Lack of cell cycle checkpoints in human cleavage stage embryos revealed by a clonal pattern of chromosomal mosaicism analysed by sequential multicolour FISH

Abstract

Multicolour fluorescence in situ hybridisation (FISH) analysis of interphase nuclei in cleavage stage human embryos has highlighted a high incidence of postzygotic chromosomal mosaicism, including both aneuploid and ploidy mosaicism. Indeed, some embryos appear to have a chaotic chromosomal complement in a majority of nuclei, suggesting that cell cycle checkpoints may not operate in early cleavage. Most of these studies, however, have only analysed a limited number of chromosomes (3–5), making it difficult to distinguish FISH artefacts from true aneuploidy. We now report analysis of 11 chromosomes in five sequential hybridisations with standard combinations of two or three probes and minimal loss of hybridisation efficiency. Analysis of a series of arrested human embryos revealed a generally consistent pattern of hybridisation on which was superimposed frequent deletion of one or both chromosomes of a specific pair in two or more nuclei indicating a clonal origin and continued cleavage following chromosome loss. With a binucleate cell in a predominantly triploid XXX embryo, the two nuclei remained attached during preparation and the chaotic diploid/triphoid status of every chromosome analysed was the same for each nucleus. Furthermore, in each hybridisation the signals were distributed as a mirror-image about the plane of attachment, indicating premature decondensation during anaphase consistent with a lack of checkpoint control.

Germ cell DNA-repair systems—possible tools in cancer research?

“…Instead, apoptosis is triggered to completely remove the blastomere or blastocyte. 16,17 This preference for an apoptotic outcome is consistent with a high expression of pro-apoptotic Bax and Bak genes and low expression of anti-apoptotic genes such as Bcl-2 and Bcl-w in MII oocytes and blastocytes. 7 This preference of the germ cell for apoptosis rather than repair is very interesting in light of synthetic lethality, a novel approach within cancer therapy that utilizes mutations to make only the cancer cell and not surrounding tissue susceptible to a tailored drug.…”

Chromosomal variation in neurons of the developing and adult mammalian nervous system

“…These data demonstrate that aneuploidy, as measured by two different techniques, can be experimentally What is the fate of aneuploid neuroblasts? One probable fate is cell death, as observed in aneuploid embryos during in vitro fertilization (15,16,38). The decreased aneuploidy we observed in the adult cortex relative to the embryonic cortex suggests that aneuploid neuroblasts may be preferentially prone to cell death during central nervous system development.…”

“…Interestingly, many of these genes are also implicated in cancer, where a commonly associated sequela is aneuploidy. Precedent exists for aneuploidy during early mammalian development (15,16) where it is thought to result in cell death. These observations led us to ask whether the number of chromosomes in neuroblasts and neurons is variable.…”

Chromosomal analysis of blastocysts from balanced chromosomal rearrangement carriers

“…Such fragments may remain or be absorbed by a neighboring blastomere (Wong et al 2010, Chavez et al 2012, creating euploid-aneuploid mosaicism, aneuploid mosaicism or even chaotic chromosome distribution pattern. Previous study also indicated that the lack of functional cell cycle checkpoint mechanisms may lead to chromosomal segregation errors during the mitotic divisions of human preimplantation embryos and thus to mosaicism (Harrison et al 2000). We estimated the percentage of mosaicism (aneuploidy) on the assumption that the biopsied tissue contains two distinct cell lines (diploid and triploid or diploid and haploid) with regard to the chromosome analyzed.…”