COLUMN ONE : Unraveling the Secrets of Genes : Genetic experiments have exploded and could hold the key to making such diseases as cystic fibrosis, Huntington’s and Lou Gehrig’s obsolete. But ethical questions remain.
Huntington’s disease. Lou Gehrig’s disease. Cystic fibrosis. Duchenne muscular dystrophy. Fragile X syndrome. Marfan syndrome.
These once-mysterious and disabling disorders and a host of others are beginning to yield their deepest secrets to the inquiring chemical scalpels of molecular biologists and geneticists.
Scientists are riding an unprecedented wave of medical research that many believe is bringing them closer and closer to the dream of a world where genetic disease is obsolete. Virtually every week, researchers report the discovery of a disease-linked gene.
Pinpointing the defective genes that cause diseases such as cystic fibrosis opens the door to what many scientists consider the ultimate cure for genetic maladies: gene therapy, the replacement of a defective gene with a healthy one.
Three years ago, there were no gene therapy experiments under way. Today, there are nearly 50, and new ones are being proposed regularly.
“Over the next 20 to 40 years, we will have the potential for eradicating the major diseases that plague the . . . population,” said molecular biologist Leroy Hood of the University of Washington. “Biotechnology will fundamentally change how we will practice medicine.”
In the 10 years since molecular biologists discovered the techniques of genetic engineering, researchers have identified the causes of more than 25 serious genetic disorders and hundreds of other genes as well. Myriad other discoveries seem just over the horizon.
These discoveries have already allowed geneticists to perform prenatal screening of high-risk groups that have family histories of diseases such as muscular dystrophy, Huntington’s disease and cystic fibrosis. But more important in the long run, advances in genetics have frequently provided the first insights into how these diseases wreak their havoc.
“Every disease we know about is either being attacked with genetics or is being illuminated through genetics,” said Nobel laureate David Baltimore, a molecular biologist at Rockefeller University in New York City.
Eventually, the new developments will provide the ultimate answer to controlling the escalating costs of health care, Hood said. As physicians learn more about the multiple genetic causes of cancer, heart disease and other widespread disorders, they will be able to practice much more preventive medicine, he said, reducing the costs of health care by minimizing the need for it.
But Hood and others also caution that the discoveries are prompting questions about the ethics of using genetic screening to identify people with inborn susceptibilities to disease and of using gene therapy.
The rapid pace of recent research is all the more remarkable because it has been a mere 40 years since two brash young scientists at the Cavendish Laboratory in Cambridge, England, deduced the structure of DNA--deoxyribonucleic acid, the genetic blueprint of life. In the April 25, 1953, issue of the British journal Nature, James D. Watson and Francis H. C. Crick proposed the now-famous double-helix structure of DNA, an event that marked the beginning of the modern era of genetics.
But that era was slow to bloom, especially in its application to human beings. It was not until 1978 that molecular biologist Y. W. Kan of UC San Francisco first used information from DNA for the prenatal diagnosis of sickle cell disease and beta-thalassemia. Both disorders involve defects in the gene that is the blueprint for hemoglobin, the oxygen-carrying molecule in red blood cells.
About the same time, scientists working in several laboratories made a discovery that made genetic engineering and the search for genes practical. They found a family of naturally occurring proteins called restriction enzymes and polymerases. Restriction enzymes split DNA at specific sites, producing useful fragments--such as genes--while polymerases stitch it back together.
These tools gave scientists the ability to isolate genes and other DNA fragments and insert them into other organisms to produce large quantities of a gene or to change the physical characteristics of the recipient. Early on, researchers at Genentech in South San Francisco introduced the gene for human insulin into bacteria to produce insulin for diabetics. Others inserted the gene for human growth hormone into pigs to produce animals with leaner meat.
Restriction enzymes are the “foundation of the genetic engineering industry,” said molecular biologist Joshua Lederberg of Rockefeller University, and have proved equally valuable in the search for genes that cause disease.
