Wednesday, April 3, 2013
Article # 444. What is Down syndrome? Trisomy 21
What is Down syndrome?
Down syndrome is a developmental disorder caused by an extra copy of chromosome 21 (which is why the disorder is also called "trisomy 21"). Having an extra copy of this chromosome means that individuals have three copies of each of its genes instead of two, making it difficult for cells to properly control how much protein is made. Producing too much or too little protein can have serious consequences. Genes on chromosome 21 that specifically contribute to the various symptoms of Down syndrome are now being identified.
How do people get Down syndrome?
Down syndrome is typically caused by what is called nondisjunction. Nondisjunction happens when a pair of chromosomes fails to separate during egg (or sperm) formation. When that egg unites with a normal sperm to form an embryo, that embryo ends up with three copies of chromosome 21 instead of the normal two. The extra chromosome is then copied in every cell of the baby's body.
Interestingly, nondisjunction events seem to occur more frequently in older women. This may explain why the risk of having a baby with Down syndrome is greater among mothers age 35 and older.
In rare cases Down syndrome is caused by a Robertsonian translocation, which occurs when the long arm of chromosome 21 breaks off and attaches to another chromosome at the centromere. The carrier of such a translocation will not have Down syndrome, but can produce children with Down syndrome.
People with Down syndrome have very distinct facial features: a flat face, a small broad nose, abnormally shaped ears, a large tongue, and upward slanting eyes with small folds of skin in the corners.
People with Down syndrome have an increased risk of developing a number of medically significant problems: respiratory infections, gastrointestinal tract obstruction (blocked digestive tract), leukemia, heart defects, hearing loss, hypothyroidism, and various eye abnormalities. They also exhibit moderate to severe mental retardation; children with Down syndrome usually develop more slowly than their peers, and have trouble learning to walk, talk, and take care of themselves.
Because of these medical problems most people with Down syndrome have a decreased life expectancy. About half live to be 50 years of age.
Two types of tests check for Down syndrome during a woman's pregnancy: screening and diagnostic tests.
Screening tests identify a mother who is likely carrying a baby with Down syndrome. The most common screening tests are the Triple Screen and the Alpha-Fetoprotein Plus. These tests measure levels of certain substances in the blood.
Alternatively, ultrasounds (which use sound waves to look inside the mother's uterus) allow the doctor to examine the fetus in the womb for the physical signs of Down syndrome.
To confirm a positive result identified in a screening test, one of the following diagnostic tests can be performed: chorionic villus sampling (CVS), amniocentesis, and percutaneous umbilical blood sampling (PUBS). Each takes a sample from the placenta, amniotic fluid, or umbilical cord, respectively, to examine the baby's chromosomes and determine if he or she has an extra chromosome 21.
If Down syndrome is not diagnosed in the womb, doctors can usually recognize it after the baby is born by the distinctive facial features. The diagnosis is confirmed with a karyotype - an examination of the baby's chromosomes.
No cure exists for Down syndrome. But physical therapy and/or speech therapy can help people with the disorder develop more normally. Screening for common medical problems associated with the disorder, followed by corrective surgery, can often improve quality of life. Moreover, enriched environments significantly increase their capacity to learn and lead a meaningful life.
Down syndrome is really the only trisomy compatible with life. Only two other trisomies have been observed in babies born alive (trisomies 13 and 18), but babies born with these trisomies have only a 5% chance of surviving longer than one year.
In 90% of Trisomy 21 cases, the additional chromosome comes from the mother's egg rather than the father's sperm.
Down syndrome is the most common genetic disorder caused by a chromosomal abnormality. It affects 1 out of every 800 to 1,000 babies.
Down syndrome was originally described in 1866 by John Langdon Down. It wasn't until 1959 that a French doctor, named Jerome Lejeune, discovered it was caused by the inheritance of an extra chromosome 21.