Mosaicism

Definition:

A person who is a 'genetic mosaic' has cells from two or more different genetic constitutions. This usually is found as a variation in the number of chromosomes in the body's cells. Normally, all body cells would have the same number of chromosomes (46). But in mosaicism, some cells may have 47 chromosomes (such as extra chromosome 21 or X chromosome in some, but not all cells).

Alternative Names:

Chromosomal mosaicism; Gonadal mosaicism

Causes, incidence, and risk factors:

There are many causes for this condition. For example, an older mother is more likely to have a child with an extra chromosome. If this extra chromosome is only present in a fraction of the cells, that person is mosaic because he or she has 2 populations of cells: one with 46 chromosomes (normal) and one with 47 (abnormal). Additional causes of mosaicism include the development of a new mutation during the early embryonic growth of a fetus and a situation where two embryos grow to be one fetus or exchange cells.

One type of mosaicism which occurs when a fetus develops a new mutation during its growth is gonadal mosaicism. In this case, the new mutation occurs in the cells that give rise to germ (egg or sperm) cells of the parent. The parent may show no signs of disease, but may have children with some risk of inheriting the mutation because there are 2 populations (mutant and normal) of germ cells in their gonads (ovaries or testes).

Symptoms:

Symptoms depend on what genetic change is mosaic. For example, if there are 2 populations of cells -- one normal and one missing an X chromosome -- the symptoms are usually milder versions of those seen in Turner syndrome. Turner syndrome results from 100% of cells lacking an X chromosome, so if only a fraction of cells lack this chromosome, the symptoms will vary from milder to none at all. The severity increases as the percentage of mosaicism increases. If 90% of cells are abnormal, symptoms are likely; if only 10% are abnormal, the person may be unaware that anything is wrong.

Gonadal mosaicism is a different situation. In that instance the mosaicism is in the parent's ovaries or testes. Any individul egg or sperm either has the mutation of not. Therefore, the mutation is present from conception and acts like a mutation in any typical (not mosaic) genetic disease. Therefore, a child born from the mutant egg or sperm will have symptoms are like those of the typical genetic disease because the child will not be mosaic.

Signs and tests:

Signs, like symptoms, depend on which genetic change is mosaic. When mosaicism results from mutation during embryonic or fetal growth, one part of the body may have signs or symptoms of disease, while another part of the body may be normal.

For chromsomal mosaicism, the test is called a karyotype or an analysis of chromsomes. The number as well as structure of the chromosomes is compared to known norms. If 2 populations are found, the blood test is usually repeated to make sure it is real. For gonadal mosaicism of other genetic diseases (such as single gene disorders), the test is the same as if the genetic change was present in 100% of cells. Testing based on analysis of DNA can be done on a man's sperm for gonadal mosaicism, but no testing is routinely available for women with gonadal mosaicism.

Mosaicism is an example where it is often helpful to do genetic testing on cells from different parts of the body. For example, one may study cells from inside the cheek (buccal smear) or skin in addition to blood. This helps to determine if one part of or fraction of the body is mosaic.

Treatment:

The treatment is the same as for the typical form of the genetic disease (wherein 100% of cells have the genetic change). However, patients with mosaicism may require less intense treatment as they may be more mildely affected.

Expectations (prognosis):

As noted above, the prognosis is heavily influenced by the degree of mosaicism. In general, patients with a high percentage of abnormal cells in their blood share the prognosis of the typical forms (100% abnormal cells) of the disease. Patients with a low percentage of abnormal cells may be quite mildly affected. In fact, they may only discover that they have mosaicism after giving birth to a child who has the typical (non-mosaic) form of a genetic disease.

Complications:

Complications, like signs and symptoms, vary with both the type and percentage of cells affected by the genetic change. One complication of gonadal mosaicism is that it becomes very difficult to give specific statistical probabilities of a child being affected by a genetic disease. If gonadal mosaicism is suspected, there is a probability that there is a "50% chance of each child inheriting a disease". And, as is the case with autosomal dominant diseases, there is a "chance between 1- 50%" of each child inheriting the disease.

Calling your health care provider:

Mosaicism is complicated, and a call to a genetic counselor and your health care provider is important under the following circumstances:

When a genetic disease appears in a family with no prior known family history of that disease.
When a person has signs/symptoms of a genetic disease that are milder, or affect only one part of the body, than is typical of that disease.

Prevention:

In the case of gonadal and chromosomal mosaicism an opportunity for prevention is available with prenatal genetic counseling and testing. An amniocentesis may reveal that 2 populations of cells are present. However, it should be recognized that many normal children result from prenatal karyotypes showing mosaicism so it is important to discuss this with your obstetrician.

Review Date: 9/10/2001
Reviewed By: David G. Brooks, M.D., Ph.D., Division of Medical Genetics, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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