Prenatal and Preimplantation Genetic Diagnosis for X-linked Alport Syndrome

By L.M. Nelson

and K. Ward
Departments of Obstetrics and Gynecology and Human Genetics
University of Utah School of Medicine, Salt Lake City, Utah

Presented at Patient, Family and Friends Day
Fourth International Workshop on Alport Syndrome
April 15, 1999, in Salt Lake City

Shortly after the X-linked form of Alport syndrome was mapped to a specific location on the X chromosome our laboratory began to offer DNA diagnostic testing for a variety of requests including carrier status, presymptomatic detection, prenatal diagnosis and evaluating the possibility of preimplantation diagnosis.

Prenatal diagnosis is the process of evaluating the presence of disease or potential disease in the unborn fetus. This enables families to make critical decisions before the birth of the baby. Prenatal diagnosis involves collecting a sample of DNA from the fetus, usually between 10-20 weeks of gestation. The collection is most commonly done using amniocentesis or sometimes by chorionic villus sampling. When linkage analysis is used the baby's sample as well as several other key family members samples are examined to determine whether the baby will develop Alport syndrome, be a female carrier or be unaffected. If the family's mutation is known, only the baby and controls need to be tested. Both of these tests give diagnostic predictions with greater than 98-99% accuracy.

Preimplantation testing is a relatively new way to look even earlier into the genes of the developing embryo. In preimplantation diagnosis a normally fertile couple will choose to undergo in-vitro fertilization, "test-tube babies," and then test each of the embryos for a genetic disease, in this case Alport syndrome, before they are transferred to the mother's womb. This amazing technology has allowed couples an even earlier decision option. Preimplantation diagnosis, however, is difficult, costly and not widely available, but is still an exciting development in "prenatal" DNA diagnostics. Preimplantation testing for Alport syndrome in a family with a known mutation is presently being offered by one center.

To date, our laboratory has offered prenatal diagnosis in 12 cases. Eleven of these cases have been diagnosed by linkage analysis, observing patterns of inheritance in a family, and one case has been diagnosed by looking specifically at the mutation that causes Alport syndrome in that family. We have also consulted in the development of a preimplantation test for Alport syndrome.

Summary taken from
PRE-IMPLANTATION GENETIC DIAGNOSIS

This is a procedure that is intended to "weed out" genetically defective embryos before they have a chance to develop. It is usually requested by prospective parents who are concerned about passing an incurable genetically based disease or disorder to their child. Typically one or both partners have been genetically screened and found to be a carrier.

The technique was developed only recently. It involves the following steps:

The woman is given drugs to produce "super-ovulation." She normally produces many eggs, which are collected.
The eggs are placed in a dish and are fertilized by donated sperm (usually from the woman's partner).
As each embryo divides to about the 8 cell level, one cell is removed.
That cell is analyzed for one or more genetic abnormalities. If it contains a genetic defect, then the embryo from which it was taken is destroyed.
Typically 3 of the embryos which are free of abnormalities are implanted in the woman's womb. The others are destroyed.
Sometimes, none of the 3 embryos develop into fetuses, and the procedure is repeated. Often one or even two embryos do live and develop into fetuses which are later born.

At this time, cells can be checked for about two dozen genetically determined diseases, including Huntington's disease, Cystic Fibrosis, Tay Sachs disease, Hemophilia A, Fragile X syndrome, Duchenne muscular dystrophy, Lesch-Nyhan syndrome - Retinitis pigmentosa, Charcot-Marie-Tooth disease, Barth's syndrome, Turner syndrome, Down's syndrome and Rett's syndrome. Female embryos can be checked for a gene that increases the propensity to develop breast cancer.

Some genetic diseases are sex-linked, e.g. they can be only passed on to male children. Even if a particular sex-linked disease cannot be detected directly, the PGD method can eliminate all of the male embryos and implant only female embryos, thus preventing the transmission of the disease.

The first PGD baby was born in 1989. (1,2) By 1997, over 30 babies had been born world-wide, following the use of this technique.

