Altered patterns of multiple recombinant events are associated with nondisjunction of chromosome 21

Centromere-proximal suppression of meiotic crossovers in Drosophila is robust to changes in centromere number, repetitive DNA content, and centromere-clustering

Accurate segregation of homologous chromosomes during meiosis depends on both the presence and the regulated placement of crossovers (COs). The centromere effect, or CO exclusion in pericentromeric regions of the chromosome, is a meiotic CO patterning phenomenon that helps prevent nondisjunction, thereby protecting against chromosomal disorders and other meiotic defects. Despite being identified nearly a century ago, the mechanisms behind this fundamental cellular process remain unknown, with most studies of the Drosophila centromere effect focusing on local influences of the centromere and pericentric heterochromatin. In this study, we sought to investigate whether dosage changes in centromere number and repetitive DNA content affect the strength of the centromere effect, using phenotypic recombination mapping. Additionally, we studied the effects of repetitive DNA function on centromere effect strength using satellite DNA-binding protein mutants displaying defective centromere-clustering in meiotic nuclei. Despite what previous studies suggest, our results show that the Drosophila centromere effect is robust to changes in centromere number, repetitive DNA content, as well as repetitive DNA function. Our study suggests that the centromere effect is unlikely to be spatially controlled, providing novel insight into the mechanisms behind the Drosophila centromere effect.

Centromere-Proximal Suppression of Meiotic Crossovers in Drosophila is Robust to Changes in Centromere Number and Repetitive DNA Content

Accurate segregation of homologous chromosomes during meiosis depends on both the presence and regulated placement of crossovers (COs). The centromere effect (CE), or CO exclusion in pericentromeric regions of the chromosome, is a meiotic CO patterning phenomenon that helps prevent nondisjunction (NDJ), thereby protecting against chromosomal disorders and other meiotic defects. Despite being identified nearly a century ago, the mechanisms behind this fundamental cellular process remain unknown, with most studies of the Drosophila CE focusing on local influences of the centromere and pericentric heterochromatin. In this study, we sought to investigate whether dosage changes in centromere number and repetitive DNA content affect the strength of the CE, using phenotypic recombination mapping. Additionally, we also studied the effects of repetitive DNA function on CE strength using satellite-DNA binding protein mutants shown to have defective centromere clustering. Despite what previous studies suggest, our results show that the Drosophila CE is robust to dosage changes in centromere number and repetitive DNA content, and potentially also to repetitive DNA function. Our study suggests that the CE is unlikely to be spatially controlled, providing novel insight into the mechanisms behind the Drosophila centromere effect.

Aging and oocyte competence: A molecular cell perspective

Follicular microenvironment is paramount in the acquisition of oocyte competence, which is dependent on two interconnected and interdependent processes: nuclear and cytoplasmic maturation. Extensive research conducted in human and model systems has provided evidence that those processes are disturbed with female aging. In fact, advanced maternal age (AMA) is associated with a lower chance of pregnancy and live birth, explained by the age-related decline in oocyte quality/competence. This decline has largely been attributed to mitochondria, essential for oocyte maturation, fertilization, and embryo development; with mitochondrial dysfunction leading to oxidative stress, responsible for nuclear and mitochondrial damage, suboptimal intracellular energy levels, calcium disturbance, and meiotic spindle alterations, that may result in oocyte aneuploidy. Nuclear-related mechanisms that justify increased oocyte aneuploidy include deoxyribonucleic acid (DNA) damage, loss of chromosomal cohesion, spindle assembly checkpoint dysfunction, meiotic recombination errors, and telomere attrition. On the other hand, age-dependent cytoplasmic maturation failure is related to mitochondrial dysfunction, altered mitochondrial biogenesis, altered mitochondrial morphology, distribution, activity, and dynamics, dysmorphic smooth endoplasmic reticulum and calcium disturbance, and alterations in the cytoskeleton. Furthermore, reproductive somatic cells also experience the effects of aging, including mitochondrial dysfunction and DNA damage, compromising the crosstalk between granulosa/cumulus cells and oocytes, also affected by a loss of gap junctions. Old oocytes seem therefore to mature in an altered microenvironment, with changes in metabolites, ribonucleic acid (RNA), proteins, and lipids. Overall, understanding the mechanisms implicated in the loss of oocyte quality will allow the establishment of emerging biomarkers and potential therapeutic anti-aging strategies. This article is categorized under: Reproductive System Diseases > Molecular and Cellular Physiology.

