Technical approaches to correction of oocyte aneuploidy

This study describes the technical approaches used in treatment of age-related oocyte aneuploidy, the efficiency of each step of nuclear transplantation into mouse and human oocytes, and the ability of germinal vesicle (GV) transplantation to restore artificially induced ooplasmic damage. Finally, it examines the possibility of constructing viable female gametes by transferring diploid somatic cell nuclei into enucleated oocytes.

GV stage mouse oocytes were collected from unstimulated ovaries, and human GV oocytes were donated from consenting patients undergoing ICSI. Stromal (somatic) cells were isolated from uterine biopsies of consenting patients. Mouse cumulus cells were obtained after ovarian stimulation. GV ooplasts prepared by removing nuclei were transplanted either with GV nuclei or with somatic cells by micromanipulation. Grafted oocytes were electrofused and cultured to allow maturation, following which they were inseminated or analysed cytogenetically. Ooplasmic dysfunction was induced by photosensitization with a mitochondria-specific fluorescent dye.

GV transplantation had an overall efficiency of 87 and 73% in the mouse and humans respectively. Maturation rates of 95 (mouse) and 64% (human) following reconstitution were comparable with those in control oocytes, as was the incidence of aneuploidy for five chromosome-specific probes after aneuploidy among the reconstituted oocytes. Photosensitization of oocytes significantly reduced the maturation rate to 4.2%, whereas 61.9% of oocytes matured after transfer of photosensitized GV karyoplasts into healthy ooplasts, with 52% of these mature oocytes being successfully fertilized by ICSI. Enucleated immature oocytes receiving mouse cumulus or human endometrial cell nuclei extruded a polar body in >40% of cases. Five out of seven successfully transferred aged human nuclei exhibited the expected number of signals with five chromosome-specific probes suggesting an appropriate chromosome separation in young ooplasm.

Nuclear transplantation itself does not appear to interfere with chromosome segregation and can possibly rescue oocytes with damaged mitochondria. Finally, immature mouse ooplasm supported separation of somatic chromosomes to expected numbers, implying that haploidization may be occurring. The roles of genetic imprinting and fidelity of chromosome segregation are unknown.