Frequency of meiotic trisomy depends on involved chromosome and mode of ascertainment

Characteristics and mechanisms of mosaicism in prenatal diagnosis cases by application of SNP array

BACKGROUND

With the application of chromosome microarray, next-generation sequencing and other highly sensitive genetic techniques in disease diagnosis, the detection of mosaicism has become increasingly prevalent. This study involved a retrospective analysis of SNP array testing on 4512 prenatal diagnosis samples, wherein the characterization of mosaicism was explored and insights were gained into the underlying mechanisms thereof.

RESULTS

Using SNP array, a total of 44 cases of mosaicism were identified among 4512 prenatal diagnostic cases; resulting in a detection rate of approximately 1.0%. The prevalence of mosaicism was 4.1% for chorionic villus sample, 0.4% for amniotic fluid, and 1.3% for umbilical cord blood. Of these cases, 29 were mosaic aneuploidy and 15 were mosaic segmental duplication/deletion. Three cases of mosaic trisomy 16 and three cases of mosaic trisomy 22 were diagnosed in the CVS samples, while four cases of mosaic trisomy 21 were detected in amniotic fluid and umbilical cord blood samples. The distribution pattern of mosaicism suggested trisomy rescue as the underlying mechanism. Structurally rearranged chromosomes were observed, including three cases with supernumerary marker chromosomes, three cases with dicentric chromosomes, and one case with a ring chromosome. All mosaic segmental duplication/deletion cases were the result of mitotic non-disjunction, with the exception of one case involving mosaic11q segmental duplication.

CONCLUSION

Improved utilization of SNP arrays enables the characterization of mosaicism and facilitates the estimation of disease mechanisms and recurrence.

Uniparental disomy: Origin, frequency, and clinical significance

Uniparental disomy (UPD) is defined as two copies of a whole chromosome derived from the same parent. There can be multiple mechanisms that lead to UPD; these are reviewed in the context of contemporary views on the mechanism leading to aneuploidy. Recent studies indicate that UPD is rare in an apparently healthy population and also rare in spontaneous abortion tissues. The most common type of UPD is a maternal heterodisomy (both maternal allele sets present). Isodisomy (a duplicated single set of alleles) or segmental loss of heterozygosity is sometimes encountered in SNP-based microarray referrals. Decisions regarding the most appropriate follow-up testing should consider the possibility of consanguinity (that will generally involve multiple regions), an imprinted gene disorder (chromosomes 6, 7, 11, 14, 15, 20), expression of an autosomal recessive disorder, and an occult aneuploid cell line that may be confined to the placenta. Upd(16)mat, per se, does not appear to be associated with an abnormal phenotype. UPD provides an insight into the history of early chromosome segregation error and understanding the rates and fate of these events are of key importance in the provision of fertility management and prenatal healthcare.

Aneuploidy in first trimester chorionic villi and spontaneous abortions: Windows into the origin and fate of aneuploidy through embryonic and fetal development

OBJECTIVE

To review the mosaic autosomal trisomies in chorionic villi sample (CVS) trophoblasts, mesenchyme, and both cell lineages and to compare them with trisomies in spontaneous abortions.

METHODS

Mosaic autosomal trisomies from 76 102 diagnostic CVS tests were classified as involving trophoblasts, involving mesenchyme, or present in both. Autosomal trisomies in products of conception were based on 18 published studies. We evaluated correlates between trisomy frequency with chromosome size or number of protein coding genes in the imbalance.

RESULTS

Distinctly different patterns of trisomy were found in trophoblasts, mesenchyme, or both. In trisomic spontaneous abortions, there was a weak, borderline significant, inverse association between frequency and trisomic chromosome size and also with the number of protein coding genes involved (r = 0.43, P = 0.04 and r = 0.39, P = 0.07, respectively). These associations became stronger after excluding trisomy 16 (r = 0.52, P = 0.01 and r = 0.64, P = 0.001, respectively). Only CVS trisomies in both trophoblasts and mesenchyme resembled the trisomies found in spontaneous abortions and these were also associated with chromosome size and protein coding genes (r = 0.42, P = 0.05 and r = 0.57, P = 0.006, respectively).

