A new test allows doctors to diagnose genetic disorders in fetuses early in pregnancy by sequencing small amounts of fetal DNA in the mother’s blood.
Non-invasive prenatal tests are already available for chromosome disorders such as Down’s syndrome. Two years ago, a similar test was developed for recessive single-gene diseases, which occur when someone has two copies of a faulty gene. That test is particularly useful when one or both parents are known to be carriers for inherited diseases such as sickle cell anaemia, haemophilia or cystic fibrosis.
Now there is a test that looks at 30 genes associated with dominant genetic diseases, which occur when someone has just one copy of a faulty gene. These usually occur because of a mutation in the sperm, egg or embryo, and are more common when one of the parents is relatively old. Around one in 600 babies is born with one of these conditions.
The test will likely be used after an ultrasound has shown that the fetus has a skeletal abnormality, or it may be offered to couples in which the father is relatively old.
If it gives a positive result for a genetic disorder, women can be offered a more invasive test to confirm the diagnosis. If that test is positive, couples would ideally be offered genetic counselling so they can make informed decisions about how to manage the pregnancy.
Jinglan Zhang at Baylor College of Medicine, Texas, and colleagues tested blood samples from 422 pregnant women, from nine weeks of gestation onwards, sent to them from clinics in the US, Europe and Asia. Some of them had abnormal results on ultrasound scans suggesting a skeletal disorder in the fetus. Thirty-two tested positive for mutations in one of the 30 genes in the test.
For 147 of the participants, the team were able to obtain results from more invasive tests or postnatal tests. Out of these, all of the follow-up tests confirmed that the initial test from the mother’s blood was correct.
“The data so far show that this test is promising and accurate to detect the mutations it’s supposed to see,” says Zhang. However, they will need results from a much larger group of women to demonstrate that the test is clinically useful.
The test uses genetic sequencing technologies that are widespread in developed countries. However, genetic counselling specialists are not available everywhere. “It’s not the technology that’s difficult to implement in different hospitals; it’s the service around this test that’s equally important,” says Zhang.