Emerging mechanisms and roles of meiotic crossover repression at centromeres

Crossover events during recombination in meiosis are essential for generating genetic diversity as well as crucial to allow accurate chromosomal segregation between homologous chromosomes. Spatial control for the distribution of crossover events along the chromosomes is largely a tightly regulated process and involves many facets such as interference, repression as well as assurance, to make sure that not too many or too few crossovers are generated. Repression of crossover events at the centromeres is a highly conserved process across all species tested. Failure to inhibit such recombination events can result in chromosomal mis-segregation during meiosis resulting in aneuploid gametes that are responsible for infertility or developmental disorders such as Down's syndrome and other trisomies in humans. In the past few decades, studies to understand the molecular mechanisms behind this repression have shown the involvement of a multitude of factors ranging from the centromere-specific proteins such as the kinetochore to the flanking pericentric heterochromatin as well as DNA double-strand break repair pathways. In this chapter, we review the different mechanisms of pericentric repression mechanisms known till date as well as highlight the importance of understanding this regulation in the context of chromosomal segregation defects. We also discuss the clinical implications of dysregulation of this process, especially in human reproductive health and genetic diseases.

Meiotic Cohesin and Variants Associated With Human Reproductive Aging and Disease

Successful human reproduction relies on the well-orchestrated development of competent gametes through the process of meiosis. The loading of cohesin, a multi-protein complex, is a key event in the initiation of mammalian meiosis. Establishment of sister chromatid cohesion via cohesin rings is essential for ensuring homologous recombination-mediated DNA repair and future proper chromosome segregation. Cohesin proteins loaded during female fetal life are not replenished over time, and therefore are a potential etiology of age-related aneuploidy in oocytes resulting in decreased fecundity and increased infertility and miscarriage rates with advancing maternal age. Herein, we provide a brief overview of meiotic cohesin and summarize the human genetic studies which have identified genetic variants of cohesin proteins and the associated reproductive phenotypes including primary ovarian insufficiency, trisomy in offspring, and non-obstructive azoospermia. The association of cohesion defects with cancer predisposition and potential impact on aging are also described. Expansion of genetic testing within clinical medicine, with a focus on cohesin protein-related genes, may provide additional insight to previously unknown etiologies of disorders contributing to gamete exhaustion in females, and infertility and reproductive aging in both men and women.

Methylenetetrahydrofolate Reductase Dimer Configuration as a Risk Factor for Maternal Meiosis I-Derived Trisomy 21

BACKGROUND

Evidence suggests that the dimer configuration of methylenetetrahydrofolate reductase (MTHFR) enzyme might be destabilized by polymorphisms in monomers at the positions C677T and A1298C. It has been observed that these polymorphisms may lead to stable (CCAA, CCAC, CCCC) and unstable (CTAA, CTAC, TTAA) enzyme dimer configurations.

OBJECTIVE

The aim of this study was to evaluate the association of the MTHFR enzyme dimer configuration and folate dietary intake with the stage of meiotic nondisjunction in mothers of children with maternally derived trisomy 21.

METHODS

A total of 119 mothers of children with maternally derived free trisomy 21 were included in the study. The mean maternal age at the time of the birth of the child with trisomy 21 was 32.3 ± 6.4 (range 16-43) years. All mothers were Caucasian. Parental origin of trisomy 21 and meiotic stage of nondisjunction was determined using short tandem repeat markers spanning from the centromere to the telomere of chromosome 21q. The MTHFR C677T and A1298C polymorphism was evaluated by PCR-RFLP.

RESULTS

Increased frequency of the MTHFR genotype combinations CTAA, CTAC, and TTAA was found in the group of mothers with meiosis I (MI) nondisjunction (p = 0.007). No differences were found between study participants regarding dietary and lifestyles habits.

CONCLUSION

The risk for MI nondisjunction of chromosome 21 was 4.6-fold higher in cases who had CTAA, CTAC, and TTAA MTHFR genotype combinations and who did not used folic acid supplements in the preconception period.

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.

