CAN GENETIC TESTING BE USEFUL FOR CHOOSING HEALTHIER EMBRYOS AND PRODUCING DESIGNER BABIES?
In addition to prenatal genome sequencing, it is now possible to analyze the DNA of early embryos, before the embryo even implants into the mother’s uterus. Specifically, after in vitro fertilization (IVF), a small number of cells (~1–6) can be removed from early embryos and analyzed by genetic tests. This procedure can be performed by biopsying an 8-cell embryo at the third day after fertilization or the trophectoderm of a blastocyst at the fifth day. In the United States, embryo biopsy typically occurs at the fifth day. Typically, only one or a handful of genes are assayed.
Presently, genetic testing of embryos is only performed in several circumstances. In cases in which a child is born with a severe genetic disease, parents often turn to IVF and embryo testing to avoid having a second child with the same disease. Similarly, if the parents are genetic carriers of a particular disease and they want to ensure that their child is not born with the genetic disease, they may chose IVF screening. IVF is often used to help solve infertility problems, and such embryos can be screened for aneuploidy. Indeed, although some aneuploid embryos can develop and result in live births, most aneuploid embryos are not implanted and many would naturally miscarry. By screening for embryos with the normal numbers of chromosomes, the success rate of IVF can be increased, along with the likelihood of having children without genetic diseases caused by aneuploidy.
Because it is now possible to determine the genome sequence of DNA from one or a few cells (albeit not yet with perfect accuracy or completeness), we can now determine the entire genome sequence of IVF embryos before they are implanted. Could we see a world in which most babies are born after screening in vitro fertilized embryos and those lacking obvious disease- causing variants are implanted into the mother? It is certainly possible and perhaps even likely. This ability to select genetically screened progeny also raises issues about selecting characteristics that are not associated with human disease, such as eye color, height, athletic ability, intelligence, and additional traits as they become better understood. These possibilities, which raise ethical concerns about how genetic information will be used, are discussed later.
It is also possible, in principle, to modify embryos by inserting or modifying genetic material. This has been performed in mice or other mammals for research purposes; genes have been inserted to fix genetic defects (e.g., cystic fibrosis in mice) or enhance traits (e.g., to produce transgenic livestock). A “reporter” gene has even been inserted into the embryos of chimpanzees. These technologies raise the prospect that it might be possible to not only select desired embryos for implantation, but to also modify them to correct disease or enhance traits before a child is born.