Babies born with Down syndrome, also known as trisomy 21, have an extra copy of all or part of chromosome 21, and so have 47 rather than 46 chromosomes. Women can be tested for this genetic signature during pregnancy, but these tests can often only take place after a certain date and can be risky to the baby. A new blood-based test would be simpler, cheaper and above all, less invasive, and may be on its way, according to two papers in the American Journal of Obstetrics & Gynecology.
Down syndrome and another chromosomal disorder, Edwards syndrome, or trisomy 18, in which the baby has an extra copy of chromosome 18, can be picked up from DNA circulating in the mother's blood using a type of genome sequencing called massively parallel shotgun sequencing (MPSS). Researchers at Aria Diagnostics have developed a cut-down version of this--Digital Analysis of Selected Regions (DANSR)--which only looks at the required chromosomes. This uses tenfold less DNA sequencing than MPSS. In Aria's study in the American Journal of Obstetrics & Gynecology, the researchers used data from DANSR and statistics (the FORTE algorithm) to work out a risk score and tested their technique on blood samples from women who were carrying babies with Down syndrome, Edwards syndrome or with no genetic changes. The test was accurate and specific.
In a second study in the American Journal of Obstetrics & Gynecology, a team of U.K. researchers asked Aria to analyze 400 blood samples from pregnant women, using the DANSR technology and FORTE algorithm. The results matched closely with those from chorionic villus sampling (CVS).
"Combining the DANSR assay with the FORTE algorithm provides a robust and accurate assessment of fetal trisomy risk," says Dr. Ken Song of Aria. "Because DANSR allows analysis of specific genomic regions, it could be potentially used to evaluate genetic conditions other than trisomy. The incorporation of additional risk information, such as from ultrasonography, into the FORTE algorithm warrants investigation."
According to The Sacramento Bee, Verinata Health is launching a test based on MPSS that will test for Down syndrome, Edwards syndrome and Patau syndrome. This will be available in March and will cost $1,200.
Down syndrome, named after a British physician, impairs cognitive and physical abilities, though this can range from mild cases, with people able to live and work independently, to severe cases, requiring full-time support. Testing for Down syndrome is through amniocentesis or CVS, which is invasive and expensive, and can increase the risk of miscarriage.
There are a lot of ethical issues tied up with the possibility of tests for Down syndrome. Can tests, even biomarker-based ones, ever be 100% accurate? Will they increase the rate of abortion, and is it right to choose which babies will live? Do the tests damage the perceived value of people with disabilities in the community, or do they just increase the options available, and help parents prepare for the birth of a child who is different? While the science is becoming cleared, the ethics will always remain hazy.
- read the press release
- see the abstract (Aria)
- see the abstract (U.K. team)
- check out the article in The Sacramento Bee