Centre for Medical Genetics, Vrije Universiteit Brussel (VUB), Universitair Ziekenhuis Brussel (UZ Brussel), Brussels, Belgium.
Research Group Reproduction and Genetics, Vrije Universiteit Brussel (VUB), Brussels, Belgium.
Hum Reprod. 2023 Dec 4;38(12):2526-2535. doi: 10.1093/humrep/dead201.
In oocytes of advanced maternal age (AMA) women, what are the mechanisms leading to aneuploidy and what is the association of aneuploidy with embryo development?
Known chromosome segregation errors such as precocious separation of sister chromatids explained 90.4% of abnormal chromosome copy numbers in polar bodies (PBs), underlying impaired embryo development.
Meiotic chromosomal aneuploidies in oocytes correlate with AMA (>35 years) and can affect over half of oocytes in this age group. This underlies the rationale for PB biopsy as a form of early preimplantation genetic testing for aneuploidy (PGT-A), as performed in the 'ESHRE STudy into the Evaluation of oocyte Euploidy by Microarray analysis' (ESTEEM) randomized controlled trial (RCT). So far, chromosome analysis of oocytes and PBs has shown that precocious separation of sister chromatids (PSSC), Meiosis II (MII) non-disjunction (ND), and reverse segregation (RS) are the main mechanisms leading to aneuploidy in oocytes.
STUDY DESIGN, SIZE, DURATION: Data were sourced from the ESTEEM study, a multicentre RCT from seven European centres to assess the clinical utility of PGT-A on PBs using array comparative genomic hybridization (aCGH) in patients of AMA (36-40 years). This included data on the chromosome complement in PB pairs (PGT-A group), and on embryo morphology in a subset of embryos, up to Day 6 post-insemination, from both the intervention (PB biopsy and PGT-A) and control groups.
PARTICIPANTS/MATERIALS, SETTING, METHODS: ESTEEM recruited 396 AMA patients: 205 in the intervention group and 191 in the control group. Complete genetic data from 693 PB pairs were analysed. Additionally, the morphology from 1034 embryos generated from fertilized oocytes (two pronuclei) in the PB biopsy group and 1082 in the control group were used for statistical analysis.
Overall, 461/693 PB pairs showed abnormal segregation in 1162/10 810 chromosomes. The main observed abnormal segregations were compatible with PSSC in Meiosis I (MI) (n = 568/1162; 48.9%), ND of chromatids in MII or RS (n = 417/1162; 35.9%), and less frequently ND in MI (n = 65/1162; 5.6%). For 112 chromosomes (112/1162; 9.6%), we observed a chromosome copy number in the first PB (PB1) and second PB (PB2) that is not explained by any of the known mechanisms causing aneuploidy in oocytes. We observed that embryos in the PGT-A arm of the RCT did not have a significantly different morphology between 2 and 6 days post-insemination compared to the control group, indicating that PB biopsy did not affect embryo quality. Following age-adjusted multilevel mixed-effect ordinal logistic regression models performed for each embryo evaluation day, aneuploidy was associated with a decrease in embryo quality on Day 3 (adjusted odds ratio (aOR) 0.62, 95% CI 0.43-0.90), Day 4 (aOR 0.15, 95% CI 0.06-0.39), and Day 5 (aOR 0.28, 95% CI 0.14-0.58).
LIMITATIONS, REASON FOR CAUTION: RS cannot be distinguished from normal segregation or MII ND using aCGH. The observed segregations were based on the detected copy number of PB1 and PB2 only and were not confirmed by the analysis of embryos. The embryo morphology assessment was static and single observer.
Our finding of frequent unexplained chromosome copy numbers in PBs indicates that our knowledge of the mechanisms causing aneuploidy in oocytes is incomplete. It challenges the dogma that aneuploidy in oocytes is exclusively caused by mis-segregation of chromosomes during MI and MII.
