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Chromosomal mosaicism refers to the presence of two or more cell lines with a different chromosome content in an individual or tissue sample. The implementation of high resolution NGS for PGS has provided more information that it is not only used for embryonic aneuploidy screening, butdetects the presence of mosaicism in trophectoderm biopsies. Recent studies have shown that transfer of mosaic embryos resulted in lower implantation rates and higher miscarriage rates as compared to euploid transfer.
This study aims to evaluate whether there is an association between chromosomal mosaicism and advanced maternal age. It also aims to compare the clinical pregnancy and implantation rate between euploid and whole chromosomal mosaic embryos transfer.
Retrospective observational study of 533 blastocyst-stage embryos analysed by NGS from IVF cycles from Sunfert International Fertility Centre over the course of 2015 to 2016. Of those analysed, 197 blastocysts were transferred. The clinical outcome obtained after transfer of whole chromosomal mosaic embryos was compared with those resulting from a control group of 190 euploid blastocysts.
Blastocyst-stage embryos underwent biopsy and trophectoderm cells are extracted for aneuploidy screening using IIlumina NGS protocol. Mosaic information for each chromosome was obtained. For this study, only whole chromosomal mosaicism is considered; segmental mosaicism were categorized under euploid embryos. These embryos were further stratified by maternal age group: <35 year old (yo), 36 to 38 yo, 39 to 40 yo, and >40 yo.
Results are summarized in Figure 1. The result showed that mosaicism rates did not vary with advancing maternal age, staying at an average of 10.14%. Euploid and aneuploidy embryos, however, did show changes in regards to age factor as the former displayed a descending trend whereas the latter displayed an increasing trend. [Display omitted]
Transfer of mosaic embryos resulted in higher clinical pregnancy rate (60% vs 57.5%) and implantation rate (57% vs 52.9%) than euploid embryos. No incidence of miscarriage was reported.
In accordance to most of the studies published, our findings showed that mosaicism is not associated with advanced maternal age. Unlike aneuploidy which arises from meiotic error, mosaicism, on the other hand is caused by impaired mitotic chromosome segregation which does not appear to increase with advancing maternal age. Therefore, while there is a direct relationship between advanced maternal age and aneuploidy, age is not a contributor to mosaicism.
On the contrary, our findings showed conflicting result in the implantation rate of mosaic embryos. Similar rate of implantation as compared to euploid transfer embryos was found in our clinical study whereas in general, transfer of mosaic embryos have lower implantation rate. This might be the consequence of small sample size of mosaic embryos transfer (<10% of total blastocysts transferred) in this clinical study. Furthermore, no incidence of miscarriage was reported.