Wednesday 29 November, 2000
Detecting Down’s Syndrome
Scientists in Hong Kong say a simple blood test can show whether an unborn baby has Down’s Syndrome. The test could one day replace the standard method of testing for Down’s, which carries a small risk of causing a miscarriage. Corinne Podger of BBC Science reports.
What is Down’s Syndrome?
Down’s Syndrome affects one in approximately 700 new-borns, and is caused when a baby is born with an extra chromosome. Our cells normally have 23 pairs of chromosomes, which determine the characteristics we inherit from our parents. But babies with Down’s have an extra chromosome in some or all of their cells. The disorder often leads to learning disorders and a distinctive physical appearance.
Whilst there are three different types of Down’s Syndrome, 95% of people with Down’s have a type known as standard trisomy 21.The reason for the presence of an extra chromosome in this instance is unknown and can occur in any pregnancy.
A genetic anomaly called a translocation results in a small number of babies to be born with Down’s Syndrome. Approximately one in 100 people with Down’s have inherited the condition from their parents. The third type is also rare and is known, as mosaic Down’s Syndrome.
Testing
The standard method of testing for Down’s is amniocentesis, where a sample of fluid is taken from the mother’s womb. This is usually carried out during the fourth month of pregnancy and is an extremely accurate method of detecting the syndrome. However this test carries a one in 100 chance of the mother miscarrying.
But now, new research at the Chinese University of Hong Kong has found a simple, inexpensive blood test can be used to test for Down’s. According to recent findings documented in the medical journal The Lancet, scientists Leo L M Poon, Tse N Leung, Tze K Lau and Y M Dennis Lo have made advances in achieving
‘the longstanding aim in medical genetics in the isolation of safe and non-invasive prenatal genetic tests.’
Up until earlier this year, it was thought that cells from a foetus couldn’t be found in its mother’s bloodstream. But in May this year, scientists in Holland showed that a baby’s cells could be detected in blood plasma – a pale liquid that is one of the components of human blood. The Hong Kong researchers have taken this discovery a step further, by showing that a mother’s plasma contains enough foetal cells to test for Down’s Syndrome.
Writing in The Lancet, the scientists explain their investigation:
‘We investigated the use of fluorescence in-situ hybridisation (FISH) techniques on maternal plasma samples.
In plasma samples obtained from three women carrying foetuses affected by trisomy 21 (Down’s Syndrome), we identified foetal cells with three chromosome-21 signals.'
| These results show the feasibility of non-invasive detection of foetal chromosomal aneuploidy by maternal plasma analysis.’ | |
Age increases risk
The prospect of a safe, non-invasive test is expected to be welcomed, especially in Western countries, where the average age of first time mothers is increasing. For reasons, which are at present unclear, the chance of a woman giving birth to a child with Down’s Syndrome increase with her age. According to the Down’s Syndrome Association (DSA) this risk increases significantly after 35 years.
However pro-life groups are expected to oppose the new test, on the grounds that women may decide to abort children with the disability. Whilst the DSA stress the point that having a baby with Down’s Syndrome is not a reason to terminate a pregnancy, they do recognise that it is a decision for individuals to make and feel that, as with all forms of testing, it is vital that counselling is given with prenatal tests.
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| History of Down’s Syndrome |
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Dr John Langdon Down first described the condition in 1866; from then on the syndrome was associated with his name. Down was the superintendent of a children’s asylum in Surrey, England when he made the distinction between children who had been termed cretins (later to be found as having the condition hypothyroidism) and those that were known as Mongoloids.
The name Mongoloid derived from the belief that these children physically looked like people from Mongolia. In the 1960s this term came under scrutiny as it was thought to be an insult and the condition became formally known as Down’s Syndrome.
In 1959 Professor Jerome Lejeune and Patricia Jacobs independently proved that Down’s was caused by the triplication of the 21st chromosome – trisomy. The causes of the syndrome due to translocation and mosaicism quickly followed. |
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