STUDY FUNDING/COMPETING INTEREST(S): Data were mined from a study funded by ESHRE. Illumina provided microarrays and other consumables necessary for aCGH testing of PBs. None of the authors have competing interests.
Data were mined from the ESTEEM study (ClinicalTrials.gov Identifier NCT01532284).
在高龄产妇（AMA）的卵子中，导致非整倍体的机制是什么，以及非整倍体与胚胎发育的关系是什么？
已知的染色体分离错误，如姐妹染色单体的过早分离，解释了极体（PB）中 90.4%的异常染色体拷贝数，这是胚胎发育受损的基础。
卵母细胞中的减数分裂染色体非整倍性与 AMA（>35 岁）相关，在该年龄段的卵子中，超过一半可能存在这种情况。这就是 PB 活检作为一种早期胚胎植入前遗传学检测非整倍体（PGT-A）的理论基础，正如在 ESHRE STudy into the Evaluation of oocyte Euploidy by Microarray analysis（ESTEEM）随机对照试验（RCT）中所进行的那样。到目前为止，对卵母细胞和 PB 的染色体分析表明，姐妹染色单体的过早分离（PSSC）、减数分裂 II（MII）非分离（ND）和反向分离（RS）是导致卵母细胞非整倍体的主要机制。
研究设计、规模、持续时间：数据来自 ESTEEM 研究，这是一项来自七个欧洲中心的多中心 RCT，旨在使用微阵列比较基因组杂交（aCGH）评估 AMA（36-40 岁）患者 PB 上的 PGT-A 的临床实用性。这包括 PB 对（PGT-A 组）的染色体成分数据，以及来自干预组（PB 活检和 PGT-A）和对照组的胚胎形态学数据，直到受精后第 6 天。
参与者/材料、设置、方法：ESTEEM 招募了 396 名 AMA 患者：干预组 205 名，对照组 191 名。对 693 对 PB 的完整遗传数据进行了分析。此外，还对 PB 活检组和对照组中从受精卵（两个原核）产生的 1034 个胚胎和 1082 个胚胎的形态进行了统计分析。
总体而言，461/693 对 PB 显示出 1162/100810 条染色体的异常分离。主要观察到的异常分离与减数分裂 I（MI）中的 PSSC 一致（n=568/1162；48.9%）、MII 或 RS 中的染色单体 ND（n=417/1162；35.9%），以及较少见的 MI 中的 ND（n=65/1162；5.6%）。对于 112 条染色体（112/1162；9.6%），我们观察到第一极体（PB1）和第二极体（PB2）中的染色体拷贝数无法用任何已知的导致卵母细胞非整倍体的机制来解释。我们观察到，RCT 的 PGT-A 臂中的胚胎在受精后第 2 天至第 6 天的形态与对照组相比没有明显差异，这表明 PB 活检不会影响胚胎质量。在对每个胚胎评估日进行年龄调整后的多级混合效应有序逻辑回归模型后，非整倍体与胚胎质量下降相关，第 3 天（调整后的优势比（aOR）0.62，95%CI 0.43-0.90）、第 4 天（aOR 0.15，95%CI 0.06-0.39）和第 5 天（aOR 0.28，95%CI 0.14-0.58）。
局限性、谨慎的原因：RS 不能通过 aCGH 与正常分离或 MII ND 区分。观察到的分离仅基于 PB1 和 PB2 的检测拷贝数，并且未通过胚胎分析得到证实。胚胎形态评估是静态的，只有一位观察者。
我们在 PB 中发现了频繁的未解释染色体拷贝数，这表明我们对卵母细胞中非整倍体形成机制的认识还不完全。这挑战了卵母细胞中非整倍体仅由 MI 和 MII 期间染色体的错误分离引起的教条。
研究资金/利益冲突：数据来自由 ESHRE 资助的一项研究。Illumina 提供了进行 PB aCGH 测试所需的微阵列和其他耗材。没有作者有利益冲突。
数据来自 ESTEEM 研究（ClinicalTrials.gov 标识符 NCT01532284）。
