DNA egg swap prevents rare diseases in babies

Sharon Bernardi, 44, with her son Edward, 20, who suffers from Leigh's disease, a form of mitochondrial disease.

LONDON (Reuters) - British scientists have mastered a controversial technique using cloning technology to prevent some incurable inherited diseases by swapping DNA between two fertilized human eggs.

Lead researcher Doug Turnbull of Newcastle University said on Wednesday he hoped the first babies free from so-called mitochondrial diseases would be born within three years.

But applying the technique in the clinic, to help women at risk of passing on the disorders, will require a change in British law that currently bans reproduction from such manipulated embryos, which would end up having three biological parents.

Around one in 6,500 children are born with serious diseases caused by malfunctioning mitochondrial DNA, leading to a range of conditions that can include fatal heart problems, liver failure, brain disorders, blindness and muscular weakness.

The Newcastle team's technique effectively replaces mitochondria, which act as tiny energy-generating batteries inside cells, so a baby doesn't inherit faults from its mother. Mitochondria are only passed down the maternal line.

"What we've done is like changing the battery on a laptop. The energy supply now works properly, but none of the information on the hard drive has been changed," Turnbull said.

"A child born using this method would have correctly functioning mitochondria, but in every other respect would get all their genetic information from their father and mother."

The researchers use a variation of the same technique used to make Dolly the cloned sheep in 1996.

Within a day of uniting egg and sperm using in vitro fertilization, nuclear DNA is removed from the embryo and implanted into a donor egg, whose own nucleus has been removed and discarded.

TWO OR THREE PARENTS?

The resulting embryo inherits nuclear DNA, or genes, from both its parents but mitochondrial DNA from a second "mother" who donated the healthy egg. In humans, about 37 genes are found in the mitochondria -- the rest of the more than 20,000 known genes are in the DNA found in the nucleus.

For critics like Josephine Quintavalle of campaign group Comment on Reproductive Ethics that makes it "a step too far in meddling with the building blocks of human life."

"No matter how small the contribution from the egg of the donor woman, the fact remains that an attempt is being made to create a three-parent child," she said.

But Alison Murdoch of the Newcastle Fertility Center, whose patients donated eggs used in the studies, told reporters such criticisms ignored the fact that all the characteristics of the baby would come from its two real parents.

Researchers in Newcastle first disclosed two years ago they had created a handful of embryos with swapped DNA, but it is only now that the process has been shown to produce viable embryos.

Writing in the journal Nature, the team said 80 embryos were created and developed in the laboratory for six to eight days to reach the blastocyst stage, comprising a ball of around 100 cells. They were then destroyed, in line with current rules.

(Editing by Tim Pearce)