New blood test for fetal genetic disease shows promise

A new method for early detection of genetic diseases in unborn babies using a simple blood test can detect the inherited condition beta-thalassemia, according to a study published in Proceedings of the National Academy of Sciences. Although the technique, which  analyses cell free fetal DNA (cffDNA) present in the mother's blood, has previously been used for detecting Down's syndrome, sex determination in pregnancies at high risk of serious genetic disorders affecting a specific sex, and 'Rhesus testing' to help spot women whose babies have a blood type incompatible with theirs, this is the first time it has been possible to detect a single-gene disorder in the developing fetus.

So-called 'Non-invasive Prenatal Diagnosis' (NIPD) can be carried out much earlier in the pregnancy than conventional method and is considered less risky than techniques such as amniocentesis, which carries up to a 1 per cent risk of miscarriage and chorionic villus sampling (CVS), which carries up to a 2 per cent risk.

The researchers, based at the Chinese University of Hong Kong 'Diseases can be diagnosed by a simple blood test taken from the mother ... and by counting the relative ratio of the mutant genes against the normal genes', said lead researcher Dennis Lo at the Chinese University of Hong Kong, who led the research.

Diseases such as beta-thalassemia, which are caused by a change in a single gene, result when the baby inherits two copies of the altered gene, one from each parent. Until now the detection of so-called 'single-gene disorders' has proved to be challenging, because they involve gene changes which differ only slightly from the mother's genetic code, perhaps by only one or two base pairs.

The new technique uses digital DNA analysis from the blood to count the number of copies of a gene. Calculating the ratio of normal to faulty copies overcomes the difficulty of distinguishing between maternal and fetal copies of the gene involved in beta-thalassemia. The accuracy of the test, however, does depend on the concentration of cffDNA in the mothers blood. Around 5-10 per cent of a pregnant woman's blood plasma comes from the fetus and  contains fetal DNA.

Dr Lo cautioned that although the test works in principle, clinical trials are some years off as the process is still expensive and inefficient.

There are concerns from anti-abortion campaigners that advances in prenatal testing for genetic diseases could lead to a rise in terminations. However, Lyn Chitty, who studies fetal medicine at University College London, said: 'there may be more tests, but people don't just have a test to decide whether to have an abortion, sometimes it's just to prepare themselves'. Dr Lo said: 'This new test saves those babies who would be incidentally aborted by invasive testing through amniocentesis'.