| ENGINEERING YOUR BABY
American Heritage.com Invention and Technology Magazine
Volume 20, Issue 3
BY BERYL LIEFF BENDERLY
A FEW DECADES AGO, PARENTS NEVER KNEW IF THEIR CHILD WAS HEALTHY UNTIL AFTER IT WAS BORN. NOW THEY CAN KNOW BEFORE PREGNANCY STARTS-RAISING ALL SORTS OF POSSIBILITIES AND CONCERNS
.FOR PROSPECTIVE PARENTS WHO CARRY THE GENES FOR a hereditary disease, one of the greatest fears is passing it to their children. Such a disease can usually be detected during pregnancy, and the pregnancy can be terminated if the parents wish. But many couples have religious or moral objections to abortion, and in any case, the procedure is always stressful. So until recently, couples at risk for a genetic condition have faced three unsatisfactory choices: Have no children, or no further children once the disease has been diagnosed; conceive a child, carry it to term, and hope it is unaffected; or conceive a child, test it during pregnancy, and possibly face a wrenching decision.
This quandary resulted from a series of advances that placed doctors' ability to diagnose genetic illnesses ahead of their ability to cure them. Until the middle of the twentieth century, prospective parents knew virtually nothing about their children until they were born. But in the 1950s amniocentesis, in which the fluid surrounding the fetus is tested, was introduced. The 1960s saw the debut of sonography, which uses sound waves to produce a visual image of the fetus. And in the 1980s chorionic villus biopsy, in which a sample of placental tissue is tested, was added to the physician's repertoire. All these techniques gave parents a wealth of information about their developing child.
Along with these advances in prenatal information came advances in genetics. After the double-helix structure of DNA was identified and described in 1953, scientists made great strides in understanding its chemistry and learning how heredity is expressed on a molecular level. More and more conditions were identified with specific genetic defects-hemophilia, cystic fibrosis, Fanconi's anemia, and many others. Yet as Dr. Mark Hughes, one of America's leading experts in hereditary disease, recalls, until the late 1980s medical genetics was a matter of "diagnose and adios." He could tell parents that their child had a certain condition, but after that, "there's nothing you can do for most of these people. They already have a child and there isn't even treatment." Knowing that they were carriers could help parents make decisions about further pregnancies, but there was no way to eliminate the risk ahead of time.
Now, however, parents can know before pregnancy starts, to a high degree of probability, that their child will be unaffected by a genetic defect. Several advances in medicine and genetics have combined to make this possible. The earliest was in-vitro fertilization (IVF), which was first performed successfully in 1978. In IVF, fertility drugs cause a woman's ovaries to release many eggs, typically around 10 to 15. These are extracted through the vagina, using a hollow needle guided by ultrasound, and are then fertilized with the father's sperm in an incubator. After 3 to 5 days of growth, one or more of the fertilized eggs are implanted in the woman's uterus. After all this, the woman ends up actually becoming pregnant about 30 percent of the time. If she doesn't, the physician can try again 10 days later with more embryos. Unused embryos can be stored in a special solution, frozen, and implanted if needed.
IVF was developed to help infertile couples have children, but toward the end of the 1980s geneticists realized that it could have another use: helping fertile couples who are in danger of passing on genetic conditions. The idea was to fertilize eggs in vitro, let them grow into multicelled embryos, and then extract a cell from each embryo and test it. If the embryo was found to be free of disease, it could be implanted in the mother, and the cell extracted for testing would not be missed (at this early stage an embryo's cells have not become specialized, so one will usually-though not always-be identical to the others). As practiced today, this procedure is called preimplantation genetic diagnosis (PGD).
Two other developments went into making PGD possible. One was the great advance in genetic science that took place in the 1970s and 1980s. This allowed researchers to find genetic markers-defects at specific spots on specific chromosomes-that indicated the presence of many genetically transmitted conditions. Once they knew where to look and what to look for, geneticists could design probes that attached themselves to chromosomes having a particular defect. So if a researcher wanted to test for Huntington's disease, for example, he or she would add a specially designed Huntington's disease probe to a sample of genetic material. If it attached itself to the sample, the defect was present.
THE FINAL PIECE IN THE PUZZLE WAS THE POLYMERASE chain reaction (PCR), which first became available in 1985 and earned the 1993 Nobel Prize in chemistry for Kary Mullis and Michael Smith. PCR allows a single molecule of DNA to be duplicated millions of times. This ability is indispensable, because to perform most PGD tests, geneticists need much more DNA than is found in the one or two cells that can be spared from a growing embryo.