Mechanisms of oocyte aneuploidy associated with advanced maternal age

It is well established that maternal age is associated with a rapid decline in the production of healthy and high-quality oocytes resulting in reduced fertility in women older than 35 years of age. In particular, chromosome segregation errors during meiotic divisions are increasingly common and lead to the production of oocytes with an incorrect number of chromosomes, a condition known as aneuploidy. When an aneuploid oocyte is fertilized by a sperm it gives rise to an aneuploid embryo that, except in rare situations, will result in a spontaneous abortion. As females advance in age, they are at higher risk of infertility, miscarriage, or having a pregnancy affected by congenital birth defects such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Turner syndrome (monosomy X). Here, we review the potential molecular mechanisms associated with increased chromosome segregation errors during meiosis as a function of maternal age. Our review shows that multiple exogenous and endogenous factors contribute to the age-related increase in oocyte aneuploidy. Specifically, the weight of evidence indicates that recombination failure, cohesin deterioration, spindle assembly checkpoint (SAC) disregulation, abnormalities in post-translational modification of histones and tubulin, and mitochondrial dysfunction are the leading causes of oocyte aneuploidy associated with maternal aging. There is also growing evidence that dietary and other bioactive interventions may mitigate the effect of maternal aging on oocyte quality and oocyte aneuploidy, thereby improving fertility outcomes. Maternal age is a major concern for aneuploidy and genetic disorders in the offspring in the context of an increasing proportion of mothers having children at increasingly older ages. A better understanding of the mechanisms associated with maternal aging leading to aneuploidy and of intervention strategies that may mitigate these detrimental effects and reduce its occurrence are essential for preventing abnormal reproductive outcomes in the human population.

Crossover Position Drives Chromosome Remodeling for Accurate Meiotic Chromosome Segregation

Interhomolog crossovers (COs) are a prerequisite for achieving accurate chromosome segregation during meiosis [1, 2]. COs are not randomly positioned, occurring at distinct genomic intervals during meiosis in all species examined [3-10]. The role of CO position as a major determinant of accurate chromosome segregation has not been previously directly analyzed in a metazoan. Here, we use spo-11 mutants, which lack endogenous DNA double-strand breaks (DSBs), to induce a single DSB by Mos1 transposon excision at defined chromosomal locations in the C. elegans germline and show that the position of the resulting CO directly affects the formation of distinct chromosome subdomains during meiotic chromosome remodeling. CO formation in the typically CO-deprived center region of autosomes leads to premature loss of sister chromatid cohesion and chromosome missegregation, whereas COs at an off-centered position, as in wild type, can result in normal remodeling and accurate segregation. Ionizing radiation (IR)-induced DSBs lead to the same outcomes, and modeling of IR dose-response reveals that the CO-unfavorable center region encompasses up to 6% of the total chromosome length. DSBs proximal to telomeres rarely form COs, likely because of formation of unstable recombination intermediates that cannot be sustained as chiasmata until late prophase. Our work supports a model in which regulation of CO position early in meiotic prophase is required for proper designation of chromosome subdomains and normal chromosome remodeling in late meiotic prophase I, resulting in accurate chromosome segregation and providing a mechanism to prevent aneuploid gamete formation.

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.

The demographic data and the high frequency of chromosome/chromatid breaks as biomarkers for genome integrity have a role in predicting the susceptibility to have Down syndrome in a cohort of Egyptian young-aged mothers

Background

Down syndrome (DS) is a common numerical chromosome disorder that has its burden on both family and community. The well-known risk factor for chromosome 21 nondisjunction is advanced maternal age which failed to explain the occurrence of Down syndrome born to mothers less than 35 years. This study aimed to assess the effect of demographic data (consanguinity, residency area, and socioeconomic state) and chromosome/chromatid breaks as biomarkers for genome integrity on the susceptibility of young mothers to have a child with Down syndrome.

Results

Fifty mothers with a history of at least one DS pregnancy before the age of 35 were compared to 50 control mothers. There was a significant increase in DS births in consanguineous parents (46%) compared to 20% in non-consanguineous ones (OR = 3.40; 95% CI = 1.4–8.20, P = 0.006). Young mothers with DS children were more likely to be from rural areas (60%) than urban areas (40%) (OR = 2.66; 95%, CI = 1.18–5.98, P = 0.017) and of a low socioeconomic status (62%) rather than a high socioeconomic status (38%) (OR = 3.80; 95%, CI = 1.65–8.74, P = 0.001).

Chromosome/chromatid breaks were detected in 76% of DS young mothers and 32% of control mothers (P < 0.001). There was an odds ratio of chromatid breaks of 8.50 (3.411–21.17) and chromosome breaks of 3.93 (1.40–11.05) with significant difference between the studied groups (P < 0.001 and P = 0.009 respectively).

