Three different origins for apparent triploid/diploid mosaics

Parental genomes segregate into distinct blastomeres during multipolar zygotic divisions leading to mixoploid and chimeric blastocysts

BACKGROUND

During normal zygotic division, two haploid parental genomes replicate, unite and segregate into two biparental diploid blastomeres.

RESULTS

Contrary to this fundamental biological tenet, we demonstrate here that parental genomes can segregate to distinct blastomeres during the zygotic division resulting in haploid or uniparental diploid and polyploid cells, a phenomenon coined heterogoneic division. By mapping the genomic landscape of 82 blastomeres from 25 bovine zygotes, we show that multipolar zygotic division is a tell-tale of whole-genome segregation errors. Based on the haplotypes and live-imaging of zygotic divisions, we demonstrate that various combinations of androgenetic, gynogenetic, diploid, and polyploid blastomeres arise via distinct parental genome segregation errors including the formation of additional paternal, private parental, or tripolar spindles, or by extrusion of paternal genomes. Hence, we provide evidence that private parental spindles, if failing to congress before anaphase, can lead to whole-genome segregation errors. In addition, anuclear blastomeres are common, indicating that cytokinesis can be uncoupled from karyokinesis. Dissociation of blastocyst-stage embryos further demonstrates that whole-genome segregation errors might lead to mixoploid or chimeric development in both human and cow. Yet, following multipolar zygotic division, fewer embryos reach the blastocyst stage and diploidization occurs frequently indicating that alternatively, blastomeres with genome-wide errors resulting from whole-genome segregation errors can be selected against or contribute to embryonic arrest.

CONCLUSIONS

Heterogoneic zygotic division provides an overarching paradigm for the development of mixoploid and chimeric individuals and moles and can be an important cause of embryonic and fetal arrest following natural conception or IVF.

Genome-wide abnormalities in embryos: Origins and clinical consequences

Ploidy or genome-wide chromosomal anomalies such as triploidy, diploid/triploid mixoploidy, chimerism, and genome-wide uniparental disomy are the cause of molar pregnancies, embryonic lethality, and developmental disorders. While triploidy and genome-wide uniparental disomy can be ascribed to fertilization or meiotic errors, the mechanisms causing mixoploidy and chimerism remain shrouded in mystery. Different models have been proposed, but all remain hypothetical and controversial, are deduced from the developmental persistent genomic constitutions present in the sample studied and lack direct evidence. New single-cell genomic methodologies, such as single-cell genome-wide haplotyping, provide an extended view of the constitution of normal and abnormal embryos and have further pinpointed the existence of mixoploidy in cleavage-stage embryos. Based on those recent findings, we suggest that genome-wide anomalies, which persist in fetuses and patients, can for a large majority be explained by a noncanonical first zygotic cleavage event, during which maternal and paternal genomes in a single zygote, segregate to different blastomeres. This process, termed heterogoneic division, provides an overarching theoretical basis for the different presentations of mixoploidy and chimerism.

Natural human chimeras: A review

The term chimera has been borrowed from Greek mythology and has a long history of use in biology and genetics. A chimera is an organism whose cells are derived from two or more zygotes. Recipients of tissue and organ transplants are artificial chimeras. This review concerns natural human chimeras. The first human chimera was reported in 1953. Natural chimeras can arise in various ways. Fetal and maternal cells can cross the placental barrier so that both mother and child may become microchimeras. Two zygotes can fuse together during an early embryonic stage to form a fusion chimera. Most chimeras remain undetected, especially if both zygotes are of the same genetic sex. Many are discovered accidently, for example, during a routine blood group test. Even sex-discordant chimeras can have a normal male or female phenotype. Only 28 of the 50 individuals with a 46,XX/46,XY karyotype were either true hermaphrodites or had ambiguous genitalia. Blood chimeras are formed by blood transfusion between dizygotic twins via the shared placenta and are more common than was once assumed. In marmoset monkey twins the exchange via the placenta is not limited to blood but can involve other tissues, including germ cells. To date there are no examples in humans of twin chimeras involving germ cells. If human chimeras are more common than hitherto thought there could be many medical, social, forensic, and legal implications. More multidisciplinary research is required for a better understanding of this fascinating subject.

