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Last Updated: Friday, 2 February 2007, 01:05 GMT
Hope for safe prenatal gene test
The test may spot genetic abnormalities in the womb
Work by scientists could lead to a non-invasive blood test to detect genetic abnormalities such as Down's syndrome in the womb.

Current non-invasive tests - such as ultrasound - are limited, and invasive tests pose a risk to the pregnancy.

The new technique works by examining samples of foetal DNA present in the mother's blood for tiny variations in the sequence of the genetic material.

US firm Ravgen Inc's work is detailed online by the Lancet.

Experts say other approaches may also bear fruit.

Most people think one of these technologies will eventually be refined enough to be useful in clinical practice
Dr Lyn Chitty
University College London Hospital

Being able to identify genetic abnormalities at an early stage not only gives parents the chance to decide whether or not to proceed with the pregnancy, but also alerts medical staff to the need for close monitoring right through to birth.

Analysing foetal DNA from a mother's blood sample has been possible for some time.

However, its effectiveness has been limited because only a small amount of the DNA makes its way into the mother's blood.

Characteristic pattern

The Ravgen team, led by Ravinder Dhallan, has been able to maximise the amount of DNA that can be recovered by treating the blood samples with a chemical called formaldehyde.

The researchers took blood samples from 60 pregnant women.

They then searched the genetic material for variations known as single nucleotide polymorphisms (SNPs).

As each chromosome has a characteristic pattern of SNPs, the researchers were able to distinguish between DNA from the mother and the foetus.

They were also able to determine whether the foetus was carrying extra copies of key chromosomes which cause genetic disease.

For example, carrying an extra copy of chromosome 21 causes Down's syndrome (trisomy 21).

Of the 60 samples tested, the technique identified the number of chromosomes correctly in 58, including two cases of trisomy 21.

One case of trisomy 21 was not identified, and one normal sample was incorrectly identified as being trisomy 21.

However, the researchers stressed their work is at a preliminary stage, and further trials were needed to fine-tune the technique.

New era

Writing in the Lancet, Alexandra Benachi and Jean-Marc Costa, of Paris-Descartes University, said the test was promising, and "opens a new era in prenatal screening".

However, they added: "Some technical issues still need to be overcome.

"First, the amount of free foetal DNA in maternal blood is low, and although the use of formaldehyde allows an increased amount to be isolated from maternal blood, yields are irregular.

"Second, Dhallan assessed only eight women in the first trimester - further testing in this stage of pregnancy will be essential."

Dr Lyn Chitty, an expert in genetics and foetal medicine at University College London Hospital, said researchers were working on other methods to develop a non-invasive test.

These include examining proteins in the mother's blood, and epigenetics - the study of spontaneous changes to the foetal DNA.

She said: "Most people think one of these technologies will eventually be refined enough to be useful in clinical practice.

"An accurate non-invasive test would mean a lot women with normal babies would not have to be put through a procedure like amniocentesis."

Professor Maj Hulten, of the University of Warwick, founded a pan-European research network to work on new prenatal tests.

She said the study was very interesting, particularly as regards the formaldehyde fixation of the blood samples, and with further refinement a non-invasive test is likely soon to become a reality.

"I would be surprised if this approach alone turned out to be the one that wins the race to become clinically useful."


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