What is the diagnostic approach for Prader Willi syndrome?

Diagnostic Approach for Prader-Willi Syndrome

DNA methylation analysis should be the first-line diagnostic test for Prader-Willi Syndrome, as it detects approximately 99% of cases regardless of the underlying genetic mechanism. 1

Initial Diagnostic Testing

Methylation analysis is the single most important test and should be ordered first when PWS is clinically suspected. 1, 2

• Perform methylation analysis using Southern hybridization with methylation-sensitive SNRPN or PW71B probes 3
• If only the maternal methylation pattern is present (absence of paternal contribution), PWS is confirmed 3, 1
• If biparental inheritance is identified (both maternal and paternal patterns present), PWS is ruled out 3, 1
• Normal methylation analysis rules out PWS and most identifiable genetic causes 3

Clinical Indications That Should Trigger Immediate Testing

• Unexplained hypotonia with poor suck in infancy requires immediate PWS testing 1, 4
• Feeding difficulties requiring assisted feeding methods (gavage, special nipples) should prompt testing 1, 5
• Failure to thrive in infancy combined with hypotonia is a critical red flag 5, 4, 2

Determining the Genetic Subtype (After Methylation Confirms PWS)

Once PWS is confirmed by methylation analysis, additional testing is necessary to identify the specific genetic mechanism for genetic counseling purposes:

• Perform FISH (fluorescence in situ hybridization) using SNRPN or other probes in the 15q11-13 region to detect deletions 3, 6
• If FISH shows a deletion (accounts for 60-75% of cases), the diagnosis is paternal deletion 1, 5, 4
• If FISH is normal (no deletion detected), perform UPD studies using PCR with microsatellite markers from 15q11-13 3
• If maternal UPD is present (both chromosome 15s from mother), this accounts for 20-30% of cases 1, 5, 4
• If biparental inheritance is identified despite abnormal methylation, an imprinting defect is present (1-3% of cases) 3, 2

Routine Chromosome Analysis

• Perform high-resolution chromosome analysis (~550 band level) routinely to rule out other chromosomal abnormalities or rare translocations involving chromosome 15 3, 1
• This is particularly important because translocations can have significantly different recurrence risks 3

When Initial Testing is Normal But Clinical Suspicion Remains High

• Consult with a clinician experienced in PWS diagnosis 3
• Consider evaluation for other conditions with overlapping phenotypes including fragile X syndrome, Williams syndrome, Smith-Magenis syndrome, or Albright hereditary osteodystrophy 3
• Referral to a research laboratory for advanced molecular investigation may be warranted 3

Critical Pitfalls to Avoid

• Do not rely on chromosome analysis alone - standard karyotyping misses approximately 20-30% of PWS cases (those with UPD or imprinting defects) 6, 2
• Do not skip methylation analysis - it is the only technique that diagnoses all three molecular genetic classes of PWS 4, 2, 7
• Do not stop at methylation confirmation - determining the specific genetic subtype (deletion vs UPD vs imprinting defect) is essential for accurate genetic counseling regarding recurrence risk 3, 1

Genetic Counseling Implications Based on Subtype

• Paternal deletion or maternal UPD: recurrence risk is low (~1%) 1
• Imprinting mutation: recurrence risk is substantial (up to 50%) 3, 1
• Translocation present in a parent: significant recurrence risk exists and requires detailed cytogenetic evaluation 3

Prenatal Diagnosis Scenarios

• When cytogenetic deletion is suspected on CVS or amniocentesis, perform FISH with SNRPN probes for confirmation 3, 1
• When trisomy 15 mosaicism is detected prenatally, perform PCR to detect UPD 15 and check paternity status 3
• When a familial or de novo translocation involving chromosome 15 is identified prenatally, perform both FISH and UPD analysis 3, 1