Diagnostic Test Directory

Molecular Genetics Laboratory

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Prader - Willi Syndrome, PWS

  • LIS Mnemonic: MBPW


    Collect whole blood in a purple top (EDTA) tube (preferred). Extracted DNA is also acceptable.

    Volume Required

    5 ml whole blood or 2 ug extracted DNA

    Minimum Required

    3 ml whole blood


    Refrigerate sample until shipment. Send the sample at room temperature with overnight delivery for receipt Monday through Friday within 24 hours of collection.


    Whole blood can be refrigerated until shipment.

    Unacceptable conditions

    Heparinized specimens, severely hemolyzed specimens, frozen, clotted or possibly commingled specimens, blood in non-sterile or leaky containers, mislabeled or inappropriately labeled specimens.

    Specimen Handling

    Do not heat, freeze or centrifuge blood before shipment. Refrigerate sample until shipment.

Days Performed

Mon - Fri 9:00am to 4:00pm


4-6 weeks



Disease Information

Clinical Features:

Prader-Willi (PWS) syndrome is characterized by severe hypotonia and feeding difficulties in early infancy, followed in later infancy or early childhood by excessive eating and gradual development of obesity (unless externally controlled). Motor milestones and language development are delayed. All individuals have some degree of mental retardation. A distinctive behavioral phenotype (temper tantrums, stubbornness, and obsessive-compulsive characteristics) is common. Hypogonadism is present in both males and females and manifests as genital hypoplasia, incomplete pubertal development, and, in most, infertility. Short stature is common; characteristic facial features, strabismus, and scoliosis may be present.

Molecular Genetics:

Prader Willi syndrome is caused by the absence of the paternally derived region (15q11-15q13) on chromosome 15. The disorder is genetically heterogeneous with ~70-75% of the patients having a large deletion on paternal chromosome 15q11-q13; about25-30% of patients having maternal uni-parental disomy (UPD) for chromosome 15; ~1% of patients carrying an Imprinting Centre (IC) mutation.

Test Methods:

Methylation sensitive PCR is performed using primers specific for the methylated (maternal) and unmethylated (paternal) alleles of the SNRPN gene. In unaffected individuals, biparental inheritance is demonstrated by the presence of both maternal and paternal products. In a child affected with Prader Willi syndrome, only the maternal allele is observed.

Detection Rate:

Methylation studies would diagnose ~99% of patients with Prader Willi syndrome (including deletions, uniparental disomy, and imprinting center mutations). If the results of methylation testing are abnormal, further testing to demonstrate the absence of the paternal allele such as: deletion testing (FISH), uniparental disomy testing, or point mutation testing in the imprinting center gene (DNA sequencing) may be considered. Methylation studies would also diagnose ~80% of patients with Angelman syndrome which is allelic (genetically related) to PWS, but clinically distinct, and results from the absence of maternally contributed region.


Test results with interpretation will be mailed and/or faxed to the referring physician or send out lab following completion of the test. Additional reports will be provided as requested.


The clinical utility of these assays is to confirm a clinical diagnosis; facilitate proper clinical management, and to alert parents to possible recurrence risk.

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