How to test for Prader-Willi syndrome?

Diagnostic Testing for Prader-Willi Syndrome

DNA methylation analysis should be the first-line diagnostic test for Prader-Willi syndrome as it can detect all three major genetic mechanisms causing PWS (deletion, uniparental disomy, and imprinting defects). 1, 2

Primary Diagnostic Approach

Step 1: Methylation Analysis

• Perform DNA methylation analysis using Southern hybridization with methylation-sensitive SNRPN or PW71B probes
• Results interpretation:
  • If only maternal methylation pattern is present → PWS is confirmed
  • If biparental methylation pattern is present → PWS is ruled out
  • If only paternal methylation pattern is present → Consider Angelman syndrome instead

Step 2: Determine Genetic Mechanism (if methylation is abnormal)

After confirming PWS by methylation analysis, determine the specific genetic mechanism:

1. FISH Analysis:

   • Use SNRPN or other probes from 15q11-13 region along with centromeric probe
   • If deletion is detected → PWS due to deletion (accounts for ~70% of cases) 1, 3
   • If no deletion is detected → Proceed to UPD testing

2. UPD Testing:

   • Perform PCR using microsatellite markers from 15q11-13
   • If maternal UPD is present → PWS due to uniparental disomy (accounts for ~25% of cases) 1, 2
   • If biparental inheritance with abnormal methylation → Likely imprinting mutation (accounts for <5% of cases) 1, 3

Alternative Diagnostic Approach

If methylation analysis is not available, an alternative approach is:

1. High-resolution chromosome analysis (550-band level) and FISH using SNRPN probe 1, 4
2. If deletion is detected → Confirm PWS with methylation analysis
3. If no deletion is detected → Perform methylation analysis to detect UPD or imprinting defects

Clinical Considerations

• Methylation analysis is the most efficient primary screening test as it can detect all three major genetic mechanisms causing PWS 1, 2
• FISH analysis alone will miss approximately 30% of PWS cases (those due to UPD or imprinting defects) 1, 5
• Real-time PCR can be used to determine the size of chromosomal deletions in PWS patients 3

Genetic Counseling Implications

• Deletion cases: Generally sporadic with low recurrence risk (~1%) 1
• UPD cases: Generally sporadic with low recurrence risk (~1%) 1
• Imprinting mutations: May have substantial recurrence risk (up to 50%) 1

Common Pitfalls to Avoid

• Relying solely on FISH analysis will miss cases caused by UPD or imprinting defects 1, 5
• Clinical diagnosis without genetic confirmation is insufficient due to overlapping features with other syndromes 1, 6
• If clinical suspicion remains high despite normal test results, consider:
  • Consultation with a clinician experienced with PWS 1
  • Evaluation for other conditions with overlapping phenotypes (e.g., fragile X, Williams syndrome) 1
  • Referral to specialized research laboratories for further investigation 1

Prenatal Testing

Prenatal testing is possible in families with a previously affected child:

• For deletion cases: FISH analysis on CVS or amniocentesis samples 1
• For UPD cases: PCR and microsatellite analysis 1
• For imprinting mutations: Testing for previously identified mutation 1