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In the Spotlight

Oocyte aneuploidies, which consist of an abnormal number of chromosome numbers in oocytes due to chromosome segregation errors during meiosis, are believed to be a major cause of miscarriages, particularly in older women, and to be responsible for a significant number of genetic disorders in offspring from these women. Nowadays, many individuals affected by recurrent pregnancy loss or concerned about aneuploidies in the offspring turn to in vitro fertilization (IVF) for help to conceive a healthy child. In these cases, embryo preimplantation genetic diagnosis/screening (PGD/PGS) is the currently available technology to determine the embryos more likely to be free of aneuploidies and genetically more suitable for transfer in order to achieve a successful pregnancy. Although PGD/PGS technology has greatly evolved in the last few years, it is still dependent on the removal of a blastomere or throphectoderm cells from the growing embryo, which not only has the potential of being detrimental to the embryo but that is also limited by the short time available to acquire results between the biopsy day and the time of transfer. In addition, most of the methods currently used for PGD/PGS cannot simultaneously detect aneuploidies and single-nucleotide variants (SNVs) associated with Mendelian diseases of severe defect phenotypes. In a recent publication in Cell (Cell, 2013; 155(7):1492-1506), Hou et al report a new approach to analyze the genome of single human oocytes, which involves only the removal of the first and second polar bodies (PB1 and PB2) of the oocyte and that allows accurate and cost-effective embryo selection for some cases of IVF while overcoming the limitations associated with the routine PGD/PGS technologies in use. Hou and colleagues used multiple annealing and loopingbased amplification cycle (MALBAC)-based sequencing technology to analyze chromatid triads of the PB1 and PB2 and the oocyte pronuclei from the same female egg donor to phase the genome of these donors with detected small nucleotide polymorphisms (SNPs) while determining the crossover maps of their oocytes. Their results show that the genome of the oocyte pronucleus, including information regarding aneuploidy and SNPs in disease-associated alleles, can be accurately deduced from the genomes of PB1 and PB2. Moreover, the results of Hou and colleagues demonstrate that MALBAC sequencing offers improved resolution and accuracy of simultaneous detection of chromosome abnormalities and heterozygous SNVs associated with Mendelian diseases. Thus, this new approach may present a remarkable alternative to commonly used PGS strategies to ensure the transfer of an embryo free of maternal aneuploidies and maternally inherited diseases. However, the disadvantage of PB biopsy is that it cannot detect aneuploidies and mutations inherited from the father, neither can it detect aneuploidies and de novo mutations arising from mitosis. Importantly, this study by Hou and colleagues also demonstrates that oocytes exhibit a higher recombination rate and higher aneuploidy rates that those that have been reported for sperm, providing the first comprehensive analyses of female meiosis recombination.