Ravgen Develops Non-Invasive Fetal DNA Test for Down’s Syndrome
Biotech company Ravgen has published a study in the Lancet that describes their technique for detecting fetal chromosome 21 in maternal blood, excess copies of which indicate Down’s syndrome.
- The unique SNP signature of fetal DNA is determined by comparing the mother’s genotype at various loci with the fathers’ and seeing where the fetus’s genotype differs from both parents.
- A comparison of average allele ratios for chromsome 13 is carried out which establishes a “normal” baseline for comparison.
- Average allele ratios are calculated for chromsome 21. If the ratio differs from that for chromsome 13, the fetus is presumed to have Down’s syndrome.
For each specific SNP examined, the parents each have to be homozygous and different from each other so that the fetus would get one of each, e.g., father has TT, mother has GG, and baby has TG. In an analysis of the mother’s blood sample containing fetal DNA, there would normally be one copy of T from the fetus, and three copies of G (2 from mom and 1 from fetus) so the ratio would be 1 T to 3 G’s.
In an analysis where there is an excess of a particular chromsome, you would have a skewed ratio. For example, in trisomy 21, if the baby had an extra chromsome with the mother’s allele, s/he would have an allele composition at that locus of TGG. An analysis of the mother’s blood sample with fetal DNA would end up with a ratio of 1 T (from the fetus) to 4 G’s (2 from mother, 2 from fetus).
Take a look at the Ravgen diagram (available in pdf) in combination with my explanation to get a better feel for their technique. (I didn’t find their explanation to be as good as mine of course!).
In the initial study, they were able to correctly calculate the number of chromosomes for 58 of 60 patients. Of the three mothers carrying a fetus with Down’s syndrome, two were correctly diagnosed. There was one false positive among the 57 samples without the extra chromosome 21. Clearly, this prenatal test is still in the early stages of development so there’s no point in stressing over false positives, false negatives, positive and negative predictive values, etc. But it’s a promising start for a DNA test that’s sure to be in demand.
NB: I particularly like the explanation given for the history of the Ravgen logo.
The dots emanating from the base of the V in Ravgen symbolize the image of DNA captured by Franklin and described by molecular physicist and crystallographer, J. D. Bernal, as “among the most beautiful X-ray photographs of any substance ever taken.”
Technology Review, February 3, 2007
Tags: ravgen, dna testing, down’s syndrome, chromomsome 21, trisomy 21, genetics, genes, dna, diseases, illness, health, fetus, prenatal testing, pregnancy
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Check out what others are saying about this post...[...] Ravgen is offering one more means of prenatal testing that is much less invasive to both the pregnant mother and her unborn child since it analyzes fetal DNA in maternal blood; only a blood draw is required. But the impact of making prenatal testing even easier than before means that more of us will be under pressure to find out all we can about our babies and to to make sure our children are as perfect as we can make them. On the other hand, it also means that parents and healthcare providers can be better prepared to handle a child with special needs once he or she is born. [...]
[...] Bonnie Ventura of Aspergian Pride notes in a 2005 interview that “Autism prenatal testing would have no purpose other than abortion. Such a test would stand in stark contrast to other prenatal tests that have legitimate medical purposes”; in contrast, she notes how a prenatal test for neural tube defects has made it possible to “cure or greatly reduce spina bifida” by surgery on a fetus who is still in the womb. Today on Genetics and Health, Dr. Hsien Hsien Lei writes about a recent conversation she had with Dr. Ravinder Dhallan, who is the Chairman and CEO of Ravgen, whose webpage notes that “if you can’t diagnose it, you can’t treat it.” Ravgen is offering a prental test that analyzes fetal DNA in maternal blood. This is a much less “invasive” test for both mother and the unborn child that only requires a blood draw. As Dr. Lei notes, Dr. Dhallen emphasizes that prenatal tests like Ravgen’s are just extra tools to be used in prenatal diagnosis. Genetic testing won’t ever replace technologies like ultrasound that’s capable of diagnosing structural abnormalities in the fetus. However, there is great promise for prenatal tests that can analyze specific genetic loci and mutations. [...]