A baby with DNA from three parents has been born using a technique known as spindle transfer in Mexico: the first child conceived this way. The child in question contains the usual genomic DNA from his biological parents, as well as mitochondrial DNA (mtDNA) from an unrelated donor; a treatment termed mitochondrial replacement therapy (MRT).
Mitochondria have long been known to contain DNA separate from genomic DNA in the nucleus. The organelle is believed to have once been a separate bacterium, which became engulfed by nucleated cells, creating a symbiotic relationship where the bacteria provides energy for the cell, while the cell provides a protective environment. However, mitochondria are also many times more likely to incur mutations, which are then possibly inherited by offspring from the mother.
Methods of Mitochondrial Donation
During spindle transfer, the nucleus of the mother’s oocyte (egg cell) is transferred to a donor egg cell with healthy mitochondria and its nucleus removed. The father’s sperm is then used to fertilize the hybrid cell.
Alternatively during pronuclear transfer, the oocyte, and the sperm are allowed to fertilize and then inserted as pronuclei in a healthy donor zygote, which has had its nucleus removed.
Jacques Cohen: Cytoplasm Transfer
Three parent babies have been born in the past. During the 90’s Jacques Cohen used a different technique called cytoplasm transfer; injecting unhealthy oocytes with cytoplasm from healthy eggs, creating a mix of functional and faulty mitochondria. Seventeen babies were born after thirty attempts. However, two fetuses lacked an X chromosome and subsequently died, while there was also a case of one miscarriage and one abortion.
Despite some failures, many of the children from Cohen’s trial have reportedly grown up to be healthy teenagers, without any apparent health problems.
Mitochondrial Donation: The Future?
So far the six-month-old boy has not encountered any problems, and he is under close observation. The team lead, John Zhang, is due to present his results on 19 October at the American Society for Reproductive Medicine meeting in Salt Lake City.
As it stands, this method of mitochondrial donation is a monumental breakthrough for parents failing to conceive healthy children, as a result of faulty mtDNA inherited from the mother. In this instance, the mother was a carrier of Leigh’s Syndrome; a neurological disorder, (caused by mitochondrial DNA mutations in 20 % of sufferers), which can lead to death three years after birth.
In the past, the couple had lost four pregnancies and two early deaths; spindle transfer offers them and many other couples new hope of conceiving a child. Male embryos are generally preferred in the process, as they will not pass on their mitochondrial DNA, avoiding any possible complications in future children.