Angelman Syndrome and Prader-Willi Syndrome

Angelman syndrome (AS) and Prader-Willi syndrome (PWS) are complex neurodevelopmental genetic disorders characterized by developmental delay and intellectual disability. AS is caused by the loss of function of maternally inherited genes within 15q11.2-q13 due to deletion, paternal uniparental disomy, ubiquitin-protein ligase E3A (UBE3A) gene variants, imprinting defects, translocation defects, or unknown causes.    PWS is caused by loss of function of paternally expressed genes in the same region (15q11.2-q13) due to deletions, maternal uniparental disomy, chromosome translocation, or imprinting defects.    Laboratory testing can be used to make a definitive diagnosis of PWS or AS, which is crucial for early intervention.  Laboratory testing is also used to identify the disease mechanism, which is important for determining recurrence risk.   

Quick Answers for Clinicians

What are the symptoms of Angelman syndrome and Prader-Willi syndrome?

Both Angelman syndrome (AS) and Prader-Willi syndrome (PWS) are associated with developmental delay and intellectual disability. AS is characterized by features such as ataxia, lack of speech, and a “happy” demeanor marked by frequent laughter, smiling, and excitability. Patients with AS frequently have microcephaly and seizures.    PWS is associated with severe hypotonia and feeding difficulties in infancy, with gradual development of hyperphagia and morbid obesity in early childhood, as well as short stature, hypogonadism, maladaptive and compulsive behaviors, and other symptoms.    

What are the genetic mechanisms that lead to Angelman syndrome or Prader-Willi syndrome, and why are they important to identify?

Most cases of Angelman syndrome (AS) and Prader-Willi syndrome (PWS) are not inherited; rather, they are due to loss of function of genes in the 15q11.2-q13 region caused by a spontaneous deletion or uniparental disomy. In addition to spontaneous deletion and paternal uniparental disomy, 11% of AS cases are linked to pathogenic variants in the UBE3A gene, which may be heritable; rare cases are linked to chromosomal translocations/rearrangements, uniparental disomy with a parental translocation, or imprinting defects with deletions in the imprinting center, all of which may be heritable. Approximately 10% of AS cases arise from currently unknown mechanisms. As with AS, rare cases of PWS may result from potentially heritable chromosomal translocations/rearrangements, uniparental disomy with a parental translocation, and imprinting defects with deletions in the imprinting center. However, PWS is not associated with variants in UBE3A or any unknown mechanisms. Although identification of the mechanism of AS or PWS does not affect treatment, it can help determine risk of recurrence in future offspring and is therefore important for genetic counseling purposes.    

When is prenatal testing for Angelman syndrome or Prader-Willi syndrome appropriate?

Although the risk of recurrence is very low in the case of a spontaneous 15q11.2-q13 deletion or most cases of uniparental disomy, prenatal testing may be offered following confirmed diagnosis in a family member for reassurance, given that a negative maternal test does not exclude the small possibility of somatic and/or germline mosaicism.  It is recommended that prenatal testing be offered if a heritable mechanism for Angelman syndrome (AS) and Prader-Willi syndrome (PWS) has been identified because of the high risk of recurrence.  Methylation analysis, fluorescence in situ hybridization (FISH), and other techniques can be used, preferably on amniotic cells.    Methylation testing is not recommended on chorionic villus samples due to incomplete methylation in early embryonic development, which may cause false-positive results. However, other molecular mechanisms responsible for AS or PWS, such as sequence variants, deletions, or chromosomal translocations, may be reliably detected in a chorionic villus sample by targeted Sanger sequencing, cytogenetic single nucleotide polymorphism microarray (CMA-SNP), or karyotyping, respectively.

Will laboratory testing diagnose all cases of Angelman syndrome?

DNA methylation testing has a clinical sensitivity of ~78%, and UBE3A gene sequencing has a sensitivity of 11% for Angelman syndrome (AS).  Laboratory testing will not always confirm a clinical diagnosis of AS because some cases are caused by a currently unknown mechanism. For more information on the clinical and analytical sensitivity of AS testing, see the Angelman Syndrome and Prader-Willi Syndrome Test Fact Sheet.

Indications for Testing

Laboratory testing for AS and PWS can be used to   :

  • Diagnose AS or PWS in infants or children with symptoms
  • Determine the risk of recurrence in future offspring of parents of a child diagnosed with AS or PWS
  • Monitor comorbidities and the effects of treatment in PWS

Laboratory Testing

Diagnosis and Determination of Recurrence Risk

Diagnosis of AS or PWS is based on clinical criteria. Laboratory testing confirms a diagnosis of PWS in >99% of cases,    whereas the diagnosis of AS can be supported by laboratory testing in ~90% of cases.  The same methods used for diagnosis can be used to identify the disease mechanism, which facilitates determination of recurrence risk.   

Summary of Recommended Laboratory Testing Strategy in AS/PWS
Testing Methoda AS Cases Detectedb,c PWS Cases Detectedb
Methylation analysis ~80% >99%
UBE3A molecular testing ~11% None (n/a)
Cytogenetic testing ~68% 65-75%
Additional genetic testing ~10% 20-30%

aTests are presented in the recommended order.

bCertain cases may be detected via multiple testing methods, but not all methods will distinguish the mechanism of disease in detected cases; a combination of testing methods may be required.

cApproximately 10% of AS cases result from currently unknown mechanisms and will not be detected.

n/a, not applicable

Methylation Analysis

The initial and most sensitive test for AS and PWS is methylation analysis of 15q11.2-q13     using methods such as methylation-sensitive polymerase chain reaction (PCR) or methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA).  Methylation analysis confirms PWS in >99% of individuals with symptoms that meet consensus criteria  and detects ~80% of AS cases. 

