Noninvasive Blood Test Could Accurately Predict Down Syndrome
One of the more difficult parts of a woman’s pregnancy is facing the results of genetic testing for conditions such as Down syndrome. Currently, a mother-to-be has a blood test that detects hormone markers that indicate a risk for such conditions and those at high risk are advised to undergo an invasive procedure such as chorionic villus sampling or amniocentesis, both of which carry risks. Chinese researchers have now developed a test that can more accurately detect trisomy 21 in a fetus, reducing referrals for more invasive testing.
Blood Test Detects Down Syndrome in over 96% of Cases
Down syndrome is the most common cause of human birth defects, occurring in about 1 in every 291 births. Children with the disorder have three copies of chromosome 21 (trisomy 21), where healthy children only have two. The extra chromosome cause problems with the way the body and brain develop, leaving children to have a disproportionally shaped head and facial features, smaller stature, decreased muscle tone at birth, and delayed mental and social development.
Children with Down syndrome also may have accompanying medical conditions such as heart defects, eye problems such as cataracts, sleep apnea, and underactive thyroid.
Although the risk of having a child with Down increases with family history (genetics) and maternal age, the American College of Obstetricians and Gynecologists recommends offering Down syndrome screening tests to all women. Screening begins in the first trimester when blood tests are drawn to measure the amount of two substances: pregnancy-associated plasma protein and human chorionic gonadotropin (hCG). Abnormal levels indicate an increased risk for chromosome abnormality.
Second trimester screenings occur between the 15th and 20th weeks of pregnancy and looks for markers such as alpha-fetoprotein (AFP). Abnormal levels of this protein may signal Down syndrome or another birth defect such as spina bifida.
Abnormal results of any of the above typically results in additional testing such as chorionic villus sampling (CVS) or amniocentesis. Both procedures carry about a 1% risk of miscarriage.
The newest blood test, however, does not rely on risk markers, but on maternal plasma DNA sequencing. The method involves testing the blood for free DNA sequences from chromosome 21. Between 10 and 20% of free DNA sequences in a pregnant woman are from the fetus, so an elevated proportion on chromosome 21 would indicate Down syndrome.
Dr. Dennis Lo MD of the Chinese University of Hong Kong and colleagues tested the method in 753 high-risk pregnancies. Using two-plex sequencing, which is more detailed, the genetic test found all 86 cases of Down syndrome. A slightly less sensitive approach called eight-plex sequencing detected 68 of the trisomy 21 cases.
Both tests did have false positives – 3 cases for the two-plex sequences and 6 for the eight-plex method. However, the existing tests have a 5% false positive rate.
If referrals for amniocentesis or chorionic villi sampling were based on the two-plex sequencing test results, Lo and colleagues argued, 98% of the invasive diagnostic procedures could be avoided. It may also have value in maternal screening of low-risk pregnancies one day, but currently the method is too costly ($2000 per test) for widespread screening.
Chiu RWK, et al "Non-invasive prenatal assessment of trisomy 21 by multiplexed maternal plasma DNA sequencing: large scale validity study." BMJ 2011; 342: c7401.