Mammalian pre-implantation chromosomal instability: species comparison, evolutionary considerations, and pathological correlations

Abstract

Pre-implantation embryo development in mammals begins at fertilization with the migration and fusion of the maternal and paternal pro-nuclei, followed by the degradation of inherited factors involved in germ cell specification and the activation of embryonic genes required for subsequent cell divisions, compaction, and blastulation. The majority of studies on early embryogenesis have been conducted in the mouse or non-mammalian species, often requiring extrapolation of the findings to human development. Given both conserved similarities and species-specific differences, however, even comparison between closely related mammalian species may be challenging as certain aspects, including susceptibility to chromosomal aberrations, varies considerably across mammals. Moreover, most human embryo studies are limited to patient samples obtained from in vitro fertilization (IVF) clinics and donated for research, which are generally of poorer quality and produced with germ cells that may be sub-optimal. Recent technical advances in genetic, epigenetic, chromosomal, and time-lapse imaging analyses of high quality whole human embryos have greatly improved our understanding of early human embryogenesis, particularly at the single embryo and cell level. This review summarizes the major characteristics of mammalian pre-implantation development from a chromosomal perspective, in addition to discussing the technological achievements that have recently been developed to obtain this data. We also discuss potential translation to clinical applications in reproductive medicine and conclude by examining the broader implications of these findings for the evolution of mammalian species and cancer pathology in somatic cells.

Keywords: Aneuploidy; IVF; cancer; chromothripsis; embryo; evolution; fragmentation; micronuclei; preimplantation.

Figure 1.

Fundamental aspects of mammalian pre-implantation development from a chromosomal perspective. Recent advances in mammalian embryology, including single cell whole genome/transcriptome analyses and time-lapse imaging, have greatly contributed to our understanding of the key characteristics in each major pre-implantation developmental stage. Time-lapse imaging has provided a non-invasive approach to monitor embryo development as well as the means to identify imaging parameters predictive of developmental success and embryo ploidy status. Embryo chromosomal integrity may be impacted by the lack of and/or inheritance of aberrant parental mRNAs, proteins, or other factors such as paternal contribution of the centrosome, which mediates the first mitotic divisions. A select group of maternal mRNAs termed maternal effect genes are recruited for translation following fertilization, whereas the remaining maternal mRNAs are degraded, a process that is essentially complete by the 2-cell stage in mouse embryos and the 8-cell stage in human embryos. Besides individual maternal proteins, multi-protein complexes, including the subcortical maternal complex (SCMC) is important for embryonic progression beyond the 2-cell stage and potential prevention of mitotic aneuploidy. This oocyte-to-embryo transition is largely dependent upon embryonic genome activation (EGA), the major wave of which occurs in the mouse at the 2-cell stage and begins in humans on day 3 at approximately the 8-cell stage. Following EGA, an embryo undergoes the processes of compaction, intracellular adhesion, and polarization to result in the formation of a morula at the 8-cell and 16- to 32-cell stage in mouse and human embryos, respectively. The majority of mammalian species, including humans, undergo cavitation to form a fluid-filled cavity called a blastocoel between days 5 and 6, whereas mouse embryos begin blastulation earlier between day 3 and 4 and bovine embryos later between day 7 and 8. There are several factors that can contribute to the generation of chromosome instability, particularly in human embryos, including cellular fragmentation, sub-chromosomal breakage and fusion, a lack of cell cycle checkpoints, and chromosomal lagging during anaphase. It appears that unlike human embryos, which respond to chromosomal aberrations by fragmenting and continuing to divide, mouse embryos deal with multi- and micronuclei formation by inducing blastomere lysis.

Figure 2.

Correlation between cellular fragmentation and multi-/micronuclei formation in cleavage-stage embryos from different mammalian species. Top row: a representative photo of (A) human embryo taken by brightfield imaging, (B) rhesus macaque embryo using Hoffmann modulation contrast, (C) bovine embryo using differential interference contrast (DIC), and (D) mouse embryo taken by darkfield illumination time-lapse imaging. Note the appearance of several cellular fragments (black or white arrows) in cleavage-stage human and non-human primate embryos, but not in mouse embryos, with a lesser extent observed in bovine embryos. Bottom row: the incidence of cellular fragmentation is highly associated with mutli- and micronuclei formation as indicated by confocal microscopy of LAMIN-B1 (green) expression in DAPI-stained (blue) cleavage-stage embryos from the different mammalian species.

Figure 3.

Potential mechanisms of aneuploidy resolution during embryo pre-implantation development. (A) Time-lapse image showing a human zygote with three cleavage furrows as it divides directly from 1-cell to 3-cells. (B) Individual imaging frames of a human embryo with cellular fragmentation demonstrating blastomeric resorption of a fragment. Adapted from Chavez et al. [2012]. (C) Multi-channel confocal analysis of histone modifications in a human morula that is undergoing cavitation to form a blastocyst reveals the presence of a large and likely polyploid blastomere excluded from the embryo. Adapted from Chavez et al. [2014].

Figure 4.

Model describing the association between chromosome mis-segregation, micronuclei, and chromosomal rearrangements. Chromosomes that are lagging during anaphase as a consequence of mitotic defects are encapsulated by nuclear envelope to form a micronucleus (MN). Within the MN, chromosomes tend to sustain frequent double-strand breaks most likely due to changes in chromatin conformation. This induces repair by non-homologous end joining (NHEJ), an error prone mechanism that can produce chromosomal aberrations. As MN may merge back with the main nucleus, this will result in the rearranged chromosomes becoming a part of the genome as described by Janssen et al. [2011].