CONCLUSION

The abnormalities seen in CVS differ from those reported in early embryos. From conception through birth, there are lineage-specific, evolving spectrums of aneuploidy in trophoblasts, mesenchyme, and fetus.

The Status of Genetic Screening in Recurrent Pregnancy Loss

Recurrent pregnancy loss is often idiopathic, but numerical and structural chromosomal abnormalities constitute an important cause. Numerical chromosomal abnormalities in the conceptus are primarily due to meiotic nondisjunction; the rate and complexity of embryonic aneuploidy are driven by female age. Structural chromosomal abnormalities (balanced translocations or inversions) can lead to unbalanced gametes depending on specific recombination and segregation patterns during meiosis. The attendant reproductive risk depends on the type of rearrangement and its parental origin. Current methods for analysis of products of conception include cytogenetics, array comparative genomic hybridization, and single nucleotide polymorphism microarray; each platform has advantages and disadvantages.

Clonal chromosomal and genomic instability during human multipotent mesenchymal stromal cells long-term culture

Background aims

Spontaneous mutagenesis often leads to appearance of genetic changes in cells. Although human multipotent mesenchymal stromal cells (hMSC) are considered as genetically stable, there is a risk of genomic and structural chromosome instability and, therefore, side effects of cell therapy associated with long-term effects. In this study, the karyotype, genetic variability and clone formation analyses have been carried out in the long-term culture MSC from human gingival mucosa.

Methods

The immunophenotype of MSC has been examined using flow cytofluorometry and short tandem repeat (STR) analysis has been carried out for authentication. The karyotype has been examined using GTG staining and mFISH, while the assessment of the aneuploidy 8 frequency has been performed using centromere specific chromosome FISH probes in interphase cells.

Results

The immunophenotype and STR loci combination did not change during the process of cultivation. From passage 23 the proliferative activity of cultured MSCs was significantly reduced. From passage 12 of cultivation, clones of cells with stable chromosome aberrations have been identified and the biggest of these (12%) are tetrasomy of chromosome 8. The random genetic and structural chromosomal aberrations and the spontaneous level of chromosomal aberrations in the hMSC long-term cultures were also described.

Conclusions

The spectrum of spontaneous chromosomal aberrations in MSC long-term cultivation has been described. Clonal chromosomal aberrations have been identified. A clone of cells with tetrasomy 8 has been detected in passage 12 and has reached the maximum size by passage 18 before and decreased along with the reduction of proliferative activity of cell line by passage 26. At later passages, the MSC line exhibited a set of cells with structural variants of the karyotype with a preponderance of normal diploid cells. The results of our study strongly suggest a need for rigorous genetic analyses of the clone formation in cultured MSCs before use in medicine.

Posttest risk calculation following positive noninvasive prenatal screening using cell-free DNA in maternal plasma

Noninvasive prenatal screening (NIPS) for fetal chromosome defects has high sensitivity and specificity but is not fully diagnostic. In response to a desire to provide more information to individual women with positive NIPS results, 2 online calculators have been developed to calculate posttest risk (PTR). Use of these calculators is critically reviewed. There is a mathematically dictated requirement for a precise estimate for the specificity to provide an accurate PTR. This is illustrated by showing that a 0.1% decrease in the value for specificities for trisomies 21, 18, and 13 can reduce the PTR from 79-64% for trisomy 21, 39-27% for trisomy 18, and 21-13% for trisomy 13, respectively. Use of the calculators assumes that sensitivity and specificity are constant for all women receiving the test but there is evidence that discordancy between screening results and true fetal karyotype is more common for older women. Use of an appropriate value for the prior risk is also important and for rare disorders there is considerable uncertainty regarding prevalence. For example, commonly used rates for trisomy 13, monosomy-X, triploidy, and 22q11.2 deletion syndrome can vary by >4-fold and this can translate into large differences in PTR. When screening for rare disorders, it may not be possible to provide a reliable PTR if there is uncertainty over the false-positive rate and/or prevalence. These limitations, per se, do not negate the value of screening for rare conditions. However, counselors need to carefully weigh the validity of PTR before presenting them to patients. Additional epidemiologic and NIPS outcome data are needed.

