First in the Family: VHL Mosaicism

VHL is generally inherited as an autosomal dominant trait. In other words, most people with von Hippel-Lindau have a parent who also has VHL. It is currently believed that if a person has VHL, the specific genetic mutation will be found in the laboratory. However these general rules do not apply to all VHL families. There are families in which a child with VHL has parents who do not have VHL. Some people with VHL do not have a VHL genetic mutation. And some unaffected parents are known to have more than one affected child. All these situations can be explained by a concept known as mosaicism (mo-ZAY-eh-sizz-em).

In order to understand the concept of mosaicism one needs a basic understanding of several biologic processes. It is necessary to understand a little bit about chromosomes, genes and the nature of genetic mutations. You need also to know how genes are passed from one generation to another at the time of conception; a process known as fertilization. After the father’s sperm (with his genes) fertilizes a mother’s egg (with her genes), an embryo carrying both sets of the parent’s genes develops through a fascinating and complex series of events.

Chromosomes and Genes

Our bodies are made up of trillions and trillions of cells. Each cell (see Figure 1) has a nucleus inside, which contains the chromosomes. The chromosomes carry the genes, made of DNA units strung together. We have 46 chromosomes, or 23 pairs of chromosomes. One chromosome of each pair is inherited from our father and the other is inherited from our mother at the time of conception. Each chromosome is made up of genes; therefore, we have two copies of every gene; one from our father and one from our mother.

Figure 1: Inside the Cell. 

We have 46 chromosomes, or 23 pairs of chromosomes. The first 22 pairs are similar in males and females and are called autosomes. The 23rd pair are the sex chromosomes which determine whether we are male or female. Males have an X and a Y chromosome and females have two X chromosomes.

Each chromosome is one long strand of DNA: a DNA helix, containing hundreds, if not thousands, of genes. Genes are the basic units of heredity, and line up on a chromosome like beads on a string. We each have around 80,000-100,000 genes that determine what we look like and how our bodies work. A gene is like a recipe; it is a list of instructions that tell the cell how to make proteins. A gene is comprised of a series of 3-letter words and each word refers to a specific protein building block. Some proteins are structural, in that they help to form tissue like the heart or bones. Other types of proteins are critical in the thousands of chemical processes that occur in our cells to make our bodies function properly.

Genetic Alteration / Mutation

All plants, animals and humans have genes that don’t work as well as they should. They have been altered or changed by a process known as mutation. The term "genetic mutation" is often thought of as negative or bad, but mutations are the basis of normal human variation, such as long fingers vs. short fingers, red hair vs. black hair, blood type A vs. blood type O, etc. However, some mutations are harmful and cause disease.

Spontaneous mutations occur in everyone, but usually enzymes repair them. Also, normal everyday life exposes us to things that are known to be mutagenic, something that alters or changes the spelling of DNA. There are some chemicals that we know are mutagenic. We know that high levels of radiation cause DNA mutations. Very rarely are we able to determine the reason or cause of the genetic mutation in a family. Alterations, or changes, in our DNA can be of several types: a spelling mistake in a single letter (point mutation), a portion of the gene can be missing (partial deletion) or an entire gene can be missing (complete deletion).

We all have 10-15 genes that don’t work as well as they should; they are altered or mutated. Mutated genes function differently; either they don’t work as well as they should or they work too hard. Once DNA has changed in a family, that change may be passed from parent to child in the sperm or egg (germline) during conception.

Pattern of Inheritance in VHL

Von Hippel-Lindau is an autosomal dominant genetic condition caused by a non-working gene on the top of chromosome #3. Autosomal dominant conditions usually occur when just one copy of a pair of genes does not work correctly. Because the VHL gene is a tumor suppressor gene, tumors and cysts arise after the second copy of the VHL gene becomes altered or mutated within a cell.

Each offspring of a parent with VHL has a 50% chance, or 1 chance in 2, of inheriting the non-working dominant gene, putting them at risk for the health problems associated with VHL. Males and females can equally inherit the mutated VHL gene. A child who does not inherit the mutated gene will not develop VHL and cannot pass VHL on to the grandchildren.

Fertilization and Early Development

Egg and sperm cells (see Figure 2) each contain one-half of the chromosomes through a process called meiosis, during which egg and sperm cells receive one copy of each chromosomal pair. Fertilization occurs when a father’s sperm and mother’s egg unite at the time of conception.

Figure 2: Fertilization

A short time later, within the fertilized cell, the DNA copies itself and the cell divides, becoming two cells, each with identical chromosomes, identical DNA and genes. This is the process of mitosis or cell division. The single cell that is formed in fertilization becomes two cells, then four cells; then eight cells, and continues until a baby is completely formed.

