Trisomy-13



Trisomy 13 is a condition caused by an extra copy of chromosome 13. It is associated with severe mental retardation and certain physical abnormalities. These abnormalities include small eyes that may exhibit a split in the iris (coloboma), an opening in the roof of the mouth (a cleft palate) and/or a cleft lip, weak muscle tone (hypotonia), skeletal abnormalities, an increased risk of heart defects, and other medical problems. Affected individuals rarely live past infancy because of the life-threatening medical problems associated with this condition.

Other Names
Trisomy 13 has many other names:
 * Patau syndrome
 * Bartholin-Patau syndrome
 * Chromosomal imbalance syndrome, pair 13, trisomy
 * Chromosome 13 trisomy syndrome
 * Complete trisomy 13 syndrome
 * D1 Trisomy

Symptoms
People with trisomy 13 have a variety of physical and neurological conditions:


 * Mental retardation
 * Movement disorders
 * Low set ears
 * Extra fingers
 * Eye defects and conditions (e.g., cataracts, retinal detachment)
 * Small brain and head
 * Cleft palate or hare lip
 * Spine and abdominal defects
 * Fingers that overlap the thumb
 * Abnormal genitalia
 * Heart defects

Causes
People with trisomy 13 have an extra copy of chromosome 13 in all or some of their cells. Normally cells in the body, except sperm or eggs, have two copies of each chromosome. Cells in people with trisomy 13 have three copies of chromosome 13. The extra genetic material (called DNA) contained in the chromosome disrupts the normal course of development, causing the characteristic features of trisomy 13. Mosaic trisomy 13 occurs if not all the cells carry the extra chromosome.

Trisomy 13 can also occur when part of chromosome 13 becomes attached to another chromosome (translocated) before or at conception. Affected people have two copies of chromosome 13, plus extra material from chromosome 13 (but not an entire copy) attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 13 and often the physical signs of the syndrome differ from those typically seen in trisomy 13.

Even though trisomy 13 involves DNA and occurs during fertilization, most cases are not inherited. Inherited conditions most often result from mutations in genes. To the contrary, trisomy 13 results from errors in cell division called nondisjunction. This process results in reproductive cells (sperm and eggs) with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 13. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 13 in each cell of the body.

Mosaic trisomy 13 is also not inherited. It occurs as a random error during cell division early in fetal development. As a result, some of the body's cells have the usual two copies of chromosome 13, and other cells have three copies of the chromosome.

Translocation trisomy 13 can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 13 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 13. Although they do not have signs of trisomy 13, people who carry this type of balanced translocation are at an increased risk of having children with the condition.

Diagnosis
Trisomy 13 can be diagnosed in the developing fetus by ultrasound or examination of cells obtained from amniotic fluid, chorionic villi, or fetal blood. The chromosomal abnormalities characteristic of the condition are visible in the nuclei of cells when viewed under a microscope.

Many fetuses with trisomy 13 are stillborn or aborted in the womb. Many of the symptoms of the condition appear at birth. Cytogenetics, or examination of cells for the chromosomal abnormalities, is used to confirm the diagnosis.

The infant may have a single umbilical artery at birth. There are often signs of congenital heart disease, such as:
 * Abnormal placement of the heart toward the right side of the chest instead of the left
 * Atrial septal defect
 * Patent ductus arteriosus
 * Ventricular septal defect
 * Gastrointestinal x-rays or ultrasound may show rotation of the internal organs.

MRI or CT scans of the head may reveal a problem with the structure of the brain. The problem is called holoprosencephaly. It is the joining together of the two sides of the brain.

Treatment
The only treatment available for surviving infants is surgery to correct physical defects. Typically surgery is only attempted when the infant reaches a suitable age.

Chances of Developing
Trisomy 13 affects approximately 1 in 10,000 newborns. The risk of having a child with trisomy 13 increases as a woman gets older.

Related Problems
Complications begin almost immediately after birth. Congenital heart disease is present in most infants with Trisomy 13. Many infants do not survive the first year of life due to severe congenital defects.

Other complications may include:
 * Breathing difficulty or lack of breathing (apnea)
 * Deafness
 * Feeding problems
 * Heart failure
 * Seizures
 * Vision problems

Clinical Trials
Clinical trials related to Trisomy 13 can be found here.

Recent discoveries

 * A 2007 study from Washington State examined the origins of Trisomy 13.
 * Another study showed that targeted ultrasound can identify fetal abnormalities in as many as 95% of fetuses with trisomy 13.
 * A study from Taiwan evaluated the survivability of Patau syndrome as related to differing genetic expression.
 * The long-term survival of trisomy 13 patients born in a medical center in Taiwan between 1985 and 2004 was studied.

History
Another name for trisomy 13 is Patau syndrome, in honor of Dr. Klaus Patau who, in 1960, was the first to recognize that the condition has a chromosomal basis. Patau syndrome has also been described in Pacific island tribes. It has been proposed that the cause of the condition in this population was atomic bomb testing.