By Rachel Hull, Biochemistry and Molecular Biology, ’19
“I decided to write this piece after stumbling across several news articles in October of this year heralding the birth of the first ever ‘three-parent baby’ and thinking to myself that something seemed to be missing in these stories. What started as some casual digging into the history of three-parent babies soon turned into a more general investigation of assisted reproductive technologies. The information I found was not only interesting but also very pertinent, as new advances could increase the role these technologies play in our society.“
Introduction and background information
Mutations in mitochondrial DNA (mtDNA) have been linked to diseases such as Kearns-Sayre syndrome, Leigh syndrome, and Pearson syndrome . In consideration of this fact, several techniques have been developed to either partially or completely replace mtDNA in embryos. One of these techniques, called spindle nuclear transfer (SNT), made headlines in September when news surfaced that five months earlier, it had been used to produce a healthy human baby . Though several publications called him the first ever “three-parent baby,” he is in fact not the only individual who technically has DNA from three people. He is merely the first baby to be born via SNT. Researchers have been dabbling with other mitochondrial replacement techniques for years, and in light of SNT’s recent debut into the public arena, an overview of these techniques seems more relevant than ever.
The field of mitochondrial replacement techniques began with an investigation of mouse zygotes in the 1990s. Ooplasmic transfer (OT), as the name implies, involves transplanting the cytoplasm from a donor to recipient oocyte, and early research showed that having these oocytes at the same developmental stage was key . In that same decade, clinical embryologist Jacques Cohen and colleagues began experimental trials at Saint Barnabas Medical Center with human patients for whom in vitro fertilization had not been successful. The team transferred 5-15% of donor cytoplasm, which contains healthy mitochondria, into recipient oocytes through either electrofusion or injection. The recipient oocytes were then fertilized . Cohen and his team performed this procedure 37 times between 1996 and 2001, resulting in the birth of 17 babies for 13 patients . In 2001, however, the Federal Drug Administration ruled that clinics performing OT on patients would require an Investigational New Drug Application, which was never approved .
Germinal vesicle transfer
In germinal vesicle transfer (GVT), another mitochondrial replacement technique, the oocyte nucleus — also known as the germinal vesicle (GV) — is removed from a donor’s oocyte and transferred into a recipient’s enucleated oocyte. The reconstructed oocyte is then matured in vitro and, finally, fertilized. This technique has primarily attracted scientists looking for a way to overcome aneuploidy, or an abnormal number of chromosomes, which is associated with aberrant meiotic spindle morphology in oocytes and is a major cause of infertility in older women. The potential applications for GVT with relation to mitochondrial diseases are clear. Since the donor’s oocytes have normal nuclear DNA and the recipient’s oocytes have normal mtDNA, this technique could theoretically produce a healthy baby. One significant concern, however, is that mutated mitochondria adjacent to the GV in the patient’s oocytes would likely be carried over to the reconstructed oocytes. To reduce this risk, new methods must be developed to remove the donor’s mtDNA before performing GVT .
Pronuclear transfer (PNT) dates back to 1983. This technique begins with the removal of the karyoplast, which consists of pronuclei and a small volume of membrane-enclosed cytoplasm, from a patient’s zygote. The karyoplast is then inserted into and fused with a recipient’s enucleated zygote using an inactivated Sendai virus or electric pulse . Like GVT, however, PNT still holds the risk that mutated mtDNA from the donor could be carried over to the recipient ; a recent study suggests that even 1% carry-over could have disastrous consequences . In addition, the use of zygotes rather than unfertilized oocytes raises ethical concerns that must be addressed if this technique is to be widely used .
Spindle nuclear transfer
SNT garnered public attention in September when it came out that five months earlier, it had been used for the first time to facilitate the birth of a baby boy. The boy’s mother carries a mutation that causes the neurodegenerative disorder Leigh syndrome. She and her husband thus contacted Dr. John Zhang, a fertility doctor at New Hope Fertility Center, looking for a solution [2, 11]. What he proposed was SNT, a specifically timed technique performed when a patient’s oocyte is at the metaphase stage of meiosis II, when the chromosomes are lined up across the middle of the cell held in place by the spindle. At this time, the nuclear genome is removed from the patient’s oocyte and inserted into a recipient’s enucleated oocyte, then fertilized . Because the patient’s oocyte chromosomes are not surrounded by a definitive nuclear membrane, specialized techniques are needed to visualize the spindle and chromosomes during manipulation. The intense precision required for SNT is a double-edged sword: it minimizes mtDNA carry-over, but is also extremely technically demanding [12, 13].
Conclusion and future outlook
Alas, the boy born in April 2016 is not the first ever “three-parent baby,” as he is not the first human to have been born from a mitochondrial replacement technique. In addition, some have pointed out that since mtDNA makes up just 0.1% of a person’s overall genome and has no effect on identity, it would perhaps be erroneous to even say that a baby produced through SNT has three parents [14, 15]. The news of this baby’s birth has generated more questions than answers, raising debate about the ethics, safety, and efficacy of not only SNT, but the other techniques as well. These methods each show promise in some way. More advances must be made, however, before any of them can be unquestionably viewed as a viable way to prevent the transmission of mitochondrial diseases.
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