Failure to recombine is a common feature of human oogenesis.

Failure of homologous chromosomes to recombine is arguably the most important cause of human meiotic nondisjunction, having been linked to numerous autosomal and sex chromosome trisomies of maternal origin. However, almost all information on these "exchangeless" homologs has come from genetic mapping studies of trisomic conceptuses, so the incidence of this defect and its impact on gametogenesis are not clear. If oocytes containing exchangeless homologs are selected against during meiosis, the incidence may be much higher in developing germ cells than in zygotes. To address this, we initiated studies of exchangeless chromosomes in fetal ovarian samples from elective terminations of pregnancy. In total, we examined more than 7,000 oocytes from 160 tissue samples, scoring for the number of foci per cell of the crossover-associated protein MLH1. We identified a surprisingly high level of recombination failure, with more than 7% of oocytes containing at least one chromosome pair that lacked an MLH1 focus. Detailed analyses indicate striking chromosome-specific differences, with a preponderance of MLH1-less homologs involving chromosomes 21 or 22. Further, the effect was linked to the overall level of recombination in the cell, with the presence of one or two exchangeless chromosomes in a cell associated with a 10%-20% reduction in the total number of crossovers. This suggests individuals with lower rates of meiotic recombination are at an increased risk of producing aneuploid offspring.

Mathematical modeling of human oocyte aneuploidy.

Aneuploidy is the leading contributor to pregnancy loss, congenital anomalies, and in vitro fertilization (IVF) failure in humans. Although most aneuploid conceptions are thought to originate from meiotic division errors in the female germline, quantitative studies that link the observed phenotypes to underlying error mechanisms are lacking. In this study, we developed a mathematical modeling framework to quantify the contribution of different mechanisms of erroneous chromosome segregation to the production of aneuploid eggs. Our model considers the probabilities of all possible chromosome gain/loss outcomes that arise from meiotic errors, such as nondisjunction (NDJ) in meiosis I and meiosis II, and premature separation of sister chromatids (PSSC) and reverse segregation (RS) in meiosis I. To understand the contributions of different meiotic errors, we fit our model to aneuploidy data from 11,157 blastocyst-stage embryos. Our best-fitting model captures several known features of female meiosis, for instance, the maternal age effect on PSSC. More importantly, our model reveals previously undescribed patterns, including an increased frequency of meiosis II errors among eggs affected by errors in meiosis I. This observation suggests that the occurrence of NDJ in meiosis II is associated with the ploidy status of an egg. We further demonstrate that the model can be used to identify IVF patients who produce an extreme number of aneuploid embryos. The dynamic nature of our mathematical model makes it a powerful tool both for understanding the relative contributions of mechanisms of chromosome missegregation in human female meiosis and for predicting the outcomes of assisted reproduction.

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.

Spore-autonomous fluorescent protein expression identifies meiotic chromosome mis-segregation as the principal cause of hybrid sterility in yeast.

Genome-wide sequence divergence between populations can cause hybrid sterility through the action of the anti-recombination system, which rejects crossover repair of double strand breaks between nonidentical sequences. Because crossovers are necessary to ensure proper segregation of homologous chromosomes during meiosis, the reduced recombination rate in hybrids can result in high levels of nondisjunction and therefore low gamete viability. Hybrid sterility in interspecific crosses of Saccharomyces yeasts is known to be associated with such segregation errors, but estimates of the importance of nondisjunction to postzygotic reproductive isolation have been hampered by difficulties in accurately measuring nondisjunction frequencies. Here, we use spore-autonomous fluorescent protein expression to quantify nondisjunction in both interspecific and intraspecific yeast hybrids. We show that segregation is near random in interspecific hybrids. The observed rates of nondisjunction can explain most of the sterility observed in interspecific hybrids through the failure of gametes to inherit at least one copy of each chromosome. Partially impairing the anti-recombination system by preventing expression of the RecQ helicase SGS1 during meiosis cuts nondisjunction frequencies in half. We further show that chromosome loss through nondisjunction can explain nearly all of the sterility observed in hybrids formed between two populations of a single species. The rate of meiotic nondisjunction of each homologous pair was negatively correlated with chromosome size in these intraspecific hybrids. Our results demonstrate that sequence divergence is not only associated with the sterility of hybrids formed between distantly related species but may also be a direct cause of reproductive isolation in incipient species.

Replication stress activates DNA repair synthesis in mitosis.

