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
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
||AS Cases Detectedb,c
||PWS Cases Detectedb
|UBE3A molecular testing
|Additional genetic testing
|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
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, as 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.
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 above 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
||Techniques for Detection
||DNA methylation, MS-MLPA, FISH,a CMA, CMA-SNP
|UPD (AS: paternal only; PWS: maternal only)
||DNA methylation, MS-MLPA, CMA-SNP,b DNA polymorphisms
|UPD with predisposing parental translocation (AS: paternal only; PWS: maternal only)
||DNA methylation, MS-MLPA, CMA-SNP,b UPD studyc
|Imprinting defect with deletion in the IC
||DNA methylation, MS-MLPA, DNA sequencing, AS IC deletion analysisd
|Imprinting defect without deletion in the IC
||DNA methylation, MS-MLPA, DNA polymorphisms
|UBE3A pathogenic variant
||Sequencing, deletion-duplication analysis
|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 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), and testing for diabetes are recommended to monitor for comorbidities.