When Three Isn't a Crowd: A Digyny Concept for Treatment-Resistant, Near-Triploid Human Cancers

Near-triploid human tumors are frequently resistant to radio/chemotherapy through mechanisms that are unclear. We recently reported a tight association of male tumor triploidy with XXY karyotypes based on a meta-analysis of 15 tumor cohorts extracted from the Mitelman database. Here we provide a conceptual framework of the digyny-like origin of this karyotype based on the germline features of malignant tumors and adaptive capacity of digyny, which supports survival in adverse conditions. Studying how the recombinatorial reproduction via diploidy can be executed in primary cancer samples and HeLa cells after DNA damage, we report the first evidence that diploid and triploid cell sub-populations constitutively coexist and inter-change genomes via endoreduplicated polyploid cells generated through genotoxic challenge. We show that irradiated triploid HeLa cells can enter tripolar mitosis producing three diploid sub-subnuclei by segregation and pairwise fusions of whole genomes. Considering the upregulation of meiotic genes in tumors, we propose that the reconstructed diploid sub-cells can initiate pseudo-meiosis producing two "gametes" (diploid "maternal" and haploid "paternal") followed by digynic-like reconstitution of a triploid stemline that returns to mitotic cycling. This process ensures tumor survival and growth by (1) DNA repair and genetic variation, (2) protection against recessive lethal mutations using the third genome.

Diploid/triploid mixoploidy: A consequence of asymmetric zygotic segregation of parental genomes

Triploidy is the presence of an extra haploid set of chromosomes and can exist in complete or mosaic form. The extra haploid set of chromosomes in triploid cells can be of maternal or paternal origin. Diploid/triploid mixoploidy is a unique form of triploid mosaicism that requires the aberrant segregation of entire parental genomes into distinct blastomere lineages (heterogoneic cell division) at the earliest zygotic divisions. Here we report on eight cases of diploid/triploid mixoploidy from our institution and conduct a comprehensive review of the literature. The parental origin of the extra set of chromosomes was determined in two cases; and, based on phenotypic evidence we propose the parental origin in the other cases. One case with complex mixoploidy appears to have a digynic origin in addition to the involvement of two different sperm. Of our eight cases, only one resulted in the birth of a live healthy child. The other pregnancies ended in miscarriage, elective termination of pregnancy, intrauterine fetal demise or neonatal death. A review of the literature and the results of our cases show that a preponderance of recognized cases of diploid/triploid mixoploidy has a digynic origin.

Triploidy mosaicism (45,X/68,XX) in an infant presenting with failure to thrive

Triploid mosaicism is a rare aneuploidy syndrome characterized by growth retardation, developmental delay, 3-4 syndactyly, microphthalmia, coloboma, cleft lip and/or palate, genitourinary anomalies, and facial or body asymmetry. In the present report, we describe a 3-month-old female presenting with failure to thrive, growth retardation, and developmental delay. A chromosomal microarray demonstrated monosomy X, but her atypical phenotype prompted further evaluation with a chromosome analysis, which demonstrated 45,X/68,XX mixoploidy. To our knowledge, this is the first report of a patient with this chromosome complement. Mosaicism in chromosomal aneuploidies is likely under-recognized and may obscure the clinical diagnosis. At a time when comparative genomic hybridization and genome sequencing are increasingly used as diagnostic tools, this report highlights the clinical utility of chromosome analysis when a molecular diagnosis is not consistent with the observed phenotype.

Developmental potential of 2n/3n mixoploid mouse embryos produced by fusion of individual second polar bodies and blastomeres of 2-cell embryos

Using 2n/3n mixoploid mouse embryos produced by fusion of individual second polar bodies (PB2s) with individual blastomeres of 2-cell embryos, the dynamics of PB2 nuclei in the host blastomeres during mitosis were examined and the fate of the 3n cell line in the mixoploid embryos was followed. Most of the PB2 nuclei were synchronised with the cell cycle of the host blastomeres and all chromosomes were incorporated into a single mitotic spindle. The majority of the mixoploid embryos developed to blastocysts with 3n cells. In conceptuses at Day 11.5 and Day 18.5 of gestation, 3n cells were recognised in both of the embryonic/fetal and placental tissues. When green fluorescent protein (GFP)-transgenic mice were used as a donor of PB2, GFP-positive 3n cells were found in more than 40% of morulae and blastocysts, indicating that the PB2 genome can be reactivated during the pre-implantation stage. GFP-positive 3n cells were non-randomly allocated in trophectoderm in blastocysts. These findings may explain the production mechanism of 2n/3n mixoploid human embryos, that is, a PB2 is incorporated into one daughter blastomere during the early cleavage period.

