Prenatal diagnosis and preimplantation genetics

One of the consequences of in vitro fertilization and the ability to detect genetic mutations early in development is a new area of medicine called preimplantation genetics. Preimplantation genetics seeks to test for genetic disease before the embryo enters the uterus. After that, many genetic diseases can still be diagnosed before a baby is born. This prenatal diagnosis can be done by chorionic villus sampling at 8–10 weeks of gestation, or by amniocentesis around the fourth or fifth month of pregnancy.

Chorionic villus sampling and amniocentesis

Chorionic villus sampling involves taking a sample of the placenta, whereas amniocentesis involves taking a sample of the amnionic fluid. In both cases, fetal cells from the sample are grown and then analyzed for the presence or absence of certain chromosomes, genes, or enzymes.

However useful these procedures have been in detecting genetic disease, they have brought with them a serious ethical concern: if a fetus is found to have a genetic disease, the only means of prevention presently available is to abort the pregnancy. The need to make such a choice can be overwhelming to prospective parents.* Indeed, the waiting time between knowledge of being pregnant and the results from amniocentesis or chorionic villus sampling has created a new phenomenon, the “tentative pregnancy.” Many couples do not announce their pregnancy during this stressful period for fear that it might have to be terminated (Rothman et al. 1995).

By using IVF, one can consider implanting only those embryos that are most likely to be healthy as opposed to aborting those fetuses that are most likely to produce malformed or nonviable children. This can be achieved by screening embryonic cells before the embryo is implanted in the womb. While the embryos are still in the petri dish (at the 6- to 8-cell stage), a small hole is made in the zona pellucida and two blastomeres are removed from the embryo (Figure 1). Since the mammalian egg undergoes regulative development (see Chapter 11), the removal of these blastomeres does not endanger the embryo, and the isolated blastomeres are tested immediately. The polymerase chain reaction technique can be used to determine the presence or absence of certain genes to be determined, and fluorescent in situ hybridization (FISH) can be used to determine whether the normal numbers and types of chromosomes are present (Kanavakis and Traeger-Synodinos 2002; Miny et al. 2002). Results are often available within 2 days. Presumptive wild-type embryos can be implanted into the uterus, while any presumptive embryos with deleterious mutations are discarded.

Figure 1 Preimplantation genetics is performed on one or two blastomeres (seen here in the pipette) taken from an early blastocyst. The polymerase chain reaction is then used to determine whether certain genes in these cells are present, absent, or mutant. (Photograph courtesy of The Institute for Reproductive Medicine and Science of St. Barnabas, Livingston, NJ.)

Sex selection and sperm selection

The same procedures that allow preimplantation genetics also enable the physician to know the sex of the embryo. Sometimes parents wish to have this information; sometimes they do not. However, knowing the sex of an embryo prior to its implantation raises the possibility that parents could decide to have only embryos of the desired sex implanted. Sex selection using preimplantation genetics is seen by many as a beneficial way of preventing X-linked diseases, but in fact it is often used as a method of simply choosing one’s offspring’s sex. Opponents of sex selection point to its possible use as a method of preventing the birth of girls in cultures where women are not as highly valued as men (see Gilbert et al. 2005; Zhu et al 2009). This has created enormous discrepancies in the sex ratio of several regions. Different countries and even different hospitals have different policies permitting preimplantation genetic diagnosis solely for the purpose of sex determination.

Literature Cited

Gilbert, S. F., A. L. Tyler, and E. J. Zackin. 2005. Bioethics and the New Embryology: Springboards for Debate. Sinauer Associates, Sunderland, MA.

Kanavakis, E. and J. Traeger-Synodinos. 2002. Preimplantation genetic diagnosis in clinical practice. J. Med. Genet. 39: 6–11.

Miny, P., S. Tercanli and W. Holzgreve. 2002. Developments in laboratory techniques for prenatal diagnosis. Curr. Opin. Obstet. Gynecol. 14: 161–168.

Rothman, K. J., L. L. Moore, M. R. Singer, U.-S. D. T. Nguyen, S. Mannino, and A. Milunsky. 1995. Teratogenicity of high vitamin A intake. N. Engl. J. Med. 333: 1369–1373.

Zhu, W. X., L. Lu, and T. Hesketh. 2009. China's excess males, sex-selective abortion, and one-child policy: Analysis of data from 2005 national intercensus survey. Brit. Med. J. 338: b1211.

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