Multicolour detection of every chromosome as a means of detecting mosaicism and nuclear organisation in human embryonic nuclei

Fluorescence in situ Hybridisation (FISH) revolutionised cytogenetics using fluorescently labelled probes with high affinity with target (nuclear) DNA. By the early 1990s FISH was adopted as a means of PGD sexing for couples at risk of transmitting X-linked disorders and later for detection of unbalanced translocations. Following a rise in popularity of PGD by FISH for sexing and the availability of multicolour probes (5-8 colour), the use of FISH was expanded to the detection of aneuploidy and selective implantation of embryos more likely to be euploid, the rationale being to increase pregnancy rates (referral categories were typically advanced maternal age, repeated IVF failure, repeated miscarriage or severe male factor infertility). Despite initial reports of an increase in implantation rates, reduction in trisomic offspring and spontaneous abortions criticism centred around experimental design (including lack of randomisation), inadequate control groups and lack of report on live births. Eleven randomised control trials (RCTs) (2004-2010) showed that PGS with FISH did not increase delivery rates with some demonstrating adverse outcomes. These RCTs, parallel improvements in culturing and cryopreservation and a shift to blastocyst biopsy essentially outdated FISH as a tool for PGS and it has now been replaced by newer technologies (array CGH, SNP arrays, qRT-PCR and NGS). Cell-by-cell follow up analysis of individual blastomeres in non-transferred embryos is however usually prohibitively expensive by these new approaches and thus FISH remains an invaluable resource for the study of mosaicosm and nuclear organization. We thus developed the approach described herein for the FISH detection of chromosome copy number of all 24 human chromosomes. This approach involves 4 sequential layers of hybridization, each with 6 spectrally distinct fluorochromes and a bespoke capturing system. Here we report previously published studies and hitherto unreported data indicating that 24 chromosome FISH is a useful tool for studying chromosome mosaicism, one of the most hotly debated topics currently in preimplantation genetics. Our results suggest that mosaic embryo aneuploidy is not highly significantly correlated to maternal age, probably due, in part, to the large preponderance of post-zygotic (mitotic) errors. Chromosome loss (anaphase lag) appears to be the most common mechanism, followed by chromosome gain (endoreduplication), however 3:1 mitotic non- disjunction of chromosomes appears to be rare. Nuclear organisation (i.e. the spatial and temporal topology of chromosomes or sub-chromosomal compartments) studies indicate that human morula or blastocyst embryos (day 4-5) appear to adopt a "chromocentric" pattern (i.e. almost all centromeric signals reside in the innermost regions of the nuclear volume). By the blastocyst stage however, a more ordered organisation with spatial and temporal cues important for embryo development appears. We have however found no association between aneuploidy and nuclear organization using this approach despite our earlier studies. In conclusion, while FISH is mostly "dead and buried" for mainstream PGS, it still has a place for basic biology studies; the development of a 24 chromosome protocol extends the power of this analysis.