Spindle nuclear transfer involves the transfer of the nuclear spindle from 1 oocyte into 1 donor oocyte from which the nuclear DNA has been removed. The reconstituted oocyte is then fertilized using conventional in vitro fertilization (IVF). New details about the first infant born via this technique suggest a viable avenue for circumventing inherited mitochondrial diseases.
John Zhang, MD, PhD, of the New Hope Fertility Center in New York City, presented updates on the patient case during a late-breaking session at the Scientific Congress & Expo of the American Society for Reproductive Medicine (ASRM) held October 15 to 19, 2016, in Salt Lake City, UT. The ASRM announced the news of the infant in a statement released on September 27, 2016.
Mitochondrial diseases are the most common form of inherited metabolic disorders and account for many inherited neurologic disorders. Mutations in mitochondrial DNA (mtDNA) are maternally inherited, with offspring expressing both wild-type and mutated mtDNA. The clinical severity of mitochondrial disease correlates with the mtDNA mutation load, with more severe disease arising in individuals who express a higher proportion of mutated mtDNA. Various techniques for minimizing the transmission of mutated mtDNA from oocytes to preimplantation embryos have been examined, but spindle nuclear transfer is the first to result in a healthy live birth.
The parents pursued conception using spindle nuclear transfer and IVF after the loss of 4 previous pregnancies and the death of 2 children—1 aged 8 months and 1 aged 6 years—who were born with Leigh syndrome. Leigh syndrome is a devastating mitochondrial disease also known as subacute necrotizing encephalomyelopathy. The presence of Leigh syndrome was confirmed by a mutated mtDNA load of more than 95%.
Mitochondrial manipulation is currently banned in the United States, although the ASRM and other groups are working to lift restrictions in certain cases. Zhang and colleagues performed the procedure at a New Hope Fertility Center site in Guadalajara, Mexico.
The mother is a 36-year-old woman who carries the mtDNA 8993 T>G mutation characteristic of Leigh syndrome. Her mutated mtDNA load is 24.5%. The patient underwent standard oocyte retrieval, resulting in 5 morphologically normal oocytes. However, genetic testing with next-generation sequencing showed that all 5 had a mutated mtDNA load of nearly 100%.
The nuclear spindle was removed from each of the 5 maternal oocytes and transferred to donor oocytes whose spindles had also been removed (Table). All 5 oocytes were successfully reconstituted and fertilized via intracytoplasmic sperm injection with the father’s sperm. Four of the 5 fertilized eggs developed into blastocysts.
Table. Results From the Spindle Nuclear Transfer
|Mean oocytes retrieved, n||5|
|Oocytes reconstituted by spindle nuclear transfer, n||5|
|Oocytes fertilized, n (% of reconstituted oocytes)||4 (80)|
|Blastocysts, n (% of oocytes fertilized)||4 (100)|
|Euploid blastocyst, n (% of blastocysts)||1 (25)|
Preimplantation genetic screening revealed that 1 blastocyst was euploid (46XY) and 3 others were aneuploid. Additional testing of the euploid blastocyst showed that the total maternal mtDNA transmission rate was 5.10%, resulting in a mutated (8993 T>G) mtDNA load of 5.73%.
The euploid blastocyst was transferred, leading to an uneventful pregnancy and delivery at 37 weeks of gestation. Testing of multiple tissue samples (cheek swab, hair follicle, circumcised foreskin, urine precipitate) showed a mean expression of 1.6% of his mother’s mtDNA. Zhang said his research team expects that this level of maternal mtDNA expression is too low to cause any clinical manifestations of Leigh syndrome.
The infant is currently 6 months of age and appears to be healthy, Zhang said. Long-term follow up will be necessary to monitor his mtDNA expression and overall health.
“We are very pleased with the result. Mitochondrial diseases are inherited through the maternal system, and by transplanting a male embryo, the mutated gene will be eliminated from this family,” he said.
Future research will explore the effects of spindle nuclear transfer on mutated mtDNA load in female offspring, who may still carry the potential to pass on the mutated mtDNA.
“This work represents an important advancement in reproductive medicine,” said Owen K. Davis MD, president of the ASRM, in the September 27, 2016, press release. “If subsequent research determines the safety and efficacy of spindle nuclear transfer, we look forward to it being an option for patients who risk transmitting mitochondrial diseases to their children,” Davis concluded.
Source: Zhang J, Liu H, Luo S, et al. First live birth using human oocytes reconstituted by spindle nuclear transfer for mitochondrial DNA mutation causing Leigh syndrome. Presented at: American Society for Reproductive Medicine 2016 Scientific Congress & Expo; October 15-19, 2016; Salt Lake City, UT. Abstract O-267.