Identifying viable embryos in IVF

At a time when IVF clinics are transferring fewer and fewer embryos and thereby placing more and more emphasis on embryo selection, investigators from Australia and Greece have for the first time used DNA fingerprinting to identify those embryos that have successfully implanted in the uterus and progressed to term. They suggest that, if DNA fingerprinting and microarray analysis can identify the genes expressed by a viable embryo, a revolutionary means of embryo selection in IVF may be possible. These techniques may make it feasible for a single embryo to be reliably transferred thus eliminating multiple pregnancies and the associated risks.

Their findings are based on a study of 48 IVF patients whose oocytes were fertilized and cultured for 5 days to the blastocyst stage. Between eight and 20 cells from the trophectoderm cell layer were then removed by biopsy from the resulting blastocysts. These samples were amplified and their gene expression analyzed using a microarray technique to target sequences of messenger RNA. One or more blastocysts were then transferred to all 48 women, 25 of whom became pregnant, with 37 babies born. In seven of these successful cases all transferred blastocysts implanted, but in 18 of them some of the blastocysts implanted and some did not, indicating that failure to implant was not due to problems with the uterus.

Once the babies were born, blood from the umbilical cord or swabs of cheek cells were taken and stored. The investigators then applied DNA fingerprinting to these samples to match them with the DNA obtained from the blastocyst trophectoderm biopsies, thereby identifying exactly which embryos grew into which babies. The microarray analysis identified which genes were expressed in the viable blastocysts.

The investigators report that their work is continuing, but already they have discovered that certain genes known to be involved in cell adhesion, cell communication, cellular metabolic processes, and response to stimuli—key processes involved in embryo implantation—were expressed in the viable blastocysts.

One of the authors of the report, Dr. David Cram, Senior Research Scientist at the Monash Immunology and Stem Cell Laboratories in Australia, said: “DNA fingerprinting is the ultimate form of biological identification, but until now it has not been used to identify the embryonic origin of resultant babies born following embryo transfer; nor has it been used for gene expression studies. We have developed a novel strategy of utilizing a combination of blastocyst biopsy, DNA fingerprinting, and microarray analysis to identify viable blastocysts among the cohorts transferred to patients. Our ultimate aim is to find out which genes are expressed by viable blastocysts.”

So far, he added, the most important findings from the research are that:
• up to 20 trophectoderm cells can be removed from a blastocyst without adversely affecting its viability and ability to implant;
• DNA fingerprinting and microarray analysis are appropriate techniques for distinguishing between viable and non-viable blastocysts;
• blastocysts have different patterns of gene expression, which, when defined, could be used to select the single most viable embryo from a group for transfer.

The ultimate goal of the procedure is, of course, guaranteed, reliable embryo selection, and that right now is the holy grail of IVF. Current morphological methods of selection are not much different today from what they were in the early days of IVF: lab technicians still rely on a visual assessment of the embryo based on cell numbers, cleavage rate, and fragmentation. However, as an IVF live birth rate of, say, 30 percent would suggest, morphological assessment is not highly predictive.

Hopes were that the techniques of preimplantation genetic diagnosis, when applied to embryo screening in the broad IVF population, would identify those embryos with chromosomal abnormalities. But so far, the three randomized trials of preimplantation genetic screening have proved inconclusive. Indeed, the latest trial, reported in 2007, showed a negative effect. Now, if microarray analysis can be used to identify those genes expressed by a viable embryo, this technique may in future provide the basis for a reliable assessment of its viability.

This was the hope expressed by another of the investigators, Dr. Gayle Jones, Research Scientist at the Monash Immunology and Stem Cell Laboratories, who said: “We believe that it will be possible to refine our gene set to a smaller number of genes that is more highly predictive of a blastocyst’s viability and ability to develop to a term pregnancy when transferred to a receptive uterus than current selection criteria. The ability to select the single most viable embryo from within a cohort available for transfer will revolutionize the practice of IVF, not only improving pregnancy rates but eliminating multiple pregnancies and the attendant complications.”

She added that one of the major stumbling blocks to worldwide acceptance of a single embryo transfer policy right now is the lack of highly predictive criteria for single embryo selection. The ability to use objective, measurable criteria rather than subjective observations should improve that predictive value and provide sufficient confidence for IVF clinics to apply single embryo transfers in all patients without a concomitant drop in pregnancy rates.

Source: Orgyn.com

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