Trisomy 21: pre-birth testing by blood sample

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The company GATC Biotech can identify the superfluous third chromosome on the basis of an examination of DNA fragments from the child in the mother's blood. Source: NHGRI

13.08.2011  - 

Determining whether an unborn child has a possible genetic hereditary disease such as Down's syndrome still necessitates that an amniotic fluid puncture, an invasive procedure that increases the risk of miscarriage. The company GATC Biotech, together with subsidiary LifeCodexx, has now found a means of detecting Trisomy 21 using the DNA fragments from the child that circulate in the mother's blood. This means that a amniotic fluid puncture is no longer required. The project has been funded to the tune of 230,000 euros by the SME Innovative program (KMU-innovativ) from the Federal Ministry of Research. At this time, the technique is being put through clinical trials; the test is anticipated to arrive on the market in late 2011.

For a long time, identifying chromosomal aberrations such as Trisomy 21 in unborn children has necessarily involved dangerous and invasive medical techniques. In amniocentesis or chorionic villus sampling, foetal cells from the amniotic fluid or placenta are collected and genetically ­analysed. Today, sophisticated ultrasound equipment enables diagnoses via the translucency of the 'nuchal fold', although this can provide only a probability. With the new method, developed at GATC (click here to view a portrait of founder Peter Pohl), further invasive testing could be ruled out in 98% of studied pregnancies that indicated a high probability of Trisomy 21. The major advantage is the low risk for both child and mother. A blood sample of just ten millilitres - easily and quickly obtained from the arm of the mother as with an ordinary blood donation -is sufficient for the procedure.

"Previous methods have focused on the extraction of intact foetal cells," says project head Wera Hofmann. "We work with cell-free foetal DNA." This is taken from the trophoblast. This cell type forms the placenta, and has the same genetic makeup as the foetus. Trophoblasts have a short lifespan. If this is exceeded, they die a programmed cell death and are flushed out of the mother's ­body. However, the DNA fragments from the nucleus of these cells are somewhat stubborn, and are still detectable in the mother's ­blood up to 24 hours after the disintegration of their host cells. There is a reason for this resistance: "We know that the genetic material is protected by the surrounding protein complexes, the nucleosomes, and degrades more slowly," says Hofmann. The cell-free foetal DNA was discovered in 1997 by scientist Dennis Lo.

DNA fragments hark clearly back to the foetus

Only about 10 to 20 percent of freely circulating DNA in a blood samples also belongs to the unborn child, but these are ultimately clearly traceable back to the foetus. This has not always been so clear-cut. The intact foetal cells extracted during a biopsy can remain in the maternal blood for decades, thereby falsifying diagnoses in all subsequent births. However, the cell-free foetal DNA from the trophoblast, which emerges in maternal blood in approximately the fourth week of pregnancy, is flushed out after the birth.

Using a high-throughput method for decoding of genetic material - Next Generation Sequencing Technology - GATC analyse the DNA fragments taken from the blood sample and compare them against a database of the known sequence of the human genome. "We conduct a quantification," says Hofmann. "We pick out the fragments we can use from the about 10 million sequenced portions of a sample." For a trisomy, statistical methods are employed to determine whether the 21st chromosome in the sample occurs more frequently than usual. In multiple pregnancies, however, this can give uncertain results - an ongoing area of testing at GATC.

Other genetic diseases in sight

For Hofmann and her team, Trisomy 21 is just the beginning - being one of the most common genetic diseases in newborns, the samples are correspondingly large. "We are also working on Trisomies 13 and 18, the Patau and Edward's syndromes," says Hofmann. "However, the clinical trials are considerably more expensive in this case, as there are less affected individuals" From the perspective of the GATC-researchers, the method could also be used in the future for other autosomal single gene diseases inherited from the mother. The difficulty at this time is in adequately and quantitatively pinpointing an inherited mutation from the mother in the unborn child. This is because the blood sample contains cell-free DNA from both the mother and the child; the GATC procedure cannot clearly distinguish the genetic information of the maternally inherited foetal chromosome.

In the US, there is already a similar method for the detection of Trisomy 21; in Germany the company is currently the only supplier, also thanks to its next-generation sequencing technologies. Since the BMBF ­funding came to an end, the procedure has been undergoing testing with clinical partners. Hofmann hopes to analyse samples from 500 expectant mothers over the course of the testing. The study should be completed by the end of 2011, after which the blood test will be put on the market. 

Author: Conny Kästner