The Biology of PWS and Similar Conditions

The underlying biology of Prader-Willi Syndrome (PWS) is rooted in a fascinating but complex genetic process called genomic imprinting. Understanding why PWS occurs requires looking at how our bodies decide which genes to use from our parents.

The Science of Genomic Imprinting

Every child inherits two sets of chromosomes: one from their mother and one from their father. For most genes, both copies are active. However, for a small number of genes, only one copy is “turned on” while the other is “silenced.” This is genomic imprinting ^[1].

In the specific area of the brain known as the 15q11-q13 region, the body depends entirely on the genes from the father to function correctly because the maternal copies are naturally silenced ^[2]. In PWS, these paternal genes are either missing or inactive, leading to the symptoms of the disorder ^[3].

The Three Genetic Causes

There are three main ways the paternal genes can be lost, and knowing which one your child has may help your doctors provide more personalized care:

1. Paternal Deletion (60–75%): A small piece of the 15th chromosome from the father is physically missing ^[4]. This is the most common form.
2. Maternal Uniparental Disomy (mUPD) (20–35%): The child inherits two copies of chromosome 15 from their mother and none from their father ^[5]. This subtype is often associated with older maternal age.
3. Imprinting Center Defects (1–4%): The genetic material from the father is present, but the “switch” that should turn the genes on is broken, leaving them silenced ^[6].

Conditions That May Look Like PWS

Because the symptoms of PWS change as a child grows, doctors must carefully distinguish it from other conditions during different stages of life.

In Infancy (The “Floppy Infant”)

If an infant has severe hypotonia (low muscle tone), doctors may consider several other conditions before confirming PWS through DNA methylation testing ^[7]:

• Spinal Muscular Atrophy (SMA): A genetic disorder affecting the motor nerve cells in the spinal cord ^[8].
• Myotonic Dystrophy: A form of muscular dystrophy that causes progressive muscle weakness ^[7].
• Infant Botulism: A rare but serious illness caused by a toxin that attacks the body’s nerves.

In Childhood (Rapid Weight Gain)

When a child begins to gain weight rapidly, other rare syndromes might be considered:

• ROHHAD Syndrome: This involves rapid-onset obesity usually after age 2, but unlike PWS, it is often accompanied by breathing issues and does not start with infant feeding problems ^[9].
• Bardet-Biedl Syndrome (BBS): Characterized by early obesity, but often includes extra fingers or toes and vision problems ^[10].
• Alström Syndrome: Includes early obesity along with progressive loss of vision and hearing ^[11].

Confirmation of PWS is typically done using DNA methylation analysis, which is the most reliable “gold standard” test for identifying the disorder across all genetic subtypes ^[4].

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