosis I. Several hypotheses regarding the cause of maternal age-related aneuploidy have been proposed, including recombination errors in early meiosis, a defective spindle assembly checkpoint in meiosis I, and deterioration of sister chromatid cohesion with age. This review discusses findings in each area, and focuses especially on recent studies suggesting that deterioration of cohesion with increasing maternal age is a leading cause of age-related aneuploidy.

Predictors of trisomy 21 in the offspring of older and younger women

BACKGROUND

Advanced maternal age is the only well-established risk factor for trisomy 21, yet the majority of affected individuals are born to younger women. To identify factors associated with the risk of trisomy 21 in the offspring of younger and older women, we analyzed data for cases with trisomy 21 from the Texas Birth Defects Registry for 1999 to 2007.

METHODS

Data were analyzed separately for younger (i.e., <35 years of age at delivery; n = 2306) and older (i.e., ≥ 35 years of age at delivery; n = 1811) women using Poisson regression.

RESULTS

After adjustment for maternal age and several other covariates, the prevalence of trisomy 21 in the offspring of women in both maternal age groups was higher in male than in female infants and in offspring of women who were Hispanic (compared with non-Hispanic white women) or who had at least one previous liveborn child compared to those with none. In the offspring of older women only, the prevalence of trisomy 21 was also significantly higher when the father was 20 to 24 years old (compared with 25 to 29 years old; adjusted prevalence ratio [aPR], 2.27; 95% confidence interval [CI], 1.47-3.49) and Hispanic (compared with non-Hispanic white; aPR, 1.34; 95% CI, 1.13-1.58) and among women with less than a high school education (compared with greater than high school).

CONCLUSIONS

This study identified several factors, in addition to maternal age, that were associated with trisomy 21 risk. In general, these factors were similar for both maternal age groups, although paternal characteristics were significantly associated with risk of trisomy 21 only in offspring of older women.

Epidemiology of Down syndrome: new insight into the multidimensional interactions among genetic and environmental risk factors in the oocyte

Down syndrome birth is attributable to multiple maternal risk factors that include both genetic and environmental challenges, but there is limited understanding of the complicated interactions among these factors. In the present study, a case-control analysis of approximately 400 infants with or without suspected Down syndrome reported between 2003 and 2009 and their parents in and around Kolkata, India, was conducted. Maternal exposure to 2 environmental risk factors (smokeless chewing tobacco and oral contraceptive pills) was recorded, and families were genotyped with microsatellite markers to establish the origin of nondisjunction errors as well as recombination patterns of nondisjoined chromosome 21. With logistic regression models, the possible interactions among all of these risk factors, as well as with maternal age, were explored. Smokeless chewing tobacco was associated with significant risk for meiosis II nondisjunction and achiasmate (nonexchange) meiosis I error among young mothers. By contrast, the risk due to oral contraceptive pills was associated with older mothers. Study results suggest that the chewing tobacco risk factor operates independently of the maternal age effect, whereas contraceptive pill-related risk may interact with or exacerbate age-related risk. Moreover, both risk factors, when present together, exhibited a strong age-dependent effect.

Demographic and medical consequences of the postponement of parenthood

BACKGROUND Across the developed world couples are postponing parenthood. This review assesses the consequences of delayed family formation from a demographic and medical perspective. One main focus is on the quantitative importance of pregnancy postponement. METHODS Medical and social science databases were searched for publications on relevant subjects such as delayed parenthood, female and male age, fertility, infertility, time to pregnancy (TTP), fetal death, outcome of medically assisted reproduction (MAR) and mental well-being. RESULTS Postponement of parenthood is linked to a higher rate of involuntary childlessness and smaller families than desired due to increased infertility and fetal death with higher female and male age. For women, the increased risk of prolonged TTP, infertility, spontaneous abortions, ectopic pregnancies and trisomy 21 starts at around 30 years of age with a more pronounced effects >35 years, whereas the increasing risk of preterm births and stillbirths starts at around 35 years with a more pronounced effect >40 years. Advanced male age has an important but less pronounced effect on infertility and adverse outcomes. MAR treatment cannot overcome the age-related decline in fecundity. CONCLUSIONS In general, women have partners who are several years older than themselves and it is important to focus more on the combined effect of higher female and male age on infertility and reproductive outcome. Increasing public awareness of the impact of advanced female and male age on the reproductive outcome is essential for people to make well-informed decisions on when to start family formation.

Germinal and Somatic Trisomy 21 Mosaicism: How Common is it, What are the Implications for Individual Carriers and How Does it Come About?

