An international team of scientists has discovered a way of testing IVF embryos using DNA fingerprinting that could one day be used to select the ones that are more likely to implant successfully and grow to full term and thereby increase the likelihood of a successful pregnancy.
The study is the work of researchers from Monash University and Monash IVF in Clayton, Victoria, Australia, Genesis Athens Hospital and Athens University in Greece, and was published on 13th May in the advance access online issue of the journal Human Reproduction.
The researchers tested the DNA of early in vitro fertilization (IVF) embryos before implanting into the womb, and then compared the results with the DNA of the healthy babies that were born, and found a cluster of genes that could be used to establish which embryos are likely to make it to full term.
In IVF treatment, a number of eggs harvested from the woman are fertilized in the lab with the man’s sperm and the fertilized eggs are allowed to grow for about 5 days until they reach the blastocyst stage when the ones that are deemed most viable are selected for implanting into the woman’s uterus.
The blastocyst stage is that reached just before a normally fertilized embryo implants itself into the uterus wall. It has an outer cell mass called the trophoblast which forms the placenta, and an inner cell mass called the embryoblast which grows into a fetus.
At present no reliable method exists to test the viability of human IVF embryos. IVF clinicians use very crude methods, including looking at the morphology (shape) of the blastocysts to select which to implant into the mother’s womb. What usually happens is that couples choose to have more than one implanted in order to maximize the chance of a successful pregnancy, but this increases the risk of multiple births which can be dangerous to mother and babies alike.
One of the researchers, Dr David Cram, senior research scientist at the Monash Immunology and Stem Cell Laboratories at Monash University, said their ultimate aim was to discover which genes are expressed by blastocysts that go on to form fetuses that make it to full term and end up as healthy babies:
“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 utilising a combination of blastocyst biopsy, DNA fingerprinting and microarray analysis to identify viable blastocysts among the cohorts transferred to patients,” he added.
For this study Cram and colleagues recruited 48 women scheduled for IVF. After their eggs had been fertilized and grown in culture for 5 days, they took between 8 and 20 cells from the trophoblast, amplified their DNA and analyzed their gene expression using micro-arrays. A micro-array is a gene probing technology based on microchips that have thousands of known DNA sequences that can be matched to DNA or messenger RNA targets to see which particular genes are being expressed in a sample.
After this, each of the 48 women had one or more blastocysts implanted in their wombs. 25 of the women became pregnant, and 37 babies were eventually born.
Of the 25 women who became pregnant, in 7 of them all the blastocysts implanted, and in 18 of them some did and some did not. This meant there was no problem with the uterus. In the 23 women who did not get pregnant, this was either because there was a problem with the uterus (for instance it could have been non-receptive) or the blastocysts were not viable.
When the babies were born, the researchers retrived blood from the umbilical cords, or took swabs of their cheeks and DNA fingerprinted these samples to find matches with the DNA fingerprints taken from the early blastocysts so they could tell which embryo each baby came from, and therefore pinpoint the viable blastocysts.
Using micro-arrays they looked to see which genes were expressed (switched on) in the viable blastocysts. At the time of writing the study the researchers were still working on this stage, but they had managed to discover a cluster of genes known to be involved in important processes during embryo implantation that were expressed in the viable blastocysts. These genes regulate processes like cell adhesion, communication, metabolism, and responding to stimuli.
Co-researcher Dr Gayle Jones, who is a senior research scientist at the Monash University’s Immunology and Stem Cell Laboratories, said they hoped to come up with a smaller subset of genes that will uniquely identify a viable embryo:
“The ability to select the single most viable embryo from within a cohort available for transfer will revolutionise the practice of IVF, not only improving pregnancy rates but eliminating multiple pregnancies and the attendant complications,” said Jones.
While the method demonstrated in this study is some way from being ready for clinical use, the researchers said it could be used to test different treatments of embryos without having to recruit large numbers of women for clinical trials. DNA fingerprinting could also be used to supplement current methods of choosing which embryos to implant, they said.
“Major improvements in IVF practice in the last decade have seen the introduction of better laboratory techniques that allow complete pre-implantation development to the blastocyst stage in vitro,” said Jones, adding that:
“One of the major stumbling blocks to worldwide acceptance of a single embryo transfer policy is the lack of highly predictive criteria to select the single most viable embryo within a cohort. “
The idea is that by using objective, measurable criteria rather than the current more subjective observations such as looking at the morphology of the blastocysts, the ability to predict which embryos are viable will improve to such an extent that IVF clinicians will be able to confidently implant single embryos without reducing pregnancy rates.
“This would effectively reduce multiple pregnancies, which is a priority in the field of assisted reproductive medicine at present,” said Jones.
“Novel strategy with potential to identify developmentally competent IVF blastocysts.”
Gayle M. Jones, David S. Cram, Bi Song, Georgia Kokkali, Kostas Pantos, and Alan O. Trounson.
Human Reproduction Advance Access published on May 13, 2008.
Sources: Journal abstract, European Society for Human Reproduction and Embryology press release.
Written by: Catharine Paddock, PhD