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Angelman syndrome and Prader Willi syndrome


Types

These two syndromes are caused by a mutation in the same region of chromosome 15, but differ in clinical presentation, as described below. The mutation is a deletion of a specific part of this chromosome.

Clinical signs

Prader Willi syndrome manifests as a combination of features; in infancy there is muscle flaccidity and dysphagia (problems with swallowing) that result in inadequate weight gain at this time. This later changes to polyphagia (overeating) and obesity that may be severe enough to cause respiratory failure. In addition, patients have developmental delay of differing degrees of severity. The genitalia are typically underdeveloped.

Angelman syndrome is marked by a decreased rate of the growth of the head towards or around the second year of life the head circumference is smaller than that of their peers and is less than the norm. They have epileptic convulsions, difficulties with balance while standing or walking, and severe mental retardation. Patients also have repetitive monotonous movements and a tendency to have unexplained attacks of laughter.

Diagnosis is made by molecular genetic tests. Sometimes the deletion is large enough for the condition to be diagnosed by a regular chromosome test.

Inheritance pattern

The mutation is the deletion of a specific region of chromosome 15. In Prader Willi syndrome, the deletion is in the chromosome 15 that was transmitted to the child from his or her father. In Angelman syndrome, the deletion is in the chromosome 15 that was transmitted to the child from his or her mother. The parents do not carry the mutation, which arises in two main ways:
  • Deletion of a specific part of chromosome 15: A new mutation, which is the deletion of a specific segment of chromosome 15, arises in the oocyte or sperm cell creating the fetus. If the segment that is deleted is from the chromosome 15 from the sperm cell, the result is Prader Willi syndrome. If the segment that is deleted is from the chromosome 15 from the oocyte, the result is Angelman syndrome. The sex of the infant is not relevant.

  • Uniparental disomy (UPD): In this situation the infant receives two normal, intact chromosome 15s from one parent, and no chromosome 15 from the other. If the infant receives two chromosome 15s from the mother and none from the father, the result is Prader Willi syndrome. Conversely, if both the chromosome 15s come from the father and there is no maternal chromosome 15, the result is Angelman syndrome. The reason why this occurs is because in this area of chromosome 15, the testis and the ovary turn on or turn off different genes (a process called genomic imprinting). In other words, in this case, in order for there to be functional completeness, the infant must receive one chromosome 15 that has been treated by the ovary and one that has been treated by the testis, i.e. there must be both a maternal and a paternal contribution.

  • There may be other mutations, possibly involving hereditary transmission of these syndromes. In these cases there is usually a family history of individuals in previous generations with one of these syndromes.

Penetrance

Full

Associated features that can be demonstrated in tests performed during pregnancy

In fetuses with Prader Willi syndrome, an attempt can be made to detect the slightly smaller genitalia by ultrasound examination.

There may be fewer than usual fetal movements.

However, these are not diagnostic signs. Failure to demonstrate these signs does not rule out these syndromes.

In all these cases it is important to refer the couple for genetic counseling, to which they should bring all data, including the results of other tests performed during the pregnancy such as alpha-fetoprotein, nuchal translucency, etc.

What is the risk of recurrence in a subsequent pregnancy?

As a rule, if there is a single case in the family, the risk is low (less than 1%), and may be even lower. In the case of UPD, there is no risk of recurrence, either for the couple who has already had an affected child or for other relatives, and the patient himself has no theoretical increased risk for having a child with the same condition. If there is a chromosomal deletion, this is usually a new mutation, and here the parents have a slight risk of recurrence in future pregnancies. This can be tested in each subsequent pregnancy. It should be noted that the patients offspring have a 50% risk of being affected.

The risk for more distant relatives depends on the number of affected individuals in the family, the degree of relationship between the relatives and the affected individuals, the results of genetic tests performed on the patient and the parents, etc. The risk is established within genetic counseling.

Molecular genetic information

The gene for the disease

A number of genes

Location

The long arm of chromosome 15.

Genetic testing

Diagnostic testing

A molecular test will determine whether there is a chromosomal deletion in the critical region of chromosome 15, and also whether the two chromosome 15s come from the same parent (UPD). This test is carried out in Israel in a number of places, and is aimed at confirming diagnoses and providing prenatal testing of amniotic fluid or chorionic villus cells.

The test for rare mutations in the genes in cases where there is neither a chromosomal deletion nor UPD is a complicated and difficult test it is performed only in special cases as research through genetic institutes.

Carrier testing

This is not practicable unless the family has many affected individuals or a point mutation (this is rare) that has been found in previous tests conducted on the patient.

Fetal testing

If there is a clinical suspicion and the couple has had an affected child from a previous pregnancy, it is advisable to undertake prenatal diagnosis. The test is performed on chorionic villus or amniotic fluid cells, and is intended to identify the common mutations (chromosomal deletion or UPD).
 
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Comments (2)


Monday, April 04, 2011 11:17 AM
(1) zach  says:


prader willi

i know a guy named willi prader


Monday, May 23, 2011 2:57 PM
(2) dina  says:


prader willi

OMG!!!!!!!!!!!!!! that is soo cool i am doing a project about the prader willi syndrome





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