Tripolar chromosome segregation drives the association between maternal genotype at variants spanning PLK4 and aneuploidy in human preimplantation embryos

Abstract

Aneuploidy is prevalent in human embryos and is the leading cause of pregnancy loss. Many aneuploidies arise during oogenesis, increasing with maternal age. Superimposed on these meiotic aneuploidies are frequent errors occurring during early mitotic divisions, contributing to widespread chromosomal mosaicism. Here we reanalyzed a published dataset comprising preimplantation genetic testing for aneuploidy in 24 653 blastomere biopsies from day-3 cleavage-stage embryos, as well as 17 051 trophectoderm biopsies from day-5 blastocysts. We focused on complex abnormalities that affected multiple chromosomes simultaneously, seeking insights into their formation. In addition to well-described patterns such as triploidy and haploidy, we identified 4.7% of blastomeres possessing characteristic hypodiploid karyotypes. We inferred this signature to have arisen from tripolar chromosome segregation in normally fertilized diploid zygotes or their descendant diploid cells. This could occur via segregation on a tripolar mitotic spindle or by rapid sequential bipolar mitoses without an intervening S-phase. Both models are consistent with time-lapse data from an intersecting set of 77 cleavage-stage embryos, which were enriched for the tripolar signature among embryos exhibiting abnormal cleavage. The tripolar signature was strongly associated with common maternal genetic variants spanning the centrosomal regulator PLK4, driving the association we previously reported with overall mitotic errors. Our findings are consistent with the known capacity of PLK4 to induce tripolar mitosis or precocious M-phase upon dysregulation. Together, our data support tripolar chromosome segregation as a key mechanism generating complex aneuploidy in cleavage-stage embryos and implicate maternal genotype at a quantitative trait locus spanning PLK4 as a factor influencing its occurrence.

Single-Cell Sequencing of Primate Preimplantation Embryos Reveals Chromosome Elimination Via Cellular Fragmentation and Blastomere Exclusion

“…While it is fairly well established that gynogenetic and androgenetic embryos can result from IVF, we speculate that a similar percentage of human embryos with uniparental origins has not yet been reported given that current preimplantation genetic screening methods do not examine parental origins of aneuploidy unless SNP arrays are used. However, SNP arrays are rarely employed by clinics for CNV analysis due to high rates of allele drop-out and the need to include parental DNA to interpret SNPs (McCoy et al 2015;McCoy et al 2018). In summary, we show that chromosomal loss from primate preimplantation embryos is due to sequestration by cellular fragments and/or non-dividing blastomeres, which may denote mechanisms to surpass aneuploidy as embryos undergo implantation and continue in development.…”

Sperm DNA damage causes genomic instability in early embryonic development

“…Direct unequal cleavage divisions are the result of multipolar spindles. Strikingly, fertilized oocytes that carry the minor allele of PLK4 are also vulnerable to multipolar spindle formation and mosaicism in embryos (16,17). Thus, spindle aberrations appear to be an important source of genomic instability in embryo development and we here identify sperm DNA damage as one possible cause.…”

“…Mosaicism is prevalent in human spontaneous abortions of natural pregnancies (13,14), indicating that the causes for the high mitotic error rate in embryos are unrelated to the IVF procedures such as the ovarian stimulation regime, fluctuations in oxygen tension or temperature, and composition of the culture medium. While advanced maternal age increases the risk for meiotic errors leading to whole embryo aneuploidies, mitotic errors and embryo mosaicism are not correlated with female age (15)(16)(17). A minor allele of the polo-like kinase 4 (PLK4) gene has been associated with tripolar chromosome segregations, leading to mitotic errors in development (16,17).…”

“…While advanced maternal age increases the risk for meiotic errors leading to whole embryo aneuploidies, mitotic errors and embryo mosaicism are not correlated with female age (15)(16)(17). A minor allele of the polo-like kinase 4 (PLK4) gene has been associated with tripolar chromosome segregations, leading to mitotic errors in development (16,17). However, PLK4 polymorphisms alone cannot explain the high prevalence of mosaicism in human embryos.…”

Making sense out of syngamy at the onset of mammalian development

“…Moreover, has anyone noticed, other than genetics aficionados, the reports of tripolar spindles all the way from human oocytes [8] to embryos [9,10]?…”