Conclusion

In addition to advanced maternal age, consanguinity, residency in rural areas, and low socioeconomic status could be considered as possible risk factors for Down syndrome. The high frequency of chromosome/chromatid breaks in young mothers with a previous history of DS children highlights the impact of genome integrity on the tendency to chromosome 21 nondisjunction. These findings are valuable in predicting having a Down syndrome baby and providing proper genetic counseling for high-risk families.

Crossover Interference, Crossover Maturation, and Human Aneuploidy

A striking feature of human female sexual reproduction is the high level of gametes that exhibit an aberrant number of chromosomes (aneuploidy). A high baseline observed in women of prime reproductive age is followed by a dramatic increase in older women. Proper chromosome segregation requires one or more DNA crossovers (COs) between homologous maternal and paternal chromosomes, in combination with cohesion between sister chromatid arms. In human females, CO designations occur normally, according to the dictates of CO interference, giving early CO-fated intermediates. However, ≈25% of these intermediates fail to mature to final CO products. This effect explains the high baseline of aneuploidy and is predicted to synergize with age-dependent cohesion loss to explain the maternal age effect. Here, modern advances in the understanding of crossing over and CO interference are reviewed, the implications of human female CO maturation inefficiency are further discussed, and areas of interest for future studies are suggested.

The roles of homologous recombination and the immune system in the genomic evolution of cancer

A variety of factors, whether extracellular (mutagens/carcinogens and viruses in the environment, chronic inflammation and radiation associated with the environment and/or electronic devices/machines) and/or intracellular (oxidative metabolites of food, oxidative stress due to inflammation, acid production, replication stress, DNA replication/repair errors, and certain hormones, cytokines, growth factors), pose a constant threat to the genomic integrity of a living cell. However, in the normal cellular environment multiple biological pathways including DNA repair, cell cycle, apoptosis and the immune system work in a precise, regulated (tightly controlled), timely and concerted manner to ensure genomic integrity, stability and proper functioning of a cell. If damage to DNA takes place, it is efficiently and accurately repaired by the DNA repair systems. Homologous recombination (HR) which utilizes either a homologous chromosome (in G1 phase) or a sister chromatid (in G2) as a template to repair the damage, is known to be the most precise repair system. HR in G2 which utilizes a sister chromatid as a template is also called an error free repair system. If DNA damage in a cell is so extensive that it overwhelms the repair system/s, the cell is eliminated by apoptosis. Thus, multiple pathways ensure that genome of a cell is intact and stable. However, constant exposure to DNA damage and/or dysregulation of DNA repair mechanism/s poses a risk of mutation and cancer. Oncogenesis, which seems to be a multistep process, is associated with acquisition of a number of genomic changes that enable a normal cell to progress from benign to malignant transformation. Transformed/cancer cells are recognized and killed by the immune system. However, the ongoing acquisition of new genomic changes enables cancer cells to survive/escape immune attack, evolve into a more aggressive phenotype, and eventually develop resistance to therapy. Although DNA repair (especially the HR) and the immune system play unique roles in preserving genomic integrity of a cell, they can also contribute to DNA damage, genomic instability and oncogenesis. The purpose of this article is to highlight the roles of DNA repair (especially HR) and the immune system in genomic evolution, with special focus on gastrointestinal cancer.

Age-Related Loss of Cohesion: Causes and Effects

Aneuploidy is a leading genetic cause of birth defects and lower implantation rates in humans. Most errors in chromosome number originate from oocytes. Aneuploidy in oocytes increases with advanced maternal age. Recent studies support the hypothesis that cohesion deterioration with advanced maternal age represents a leading cause of age-related aneuploidy. Cohesin generates cohesion, and is established only during the premeiotic S phase of fetal development without any replenishment throughout a female's period of fertility. Cohesion holds sister chromatids together until meiosis resumes at puberty, and then chromosome segregation requires the release of sister chromatid cohesion from chromosome arms and centromeres at anaphase I and anaphase II, respectively. The time of cohesion cleavage plays an important role in correct chromosome segregation. This review focuses specifically on the causes and effects of age-related cohesion deterioration in female meiosis.

