受精卵の染色体検査により体外受精の成績向上を　

2000/10/23　受精卵の着床前診断の新しい方法はすべての染色体について異常を調べることができるようになった。ヒトの場合、排卵した卵の60％が受精するが25%しか児に発育しないといわれ、多くは染色体に異常があり極初期に段階で流産となっている。体外受精で得られた受精卵について染色体の異常を調べることにより、発育可能な卵を選び妊娠率の向上を期待できると考えられる。

Test May Help Chances of IVF Embryo Survival
By Nicolle Charbonneau
HealthScout Reporter

MONDAY, Oct. 23 (HealthScout) -- A new method of examining every chromosome in embryos considered for in vitro fertilization could tell doctors which ones are more likely to miscarry.

The test could improve the chances that the carefully prepared embryos used for in vitro fertilization (IVF) will survive and grow into a viable fetus. The findings were presented today at the annual meeting of the American Society for Reproductive Medicine in San Diego and appear in the current issue of Molecular Human Reproduction.

Until now, the most sophisticated tests have been able to look at only about five types of chromosomes in each cell. This test, which combined whole genome amplification and comparative genomic hybridization, was able to study all of the chromosomes.

Compared to animals, humans have a high rate of early pregnancy failure. While fertilization may occur in 60 percent of cycles, the chance of achieving a viable pregnancy is only 25 percent. Researchers have suspected that chromosomal abnormalities are behind this high rate of early miscarriage, but this is the first study to hint at how common such abnormalities may be.

In this study, doctors at University College London examined 12 embryos donated by patients in accordance with the British Human Fertilization and Embryology Authority. After fertilization, the embryos were cultured in the laboratory, then prepared for analysis.
Renee Martin, a human geneticist at the University of Calgary in Alberta and a past president of the Canadian Fertility and Andrology Society, says that in current pre-implantation genetic diagnosis, doctors must leave some cells in the embryo untouched. Finding that, say, two out of eight cells in an embryo have normal chromosomes is a good indicator that the others are fine. But it's possible that the other cells carry abnormalities from a different cell line.

Only 3 in 12 embryos normal
"The big advantage in this study is that they are studying every single chromosome, and we haven't been doing that up until now," says Martin. "Most centers study the chromosomes that are seen commonly to have abnormalities: 13, 16, 18, 21, X and Y."

Of the 12 embryos studied, the British researchers found that only three were completely normal. Of the remaining nine, three had no normal cells at all.

"Almost all chromosome abnormalities will miscarry, because they're a very large abnormality within the embryo or they won't [allow the embryo to] implant," says study co-author Joyce Harper, a senior lecturer in human genetics and embryology.

"We know, for example, that as women get older, the chromosomes can become more abnormal," says Harper, pointing out that as women age, for example, their risk of conceiving a fetus with Down syndrome -- caused by an extra chromosome -- increases. "Also, as women get older, they have more of a chance of miscarrying due to chromosome abnormalities."

Harper says that one potential application that should be studied is whether this procedure could be used to help older women to choose embryos without potentially fatal abnormalities.

Gail Stetten, a cytogeneticist at Johns Hopkins Medical Institutions in Baltimore, agrees. "If you put back something that has at least one normal cell in it, you'll be more likely to have a pregnancy and less likely to have a spontaneously lost pregnancy," says Stetten. "I think it's hopeful that we'll be able, in the near future, to use a pre-implantation diagnostic technique to make IVF more successful."

But Harper stresses that this research involved only a small number of embryos. As well, she points out that this technique doesn't look for genetic abnormalities. "All this procedure does is it looks at the chromosomes within the embryo," she says. "This procedure is very much in the research stage."

"It's not even possible to use this procedure in the pre-implantation diagnosis time frame," says Harper. IVF doctors can currently only keep the embryo outside the womb for six days, and this procedure still takes longer than that. "It may be possible in five years or so if the research shows that older women's embryos are definitely more susceptible to these chromosome imbalances."