Dr. Perry Phillips, an obstetrician and gynecologist, is one of the directors of IVF Canada. He said: "This is the beginning of the end of genetic disease. That's the dream of medicine. It's our dream. This should have the same impact [that] antibiotics did to bacterial disease."

Objections to the Procedure:

The cell that is removed could conceivably have developed into a fetus. But the testing will destroy it. Many individuals who believe that a fertilized egg is a full human being and deserves full protection would consider this an act of murder. The fertilized cells that are not implanted are destroyed. Again, some regard this as murder.
Dr. Patricia Baird, a geneticist who led the Canadian Royal Commission on New Reproductive Technologies said: "Because there is so much money involved, there is a real danger of premature, unwise application of this procedure." We find this comment confusing. The procedure would cost in the vicinity of $4,500 to $7,000 in US funds. It would seem to be more likely that because of the money involved, the procedure would only be applied in unusual cases after much careful thought.
A genetically defective fertilized egg, if allowed to mature and cause a live birth, would not necessarily generate a disorder or disease in the individual. Various genetic variations (called alleles) have a penetrance factor, which is a measure of their effectiveness or power. For example, the allele which causes Huntington's Disease has a 100% penetrance: if you have the allele, you will certainly develop the disease. But other genetically determined conditions have a much lower penetrance: left handedness is only about 15%. Thus, many embryos would be killed which would never have caused a disease or disorder. Some genetically caused diseases only develop symptoms when the person is in their 30's or 40's. By that time, a cure might have been found.
The procedure could be the start of a slippery slope. Perhaps embryos would be eliminated that might leave the individual at higher risk for heart disease, or stroke, or obesity, etc. And there is the possibility that the procedure could be used to eliminate female embryos, or embryos that would grow into adulthood with a minority sexual orientation.

Advantages to the Procedure:

Most genetic testing now is done through amniocentesis when the fetus is 12 to 16 weeks old. This consists of drawing a sample of the amniotic fluid from around the fetus and examining a floating cell from the fetus. If the analysis shows that the fetus is genetically defective, then the parents have the option of aborting the fetus. Essentially all couples do elect to have an abortion. This is far more distressing to most couples, because the fetus is so developed. The Pre-Implantation Genetic Diagnosis technique is performed before pregnancy begins. This avoids "the stress, emotional trauma and subsequent moral dilemma" of amniocentesis.
Some adults who know that they are carriers of a genetically transmitted disease decide to not have children. The Pre-Implantation Genetic Diagnosis procedure allows them to have a child with full assurance that it would not be carrying that disease. (Of course, the child could be born with other malformations, diseases and disorders that were not tested for.)
If the procedure became widespread, the incidence of many diseases would be reduced. The procedure could significantly reduce the cost of medical systems in North America. Treatment of some genetic diseases can easily cost multiple-millions of dollars over the lifetime of a single individual.

REFERENCES:
1. Anuja Dokras, M.D., Ph.D., Pre-Implantation Genetic Diagnosis, Vol.1 No.5.
2. Fact Sheet: Preimplantation Genetic Diagnosis from the American Society for Reproductive Medicine, 1996-DEC.
3. S. H. Black, Preimplantation Genetic Diagnosis and Sperm Separation. See also Custom Preimplantation Genetic Testing for Rare Genetic Diseases.

4. S.A. Beyler, "Diagnosis of Genetic Diseases in the Preimplantation Embryo." Lab Med, 1993; 24:642-647.

USEFUL WEB LINKS
A collection of links from Yahoo! to sites concerned with PGD and related issues.
Ask Jeeves!, super-search site that allows you to ask "normal" questions. Possibly a good way to keep track of new information on the Internet concerning PGD, related topics or anything else. (Questions are answered by computer-aided humans!)
The Research Genetics Institute in Chicago. A commercial organization that offers PGD—including for Alport syndrome—and other genetic testing services.
The Alport Syndrome Home Page, which includes several links to information on genetic testing.
The University of Utah DNA Diagnostic Laboratory, which offers both genetic linkage testing and direct mutation testing for the X-linked form of Alport Syndrome.
The National Center for Biotechnology Information, containing many links to sites describing current progress in the analysis of the human genome.