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).

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:

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.