Mitochondrial replacement by genome transfer in human oocytes: Efficacy, concerns, and legality

Abnormalities of Oocyte Maturation: Mechanisms and Implications

The elucidation of oocyte maturation mechanisms is paramount for advancing embryo development within the scope of assisted reproductive technologies (ART). Both cytoplasmic and nuclear maturation represent intricate processes governed by tightly regulated cellular pathways, which are essential for ensuring the oocyte's competence for fertilization and subsequent embryogenesis. A comprehensive grasp of these mechanisms is vital, as the maturation stage of the oocyte significantly influences chromosomal integrity, spindle formation, and its ability to support the initial stages of embryonic development. By leveraging this knowledge, we can enhance in vitro fertilization (IVF) protocols, refining ovarian stimulation regimens and culture conditions to improve oocyte quality. This, in turn, has the potential to boost pregnancy rates and outcomes. Further research in this area will contribute to the development of novel interventions that aim to increase the efficacy of preimplantation embryonic development, offering new opportunities for individuals undergoing fertility treatments.

Human ovarian aging is characterized by oxidative damage and mitochondrial dysfunction

STUDY QUESTION

Are human ovarian aging and the age-related female fertility decline caused by oxidative stress and mitochondrial dysfunction in oocytes?

SUMMARY ANSWER

We found oxidative damage in oocytes of advanced maternal age, even at the primordial follicle stage, and confirmed mitochondrial dysfunction in such oocytes, which likely resulted in the use of alternative energy sources.

WHAT IS KNOWN ALREADY

Signs of reactive oxygen species-induced damage and mitochondrial dysfunction have been observed in maturing follicles, and even in early stages of embryogenesis. However, although recent evidence indicates that also primordial follicles have metabolically active mitochondria, it is still often assumed that these follicles avoid oxidative phosphorylation to prevent oxidative damage in dictyate arrested oocytes. Data on the influence of ovarian aging on oocyte metabolism and mitochondrial function are still limited.

STUDY DESIGN, SIZE, DURATION

A set of 39 formalin-fixed and paraffin-embedded ovarian tissue biopsies were divided into different age groups and used for immunofluorescence analysis of oxidative phosphorylation activity and oxidative damage to proteins, lipids, and DNA. Additionally, 150 immature oocytes (90 germinal vesicle oocytes and 60 metaphase I oocytes) and 15 cumulus cell samples were divided into different age groups and used for targeted metabolomics and lipidomics analysis.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Ovarian tissues used for immunofluorescence microscopy were collected through PALGA, the nationwide network, and registry of histo- and cytopathology in The Netherlands. Comprehensive metabolomics and lipidomics were performed by liquid-liquid extraction and full-scan mass spectrometry, using oocytes and cumulus cells of women undergoing ICSI treatment based on male or tubal factor infertility, or fertility preservation for non-medical reasons.

MAIN RESULTS AND THE ROLE OF CHANCE

Immunofluorescence imaging on human ovarian tissue indicated oxidative damage by protein and lipid (per)oxidation already at the primordial follicle stage. Metabolomics and lipidomics analysis of oocytes and cumulus cells in advanced maternal-age groups demonstrated a shift in the glutathione-to-oxiglutathione ratio and depletion of phospholipids. Age-related changes in polar metabolites suggested a decrease in mitochondrial function, as demonstrated by NAD+, purine, and pyrimidine depletion, while glycolysis substrates and glutamine accumulated, with age. Oocytes from women of advanced maternal age appeared to use alternative energy sources like glycolysis and the adenosine salvage pathway, and possibly ATP which showed increased production in cumulus cells.

LIMITATIONS, REASONS FOR CAUTION

The immature oocytes used in this study were all subjected to ovarian stimulation with high doses of follicle-stimulating hormones, which might have concealed some age-related differences.

WIDER IMPLICATIONS OF THE FINDINGS

Further studies on how to improve mitochondrial function, or lower oxidative damage, in oocytes from women of advanced maternal age, for instance by supplementation of NAD+ precursors to promote mitochondrial biogenesis, are warranted. In addition, supplementing the embryo medium of advanced maternal-age embryos with such compounds could be a treatment option worth exploring.

STUDY FUNDING/COMPETING INTEREST(S)

The study was funded by the Amsterdam UMC. The authors declare to have no competing interests.

TRIAL REGISTRATION NUMBER

N/A.

Clinical Approaches for Mitochondrial Diseases

Mitochondria are subcontractors dedicated to energy production within cells. In human mitochondria, almost all mitochondrial proteins originate from the nucleus, except for 13 subunit proteins that make up the crucial system required to perform 'oxidative phosphorylation (OX PHOS)', which are expressed by the mitochondria's self-contained DNA. Mitochondrial DNA (mtDNA) also encodes 2 rRNA and 22 tRNA species. Mitochondrial DNA replicates almost autonomously, independent of the nucleus, and its heredity follows a non-Mendelian pattern, exclusively passing from mother to children. Numerous studies have identified mtDNA mutation-related genetic diseases. The consequences of various types of mtDNA mutations, including insertions, deletions, and single base-pair mutations, are studied to reveal their relationship to mitochondrial diseases. Most mitochondrial diseases exhibit fatal symptoms, leading to ongoing therapeutic research with diverse approaches such as stimulating the defective OXPHOS system, mitochondrial replacement, and allotropic expression of defective enzymes. This review provides detailed information on two topics: (1) mitochondrial diseases caused by mtDNA mutations, and (2) the mechanisms of current treatments for mitochondrial diseases and clinical trials.

