Comprehensive chromosome screening and follow-up assessment of large numbers of cells provided a unique insight into the cytogenetics of human blastocysts. Meiotic and post-zygotic errors leading to mosaicism were common. However, most mosaic blastocysts contained no normal cells. Hence, CGH or aCGH TE analysis is an accurate aneuploidy detection tool and may assist in identifying viable euploid embryos with higher implantation potential.

No abstract

Female meiosis is comprised by two cell divisions, meiosis I (MI) and II (MII) and two different stages at which the development of the oocyte is temporarily halted. In the case of MI, this pause can potentially last for four to five decades. This added layer of complexity distinguishes female gametogenesis from its male counterpart. The single most important genetic factor impacting human reproductive success is aneuploidy. Aneuploid embryos may undergo permanent arrest during preimplantation development, fail to implant or spontaneously abort. Most aneuploidies originate during female meiosis and become increasingly common with advancing maternal age. To shed further light on the nature of aneuploidy in human oocytes, we utilized comparative genomic hybridization (CGH) to provide a detailed cytogenetic analysis of 308 first and second polar bodies (PBs). These were biopsied from fertilized oocytes, generated by 70 reproductively older women (average maternal age of 40.8 years). The total oocyte abnormality rate was 70%, and MII anomalies predominated over MI (50% aneuploidy rate versus 40.3%). Both whole chromosome non-disjunction and unbalanced chromatid predivision were seen, but the latter was the dominant MI aneuploidy-causing mechanism. Chromosome losses occurred more frequently than chromosome gains, especially during MI. Chromosomes of all sizes were found to participate in aneuploidy events, although errors involving smaller chromosomes were more common. These data reveal the spectrum of aneuploidies arising after each meiotic division, indicating that oocyte-derived abnormalities present at conception differ from those observed in established pregnancies. It is also clear that advancing maternal age had a significant adverse effect on female meiosis, and that this effect is most pronounced in MII. Indeed, our data suggest that MII may be more susceptible to age-related errors than MI.

The clinical application of a new, widely applicable method known as Karyomapping to carry out a total of 55 clinical cases of preimplantation genetic diagnosis (PGD) for single gene disorders is reported. Conventional polymerase chain reaction (PCR) testing was carried out in parallel to the new method for all cases. Clinical application of Karyomapping in this study resulted in three live births and nine clinical pregnancies out of 20 cases with a transfer. All in all, results presented in this study indicate that Karyomapping is a highly efficient, accurate and robust method for PGD of single gene disorders. Karyomapping can offer a more comprehensive assessment of the region of interest than conventional PCR analysis, allowing for more embryos to receive diagnosis (99.6% versus 96.8%), whereas its wide applicability reduces substantially the time that patients have to wait before starting their in vitro fertilization (IVF) cycle. Nonetheless, inclusion of elements of conventional PCR methodology, such as direct mutation detection, may be required in cases in which the gene of interest is in a region with reduced single nucleotide polymorphism (SNP) coverage (e.g. telomeric regions), when offering PGD for consanguineous couples, or in cases where no samples from additional family members are available.

One of the most important factors in increasing the screening potential of preimplantation genetic diagnosis (PGD) for aneuploidy is to increase the number of chromosomes analysed. Inclusion of chromosomes 8, 14 and 20 to the standard set of chromosomes X, Y, 13, 15, 16, 17, 18, 21 and 22 allows the analysis of 12 chromosomes in three rounds of fluorescent in-situ hybridization (FISH) without decreasing the efficiency of the technique. Pregnancy rate was significantly increased when only embryos that had been diagnosed as normal for the 12 chromosomes analysed were transferred compared with transfer of embryos with any abnormality for chromosomes 8, 14 or 20 (P < 0.05). This study proves that the high efficiency and practical feasibility of FISH analysis of 12 chromosomes in PGD for aneuploidy is a superior approach than the standard nine-chromosome analysis in order to screen for abnormalities.