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 (UPD), 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 UPD, 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 (UPD). In addition to spontaneous deletion and paternal UPD, 11% of AS cases are linked to pathogenic variants in the UBE3A gene, which may be heritable. Rare AS cases are linked to chromosomal translocations/rearrangements, UPD 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, UPD 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?

Prenatal testing for Angelman syndrome (AS) or Prader-Willi syndrome (PWS) is recommended for families with a previously affected child when the underlying mechanism of AS or PWS is known.

Prenatal testing may also be considered for:

A family with a previously affected child in whom the underlying mechanism for AS or PWS was spontaneous 15q11.2-q13 deletion or uniparental disomy (UPD)
A pregnancy without a family history of AS or PWS if noninvasive prenatal screening (NIPS) shows an increased risk
A pregnancy without a family history of AS or PWS if previous fetal cytogenetic testing indicates abnormalities involving chromosome 15 (eg, absence of heterozygosity, balanced Robertsonian translocation, 15q11.2-q13 deletion, mosaic monosomy/trisomy, de novo translocation, and/or a supernumerary chromosome)

What prenatal laboratory testing is appropriate for Angelman syndrome and Prader-Willi Syndrome?

The prenatal testing approach for Angelman syndrome (AS) or Prader-Willi syndrome (PWS) varies based on the underlying mechanism previously identified. That is, the test methods used for at-risk fetuses must detect the type of causative variant present in the initial proband. Both amniocentesis and chorionic villi sampling are prenatal diagnostic options to detect sequencing and copy number variants. Methylation testing may be performed on samples from amniocentesis. Methylation testing is not recommended on chorionic villus samples due to incomplete methylation in early embryonic development, which may result in uninterpretable or false-positive results. Chromosomal translocations may be assessed by karyotyping.

Will laboratory testing diagnose all cases of Angelman syndrome?

DNA methylation testing has a clinical sensitivity of approximately 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.

Indications for Testing

Laboratory testing for AS and PWS can be used to:

Diagnose AS or PWS in symptomatic infants and children
Diagnose AS or PWS in at-risk fetuses
Determine the risk of recurrence in future offspring of parents who have a child with AS or PWS
Monitor comorbidities and the effects of treatment in individuals with 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 approximately 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 Method^a	AS Cases Detected (%)^b,c	PWS Cases Detected (%)^b
Methylation analysis	Approximately 80	>99
UBE3A molecular testing	Approximately 11	n/a
Cytogenetic testing	Approximately 68	65-75
Additional genetic testing	Approximately 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 approximately 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 deletions, thereby determining if the expected phenotype is AS or PWS in cases with an identified deletion. For positive cases, MS-MLPA can also distinguish deletions of the AS/PWS critical region from UPD/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) or genomic microarray can be used in the evaluation of AS or PWS following an abnormal methylation result to confirm deletions in 15q11.2-q13. Normal FISH or genomic microarray studies do not exclude a diagnosis of AS or PWS. Karyotyping may also be used to detect chromosomal translocations or rearrangements.

Genomic microarray will detect deletions in 15q11.2-q13 and absence of heterozygosity (AOH) involving chromosome 15, which indicate possible UPD. However, having no AOH of the AS/PWS critical region does not exclude UPD. Genomic microarray will not detect chromosomal translocations; however, defining the breakpoints of a deletion with microarray modifies the risk that the deletion is part of an unbalanced translocation.

Additional Genetic Testing

If deletions or other abnormalities are not detected by the previously described techniques, DNA polymorphism testing (eg, PCR) of the patient and both parents can be used to distinguish between UPD and imprinting defects. This testing 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^a	Cases (%)	Recurrence Risk (%)
15q11.2-q13 de novo deletion	DNA methylation, MS-MLPA, FISH,^b CMA, CMA-SNP	AS: 65-75 PWS: 60-70	<1
Chromosomal translocation/rearrangement	Karyotype^c	AS: <1 PWS: <1	≤50
De novo UPD (AS: paternal; PWS: maternal)	DNA methylation, MS-MLPA, CMA-SNP,^d UPD study^e	AS: 3-7 PWS: approximately 30-40	<1
UPD with predisposing chromosomal abnormality	DNA methylation, MS-MLPA, CMA-SNP,^d UPD study^e	AS: <1 PWS: <1	≤100
Imprinting defect with deletion in the imprinting center	DNA methylation, MS-MLPA, DNA sequencing, AS imprinting center deletion analysis^f	AS: 0.3 PWS: <0.5	≤50
Imprinting defect without deletion in the imprinting center	DNA methylation, MS-MLPA, DNA polymorphisms	AS: 2.5-3 PWS: 2-4	<1
UBE3A pathogenic variant	UBE3A sequencing and deletion-duplication analysis	AS: 11 PWS: n/a	≤50
Currently unknown	Unknown	AS: 10 PWS: n/a	Unknown
^aTechniques for initial detection of mechanism; additional testing may be required to differentiate from other mechanisms. ^bWith use of specific probes. ^cNot for initial evaluation; use to assess etiology and/or at-risk relatives. ^dDoes not detect all cases of UPD. ^eDNA polymorphism testing using samples from the patient and both parents. ^fAnalysis via quantitative PCR, long-range PCR, MLPA, or gene-targeted microarray using samples from the patient and both parents. CMA-SNP, single nucleotide polymorphism chromosomal microarray; n/a, not available Sources: Dagli, 2017; Driscoll, 2017

Monitoring

Monitoring in Prader-Willi Syndrome

Insulin-like 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.