Therapeutic Cloning Takes A Big Step Forward

Leber’s Hereditary Optic Neuropathy
Leigh Syndrome
Myoneurogenic Gastrointestinal Encephalopathy

What do these disorders have in common? They are all mitochondrial diseases.



What is a mitochondrial disease? Mitochondria are the parts of a cell that are responsible for generating energy. Mitochondria contain DNA, just like the nucleus does. Mutations in mitochondrial DNA can cause them not to function correctly, and some of these mutations cause the diseases listed above (and others).

When a baby is conceived, we think of the baby’s DNA as coming half from the mother and half from the father. That is true for the DNA in the nucleus, but not true for mitochondrial DNA. All mitochondrial DNA is inherited from your mother.

As a side note, the fact that all mitochondrial DNA is inherited from the mother is the basis for anthropological studies that allow scientists to trace the origins of humanity back to “Mitochondrial Eve” in Africa.


Since all mitochondrial DNA is inherited from the mother, replacing mitochondrial DNA that carries a disease-causing mutation with mitochondrial DNA from another source would prevent a woman from passing on such a disease to her children.

That is the basis for the work by Shoukhrat Mitalipov’s group at the Oregon Health and Science University published online in Nature on August 26. This group took an egg from a female monkey and removed the nucleus, replacing it with the nucleus from a different female. This hybrid egg, with nuclear DNA from one female and mitochondrial DNA from another female, was fertilized, and the resulting embryo was allowed to develop. Not every embryo developed normally, but some developed into seemingly normal monkeys like the ones shown above.

This approach would allow a couple with a family history of mitochondrial disease on the mother’s side to have children with mitochondria donated by an unrelated woman, and they could have children with no risk of developing the disease.

The monkeys described in the paper have genetic contributions from 3 adults. Does that mean there are three parents? I guess that depends on how you define “parent.” If you are a parent simply by virtue of having contributed DNA to a child, then yes, these monkeys have 3 parents. Of course, if a parent is defined as the adults who raise you, then a child conceived in this way would have 2 parents. The ones who raise the child.

Experiments like these raise huge ethical issues. Therapeutic cloning like this introduces changes directly into the germline (the DNA that is passed from parent to offspring), something that has long been taboo in the mainstream scientific community. It also raises interesting custody issues (does the donor of the mitochondrial DNA have any parental rights?). I’d be interested to hear what people think about this.

Like it or not, we are on the verge of a new era of genetic medicine. Some aspects of genetic medicine will not be controversial (tailoring medical treatments based on the presence of specific mutations), but other aspects are sure to raise questions (like the experiments described in this paper). It is critical that we begin to have serious, society-wide discussions about these issues, before it is too late and the discussions become arguments.

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