Inefficient Crossover Maturation Underlies Elevated Aneuploidy in Human Female Meiosis

Meiosis is the cellular program that underlies gamete formation. For this program, crossovers between homologous chromosomes play an essential mechanical role to ensure regular segregation. We present a detailed study of crossover formation in human male and female meiosis, enabled by modeling analysis. Results suggest that recombination in the two sexes proceeds analogously and efficiently through most stages. However, specifically in female (but not male), ∼25% of the intermediates that should mature into crossover products actually fail to do so. Further, this "female-specific crossover maturation inefficiency" is inferred to make major contributions to the high level of chromosome mis-segregation and resultant aneuploidy that uniquely afflicts human female oocytes (e.g., giving Down syndrome). Additionally, crossover levels on different chromosomes in the same nucleus tend to co-vary, an effect attributable to global per-nucleus modulation of chromatin loop size. Maturation inefficiency could potentially reflect an evolutionary advantage of increased aneuploidy for human females.

Prophase I: Preparing Chromosomes for Segregation in the Developing Oocyte

Formation of an oocyte involves a specialized cell division termed meiosis. In meiotic prophase I (the initial stage of meiosis), chromosomes undergo elaborate events to ensure the proper segregation of their chromosomes into gametes. These events include processes leading to the formation of a crossover that, along with sister chromatid cohesion, forms the physical link between homologous chromosomes. Crossovers are formed as an outcome of recombination. This process initiates with programmed double-strand breaks that are repaired through the use of homologous chromosomes as a repair template. The accurate repair to form crossovers takes place in the context of the synaptonemal complex, a protein complex that links homologous chromosomes in meiotic prophase I. To allow proper execution of meiotic prophase I events, signaling processes connect different steps in recombination and synapsis. The events occurring in meiotic prophase I are a prerequisite for proper chromosome segregation in the meiotic divisions. When these processes go awry, chromosomes missegregate. These meiotic errors are thought to increase with aging and may contribute to the increase in aneuploidy observed in advanced maternal age female oocytes.

Mechanisms of Aneuploidy in Human Eggs

Eggs and sperm develop through a specialized cell division called meiosis. During meiosis, the number of chromosomes is reduced by two sequential divisions in preparation for fertilization. In human female meiosis, chromosomes frequently segregate incorrectly, resulting in eggs with an abnormal number of chromosomes. When fertilized, these eggs give rise to aneuploid embryos that usually fail to develop. As women become older, errors in meiosis occur more frequently, resulting in increased risks of infertility, miscarriage, and congenital syndromes, such as Down's syndrome. Here, we review recent studies that identify the mechanisms causing aneuploidy in female meiosis, with a particular emphasis on studies in humans.

Insights into epigenetic landscape of recombination-free regions

Genome architecture is shaped by gene-rich and repeat-rich regions also known as euchromatin and heterochromatin, respectively. Under normal conditions, the repeat-containing regions undergo little or no meiotic crossover (CO) recombination. COs within repeats are risky for the genome integrity. Indeed, they can promote non-allelic homologous recombination (NAHR) resulting in deleterious genomic rearrangements associated with diseases in humans. The assembly of heterochromatin is driven by the combinatorial action of many factors including histones, their modifications, and DNA methylation. In this review, we discuss current knowledge dealing with the epigenetic signatures of the major repeat regions where COs are suppressed. Then we describe mutants for epiregulators of heterochromatin in different organisms to find out how chromatin structure influences the CO rate and distribution.

Oocyte development, meiosis and aneuploidy

Meiosis is one of the defining events in gametogenesis. Male and female germ cells both undergo one round of meiotic cell division during their development in order to reduce the ploidy of the gametes, and thereby maintain the ploidy of the species after fertilisation. However, there are some aspects of meiosis in the female germline, such as the prolonged arrest in dictyate, that appear to predispose oocytes to missegregate their chromosomes and transmit aneuploidies to the next generation. These maternally-derived aneuploidies are particularly problematic in humans where they are major contributors to miscarriage, age-related infertility, and the high incidence of Down's syndrome in human conceptions. This review will discuss how events that occur in foetal oocyte development and during the oocytes' prolonged dictyate arrest can influence meiotic chromosome segregation and the incidence of aneuploidy in adult oocytes.

