Prader-Willi Syndrome (PWS)

Prader-Willi syndrome (PWS) is a rare neurobehavioral condition that affects multiple organ systems. PWS is characterized by severe hypotonia, feeding difficulties, developmental delays, and poor growth in infancy followed by chronic hyperphagia leading to obesity in early childhood. Behavioral problems are common in individuals with PWS and include stubbornness, temper tantrums, and obsessive-compulsive habits such as skin picking.¹

Genetic loss-of-function variations on chromosome 15 cause the development of PWS.¹ Abnormal DNA methylation occurs within the Prader-Willi critical region (PWCR) of chromosome 15 at the position q11-q13.²

Approximately 70% of PWS cases are due to paternal chromosome 15 deletions. Around 25% of PWS cases are individuals who inherit 2 maternal copies of chromosome 15, known as maternal uniparental disomy (UPD). Rare cases of PWS develop due to chromosomal translocations or other genetic changes that inactivate genes on the paternal copy of chromosome 15.¹

Genetic Testing for PWS

Genetic testing is required to confirm a diagnosis of PWS when it is suspected based on clinical presentation. Due to the underlying genetic etiology of PWS, molecular genetic testing should include DNA methylation analysis and detection of the chromosome 15 deletion or other chromosome 15 abnormalities.³ Early diagnosis with genetic testing should be encouraged, as it allows for earlier interventional therapies. Consensus diagnostic criteria have been established to support an official PWS diagnosis.³

Read more about PWS diagnosis 

DNA Methylation Testing

Initial diagnostic testing for PWS should include DNA methylation analysis, which detects DNA methylation abnormalities and confirms the clinical suspicion of PWS in approximately 99% of cases, regardless of PWS subtype.^3,4 

DNA methylation testing consists of polymerase chain reaction (PCR) testing using DNA primers combined with Southern blot hybridization of the SNRPN probe for the chromosome 15q11-q13 region or methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA).^3,5 The recent development of droplet digital PCR technology allows for an analysis of the deletion status of chromosome 15q11-q13 and identification of the size of the deletion, if present.³

If DNA methylation abnormalities are identified, further testing via fluorescence in situ hybridization (FISH) testing along with karyotyping and chromosomal or high-resolution microarray testing with single nucleotide polymorphism (SNP) and copy number variant (CNV) probes is recommended to determine PWS subtype. DNA methylation testing alone cannot confirm the PWS subtype.³ 

Read more about PWS types

Fluorescence In Situ Hybridization 

DNA is extracted and isolated from the patient’s venous blood sample. This sample undergoes FISH testing, which is a chromosomal analysis technique that enables cytogenetic localization of a DNA sequence to detect deletions or translocations in a specific gene or chromosomal region.³ 

FISH testing for PWS uses a fluorescently labeled probe that corresponds to the SNRPN locus of the PWCR on chromosome 15 to detect deletions in this region. If no deletions or rearrangements are present, the fluorescent probe will generate a single fluorescent signal at chromosome 15q11-q13.³

FISH tests are not sensitive enough for consideration as a first-line molecular test to confirm PWS, and they cannot identify whether the source of the chromosomal deletion in the 15q11-q13 region is paternal (resulting in PWS) or maternal (resulting in Angelman syndrome). Further determination of PWS genetic subtypes requires microarray testing.³

Read more about PWS differential diagnosis

Chromosomal or High-Resolution Microarray Testing

Chromosomal or high-resolution microarray testing uses SNP or CNV probes to identify the following areas across the whole genome³: 

• Identical DNA patterns on chromosomes called regions of homozygosity (ROH), which suggest consanguinity
• Large areas greater than 8 megabases (Mb) in size with loss of heterozygosity (LOH), which is suggestive of contiguous or segmental UPD (including maternal UPD in PWS)

Testing for maternal UPD requires DNA samples from both parents and the child with PWS.⁵

DNA Polymorphism Analysis

DNA polymorphism analysis determines the inheritance pattern by assessing for biparental inheritance or maternal-only inheritance. This analysis differentiates between maternal uniparental heterodisomy and an imprinting center defect due to epimutation. In contrast, individuals with the maternal isodisomy PWS subtype demonstrate LOH along the entire length of chromosome 15 without evidence of DNA polymorphism.³

DNA Sequence Analysis

DNA sequence analysis is reserved for rare cases of imprinting center mutations when DNA methylation testing detects only maternally methylated alleles, FISH testing detects no deletions, and DNA polymorphism analysis suggests biparental inheritance.^3,5 

For this test, DNA from both biological parents is extracted and isolated from a venous blood sample, saliva, or buccal cells. DNA sequence analysis defines the smallest region of overlap for the Prader-Willi imprinting center. This test identifies imprinting center microdeletions or asymptomatic epimutations and helps to determine the risk of having other children with PWS. Paternal microdeletions of the Prader-Willi imprinting center increase the risk of PWS recurrence to 50%, whereas epimutations result in a risk of recurrence of less than 1%.^3,5

Read more about PWS genetics

Pharmacogenetic Testing

Pharmacogenetic testing to assess the status of the cytochrome (CYP) p450 hepatic enzyme system is frequently performed in clinical practice. Variations in genes that encode this CYP system can affect how a person metabolizes specific medications and therefore can influence selection of the type or dosage of these medications.³ 