Just as crucial was the 1983 discovery of an enzymatic process called polymerase chain reaction (PCR) by Kary Mullis, then a molecular biologist at Cetus Corp. in San Francisco--a feat for which he won the 1993 Nobel Prize for chemistry. PCR, Lederberg said, led to “democratization of molecular biology” by allowing a single piece of DNA in a messy mixture from a cell to be fished out and reproduced in large quantities, at low cost and with simple instruments. PCR played a key role in the isolation of the defective genes that cause cystic fibrosis and Fragile X syndrome.
“High school students do experiments today that would have been doctoral dissertations 15 years ago,” Lederberg said.
The pace of discovery is likely to become even more furious. Researchers are in the third year of the $3-billion, 15-year Human Genome Project, whose ambitious goal is nothing less than decoding the human genome--”the complete set of instructions for making a human being,” in the words of biochemist Robert Sinsheimer of UC Santa Barbara. Their aim is not only to locate and identify the estimated 100,000 human genes that make up about 10% of the genome, but also to decipher the sequence of the 3 billion individual chemicals, called bases, that compose the entire genome. The sequence is the order in which bases are strung along the DNA chain, providing information in the same way that letters form a sentence.
Including work done before the initiation of the Human Genome Project, researchers had determined the sequence of a grand total of 3,837 genes by the end of March. Only a relative handful of the identified genes cause disease. The rest are healthy genes whose functions in the cell are known.
The search for new genes relies heavily on two key tools whose uses are inseparable: large human families and genetic markers--short fragments of DNA with a unique sequence. Scientists home in on defective genes by identifying markers that are present only in family members that suffer from the inherited disorder.
Looking for a specific gene in the mammoth human genome without any signposts is much like looking for a needle in a haystack. Genetic markers provide these signposts and allow researchers to navigate their way through the immense maze of the genome.
But the markers by themselves are of limited value for finding the causes of genetic diseases. Identification of the defective genes also requires the use of a family with a history of the disease, and the bigger the family the better.
The use of families relies on the fact that each person’s DNA is slightly different from that of everybody else. When one person’s genome is chopped up with restriction enzymes, it will produce a pattern of genetic markers that is different from that of other people.
The key in using families is to locate genetic markers that are inherited by people with the disease but not by healthy family members. When such inheritance occurs, it means that the marker lies close to the defective gene. Researchers then search for other markers in that same region, hoping to find a series of markers that get closer and closer to the defective gene until the gene is identified.
The markers can be useful for genetic screening of newborns and adults in affected families. For years, researchers have used a particular pattern of markers in families with Huntington’s disease to show with 95% accuracy whether individuals will develop the disease. The closer the marker is to the actual gene, the higher the percentage of success.
The families are so important that the search for them has led geneticists to some otherwise unlikely research sites. Geneticist Raymond White moved his laboratory from the University of Massachusetts to the University of Utah because of the large number of Mormon families there. The Church of Jesus Christ of Latter-day Saints, headquartered in Salt Lake City, not only encourages its members to produce large families, but also urges them to compile extensive genealogies.
After arriving in Utah, White collected DNA from 46 healthy families of three generations--four grandparents, two parents and at least six children. Over the years, he has put together more than 500 markers scattered throughout their genomes. By comparing these markers to those in families with a history of a particular disease, it is possible to locate the gene.
Using the markers from a family with a history of colon cancer, White, Utah geneticist Mark Skolnick and their colleagues identified the defective gene on Chromosome 5 that causes familial adenomatous polyposis, which is responsible for about 1% of all cases of colon cancer.
The markers were also crucial to White’s discovery of the gene for neurofibromatosis, a disfiguring disease often referred to incorrectly as “elephant man’s disease,” and the identification of the gene for cystic fibrosis by molecular geneticists Francis Collins of the University of Michigan and Lap-Chee Tsui of the Hospital for Sick Children in Toronto. White’s DNA markers have also become the core of the collection at the Centre d’Etude du Polymorphisme Humain in Paris, which provides them to researchers around the world.