Detecting somatic mosaicism: considerations and clinical implications

Human disease is rarely a matter of all or nothing; variable expressivity is generally observed. Part of this variability is explained by somatic mosaicism, which can arise by a myriad of genetic alterations. These can take place at any stage of development, possibly leading to unusual features visible at birth, but can also occur later in life, conceivably leading to cancer. Previously, detection of somatic mosaicism was extremely challenging, as many gold standard tests lacked the necessary resolution. However, with the advances in high-throughput sequencing, mosaicism is being detected more frequently and at lower levels. This raises the issue of normal variation within each individual vs mosaicism leading to disease, and how to distinguish between the two. In this article, we will define somatic mosaicism with a brief overview of its main mechanisms in concrete clinical examples, discuss the impact of next-generation sequencing technologies in its detection, and expand on the clinical implications associated with a discovery of somatic mosaicism in the clinic.

Full trisomy 5 in a sample of spontaneous abortion and Arias Stella reaction

BACKGROUND

Historically, 50% of spontaneously expelled abortuses have been thought to be chromosomally abnormal; about 60% are trisomies. In general, trisomy 16 is the most frequent chromosomal abnormality, followed by trisomy 21 and trisomy 22. So far only 1 case of a female fetus with multiple congenital malformations associated with full trisomy 5 has been described.

REPORT

We present a case of de novo full trisomy 5 in a spontaneous abortion sample. A young couple with normal constitutional karyotype experienced the second spontaneous abortion at 9 weeks of gestation, with the cytogenetic formula 47,XX,+5 in all analyzed cells.

CONCLUSIONS

The routine cytogenetic analysis of miscarriages is still an uncommon practice, but it can have a great impact on the management of couples with repeated pregnancy wastage. Besides of the obvious cost benefit for health care, such analysis would help the physician to decide about future patient management, as well as planning the genetic counseling.

Mechanisms of mosaicism, chimerism and uniparental disomy identified by single nucleotide polymorphism array analysis

Mosaic aneuploidy and uniparental disomy (UPD) arise from mitotic or meiotic events. There are differences between these mechanisms in terms of (i) impact on embryonic development; (ii) co-occurrence of mosaic trisomy and UPD and (iii) potential recurrence risks. We used a genome-wide single nucleotide polymorphism (SNP) array to study patients with chromosome aneuploidy mosaicism, UPD and one individual with XX/XY chimerism to gain insight into the developmental mechanism and timing of these events. Sixteen cases of mosaic aneuploidy originated mitotically, and these included four rare trisomies and all of the monosomies, consistent with the influence of selective factors. Five trisomies arose meiotically, and three of the five had UPD in the disomic cells, confirming increased risk for UPD in the case of meiotic non-disjunction. Evidence for the meiotic origin of aneuploidy and UPD was seen in the patterns of recombination visible during analysis with 1-3 crossovers per chromosome. The mechanisms of formation of the UPD included trisomy rescue, with and without concomitant trisomy, monosomy rescue, and mitotic formation of a mosaic segmental UPD. UPD was also identified in an XX/XY chimeric individual, with one cell line having complete maternal UPD consistent with a parthenogenetic origin. Utilization of SNP arrays allows simultaneous evaluation of genomic alterations and insights into aneuploidy and UPD mechanisms. Differentiation of mitotic and meiotic origins for aneuploidy and UPD supports existence of selective factors against full trisomy of some chromosomes in the early embryo and provides data for estimation of recurrence and disease mechanisms.

