Diagnostics: DNA to deliver safer pregnancies

Technology to analyse tiny traces of DNA is now so sensitive that it can diagnose disease by detecting rogue mutations circulating in human blood. The most important future application of "cell-free DNA diagnosis", as it is called, is likely to be in oncology - detecting cancer genes shed by tumours elsewhere in the body - but an early commercial use of the technique is in prenatal diagnosis of genetic disorders in the foetus.

Premaitha Health, based in Manchester, is the first European company to offer non-invasive prenatal testing of this sort. The test is much more accurate than the current technique, which requires sampling of foetal DNA from the womb or placenta, and which produces many false positive results.

Premaitha's new Iona test detects Down's syndrome (resulting from an extra copy of chromosome 21) as well as the much rarer Patau and Edwards' syndromes (an extra chromosome 13 and 18 respectively). Like similar tests developed in the US and China, it depends on the fact that some foetal DNA leaks from the placenta into the maternal bloodstream.

Last month Premaitha announced its first National Health Service contract in the UK, with St George's NHS trust in southwest London, as well as deals in continental Europe. "This is great news for the trust and for pregnant women in the UK," says Basky Thilaganathan, consultant obstetrician at St George's. "Non-invasive pregnancy testing has the potential to make a huge difference to the care and reassurance that clinicians can offer to pregnant women."

For the past 20 years or so, screening for Down's syndrome has relied on the "combined test". This uses an ultrasound examination plus a maternal blood test at around the 11th week of pregnancy to measure levels of certain proteins that tend to be raised in cases of foetal abnormality. But these are not very reliable indicators, and a positive result has to be confirmed by an invasive test - sampling amniotic fluid from the womb or "chorionic villus" cells from the placenta - which poses a significant risk to the pregnancy.

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< > Typically, about 15 per cent of DNA circulating in the blood of a pregnant woman comes from the foetus. The new test doesn't need to distinguish this from maternal DNA. Rather, it analyses all DNA in the blood sample and looks for any increases in the genetic material associated with a particular chromosome.

Down's syndrome in the foetus raises the proportion of DNA from chromosome 21 from 1.3 per cent to 1.4 per cent of total DNA in the sample, says Robert Old, molecular biologist at the University of Warwick. This small increase can be measured reliably with the latest technology. Moreover, the results are available within two days, which is significantly quicker than the current combined test.

The new technology is more expensive than its predecessor but its accuracy and speed justify its introduction across the NHS, says Thilaganathan. How quickly this happens will depend partly on the UK National Screening Committee, which is due to assess DNA-based antenatal testing in June.

Meanwhile, Premaitha's chief executive Stephen Little is looking ahead to cell-free DNA diagnosis in oncology, a far larger market. "Some tumours shed a lot of cancer DNA into the bloodstream," he says. "The first applications will probably be to follow the progress of existing, known tumours without the need for biopsies, rather than to diagnose new cases."

Photograph: Fetal Medicine Unit, St. George's Hospital NHS Foundation Trust

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