Cell Differentiation

In the first few days after fertilization each cell of the early embryo is capable of becoming any type of cell (a blood cell, a nerve cell, a bone cell, etc.). Four days after fertilization the embryo contains around 30 cells; at this point the cells begin to specialize. For example, one of the cells develops into the nervous system; another specializes to become the skin; others become the circulatory system (blood vessels); still others become the kidneys. This is the process of cell differentiation, which will have more meaning when mosaicism is discussed below.

Inherited Mutation

Most people who have VHL inherited the VHL gene from a parent who is also affected with VHL. We would expect every cell of the offspring to have one chromosome #3 that carries the VHL mutation and one chromosome #3 from the unaffected parent who has a correct "spelling" of the VHL gene. At the time of fertilization, the child received the mutated VHL gene from the affected parent, and every cell thereafter will have the same VHL mutation.

New Mutation

In every VHL family the gene mutation began as a brand new event in someone; a grandparent, a great grandparent or a parent. From the time the first heritable VHL mutation occurs, the family from that point on has an autosomal dominant genetic condition that may be passed down through the generations. It is often difficult to determine in a family exactly where, when or why the VHL gene mutated.

Mosaicism

Sometimes we identify people who have two types of body cells. In this case, as it relates to the VHL, there would be some cells with the VHL mutation and some cells without the mutation. This is what we call "mosaicism". The word mosaicism basically means the person has two types of cells, or cell lines. Most, if not all, autosomal dominant conditions are known to have some people who are "mosaics" for that condition. If you test many different cells of their bodies, you will find some cells with the mutation and some cells without the mutation. We do not know precisely, because it is often difficult to be certain a particular person does not a have a second cell line.

To understand mosaicism, let us go back to fertilization. Let us presume that a mother’s egg and father’s sperm each have two working copies of the VHL gene (that is, the gene is "spelled" correctly). They do not have VHL. Fertilization occurs and the first few cell divisions result in identical cells containing two VHL genes that are spelled correctly. At this point there are 8 cells and each of these cells can become any type of cell; differentiation has not yet occurred. We are now ready to learn what happens when a mutation occurs at this stage.

Somatic Mosaicism

A somatic (so-MAT-ik) mutation (see Figure 3) is a gene mutation that occurs in a cell of the body that is not a germ cell (not eggs or sperm). Because somatic mutations do not come from the eggs or sperm, they were not passed down from a previous generation.

Figure 3: Somatic Mosaicism. A portion of developing tissue will have the mutated VHL gene. Thus VHL may develop in some, but not all tissue sites.

Let us presume that when the pre-embryo is eight cells in number (differentiation has not occurred), the VHL gene in one of these cells is altered or mutated. One cell now has one #3 chromosome that has a VHL mutation, and one copy of the VHL gene that is spelled correctly. The VHL genes in the other 7 cells remain unchanged; they work well. The eight cells continue to divide.

All cells that are derived from the one cell in which the mutation occurred will also have the VHL mutation. All cells that derive from the 7 non-altered cells will be identical; they will not have the VHL mutation. This person now has two cell lines: some cells with the VHL mutation and some cells that do not have the VHL mutation. These cells will go on to differentiate into the various organs in the body.

For example, let us presume the VHL gene mutation occurred after the cells became specialized. Many of the cells are now specialized; let us presume that the genetic mutation occurred in one of the first few cells that become the central nervous system (brain and spinal cord). The cells that become the kidney do not have a VHL mutation, and as such, will not develop VHL-related health problems.

However, the brain and spinal cord that have the mutation may very well develop VHL-related problems. In this situation, analysis of a blood sample may not find a mutation, because the mutation occurred in the cells that became the brain and spinal cord.

Gonadal Mosaicism

Another example of mosaicism (see Figure 4) is when a somatic VHL mutation occurs in the germline tissue, creating two types of germ cells. In other words, some egg and sperm cells have the mutation; others do not. This is called germline or gonadal mosaicism. In this situation, the person does not have a VHL mutation in any other cell in the body. Therefore, he or she does not have VHL-related problems, andthere is no family history of VHL. But because the gonads are affected, a portion of this person’s eggs or sperm have a mutated copy of the VHL gene. When this person has a child, there is a chance that the child will have VHL.

Figure 4: Germline or Gonadal Mosaicism. Some of the egg or sperm cells have a VHL gene mutation.

