Relationship of morphokinetics and gene expression in the generation of aneuploidies in the human embryo

Resumen

Over the last 20 years, the number of assisted reproduction procedures has drastically increased, and this trend is expected to continue as parenthood is postponed. In Europe, more than half a million in vitro fertilization (IVF) cycles are performed annually, resulting in 100,000 newborns. With the increased use of assisted reproduction, a concomitant increase in success rates would also be expected. However, the pregnancy rate per cycle has remained constant over the same 10-year range, despite important progress in the field. The majority of women using assisted reproduction are over 35 years old, and this needs to be taken into consideration since the rate of chromosomal abnormalities in the oocytes increases with age. As a consequence, advanced maternal age may result in an increase in the number of aneuploid embryos, which would be translated into high miscarriage rates and low live-birth rates. Since we are not able to modify the IVF population or to change embryo quality, our goal should be to focus on improving IVF techniques. Such improvement necessitates a better understanding of the etiology of embryo aneuploidies. Recent advances in imaging and molecular and genetic analyses are postulated as promising strategies to unveil the mechanisms involved in aneuploidy generation. Thus, our goal was to analyse, simultaneously in the same human embryo, morphology, kinetics, transcriptomics and genetics to find a correlation between these parameters in the origin of aneuploidies. Here we combine time-lapse, complete chromosomal assessment and single-cell real-time quantitative PCR to simultaneously obtain information from all cells that compose a human embryo until the approximately eight-cell stage. Our data indicate that the chromosomal status of aneuploid embryos correlates with significant differences in the kinetic pattern when compared with euploid embryos, especially in the duration of the first mitotic phase. We also demonstrate that embryo kinetics is affected by the existence of irregular divisions during development, and this should be taken into consideration for future studies. Gene expression analyses reveal that embryonic genome activation starts as early as the zygote stage. Moreover, gene expression profiling suggests that a subset of genes is differentially expressed in aneuploid embryos during the first 30 hours of development. Thus, we propose that the chromosomal fate of an embryo is likely determined as early as the pronuclear stage and may be predicted by a 12-gene transcriptomic signature. Finally, the atlas generated from the data obtained in this study shows the high variability underlying human embryo development. While euploid embryos seem to follow a uniform development without high variability for morphokinetics, aneuploid embryos may follow different pathways, overlapping with and mimicking euploid embryos in some cases, making them hard to differentiate.