Evaluation of triploid<-->diploid and trisomy-3<-->diploid mouse chimeras as models for investigating how lineage restriction occurs in confined placental mosaicism

Human confined placental mosaicism (CPM), where the placental trophoblast is mosaic for a chromosome abnormality but the fetus is chromosomally normal, can cause problems for prenatal diagnosis, but its causes are poorly understood. Tetraploid↔diploid chimeras provide a model for the development of one type of CPM, but animal models for other types of restricted mosaicism are needed. The objective of the present study was to evaluate triploid↔diploid and trisomy-3↔diploid chimeric mouse conceptuses as new models for investigating the development of restricted mosaicism. Novel stocks of mice were generated to produce triploid and trisomy-3 embryos that could be identified by DNA in situ hybridisation to a chromosome 3 transgenic marker. Triploid↔diploid and trisomy-3↔diploid mouse chimeras were produced by embryo aggregation, and the contribution of triploid or trisomy-3 cells was analysed in the fetus and extraembryonic tissues. Only two trisomy-3↔diploid chimeras were analysed but trisomy-3 cells contributed well to all lineages, so these chimeras did not show restricted mosaicism. In contrast, triploid cells usually contributed poorly to all lineages in the ten 3n↔2n chimeras analysed. They contributed more to the primitive endoderm derivatives than other lineages and were present in the primitive endoderm derivatives of all ten chimeras, but excluded from fetuses and trophectoderm derivatives in some cases. This pattern of restricted mosaicism differs from that reported for tetraploid cells in tetraploid↔diploid chimeras, and triploid↔diploid chimeras may provide a useful model for the development of some types of restricted mosaicism in human conceptuses.

Normal female phenotype and ovarian development despite the ovarian expression of the sex-determining region of Y chromosome (SRY) in a 46,XX/69,XXY diploid/triploid mosaic child conceived after in vitro fertilization-intracytoplasmic sperm injection

CONTEXT

Diploid/triploid mosaicism (mixoploidy) is a rare chromosomal abnormality characterized by mental and growth retardation, hypotonia, and dysmorphic features such as facial asymmetry, low-set ears, and syndactyly. All 46,XX/69,XXY cases fall into three phenotypic groups: male with testicular development, ovotestis disorder of sex development (DSD), or undervirilized male DSD. All phenotypic females with diploid/triploid mosaic reported so far had 46,XX/69,XXX karyotype.

PATIENT

We report an 8-year-old girl conceived after in vitro fertilization-intracytoplasmic sperm injection with normal internal/external genital and ovarian development despite 46,XX/69,XXY mosaicism and normal expression of sex-determining region of Y chromosome (SRY) in her gonads.

INTERVENTION

Because of the increased risk of gonadoblastoma resulting from Y chromosome mosaicism, her ovaries were removed by laparoscopy. Ovarian tissue was analyzed histologically as well as by fluorescence in situ hybridization, PCR, and RT-PCR amplification to determine the localization of Y chromosome and expression of SRY and DAX1 mRNA. Methylation-specific PCR was used to assess the inactivation pattern of X chromosomes.

RESULTS

By laparoscopy, internal female genital anatomy appeared to be normal. Cytogenetic and molecular methods confirmed the presence of intact and functionally active Y chromosome in the ovary. Strikingly, histological assessment of the gonads showed normal ovarian architecture with abundant primordial follicles despite the presence of the Y chromosome in ovarian follicles and the expression of SRY mRNA in gonadal tissue.

