For decades doctors and pregnant women have relied on amniocentesis – taking a sample of DNA from the amniotic fluid – to diagnose certain disorders in unborn babies, despite the risk of triggering a miscarriage.
Now scientists in Hong Kong have made a groundbreaking discovery which will make it possible for doctors to detect a whole range of genetic abnormalities in foetuses by simply testing the mother’s blood.The team at the Chinese University of Hong Kong have become the first in the world to map out the entire genome – the DNA sequence – of an unborn baby from a sample of blood taken from a woman 12 weeks pregnant.
The discovery opens the door to the development of a maternal blood test capable of detecting multiple genetic disorders which could replace more invasive and potentially risky tests such as amniocentesis.
Previously, it was not known whether the complete DNA of the baby was present in the mother’s blood, and tests could only look for one disorder at a time.
The discovery was “a quantum leap,” said the team of scientists, who were also responsible for first discovering the presence of foetal DNA in maternal blood in 1997.
Dennis Lo Yuk-ming, who led the research, said the process had been extremely challenging and likened it to assembling a million-piece jigsaw puzzle.
Foetal DNA is now known to be carried into the mother’s blood in cells via the placenta, but the difficulty lies in isolating the baby’s DNA from the mother’s.
To do so, the scientists developed a method of detecting first the fragments of the baby’s DNA inherited from the father, and then picking up the rest by detecting variations in the concentration of mother’s genes.
“This is similar to adding tens of millions of pieces from another jigsaw puzzle and then trying to reassemble the first one,” Lo said.
By piecing together the chromosomes inherited from each parent, the scientists were able to complete the puzzle and reconstruct the baby’s genome.
Lo said potentially this procedure could be used to detect a range of genetic disorders such as alpha and beta thelassaemia – forms of anaemia – and cystic fibrosis as early as nine weeks into the pregnancy.
Amniocentesis, the most common current method for isolating foetal DNA, involves taking a sample by needle from the amniotic fluid which surrounds the baby, usually between the 15th and 20th week of pregnancy.
The procedure carries a 1 per cent risk of miscarriage compared to no additional risk of miscarriage from a maternal blood test.
However, Lo said the blood test would be very expensive and it had cost them 1.5 million Hong Kong dollars ($190,000) to map out the entire genome.
“The next step will be to work on how to bring the cost down. I think we could probably get it to 10,000 to 20,000 Hong Kong dollars per test if we just detected genes which are mutated in particular populations,” Lo said.
This would involve only reconstructing parts of the genome suspected of carrying specific disorders in certain geographic areas or populations.
“This is called targeted sequencing rather than the random sequencing we used in the case we reported,” Lo said. “It will be like doing just the corner of that million-piece jigsaw, rather than the whole thing.”
