Chromosome 20 trisomy

Chromosome 20 trisomy, (also called trisomy 20) is a condition in which a fetus or individual has an extra full or partial copy of chromosome 20 in some or all of of his/her cells. An extra full copy of chromosome 20 in all of a person's cells is rare, and almost all fetuses with this do not survive past the first trimester of pregnancy.^[1] The presence of an extra copy of only part of chromosome 20 is called partial trisomy 20; and an extra copy of chromosome 20 in only some of a person's cells is called mosaic trisomy 20. Mosaic trisomy 20 is the most common type of chromosome 20 trisomy and is one of the more common chromosomal abnormalities found during prenatal diagnostic testing. Studies have shown that the child is normal in the vast majority of prenatally diagnosed individuals. However, features that have been reported include spinal abnormalities (including spinal stenosis, vertebral fusion, and kyphosis), hypotonia (decreased muscle tone), lifelong constipation, sloped shoulders, and significant learning disabilities despite normal intelligence.^[2] Trisomy 20 usually results from an error that occurs when an egg or sperm cell develops (before fertilization); mosaic trisomy 20 usually results from errors in cell division soon after fertilization.^[3]

Chromosomal abnormalities usually result from an error that occurs when an egg or sperm cell develops. It is not always known why these errors occur, but it is thought that nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in a fetus or child. Egg and sperm cells should each contain 23 chromosomes. When they join together, they form a fertilized egg with 46 chromosomes. Sometimes, something goes wrong before fertilization. An egg or sperm cell may divide incorrectly, resulting in an egg or sperm cell with too many or too few chromosomes. When this cell with the wrong number of chromosomes joins with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality. If an egg or sperm cell has 24 chromosomes because either has an extra copy of chromosome 20, this can cause trisomy 20 (the resulting embryo will have 3 copies of chromsome 20). In most cases, an embryo with 3 full copies of chromosome 20 in all cells does not survive and the pregnant woman has a miscarriage, often very early in pregnancy.

Other types of errors can alter the structure of one or more chromosomes. Individuals with structural chromosomal abnormalities usually have the normal number of chromosomes. However, small pieces of a chromosome (or chromosomes) may be duplicated, and in the case of part of chromosome 20 being duplicated, it would be called partial trisomy 20 (because there are 3 copies of only part of chromosome 20 in each cell).

Errors in cell division involving chromosome 20 can also occur soon after fertilization, which can cause mosaicism, a condition in which an individual has cells with different genetic makeups. Individuals with the mosaic form of trisomy 20 have an extra chromosome 20 in some, but not all, of their cells. Some individuals with chromosomal mosaicism may be mildly affected or apparently not affected at all, but the severity of the condition may depend on the number of abnormal cells that are present. Other individuals may be severely affected.^[3]

Most cases of chromosome 20 trisomy are not inherited and result from random events during the formation of eggs and sperm in healthy parents. An error in cell division, called nondisjunction, results in an egg or sperm cell with an abnormal number of chromosomes. If an egg or sperm cell gains an extra copy of chromosome 20, the embryo will have an extra chromosome 20 in each cell of the body. For more information about the causes of different types of chromosome 20 trisomy (full, partial or mosaic trisomy 20) click here.

Translocation trisomy 20 can be inherited. A  healthy person (that does not have trisomy 20) can carry a rearrangement of genetic material between chromosome 20 and another chromosome (called a translocation). These rearrangements are called balanced translocations because there is no extra material from chromosome 20. A person with a balanced translocation involving chromosome 20 has an increased chance of passing extra material from chromosome 20 to his/her child, which would result in a form of trisomy 20 in the child.^[4]

After conducting a thorough search of available literature, there do not seem to be any reports of individuals with full (non-mosaic) trisomy 20 that reach either the age, or capability, of reproduction. There is a lot of uncertainty about whether there are increased risks to the children of individuals with a mosaic trisomies. It is theoretically possible for individuals with mosaic trisomy 20 to have germline mosaicism for trisomy 20, which would increase risks for their children. This means that the individual with mosaic trisomy 20 (one who has 3 copies of chromosome 20 in only a portion of the somatic cells of the body) may also have this mosaicism in his/her sperm or egg cells; if a sperm or egg cell with an extra copy of chromosome 20 contributes to an embryo, the resulting embryo could then have an extra copy of chromosome 20 in all of its cells.  Unfortunately, it is not possible to determine whether an individual has germline mosaicism; to do so would mean that every egg or sperm would have to be tested.^[5]