Ask a Genetic Counselor: Common Questions about Prader-WiIli Syndrome

By: Elizabeth Kearney, MS, CGC, MBA

What is Prader-Willi syndrome?

Prader-Willi syndrome (PWS) is a genetic condition that affects growth and development. People with PWS have intellectual deficits, or learning and sleep problems, and typically have extreme hunger as children, which leads to obesity. They also have some common characteristics in their appearance and health, such as small hands and feet, muscle weakness, and shorter than average height.

What causes Prader-Willi syndrome?

PWS happens when a person has an unusual copy of chromosome 15. Chromosomes are inside cells and contain all of the genetic information, or DNA, that tells the body how to grow, develop, and function. Most people have 46 chromosomes, or 23 pairs. Each pair typically has one copy from the mother and one from the father. If a person has missing pieces or unusual forms of chromosomes, it can affect their development and their health.

Portions of chromosome 15 function differently when they come from the mother versus the father because of a process called imprinting. Imprinting is a normal process of modifying DNA to affect whether certain genes are active in a cell. Because of imprinting, a person needs to have a full copy of chromosome 15 from the mother and a full copy from the father to have typical development.

Because the region of chromosome 15 involved with Prader-Willi syndrome has an imprinting effect, there are multiple causes of PWS.

Paternal deletion:

The most common form of PWS is when people are missing a piece of chromosome 15 from their father.

Maternal uniparental disomy (UPD):

Sometimes, a person has two copies of chromosome 15 from the mother, and no copies from the father. This is called uniparental disomy, or maternal UPD. People with maternal UPD of chromosome 15 have PWS. Maternal UPD is the second most common cause of PWS.

Translocation:

Sometimes PWS results from a chromosome rearrangement, or translocation. A balanced translocation means that a person has all the right amounts of chromosomal material, but the chromosomes are arranged, or packaged, differently. A person with an unbalanced translocation may have extra or missing pieces of chromosomes. If that unbalanced translocation results in the person missing a portion of the chromosome15 from the father, the person will have features of PWS. It’s also possible that a translocation could cause PWS by disrupting critical genes involved with imprinting (instead of causing a deletion). Translocations are a rare cause of PWS.

Imprinting disorder:

In rare forms of PWS, the person may have the normal amount of chromosome 15 from the father, but the proper imprinting did not occur. Imprinting disorders are rare.

If a family member has Prader-Willi syndrome, does that mean that others in the family may have a child with PWS, too?

Prader-Willi syndrome is a genetic condition, but that doesn’t necessarily mean it runs in a family or is inherited. In fact, most parents who have a child with Prader-Willi syndrome have a very low chance (1% or lower) to have another child with PWS. In the more rare forms of PWS, the chances for a second child, or for a close blood relative to also have a child with PWS, may be higher.

Ultimately, genetic testing can confirm the diagnosis of Prader-Willi syndrome when it is suspected in a person. However, the type of genetic testing is very important because of the different possible genetic causes of PWS. Not every genetic test is the same, and the right type of testing is important for providing information about the chances for someone else in the family to have PWS. People who are planning for pregnancy and have a family member with PWS may need to talk with a genetics professional, like a medical geneticist, a doctor who specializes in genetics, or a genetic counselor.

What should parents of a child with PWS know if they are pregnant or planning for pregnancy?

As stated above, most of the time, the chances for another family member with PWS is low. However, the earlier people start planning for pregnancy, the more time to arrange for genetic testing and the more options they will have available. Many people don’t realize that they can use in vitro fertilization or IVF to test embryos for the condition that causes PWS in many families and only transfer embryos that are not affected by PWS. Others may decide to use an egg or sperm donor, depending on the genetic basis of PWS in their families. People may test a pregnancy through procedures called chorionic villus sampling (CVS) or amniocentesis to test the pregnancy, which involve getting a small sample of the developing baby’s cells for DNA testing. Amniocentesis and CVS have a small chance for pregnancy loss, so prospective parents may want to weigh the benefits of testing against this risk.

Will standard prenatal genetic testing identify PWS?

Types of genetic testing offered during pregnancy vary. Even when state-of-the-art genetic testing is used, the number and types of conditions that can be detected may depend on which laboratory performs the testing. Therefore, people who have a family member with PWS may want to talk with a genetic counselor, who is specially trained in genetics, to understand their personalized likelihood to have a child with PWS. Even if a woman’s prenatal or preconception care provider tells them that they will order genetic testing, it is best to find out what type of testing and check with a genetic specialist like a genetic counselor. Genetic science is changing rapidly, and it can be difficult for healthcare professionals to order the appropriate testing.

Is there a cure for PWS?

At this time, there is no cure for PWS. However, some treatments are available to manage symptoms to help a child meet his or her greatest potential. Ongoing research continues to advance the treatment options for people with PWS.

For more information about a current study for sleep disorders in PWS, click here.

Sources:

Prader-Willi Syndrome. GeneReviews, NIH 2017. Accessed 7/27/2021.https://www.ncbi.nlm.nih.gov/books/NBK1330/
Prader-Willi Syndrome. Online Mendelian Inheritance in Man,. 2020. Accessed 7/27/2021 https://www.omim.org/entry/176270
Butler, MG, Miller JL, and JL Forster. Prader-Willi Syndrome - Clinical Genetics, Diagnosis and Treatment Approaches: An Update. Curr Pediatr Rev. 2019 Nov; 15(4): 207–244.

Elizabeth Kearney, MS, CGC, MBA is a licensed genetic counselor and founder of Mainstream Genomics, LLC, which aims to help people who are pregnant or planning for pregnancy find the right genetic services for them. She is a past President of the National Society of Genetic Counselors and speakers nationally and regionally as a thought leader in genetic service delivery. Liz has a Master’s in Human Genetics From the University of Michigan and an MBA from Kellogg School of Management at Northwestern University.