Earlier second polar body transfer and further mitochondrial carryover removal for potential mitochondrial replacement therapy

The second polar body (PB2) transfer in assisted reproductive technology is regarded as the most promising mitochondrial replacement scheme for preventing the mitochondrial disease inheritance owing to its less mitochondrial carryover and stronger operability. However, the mitochondrial carryover was still detectable in the reconstructed oocyte in conventional second polar body transfer scheme. Moreover, the delayed operating time would increase the second polar body DNA damage. In this study, we established a spindle-protrusion-retained second polar body separation technique, which allowed us to perform earlier second polar body transfer to avoid DNA damage accumulation. We could also locate the fusion site after the transfer through the spindle protrusion. Then, we further eliminated the mitochondrial carryover in the reconstructed oocytes through a physically based residue removal method. The results showed that our scheme could produce a nearly normal proportion of normal-karyotype blastocysts with further reduced mitochondrial carryover, both in mice and humans. Additionally, we also obtained mouse embryonic stem cells and healthy live-born mice with almost undetectable mitochondrial carryover. These findings indicate that our improvement in the second polar body transfer is conducive to the development and further mitochondria carryover elimination of reconstructed embryos, which provides a valuable choice for future clinical applications of mitochondrial replacement.

Mitochondria Have Made a Long Evolutionary Path from Ancient Bacteria Immigrants within Eukaryotic Cells to Essential Cellular Hosts and Key Players in Human Health and Disease

Mitochondria have made a long evolutionary path from ancient bacteria immigrants within the eukaryotic cell to become key players for the cell, assuming crucial multitasking skills critical for human health and disease. Traditionally identified as the powerhouses of eukaryotic cells due to their central role in energy metabolism, these chemiosmotic machines that synthesize ATP are known as the only maternally inherited organelles with their own genome, where mutations can cause diseases, opening up the field of mitochondrial medicine. More recently, the omics era has highlighted mitochondria as biosynthetic and signaling organelles influencing the behaviors of cells and organisms, making mitochondria the most studied organelles in the biomedical sciences. In this review, we will especially focus on certain 'novelties' in mitochondrial biology "left in the shadows" because, although they have been discovered for some time, they are still not taken with due consideration. We will focus on certain particularities of these organelles, for example, those relating to their metabolism and energy efficiency. In particular, some of their functions that reflect the type of cell in which they reside will be critically discussed, for example, the role of some carriers that are strictly functional to the typical metabolism of the cell or to the tissue specialization. Furthermore, some diseases in whose pathogenesis, surprisingly, mitochondria are involved will be mentioned.

Genome transfer technique for bovine embryo production using the metaphase plate and polar body

Despite many studies in humans and mice using genome transfer (GT), there are few reports using this technique in oocytes of wild or domestic animals. Therefore, we aimed to establish a GT technique in bovine oocytes using the metaphase plate (MP) and polar body (PB) as the sources of genetic material. In the first experiment, GT was established using MP (GT-MP), and a sperm concentration of 1 × 106 or 0.5 × 106 spermatozoa/ml gave similar fertilization rates. The cleavage rate (50%) and blastocyst rate (13.6%) in the GT-MP group was lower than that of the in vitro production control group (80.2% and 32.6%, respectively). The second experiment evaluated the same parameters using PB instead of MP; the GT-PB group had lower fertilization (82.3% vs. 96.2%) and blastocyst (7.7% vs. 36.8%) rates than the control group. No differences in the amount of mitochondrial DNA (mtDNA) were observed between groups. Finally, GT-MP was performed using vitrified oocytes (GT-MPV) as a source of genetic material. The cleavage rate of the GT-MPV group (68.4%) was similar to that of the vitrified oocytes (VIT) control group (70.0%) and to that of the control IVP group (81.25%, P < 0.05). The blastocyst rate of GT-MPV (15.7) did not differ neither from the VIT control group (5.0%) nor from the IVP control group (35.7%). The results suggested that the structures reconstructed by the GT-MPV and GT-PB technique develop in embryos even if vitrified oocytes are used.

Enucleated oocyte donation: first for infertility treatment, then for mitochondrial diseases

There seems to be a consensus that enucleated oocyte donation (EOD) should only be used to reduce the risk of having a child with mitochondrial disorders. However, this paper argues that in the initial phase in which we are at the moment, EOD should first be used to remedy infertility caused by poor oocyte quality or poor embryonic development. That learning period will allow researchers to improve their technical skills and the knowledge of the best procedure before starting on high-risk cases. Mitochondrial carryover of pathologic mtDNA is the main cause of concern for the offspring. That risk does not exist in infertility cases. The application of EOD to treat infertility should at present be performed in a clinical research setting to obtain more evidence about efficacy and safety.