It is well known that varying degrees of mosaicism for Trisomy 21, primarily a combination of normal and Trisomy 21 cells within individual tissues, may exist in the human population. This involves both Trisomy 21 mosaicism occurring in the germ line and Trisomy 21 mosaicism documented in different somatic tissues, or indeed a combination of both in the same subjects. Information on the incidence of Trisomy 21 mosaicism in different tissue samples from people with clinical features of Down syndrome as well as in the general population is, however, still limited. One of the main reasons for this lack of detailed knowledge is the technological problem of its identification, where in particular low grade/cryptic Trisomy 21 mosaicism, i.e. occurring in less than 3-5% of the respective tissues, can only be ascertained by fluorescence in situ hybridization (FISH) methods on large cell populations from the different tissue samples.In this review we summarize current knowledge in this field with special reference to the question on the likely incidence of germinal and somatic Trisomy 21 mosaicism in the general population and its mechanisms of origin. We also highlight the reproductive and clinical implications of this type of aneuploidy mosaicism for individual carriers. We conclude that the risk of begetting a child with Trisomy 21 Down syndrome most likely is related to the incidence of Trisomy 21 cells in the germ line of any carrier parent. The clinical implications for individual carriers may likewise be dependent on the incidence of Trisomy 21 in the relevant somatic tissues. Remarkably, for example, there are indications that Trisomy 21 mosaicism will predispose carriers to conditions such as childhood leukemia and Alzheimer's Disease but there is on the other hand a possibility that the risk of solid cancers may be substantially reduced.

Polymorphism C1420T of Serine hydroxymethyltransferase gene on maternal risk for Down syndrome

Recent researches have investigated the factors that determine the maternal risk for Down syndrome (DS) in young woman. In this context, some studies have demonstrated the association between polymorphisms in genes involved on folate metabolism and the maternal risk for DS. These polymorphisms may result in abnormal folate metabolism and methyl deficiency, which is associated with aberrant chromosome segregation leading to trisomy 21. In this study, we analyzed the influence of the polymorphism C1420T in Serine hydroxymethyltransferase (SHMT) gene on maternal risk for DS and on metabolites concentrations of the folate pathway (serum folate and plasma homocysteine and methylmalonic acid). The study group was composed by 105 mothers with DS children (case group) and 185 mothers who had no children with DS (control group). The genotype distribution did not show significant statistical difference between case and control mothers (P = 0.24) however a protective effect between genotypes CC (P = 0.0002) and CT (P < 0.0001) and maternal risk for DS was observed. Furthermore, the SHMT C1420T polymorphism (rs1979277) does not affect the concentration of metabolites of folate pathway in our DS mothers. In conclusion, our data showed a protective role for the genotypes SHMT CC and CT on maternal risk for DS. The concentrations of metabolites of folate pathway did not differ significantly between the genotypes SHMT.

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

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

Association of micronucleus frequency with neurodegenerative diseases

Micronuclei (MNi) can originate either from chromosome breakage or chromosome malsegregation events and are therefore ideal biomarkers to investigate genomic instability. Studies in peripheral lymphocytes of patients with neurodegenerative diseases, mainly Alzheimer's disease (AD) and Parkinson's disease (PD), revealed an increased micronucleus (MN) frequency in both disorders but originating mainly from chromosome malsegregation events in AD and from chromosome breakage events in PD. Studies in other neurodegenerative diseases are largely missing, and some data in premature ageing disorders characterised by neurodegeneration and/or neurological complications, such as Ataxia telangiectasia, Werner's syndrome, Down's syndrome (DS) and Cockayne's syndrome, indicate that MNi increase with ageing in cultured cells. An increased frequency of aneuploidy characterises several tissues of AD patients, as well as of individuals at increased risk to develop AD, such as mothers of DS individuals and DS subjects themselves. The use of the buccal MN cytome assay in AD and DS subjects allowed finding significant changes in the MN frequency as well as other cellular modifications reflecting reduced regenerative capacity compared to age- and gender-matched controls. These changes in buccal cytome ratios may prove useful as potential future diagnostics to identify individuals of increased risk for these disorders.

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

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

Polymorphisms in folate-metabolizing genes, chromosome damage, and risk of Down syndrome in Italian women: identification of key factors using artificial neural networks

BACKGROUND

Studies in mothers of Down syndrome individuals (MDS) point to a role for polymorphisms in folate metabolic genes in increasing chromosome damage and maternal risk for a Down syndrome (DS) pregnancy, suggesting complex gene-gene interactions. This study aimed to analyze a dataset of genetic and cytogenetic data in an Italian group of MDS and mothers of healthy children (control mothers) to assess the predictive capacity of artificial neural networks assembled in TWIST system in distinguish consistently these two different conditions and to identify the variables expressing the maximal amount of relevant information to the condition of being mother of a DS child.The dataset consisted of the following variables: the frequency of chromosome damage in peripheral lymphocytes (BNMN frequency) and the genotype for 7 common polymorphisms in folate metabolic genes (MTHFR 677C>T and 1298A>C, MTRR 66A>G, MTR 2756A>G, RFC1 80G>A and TYMS 28bp repeats and 1494 6bp deletion). Data were analysed using TWIST system in combination with supervised artificial neural networks, and a semantic connectivity map.

