13 October 2008
ByAppeared in BioNews 479
Scientists at Stanford University in California, US have developed a new non-invasive prenatal test for Down's syndrome. Stephen Quake and colleagues successfully identified presence and absence of fetal chromosomal abnormality in 18 pregnant women from maternal blood samples. The work is published this month in the journal Proceedings of the National Academy of Sciences.
Currently, the two techniques available for prenatal diagnosis of Down's syndrome are both invasive and carry a risk of miscarriage. CVS (chorionic villus sampling) and amniocentesis carry a two per cent and one per cent chance of miscarriage respectively. Dr Quake's procedure does not bring any risk of miscarriage and can also be performed at an earlier point in pregnancy (a few weeks as opposed to ten and 15 weeks for CVS and amniocentesis). Earlier diagnosis of Down's syndrome would also allow parents more time to think about test results - especially important if they are considering terminating an affected pregnancy.
As well as being less invasive and accurate at an earlier stage in pregnancy, Dr Quake's test would also be simpler and less expensive to perform than test currently available. Dr Lyn Chitty, an expert in genetics and fetal medicine at University College London Hospital, said: 'This is a potentially exciting development which may take us closer to a safer, non-invasive test for Down's syndrome'. Earlier prenatal tests would also improve the management of treatable conditions such as congenital adrenal hyperplasia (CAH), which causes the genitalia of female fetuses to develop abnormally. CAH can easily be treated with steroids during pregnancy if diagnosed.
Down's syndrome occurs when a fetus has three copies of chromosome 21 instead of two. Dr Quake's method uses 'shotgun sequencing' to sequence short fragments of 'cell-free' fetal DNA present in the mother's blood. The fragments can then be matched to specific chromosomes and the proportion from chromosome 21 determined. It is still early days, however, and Dr Quake is keen to run larger studies to verify these exciting initial findings.
An alternative method which relies on determining if equal amounts of chromosome 21 come from each parent is already being commercialised in the US. However, that method, developed by Dr Dennis Lo of the Chinese University in Hong Kong, is optimised for the US population, which has a different ethnic structure to other countries, so the current test may not work as effectively outside the US. It also relies upon having the father present and available for testing as well as the mother. Dr Quake's test does not suffer from these limitations, 'Our approach works on 100% of the population, independent of ethnicity', he said.