Genetic tests designed to weed out embryos that are unlikely to grow into healthy babies after in vitro fertilization (IVF) are often administered to couples receiving treatment even though it seems to have little impact on pregnancy rates. A new study involving higher-resolution genetic screening throws the practice into new doubt by showing that most of the cells in even healthy embryos have such chromosomal defects.
Evelyne Vanneste and her colleagues at the Catholic University of Leuven, in Belgium, used new, higher-resolution screening techniques to analyze cells from three- or four-day-old embryos from 23 fertile couples aged less than 35. Embryos are typically analyzed at this stage of development because less mature embryos contain less information, and more developed embryos are more difficult to transfer.
Vanneste and her colleagues found that more than 90 percent of the cells had some chromosomal abnormalities, a finding that goes some way toward explaining why humans have such low fertility rates in general. But it also means that some usable embryos may be discarded following screening.
Preimplantation genetic screening (PGS) usually involves either polymerase chain reaction (PCR), which detects genetic disorders by amplifying a specific chunk of mutated DNA, or fluorescent in-situ hybridization (FISH), which allows chromosomes to be checked for structural flaws against normal chromosomes but cannot screen all chromosomes simultaneously. As a result, chromosomal problems that may prevent a successful pregnancy can be missed.
Vanneste and her colleagues used two newer tools–a SNP array and a BAC array–to look for chromosomal errors across the whole genome. The SNP array can identify variations in short pieces of DNA, while the BAC array can analyze larger chromosome chunks for structural errors. The team studied cells from 23 embryos taken from nine couples with normal fertility that were undergoing IVF treatment to exclude embryos with specific genetic diseases. To the researchers’ surprise, they found that 90 percent of the cells had duplicated or missing chunks of chromosomes. Not only that, but the errors changed in different cells taken from the same embryo.
This suggests that human embryos naturally have high chromosome instability, at least during the first few rounds of cell division. The same has been observed in macaques but not in mice, says Vanneste, and the evolutionary reason for it is not yet clear. “Possibly, instability is a mechanism that can rapidly generate genetic diversity, thus allowing more rapid adaptation to changing environments,” she says.
Vanneste’s results may partly explain humans’ relatively low fertility rate of about 30 percent, but the rate is still much higher than the 10 percent of apparently genetically normal embryos found by her team. This means that many embryos must go on to develop into healthy babies even though their chromosomes have defects at this stage. As the embryo grows, complementary mutations in its cells may compensate for each other, or cells without chromosome defects may preferentially populate the embryo.
The benefits of preimplantation screening are already hotly debated, since chromosome errors occur frequently in older women who may produce few eggs to begin with which stand a worse chance of successful impregnation following invasive biopsies to remove cells for genetic screening.
“None of us are really normal, so we are actually wasting our time trying to screen for normality,” says Stuart Lavery, a consultant specializing in reproductive medicine at Hammersmith Hospital, in London, U.K. “If you screen hard enough, you will never find a normal embryo.”
Neither Lavery nor Vanneste suggests giving up on IVF screening completely, but they both argue that chromosomes from more or less developed embryos may provide more reliable results because chromosomal instability is not as much of an issue then. “Changing the time point of genomic analysis to either an earlier-stage polar body analysis or to a later stage by blastocyst biopsy might be a better approach towards selecting genetically normal embryos for transfer,” Vanneste says.
Polar bodies are cells left over from the meiotic cell divisions that form the egg that contain only DNA from the mother and therefore cannot show errors from the father or arising after fertilization took place. But the method is gentle on the embryo and chromosomal errors present in polar bodies are carried by all cells of the embryo, providing a strong justification for discarding embryos with errors.
In contrast, the human blastocyst has around 100 cells at five days old, some of which will form the placenta rather than the tissues of the fetus. This permits removal of several cells, which makes genetic analysis easier and includes DNA derived from the father. Chromosomal instability is less pronounced at this stage but until recently, it hasn’t been practical to analyze blastocyst cells because it is much more difficult to transfer them. But newer techniques that involve freezing embryos are making blastocyst analysis followed by later transfer more practical.
Lavery says that polar body analysis might particularly benefit older women with only a few precious eggs left. For younger women, blastocyst biopsies may offer more promising results, he says.
SNP and BAC arrays are still relatively expensive, however. Another, less costly method for analyzing human chromosomes, called comparative genomic hybridization (CGH), is being developed by Elpida Fragouli from the Nuffield Department of Obstetrics and Gynaecology, at the University of Oxford, and Reprogenetics, in the United Kingdom. This approach allows all 23 pairs of human chromosomes from blastocysts to be examined at slightly lower resolution.
With CGH, DNA extracted from the embryo is amplified, labeled green, and mixed with normal reference DNA that has been labeled red. The mixture is then spread onto slides along with metaphase (early stage) chromosomes to which the DNA mix binds. The chromosomes are condensed into distinct shapes, and the ratio of green to red fluorescence along the length of each chromosome indicates whether the embryo’s chromosomes have lost or duplicated noticeable chunks. Preliminary clinical results using the technique on blastocyst embryos have been promising, Fragouli says.