Association between telomere length and chromosome 21 nondisjunction in the oocyte

Chromosome 21 nondisjunction in oocytes is the most common cause of trisomy 21, the primary chromosomal abnormality responsible for Down syndrome (DS). This specific type of error is estimated to account for over 90 % of live births with DS, with maternal age being the best known risk factor for chromosome 21 nondisjunction. The loss of telomere length and the concomitant shortening of chromosomes are considered a biological marker for aging. Thus, we tested the hypothesis that mothers who had a maternal nondisjunction error leading to a live birth with DS (n = 404) have shorter telomeres than mothers with live births without DS (n = 42). In effect, our hypothesis suggests that mothers of children with DS will appear "biologically older" as compared to the mothers of euploid children. We applied a quantitative PCR assay to measure the genome-wide relative telomere length to test this hypothesis. The results of our study support the hypothesis that young mothers of DS babies are "biologically older" than mothers of euploid babies in the same age group and supports telomere length as a biomarker of age and hence risk for chromosome nondisjunction.

The genetics of folate metabolism and maternal risk of birth of a child with Down syndrome and associated congenital heart defects

Almost 15 years ago it was hypothesized that polymorphisms of genes encoding enzymes involved in folate metabolism could lead to aberrant methylation of peri-centromeric regions of chromosome 21, favoring its abnormal segregation during maternal meiosis. Subsequently, more than 50 small case-control studies investigated whether or not maternal polymorphisms of folate pathway genes could be risk factors for the birth of a child with Down syndrome (DS), yielding conflicting and inconclusive results. However, recent meta-analyses of those studies suggest that at least three of those polymorphisms, namely MTHFR 677C>T, MTRR 66A>G, and RFC1 80G>A, are likely to act as maternal risk factors for the birth of a child with trisomy 21, revealing also complex gene-nutrient interactions. A large-cohort study also revealed that lack of maternal folic acid supplementation at peri-conception resulted in increased risk for a DS birth due to errors occurred at maternal meiosis II in the aging oocyte, and it was shown that the methylation status of chromosome 21 peri-centromeric regions could favor recombination errors during meiosis leading to its malsegregation. In this regard, two recent case-control studies revealed association of maternal polymorphisms or haplotypes of the DNMT3B gene, coding for an enzyme required for the regulation of DNA methylation at centromeric and peri-centromeric regions of human chromosomes, with risk of having a birth with DS. Furthermore, congenital heart defects (CHD) are found in almost a half of DS births, and increasing evidence points to a possible contribution of lack of folic acid supplementation at peri-conception, maternal polymorphisms of folate pathway genes, and resulting epigenetic modifications of several genes, at the basis of their occurrence. This review summarizes available case-control studies and literature meta-analyses in order to provide a critical and up to date overview of what we currently know in this field.

Meiosis and maternal aging: insights from aneuploid oocytes and trisomy births

In most organisms, genome haploidization requires reciprocal DNA exchanges (crossovers) between replicated parental homologs to form bivalent chromosomes. These are resolved to their four constituent chromatids during two meiotic divisions. In female mammals, bivalents are formed during fetal life and remain intact until shortly before ovulation. Extending this period beyond ∼35 years greatly increases the risk of aneuploidy in human oocytes, resulting in a dramatic increase in infertility, miscarriage, and birth defects, most notably trisomy 21. Bivalent chromosomes are stabilized by cohesion between sister chromatids, which is mediated by the cohesin complex. In mouse oocytes, cohesin becomes depleted from chromosomes during female aging. Consistent with this, premature loss of centromeric cohesion is a major source of aneuploidy in oocytes from older women. Here, we propose a mechanistic framework to reconcile data from genetic studies on human trisomy and oocytes with recent advances in our understanding of the molecular mechanisms of chromosome segregation during meiosis in model organisms.

TroX: a new method to learn about the genesis of aneuploidy from trisomic products of conception

MOTIVATION

An estimated 10-30% of clinically recognized conceptions are aneuploid, leading to spontaneous miscarriages, in vitro fertilization failures and, when viable, severe developmental disabilities. With the ongoing reduction in the cost of genotyping and DNA sequencing, the use of high-density single nucleotide polymorphism (SNP) markers for clinical diagnosis of aneuploidy and biomedical research into its causes is becoming common practice. A reliable, flexible and computationally feasible method for inferring the sources of aneuploidy is thus crucial.