In patients with PWS, pharmacogenetic testing may predict weight gain due to certain atypical antipsychotics prescribed to address behavioral or psychological challenges.³

Read more about PWS treatment

Prenatal Genetic Testing

Although not frequently performed, a prenatal diagnosis of PWS may be confirmed via chorionic villus sampling and amniocentesis in cases of reduced fetal movement and polyhydramnios.⁶

Laboratory Testing and Imaging Studies for PWS

In addition to genetic testing, other laboratory tests and imaging studies can help identify comorbidities and complications of PWS, such as type 2 diabetes mellitus, growth problems, central adrenal insufficiency, sleep disorders, and osteoporosis/osteopenia.⁵

Laboratory testing includes^5,7-10:

• Fasting serum insulin-like growth factor-1 (IGF-1) and insulin-like growth factor binding protein-3 (IGFBP-3), which are decreased in patients with PWS and help screen for underlying growth hormone deficiency
• Growth hormone levels, which are decreased in patients with PWS
• Thyroid stimulating hormone (TSH) levels, which are low in patients with PWS due to hypopituitarism causing secondary hypothyroidism or TSH deficiency
• Levels of cortisol and adrenocorticotropic hormone (ACTH) in the morning, which are either both low or ACTH may be normal while cortisol is low, resulting in central adrenal insufficiency
• Oral glucose tolerance test (OGTT) and insulin testing to screen for impaired glucose tolerance, insulin resistance, or diabetes

Read more about PWS complications

Imaging studies include^5,11,12:

• Dual energy X-ray absorptiometry (DEXA) scans to screen for osteopenia or osteoporosis
• X-rays to detect scoliosis or hip dysplasia
• Magnetic resonance imaging (MRI) of the brain to assess for hypopituitarism
• Chest radiography to rule out cor pulmonale
• Abdominal radiography, computed tomography (CT) scans, or abdominal ultrasound to rule out acute gastric dilation or rupture, necrosis, cholelithiasis, and pancreatitis, especially in the presence of vomiting or abdominal pain in individuals with PWS
• Multiple sleep latency studies to rule out narcolepsy or sleep apnea

Read more about PWS comorbidities

References

1. Prader-Willi syndrome. MedlinePlus. Updated May 13, 2022. Accessed July 22, 2023.
2. Driscoll DJ, Miller JL, Cassidy SB. Prader-Willi syndrome. In: Adam MP, Mirzaa GM, Pagon RA, et al., eds. GeneReviews^® [Internet]. Seattle, WA: University of Washington, Seattle; 1993-2023. October 6, 1998. Updated March 9, 2023. Accessed July 22, 2023.
3. Butler MG, Miller JL, Forster JL. Prader-Willi syndrome – clinical genetics, diagnosis and treatment approaches: an update. Curr Pediatr Rev. 2019;15(4):207-244. doi:10.2174/1573396315666190716120925
4. Diagnosis and treatment. Foundation for Prader-Willi Research. Accessed July 22, 2023.
5. Scheimann A. Prader-Willi syndrome workup: laboratory studies. Medscape. Updated August 27, 2021. Accessed July 22, 2023.
6. Goldstone AP, Holland AJ, Hauffa BP, Hokken-Koelega AC, Tauber M. Recommendations for the diagnosis and management of Prader-Willi syndrome. J Clin Endocrinol Metab. 2008;93(11):4183-4197. doi:10.1210/jc.2008-0649
7. Aycan Z, Baş VN. Prader-Willi syndrome and growth hormone deficiency. J Clin Res Pediatr Endocrinol. 2014;6(2):62-67. doi:10.4274/jcrpe.1228
8. Clark K. Thyroid issues in PWS. Prader-Willi Syndrome Association (USA). July 2017. Accessed July 22, 2023.
9. Kusz MJ, Gawlik AM. Adrenal insufficiency in patients with Prader-Willi syndrome. Front Endocrinol (Lausanne). 2022;13:1021704. doi:10.3389/fendo.2022.1021704
10. Qian Y, Xia F, Zuo Y, et al. Do patients with Prader-Willi syndrome have favorable glucose metabolism? Orphanet J Rare Dis. 2022;17:187. doi:10.1186/s13023-022-02344-3
11. Scheimann A. Prader-Willi syndrome workup: imaging studies. Medscape. Updated August 27, 2021. Accessed July 22, 2023.
12. Stevenson DA, Heinemann J, Angulo M, et al. Gastric rupture and necrosis in Prader-Willi syndrome. J Pediatr Gastroenterol Nutr. 2007;45(2):272-274. doi:10.1097/MPG.0b013e31805b82b5

Reviewed by Hasan Avcu, MD, on 7/25/2023.

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Maria Arini Lopez, PT, DPT, CSCS, CMTPT, CIMT

Maria Arini Lopez, PT, DPT, CSCS, CMTPT, CIMT is a freelance medical writer and Doctor of Physical Therapy from Maryland. She has expertise in the therapeutic areas of orthopedics, neurology, chronic pain, gastrointestinal dysfunctions, and rare diseases especially Ehlers Danlos Syndrome.