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.

Copy number analysis of meiotic and postzygotic mitotic aneuploidies in trophectoderm cells biopsied at the blastocyst stage and arrested embryos.

Preimplantation genetic testing for aneuploidy (PGT-A) by copy number analysis is now widely used to select euploid embryos for transfer. Whole or partial chromosome aneuploidy can arise in meiosis, predominantly female meiosis, or in the postzygotic, mitotic divisions during cleavage and blastocyst formation, resulting in chromosome mosaicism. Meiotic aneuploidies are almost always lethal, however, the clinical significance of mitotic aneuploidies detected by PGT-A is not fully understood and healthy live births have been reported following transfer of mosaic embryos. Here, we used single nucleotide polymorphism genotyping of both polar bodies and embryo samples to identify meiotic aneuploidies and compared copy number changes for meiotic and presumed mitotic aneuploidies in trophectoderm cells biopsied at the blastocyst stage and arrested embryos. PGT-A detected corresponding full copy number changes (≥70%) for 36/37 (97%) maternal meiotic aneuploidies. The number of presumed mitotic copy number changes detected exceeded those of meiotic origin. Although mainly in the mosaic range, some of these mitotic aneuploidies had copy number changes ≥70% and would have been identified as full aneuploidies. Interestingly, many arrested embryos had multiple mitotic aneuploidies across a broad range of copy number changes, which may have arisen through tripolar spindle and other mitotic abnormalities.

Abnormal cleavage and developmental arrest of human preimplantation embryos in vitro.

Despite improvements in culture conditions and laboratory techniques still only about 50% of human embryos reach the blastocyst stage of development in vitro. While many factors influence embryo development, aberrant cleavage divisions have only recently been shown to directly affect the genome in individual cells of human embryos resulting in chromosome loss, mosaicism and cell arrest. In this article we review the current literature in the area of aberrant cleavage in human embryos and its effect on blastocyst development. Further to this, we propose a series of common abnormal cleavage events, with particular attention to timing and frequency, and illustrate how these might influence a number of different embryo fates.

High implantation and clinical pregnancy rates with single vitrified-warmed blastocyst transfer and optional aneuploidy testing for all patients.

This study reports the results of a 2-year long IVF programme ('One by One') in which all patients (median age 40 years; range 27-45 years) were offered preimplantation genetic testing for aneuploidy (PGT-A) and had all blastocysts vitrified (freeze-only), followed later by single vitrified-warmed blastocyst transfer (vSET) in managed cycles. Between January 2016 and December 2017, a total of 155 patients started 222 treatment cycles and 99 (45%) cycles resulted in one or more vitrified blastocysts (untested or with normal copy number for all chromosomes) available for transfer. Seventeen patients (11%) aged ≤35 years opted out of PGT-A. Over this period, 85 vSETs in 74 patients resulted in an implantation rate of 80% (68/85) and a singleton clinical pregnancy rate of 66% (56/85). Cumulative live birth rates will not be known for 1-2 years. Nevertheless, these high success rates with vSET confirm larger studies using selected patients and are likely to deliver similar, if not higher, live birth rates per cycle started than rates typically reported in national registries with conventional IVF and transfer of one or more fresh and/or frozen embryos.