In 1987 a British research group published proof that PGD was feasible, at least in mice. But the strong differences between rodents and humans took several more years to overcome, according to Alan Handyside, a member of that British team who is now a professor of developmental biology at the University of Leeds. Research groups on both sides of the Atlantic took up the challenge. In April 1990 Handyside and his colleagues Elena Kontogianni, Kate Hardy, and Robert Winston reported the first successful use of PGD in humans.
The condition they were looking for was adrenoleukodystrophy, the fatal "Lorenzo's oil" disease, which occurs only in males. Using a laser, they bored tiny holes in the coatings of IW-created embryos from two women who carried the gene for this condition. Then, with a tiny pipette and very delicate suction, they removed a cell from each embryo. By examining the chromosomes in the removed cells, they were able to identify and transfer exclusively female embryos, ensuring that the offspring would be free of this disease. Soon afterward groups in America and elsewhere began performing PGD, and as they developed the ability to test not just for gender but for many specific diseases and conditions, the applications were greatly expanded.
A different PGD method exists, however, one that does not involve extracting a cell from an embryo. This method, which is much more rarely employed, makes use of polar bodies, which are the leftover cells cast off when an egg cell is created. In the human body a normal cell contains 46 chromosomes, 23 from the father and 23 from the mother, arranged in pairs. Every cell, no matter where in the body it is found, contains the identical set of 46-except for sperm and egg cells, which contain only one chromosome from each pair, for a total of 23. Chance determines which chromosome from each pair will make it into the egg and which will get left behind in the polar body. When the sperm fertilizes the egg, the resulting merged cell has a complete complement of 46 chromosomes.
The polar body stays within the egg after the splitting process, though it plays no part in fertilization or the growth of the embryo. Therefore it can be removed and tested without creating any harm. Before undertaking polar-body analysis, the geneticist procures an example of the mother's complete genome. Subtracting the contents of the polar body from the complete genome reveals what chromosomes are in the egg. If a potential problem exists with one of these chromosomes, the egg can be discarded.
Obviously, polar-body analysis works only with conditions that are transmitted through the mother. The advantage is that since it is performed before fertilization, it does not involve manipulation of an embryo, so it sidesteps some moral and religious objections as well as government funding restrictions. But the uncertainties involved in polar-body analysis make it advisable to verify it by analyzing a second polar body, which is cast off during fertilization. This one contains the same 23 chromosomes as the egg. Since this analysis must be done after fertilization, it negates some of the method's presumed ethical advantages, though it still avoids removing cells from the embryo itself
Whether the genetic material comes from an embryonic cell (known as a blastomere) or a polar body, the next step after acquisition is to test it. With blastomeres, since an embryo takes three days after fertilization to reach the required size of 6 to 8 cells, and embryos must be transferred to the uterus on the fifth day, only 48 hours remain for analysis. In many cases, precious time is lost in transporting the cells from an IVF clinic to a suitably equipped genetic laboratory, of which there are only a few in the United States.
To check for a defect on a particular gene, the geneticist first removes the relevant portion of DNA from the cell or polar body. The extracted piece of DNA is then multiplied using PCR. This process creates essentially limitless numbers of identical bits of DNA by mimicking DNAs natural "zipper" action for duplicating itself. In PCR, a sample of DNA is placed in a machine along with a "broth" of the four nucleotides that compose DNA, plus a special heat-resistant enzyme and some short strings of nucleotides to help get the process started. The sample then undergoes alternating cycles of heating and cooling. When heated, DNA's two strands separate. As they cool, the enzyme attaches free nucleotides from the broth to the separated single strands to form new, complementary second strands. Two identical lengths of DNA now exist in place of the original one. Repeating the process multiplies the DNA exponentially, producing millions of copies in a matter of hours.
It sounds fairly straightforward, but several serious problems can crop up. The sample can be contaminated with extraneous DNA from lab personnel or equipment-remember, a single molecule is enough-and flaws in amplification can occur, the same as in any situation where multiple generations of copies are made from an original. Another possible problem happens when one member of a chromosome pair amplifies much worse than the other, or even does not amplify at all. Geneticists use elaborate analytic methods to overcome these problems, and while no fix is foolproof, PGD done by reputable labs is now reliable in nearly 90 percent of cases. Still, the possibility always exists that a diagnostic test can miss a mutation. Errors of this type occurred in some early PGD cases but were caught by amniocentesis during pregnancy. Women undergoing PGD are therefore always advised to double-check the results with amniocentesis or chorionic villus biopsy if pregnancy occurs.
