Non-meiotic chromosome instability in human immature oocytes

Abstract

Aneuploidy has been a major issue in human gametes and is closely related to fertility problems, as it is known to be present in cleavage stage embryos and gestational losses. Pre-meiotic chromosome abnormalities in women have been previously described. The aim of this study is to assess the whole-chromosome complement in immature oocytes to find those abnormalities caused by mitotic instability. For this purpose, a total of 157 oocytes at the germinal vesicle or metaphase I stage, and discarded from IVF cycles, were analysed by CGH. Fifty-six women, between 18 and 45 years old (mean 32.5 years), including 32 IVF patients (25-45 years of age) and 24 IVF oocyte donors (18-33 years of age), were included in the study. A total of 25/157 (15.9%) of the oocytes analysed, obtained from three IVF clinics, contained chromosome abnormalities, including both aneuploidy (24/157) and structural aberrations (9/157). Independently of the maternal age, the incidence of abnormal oocytes which originated before meiosis is 15.9%, and these imbalances were found in 33.9% of the females studied. This work sheds light on the relevance of mitotic instability responsible for the generation of the abnormalities present in human oocytes. European Journal of Human Genetics ( Keywords: premeiotic instability; immature oocytes; aneuploidy; segmental imbalances; germline mitotic abnormalities INTRODUCTION Aneuploidy has been a major issue in human gametes, and is highly related to fertility problems. It is known to be present in cleavage stage embryos and gestational losses. 1,2 The prevailing mechanism causing aneuploidy in humans concerns maternal meiotic mal-segregation. Several studies have focused their interest on oocyte aneuploidy, mainly those which originate during the first and second meiotic divisions. 1 For this purpose, first and second polar bodies (1PB and 2PB) and the corresponding metaphase II (MII) oocytes have been analysed, describing aneuploidies of meiotic origin, mainly caused by chromosome non-disjunction and/ or sister chromatid predivision. 3 Meiotic studies have been performed in oocytes using a great variety of cytogenetic methodologies. R-banding karyotyping on 1397 oocytes showed aneuploidies in 10.8% of the cells studied. 4 An aneuploidy rate of 47.5% was observed when 1PB-MII oocyte doublets were analysed by 9-chr FISH. 5 In studies where the wholechromosome complement was assessed by spectral karyotyping, aneuploidy rates from 16.7% (10/60) 6 to 42.5% (20/47) 7 were detected. Using comparative genomic hybridisation (CGH), a great variability of aneuploidy rates was found, when different wholegenome amplification methodologies were used to amplify the DNA from single cells, and aneuploidy rates found in 1PB-MII oocyte doublets varied from 3% in oocyte donors 8 to 65% in IVF patients. 9

How common is germinal mosaicism that leads to premeiotic aneuploidy in the female?

“…A more recent study by the same Spanish group analysed 157 immature oocytes (GV or MI stage) from both IVF patients and IVF donors by metaphase CGH. Fifty-six women of average age 32.5 years included 32 IVF patients (range 25–45 years) and 24 oocyte donors (range 18–33 years) [46]. Overall, 24 oocytes of the 157 showed premeiotic aneuploidy of whole chromosomes (15.3%).…”

Double-factor preimplantation genetic diagnosis: monogenic and cytogenetic diagnoses analyzing a single blastomere

“…This aneuploidy incidence is higher than expected, but the presence of three out of seven women aged between 36 and 37 years should be considered as an influencing factor as well as the fact that not only aneuploidies but also segmental imbalances are considered and informed in the preimplantation genetic screening (PGS) report. Moreover, previous studies reported considerable rates of oocyte aneuploidy even in young women …”

“…Moreover, previous studies reported considerable rates of oocyte aneuploidy even in young women. 13,14 The aneuploid events affected all of the chromosomes. In the present work, 44.8% of aneuploid blastomeres had at least three or more chromosomes simultaneously altered.…”

Novel Double Factor PGT strategy analyzing blastocyst stage embryos in a single NGS procedure

“…[ 2 – 4 ] Moreover, a high incidence of aneuploidy has also been reported in embryos from in vitro fertilization cycles[ 3 , 5 , 6 ] and in oocytes, even from young donors. [ 4 , 7 , 8 ] The Double Factor-PGT (DF-PGT) allows to transfer not only healthy embryos, free from the monogenic disease, but also embryos with the highest implantation potential according to its cytogenetic characterization. [ 9 , 10 ] This should produce an increase of pregnancy and birth rates in PGT cycles and consequently should reduce the incidence in the populations of genetic illnesses.…”