Meiotic and mitotic aneuploidies drive arrest of in vitro fertilized human preimplantation embryos

Background The high incidence of aneuploidy in early human development, arising either from errors in meiosis or postzygotic mitosis, is the primary cause of pregnancy loss, miscarriage, and stillbirth following natural conception as well as in vitro fertilization (IVF). Preimplantation genetic testing for aneuploidy (PGT-A) has confirmed the prevalence of meiotic and mitotic aneuploidies among blastocyst-stage IVF embryos that are candidates for transfer. However, only about half of normally fertilized embryos develop to the blastocyst stage in vitro, while the others arrest at cleavage to late morula or early blastocyst stages. Methods To achieve a more complete view of the impacts of aneuploidy, we applied low-coverage sequencing-based PGT-A to a large series (n = 909) of arrested embryos and trophectoderm biopsies. We then correlated observed aneuploidies with abnormalities of the first two cleavage divisions using time-lapse imaging (n = 843). Results The combined incidence of meiotic and mitotic aneuploidies was strongly associated with blastocyst morphological grading, with the proportion ranging from 20 to 90% for the highest to lowest grades, respectively. In contrast, the incidence of aneuploidy among arrested embryos was exceptionally high (94%), dominated by mitotic aneuploidies affecting multiple chromosomes. In turn, these mitotic aneuploidies were strongly associated with abnormal cleavage divisions, such that 51% of abnormally dividing embryos possessed mitotic aneuploidies compared to only 23% of normally dividing embryos. Conclusions We conclude that the combination of meiotic and mitotic aneuploidies drives arrest of human embryos in vitro, as development increasingly relies on embryonic gene expression at the blastocyst stage

Investigating the significance of segmental aneuploidy findings in preimplantation embryos

Segmental aneuploidies (SAs) are structural imbalances, namely, gains or losses, involving a chromosomal segment. Most preimplantation genetic testing platforms can detect segmental imbalances greater than 5–10 Mb, either full or mosaic; however, questions remain about clinical significance. An in-depth review was performed to determine the accuracy, frequency, and types of SAs detected in preimplantation embryos. A comprehensive search of the literature revealed an incidence of approximately 8.15% in preimplantation embryos, compared with a prevalence of 3.55% in prenatal diagnosis samples. Several studies have used rebiopsy analysis to validate the accuracy and reproducibility of such findings in blastocyst-stage embryos. A comparison of these studies yielded a mean confirmation rate of SAs slightly higher than 30%. This result could be attributed to their mitotic origin as well as to the technical limitations of preimplantation genetic testing. In addition, the few available studies in which embryos with a segmental finding were transferred in utero are analyzed to discuss the reproductive competence of such embryos. Except for 1 study, all outcomes were described for segmental embryos in a mosaic state. As a result, there is still insufficient evidence to provide accurate information about the effect of segmental imbalances on embryonic reproductive competence and to determine gestational and newborn risks

Artificial oocyte activation with calcium ionophore does not cause a widespread increase in chromosome segregation errors in the second meiotic division of the oocyte

Objective To study the effect of artificial oocyte activation (AOA) on chromosome segregation errors in the meiotic divisions. Design Prospective cohort study with historical control. Setting Private/academic IVF centers. Patient(s) Fifty-six metaphase II oocytes were donated from 12 patients who had undergone IVF between June 2008 and May 2009. Intervention(s) Oocytes were activated by 40 minutes' exposure to 100 ?M calcium-ionophore. The activated oocyte was tubed and analyzed by array comparative genomic hybridization and/or single-nucleotide polymorphism genotyping and maternal haplotyping (meiomapping). A control sample of embryos derived from normally fertilized oocytes was included for comparison. Main Outcome Measure(s) Incidence of chromosome segregation errors in artificially activated and normally fertilized oocytes in relation to pronuclear evaluation. Result(s) Of 49 oocytes that survived the warming procedure, thirty-nine (79.6%) activated. Most activated normally, resulting in extrusion of the second polar body and formation of a single or no pronucleus (2PB1PN: 30 of 39, 76.9%; or 2PB0PN: 5 of 39, 12.8%). Twenty-seven of these were analyzed, and 16 (59.3%) were euploid, showing no effect of AOA on meiotic segregation. Single-nucleotide polymorphism analysis of normally activated oocytes confirmed normal segregation of maternal chromosomes. No difference in the proportion of meiosis II type errors was observed between artificially activated oocytes (28.6%; 95% confidence interval 3.7%–71.0%) compared with embryos obtained from normally fertilized oocytes (44.4%; 95% confidence interval 13.7%–78.8%). The abnormally activated oocytes, with ?2PN (4 of 39, 10.3%) were diploid, indicating a failure to coordinate telophase of meiosis II with polar body extrusion. Conclusion(s) From this preliminary dataset, there is no evidence that AOA causes a widespread increase in chromosome segregation errors in meiosis II. However, we recommend that it be applied selectively to patients with specific indications

Karyomapping identifies second polar body DNA persisting to the blastocyst stage: implications for embryo biopsy

Blastocyst biopsy is now widely used for both preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD). Although this approach yields good results, variable embryo quality and rates of development remain a challenge. Here, a case is reported in which a blastocyst was biopsied for PGS by array comparative genomic hybridization on day 6 after insemination, having hatched completely. In addition to a small trophectoderm sample, excluded cell fragments from the subzonal space from this embryo were also sampled. Unexpectedly, the array comparative genomic hybridization results from the fragments and trophectoderm sample were non-concordant: 47,XX,+19 and 46,XY, respectively. DNA fingerprinting by short tandem repeat and amelogenin analysis confirmed the sex chromosome difference but seemed to show that the two samples were related but non-identical. Genome-wide single nucleotide polymorphism genotyping and karyomapping identified that the origin of the DNA amplified from the fragments was that of the second polar body corresponding to the oocyte from which the biopsied embryo developed. The fact that polar body DNA can persist to the blastocyst stage provides evidence that excluded cell fragments should not be used for diagnostic purposes and should be avoided when performing embryo biopsies as there is a risk of diagnostic errors

Live birth after PGD with confirmation by a comprehensive approach (karyomapping) for simultaneous detection of monogenic and chromosomal disorders.

Preimplantation genetic diagnosis (PGD) for monogenic disorders has the drawback of time and cost associated with tailoring a specific test for each couple, disorder, or both. The inability of any single assay to detect the monogenic disorder in question and simultaneously the chromosomal complement of the embryo also limits its application as separate tests may need to be carried out on the amplified material. The first clinical use of a novel approach ('karyomapping') was designed to circumvent this problem. In this example, karyomapping was used to confirm the results of an existing PGD case detecting both chromosomal abnormalities and a monogenic disorder (Smith-Lemli-Opitz [SLO] syndrome) simultaneously. The family underwent IVF, ICSI and PGD, and both polar body and cleavage stage biopsy were carried out. Following whole genome amplification, array comparative genomic hybridisation of the polar bodies and minisequencing and STR analysis of single blastomeres were used to diagnose maternal aneuploidies and SLO status, respectively. This was confirmed, by karyomapping. Unlike standard PGD, karyomapping required no a-priori test development. A singleton pregnancy and live birth, unaffected with SLO syndrome and with no chromosome abnormality, ensued. Karyomapping is potentially capable of detecting a wide spectrum of monogenic and chromosome disorders and, in this context, can be considered a comprehensive approach to PGD