|Year : 2021 | Volume
| Issue : 1 | Page : 9-13
Is three-parent IVF the answer to preventing mitochondrial defects?
Department of Biological Sciences, Sunandan Divatia School of Science, NMIMS (Deemed-to-be University), Mumbai, Maharashtra, India
|Date of Submission||17-May-2021|
|Date of Decision||05-Jun-2021|
|Date of Acceptance||05-Jun-2021|
|Date of Web Publication||30-Jun-2021|
Ms. Shloka Shetty
Department of Biological Sciences, Sunandan Divatia School of Science, NMIMS (Deemed-to-be University), Bhaidas Sabhagriha Building, Bhaktivedanta Swami Marg, Vile Parle (W), Mumbai - 400 056, Maharashtra
Source of Support: None, Conflict of Interest: None
Mitochondrial DNA (mtDNA), as its name implies, is an embodiment of the mitochondrial genetic information that constitutes about 1% of the mammalian genome. It fills a vital niche in tracing matrilineality; the mtDNA is inherited solely from the mother and plays a crucial role in genealogical research. Even a single mutation in the mtDNA can have debilitating and life-altering consequences. Mothers carrying mtDNA mutations will inevitably pass it on to the future generation. Three-parent In vitro fertilization (IVF), a breakthrough technique, shows promising potential to prevent mothers with mtDNA defects from passing it on to their future generation, while also maintaining the genetic link to their posterity. In this review, I delve into the intricacies of this technique, compare and analyze the difference between maternal spindle transfer and pronuclear transfer, discuss the prospective therapeutic effects, and highlight the ethical concerns surrounding this procedure. Considering the various challenges and ethics of this contentious technique, the paper seeks to answer the rousing question – Is three-parent IVF the answer to preventing mitochondrial defects?
Keywords: Maternal spindle transfer, mitochondrial donation, mitochondrial replacement therapy, pronuclear transfer, three-parent IVF
|How to cite this article:|
Shetty S. Is three-parent IVF the answer to preventing mitochondrial defects?. Biomed Res J 2021;8:9-13
| Introduction|| |
The mitochondrion is a vital eukaryotic cellular organelle that regulates cellular energy production through oxidative phosphorylation. It plays a pivotal role in energy metabolism and is most prevalent in tissues with high energy demand such as the liver, heart, muscle, and kidney. The mitochondrial DNA (mtDNA), comprising 37 genes, is the linchpin of the mitochondrial genome and is responsible for maintaining mitochondrial functions depending on the normal to mutated mtDNA ratio per cell. According to recent estimates, mtDNA disorders which can lead to severe morbidity, are more prevalent than has been previously appreciated. Another study suggests that one of the main reasons for such high mitochondrial mutation rate is decreased POLG (DNA polymerase gamma) fidelity; POLG is a nuclear genome-encoded product, which plays a key role in mitochondrial replication.
Mitochondrial disorders arise in two ways: through defects within the mitochondria found in the cytosol or through defective genes in the nucleus which play an integral role in maintaining mitochondrial functions. The incurable diseases arising from the former are maternally inherited. Although sperm also carries mtDNA, it is specifically removed by ubiquitination. The only treatment currently available for these defects is limited to the care and lifelong management of symptoms with diets and anticonvulsants for possible seizures, exercise, and supplements.
The tangled interaction between mitochondrial and nuclear genome makes it especially challenging to treat heterogeneous mtDNA mutations, which are so called as the same mutations lead to different phenotypic manifestations in different individuals. There is no curative treatment available that corrects the biochemical defects caused by these mutations. However, recent studies show that the possibility of giving birth to a child without passing on maternal mtDNA defects is drawing closer to reality. Fatal diseases could be prevented by either of the two novel three-parent IVF techniques: maternal spindle transfer (MST) and pronuclear transfer (PNT). While both the procedures are designed to prevent transmission of defective mtDNA, there are considerable germline, safety, and technical differences which will be discussed in this review. The review aims to encapsulate and provide a detailed analysis of three-parent IVF while also closely assessing the scope for future research, ethical questions, and concerns raised.
| Maternal Spindle Transfer versus Pronuclear Transfer|| |
MST uses assisted reproductive technologies to obtain eggs from the donor and intending mother to extract their chromosome. Although the mother's enucleated oocyte along with the donor's chromosomes is discarded, the intending mother's chromosomes are transferred to the donor's now enucleated oocyte. Subsequently, the reconstructed oocyte is fertilized in vitro and transferred to the intending mother's uterus.
