Scientists have pieced together the entire DNA sequence of an 18-week-old fetus without having to use anyinvasive tests that could result in a miscarriage — an advance that offers a glimpse of the future of prenataltesting. Using blood drawn from the mother and a sample of saliva from the father, theresearchers were able to scan the fetus' genome and determinewhether it contained any of the myriad single-letter changes in theDNA code that can cause a genetic disorder . They could even pinpoint which mutations were inherited from Mom,which came from Dad, and which were brand-new. If the technique is refined and the technology becomes inexpensive— as many experts anticipate — this type of prenataltesting could provide prospective parents with a simple, risk-freeway to screen for a broad array of simple genetic disorders,according to the authors of a report in Thursday's edition ofScience Translational Medicine. |
The work is based on the fact that small fragments of fetal DNAcirculate in the blood of pregnant women. Several biotech companies are developing tests that capture thoseDNA fragments and screen them for signs of Down syndrome and other disorders that result from having an extra copy of anentire chromosome . But that type of screening is far easier than searching forsingle-letter variations in individual genes, said senior authorJay Shendure, a geneticist at the University of Washington in Seattle. An additional chromosome is "the equivalent of an extra chapter ina book," he said. "What we're trying to do is pick up a typo in aword." To set about their task, Shendure's team started by sequencing thegenome of an anonymous pregnant woman, using a complete sample ofher DNA obtained from her blood cells.
They also sequencedfree-floating DNA fragments extracted from her blood plasma,repeating their work until they had decoded every part of the humangenome 80 times. That plasma contained a mix of 10% fetal DNA and 90% maternal DNA,all in tiny fragments. The scientists needed to be able to tellwhich pieces were from the mother and which belonged to the fetus. To solve that problem, the scientists relied on the fact thatgenetic material is inherited in long strands of DNA, calledchromosomes — and that tiny genetic variations on the samechromosome are usually inherited together, in blocks known ashaplotypes. If a given haplotype was present in the fetus as wellas in the mother, it would be detected in the plasma in extraamounts.
The scientists also sequenced the father's DNA, which was extractedfrom saliva. This allowed the team to figure out whether geneticvariations in the fetus that didn't match the mother were inheritedfrom the father or were new mutations. On average, about 50 newmutations show up in a fetus. The scientists checked their results against a blood sample takenfrom the baby's umbilical cord after birth. Their calculations weremore than 98% correct, they found, and they had detected 39 out of the 44 newmutations.
None of those mutations had known medical consequences,the researchers said. This approach could be used to devise a single test to screen forthe 3,000 known disorders that are caused by mistakes in singlegenes. Individually, they are rare, but together they affect about1% of births. Technology like this could lead to more widespread screening offetuses for genetic disorders that could benefit from earlytreatment, said Dr. Joe Leigh Simpson, senior vice president forresearch and global programs for the March of Dimes in WhitePlains, N.Y.
It might even help doctors identify women atheightened risk for problems such as pre-term birth, he said. email@example.com.
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