Oncogene-induced DNA replication stress has been implicated as a driver of tumorigenesis. Many chromosomal rearrangements characteristic of human cancers originate from specific regions of the genome called common fragile sites (CFSs). CFSs are difficult-to-replicate loci that manifest as gaps or breaks on metaphase chromosomes (termed CFS 'expression'), particularly when cells have been exposed to replicative stress. The MUS81-EME1 structure-specific endonuclease promotes the appearance of chromosome gaps or breaks at CFSs following replicative stress. Here we show that entry of cells into mitotic prophase triggers the recruitment of MUS81 to CFSs. The nuclease activity of MUS81 then promotes POLD3-dependent DNA synthesis at CFSs, which serves to minimize chromosome mis-segregation and non-disjunction. We propose that the attempted condensation of incompletely duplicated loci in early mitosis serves as the trigger for completion of DNA replication at CFS loci in human cells. Given that this POLD3-dependent mitotic DNA synthesis is enhanced in aneuploid cancer cells that exhibit intrinsically high levels of chromosomal instability (CIN(+)) and replicative stress, we suggest that targeting this pathway could represent a new therapeutic approach.

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.

xnd-1 regulates the global recombination landscape in Caenorhabditis elegans.

Meiotic crossover (CO) recombination establishes physical linkages between homologous chromosomes that are required for their proper segregation into developing gametes, and promotes genetic diversity by shuffling genetic material between parental chromosomes. COs require the formation of double strand breaks (DSBs) to create the substrate for strand exchange. DSBs occur in small intervals called hotspots and significant variation in hotspot usage exists between and among individuals. This variation is thought to reflect differences in sequence identity and chromatin structure, DNA topology and/ or chromosome domain organization. Chromosomes show different frequencies of nondisjunction (NDJ), reflecting inherent differences in meiotic crossover control, yet the underlying basis of these differences remains elusive. Here we show that a novel chromatin factor, X non-disjunction factor 1 (xnd-1), is responsible for the global distribution of COs in C. elegans. xnd-1 is also required for formation of double-strand breaks (DSBs) on the X, but surprisingly XND-1 protein is autosomally enriched. We show that xnd-1 functions independently of genes required for X chromosome-specific gene silencing, revealing a novel pathway that distinguishes the X from autosomes in the germ line, and further show that xnd-1 exerts its effects on COs, at least in part, by modulating levels of H2A lysine 5 acetylation.

Polymorphisms in folate pathway genes are not associated with somatic nondisjunction in turner syndrome.

Folate metabolism dysfunction can lead to DNA hypomethylation and abnormal chromosomal segregation. Previous investigations of this association have produced controversial results. Here we performed a case-control study in patients with Turner syndrome (TS) to determine the effects of genetic polymorphisms of folate pathway genes as potential risk factors for somatic chromosomal nondisjunction. TS is a useful model for this investigation because patients with TS show a high frequency of chromosome mosaicism. Here we investigated the possible association of polymorphisms of the MTHFR gene with TS risk, which has been previously investigated with controversial results. We also examined the effects of MTR, RFC1, and TYMS gene polymorphisms in TS for the first time. The risk was evaluated according to allelic and genotype (independent and combined) frequencies among 70 patients with TS and 144 age-matched healthy control subjects. Polymorphism genotyping was performed by PCR, PCR-RFLP, and PCR-ASA. The polymorphisms MTHFR 677C>T and 1298A>C, MTR 2756A>G, RFC1 80G>A, and TYMS 2R/3R-alone or in combinations-were not associated with the risk of chromosomal aneuploidy in TS. In conclusion, our present findings did not support a link between impaired folate metabolism and abnormal chromosome segregation leading to somatic nondisjunction in TS patients.

Maternal risk for down syndrome and polymorphisms in the promoter region of the DNMT3B gene: a case-control study.

