Chromosomal mosaicism; Mosaicism
A person who is a “genetic mosaic” has cells in his or her body with different genetic makeup.
This usually is 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).
Causes, incidence, and risk factors
There are many causes of mosaicism. 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 in the child, that person is mosaic because his or her body has some cells with 46 chromosomes (normal) and some cells with 47 (abnormal).
Another cause of mosaicism is the development of a new mutation during the early embryonic growth of a fetus which affects some, but not all, cells.
Yet another type of mosaicism, called gonadal mosaicism, happens when a mutation occurs in a developing fetus’ egg or sperm cells. A male with this condition may not show any signs of a genetic disease himself, but will carry some sperm that have potentially harmful mutations. When he later has his own children, those children carry some risk of inheriting the mutation (depending on whether the pregnancy is the result of a mutant or normal sperm). The same is true for a female with gonadal mosaicism (i.e., some of her eggs develop mutations before she was born).
The symptoms depend on what genetic change is mosaic.
For example, if a person has two 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 when 100% of cells lack an X chromosome. 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 case, the mosaicism is in the parent’s ovaries or testes. Any individul egg or sperm either has the mutation or not. Therefore, if conception involves one of these mutant sex cells, the resultant child will not be mosaic, but will simply have the genetic disease caused by that particular mutation.
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 a genetic disease, while another part of the body may be normal.
For chromosomal mosaicism, the test is called a karyotype (an analysis of your chromosomes). Both the number and structure of the chromosomes is examined. If two different populations are found, the blood test is usually repeated to make sure the result is accurate. If it is, mosaicism is diagnosed.
For gonadal mosaicism that could cause a genetic disease in the offspring (such as a single gene disorder), the appropriate tests are the same as if the genetic change was present in 100% of the cells. DNA analysis can be done on a man’s sperm to look for gonadal mosaicism, but no testing is routinely available for women with gonadal mosaicism. This is because it is difficult to obtain a woman’s eggs for testing.
Mosaicism is an example where it is often helpful to do genetic testing on cells from different parts of the body. For example, studying cells from inside the cheek (buccal smear) or from the skin in addition to blood cells. This helps to determine if one part of or fraction of the body is mosaic.
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 because they may be more mildly affected (some of their cells are normal).
The outlook is heavily influenced by the degree of mosaicism. In general, patients with a high percentage of abnormal cells in their blood share the same prognosis as people with the typical form (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, 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 because the percentage of affected sex cells is unknown.
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.
In the case of gonadal and chromosomal mosaicism, prenatal genetic counseling is available when amniocentesis reveals that two populations of cells are present in the fetus. However, it should be recognized that many many normal children result from prenatal karyotypes showing mosaicism, so it is important to discuss this with your obstetrician and genetic counselor.
by Brenda A. Kuper, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.