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Tracking Down the VHL Gene
[Note 12/97: The VHL gene was located in June 1993. This article is a good history of how they closed in on the gene and the reasons that motivated the search. See also VHL Tumor Suppressor Gene Found!, June 1993.]
"I’m thinking of a number between one and one hundred." One child chooses a number, and the other must guess the number by asking yes and no questions. Looking for a gene is a similar undertaking, but on a much broader scale. It is somewhat like looking for a particular house, not knowing at the beginning which continent the house is on.
There has been considerable progress in the molecular genetics of VHL disease. Dr. Bernd Seizinger and his team, then at the Massachusetts General Hospital, began by doing genetic linkage studies on nine families with VHL. They collected genetic markers which had already been identified, and tried them on samples from affected individuals.
The researchers watched the pattern of inheritance of particular genetic markers in VHL families to see whether they were present in the affected parent and not present in the unaffected parent, present in affected children and not in unaffected children. This method was used to determine whether a particular marker was associated or "linked" with the VHL gene.
Researchers then compared these inheritance patterns with the inheritance patterns of that same marker in a control study, a Venezuelan family whose genes were mapped in great detail by Dr. Nancy Wexler for the purpose of studying Huntington’s Disease. While this family is affected with Huntington’s Disease, it is normal for VHL, and therefore provides a good reference.
They found that one marker which mapped to chromosome 3 was pretty tightly linked to VHL, and announced this finding in 1988. This finding has since been confirmed by Dr. Berton Zbar at the National Cancer Institute, by Dr. Eamonn Maher at Cambridge University, by Dr. David I. Smith at Wayne State University, and by Dr. Jeff Vance at Duke University Medical Center.
Now at least they knew which continent the house was on. With an ever-growing group of families participating in the study, they continued to do comparative analysis of the genetic information in the family samples. The researchers examined the markers found on chromosome 3 in each in sample, and recorded the pattern for family members with and without clinically diagnosed VHL. Using additional markers on chromosome 3, and LINKAGE, a computer program from the Howard Hughes Medical Institute, to calculate the probability of linkage of a particular area to VHL, they have narrowed the search to an area known as 3p25-p26, near the tip of chromosome 3.
They are in the right general area. While these markers may seem close, there are between 5 and 8 million base pairs of genes between them. The house we’re looking for is somewhere in the Middle West. There is still a long way to go.
Dr. Berton Zbar and his team at the National Cancer Institute, working in parallel with the Seizinger team, have discovered another set of five markers which take him to the same area, and which can also be used to predict whether the VHL gene is present.
The markers find genes which are found throughout the normal population. "The marker is unrelated in any functional sense to the VHL gene. It is only related in its proximity to the VHL gene," says Corinne Boehm, director of the DNA Diagnostics Laboratory at Johns Hopkins Center for Medical Genetics. "Because these markers are so closely located on chromosome 3, they usually get inherited together as a complex with the gene, and that pattern can be used to predict the probability of VHL inheritance in other family members."
Even though the gene itself has not yet been identified, Dr. Zbar finds that some arrangement of these markers tends to be inherited along with the VHL gene in a consistent pattern in a given family. While the pattern in each family will be different, once a family’s unique pattern has been worked out it can be used to do some initial testing to determine whether someone in this family is at risk of developing VHL cysts and tumors.
"But," Dr. Zbar cautions, "because the test does not measure the disease gene itself, errors in risk prediction can occur." Where informative markers are present on both sides of the gene, the accuracy of the test is above 99%. This is the case in only about 44% of the families studied. It is possible to improve the ease of risk prediction by isolating more markers, or by locating the gene itself.
Why so many errors in our DNA?
Determining risk: "When it is not possible to predict whether an individual in a VHL family is a VHL gene carrier, that individual must be screened as though they were carriers of the VHL gene. Otherwise, persons who do not yet have symptoms but who may have potentially dangerous kidney and other tumors may go untreated." Dr. Zbar is hopeful that the gene will be isolated within a year.
Dr. Vance at Duke is working with yet another group of markers, building on the work of the Seizinger team. "Some markers are not very useful in every family," says Dr. Vance. "We are trying to make more informative markers." Dr. Maher at Cambridge is following a similar pattern to that of Drs. Zbar, Vance, and Seizinger.
Dr. Smith and his team at Wayne State are using yet another technique called Yeast Artificial Chromosomes (YAC) which latch on to twenty points along chromosome three and essentially form overlapping markers which map the entire area where the VHL gene is located. They are now in the process of isolating genes in this region and testing them individually as candidates for the VHL gene. "It is amazing how fast the research is moving now," says Dr. Smith. "The tools are getting better, the reagents are getting better — it’s a very exciting time."
What does this gene do? "The hypothesis," says Dr. Jean Whaley of the Seizinger team, "is that the VHL gene is one which normally produces a protein relating to cell growth or DNA replication. When someone has VHL, the ability to produce this protein is in some way deactivated or altered." To determine just what its function is, researchers must first isolate the specific gene, and study its normal activity in the laboratory. They also have to show clearly that this gene is in fact normal in unaffected people and deactivated in people affected with VHL, and that this inactive form of the gene is present in VHL tumor tissue.
