Researchers have designed a method to diagnose Down syndrome prenatally using the mother’s blood. This minimizes risks dramatically, compared with traditional methods that require a giant needle to be inserted into the uterus or placenta.
People with Down syndrome have three copies of chromosome 21 – called trisomy-21 – rather than the usual two, causing mental impairment and heart defects. This is the world’s most common genetic condition, affecting about one in every 600 births.
Although certain markers in a mother's blood can tip off doctors that a fetus is at higher risk of Down syndrome, only invasive and expensive procedures such as amniocentesis – which requires poking a needle into the uterus to obtain a fluid sample – can give a 99% accurate answer [Science].
But these risky tests cause miscarriage in up to 2% of cases, so the diagnosis is only made available to high risk women, which fails to catch all cases.
The new test eliminates the risk of miscarriage, says study author Philippos Patsalis of the Cyprus Institute of Neurology and Genetics. "This method can apply to all pregnancies not only those at risk," he adds.
Instead of sequencing entire genomes, Patsalis and colleagues took a close look at the differences between ‘DNA methylation patterns’ of the mother and fetus. Science explains:
Here, chemical compounds called methyl groups attach to regions of DNA and, like a volume knob on a stereo, turn the activity of a gene up or down. Fetuses have methylation patterns that are different from their mothers', which allowed Patsalis's group to easily distinguish DNA from the two in the mother's blood.
They used these mom vs. fetus differences to detect extra copies of fetal chromosome in the mother’s blood sample. And it turns out that chromosome 21 has its own pattern of DNA methylation, helping the researchers find the extra copies more easily.
Among 80 women who were 11 to 14 weeks pregnant, the method was 100% accurate, using just standard equipment available in any basic diagnostic laboratory. The new test would take less than 5 days for results and won’t require specialized lab equipment or know-how, making it easy to introduce it into genetic diagnostic labs around the world.
The team has plans for larger-scale clinical trials involving 1,000 women on four continents in the coming months. The simplicity, low cost, and potential for spotting other genetic conditions makes them confident, Patsalis says, that the method "could be introduced in a clinical setting within 2 years."
The test can identify Down syndrome in the 11th week of pregnancy, early enough for a woman to end her pregnancy if she chooses, Patsalis says. This method could encourage more couples to take the test, he adds, and therefore slowly eradicate the disease.
The study was published in Nature Medicine this week.
Image: National Institute of General Medical Sciences, NIH
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