RESULTS

TWIST system selected 6 variables (BNMN frequency, MTHFR 677TT, RFC1 80AA, TYMS 1494 6bp +/+, TYMS 28bp 3R/3R and MTR 2756AA genotypes) that were subsequently used to discriminate between MDS and control mothers with 90% accuracy. The semantic connectivity map provided important information on the complex biological connections between the studied variables and the two conditions (being MDS or control mother).

CONCLUSIONS

Overall, the study suggests a link between polymorphisms in folate metabolic genes and DS risk in Italian women.

Chromosome 21 non-disjunction and Down syndrome birth in an Indian cohort: analysis of incidence and aetiology from family linkage data

We analysed the family linkage data obtained from short tandem repeat (STR) genotyping of 212 unrelated Indian families having a single Down syndrome (DS) baby each, in order to explore the incidence and aetiology of this human aneuploidy in our cohort. The estimated values of maternal meiotic I and meiotic II non-disjunction (NDJ) errors of chromosome 21 (Ch 21) were ~78 and ~22%, respectively. Within the paternal outcome group, about 47 and 53% were accounted for NDJ at meiosis I and meiosis II, respectively. We estimated only ~2% post-zygotic mitotic errors. The comparison of average age of conception between controls and DS-bearing mothers revealed a significant difference (P<0·001) with DS-bearing women were on an average older than controls and meiotic II non-disjoined mothers were oldest among meiotic outcome groups. Our linkage analysis suggested an overall reduction in recombination by more than 50% on meiotic I non-disjoined maternal Ch 21 with error prone to susceptible chiasma formation within the ~5·1 kbp segment near the telomeric end. We stratified meiotic I non-disjoined women in three age groups, viz. young (⩽28 years), middle (29–34 years) and old (⩾35 years) and found linear decrease in the frequency of achiasmate meiosis from the young to the old group. In contrary, a linear increase in the multiple chiasma frequency from the young to the old group was observed. Considering these results together, we propose that the risk factors for Ch 21 NDJ are of two types, one being ‘maternal age-independent’ and the other being ‘maternal age-dependent’. Moreover, a comparison of our present Indian dataset with that of other published data of ethnically different populations suggested that the genetics that underlies the NDJ of Ch 21 is probably universal irrespective of racial difference across human populations. The present study is the first population-based report on any DS cohort from the Indian subcontinent and our work will help future workers in understanding better the aetiology of this birth defect.

On the paternal origin of trisomy 21 Down syndrome

BACKGROUND

Down syndrome (DS), characterized by an extra free chromosome 21 is the most common genetic cause for congenital malformations and learning disability. It is well known that the extra chromosome 21 originates from the mother in more than 90% of cases, the incidence increases with maternal age and there is a high recurrence in young women. In a previous report we have presented data to indicate that maternal trisomy 21 (T21) ovarian mosaicism might provide the major causative factor underlying these patterns of DS inheritance. One important outstanding question concerns the reason why the extra chromosome 21 in DS rarely originates from the father, i.e. in less than 10% of T21 DS cases. We here report data indicating that one reason for this parental sex difference is a very much lower degree of fetal testicular in comparison to ovarian T21 mosaicism.

RESULTS

We used fluorescence in situ hybridisation (FISH) with two chromosome 21-specific probes to determine the copy number of chromosome 21 in fetal testicular cell nuclei from four male fetuses, following termination of pregnancy for a non-medical/social reason at gestational age 14-19 weeks. The cells studied were selected on the basis of their morphology alone, pending immunological specification of the relevant cell types. We could not detect any indication of testicular T21 mosaicism in any of these four male fetuses, when analysing at least 2000 cells per case (range 2038-3971, total 11.842). This result is highly statistically significant (p < 0.001) in comparison to the average of 0.54% ovarian T21 mosaicism (range 0.20-0.88%) that we identified in eight female fetuses analysing a total of 12.634 cells, as documented in a previous report in this journal.

CONCLUSION

Based on these observations we suggest that there is a significant sex difference in degrees of fetal germ line T21 mosaicism. Thus, it would appear that most female fetuses are T21 ovarian mosaics, while in sharp contrast most male fetuses may be either very low grade T21 testicular mosaics or they may be non-mosaics. We further propose that this sex difference in germ line T21 mosaicism may explain the much less frequent paternal origin of T21 DS than maternal. The mechanisms underlying the DS cases, where the extra chromosome 21 does originate from the father, remains unknown and further studies in this respect are required.