Understanding How Genotype Influences Phenotype

by Sarah Nielsen, Genetic Counselor

Editor’s Note: The “genotype” is the makeup of the altered genes—where is the alteration in the gene?—and the “phenotype” is the set of symptoms and tumors experienced by a person with that genotype. By studying how families and family members are the same or different, we are learning more about how the VHL protein functions in the cell connecting with other proteins and enzymes to regulate normal processes, and what happens when one of those connections is changed.

Von Hippel-Lindau (VHL) disease is what is described as an inherited endocrine tumor syndrome. Breaking this down, it means that VHL is passed down in families and causes an increased risk for a variety of tumors that may be cancerous or not, and can occur in endocrine glands (whose job is to produce hormones) as well as non-endocrine organs. Some examples of organs that can be affected by VHL-related tumors include the kidney, adrenal glands, pancreas, eyes, brain, and spine. VHL has 90% “penetrance” by age 65, which means that if you have an alteration in the VHL gene, there is a 90% chance that by age 65 you will have shown at least one symptom. This paper focuses on one specific type of VHL, type 2A, which is usually associated with a high risk for adrenal gland tumors (called pheochromocytoma, or “pheo”), eye tumors (retinal angioma, or “RA”), and brain/spine tumors (hemangioblastoma, or “HB”), and a low risk for kidney and pancreatic tumors. Specifically, the paper takes a closer look at pheochromocytoma, the most common tumor type in VHL type 2A. Pheo is a very dangerous tumor that can become cancerous. Even if it doesn’t metastasize, if it is left untreated, it can result in death due to complications of severe hypertension (high blood pressure).

VHL disease is caused by a mutation, or change, in a gene called a tumor suppressor. Genes are like packages of information that tell our body how to function and determine what we look like. Tumor suppressors are specific types of genes that prevent tumors from forming, so when there is a mutation in the gene, it is more likely that tumors will develop. The gene that causes VHL is also named VHL and is located on chromosome number 3. We receive two copies of each of our 23 chromosomes, one from our mother and one from our father. It only takes a change in one of our chromosome 3s to have VHL disease. This study compared two very large families with VHL type 2A that both have German ancestry (from different regions of Germany) and have settled in the Western Pennsylvania area.

Although the families have the same type of VHL, they have different mutations in the VHL gene. They both have “missense” mutations, which means only one nucleotide, or letter, has been changed in the spelling of the gene, but those letters are different in each family. Both “misspellings” prevent the VHL gene from doing its job and result in an increased risk for the tumors associated with VHL type 2A. The mutation in Family 1 is called Y112H and the mutation in Family 2 is called Y98H. These are the two most common mutations found in VHL type 2A families. This paper looks at how these different mutations might result in differences in how the disease is expressed, especially related to pheo.

Because members of both of these families had been followed for decades at the University of Pittsburgh, and their VHL-specific medical and genetic counseling records were available for review, family members were re-contacted and extensive interviews were conducted. After all the relevant information was collected, pedigrees (family trees) were constructed for each family. All the members with VHL were tabulated, noting which types of tumors had been diagnosed and at what age. Then the characteristics of the pheos diagnosed in each family were examined more closely with regard to specific clinical and pathological features, such as the age at first diagnosis, chemicals produced, number, location, size, and weight, and what the tumors looked like under the microscope.

It was discovered that the overall expression of the disease was very similar between the families. In Family 1 (Y112H mutation), out of 108 family members determined to be at risk, 49 members were confirmed to have VHL (45%); 71% of these individuals have shown at least one symptom of VHL. Pheo was the most frequent tumor type (86%), followed by RA (37%), HB (17%); there was only one case of kidney cancer. Similarly, in Family 2 (Y98H mutation), 65 out of 131 at-risk members were confirmed to have VHL (50%); 72% have had at least one symptom of VHL. Pheo was the most frequent tumor type (79%), followed by HB (26%), RA (19%); again, there was only one case of kidney cancer.

Differences between the families were discovered regarding pheo expression. The most significant differences were that the average age of pheo diagnosis was 9 years younger in Family 2 compared to Family 1 (20 years old vs. 29 years old), pheo in Family 2 was more likely to be multiple (multifocal), and pheo in Family 1 was more likely to be cancerous and result in death. Additional differences that were not as strong between the families were that in Family 1 pheo was more likely to be diagnosed by symptoms rather than routine screening and additional pheos were diagnosed sooner after first diagnosis; in Family 2, pheo was more likely to be found outside the adrenal glands (a special type of pheo called paraganglioma). Size, weight and frequency of left- or right-sided pheos were found to be the same between the two families.

The limitations of this study are that, although the families were large, it is still a relatively small sample size, so we cannot say that these findings hold true for all families with these mutations. Also, because there were many members in each family, we could not ensure that they were all followed properly regarding their VHL, and some members did not always come on schedule for their screening tests. Some information used in the study was verbal and could not be verified by medical documents. Lastly, we were unable to take into account environment and lifestyle differences between the families that could have contributed to the differences observed.

The take-home points of this study are multiple:

1) it is important to screen for pheo beginning in childhood (pheos occurred as early as 6 years old in the study);
2) children who have pheo or paraganglioma should be tested for VHL gene mutations;
3) if individuals with a Y98H mutation have abnormal blood or urine, they should have a special type of body scan (called a MIBG scan) that is able to detect pheos that occur outside the adrenal gland.

4) it may be beneficial for individuals with a Y112H mutation undergoing surgery to remove a pheo to have an “open resection,” which is more invasive than a “laparoscopic resection,” but is more likely to remove all of a pheo that may be cancerous.*

*Nielsen SM, et. al. Genotype-Phenotype Correlations of Pheochromo­cytoma in Two Large von Hippel-Lindau (VHL) Type 2A Kindreds With Different Missense Mutations. Am J Med Genet A. 2011 Jan;155A(1):168-73. PMID: 1204227

As printed in the VHL Family Forum 19:2, April/May 2011. For permission to reprint, please contact VHL Family Alliance, editor@vhl.org. Further information is available from the VHL Family Alliance, info@vhl.org.