Parental Origin and Cell Stage Errors in X-Chromosome Polysomy 49, XXXXY

Polysomy 49, XXXXY is a rare sex chromosome aneuploidy syndrome characterized by mental retardation, severe speech impairment, craniofacial abnormalities, multiple skeletal defects and genital abnormalities. We describe a patient with 49, XXXXY syndrome who had many characteristics of Fraccaro syndrome; language impairment, mongoloid slant, epicanthal folds, cryptorchidism, umbilical hernia and dysmyelinization in his brain. A GTG-banding technique was used for karyotype analysis of peripheral blood cell cultures. The parental origin of polysomy X was identified by using quantitative fluorescent polymerase chain reaction (QF-PCR) with seven short tandem repeat (STR) markers specific for the X/Y-chromosome which revealed that all the X-chromosomes were of maternal origin. This report provides evidence for successive non disjunctions in maternal meiosis I and II.

Inducing segmental aneuploid mosaicism in the mouse through targeted asymmetric sister chromatid event of recombination

Loss or gain of whole chromosomes, or parts of chromosomes, is found in various pathological conditions, such as cancer and aneuploidy, and results from the missegregation of chromosomes during cellular division or abnormal mitotic recombination. We introduce a novel strategy for determining the consequences of segmental aneuploid mosaicism, called targeted asymmetric sister chromatin event of recombination (TASCER). We took advantage of the Cre/loxP system, used extensively in embryonic stem cells for generating deletions and duplications of regions of interest, to induce recombination during the G2 phase. Using two loxP sites in a Cis configuration, we generated in vivo cells harboring microdeletions and microduplications for regions of interest covering up to 2.2 Mb. Using this approach in the mouse provides insight into the consequences of segmental aneuploidy for homologous regions of the human chromosome 21 on cell survival. Furthermore, TASCER shows that Cre-induced recombination is more efficient after DNA replication in vivo and provides an opportunity to evaluate, through genetic mosaics, the outcome of copy number variation and segmental aneuploidy in the mouse.

Recurrent miscarriage

Many human conceptions are genetically abnormal and end in miscarriage, which is the commonest complication of pregnancy. Recurrent miscarriage, the loss of three or more consecutive pregnancies, affects 1% of couples trying to conceive. It is associated with psychological morbidity, and has often proven to be frustrating for both patient and clinician. A third of women attending specialist clinics are clinically depressed, and one in five have levels of anxiety that are similar to those in psychiatric outpatient populations. Many conventional beliefs about the cause and treatment of women with recurrent miscarriage have not withstood scrutiny, but progress has been made. Research has emphasised the importance of recurrent miscarriage in the range of reproductive failure linking subfertility and late pregnancy complications and has allowed us to reject practice based on anecdotal evidence in favour of evidence-based management.

Meiotic chromosome abnormalities in human spermatogenesis

The last few years have witnessed an explosion in the information about chromosome abnormalities in human sperm and the meiotic events that predispose to these abnormalities. We have determined that all chromosomes are susceptible to nondisjunction, but chromosomes 21 and 22 and, especially, the sex chromosomes have an increased frequency of aneuploidy. Studies are just beginning on the effects of potential mutagens on the chromosomal constitution of human sperm. The effects of pesticides and cancer therapeutic agents have been reviewed. In the last decade, there has been a great impetus to study chromosome abnormalities in sperm from infertile men because the advent of intracytoplasmic sperm injection (ICSI) made it possible for these men to father pregnancies. A large number of studies have demonstrated that infertile men have an increased frequency of chromosomally abnormal sperm and children, even when they have a normal somatic karyotype. Meiotic studies on the pachytene stage of spermatogenesis have demonstrated that infertile men have impaired chromosome synapsis, a significantly decreased frequency of recombination, and an increased frequency of chromosomes completely lacking a recombination site. Such errors make these cells susceptible to meiotic arrest and the production of aneuploid gametes.