Potential benefits for everyone: By studying inherited forms of tumor conditions, like VHL, scientists hope to unlock the mysteries of tumor conditions in the normal population. One interesting hypothesis is that there are two copies of this gene, one inherited from each parent. Both copies need to be deactivated in order for the cell to lose its control over cell growth. In a person with VHL, there is one normal copy and one inactive copy, so it is only necessary for one to drop out of function. Normal cell division and gene replication is complex. It is usually accurate, but occasionally a mistake is made during this replication process.
Why? "We often think of cancer as a disease of older people," says Dr. Whaley, "because statistically more older people have cancer. For example, the average age of onset of kidney cancer in the general population is about 62. In persons with VHL, the age of onset is significantly younger — about 45, and often much younger. Perhaps there is some influence from the environment — from exposure to sunlight, food additives, air and water pollution, smoking, or any of a number of other possibilities — which causes this tumor suppressor to become deactivated. In persons with VHL, this happens earlier because only half of the system is active to begin with. But by understanding this mechanism we would learn useful lessons for the entire population."
Specifically for VHL families, the immediate short-term goal is to be able to do reliable predictive testing, and with the methods which Dr. Zbar and other researchers are proposing, we are part-way there. "The clinical examinations are costly, inconvenient, and may not yield definitive diagnostic information," says Dr. Gladys Glenn of the Cancer Diagnostic Branch, National Institutes of Health. "If it were possible to identify disease gene carriers, medical attention could be directed to those individuals at high risk of tumor development."
Dr. Maher is following patients clinically to determine the value of screening and presymptomatic diagnosis in reducing the severity of VHL.
Carrier Detection Testing: Using genetic analysis, the Johns Hopkins Center for Medical Genetics in Baltimore, Maryland, and Addenbrookes Hospital in Cambridge, England, are beginning to perform some testing in families. In order for a member of a VHL family to be tested, there usually need to be DNA samples from two affected family members. The Center asks that you work with a local geneticist to do the background collection of family history information, and to provide you with a local team to assist you in analyzing the results. Your geneticist can contact the lab to determine whether the testing would be useful in your family.1
"It is also important for families to realize," says Ms. Boehm, "that this testing is still experimental and is not 100% reliable. At this point it should be used primarily to determine levels of risk in an affected family, to tell who should be followed more closely than others. In some families with the right kinds of markers present, it may even be reliable enough to do prenatal testing. In families where we have sufficient samples it can be used to determine who is a carrier of VHL, and can therefore provide some useful genetic counseling information."
The longer term possibilities and dreams are endless. Once the gene has been cloned and we understand its operation, we might discover a missing protein or enzyme, or the operation of the tumor suppressor, and learn how to stimulate the tumor suppression mechanism artificially. Dr. Seizinger’s lab has recently moved to Bristol-Myers Squibb Laboratories in New Jersey. Clearly Squibb is gambling that this research will lead to a commercially viable treatment — or cure! — for cancer. It will probably take another decade or two, but the prospects are more real now than ever before.
Be Part of a Modern Miracle: At this point, there is a race to identify the VHL gene, with teams in a friendly mix of collaboration and competition with one another. This is good news for VHL families, since it means that researchers are highly motivated, and are making good progress toward identifying the gene. It is in our interest to keep them all going, and to supply them with as much information as we can. They are using different techniques. No matter which team identifies the gene first, we gain the diagnostic information we need for our families.
Families from various part of the United States including Hawaii, from Canada, the Netherlands, France, Germany, and Iceland have participated in the U.S. studies, and families in Europe are participating in a parallel study in England under Dr. Eamonn Maher. This work would not have been possible without the willingness of patients and physicians to provide family history information, blood, and tumor tissue samples. Every team we spoke with was effusively grateful for the cooperation of their participants.
Researchers still need more families, especially ones where there are a number of affected individuals. In addition to blood samples, they need samples of tumor tissue. To keep the research moving quickly, contributions of blood and tumor tissue to the teams would be appreciated.2 Dr. Vance is willing to coordinate collection of samples and ensure that samples are made available to all four U.S. teams.
Before any surgery, please have your surgeon call the team or teams of your choice to arrange for quick delivery of fresh tissues, along with your written request that they be made available also to additional teams. Sharing of samples requires your permission in writing and the team’s agreement in advance.
Your participation could be just what is needed to create the breakthrough we are all waiting for.
Notes:
1. To arrange for carrier detection testing, ask your Geneticist to contact Ms. Corinne Boehm, DNA Diagnostics Laboratory, Center for Medical Genetics, Johns Hopkins Hospital, Baltimore, MD 21205, Tel: 1-410-955-0483, Fax: 1-410-955-0484; or contact Dr. Eamonn R. Maher, Clinical Genetics, Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, U.K., Tel: 44-223-216446, Fax: 44-223-217054. [Note 12/97: At this time DNA testing works for 93% of the families in labs with the most modern equipment. Ask the lab what their success rate is. See the list of DNA testing centers for VHL.]
2. The research teams are as follows. If you would like more than one team to have access to your samples, you have to make your wishes known in writing, and get the recipient’s agreement in advance. [Note 12/97: this list is no longer correct. The basic research done by these teams has been completed, and the gene has been found. For the current state of research on VHL, please see http://www.vhl.org/research
References: Glenn et al., JAMA, 267:9 (1992), 1226-1231; Seizinger et al., Proc. Natl. Acad. Sci. 88 (1991), 2864-2868; Pines et al., Blazing a Genetic Trail, Howard Hughes Medical Institute, 1991.
as published in March 1993, VHLFF 1:1 |
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