受精胚の異常を見つける新技術

体外受精により得られた受精胚のうち染色体の正常な胚のみを選んで移植することにより流産や異常児の出産を防ぐ方法が試みられている。Medical Center in West Orange　(New Jersey) のグループは第２極体をcomparative genomic hybridization (CGH)で調べる方法を開発したが、まだ研究の余地があるという。

New Test Picks Up for More Embryo Defects
Mon Sep 16, 1:41 PM ET
NEW YORK (Reuters Health) - A new technique appears to be a promising way to test embryos for more genetic defects, increasing the chance that a woman undergoing in vitro fertilization (IVF) can achieve a healthy pregnancy.
Currently, IVF patients may undergo preimplantation genetic diagnosis (PGD) including a test in which a single cell is removed from the embryo and examined. The test can detect certain genetic defects or can be used to pick up chromosomal abnormalities, which increase the risk of miscarriage.
Preimplantation genetic diagnosis allows doctors to select the embryos that are most likely to result in a normal pregnancy and a healthy baby. However, the testing process is somewhat limited--doctors can usually only check out 6 to 9 chromosomes per cell, although cells typically contain 23 pairs of chromosomes.
Although a more comprehensive test can be done called comparative genomic hybridization (CGH), it takes too long--once an egg and sperm are mixed in a laboratory dish, the resulting embryos must be transferred into a woman's uterus within 4 days.
According to a report in the journal Fertility and Sterility, doctors at St. Barnabas Medical Center in West Orange, New Jersey, were able to cut the time by testing polar bodies--packets of chromosomes left over after an egg cell divides--using a quicker version of CGH. While half of the egg cell's contents go on to mix with the DNA of sperm and form an embryo, the other half is discarded into the polar body.
If an abnormal number of chromosomes are present in the polar body, it's a good bet that the developing embryo is abnormal as well--and likely to end in miscarriage, or conditions such as Down syndrome. What's more, polar bodies can be tested right away, allowing all chromosomes to be examined, while an embryo can be tested only 3 days after fertilization.
In the study, the researchers used polar body testing combined with traditional embryo analysis in a single 40-year-old woman undergoing IVF but having difficulty becoming pregnant.
The study authors retrieved 12 eggs from the woman. Testing of 10 polar bodies showed that 9 of the 10 were abnormal, having either too many or not enough chromosomes. Only 7 of the 12 eggs developed into embryos, and traditional testing confirmed the results of the polar body testing.
Only one resulting embryo was deemed normal, according to the test. Although the embryo was transferred, the woman did not become pregnant. The findings are not unusual, given that only half of all human conceptions are thought to result in a live birth. And in IVF, only 5% to 30% of embryos transferred into the uterus result in a live birth.
The researchers note that the testing process is labor intensive, and "more advances will be required before comparative genomic hybridization can be offered to a larger population of infertile patients."
Still, the technique "could help to extend the improvements in IVF success rates already achieved using the limited chromosomal tests currently available," Dr. Dagan Wells and colleagues conclude.
The technique "requires further development before wider clinical application can be considered but holds great promise for the future," the researchers add.
SOURCE: Fertility and Sterility 2002;78:543-549.