BACKGROUND: Epigenetic changes leading to improper methylation of the pericentromeric region of chromosome 21 may contribute to the nondisjunction of this chromosome. Polymorphisms in the DNA Methyltransferase 3B (DNMT3B) gene, one of the crucial gene of the folate metabolism, affects the activity of the enzyme and increases the susceptibility of nondisjunction in mothers of Down syndrome children (MDS). METHODS: Considering this hypothesis we investigated the association of single nucleotide polymorphisms in the promoter region of the DNMT3B gene (rs1569686 -579G>T; rs2424913 -149C>T) with a predisposition of mothers to deliver a Down syndrome (DS) child. The study was performed on DNA samples from 150 MDS and 172 control mothers. Transmission disequilibrium tests were performed on 103 DS trio families. Genotyping was done using a polymerase chain reaction-restriction fragment length polymorphism method. RESULTS: With respect to the single nucleotide polymorphisms studied, no significant difference was observed in the genotypes and alleles frequency distributions between MDS and control mothers. The frequency of the DNMT3B-579G allele was, respectively, 0.34 in MDS and 0.33 in control mothers whereas the frequency of the DNMT3B-149C allele was respectively 0.31 in MDS and 0.26 in control mothers. No significant deviation in genotypic combinations as well as in transmission disequilibrium tests analysis was observed. However, a strong linkage disequilibrium was observed with significant differences in the distribution of G-T and G-C haplotypes among case and control mothers. CONCLUSION: Although the above studied polymorphisms of DNMT3B may not be an independent risk factor it might be possible that certain allelic combinations (G-T) are. This finding suggests that DNMT3B might be a maternal risk factor for DS in our Indian cohort. Replication studies are required to confirm these findings.

A conditional mutation in Arabidopsis thaliana separase induces chromosome non-disjunction, aberrant morphogenesis and cyclin B1;1 stability.

The caspase family protease, separase, is required at anaphase onset to cleave the cohesin complex, which joins sister chromatids. However, among eukaryotes, separases have acquired novel functions. Here, we show that Arabidopsis thaliana radially swollen 4 (rsw4), a temperature-sensitive mutant isolated previously on the basis of root swelling, harbors a mutation in At4g22970, the A. thaliana separase. Loss of separase function in rsw4 at the restrictive temperature is indicated by the widespread failure of replicated chromosomes to disjoin. Surprisingly, rsw4 has neither pronounced cell cycle arrest nor anomalous spindle formation, which occur in other eukaryotes upon loss of separase activity. However, rsw4 roots have disorganized cortical microtubules and accumulate the mitosis-specific cyclin, cyclin B1;1, excessive levels of which have been associated with altered microtubules and morphology. Cyclin B1;1 also accumulates in certain backgrounds in response to DNA damage, but we find no evidence for aberrant responses to DNA damage in rsw4. Our characterization of rsw4 leads us to hypothesize that plant separase, in addition to cleaving cohesin, regulates cyclin B1;1, with profound ramifications for morphogenesis.

Numerical chromosome disorders in the common marmoset (Callithrix jacchus)--comparison between two captive colonies.

BACKGROUND: Chromosomal analyses were performed for marmosets from two colonies - Deutsches Primatenzentrum (DPZ) and Biomedical Primate Research Centre (BPRC). Chlorine-based disinfectants are used in DPZ; no chemical disinfection is applied in BPRC. METHODS: The rates of chromosomal non-disjunction, polyploidy and endoreduplication were investigated after G-banding. RESULTS: For DPZ monkeys, the mean rates of non-disjunction were 7.6% for bone marrow and 11.3% for lymphocytes. The polyploidy level was 2.5% in bone marrow and 0.8% in blood. Frequency of endoreduplication in bone marrow and in leucocytes was 0.5% and 0.8%, respectively. For BPRC, the rate of non-disjunction in leucocytes (1.3%) was significantly lower than that for DPZ; the polyploidy rate (0.2%) in blood was lower than that in DPZ; endoreduplication was not observed. CONCLUSION: The levels of chromosomal disorders are elevated for DPZ colony. We suggest that the increased rate of chromosomal disorders in DPZ marmosets can be related to the chemical disinfection of their environment.

Cell biology: nondisjunction, aneuploidy and tetraploidy.

One simple, widely accepted mechanism for generating an aberrant chromosome number, or aneuploidy, is through nondisjunction--a chromosome distribution error that occurs during mitosis when both copies of a duplicated chromosome are deposited into one daughter cell and none into the other. Shi and King challenge this view, concluding that nondisjunction does not yield aneuploid cells directly, but instead gives rise to tetraploid cells that may subsequently become aneuploid through further division. Here we show that the direct result of chromosome nondisjunction is gain or loss of a single chromosome, which results in near-diploid aneuploidy, not tetraploidy. We suggest that chromatin trapped in the cytokinetic cleavage furrow is the more likely reason for furrow regression and tetraploidization.

Mosaic trisomy 13: understanding origin using SNP array.