Chromosome 21 mosaic human preimplantation embryos predominantly arise from diploid conceptions

OBJECTIVE

High rates of chromosomal mosaicism in human IVF embryos question the accuracy of preimplantation genetic diagnosis, and, with the majority of embryo transfers still resulting in no pregnancy, chromosomal mosaicism is likely to be a contributing factor to human IVF failure. The aim of this study was to investigate the origin and nature of chromosome 21 (Ch21) cell division errors in human IVF embryos.

DESIGN

Perform single cell Ch21 allelic profiling on human IVF embryos.

SETTING

Academic research environment.

PATIENT(S)

Women of advanced maternal age (> 35 yrs) (n = 65) undergoing infertility treatment; and amniocytes/chorionic cells from trisomy 21 pregnancies (n = 28).

INTERVENTION(S)

Cells were analyzed by single cell allelic profiling,

MAIN OUTCOME MEASURE(S)

The origin and nature of cell division errors.

RESULT(S)

The vast majority of Ch21 mosaic embryos (approximately 80%) originated from diploid conceptions. In contrast, all fetal trisomy 21 originated from aneuploid conceptions. Increasing maternal age was significantly associated with aneuploid conceptions, meiotic cell division error, and adverse pregnancy outcome (P < .05). The mean daily FSH dose that produced embryos with normal Ch21 cell division was significantly lower than the mean daily FSH dose that produced embryos with mitotic Ch21 cell division errors (P < .01) and embryos with meiotic cell division errors (P < .05).

CONCLUSION(S)

Chromosomal mosaicism of Ch21 in human IVF embryos predominantly originate from diploid conceptions. Further understanding of chromosomal mosaicism with respect to IVF parameters, such as daily FSH dose, may eventually lead to improvements in IVF outcomes.

Double trisomy in spontaneous miscarriages: cytogenetic and molecular approach

BACKGROUND

Although single trisomy is the most common chromosomal abnormality observed within first trimester spontaneous abortions (SA) (>50%), double trisomy (DT) ranges from 0.21 to 2.8% in the literature. Since little is known about mechanisms underlying DT, we report the results of our experience with 517 SA, establishing parental origin and cell stage of non-disjunction when possible in DT cases, and making a revision of those previously reported.

METHODS

Cytogenetic analysis was performed in all aborted specimens. Quantitative fluorescent PCR (QF-PCR) and multiplex ligation-dependent probe amplification (MLPA) were performed in DT cases in order to assess parental origin and stage of error of aneuploidy in addition to its reliability in detecting aneuploidies.

RESULTS

Karyotyping was successful in 321 miscarriages; the rate of DT was 2.18%. Among the seven DT cases reported, three new combinations were found. Maternal origin was established for all DT SA analysed. Meiotic stage of error was presumed meiosis I (MI) for 48,XX+15+22 and 48,XX+8+21, meiosis II (MII) for 48,XXX+18, and MII and MI respectively for 48,XY+18+22. Molecular results agreed with cytogenetic results.

CONCLUSIONS

Similar maternal age-related mechanisms could be implicated in both single and double trisomy. Molecular techniques could be useful in diagnosing not only single but multiple aneuploidy and determining its origin. This will improve our knowledge about mechanisms underlying human aneuploidy, and enable appropriate genetic counselling.

Trisomy 2 due to a 3:1 segregation in an abortion studied by QF-PCR and CGH

Balanced reciprocal translocation is one of the known causes of recurrent spontaneous abortions. Cytogenetic studies of unbalanced miscarriages are difficult due to the growth failure of early loss and usually macerated abortions. We present a molecular study of an abortion in which the father carries a balanced reciprocal translocation t(2;17)(q32.1;q24.3) using QF-PCR and CGH techniques. DNA analysis showed the presence of a trisomy 2 due to a 3:1 interchange segregation. Recombinant events could also be investigated by comparing DNA samples from the family. We propose QF-PCR in addition to CGH as an efficient diagnostic method to improve our knowledge of unbalanced offspring in balanced translocation carriers.

