Mitochondrial replacement therapy can prevent the transmission of mitochondrial diseases from mother to child. But some experts are urging caution
IN SEPTEMBER, using a technique called mitochondrial replacement therapy, the researchers combined DNA from two different women (mother which carried a fatal genetic defect in mitochondrial genome, healthy woman in terms of its mitochondrial genome) and one man (father) to bypass the defect and produce a healthy baby boy — one with three biological parents.
Mitochondria are the energy powerhouses inside our cells, and they carry their own DNA, separate from our nuclear genome. The procedure replaced defective mitochondrial DNA in the mother’s egg with healthy mitochondria from a donor woman’s egg, and the hybrid egg was then fertilized with the father’s sperm.
The team was forced to perform this method in Mexico, because the technique has not yet been approved in the United States.
The boy was an astonishing gift for the birth mother. A mutation in the mitochondria in her eggs causes Leigh Syndrome, a progressive neurological disorder, and over a period of twenty years, that mutation was linked to four miscarriages and the eventual death of her two children.
However, an analysis in Nature suggested that in roughly 15 percent of cases, mitochondrial replacement could fail and actually allow fatal defects to return, or even increase a child’s vulnerability to new ailments. It was tated that there was a potential for conflicts between transplanted and original mitochondrial genomes and pairing mothers whose mitochondria share genetic similarities, are needed to avoid potential tragedies.
The danger lies in the fact that mitochondria are in some ways like aliens inside our cells. Two billion years ago they were free-floating bacteria basking in the primordial soup. Then one such microbe merged with another free-floating bacterium, and over evolutionary time, the two formed a complete cell. The bacteria eventually evolved into mitochondria, migrating most of their genes to the nucleus and keeping just a few dozen, largely to help them produce energy.
Today, our nuclear genome contains around 20,000 genes, while a scant 37 genes reside in the mitochondria. And yet, the two genomes are intensely symbiotic: 99 percent of the proteins that mitochondria import are actually made in the nucleus.
Mitochondria also still divide and replicate like the bacteria they once were, and constant replication means that mutations arise 10 to 30 times more often than in nuclear genes. If too many mitochondria become dysfunctional, the entire cell suffers and serious health problems can result. That’s because when mitochondria falter, the bioenergetics of the cell itself is compromised.
A three-parent baby could solve the problem by overriding faulty mitochondria, but it also raises the stakes, because the procedure does not completely replace defective mitochondria with healthy ones. When the mother’s nucleus is transferred, it’s like a plant dug up out of ground — a bit of the original soil (in this case, the mother’s mitochondria) is still clinging to the roots. That creates a situation that never happens in nature: Two different mitochondrial genomes from two different women are forced to live inside the same cell.