BACKGROUND: Trisomy 13 occurs in 1/10,000-20,000 live births, and mosaicism accounts for 5% of these cases. Phenotype and outcome of mosaic trisomy 13 are variable and poorly understood. Microsatellite analyses of trisomy 13 have indicated the high incidence of maternal meiotic origin and reduced recombination, but no study has focused on mosaic trisomy 13 in live born patients. METHODS AND RESULTS: Single-nucleotide polymorphism (SNP) array, fluorescence in situ hybridisation and bioinformatics analyses were performed in three cases of mosaic trisomy 13. Two cases of complete mosaic trisomy 13 originated from meiosis I non-disjunction followed by trisomic rescue; one had crossovers resulting in segmental uniparental disomy in the disomic line, and one had no crossover. Mosaicism for partial trisomy 13 in the third complex case either arose from meiosis II non-disjunction without crossover or in early mitosis followed by anaphase lags. The extra chromosome 13 was maternal in origin in all three cases. Mosaicism percentage calculated from B allele frequencies ranged from 30 to 50. CONCLUSIONS: Genotypes and copy number information provided by SNP array allow determination of parental origin and uniparental disomy status and direct quantification of mosaicism. Such information may lead to a better understanding of mechanisms underlying mosaic aneuploidies and the observed phenotypic variability and better prediction of recurrent risk.

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.

Chromosome nondisjunction yields tetraploid rather than aneuploid cells in human cell lines.

Although mutations in cell cycle regulators or spindle proteins can perturb chromosome segregation, the causes and consequences of spontaneous mitotic chromosome nondisjunction in human cells are not well understood. It has been assumed that nondisjunction of a chromosome during mitosis will yield two aneuploid daughter cells. Here we show that chromosome nondisjunction is tightly coupled to regulation of cytokinesis in human cell lines, such that nondisjunction results in the formation of tetraploid rather than aneuploid cells. We observed that spontaneously arising binucleated cells exhibited chromosome mis-segregation rates up to 166-fold higher than the overall mitotic population. Long-term imaging experiments indicated that most binucleated cells arose through a bipolar mitosis followed by regression of the cleavage furrow hours later. Nondisjunction occurred with high frequency in cells that became binucleated by furrow regression, but not in cells that completed cytokinesis to form two mononucleated cells. Our findings indicate that nondisjunction does not directly yield aneuploid cells, but rather tetraploid cells that may subsequently become aneuploid through further division. The coupling of spontaneous segregation errors to furrow regression provides a potential explanation for the prevalence of hyperdiploid chromosome number and centrosome amplification observed in many cancers.

Heterochromatin-mediated association of achiasmate homologs declines with age when cohesion is compromised.

Normally, meiotic crossovers in conjunction with sister-chromatid cohesion establish a physical connection between homologs that is required for their accurate segregation during the first meiotic division. However, in some organisms an alternative mechanism ensures the proper segregation of bivalents that fail to recombine. In Drosophila oocytes, accurate segregation of achiasmate homologs depends on pairing that is mediated by their centromere-proximal heterochromatin. Our previous work uncovered an unexpected link between sister-chromatid cohesion and the fidelity of achiasmate segregation when Drosophila oocytes are experimentally aged. Here we show that a weak mutation in the meiotic cohesion protein ORD coupled with a reduction in centromere-proximal heterochromatin causes achiasmate chromosomes to missegregate with increased frequency when oocytes undergo aging. If ORD activity is more severely disrupted, achiasmate chromosomes with the normal amount of pericentric heterochromatin exhibit increased nondisjunction when oocytes age. Significantly, even in the absence of aging, a weak ord allele reduces heterochromatin-mediated pairing of achiasmate chromosomes. Our data suggest that sister-chromatid cohesion proteins not only maintain the association of chiasmate homologs but also play a role in promoting the physical association of achiasmate homologs in Drosophila oocytes. In addition, our data support the model that deterioration of meiotic cohesion during the aging process compromises the segregation of achiasmate as well as chiasmate bivalents.

Natural variation of outcrossing in the hermaphroditic nematode Pristionchus pacificus.

BACKGROUND: Evolution of selfing can be associated with an increase in fixation of deleterious mutations, which in certain conditions can lead to species extinction. In nematodes, a few species evolved self-fertilization independently, making them excellent model systems to study the evolutionary consequences of this type of mating system. RESULTS: Here we determine various parameters that influence outcrossing in the hermaphroditic nematode Pristionchus pacificus and compare them to the better known Caenorhabditis elegans. These nematode species are distinct in terms of genetic diversity, which could be explained by differences in outcrossing rates. We find that, similarly to C. elegans, P. pacificus males are generated at low frequencies from self-fertilizing hermaphrodites and are relatively poor mating partners. Furthermore, crosses between different isolates reveal that hybrids have lower brood sizes than the pure strains, which is a sign of outbreeding depression. In contrast to C. elegans, P. pacificus has lower brood sizes and the male X-bearing sperm is able to outcompete the X-nullo sperm. CONCLUSION: The results indicate that there is no evidence of any selection acting very strongly on P. pacificus males.

