DNA From a Third Parent May Help Babies Avoid Inherited mtDNA Disease

Mitochondrial donation through pronuclear transfer resulted in several live births and reduced transmission of pathogenic mitochondrial DNA (mtDNA) variants, researchers reported.

The study involved 22 women with pathogenic mtDNA variants who underwent an intracytoplasmic sperm injection procedure for pronuclear transfer to reduce the transmission of these variants to their children. This resulted in eight live births and one ongoing pregnancy, reported Louise A. Hyslop, PhD, of Newcastle Fertility Centre in England, and colleagues.

All babies had no signs of mtDNA disease. Heteroplasmy levels in the blood ranged from undetectable to 16%. Levels of the maternal pathogenic mtDNA variant were 95-100% lower in six newborns and 77-88% lower in two newborns compared with corresponding enucleated zygotes, they reported in the New England Journal of Medicine (NEJM).

“Mitochondrial donation by pronuclear transfer involves transplantation of nuclear genome from a fertilized egg from the affected woman to an enucleated fertilized egg donated by an unaffected woman,” Hyslop and co-authors explained. “Thus, pronuclear transfer offers affected women the potential to have a genetically related child with a reduced risk of mtDNA disease.”

In comparison, an additional 39 women in the study were offered preimplantation genetic testing (PGT), which resulted in 18 live births. Heteroplasmy levels were known for 10 of the 18 infants and ranged from undetectable to 7%.

“The reduced levels of heteroplasmy in infants born to women carrying homoplasmic variants provides grounds for optimism,” Hyslop’s team wrote. “However, until more is known about its efficacy, mitochondrial donation should be regarded as a risk-reduction strategy.”

In an accompanying editorial, Julie Steffann, MD, PhD, of Paris Cité University, noted that “Pathogenic, damaging variants in mtDNA are responsible for severe maternally inherited disorders with a high risk of transmission,” including MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes) syndrome, Leber’s hereditary optic neuropathy, and NARP (neuropathy, ataxia, and retinitis pigmentosa) syndrome.

While PGT is the standard way to reduce pathogenic mtDNA load, it’s not an option for women with high levels of mtDNA heteroplasmy or homoplasmic pathogenic mtDNA variants.

Accompanying editorial author Robin Lovell‑Badge, PhD, of the Francis Crick Institute in London, explained that “the approach involving PGT requires that at least one derived embryo has a sufficiently low level of heteroplasmy (in which mtDNA is a mix of two or more sequences) to justify intrauterine transplantation (i.e., derived progeny would have a substantially reduced risk of disease), an impossible outcome for women who have homoplasmy (in which all copies of mtDNA have the same sequence) or who have very high levels of heteroplasmy.”

However, the idea of mitochondrial replacement as a solution has been around for 30 years, he said.

Three main methods of mitochondrial replacement currently can be used “if we set aside the possibility of genome editing of mtDNA, which is currently too risky,” said Lovell-Badge. They include pronuclear transfer, which is carried out after fertilization and was used by Hyslop’s group, maternal-spindle transfer carried out between unfertilized oocytes, and polar-body transfer.

“Pronuclear transfer is a relatively established method, with substantial preclinical research,” he noted.

Although it’s hard to compare pronuclear transfer with PGT, given the differences in diseases and mtDNA variants among the women, having the group that underwent PGT as the comparison group “strengthens the conclusion that pronuclear transfer is a valid method to reduce the risk of mtDNA disease,” said Lovell‑Badge.

“Many of the outcomes assessed were nearly equivalent in the study groups, including similar percentages of patients who had a pregnancy, although the percentage of patients who had embryo loss was higher after pronuclear transfer than after PGT, for reasons that are unclear,” he pointed out. “The fact that eight apparently healthy babies were born after pronuclear transfer (without which mitochondrial disease would have developed in these babies) is a clear demonstration of the usefulness of this method among women for whom PGT is not an option.”

In the study, all eggs were fertilized by intracytoplasmic sperm injection. Pronuclear transfer was performed at 8-13 hours after intracytoplasmic sperm injection, and intact zygotes were cultured for 5-6 days.

For PGT, one blastomere was removed on day 3 after intracytoplasmic sperm injection for quantitative analysis of mtDNA heteroplasmy, and embryos were cultured for an additional 2-3 days. Suitable quality embryos were then used for fresh intrauterine transfer or vitrified for a future frozen embryo transfer. In PGT patients, embryos with less than 30% heteroplasmy were preferentially used for transfer or vitrification.

“Multidisciplinary advice regarding the optimal choice for an individual woman is tailored to the specific mtDNA variant and the woman’s own views on risk reduction,” the researchers advised.

In an NEJM brief report, Douglass M. Turnbull, PhD, of Newcastle University in England, and colleagues detailed the familial diseases in the cohort of eight live births. These included blindness, severe cardiomyopathy, and fatal cardiomyopathy or child death. Among the eight births, hyperlipidemia and cardiac arrhythmia developed in one child whose mother had hyperlipidemia during pregnancy, but both conditions were responsive to treatment. Infant myoclonic epilepsy developed in another child, who had spontaneous remission. At the time of publication, all children had made normal developmental progress.

Researchers said it will be key to follow these babies to monitor long-term health outcomes and extent of heteroplasmy of the offspring.