“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.