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Non-invasive pregnancy test shows promise

6 February 2007
Appeared in BioNews 394

A new study offers 'proof of principle' for a non-invasive pregnancy test that can detect conditions such as Down syndrome using samples of the mother's blood. The findings, published online in the Lancet journal, show that the technique correctly diagnosed 58 out of 60 samples, with one case of Down syndrome going undetected and a normal sample incorrectly diagnosed as affected. Dr Ravinder Dhallan, chief executive of Ravgen Inc, which carried out the work, said that 'next we will do a larger study and start building the infrastructure to go to market'.

Researchers have been trying for some time to develop new techniques for carrying out genetic tests during pregnancy, to replace current invasive methods such as amniocentesis. The new methods rely on tiny amounts of 'free fetal DNA' found in the mother's blood. A small number of cells from the fetus always find their way into the mother's bloodstream, where they are attacked by their mother's immune system. The fetal cells break apart, releasing their DNA.

One of the challenges facing researchers has been that when a sample of the mother's blood is taken, her cells burst, releasing large amounts of maternal DNA that swamp the tiny amounts of fetal DNA present. In earlier work, Ravgen showed that they could potentially overcome this problem by adding formaldehyde to the blood sample, causing the mother's cells to harden 'like ping pong balls'. This increased the proportion of fetal DNA in the sample to around 25 per cent of the total. But the problem of distinguishing between the fetal and maternal DNA remained.

In the latest study, the researchers used a battery of 'SNPs' (single nucleotide polymorphisms) - single DNA 'letters' that vary between individuals - to study the chromosomes of the mother, father and fetus. Since Down syndrome is caused by an extra copy of chromosome 21, its presence can, in theory, be detected by a difference in the ratio of maternal and fetal SNPs. That ratio should be the same for all chromosomes, says Dhallan, adding 'if it's not, you have a problem'.

Dr Michael Katz, of US charity the March of Dimes, called the test 'a good system if it can be sustained by more extensive tests. It also dovetails with the current suggestion by the American College of Obstetricians and Gynecologists that prenatal diagnosis should be applied to all women'. However, he cautioned that 'they have one mistake one way and one the other way. That's a little too risky if you compare this with invasive techniques where you have virtually 100 per cent accuracy. Also, one can never be absolutely certain that this is the biological father. In addition, there may be some times when the father is not available'.

SOURCES & REFERENCES
Accurate, Safe Prenatal test for Birth Defects
HealthNews-Stat |  2 February 2007
Experimental Prenatal Test Helps Spot Birth Defects
The Washington Post |  2 February 2007
Non-invasive Down's syndrome test shows promise
New Scientist |  2 February 2007
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