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

OBJECTIVES

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

CASE

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

METHOD

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

CONCLUSION

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

X chromosome inactivation patterns in Russell-Silver syndrome patients and their mothers

Russell-Silver syndrome (RSS) is a disorder characterized by pre- and post-natal growth deficiency, triangular facies, relative macrocephaly, and body asymmetry. Maternal uniparental disomy for chromosome 7 has been found in approximately 10% of RSS cases, while the cause in the remaining cases is unknown. Although most cases of RSS are sporadic, at least 25 families have been reported with familial RSS and inheritance patterns of RSS consistent with an X-linked dominant mutation. Thus, we hypothesized that skewed X-chromosome inactivation (XCI) could be increased in some females with RSS as a consequence of a tendency to have the mutant allele on the predominantly active chromosome. Alternatively, unaffected mothers of children with RSS may tend to be skewed due to preferential inactivation of the mutant allele. In support of this last hypothesis, a significant increase in extremely skewed XCI (>95%) was found in mothers of children with RSS (4 of 21, 19%) compared to controls (2 of 101, 2%) (P = 0.008). However, an increase in skewed XCI was also observed in female patients who had unexplained short stature but did not fulfill the criteria for RSS (3 of 17, 18%) (P = 0.02), but not in those diagnosed as RSS (0 of 7, n.s.). Different mechanisms may underlie the increase in skewing in each group, possibly being due to different X-linked mutations or growth restriction during very early in utero development.

Cytogenetic analysis of human blastocysts

The human blastocyst is key to understanding the aetiology of constitutional chromosome abnormalities in our species.

OBJECTIVES

To investigate the range and incidence of chromosome abnormalities in a large series of human blastocysts, using classic cytogenetic techniques.

METHODS

Using thymidine, cell division is synchronized in spare five-to-eight-day-old human blastocysts generated by IVF. A simple acetic acid disaggregation step produces discrete metaphases for G-band analysis. Subsequent FISH analysis of both metaphase and interphase nuclei allows further exploration of an abnormality detected by G-banding, including the investigation of any mosaicism.

RESULTS

A total of 438 blastocysts have been prepared. Where analysis was possible, 3% appeared polyploid (mainly tetraploid), 29% were diploid : tetraploid mosaics and 68% were uniformly diploid. Abnormalities observed include triploidy, trisomy 16, trisomy 2, trisomy for unidentifiable D-group chromosome, mosaic trisomy 3, and mosaic trisomy 3 and trisomy 7.

CONCLUSION

Comparison of results with existing data from both first trimester pregnancies and cleavage stage embryos suggests significant loss of haploid and monosomic embryos, as well as loss of some trisomies, prior to the blastocyst stage. It appears that the general range and incidence of most main groups of constitutional abnormalities observed in the first trimester (including mosaic forms) are in place by the blastocyst stage.

Maternal uniparental isodisomy 20 in a foetus with trisomy 20 mosaicism: clinical, cytogenetic and molecular analysis

The clinical significance of trisomy 20 mosaicism detected prenatally remains uncertain due to the rarity of liveborn cases with inconsistent clinical findings, and lack of long-term follow-up and outcome. We describe a case of true trisomy 20 mosaicism in a liveborn girl with maternal uniparental isodisomy of chromosome 20 in the diploid blood cells. Trisomy 20 mosaicism was originally detected in amniotic fluid (98%) and was confirmed in the term placenta (100%), as well as in the blood (10%) and urine sediment (100%) of the neonate. There was intrauterine and postnatal growth retardation, but otherwise the newborn manifested no gross abnormalities. At 9 months of age moderate psychomotor retardation, central hypotonia with peripheral hypertonia, numerous minor morphogenetic variants, marked kyphosis, and extensive Mongolian spot were observed. To our knowledge this represents the first case of trisomy 20 mosaicism detected prenatally and confirmed in different tissues of the newborn, where uniparental disomy was demonstrated in the diploid cell line. The clinical and laboratory findings in our patient are compared with those of five previously reported cases of UPD20, suggesting that maternal UPD20 might be associated with a characteristic phenotype.

