Vitrification affects nuclear maturation and gene expression of immature human oocytes

Vitrification of oocytes is a fast-freezing technique, which may affect the quality of the human oocyte, and consequently affects the embryo development, pregnancy and birth. The aim of the current study was to investigate the consequence of in-vitro vitrification on maturation status of immature human oocytes, additionally, expression levels of stress, and apoptosis related genes.
The total of 213 human immature oocytes which routinely discarded from assisted reproduction clinics were collected and divided into two groups including: (I) fresh germinal vesicle (GV) oocytes (n=106) (matured in-vitro (fIVM) , and (II) GV oocytes (n=107) that initially vitrified, then matured in in-vitro (vIVM). After 36 hours of incubation, the oocytes were evaluated for nuclear maturation and expression level of DNA methyltransferase (DNMT1), stress related genes (Sod1 and Hsp70), and apoptotic related genes (Bax and Bcl-2) by quantitative Real-Time PCR.
Oocyte maturation rates were reduced in vIVM compared to fIVM oocytes (P=0.001). The expression of stress (Sod1 and Hsp70), and apoptotic-related genes (Bax and Bcl-2) in vIVM were significantly higher compared to the fIVM group. Additionally, pro-apoptotic gene up-regulated 4.3 times more than anti-apoptotic gene in vIVM oocyte. However, DNMT1 gene expression was reduced in vIVM oocyte (P = 0.047).
The low survival rate of vitrified In-vitro matured GV oocytes could definitely be explained by the alterations of their gene expression profile.