The Incidence of Mosaicism for Individual Chromosome in Human Blastocysts Is Correlated With Chromosome Length

Abstract

Mosaicism, known as partial aneuploidies, mostly originates from mitotic errors during the post-zygotic stage; it consists of different cell lineages within a human embryo. The incidence of mosaicism has not been shown to correlate with maternal age, and its correlation with individual chromosome characteristics has not been well investigated. In this study, the results of preimplantation genetic testing for aneuploidy (PGT-A) derived from 4,036 blastocysts (930 IVF couples) were collected from 2015 to 2017. Via next-generation sequencing for comprehensive chromosome screening, embryo ploidy was identified as aneuploid, mosaic, and euploid. Total mosaicism was classified into two categories: "mosaic euploid/aneuploidy" (with mosaic aneuploidy between 20 and 80%) and "mosaic and aneuploidy" (a uniformly abnormal embryo superimposed with mosaic aneuploidies). Frequency of mosaicism was analyzed according to the function of chromosomal lengths, which divides involved chromosomes into three groups: group A (156-249 Mb), group B (102-145 Mb), and group C (51-90 Mb). The results show that the aneuploidy was more frequent in group C than in group A and group B (A: 23.7%, B: 35.1, 41.2%, p < 0.0001), while the mosaicism was more frequent in group A and group B than in group C [(Mosaic euploid/aneuploid) A: 14.6%, B: 12.4%, C: 9.9%, p < 0.0001; (mosaic and aneuploid) A: 21.3%, B: 22.9%, C: 18.9%, p < 0.0001; (Total mosaicism) A: 35.9%, B: 35.3%, C: 28.8%, p < 0.0001]. The significantly higher frequency of aneuploidy was on the shorter chromosome (< 90 Mb), and that of mosaicism was on the longer chromosomes (> 100 Mb). The length association did not reach significance in the patients with advanced age (≥ 36 years), and of the chromosome-specific mosaicism rate, the highest prevalence was on chromosome 14 (5.8%), 1 (5.7%), and 9 (5.6%). Although the length association was observed via group comparison, there may be affecting mechanisms other than chromosomes length. Eventually, twenty patients with mosaic embryo cryotransfers resulted in six live births. No significant correlation was observed between the transfer outcomes and chromosome length; however, the analysis was limited by small sample size.

Keywords: chromosome length; mitotic aberration; mosaicism; next-generation sequencing; preimplantation genetic testing for aneuploidy.

FIGURE 1

Higher frequency of mosaicism in longer chromosomes. The frequency of aneuploidy differed among the three groups as a function of chromosome length (A-C). The frequency of aneuploidy was significantly higher within the shorter chromosomes in the overall patients (A) and patients with advanced maternal age (≥ 36 years) (B), but the trend was not observed in the patients with young age (≤ 32 years) (C). The frequency of mosaicism differed among the three groups as a function of chromosome length (D-F). Both the frequencies of mosaic euploid/aneuploid and mosaic and aneuploid were significantly higher within the longer chromosomes in the overall patients (D) and the patients with young age (≤ 32 years) (F), but the significance was not reached in the patients with advanced maternal age (≥ 36 years) (E). The power of significance in the figures were shown by the p-values, and p≥ 0.05 is defined as without significance, which would not be labeled.

FIGURE 2

Mosaicism and aneuploidy across individual chromosomes. The frequency of aneuploidy across individual chromosomes was shown (A-C). The highest frequencies in the overall patients occurred on chromosome 22, 16, and 15 (A). The highest frequencies in the patients with advanced maternal age (≥ 36 years) occurred on chromosomes 22, 16, and 15 (B). The highest frequencies in the patients with young age (≤ 32 years) occurred on chromosomes 16, 8 and 9 (C). The frequency of mosaicism, including both the mosaic euploid/aneuploid and mosaic and aneuploid, across individual chromosomes was shown (D-F). The highest frequency of mosaicism in the overall patients occurred on chromosomes 14, 1 and 9 (D). The highest frequency of mosaicism in the patients with advanced maternal age (≥ 36 years) occurred on chromosomes 21,14 and 9 (E). The highest frequency of mosaicism in the patients with young age (≤ 32 years) occurred on chromosomes 5, 7, 3, and 21 (F).

FIGURE 3

Clinical outcomes of transferring embryos with mosaic euploid/aneuploid in 20 patients were shown. Among the three groups as a function of chromosome length, the clinical pregnancy rate (CPR), implantation rate (IR), ongoing pregnancy rate (OPR), and live birth rate (LBR) were not significantly correlated with chromosome length. The power of significance in the figures were shown by the p-values, and p≥ 0.05 is defined as without significance, which would not be labeled.