Everyone has two copies of the Ube3a gene, one from their mother and one from their father. In the brain, due to a biological quirk called imprinting, we only use the Ube3a gene we inherit from our mother – the paternal copy is ‘switched off’.
- Deletion (Del +)
The majority of individuals with Angelman Syndrome (approx. 70%) are missing the region of chromosome 15 containing the mother’s copy of Ube3a – it has been deleted in the AS individual. This happens solely in the affected child – the mother is unlikely to have a further child with AS and any siblings of the person with AS are unlikely to have children with AS.
- Uni-Parental Disomy (UPD)
In some cases (approx. 1%), a person with Angelman Syndrome has two copies of the “switched off” paternal Ube3a gene. In this case, all of the chromosome 15 is present, but neither Ube3a copy is “switched on”.
- Ube3a Mutation (Ube3a)
Some individuals (approx. 11%) have mutations (simple changes to the genetic code) in the maternal Ube3a gene that prevent it from being functional. This form of Angelman Syndrome can be hereditary, referral to a genetic councillor is advised.
- Imprinting Centre Defect (ICD)
Sometimes (approx. 5% of people with AS) there are changes in the Ube3a gene that cause the maternal copy to act as if it was the paternal copy. These are called imprinting defects. This form of Angelman Syndrome can be hereditary, referral to a genetic councillor is advised.
A very small number of people with AS have a “mosaic” form of AS, whereby a percentage of cells in the brain have a normal copy of the maternal Ube3a. Individuals with this type of AS tend to have a milder form of the disorder.
- Clinical Diagnosis
In approximately 10% of cases diagnosed to date, the cause is unknown. These individuals probably do not have Angelman Syndrome at all and further genetic testing is required. Information on testing can be found here.
Statement regarding Angelman Syndrome/Prader-Willi Syndrome.
AngelmanUK is concerned to hear that a number of professionals are diagnosing people with a condition called ‘Praderman Syndrome’ or with a dual diagnosis of Angelman Syndrome and Prader-Willi Syndrome. Along with the Prader-Willi Syndrome Association UK (PWSA UK) and the International PWS organisation Clinical and Scientific Advisory Board (CSAB) we would like to issue a joint statement to the effect that this is not an accurate diagnosis and should not be used. “The term ‘Praderman Syndrome’ is unhelpful, confusing and inaccurate. Dr Prader never saw any of these patients and thus he could not describe a condition bearing his name”
People with the mosaic form of Angelman Syndrome exhibit somewhat higher functional skills and abilities than those with other forms of Angelman Syndrome and frequently have advanced verbal skills in comparison to others with other forms of AS. This is sometimes confusing to professionals who may not have seen this phenotype before. However the symptoms and characteristics of Angelman Syndrome and Prader-Willi Syndrome are very different.
Many people with AS also exhibit hyperhpagia and other food related behaviours and this is sometimes seen more in people with the mosaic form of AS, possibly due to their increased physical abilities.
More information on this can be found on the FIND Resources website.
Should your loved one or patient exhibit characteristics of both conditions it would be prudent to request further genetic testing for the Mosaic Imprinting Centre Defect form of Angelman Syndrome (Mosaic ICD). This is also sometimes called ‘Atypical Angelman Syndrome’ by some professionals.
For further information about Mosaic AS please see www.ncbi.nlm.nih.gov/pubmed/28211971 (subscription may be required.)
For information about Prader-Willi Syndrome please see www.pwsa.co.uk