The origin of abnormalities in recurrent aneuploidy/polyploidy

Recurrent miscarriage due to sporadic chromosomal abnormalities may simply be a consequence of the dramatic increase of trisomic conceptions with increased maternal age. However, it is also possible that some couples are at increased risk of abnormalities as a result of gonadal mosaicism, factors affecting chromosome structure and segregation, increased sperm aneuploidy in the male partner, or accelerated "aging" of the ovaries. We report cytogenetic and molecular findings from 122 spontaneous abortions (SAs) from 54 couples who were ascertained as having two or more documented aneuploid or polyploid SAs. The distribution of abnormalities in this group was similar to those from 307 SAs that involved chromosome abnormalities and were diagnosed at the same center but did not involve documented recurrent aneuploidy/polyploidy. Although recurrence of the same abnormality was observed in eight families, this number was equal to that expected by chance, indicating that gonadal mosaicism is rarely the explanation for recurrence. The origin of the abnormality was determined in 37 SAs from 23 of the couples in the study. A maternal meiotic origin was involved in 30 trisomies and in 1 triploid SA; 3 additional maternal trisomies were of possible somatic origin. A paternal origin was found in the remaining two trisomies and in one triploid SA. In addition, one double trisomy was the consequence of both a maternal and a paternal meiotic error. These results confirm that the etiology of trisomy is predominantly a result of meiotic errors related to increased maternal age, regardless of whether the couple has experienced one or multiple aneuploid SAs. Furthermore, this is true even when a second SA involves the same abnormality. Nonetheless, these data do not exclude some population variability in risk for aneuploidy.

Two cases of confined placental mosaicism for chromosome 4, including one with maternal uniparental disomy

Two cases of trisomy 4 mosaicism are reported including one with molecularly confirmed uniparental disomy (UPD) of chromosome 4. Cytogenetic analysis of a chorionic villus sample (CVS) in Case 1 showed complete trisomy 4 in trophoblast and diploidy in chorionic stroma. Amniotic fluid analysis demonstrated a 46,XX complement. After intrauterine fetal death at 30 weeks, molecular analysis confirmed the presence of trisomy 4 of maternal meiotic origin, while fetal tissues showed maternal UPD for chromosome 4. Cultured CVS in Case 2 revealed trisomy 4 in 2/30 cells analyzed. This pregnancy resulted in a healthy livebirth with biparental inheritance of chromosome 4. Molecularly confirmed UPD4 has not been previously reported, and therefore, although the adverse outcome in Case 1 is likely due to the trisomy 4 in the placenta, an imprinting effect associated with UPD4 cannot be excluded.

Mechanisms leading to uniparental disomy and their clinical consequences

Uniparental disomy (UPD) refers to the situation in which both copies of a chromosome pair have originated from one parent. In humans, it can result in clinical conditions by producing either homozygosity for recessive mutations or aberrant patterns of imprinting. Furthermore, UPD is frequently found in conjunction with mosaicism for a chromosomally abnormal cell line, which can also contribute to phenotypic abnormalities. Investigations into the mechanisms by which UPD may arise have helped to expand our general awareness of the impact of chromosomal abnormalities and chromosomal mosaicism in normal human development. Specifically, it appears that errors in the transmission of a chromosome from parent to gamete and during early somatic cell divisions are remarkably common but that embryo and cell selection during early embryogenesis help to ensure the presence of a numerically balanced chromosome complement in the developing fetus. UPD is also likely to occur within a portion of cells in all individuals simply as a consequence of somatic recombination occurring during mitotic cell divisions. This can be an important step in cancer development as well as a contributing factor to other late onset diseases. This review summarizes mechanisms by which UPD may arise and their associated clinical consequences.