CONCLUSION

This case illustrates that normal ovarian development is possible in the presence of Y chromosome in ovarian follicles and despite the expression of SRY in ovarian tissue. Furthermore, this is the first documented case of mixoploidy after in vitro fertilization-intracytoplasmic sperm injection and the only phenotypic female with 46,XX/69,XXY karyotype.

Congenital hyperinsulinism and mosaic abnormalities of the ploidy

BACKGROUND

Congenital hyperinsulinism and Beckwith-Wiedemann syndrome both lead to beta islet hyperplasia and neonatal hypoglycaemia. They may be related to complex genetic/epigenetic abnormalities of the imprinted 11p15 region. The possibility of common pathophysiological determinants has not been thoroughly investigated.

OBJECTIVE

To report abnormalities of the ploidy in two unrelated patients with congenital hyperinsulinism.

METHODS

Two patients with severe congenital hyperinsulinism, one overlapping with Beckwith-Wiedemann syndrome, had pancreatic histology, ex vivo potassium channel electrophysiological studies, and mutation detection of the encoding genes. The parental genetic contribution was explored using genome-wide polymorphism, fluorescent in situ hybridisation (FISH), and blood group typing studies.

RESULTS

Histological findings diverged from those described in focal congenital hyperinsulinism or Beckwith-Wiedemann syndrome. No potassium channel dysfunction and no mutation of its encoding genes (SUR1, KIR6.2) were detected. In patient 1 with congenital hyperinsulinism and Beckwith-Wiedemann syndrome, paternal isodisomy for the whole haploid set was homogeneous in the pancreatic lesion, and mosaic in the leucocytes and skin fibroblasts (hemihypertrophic segment). Blood group typing confirmed the presence of two erythroid populations (bi-parental v paternal only contribution). Patient 2 had two pancreatic lesions, both revealing triploidy with paternal heterodisomy. Karyotype and FISH analyses done on the fibroblasts and leucocytes of both patients were unremarkable (diploidy).

CONCLUSIONS

Diploid (biparental/paternal-only) mosaicism and diploid/triploid mosaicism were present in two distinct patients with congenital hyperinsulinism. These chromosomal abnormalities led to paternal disomy for the whole haploid set in pancreatic lesions (with isodisomy or heterodisomy), thereby extending the range and complexity of the mechanisms underlying congenital hyperinsulinism, associated or not with Beckwith-Wiedemann syndrome.

Hydatidiform mole and triploidy: the role of genomic imprinting in placental development

Genomic imprinting, the differential expression of paternal and maternal alleles, is involved in the regulation of embryonic and fetal growth and development. In this review, we focus on the genetics of a disorder caused by a global defect in genomic imprinting, the hydatidiform mole. The ratio between the maternal and paternal genomes is critical in determining the development of both the embryonic and extraembryonic tissues, with an excess of paternally derived chromosomes leading to a complete (no maternal genome) or partial (lower amount of maternal chromosomes) mole. The recent identification and molecular studies in biparental complete moles may yield more insight into the regulation of imprinting during gametogenesis.

Prenatal diagnosis of minute supernumerary marker chromosomes

The identification of supernumerary marker chromosomes (SMC) at prenatal diagnosis is problematic, particularly for the prediction of phenotype. The assessment of phenotypic risk is based on the size, morphology and origin of the SMC. Fluorescence in situ hybridization (FISH) characterization and family studies are also employed to aid in determining the significance of a prenatally ascertained SMC. Generally, SMC containing euchromatin are more likely to be associated with abnormal phenotypes and SMC without euchromatin are more likely to result in normal phenotypes. The smallest of SMC, minute SMC (minSMC) appear as dot-like or centric fragments and are particularly difficult to identify and characterize. Previous empirical observations suggested that the risk of phenotypic abnormality in prenatally ascertained minSMC was < or = 5%. We identified minSMC in chorionic villus samples (CVS) or amniocytes from 11 unrelated pregnancies. The chromosomal origin of each minSMC was identified by sequential FISH analysis with chromosome-specific centromere probes. Further FISH analysis with whole chromosome paint probes was undertaken to assess each minSMC for the presence or absence of euchromatin, since the presence of euchromatin may be associated with a higher risk of abnormality. Two minSMC were shown to have euchromatin. The first, a minSMC(12) was found in CVS but not confirmed in amniocytes, indicating confined placental mosaicism. The second, a minSMC derived from chromosome 19, was associated with ultrasound abnormalities. Apart from a case with mild speech delay, the remaining minSMC cases without detectable euchromatin had a normal outcome at birth and/or on longer term follow-up. Additional FISH analyses with a telomeric repeat probe showed no signal on any of the minSMC tested, suggesting that they were ring chromosomes in structure. These data further support the concept that minSMC containing euchromatin are more likely to be associated with an abnormal phenotype, although as more data are collected, this may vary by chromosome of origin. The absence of detectable euchromatin, while not guaranteeing a normal result, is most likely to have a normal outcome. The present report and previous studies do not yet allow any significant adjustment of the empirical < or = 5% risk estimate for minSMC identified at prenatal diagnosis. However, reporting of additional cases with characterization of the minSMC and particularly with long-term follow-up will, in time, allow for more accurate risk estimates and provide prognostic information.