PNT involves the creation of two zygotes in vitro [Figure 1]; one of them is created using the donated oocyte and the father's (sperm donor) sperm whereas the other is created using the intending parent's sperm and oocyte. The parent's pronuclei are removed within the first 24 h postfertilization. The pronuclei contained in the cell produced with the donor's oocyte and the enucleated cell produced with the intending mother's oocyte are subsequently discarded. Further, the maternal and paternal pronuclei are transferred to the cytoplasm of the donor's oocyte. The remodeled zygote is then transferred to the expecting mother's uterus. [Table 1] shows a comparative analysis between the two techniques.
|Table 1: Comparison between maternal spindle transfer and pronuclear transfer|
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One of the studies, investigating the feasibility of mtDNA donation using MST, showed normal euploid karyotype in the cell line and exclusive donor mtDNA. The significantly low levels of mtDNA carryover are one of the major findings of this study (<2%).
| Successful Cases of Three-Parent IVF|| |
Here, two of the major cases reported will be reviewed with details about the experimental procedure that was used, while also discussing the mtDNA profile seen in the fetus.
- In one of the first cases, pregnancy was achieved in a 30-year-old patient with unexplained infertility who had undergone two failed IVF cycles and had consistent embryo arrest at the two-celled stage. In her third cycle using the PNT technique [Figure 2], pronuclei from 80% of the oocytes fertilized were subsequently transferred to the donor's enucleated cytoplasm. The methods used were pronuclear karyotype fusion, DNA extraction, and mitochondrial genotype analysis. Half of the viable embryos were implanted into the intending mother's uterus which led to a triplet pregnancy. There was no trace of the intending mother's mtDNA in the fetus, whose mitochondrial profile was identical to the donor's. On the other hand, the DNA fingerprinting results, using restriction fragment length polymorphism, matched the mother's genetic fingerprinting, indicating that the maternal nuclear DNA had indeed been transferred to the fetus
- On April 16, 2016, a boy was born to a 33-year-old patient, who was a carrier of Leigh's disease (mitochondrial genome mutation 8993T), using oocyte spindle transfer. The patient had already lost a child to the disease and had suffered four pregnancy losses; she finally opted for spindle transfer which was preferred over PNT due to religious reasons (no destruction of the zygote). Most of the oocytes retrieved were degenerate, only 30% were metaphase II oocytes (had a polar body). The process of spindle transfer was performed using electrofusion, through which an opening was made in the zona pellucida and the spindle was aspirated using a pipette [Figure 3]. Postseparation, it was transferred to the enucleated donor oocyte cytoplasm. Motile sperm which had been frozen and thawed was then injected into the oocyte within 1 h of fusion. Furthermore, blastocyst biopsy, whole mtDNA sequencing analysis, and array comparative genomic hybridisation (aCGH) test showed that one of the blastocysts was euploid which eventually led to the birth of a boy at 37 weeks of gestation. The case strongly suggests that MST can be explored as a technique to prevent the transmission of mutant mtDNA to the offspring.
|Figure 2: Pronuclear transfer from the recipient oocyte into the cytoplasm of the donor oocyte. 2PN stands for 2 pronuclei|
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|Figure 3: (A) The patient's oocyte immobilized by a holding pipette and approached by a transfer pipette; (B) suction of spindle into biopsy pipette; (C) spindle removed from the patient's oocyte; (D) transfer of the removed spindle into the perivitelline space of the enucleated donor oocyte. Arrows indicate the spindle. Scale bar, 10 μm.|
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| Ethics and Concerns|| |
Although three-parent IVF is a promising breakthrough with a lot of scope for future research, there are some ethical arguments and concerns that need to be considered before this technique can be practiced globally. Currently, the UK is the only country to legalize and approve this procedure with tight regulations.
Mitochondrial DNA heteroplasmy
mtDNA heteroplasmy, which is the co-existence of two or more mtDNA variants within the same cell, and mtDNA overload or high mutant load of mtDNA in the cell are some of the major concerns revolving around this procedure. The adult human body comprises approximately 37.2 trillion cells; if the chances of survival increase with a specific mtDNA mutation, their proliferation or division could drastically raise the level of heteroplasmy. In one of the clinical studies, 13 embryos were examined from four patients for mtDNA heteroplasmy using mtDNA fingerprinting. They found that even though there was no nuclear inheritance from the donor, there was the presence of a small proportion of maternal mtDNA in the samples indicating heteroplasmy from both the donor as well as the intending mother which could replicate during fetal development. There are further concerns raised about the effect of mtDNA heteroplasmy on the completion of meiosis. Nuclear mitochondrial compatibility and its impact on mitochondrial function following this procedure also need to be investigated.
Unknown long-term effects
Considering this technique is still in the experimental phase, there are certain risks and challenges associated with its outcomes; For example, the chances of miscarriage and the fetus being born with unknown disorders is high. There is also uncertainty about the possible future ill effects in the baby born to this procedure since a combination of mtDNA from more than one origin can be harmful and lead to unforeseen diseases, due to which careful monitoring of reported cases is crucial. Several arguments raised the question of the benefit to harm scale of this technique. There are multiple options available for patients such as egg or embryo donation as well as adoption; these alternatives are considerably safer, more effective, and have a low-risk profile. Hence, the only reason for intending parents to opt for three-parent IVF is the desire to have a child with their own genetic link. However, this raises a question – Is the “wish” to have a genetic link to the child enough to disregard the risks and consequences of this controversial technique?
