P&S Journal: Spring 1994, Vol.14, No.2
Is it possible to give a laboratory animal a genetically transmitted human disease, such as sickle cell anemia or cystic fibrosis? It is, and the method has become a very powerful tool in investigating the expression of genetic traits.
In the procedure called transgenics, mice are injected with foreign DNA early in embryonic development, causing them to express one or more genes from another mouse or from another species of animal, including humans. They then pass this trait on to subsequent generations.
Dr. Frank Costantini, professor of genetics and development, has been working with two types of transgenic mice since 1980 in his investigations of various gene mutations. One type is created by injecting an isolated cloned gene (raw DNA) into the pronucleus (sperm or egg nucleus) of a one cell-stage embryo, immediately following fertilization but before the pronuclei have fused. To accomplish this, a female mouse is given fertility hormones to increase ovulation; following copulation, her eggs are removed, injected with the gene, and kept in culture until they have reached the blastula stage (64 cells) when they are implanted into foster mothers.
When the DNA is injected it is incorporated into the pronuclei, where it is joined to the chromosomes at random sites. It could disrupt another gene and cause an unexpected mutation, but most often it goes into DNA that lies between genes or is not incorporated at all. If the foreign gene does become part of the host DNA, in a spot where it can be expressed, a transgenic mouse is created. This means that the new gene must be accompanied by, or insert itself near, the correct regulatory mechanisms to turn it on and off. It is hit or miss, but with sufficient numbers, a certain percentage of mice will be born with evidence of the desired new genetic trait. These can be bred together to perpetuate the mutation.
Despite limitations, this is a relatively quick and easy way to determine the effect of a certain gene, such as the formation of a new protein or the expression of a disease. The second type of transgenesis that Dr. Costantini studies in mice utilizes a process called homologous recombination, in which a specific gene may be integrated into the right location in a chromosome by correct pairing of nucleotide bases. This is accomplished by removing cells from the donor embryo in the 64-cell blastocyst stage, some of which form what is known as the inner cell mass. These embryonic stem cells (ES cells) have the potential of becoming any part of the body. After the donor ES cells are mutated by homologous recombination, they can be kept in culture for extended periods, retaining their genetic properties, and distributed to any laboratory in the world.
Methods have been devised to screen out all the ES cells that lack this integration, which occurs only in about 1:1000 cells, leaving only cells containing the desired gene. At any time, the mutated ES cells can be injected into a blastocyst taken from another mouse where they become part of the inner cell mass of the host. There are now two colonies of ES cells, donor and host, which are the precursors of every cell in the body, including the germ cells. Thus, for example, if the blastocyst is destined to be a male mouse, some sperm will carry only donor genes and some only host genes. If a brown mouse provides the donor ES cells, and an albino mouse egg is the recipient, the resulting mouse, called a chimeric mouse, will be white with brown patches because both the donor and recipient genes are being expressed independently. If, however, a male chimeric mouse is mated to an albino female, the offspring will be either brown or white, depending on which sperm from the chimeric mouse fertilizes the albino's egg.
With this technology it is possible to take any gene of interest, identify and isolate it, and develop mice with a mutation in that gene. This procedure takes longer, at least two generations of mice for the gene to be expressed. If the trait is recessive, it will take three. However, it is not random and allows for point mutations. It is also known as "gene targeting" because it allows the investigator to make changes within a specific gene. Using either of these methods, it is possible to make transgenic mice with a human gene, such as the gene for sickle cell anemia, and produce mice that make SS hemoglobin. Human disease can then be studied in laboratory animals and treatments devised. The procedure has been used in agriculture in the creation of transgenic fruits or vegetables.
The potential use of transgenics in human embryos poses moral and ethical dilemmas. While few would dispute its benefit in the elimination of hereditary disease, the technique could be expanded to include other "undesirable" traits, and create a "Brave New World" environment in which babies are made to order. Discussions about such future dilemmas are already under way.