Experimentally produced diploid〈-〉triploid mouse chimaeras develop up to adulthood

Spontaneous diploid-triploid chimaeras occur sporadically in various mammalian species including man, but so far have never been produced experimentally. In order to get a deeper insight into the developmental consequences of this anomaly, we have developed two procedures that enabled for the first time to produce routinely diploid-triploid embryos, foetuses, and animals in the mouse. These procedures are: (1) aggregation of cleaving diploid embryos with triploid embryos produced by suppression of the second polar body in zygotes, and (2) fusion of a haploid karyoplast with one blastomere of the two-cell diploid embryos. The first procedure yielded 23 living and 6 dead postimplantation embryos and foetuses (age: 8th-19th day) out of which 22 were chimaeric. In addition, three chimaeric neonates reached adulthood. Two animals were fertile, and one--an overt chimaera--was an infertile male. The rate of postimplantation development of aggregation chimaeras was normal or only slightly retarded, and with one exception the foetuses were morphologically normal. Generally, the highest contribution of the 3n component in extra-embryonic structures was noted in the yolk sac, and usually it was higher than its contribution to the organs of the body. Chimaerism was most often noted in the liver, the heart, the intestine, and the lungs. Participation of triploid cells to all tissues studied, both in the body and in extra-embryonic structures, appeared to decrease slightly as development progressed. The second procedure yielded 10 foetuses and 6 adults. Three foetuses were chimaeric. Six fertile adults were probably non-chimaeras: the triploid component was absent in the coat and in the blood.

Triploid mosaicism in a 45,X/69,XXY infant

We report on an infant referred for chromosome analysis during the neonatal period due to ambiguous genitalia. The genitalia appeared male with bilaterally palpable testes, penoscrotal hypospadias, chordee, and a bifid scrotum. Chromosome analysis and interphase FISH analysis of lymphocytes showed a 45,X karyotype and no evidence for SRY in 200 nuclei examined, respectively. Subsequent chromosome analysis of fibroblasts revealed a 69,XXY karyotype. Molecular studies were carried out to determine the etiology of the chromosome findings. Results indicated that the two cell lines are mosaic rather than chimeric and that the triploidy resulted from delayed dispermy rather than delayed polar body inclusion. To our knowledge this is the first reported living individual with (near) diploid/triploid mosaicism for 45,X/69,XXY.

A clinical report of a patient with two abnormal cell lines: 46,XX,del(21)(q22.1) and 47,XX,+3

Mosaicism for two chromosomally abnormal cell lines in the absence of a normal cell line is exceedingly rare. We report a patient with developmental and growth delay, mild dysmorphic features, a history of hypertension and hepatoblastoma who was found to be mosaic for two chromosomally abnormal cell lines. The cell lines, one containing a terminally deleted chromosome 21, the other trisomy 3, were found in her blood. Fibroblasts and hepatoblastoma tumor cells revealed only the presence of the deleted 21 cell line. Microsatellite marker analysis suggests a mosaic rather than chimeric etiology for the cell lines. This case is exceptional in that the presence of either of these two cell lines alone is uncommon; finding both of these cell lines in an individual appears to be unique.