IVF breakthrough modifies genes

Researchers in the US have created children with a genetic modification which could be passed on to future generations - so-called germline engineering, illegal in Britain.

The nearly 30 children, born as a result of IVF treatment for infertile women, effectively have three parents. They are said to be healthy.

The technique is less startling than might at first appear since it does not involve genetically modifying any of the estimated 30,000 genes in the nucleus of the children's cells, the DNA collectively referred to as the human genome.

Researchers were focusing on another, much smaller set of 27 genes in a component of the cell called the mitochondrion, which produces a chemical called ATP, essential to life.

In normal human reproduction, the human embryo is created when a man's sperm carries its 23 gene-bearing chromosomes into a woman's egg, where it combines with her 23 chromosomes to make up the full set of genes.

The mitochondria are in another part of the egg, and are passed down, complete, from a mother to her children.

In the infertile women being treated in the US, scientists believed defects in their eggs could best be treated by adding part of a donor egg. This had the side-effect of introducing the donor's mitochondria into the cells of the embryo.

In effect, the children have two mothers and one father, although the vast bulk of their genes come from their "natural" parents.

The research, at the Institute for Reproductive Medicine and Science at St Barnabas in New Jersey, shows how the desire of infertile men and women to have children is stretching the boundaries of the science and ethics of human life.

The children have two sets of mitochondrial DNA (mtDNA) in every cell of their bodies. It is not clear whether this will affect their health, or what mitochondria they will pass on to their offspring.

The main US genetics research watchdog did not step in to decide whether the research should proceed because nucleic DNA was not involved.

This decision was criticised by the journal Science, which warned that it was wrong to assume mtDNA did not affect the wider workings of the human mind and body.

The birth of Emma, the first child to be born through this technique, was reported in the medical press in 1997, but few realised the significance.

In March the St Barnabas researchers reported in the journal Human Reproduction that they had checked the mtDNA in the blood of two of the nearly 30 children born worldwide using the technique. They found that the children, both aged one, did indeed have mtDNA from two "mothers". "This report is the first case of human germline genetic modification resulting in normal healthy children," they wrote.