PROSPECTIVE parents at risk of passing inherited diseases to their children will be able to have embryos screened for almost any defective gene using a revolutionary new test developed by British scientists.

The technique, which is more accurate and efficient than current methods, dramatically expands the range of genetic mutations that can be detected before embryos are implanted in the womb, promising a “paradigm shift” in screening technology.

About 200 inherited defects, including Huntington’s disease and the commonest form of cystic fibrosis, can already be identified in embryos using a method called pre-implantation genetic diagnosis (PGD).

The procedure developed at Guy’s and St Thomas’ Hospital in London, known as pre-implantation genetic haplotyping (PGH), will work for thousands of other heritable conditions, most of which are too rare or complicated to be pinpointed by existing means.

It will also help families at risk of diseases that usually afflict only boys, such as Duchenne muscular dystrophy and haemophilia. At present screening for these “X-linked” disorders involves selecting only female embryos, but it will now be possible to look directly for the genetic errors that are responsible and use normal male embryos as well.

This will add to couples’ chances of having a baby, as 50 per cent more embryos will be suitable for use, but it could raise controversy over sex-selection. There will be occasional cases in which doctors have a choice of implanting embryos they know to be male or female for non-medical reasons, though scientists emphasised that this will be rare.

Also contentious is the test’s potential to eliminate inherited diseases from susceptible families. It can detect not only embryos that will become ill, but also carriers that will remain healthy but capable of passing rogue genes down to their own offspring.

Though doctors at St Thomas’ have no intention of using it in this way, the prospect of weeding out embryos that would grow into healthy people is new ethical ground.

Professor Peter Braude, who supervised the research team, said that the test would give thousands more couples who carry genetic illnesses a chance of having healthy children.

“We are very excited about this,” he said. “It is a paradigm shift, a big, big change.

“Because we don’t now have to know the precise details of every mutation we want to screen for, it opens the door for all sorts of disorders. It will make diagnosis more accurate, and improve success rates.”

The test has been licensed by the Human Fertilisation and Embryology Authority, and the St Thomas’ team has tried it seven times. They have five ongoing pregnancies, the most advanced of which is at 16 weeks.

Two of the pregnant patients carry a rare cystic fibrosis gene, two carry the Duchenne muscular dystrophy gene, and one has had hydatidiform mole, a condition in which the placenta becomes cancerous.

These results will be presented today at the European Society of Human Reproduction and Embryology conference in Prague, and are also published in the journal Reproductive Biomedicine Online.

The test gets around the limitations of the present technique in two ways. First, a new technique called multiple displacement analysis is used to expand the amount of DNA collected from the embryonic cell. This then enables scientists to scan not for the disease-causing mutation, but for a whole suite of genetic markers, or haplotype, that is inherited along with the rogue gene. This greatly broadens its scope to pick up rare DNA errors.

Pam Renwick, who led the research, said: “The great advantage of this test is that you can use it for all families, regardless of the mutation they carry in their genomes.”

With cystic fibrosis, for example, the St Thomas’ centre was previously able to screen embryos only when both prospective parents were carriers of the most common mutation that causes the lung disorder.

The team has plans to extend the technique to several other rare genetic conditions, including myotonic dystrophy, Prader-Willi syndrome and Angelman syndrome.

Alison Lashwood, consultant nurse at St Thomas’, said that one of the greatest benefits would be improved success rates. Very rarely, couples may produce multiple male and female embryos that are disease-free and of comparable quality, she said, though this had not happened yet. In this case, the clinic would offer the couple their choice of trying first for a boy or a girl, and the other embryos would be frozen.

“This isn’t about sex-selection, it is about having a healthy child,” she said. “Usually, the problem is getting one good quality, healthy embryo.”

Marita Pohlschmidt, director of research at the Muscular Dystrophy Campaign, said that the test would give couples who had opted for IVF more freedom. “[It] will hopefully increase the chance of success and reduce the strain on couples during this often intensely emotional time.”

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