The search for the gene that causes Huntington’s disease revolved around an even larger family in a more unusual location. Molecular biologist Nancy Wexler of Columbia University compiled a pedigree of nearly 11,000 residents of the remote village of San Luis on Lake Maracaibo in Venezuela. All 11,000 were descendants of a single woman with Huntington’s disease who moved to San Luis in the 18th Century, and DNA isolated from about 2,000 relatives was critical in the discovery of the Huntington’s gene, which was announced in March.
Researchers are still looking for appropriate families to study for other disorders, such as Alzheimer’s disease, a degenerative mental disorder of aging. “There are a lot of good molecular biologists involved in Alzheimer’s disease,” Wexler said, “but they just don’t have the families to work with.”
The discovery of the gene that causes an inherited disorder has many potential ramifications. At the very least, the discovery enables prenatal identification of individuals with the disease, allowing parents to take appropriate action--whether it is having an abortion in the case of very serious disorders such as Huntington’s or increased vigilance for the onset of the disorder, as is the case with genes that cause colon cancer.
Widespread prenatal screening for beta-thalassemia, for example, has largely eradicated the disorder in Sardinia, where it was once common, and has greatly reduced the incidence of Tay-Sachs disease in the United States.
The discovery of a gene can also open the door to new forms of therapy. When researchers this year discovered that amyotrophic lateral sclerosis, better known as Lou Gehrig’s disease, is caused by a defect in the body’s antioxidant system, they immediately began planning trials with antioxidant medications such as Vitamin E.
Researchers are working to identify the function of the defective gene that causes Huntington’s disease in the hopes that new therapies will be possible there as well.
But the discovery of a gene does not mean that successful drug therapy is inevitable. Scientists have known the cause of sickle cell disease since the beginning of the age of molecular biology but have not been able to design a therapy based on it. The same is true for Down’s syndrome.
“Having localized and characterized a gene doesn’t mean we can do anything about it,” Hood said. “Every isolated gene is a unique story.”
The cure for diseases such as sickle cell may ultimately involve gene therapy, in which a healthy gene is inserted into a patient’s genome to correct the disorder. Gene therapy is being used to treat such conditions as severe combined immunodeficiency disease, in which the victim lacks an enzyme that is crucial to the functioning of the immune system, and cystic fibrosis.
Such therapy may also prove useful for combatting a number of other disorders, such as cancer, and even for diseases that are caused by viruses, such as AIDS. Close to 100 gene therapy projects are approved or under way around the world.
The identification of defective genes is also raising a number of ethical questions about the use--and potential misuse--of genetic information. Scientists recently discovered the gene that causes the major inherited form of colon cancer. Virtually every person who has this gene will develop cancer, and that prompts several concerns, including: Will people who have the gene be able to get health insurance? Will employers hire them? Will they become genetic outcasts?
Genetic Blueprint
Humans have an estimated 100,000 genes, each of which serves as the recipe for a protein--the fundamental unit of heredity. Here are the basic elements of the genetic code:
(Cell drawing): Each of the 100 trillion cells in the human body (except red blood cells) contains the complete genetic blueprint of a human in the form of deoxyribonucleic acid, DNA.
(With chromosome drawing): DNA is packaged into 23 pairs of chromosomes. One chromosome in each pair comes from the father, one from the mother.
(With helix detail): The long, fragile DNA molecule is made up of two intertwined strands called a double helix. The two strands have four types of compounds called bases. The four bases--adenine, cytosine, guanine and thymine--constitute the ‘alphabet’ that makes up the genetic code.
(With drawing of a protein molecule): The information encoded in DNA is used to build chains of amino acids in protein molecules. Proteins are the workhorses of cells, providing structure and carrying out all the chemical reactions that occur in metabolism.
DNA compound bases: A, C, G and T
Researched by VICKY McCARGAR / Los Angeles Times
