Genetic: Patau Syndrome
Genetic: Patau Syndrome
Patau syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 13 or by a portion of chromosome 13 translocated (attached) to another chromosome plus two copies of chromosome 13.
Babies born with Patau syndrome have a characteristic set of facial, neurological, and heart defects and a very high mortality rate.
Estimates of the frequency of Patau syndrome range from one in every 5,000 live births to one in every 12,000. The syndrome occurs with equal frequency in all races and countries. The chief risk factor for Patau syndrome is the age of the mother; the average age of mothers of babies with the syndrome is thirty-one.
It is thought that male and female fetuses are equally likely to be affected at the time of conception; however, the mortality rate among boys is higher at all points along the baby's development before and after birth.
Patau syndrome can result from genetic errors during the formation of germ cells (eggs and sperm) or during cell division shortly after the egg is fertilized by the sperm. The most common form of Patau syndrome, which accounts for about 75 percent of cases, occurs when an egg or sperm carrying two copies of chromosome 13 is involved in conception. The reason for the extra copy in the abnormal germ cell is a genetic error called nondisjunction. During the normal process of germ cell formation, the paired chromosomes in the cell divide so that each daughter cell has only one member of the pair. In nondisjunction, one daughter cell gets both members of the chromosome pair and the other cell has none. If a germ cell carrying two copies of chromosome 13 is fertilized by a normal
germ cell from the other parent, the child will have three copies of chromosome 13. This genetic error is called a full trisomy 13.
About 20 percent of cases of Patau syndrome develop when a part of chromosome 13 becomes attached to chromosome 14 either before or at the moment of conception. This type of genetic error is called a translocation. The child will be born with two copies of chromosome 13 plus some extra genetic material from chromosome 13 attached to chromo-some 14. A child with this type of genetic error is said to have partial trisomy 13.
About 5 percent of cases of Patau syndrome occur in children who have some body cells with the extra copy of chromosome 13 and some body cells without the extra copy. This condition is called mosaic Patau. It is thought to result from random errors in cell division during the early stages of fetal development. Children with mosaic Patau are usually less severely affected than those with full or partial trisomy 13 and may lack many of the features associated with Patau syndrome.
The symptoms of Patau syndrome include a high mortality rate even before birth. Many embryos with full trisomy 13 die during pregnancy or are expelled from the mother's womb in what is called a spontaneous abortion or miscarriage. Those who survive until birth usually live only a few days; 69 percent die of breathing problems and another 13 percent die during this period from heart defects. Although some children with
Patau syndrome do not have all the physical features that characterize the syndrome, the following are considered typical:
- Holoprosencephaly. This term refers to thefailureof theinfant's brain to divide into two equal halves during its development before birth. Babies with holoprosencephaly usually have facial abnormalities, including cleft lip, cleft palate, a misshapen or completely absent nose, and abnormally small eyes placed unusually close together. The entire head may also be unusually small.
- Heart defects. Eighty percent of infants with Patau syndrome are born with one or more heart defects, including an opening between the two lower chambers of the heart or the heart being situated on the right rather than the left side of the body.
- Polydactyly. This term refers to the presence of extra fingers or toes.
- Spina bifida. Spina bifida is a condition in which the spinal cord is partially open at birth instead of being covered by the bones of the spinal column.
- Abnormal genitalia.
- Rocker-bottom feet. This term is used to describe abnormally long and slender feet with pointed heels turned outward like the bottom rails of a rocker.
- Low-set ears and abnormalities of the inner ear.
- Abnormal kidneys.
Rasmus Batholin and Klaus Patau
The two doctors who are responsible for identifying Patau syndrome are Rasmus Bartholin (1625–1698), a Danish doctor, and Klaus Patau (1908–1975), an American geneticist who was born in Germany. Bartholin was a professor of medicine at the University of Copenhagen who wrote the first description of a baby with Patau syndrome in 1657. His discovery is the reason why the syndrome is sometimes called Bartholin-Patau syndrome.
Doctors in the seventeenth century did not have the research tools that allow modern doctors to study the way human cells reproduce, and so Bartholin could not identify the syndrome he described as a genetic disorder. It was not until the late 1950s when Patau, a researcher in the Department of Genetics at the University of Wisconsin-Madison, was able to identify the extra chromosome 13 as the cause of the syndrome that was later named for him. Patau published the first article about Patau syndrome in a British medical journal in 1960.
Diagnosis of Patau syndrome is usually made on the basis of the child's appearance at birth. It can be diagnosed before birth on the basis of ultra-sound studies during the first three months of pregnancy; a sample of the
mother's blood plasma; or by genetic analysis of cells taken from the baby's blood or the amniotic fluid that surrounds the baby inside the womb.
There is no treatment for Patau syndrome itself. Infants who survive the first two days often require two or more weeks of treatment in an intensive care unit (ICU). Those who survive the first six months may be treated with surgery for specific heart defects and other abnormalities; however, their mental retardation cannot be corrected by surgery and they remain at high risk of developing severe curvature of the spine in adolescence. They also have an increased risk of developing cancer.
The prognosis for children with Patau syndrome is very poor. Of those who do not die before birth, the average length of survival is 2.5 days. Only one child in twenty lives longer than six months. A few children live into their teens, and there are a few case reports of people living into their early twenties with Patau syndrome. Most long-term survivors are female.
Since Patau syndrome is thought to be caused by a spontaneous genetic mutation rather than an inherited genetic defect, there is no way to prevent it. Pregnant women over thirty-five should have tests during the first trimester (three-month period) of pregnancy to screen for the syndrome. These tests may involve ultrasound studies, which can detect abnormalities in the baby's heart or facial development, followed by a photographic analysis of cells taken from the fluid that surrounds the baby in the womb. This analysis, or karyotype, is needed to distinguish Patau syndrome from other genetic disorders that can cause heart defects and facial abnormalities. Doctors recommend that the parents of a child with Patau syndrome should consult a genetic counselor for advice about future pregnancies.
It is not known whether the increasing numbers of women having children in their thirties or forties will lead to an increase in the number of
children born with Patau syndrome. Many women whose fetuses are diagnosed with Patau syndrome choose to end their pregnancies before childbirth.
There are online support groups for families of children with trisomy 13; one posts photo albums of long-term survivors of Patau syndrome as well as information about the disorder and frequent updates about medical research.
WORDS TO KNOW
Germ cell: A cell involved in reproduction. In humans the germ cells are the sperm (male) and egg (female). Unlike other cells in the body, germ cells contain only half the standard number of chromosomes.
Holoprosencephaly: A disorder in which a baby's forebrain does not develop normally. The infant's brain fails to divide into two cerebral hemispheres; this failure in turn leads to facial deformities and abnormal brain structure and function.
Karyotype: A photomicrograph of the chromosomes in a single human cell. Making a karyotype is one way to test for genetic disorders.
Mosaicism: A condition in which a person has some body cells containing an abnormal number of chromosomes and other cells containing the normal number. Mosaicism results from random errors during the process of cell division that follows conception.
Nondisjunction: A genetic error in which one or more pairs of chromosomes fail to separate during the formation of germ cells, with the result that both chromosomes are carried to one daughter cell and none to the other. If an egg or sperm with a paired set of chromosomes is involved in the conception of a child, the child will have three chromosomes in its genetic makeup, two from one parent and one from the other.
Translocation: A genetic error in which a part of one chromosome becomes attached to another chromosome during cell division.
Trisomy: A type of genetic disorder in which a cell contains three copies of a particular chromo-some instead of the normal two. Patau syndrome is one of several trisomies.
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