A completely different PGD method, applicable to a different group of genetic irregularities, analyzes whole chromosomes instead of selected bits of them. It does not require PCR. The method is effective for sex selection as well as for a condition called aneuploidy, in which a person has too many or too few chromosomes. Sex-linked conditions that can be avoided with sex selection include hemophilia, Lesch-Nyhan syndrome, Duchenne/Becker muscular dystrophy, and a number of other recessive single-gene disorders. These mutations are inherited from the mother and carried on the X chromosome.
If the embryo gets a Y chromosome from the father, resulting in a boy, the defect on the X chromosome will be expressed in the child. But if the embryo gets an X chromosome from the father, resulting in a girl, it will almost always override the recessive trait from the mother's X chromosome. Having only girls, then, is a simple way to avoid these conditions. Today, with more sensitive testing, geneticists can examine a male embryo to see if it carries one of these mutations. When the first human PGD, involving adrenoleukodystrophy, was accomplished, however, no such method existed that could be performed in the short time available, so the parents chose to select only female embryos.
As for aneuploidy, examples include Down syndrome, which results from an extra copy of chromosome 21; Klinefelter syndrome, in which boys have an extra X chromosome, yielding an XXY configuration; and Turner syndrome, in which girls have only one X chromosome. These and other common aneuploidies often prevent embryos from implanting or cause miscarriages. If the pregnancy does proceed to term, the child may be afflicted with learning or language difficulties, sterility, short stature, or other problems, which can be severe or even fatal. Embryos with aneuploidy become increasingly common as women age. The same is true for another condition called translocation, in which the exchange of genetic material during cell division goes awry and members of different chromosome pairs trade DNA. Since these problems make successful pregnancies less likely, another use for PGD is to help infertile couples increase their chances of having a child, even if they do not carry any known genetic abnormalities.
The technique most commonly used to detect chromosomal anomalies is called fluorescence in situ hybridization, or FISH. In the first step of that process, the geneticist decides which condition or conditions to test for and prepares appropriate probes that will attach, or hybridize, to the target segments of DNA. After putting a biopsied cell on a slide, the geneticist adds the probes, each of which is tagged with a fluorescent dye that glows in a particular color under ultraviolet light.
Once hybridization has occurred and surplus probes have been rinsed away, the geneticist looks at the cell through a special microscope. Glowing spots on the chromosomes reveal which probes have hybridized and thus which abnormalities are present. Three gleaming dots when there should be only two indicates Down syndrome, for example. Other spot patterns can pinpoint translocations of pieces of DNA. FISH cannot reveal all possible chromosomal abnormalities, however, because probes exist for only some of them. Faulty diagnoses can also occur if, for example, overlapping chromosomes mask spots of color hidden underneath, or if only partial hybridization occurs between chromosomes and probes
Other techniques may soon join the PGD arsenal. Comparative genomic hybridization, which examines all of a cell's chromosomes instead of just specific ones chosen in advance, shows promise of exceeding FISH in accuracy and reliability. It has already identified aneuploidies invisible to previous methods. In this technique, a special PCR procedure amplifies a cell's complete genome and dyes the newly created chromosomes a fluorescent color, frequently green. Then a standard set of chromosomes is amplified and dyed some other color, usually red. The two sets are allowed to hybridize to each other, with a computer analyzing the resulting colors. Complete hybridization of the standard and test chromosomes shows up as brown; this indicates that the sample cell is normal. Green areas indicate extra chromosomes in the genome being tested, and red areas indicate where all or part of a test chromosome is missing.
PGD has become a routine (if expensive) procedure, but on its introduction in 1990, it was both a scientific milestone and a political sensation. In Britain, intense public discussion about the manipulation of human embryos was already under way, prompted by earlier work with IVF as well as other technologies, existing and projected. Parliament was considering a comprehensive ban on human embryo research. As Alan Handyside recalls, PGD proved "a wonderful advocate for human embryo research..We published the first successful pregnancies in the week of the debate in Parliament."
