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

Abstract

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.

Keywords: IVF; Meiosis; Mitosis; Monosomy; Preimplantation genetic testing; Time-lapse; Trisomy.

Fig. 1

Developmental outcomes of 1232 normally fertilized (2PN) embryos and their associated PGT-A results. A total of 909 (73.8%) of embryos were tested with PGT-A, including 297 of 610 (48.7%) arrested embryos and 612 of 622 (98.4%) blastocysts. Early arrest: cleavage stages; ≤ 10 cells. Mid arrest: > 10 cells, but pre-compact morula. Late arrest: compact morula to cavitating (non-expanded) blastocyst

Fig. 2

Chromosome-specific rates and age associations for putative meiotic- and mitotic-origin aneuploidies. A Chromosome-specific counts of putative meiotic (i.e., full copy number change; left panel) and putative mitotic (i.e., intermediate copy number change; right panel) whole-chromosome gains and losses observed in blastocysts (top panel) and arrested embryos (bottom panel) as determined with PGT-A. Only aneuploidies of entire autosomes are depicted, as distinguishing meiotic and mitotic origins of segmental aneuploidies and aneuploidies affecting sex chromosomes poses unique challenges (see the “Methods” section). B Observed rates of meiotic (left panel) and mitotic (right panel) aneuploidy in relation to maternal age, including both arrested embryos and blastocysts. Each data point represents a distinct IVF case. Lines represent predictions from binomial generalized linear models fit to the data, with standard errors of the predictions indicated in gray

Fig. 3

Contrasting chromosomal and cellular characteristics of arrested embryos with developing blastocysts. A Counts of arrested embryos versus developing blastocysts, stratified by PGT-A copy number result category. B Counts of arrested embryos versus developing blastocysts, stratified by the total number of aneuploid chromosomes. C Counts of arrested embryos versus developing blastocysts, stratified by the number of cells present after the first mitotic division

Fig. 4

Data (left panels) and statistical modeling (right panels) of the proportion/probability of embryo arrest, stratifying on various patterns of chromosome copy number or cell division. Error bars denote 95% confidence intervals of estimates. A Proportion of arrested (red) versus unarrested (blue) embryos, stratifying on chromosome copy number pattern, as assessed by PGT-A. B Statistical modeling of the data from panel A. C. Proportion of arrested (red) versus unarrested (blue) embryos, stratifying on the number of aneuploid chromosomes. D Statistical modeling of data from panel C. E Proportion of arrested (red) versus unarrested (blue) embryos, stratifying on the number of cells observed after the first mitotic division, where 2 is normal. F Statistical modeling of data from panel E

Fig. 5

Chromosome copy number results (as assessed via PGT-A) across all tested embryos, stratifying by stage at arrest (see the “Methods” section for description of arrested embryos) or morphological grade (for embryos that formed blastocysts). ICM grade is listed first, and TE grade is listed second. A Copy number results assigned to categories, as described in Table 1. B Copy number results summarized as counts of aneuploid chromosomes