Oocytes with impaired meiotic maturation contain increased mtDNA deletions

Induction of meiotic competence is a major goal of the controlled ovarian stimulation used in ART. Do factors intrinsic to the oocyte contribute to oocyte maturation? Deletions in mtDNA accumulate in long-lived post mitotic tissues and are found in human oocytes. If oogenesis cleanses the germline of deleterious deletions in mtDNA, meiotically competent oocytes should contain lower levels of mtDNA deletions vs. meiotically arrested oocytes. We tested this hypothesis using a novel PCR assay for a deletion ratio in human oocytes derived from IVF.

A real-time PCR assay was developed to measure total mtDNA copy number (mtDNA_CN) and mtDNA harboring the 5 Kb "common deletion" to enable calculation of the mtDNA deletion ratio (mtDNA_DR) in 143 cultured oocytes. Kruskal-Wallis test was carried out to compare the total mtDNA_CN and the mtDNA_DR among oocytes which matured to metaphase II (MII) vs. oocytes arrested at GV or metaphase I (MI).

51.75% of oocytes reached MII, and 17% remained at MI. Mean mtDNADR in GV, MI and MII oocytes were 27.87%, 31.88% and 20.05%, respectively. The difference in deletion ratios between GV and MII and between MI and MII stages was statistically significant p < 0.001 and p = 0.034, respectively. Additionally, patient age was found to be positively correlated with time to Polar body extrusion (- 0.278 Pearson correlation).

Oocytes with impaired meiotic maturation contain an increased load of mtDNA deletions. This is the first report of an association between the mtDNA deletion ratio and human oocyte maturation in vitro.