Possible Scenarios:

For the purpose of this discussion, the terms "unaffected" and "affected" will be used. It used to be that a person was determined to have VHL based on "test results". There was only one type of testing; these were the clinical tests such as X-rays, MRI’s and CT scans that are done to monitor the organs affected by the altered VHL gene.

However, around 5 years ago, the VHL gene test became available. Now we can "test" a person who is at risk of inheriting the VHL mutation by checking the DNA to see if he or she carries the known gene mutation in the family. When we use the word "test for VHL", it can be confusing. Does this mean "clinical test" or "DNA test"? In this next section, the term "unaffected" means that both DNA and clinical testing do not find evidence of VHL.

Parents do not have VHL,

but have one or more children with VHL:

There are two possibilities. The most likely scenario is that the VHL gene mutated in a cell that gave rise to an egg or sperm, such that the fertilized egg (the very first cell) contained the VHL mutation. The child that is created now has a VHL mutation in every cell. Because the parents do not have the VHL mutation in any other cell, they do not have VHL, will not develop VHL and cannot pass a VHL mutation to another child they may have. A new mutation occurred in a single cell that gave rise to an egg or sperm. That one germ cell conceived that child, but all the other germ cells are free of the mutation. Therefore, the risk of VHL in another child is extremely low.

Another possibility is that one parent has gonadal mosaicism; some germ cells (eggs or sperm) have the mutation; some do not. In this scenario, both parents are not affected by VHL because the VHL mutation they have is found only in their germ cells. There is no evidence of VHL by clinical tests and no DNA mutation can be found in a blood sample. However, it is possible for this parent to have more than one affected child with VHL. If their child is conceived by a sperm or egg that has the mutation, that child will be affected. If the child is conceived by a sperm or egg without the mutation, the child will not have VHL and cannot pass VHL to his or her offspring. The risk of having multiple affected children (recurrence risk) depends on the degree to which the germ cells are affected.

Some features of VHL,

but no DNA mutation detected:

In this situation, we presume that a somatic mutation occurred early in the development of the embryo. Therefore, multiple tissues may carry the mutation, such as the kidney, central nervous system, eye and in some cases including germ cells (eggs and sperm). The DNA test is generally done with a blood sample. If the blood DNA test does not demonstrate a VHL mutation in a person who has VHL, we presume the cell that was destined to become the blood system did not carry the mutation. Because some systems in the body have the mutation, we can expect some features of VHL. But the gene mutation may not be detected in a blood test.

The tissues that are affected depends on where, in the process of development, the mutation first occurred. It is possible for one tissue or many tissues to be affected.

If a person has VHL symptoms but a negative DNA test using DNA extracted from blood, other types of tissue can be analyzed. Skin tissue is an easy tissue to obtain and analyze in a research lab. A small sample of skin can be taken by a skin biopsy, which is a simple procedure done under local anesthesia in an office setting. Generally no stitches are needed. Another easily obtained cell type are those found in cheek cells which are obtained by swishing mouthwash for a few minutes and spitting into a cup. Cells from the skin and cheek cells may demonstrate two different cell origins.

r to be certain that a person is mosaic for the VHL mutation, one would have to test cells in all body systems, which is not realistic. It may be possible in a specialized research laboratory to test sperm for the presence of a VHL mutation, but the sperm cell is destroyed by the analysis.

It is difficult to guarantee that VHL mosaicism in a single individual is not present. Estimates about the frequency of mosaicism in specific autosomal dominant conditions are based on observation of disease occurrence in families and by laboratory analysis of available tissues. We currently estimate the incidence of mosaicism in VHL to be approximately 5%. Currently 23% of the families we are seeing have one person with VHL, with no VHL previously in the family.

Genetic Counseling and medical screening in these cases requires a step-wise approach using DNA testing, and adding clinical examinations as indicated.

If you are the first in your family to have VHL:

1. If a mutation is found, then there may be VHL mosacism in one of your parents, perhaps at a low level without symptoms.

 

2. If a mutation is not found, and if the test was done at a lab with a 99% hit rate (e.g. Pennsylvania or Padua),² it is possible that you, yourself, are mosaic for VHL, and your parents are not expected to be at risk for VHL. Consult a geneticist to explore this possibility.

 

3. In either case, whether the mutation is found or not, your children are considered to be at risk until their own DNA test is shown to be negative. See the VHL Handbook for suggested screening for children.

 

4. NIH plans further studies of VHL mosaicism.

As printed in the VHL Family Forum 8:2, June 2000.  For permission to reprint, please contact VHL Family Alliance, editor@vhl.org. Further information is available from the VHL Family Alliance, info@vhl.org.