The timing was not accidental. The editor of the journal Nature "felt that the public should be informed of the potential benefits of embryo research, and so we were encouraged to submit the manuscript," Handyside says. The rapidly written paper was "accepted within three or four days." In the end Parliament voted to permit embryo research under government regulation.
Americans were also active in the race for PGD, and the United States took a very different approach. Not long after the British group's breakthrough, a Chicago team became the first to use polar-body analysis in human PGD. In addition, Mark Hughes and other Americans were working with Handyside's group in England. Together they extended the technology from whole chromosomes to small pieces of DNA and "did the world's first cases of this for specific genetic disease."
Then, Hughes recalls, the Americans "brought the technology back to the States," where government regulations forbade (as they still do) the use of federal money for any research involving human embryos. Far from preventing embryo research, this permitted the field to continue its rapid advance supported exclusively with private funds and effectively outside of federal control. As a result, decisions on when and how to use PGD in this country rest not with any official authority but with individual researchers and practitioners.
AS THE TECHNOLOGY HAS EVOLVED, SO HAVE ITS APPLIcations. Hughes, for example, initially resolved to use it only for "the most serious genetic disease" and not for traits that do not threaten a child's health. Then in 1994 a couple with a daughter dying of severe combined immune deficiency syndrome (SCIDS), the fatal "bubble boy" disease, asked Hughes to help them have a healthy baby by screening for the syndrome. He agreed, and preparations began.
Shortly afterward the parents made a second request, which, Hughes says, "blew my socks off." They wanted him to use genetic analysis not just to screen for SCIDS but also to make sure that the baby's bone-marrow type would be compatible with that of their sick daughter. A bone-marrow transplant could save the daughter's life. Such a transplant would take place while the baby was less than a year old and would put it at very little risk.
This request "was crossing a line," Hughes recalls, because it involved screening for a trait, not a disease. Therefore "we couldn't do this until we had thought it through." Hughes explored the question at length with bioethicists and others. But before he reached a decision, the girl's father confronted him and forcefully reminded him that a child's life was at stake and that he and his wife would love the second child just as much as the first.
"I don't know what is right from a whole societal perspective," Hughes says, "and I don't know what's right for everybody, but I knew at that moment what was right as a physician and scientist for that patient." He selected a suitable embryo, a healthy baby was born, the transplant succeeded, and the sick child lived. Now, says Hughes, his team sees this type of case "two or three times a week." Selecting an embryo for bonemarrow suitability has become a commonplace procedure.
Not all ethical questions can be resolved so neatly. Some observers fear that the power to choose embryos with particular traits might be used not just to save lives but to give favored children social advantages. Could parents buy themselves designer babies with musical talent, athletic ability, desirable looks, or superior intelligence?
PGD practitioners suggest that this risk is small, since the procedure does not implant genes but merely selects among those that are already present in the embryo. "If you don't have genes for blond hair, you're not going to find embryos with them," Handyside says. "The best way to design a baby is to choose your partner." And, adds Hughes, "No one in their right mind would go through IVF if they didn't have to.. Nobody is going to use [PGD] for trivia." But not everyone has the same idea of what constitutes trivia. Gender, for example, is already widely, if often surreptitiously, determined by use of PGD.
Some critics argue that preventing the birth of children with non-life-threatening traits that some consider undesirable will devalue the lives of people who already have those traits. Philippa Taylor of the Centre for Bioethics and Public Policy, a Christian-oriented British group, asserts that "PGD involves selection on the grounds that some lives are not thought worth living.. It is difficult to believe that in a society that has overcome its fears of disability and truly considers disabled people as equal members of the community, there would be such a strong interest in PGD (or indeed prenatal screening)."
Taylor predicts that the fewer people there are with a given disability, the less attention and funding for research and services it will get. She also says that "PGD (and prenatal diagnosis) promote the idea that it is part of responsible parenthood to avoid the birth of a handicapped child." Yet many people believe just that and see modern technology as the way to achieve it.
But even if PGD presents society with unprecedented ethical challenges, it has forever transformed medical genetics from a field with nothing to offer many afflicted families into one that has changed thousands of lives for the better. PGD frees couples from the fear of transmitting misery to new generations, permits others to bear longed-for children, and allows children who might have been doomed to premature and painful deaths to live healthy lives.
BERYL LIEFF BENDERLY is a freelance writer in Washington, D.C. Her most recent article in Invention & Technology, about the development of chemotherapy, appeared in Summer 2002.