Segmental aneuploid hotspots identified across the genome concordant on reanalysis

A systematic review and meta-analysis of the diagnostic accuracy after preimplantation genetic testing for aneuploidy

OBJECTIVE

Aneuploidy accounts for many pregnancy failures and congenital anomalies. Preimplantation genetic testing for aneuploidy (PGT-A) is a screening test applied to embryos created from in vitro fertilization to diminish the chance of an aneuploid conception. The rate of misdiagnosis for both false aneuploidy (false positive) and false euploidy (false negative) test results is unknown. The objective of this study was to determine the rate of misclassification of both aneuploidy and euploidy after PGT-A.

DATA SOURCES

We conducted a systematic review and meta-analysis. We searched Medline, Embase, Cochrane Central, CINAHL and WHO Clinical Trials Registry from inception until April 10, 2024. The protocol was registered in International Prospective Register of Systematic Reviews (PROSPERO CRD 42020219074).

METHODS OF STUDY SELECTION

We included studies that conducted either a pre-clinical validation of the genetic platform for PGT-A using a cell line, studies that compared the embryo biopsy results to those from the whole dissected embryo or its inner cell mass (WE/ICM), and studies that compared the biopsy results to prenatal or postnatal genetic testing.

TABULATION, INTEGRATION, AND RESULTS

Two independent reviewers extracted true and false positives and negatives comparing biopsy results to the reference standard (known karyotype, WE/ICM, pregnancy outcome). For preclinical studies, the main outcome was the positive and negative predictive values. Misdiagnosis rate was the outcome for pregnancy outcome studies. The electronic search yielded 6674 citations, of which 109 were included. For WE/ICM studies (n=40), PPV was 89.2% (95% CI 83.1-94.0) and NPV was 94.2% (95% CI 91.1-96.7, I2=42%) for aneuploid and euploid embryos, respectively. The PPV for mosaic embryos of either a confirmatory mosaic or aneuploid result was 52.8% (95% CI 37.9-67.5). For pregnancy outcome studies (n=43), the misdiagnosis rate after euploid embryo transfer was 0.2% (95% CI 0.0-0.7%, I2=65%). However, the rate for mosaic transfer, with a confirmatory euploid pregnancy outcome, was 21.7% (95% CI: 9.6-36.9, I2=95%).

CONCLUSION

The accuracy of an aneuploid result from PGT-A is excellent and can be relied upon as a screening tool for embryos to avoid aneuploid pregnancies. Similarly, the misdiagnosis rate after euploid embryo transfer is less than 1%. However, there is a significant limitation in the accuracy of mosaic embryos.

Predicting frequency distributions of blastocyst biopsy genotypes by their discrete cohort size using the binomial theorem

RESEARCH QUESTION

How are blastocyst biopsy genotypes distributed as a function of cohort size, and how well does the binomial theorem predict the specific distribution of euploid genotypes across different cohort sizes?

DESIGN

This retrospective observational study included 1065 autologous blastocyst biopsies from 206 consecutive biopsy cases at a single centre. Cohorts were classified into three mutually exclusive categories: euploid+ (containing at least one euploid), euploid-mosaic/segmental+ and meiotic (containing whole chromosome errors alone), stratified by patient age (<37 years versus ≥37 years) and cohort size (10 discrete groups). 'Observed' distributions of individual euploid blastocysts/cohort were compared with binomially 'expected' distributions using the overall probability for euploidy for each age group.

RESULTS

For patients aged <37 years, cohorts were predominantly euploid+ (85.7%). The odds of being euploid increased for each 1-unit increase in cohort size. For each size group, the observed distribution of euploid blastocysts/cohort was closely concordant with distributions predicted binomially in the largest three cohort size quartiles. In contrast, cohort categories in the smallest quartile showed greater discordance. For patients aged ≥37 years, 48.5% of cohorts were euploid+ cohorts. While the odds of being euploid also increased for each 1-unit increase in cohort size, the observed peak number of euploid blastocysts/cohort was lower than predicted binomially in all size quartiles.

CONCLUSIONS

The binomial theorem can predict the probability distributions of euploid genotypes in cohort sizes exceeding four in patients aged <37 years without a biopsy. However, in smaller cohort sizes for patients aged >37 years and all cohort sizes in patients aged ≥37 years, there were fewer euploid blastocysts/cohort than predicted binomially; thus, untransferred mosaic/segmental genotypes represent a repository of pregnancy potential.

The chromosomal challenge of human embryos: Mechanisms and fundamentals

Chromosomal abnormalities in human pre-implantation embryos, originating from either meiotic or mitotic errors, present a significant challenge in reproductive biology. Complete aneuploidy is primarily linked to errors during the resumption of meiosis in oocyte maturation, which increase with maternal age, while mosaic aneuploidies result from mitotic errors after fertilization. The biological causes of these abnormalities are increasingly becoming a topic of interest for research groups and clinical specialists. This review explores the intricate processes of meiotic and early mitotic divisions in embryos, shedding light on the mechanisms that lead to changes in chromosome number in daughter cells. Key factors in meiotic division include difficulties in spindle assembly without centrosomes, kinetochore (KT) orientation disturbances, and inefficient cell-cycle checkpoints. The weakening of cohesion molecules that bind chromosomes, exacerbated by maternal aging, further complicates chromosomal segregation. Mitotic errors in early development are influenced by defects in sperm centrosomes, KT misalignment, and the gradual depletion of maternal regulatory factors. Coupled with the inactive or partially active embryonic genome, this depletion increases the likelihood of chromosomal aberrations. While various theoretical mechanisms for these abnormalities exist, current data remain insufficient to determine their exact contributions. Continued research is essential to unravel these complex processes and improve outcomes in assisted reproductive technologies.

Insights into embryonic chromosomal instability: mechanisms of DNA elimination during mammalian preimplantation development

Mammalian preimplantation embryos often contend with aneuploidy that arose either by the inheritance of meiotic errors from the gametes, or from mitotic mis-segregation events that occurred following fertilization. Regardless of the origin, mis-segregated chromosomes become encapsulated in micronuclei (MN) that are spatially isolated from the main nucleus. Much of our knowledge of MN formation comes from dividing somatic cells during tumorigenesis, but the error-prone cleavage-stage of early embryogenesis is fundamentally different. One unique aspect is that cellular fragmentation (CF), whereby small subcellular bodies pinch off embryonic blastomeres, is frequently observed. CF has been detected in both in vitro and in vivo-derived embryos and likely represents a response to chromosome mis-segregation since it only appears after MN formation. There are multiple fates for MN, including sequestration into CFs, but the molecular mechanism(s) by which this occurs remains unclear. Due to nuclear envelope rupture, the chromosomal material contained within MN and CFs becomes susceptible to double stranded-DNA breaks. Despite this damage, embryos may still progress to the blastocyst stage and exclude chromosome-containing CFs, as well as non-dividing aneuploid blastomeres, from participating in further development. Whether these are attempts to rectify MN formation or eliminate embryos with poor implantation potential is unknown and this review will discuss the potential implications of DNA removal by CF/blastomere exclusion. We will also extrapolate what is known about the intracellular pathways mediating MN formation and rupture in somatic cells to preimplantation embryogenesis and how nuclear budding and DNA release into the cytoplasm may impact overall development.