RESULTS

We propose a new method, TroX, for analyzing human trisomy data using high density SNP markers from a trisomic individual or product of conception and one parent. Using a hidden Markov model, we infer the stage of the meiotic error (I or II) and the individual in which non-disjunction event occurred, as well as the crossover locations on the trisomic chromosome. A novel and important feature of the method is its reliance on data from the proband and only one parent, reducing the experimental cost by a third and enabling a larger set of data to be used. We evaluate our method by applying it to simulated trio data as well as to genotype data for 282 trios that include a child trisomic for chromosome 21. The analyses show the method to be highly reliable even when data from only one parent are available. With the increasing availability of DNA samples from mother and fetus, application of approaches such as ours should yield unprecedented insights into the genetic risk factors for aneuploidy.

AVAILABILITY AND IMPLEMENTATION

An R package implementing TroX is available for download at http://przeworski.uchicago.edu/.

An examination of the relationship between hotspots and recombination associated with chromosome 21 nondisjunction

Trisomy 21, resulting in Down Syndrome (DS), is the most common autosomal trisomy among live-born infants and is caused mainly by nondisjunction of chromosome 21 within oocytes. Risk factors for nondisjunction depend on the parental origin and type of meiotic error. For errors in the oocyte, increased maternal age and altered patterns of recombination are highly associated with nondisjunction. Studies of normal meiotic events in humans have shown that recombination clusters in regions referred to as hotspots. In addition, GC content, CpG fraction, Poly(A)/Poly(T) fraction and gene density have been found to be significant predictors of the placement of sex-averaged recombination in the human genome. These observations led us to ask whether the altered patterns of recombination associated with maternal nondisjunction of chromosome 21 could be explained by differences in the relationship between recombination placement and recombination-related genomic features (i.e., GC content, CpG fraction, Poly(A)/Poly(T) fraction or gene density) on 21q or differential hot-spot usage along the nondisjoined chromosome 21. We found several significant associations between our genomic features of interest and recombination, interestingly, these results were not consistent among recombination types (single and double proximal or distal events). We also found statistically significant relationships between the frequency of hotspots and the distribution of recombination along nondisjoined chromosomes. Collectively, these findings suggest that factors that affect the accessibility of a specific chromosome region to recombination may be altered in at least a proportion of oocytes with MI and MII errors.

Maternal obesity and risk of Down syndrome in the offspring

OBJECTIVE

The objective of this article is to determine if maternal obesity is associated with an increased risk of Down syndrome in the offspring and whether the risk estimates for trisomy 21 based on combined screening is affected by maternal body mass index (BMI).

METHODS

Study group I consisted of a nationwide cohort of 1 568 604 women giving birth; outcome was infants born with Down syndrome [Correction made here after initial online publication.]. Adjustment was made for maternal age. Study group II consisted of 10 224 women undergoing 1st trimester combined screening. Outcome was risk assessment for Down syndrome. All women were divided into six BMI groups, and outcomes were evaluated over the BMI strata with BMI 18.5 to 24.9 as reference and correcting for maternal age.

RESULTS

Obese women had an increased risk for giving birth to an infant with Down syndrome compared with normal-weight women, BMI 30 to 34.9 odds ratio (OR) 1.31 [95% confidence interval (CI) 1.10-1.55], BMI 35 to 39.9 OR 1.12 (95% CI 0.82-1.53), BMI ≥ 40 OR 1.56 (95% CI 1.00-2.43). The observed and the expected numbers of women with a risk of Down syndrome >1/300 based on 1st trimester combined screen and maternal age were similar in each BMI group.

CONCLUSION

Maternal obesity seems to increase the risk for Down syndrome births. The risk estimate for Down syndrome with 1st trimester combined screening is unaffected by BMI.

Evidence for dysregulation of genome-wide recombination in oocytes with nondisjoined chromosomes 21

In oocytes with nondisjoined chromosomes 21 due to a meiosis I (MI) error, recombination is significantly reduced along chromosome 21; several lines of evidence indicate that this contributes to the nondisjunction event. A pilot study found evidence that these oocytes also have reduced recombination genome-wide when compared with controls. This suggests that factors that act globally may be contributing to the reduced recombination on chromosome 21, and hence, the nondisjunction event. To identify the source of these factors, we examined two levels of recombination count regulation in oocytes: (i) regulation at the maternal level that leads to correlation in genome-wide recombination across her oocytes and (ii) regulation at the oocyte level that leads to correlation in recombination count among the chromosomes of an oocyte. We sought to determine whether either of these properties was altered in oocytes with an MI error. As it relates to maternal regulation, we found that both oocytes with an MI error (N = 94) and their siblings (N = 64) had reduced recombination when compared with controls (N = 2723). At the oocyte level, we found that the correlation in recombination count among the chromosomes of an oocyte is reduced in oocytes with MI errors compared with that of their siblings or controls. These results suggest that regulation at the maternal level predisposes MI error oocytes to reduced levels of recombination, but additional oocyte-specific dysregulation contributes to the nondisjunction event.