Although methylation analysis does not always distinguish the causative mechanism of AS or PWS, MS-MLPA can be used to simultaneously assess methylation and copy number changes, thereby distinguishing between deletions that cause uniparental disomy and those that cause imprinting center defects.  

UBE3A Molecular Testing

Testing for UBE3A variants is recommended for patients with AS if DNA methylation analysis fails to yield a diagnostic result,  given that approximately 11% of AS cases have been linked to pathogenic variants in the UBE3A gene. Sequencing should be performed first, followed by deletion/duplication analysis if sequencing fails to detect a pathogenic variant. 

Cytogenetic Testing

Fluorescence in situ hybridization (FISH), typically performed with a karyotype, can be used in the evaluation of AS (if a pathogenic UBE3A variant is not identified) or PWS following methylation analysis to confirm deletions in 15q11.2-q13.   FISH may also be used to detect chromosomal translocations or rearrangements with the appropriate probes.   FISH should not be used alone because negative results do not exclude a diagnosis of AS or PWS. 

Chromosomal microarray (CMA) will detect deletions in 15q11.2-q13, whereas cytogenetic single nucleotide polymorphism (SNP) microarray (CMA-SNP) will detect deletions and most cases of uniparental disomy due to deletion of the maternal or paternal critical region.   However, CMA or CMA-SNP will not detect chromosomal translocations. 

Additional Genetic Testing

If deletions or other abnormalities are not detected by the previously described techniques, DNA polymorphism testing (via PCR, MLPA, or gene-targeted microarray) of the patient and both parents can be used to distinguish between uniparental disomy and imprinting defects.   DNA sequencing of the patient and both parents may also be useful to distinguish between imprinting center deletions and epimutations that lead to imprinting defects.  Some cases of AS may present similarly to Rett syndrome or another MECP2-related disorder, which can be ruled out by molecular testing of the MECP2 gene.

Recurrence Risk Based on Mechanism in AS and PWS
Mechanism Techniques for Detection Cases Recurrence Risk
15q11.2-q13 deletion DNA methylation, MS-MLPA, FISH,a CMA, CMA-SNP

AS: 65-75%

PWS: 65-75%

<1%
Chromosomal translocation/rearrangement FISH

AS: <1%

PWS: <1%

≤50%
UPD (AS: paternal only; PWS: maternal only) DNA methylation, MS-MLPA, CMA-SNP,b DNA polymorphisms

AS: 2-7%

PWS: 20-30%

<1%
UPD with predisposing parental translocation (AS: paternal only; PWS: maternal only) DNA methylation, MS-MLPA, CMA-SNP,b UPD studyc

AS: <1%

PWS: <1%

≤100%
Imprinting defect with deletion in the IC DNA methylation, MS-MLPA, DNA sequencing, AS IC deletion analysisd

AS: 0.5%

PWS: <0.5%

≤50%
Imprinting defect without deletion in the IC DNA methylation, MS-MLPA, DNA polymorphisms

AS: 2.5%

PWS: 2%

<1%
UBE3A pathogenic variant Sequencing, deletion-duplication analysis

AS: 11%

PWS: n/a

≤50%
Currently unknown Unknown

AS: 10-15%

PWS: n/a

Unknown

aWith use of specific probes.

bDoes not detect all cases of UPD.

cDNA polymorphism testing using samples from patient and both parents.

dAnalysis via quantitative PCR, long-range PCR, MLPA, or gene-targeted microarray using samples from patient and both parents.

IC, imprinting center; UPD, uniparental disomy​

Sources: Dagli, 2017 ; Driscoll, 2017 

Monitoring

Monitoring in Prader-Willi Syndrome

Insulinlike growth factor 1 (IGF-1) and growth hormone status testing is recommended to monitor the success of growth hormone treatment.  Testing for hypothyroidism, including thyroid-stimulating hormone (TSH) and free thyroxine (T4) tests, and testing for diabetes are recommended to monitor for comorbidities. 

ARUP Laboratory Tests

Diagnosis

Methylation Analysis

Preferred initial diagnostic test for AS or PWS

Prenatal testing for AS or PWS

Use to identify cases resulting from molecular mechanisms that produce abnormal methylation patterns

UBE3A Test

Second-tier test for diagnosis of AS

Cytogenetic Tests

Follow-up test after methylation analysis in PWS and AS

Additional Genetic Tests

Use to screen for fetal microdeletions causing AS, PWS, and other syndromes

Use to rule out an MECP2 gene mutation in individuals with clinical features of AS who lack a molecular abnormality involving 15q11.2-q13

Useful when a pathogenic familial variant identifiable by sequencing is known

Monitoring in PWS

Preferred test for screening and monitoring of thyroid function

Use in conjunction with TSH test in cases of suspected secondary hypothyroidism

Use to assess for diabetes mellitus or impaired glucose tolerance

Use to assess growth hormone levels

Medical Experts

Contributor
Contributor

Mao

Rong Mao, MD, FACMG
Professor of Clinical Pathology, University of Utah
Section Chief, Molecular Genetics and Genomics, ARUP Laboratories

References