Comprehensive analysis of karyotypic mosaicism between trophectoderm and inner cell mass

Abstract

Aneuploidy has been well-documented in blastocyst embryos, but prior studies have been limited in scale and/or lack mechanistic data. We previously reported preclinical validation of microarray 24-chromosome preimplantation genetic screening in a 24-h protocol. The method diagnoses chromosome copy number, structural chromosome aberrations, parental source of aneuploidy and distinguishes certain meiotic from mitotic errors. In this study, our objective was to examine aneuploidy in human blastocysts and determine correspondence of karyotypes between trophectoderm (TE) and inner cell mass (ICM). We disaggregated 51 blastocysts from 17 couples into ICM and one or two TE fractions. The average maternal age was 31. Next, we ran 24-chromosome microarray molecular karyotyping on all of the samples, and then performed a retrospective analysis of the data. The average per-chromosome confidence was 99.95%. Approximately 80% of blastocysts were euploid. The majority of aneuploid embryos were simple aneuploid, i.e. one or two whole-chromosome imbalances. Structural chromosome aberrations, which are common in cleavage stage embryos, occurred in only three blastocysts (5.8%). All TE biopsies derived from the same embryos were concordant. Forty-nine of 51 (96.1%) ICM samples were concordant with TE biopsies derived from the same embryos. Discordance between TE and ICM occurred only in the two embryos with structural chromosome aberration. We conclude that TE karyotype is an excellent predictor of ICM karyotype. Discordance between TE and ICM occurred only in embryos with structural chromosome aberrations.

Human embryo mosaicism: did we drop the ball on chromosomal testing?

“…However, up until now, this concept is not well supported by the data obtained from early PGS platforms. For example, Johnson et al (2010) and Northrup et al (2010) found that aneuploid blastocysts displayed no evidence of preferential segregation of abnormalities to the TE, and each blastomere of the early cleavage-stage human embryo could participate in both trophectoderm and inner cell mass formation [8,22]. Therefore, an abnormal blastomere would have the same chance of ending up in the trophectoderm as in the inner cell mass.…”

“…However, even with a chromosomally abnormal TE biopsy result, a possibility of a chromosomally normal fetus remains. One possible explanation for this observation is embryonic mosaicism leading to biopsied blastomeres that are not representative of the whole embryo [8]. In the presence of mosaicism, it is possible that chromosomes are different among TE cells, or between TE cells and ICM cells.…”

Preimplantation genetic testing for aneuploidy: what technology should you use and what are the differences?

“…One of the most relevant of these reasons is that of embryo mosaicism. In many embryos, there are multiple different cell lines-a condition known as mosaicism [24][25][26][27]. This means that that the concept of all cells being derived from a common initial division cannot be wholly accurate.…”

“…Mosaicism can exist within the cells comprising the trophectoderm [24]. The ability to identify aneuploid/euploid mosaicism within the trophectoderm presents a significant challenge, and strategies have been implemented to overcome this issue.…”

Preimplantation Genetic Screening

“…Using SNP arrays to obtain this level of information requires the additional SNP analysis of the couple undergoing IVF, and entail a high level of bioinformatics analysis. The groups who developed this type of technology gave their particular method specific names, such as "Parental support" [48,53], "Karyomapping" [51] or "siChilds" [50]. The rates of meiotic and mitotic abnormalities found using SNP arrays were in concordance with those found with aCGH: Johnson et al found 9/26 cleavage stage embryos to carry a meiotic abnormality while 15/26 carried a mitotic abnormality while they found no evidence in their data set of uniparental disomy [48].…”