Maturation arrest of human oocytes as a cause of infertility: case report

Mammalian oocyte therapies

In assisted human reproduction, the cytoplasm of oocytes recovered from follicles is often abnormal. Its lower quality, especially in older patients, may be responsible for certain chromosomal abnormalities or developmental arrest. Thus, the deficiency of some vital molecules, which are necessary for oocyte maturation, can be the cause of infertility in women. Moreover, mutated mitochondrial DNA (mtDNA) that is located in the oocyte cytoplasm might be transmitted to offspring. With the advance of new micromanipulation techniques like the oocyte nucleus replacement or cytoplasmic transfer, some of these abnormalities could be theoretically eliminated. In this review, we briefly discuss some of these approaches and their potential use in assisted human reproduction.

Characterization and mutation analysis of the human () gene in women with unexplained infertility

OBJECTIVE

Formin-2 (Fmn2) mutant mice produce oocytes with meiosis I arrest. Our aim was to describe the human FORMIN-2 (FMN2) gene and to identify DNA sequence polymorphisms in patients with unexplained infertility and multiple failed IVF cycles.

DESIGN

Institutional review board-approved observational case-control study.

SETTING

Infertility center and university hospital.

PATIENT(S)

Sixty-two fertile controls and seven subjects with unexplained infertility.

INTERVENTION(S)

BLASTP (www.ncbi.nlm.nih.gov) was used to map the genomic DNA and complementary DNA sequence of FMN2. Genomic DNA was extracted from blood leukocyte samples. The polymerase chain reaction was used to amplify FMN2 gene exons for analysis by denaturing gradient gel electrophoresis.

MAIN OUTCOME MEASURE(S)

Characterization of the FMN2 gene and identification of fragment melting polymorphisms (FMPs).

RESULT(S)

FMN2 includes 411,960 base pairs (bp) of DNA with 6,204 bp in 18 exons. There was no difference in FMN2 FMP allele frequencies between the controls and subjects. One patient was homozygous for one FMP.

CONCLUSION(S)

The human FMN2 gene is conserved between evolutionarily diverse vertebrates. It is likely that FMN2 has the same function as Fmn2 in the mouse (i.e., maintenance of the meiotic spindle). Prospective identification of patients with meiosis I arrest is necessary to determine whether FMN2 mutations are a cause of unexplained infertility.

A synthetic analogue of meiosis-activating sterol (FF-MAS) is a potent agonist promoting meiotic maturation and preimplantation development of mouse oocytes maturing in vitro

BACKGROUND

Follicular fluid-meiosis-activating sterol (FF-MAS) is a factor present in the pre-ovulatory follicle during the time of oocyte maturation. In mouse oocytes maturing in vitro, FF-MAS promotes the completion of meiotic maturation to metaphase II (MII) and improves competence to complete the 2-cell stage to blastocyst transition. We produced analogues of FF-MAS and selected three on the basis of potency to promote the resumption of meiosis by mouse oocytes maintained in meiotic arrest by hypoxanthine. The objective of this study was to determine whether these FF-MAS analogues also affect the quality of oocytes maturing in vitro with respect to the completion of meiotic maturation and augmenting the frequency of development to the blastocyst stage after fertilization in vitro.

METHODS

Cumulus cell-enclosed oocytes were isolated from the small antral follicles of 18 or 20 day post-natal mice. These oocytes normally have a reduced competence to complete meiotic maturation and preimplantation embryo development. Oocytes were isolated at the germinal vesicle stage and matured in vitro using media supplemented with 0.1% ethanol, 1 micromol/l FF-MAS, or 0.1-10 micromol/l FF-MAS analogues ZK255884 (884), ZK255933 (933) and ZK255991 (991). Oocytes that progressed to MII were fertilized in vitro and the percentage developing to the 2-cell and blastocyst stages was determined.

RESULTS

At 1 micromol/l, 991 and 933 increased the portion of oocytes progressing to MII, whereas the lowest dose of 991 and 884 was ineffective. Treatment of maturing oocytes with either 0.1 or 1 micromol/l 933 dramatically increased oocyte competence to complete preimplantation development.

CONCLUSIONS

The synthetic analogue of FF-MAS, ZK255933, is a potent agonist that improves the quality of mouse oocytes matured in vitro. This compound may therefore have therapeutic value for treatment of oocytes from women undergoing therapy for infertility owing to poor oocyte quality.

Distribution of mitochondria in reconstructed mouse oocytes

It has been suggested that nucleus replacement (transfer) may be used as an efficient oocyte therapy in order to prevent transmission of mutated mitochondrial DNA from mother to offspring in humans. The essential and not yet answered question is how mitochondria surrounding the karyoplast will be distributed in the newly reconstructed oocytes. In our model experiments, we have evaluated the distribution of mitochondria in reconstructed immature mouse oocytes when germinal vesicle karyoplasts, with labeled mitochondria, were fused to unlabeled cytoplasts. The penetration of mitochondria from karyoplasts into cytoplasts can be detected almost immediately after the beginning of fusion. In immature reconstructed oocytes, mitochondria are first located in the oocyte center but they are homogenously distributed within the whole cytoplasm before the completion of maturation. Fusion of oocytes at different stages of maturation suggests that the speed of mitochondria distribution is cell cycle dependent.