Etiology of Down syndrome: Evidence for consistent association among altered meiotic recombination, nondisjunction, and maternal age across populations.

Down syndrome caused by meiotic nondisjunction of chromosome 21 in humans, is well known to be associated with advanced maternal age, but success in identifying and understanding other risk factors has been limited. Recently published work in a U.S. population suggested intriguing interactions between the maternal age effect and altered recombination patterns during meiosis, but some of the results were counter-intuitive. We have tested these hypotheses in a population sample from India, and found that essentially all of the results of the U.S. study are replicated even in our ethnically very different population. We examined meiotic recombination patterns in a total of 138 families from the eastern part of India, each with a single free trisomy 21 child. We genotyped each family with a set of STR markers using PCR and characterized the stage of origin of nondisjunction and the recombination pattern of maternal chromosome 21 during oogenesis. Our sample contains 107 maternal meiosis I errors and 31 maternal meiosis II errors and we subsequently stratified them with respect to maternal age and the number of detectable crossover events. We observed an association between meiosis I nondisjunction and recombination in the telomeric 5.1 Mb of chromosome 21. By contrast, in meiosis II cases we observed preferential pericentromeric exchanges covering the proximal 5.7 Mb region, with interaction between maternal age and the location of the crossover. Overall reduction of recombination irrespective of maternal age is also evident in meiosis I cases. Our findings are very consistent with previously reported data in a U.S. population and our results are the first independent confirmation of those previous reports. This not only provides much needed confirmation of previous results, but it suggests that the genetic etiology underlying the occurrence of trisomy 21 may be similar across human populations.

Aurora kinase B, epigenetic state of centromeric heterochromatin and chiasma resolution in oocytes.

Aurora kinases comprise a family of phosphoproteins performing multiple functions in mitosis and meiosis. Because Aurora kinase B (AURKB) expression is altered in aged oocytes and there is only limited information on its function in meiosis, it was decided to study the spatial distribution and co-localization of AURKB with other regulatory proteins at centromeres during mouse oocyte maturation. AURKB associates with chromosomes after germinal vesicle breakdown, is enriched at centromeres from prometaphase I and transits to the spindle midzone at late anaphase I. Preferential inhibition of AURKB by low concentrations of ZM 447439 inhibitor prevents polar body formation and affects spindle formation and chromosome congression at meiosis I, associated with expression of BubR1 checkpoint protein at kinetochores. Release of cohesion between sister chromatids appears inhibited resulting in failure of chiasma resolution in oocytes progressing to anaphase I. Concomitantly, the inhibitor reduces histone H3 lysine 9 trimethylation at centromeric heterochromatin and affects chromosome condensation. The cytokinesis arrest protects young, healthy oocytes from errors in chromosome segregation although increasing polyploidy. This study shows that changes in activity of AURKB may increase risks for chromosome non-disjunction and aneuploidy in mammalian oocytes, irrespective of age.

Double aneuploidy (48,XXY,+21) of maternal origin in a child born to a 13-year-old mother: evaluation of the maternal folate metabolism.

The occurrence of non-mosaic double trisomy is exceptional in newborns. In this paper, a 48,XXY,+21 child, the parental origin of the extra chromosomes and the evaluation of the maternal folate metabolism are presented. The infant was born to a 13-year-old mother and presented with the typical clinical features of Down syndrome (DS). The origin of the additional chromosomes was maternal and most likely resulted from errors during the first meiotic division. Molecular analysis of 12 genetic polymorphisms involved in the folate metabolism revealed that the mother is heterozygous for the MTHFR C677T and TC2 A67G polymorphisms, and homozygous for the mutant MTRR A66G polymorphism. The maternal homocysteine concentration was 4.7 miromol/L, a value close to the one considered as a risk factor for DS in our previous study. Plasma methylmalonic acid and serum folate concentrations were 0.17 micromol/L and 18.4 ng/mL, respectively. It is possible that the presence of allelic variants for the folate metabolism and Hey concentration might have favored errors in chromosomal disjunction during gametogenesis in this young mother. To our knowledge, this is the first patient with non-mosaic Down-Klinefelter born to a teenage mother, resulting from a rare fertilization event combining an abnormal 25,XX,+21 oocyte and a 23,Y spermatozoon.