Three-Way Parenting

Three-way parenting, in the embryologic sense, is a topic currently in the news due to the recent birth of a male baby, conceived through assisted reproductive techniques, who has inherited DNA from his two parents as well as a third party, an anonymous egg donor.

The DNA inherited from his parents is nuclear DNA, while the DNA he inherited from the egg donor is mitochondrial DNA.

  • Nuclear DNA accounts for the overwhelming majority of genes we possess.
  • Mitochondria are “organelles” within the cytoplasm of all cells that allow the cell to conduct proper metabolism and energy creation.

The child’s mother is an asymptomatic “carrier” for Leigh Syndrome, a severe and fatal neurologic disorder that took the lives of her two other children. The disorder stems from having dysfunctional mitochondria whose DNA carries mutations. Scientists have long realized that getting normal mitochondria into the doomed cells would hold the potential to, in essence, cure the disease.

Origins of Three-Way Parenting

Three-way parenting is actually not new. In 2001, embryologists in Livingston, New Jersey reported on the birth of a baby born possessing DNA from three parents. Their technique, called “cytoplasmic transfer,” involved injecting cytoplasm from donor oocytes into the fertilized oocyte of the would-be mother. This “younger” cytoplasm, with its likely healthier component of mitochondria, was presumed to help overcome the natural negative effects that aging tends to have on oocyte mitochondria, which are prone to mutations as time goes by. It was, essentially, designed to help overcome age-associated oocyte quality issues. At the time, the scientists were criticized for advancing a technique without fully understanding its possible negative ramifications.

Three-Way Parenting Success

As time passed further experimentation suggested the technique held promise, but was perhaps best restricted to helping parents overcome the transmission of mitochondrial genetic disease to their offspring. The method currently approved for use in the United Kingdom is called pronuclear transfer and involves the following:

  • Both the mother’s egg and a donor’s egg is fertilized with the father’s sperm.
  • Before the fertilized eggs start dividing into early-stage embryos, each nucleus is removed.
  • The nucleus from the donor’s fertilized egg is discarded and replaced by that from the mother’s fertilized egg.

The resulting embryo has all of the mother’s nuclear DNA, but none of her mitochondrial DNA that resides exclusively in the cytoplasm. Presumably, the donor cytoplasm contains healthy mitochondria allowing the embryo, and ultimately baby, to thrive. This three-way parenting technique has indeed been applied successfully.

Using Three-Way Parenting to Create One, And Only One, Embryo

The latest three-way parenting technique was designed to adhere to the religious convictions of parents who wished to avoid creating embryos that would later be discarded. Called spindle nuclear transfer, this new technique enabled doctors to remove the nucleus from one of the mother’s eggs and insert it into a donor egg that had had its own nucleus removed. The resulting egg – with nuclear DNA from the mother and mitochondrial DNA from a donor – was then fertilized with the father’s sperm. Hence only one embryo was made.

The hope is that the child born from this technique will grow to be healthy and show no symptoms of Leigh Syndrome. It would appear this will be the case given that the baby’s cells have been analyzed and found to contain a high percentage of “donor,” hence, normal mitochondria.

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