Altered incidence of meiotic errors and Down syndrome birth under extreme low socioeconomic exposure in the Sundarban area of India

We conducted a survey to analyze the genetic epidemiology of trisomy 21 Down syndrome births in the Sundarban delta region of India. In this region, inhabitants are chiefly from marginalized poor tribal communities and have lived in extremely low socioeconomic condition for several generations. Microsatellite genotyping revealed an meiosis I/meiosis II ratio that is different from the previous reports on the Down syndrome populations from other parts of the world. Analyses of distribution of achiasmate nondisjunction at maternal meiosis I in interaction with different maternal age groups (young, middle, and old) revealed a very concordant pattern to that of urban and semi-urban Down syndrome cases previously studied by our group. However, the frequency of achiasmate meiosis is much lower, which suggests that extreme low socioeconomic exposure imparts risk of chromosomal nondisjunction even when the maternal chromosomes 21 engage in proper chiasma formation at prophase I of oogenesis.

The association of low socioeconomic status and the risk of having a child with Down syndrome: a report from the National Down Syndrome Project

PURPOSE

Advanced maternal age and altered recombination are known risk factors for Down syndrome cases due to maternal nondisjunction of chromosome 21, whereas the impact of other environmental and genetic factors is unclear. The aim of this study was to investigate an association between low maternal socioeconomic status and chromosome 21 nondisjunction.

METHODS

Data from 714 case and 977 control families were used to assess chromosome 21 meiosis I and meiosis II nondisjunction errors in the presence of three low socioeconomic status factors: (i) both parents had not completed high school, (ii) both maternal grandparents had not completed high school, and (iii) an annual household income of <$25,000. We applied logistic regression models and adjusted for covariates, including maternal age and race/ethnicity.

RESULTS

As compared with mothers of controls (n = 977), mothers with meiosis II chromosome 21 nondisjunction (n = 182) were more likely to have a history of one low socioeconomic status factor (odds ratio = 1.81; 95% confidence interval = 1.07-3.05) and ≥2 low socioeconomic status factors (odds ratio = 2.17; 95% confidence interval = 1.02-4.63). This association was driven primarily by having a low household income (odds ratio = 1.79; 95% confidence interval = 1.14-2.73). The same statistically significant association was not detected among maternal meiosis I errors (odds ratio = 1.31; 95% confidence interval = 0.81-2.10), in spite of having a larger sample size (n = 532).

CONCLUSION

We detected a significant association between low maternal socioeconomic status and meiosis II chromosome 21 nondisjunction. Further studies are warranted to explore which aspects of low maternal socioeconomic status, such as environmental exposures or poor nutrition, may account for these results.

Preconception folic acid supplementation and risk for chromosome 21 nondisjunction: a report from the National Down Syndrome Project

Both a lack of maternal folic acid supplementation and the presence of genetic variants that reduce enzyme activity in folate pathway genes have been linked to meiotic nondisjunction of chromosome 21; however, the findings in this area of research have been inconsistent. To better understand these inconsistencies, we asked whether maternal use of a folic acid-containing supplement before conception reduces risk for chromosome 21 nondisjunction. Using questionnaire data from the National Down Syndrome Project, a population-based case-control study, we compared the use of folic acid-containing supplements among mothers of infants with full trisomy 21 due to maternal nondisjunction (n = 702) and mothers of infants born with no major birth defects (n = 983). Using logistic regression, adjusting for maternal age, race/ethnicity, and infant age at maternal interview, we found no evidence of an association between lack of folic acid supplementation and maternal nondisjunction among all case mothers (OR = 1.16; 95% CI: 0.90-1.48). In analyses stratified by meiotic stage and maternal age (<35 or ≥35 years), we found an association among older mothers experiencing meiosis II nondisjunction errors (OR = 2.00; 95% CI: 1.08-3.71). These data suggest that lack of folic acid supplementation may be associated specifically with MII errors in the aging oocyte. If confirmed, these results could account for inconsistencies among previous studies, as each study sample may vary by maternal age structure and proportion of meiotic errors.