Identifying parental and cell-division origins of aneuploidy in the human blastocyst

Abstract

Preimplantation genetic testing commonly employs simplistic copy-number analyses to screen for aneuploidy in blastocyst trophectoderm biopsies. Interpreting intermediate copy number alone as evidence of mosaicism has led to suboptimal estimation of its prevalence. Because mosaicism originates from mitotic nondisjunction, utilizing SNP microarray technology to identify the cell-division origins of aneuploidy might provide a more accurate estimation of its prevalence. The present study develops and validates a method of determining the cell-division origin of aneuploidy in the human blastocyst by using both genotyping and copy-number data in parallel. The concordance of predicted origins with expected results was demonstrated in a series of truth models (99%-100%). This included determination of X chromosome origins from a subset of normal male embryos, determination of the origins of translocation chromosome-related imbalances via embryos from couples with structural rearrangements, and prediction of either mitotic or meiotic origins via multiple rebiopsies of embryos with aneuploidy. In a cohort of blastocysts with parental DNA (n = 2,277), 71% were euploid, 27% were meiotic aneuploid, and 2% were mitotic aneuploid, indicating a low frequency of bona fide mosaicism in the human blastocyst (mean maternal age: 34.4). Chromosome-specific trisomies in the blastocyst were also consistent with observations previously established in products of conception. The ability to accurately identify mitotic-origin aneuploidy in the blastocyst could benefit and better inform individuals whose IVF cycle results in all aneuploid embryos. Clinical trials with this methodology might also help provide a definitive answer regarding the reproductive potential of bona fide mosaic embryos.

Keywords: meiosis; microarray; mitosis; mosaicism; preimplantation genetic testing; trisomy.

Figure 1

Method of identifying monosomies of mitotic origin Bar graphs indicate predicted copy number per chromosome. All three examples have a monosomy in chromosome 4 (the maternal meiotic monosomy sample also contains monosomies in chromosomes 10 and 16). For each example, the top-colored dot plots indicate the predicted source of alleles present on chromosome 4; maternal (M) in red, paternal (P) in green, and intermediate in light blue. The bottom dot plots in dark blue indicate b-allele frequency ratios on chromosome 4 for all samples. The meiotic monosomy samples show loss of heterozygosity (LOH) throughout, and the embryonic monosomy shows signal in the heterozygous region of the plot as a result of the presence of normal and abnormal cells.

Figure 2

Male-embryo X chromosomes as positive controls Workflow and results of predicting the parental origin of the X chromosome in male karyotype embryos.

Figure 3

Unbalanced-embryo-derivative chromosomes as positive controls Workflow and results of predicted parental and cell-division origin of segmental imbalances in embryos from couples that harbor a chromosomal translocation. (PGT-SR: preimplantation genetic testing of structural rearrangements).

Figure 4

Rebiopsies as positive controls Workflow and results of predicting cell-division and parental origin of aneuploidy from a single biopsy with confirmation from rebiopsies.

Figure 5

Aneuploidy classification by copy number Predicted copy number of chromosomes after classification by b-allele frequency analysis between meiotic and mitotic (embryonic) errors.

Figure 6

Classification of chromosomal abnormalities across age groups (A)Breakdown of embryos in experimental cohort by ploidy group (euploid or aneuploid, including monosomy, trisomy, and multiple chromosomal abnormalities). (B) Incidence of cell-division origin of aneuploidy by maternal age group. (C) Proportions of trisomies of MI, MII, and embryonic origin by maternal age group.

Figure 7

Incidence of aneuploidy by chromosome and comparison of origin between products of conception (POCs) and human blastocysts (A) Monosomy and trisomy occurrences by chromosome. The y axis indicates the event count, and the x axis indicates the affected chromosome. (B) Comparison of common aneuploidies in POCs^, and blastocysts.

Figure 8

Cell-division origins of trisomy in the human blastocyst Frequencies of maternal, paternal, and embryonic trisomies by chromosome in the human blastocyst.