Tri-directional anaphases as a novel chromosome segregation defect in human oocytes.

Study Question: What are the chromosome segregation errors in human oocyte meiosis-I that may underlie oocyte aneuploidy?

Summary Answer: Multiple modes of chromosome segregation error were observed, including tri-directional anaphases, which we attribute to loss of bipolar spindle structure at anaphase-I.

What Is Known Already: Oocyte aneuploidy is common and associated with infertility, but mechanistic information on the chromosome segregation errors underlying these defects is scarce. Lagging chromosomes were recently reported as a possible mechanism by which segregation errors occur.

Study Design, Size, Duration: Long-term confocal imaging of chromosome dynamics in 50 human oocytes collected between January 2015 and May 2016.

Participants/materials, Setting, Methods: Germinal vesicle (GV) stage oocytes were collected from women undergoing intracytoplasmic sperm injection cycles and also CD1 mice. Oocytes were microinjected with complementary RNAs to label chromosomes, and in a subset of oocytes, the meiotic spindle. Oocytes were imaged live through meiosis-I using confocal microscopy. 3D image reconstruction was used to classify chromosome segregation phenotypes at anaphase-I. Segregation phenotypes were related to spindle dynamics and cell cycle timings.

Main Results And The Role Of Chance: Most (87%) mouse oocytes segregated chromosomes with no obvious defects. We found that 20% of human oocytes segregated chromosomes bi-directionally with no lagging chromosomes. The rest were categorised as bi-directional anaphase with lagging chromosomes (20%), bi-directional anaphase with chromatin mass separation (34%) or tri-directional anaphase (26%). Segregation errors correlated with chromosome misalignment prior to anaphase. Spindles were tripolar when tri-directional anaphases occurred. Anaphase phenotypes did not correlate with meiosis-I duration (P = 0.73).

Large Scale Data: Not applicable.

Limitations, Reasons For Caution: Oocytes were recovered at GV stage after gonadotrophin-stimulation, and the usual oocyte quality caveats apply. Whilst the possibility that imaging may affect oocyte physiology cannot be formally excluded, detailed controls and justifications are presented.

Wider Implications Of The Findings: This is one of the first reports of live imaging of chromosome dynamics in human oocytes, introducing tri-directional anaphases as a novel potential mechanism for oocyte aneuploidy.

Study Funding/competing Interest(s): This study was funded by grants from Fondation Jean-Louis Lévesque (Canada), CIHR (MOP142334) and CFI (32711) to GF. JH is supported by Postdoctoral Fellowships from The Lalor Foundation and CIHR (146703). The authors have no conflict of interest.