Disruption of maternal lineage
As mentioned before, mtDNA can only be passed from the mother to future generations. Tracing matrilineality using mtDNA analysis has been one of the prime interests of genealogical researchers, who focus on tracing back mammalian ancestry as a part of understanding an individual's identity and family tree. Evolutionary and anthropological research also uses mtDNA to understand the migration patterns and demographic behavior of humans. The three-parent IVF and the mitochondrial donation would derange this matriarchal lineage and put forward a very confusing picture while tracing back the ancestry. Although this may not seem like a pertinent concern, it could have some serious personal and cultural implications toward the acceptance of this technique.
Three-parent IVF is especially controversial because it involves germline modifications which will also be passed on to and thus influence the posterity. Most of the preclinical experiments and research are conducted on nonhuman primates as this technique involves the destruction of embryos which raises several ethical concerns. One of the advantages of MST over PNT is that it is performed using unfertilized oocytes and thus does not lead to the creation or destruction of an embryo. Since mtDNA is maternally inherited, female embryos born to this technique will be a case of germline intervention as their progeny will be free from the mitochondrial disorder but at the same time will also be prone to any ill effects caused as a result of this experimental procedure. Thus, this technique is usually recommended for producing male embryos as they will not cause any germline modification or influence the future generation.
| Is “Three-Parent” Babies a Good Description for Children Born to the Procedure?|| |
Terminologies play a very important role in how a procedure will be received and accepted by the public. Terms such as “genetic modification,” “germline intervention,” and “three-parent embryos” evoke concerns relating to the promotion and attitude toward the concept of designer babies. The biological role and involvement of the mitochondrial donor as a parent is often questioned. The genetic contribution coming from the donor is <0.1% of the total genome, thus only comprising an exiguous proportion of the child's genetic material. Further arguments question the role of mtDNA in shaping a person's personality and characteristics as compared to the nuclear DNA. Many close reproductive techniques such as surrogacy and oocyte donation involving three parents already exist; however, they are considered to have biparental origins. The difference here is that in this case, instead of simply providing assistance, the donor contributes to the mitochondrial genome of the offspring and there is a transmission of genes coming from three different sources. The UK government regulations do not allow the donor's identity to be revealed. Keeping in mind that the key contribution of mtDNA is oxidative phosphorylation and mitochondrial replication and also considering that it does not influence heritable physiological functions, should this small fraction of genes be considered a part of the individual's genetic identity? Is this enough support for one to label a baby born to this procedure as a three-parent baby?
| Conclusion and Future Research|| |
To condense the essence of this review – mitochondria, one of the most pivotal organelles of eukaryotic cells is responsible for the basic functioning and energy metabolism of organisms. It has its own set of mtDNA (endosymbiotic theory) which is directly responsible for carrying out mitochondrial functions. Although it is not as plenteous as the nuclear genome, even a single mutation in mtDNA could lead to some fatal defects which will be maternally transmitted to future generations. Limited treatment options focusing on alleviating the symptoms have been the only course of action available for decades.
The novel technology of three-parent IVF is the first of its kind to allow mothers with mtDNA defects to conceive without passing on these disorders to their children. The birth of the first healthy boy born to this technique by Zhang et al. opened a whole dimension of potential applications of this procedure, taking us one step closer toward the goal of rendering three-parent IVF as a safe and effective treatment. Having said that, this also sparks a polemic attitude among researchers and the public, similar to what they have toward the concept of designer babies. This technique allows one to create and destroy embryos that would not have existed otherwise; hence, several arguments question the ethics and consequences of playing “GOD.” If clinical studies can provide more backing to prove the necessity of this treatment, focus on a higher benefit to risk ratio, and battle its constant comparison to genetically modified designer babies, it could instill more interest and help scrap the concerns surrounding it.
The most striking aspect of this procedure is the involvement of transmission of genes from three sources which also leads us to wonder about the role of the donor in the baby's life.
Similar assisted reproductive techniques are considered to have a biparental origin. If more work could be done on understanding the role and impact of the donated mtDNA on future generations, one would get a better picture and evidence to determine if the contribution from the donor is sufficient to consider this treatment as tri parental.
Close analysis of reported and experimental cases needs to be done to understand the unforeseen consequences and possible long-term effects seen in babies born to this procedure. Since mtDNA is maternally inherited, giving birth to boys is not considered a germline intervention and won't have a consequential impact on future generations; more work needs to be focused on the long-term effects of giving birth to girls using this treatment. If we can find a way around these blockades, we will eventually find ourselves in a world where three-parent IVF is the ultimate key and answer to preventing inherited mitochondrial defects.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]