Angelman Syndrome and Prader-Willi Syndrome (15q11-13 Imprinting Disorders)

Shared Genetic Basis And Genomic Imprinting

Both Prader-Willi syndrome (PWS) and Angelman syndrome (AS) result from the loss of function of genes in the imprinted region of chromosome 15q11.2-q13.
Genomic imprinting involves parent-of-origin specific gene expression mediated by DNA methylation and histone modification.
Paternally expressed genes (SNRPN, NDN, MAGEL2, MKRN3) are critical for preventing PWS, while the maternally expressed gene UBE3A is critical for preventing AS.
Diagnosis for both conditions initially requires methylation analysis, which detects greater than 99% of cases, followed by specific testing to distinguish the exact genetic mechanism.

PWS is a neurodevelopmental disorder caused by the lack of paternally expressed genes at the 15q11-13 locus.
Loss of genes like SNRPN leads to severe hypothalamic dysfunction, which disrupts appetite regulation, growth hormone (GH) production, and sex hormone synthesis.

Neonatal And Infancy Phase: Severe hypotonia (Approach to a floppy infant), weak cry, poor suck, failure to thrive, and frequent requirement for nasogastric feeding.
Childhood Phase (2-6 Years): Development of hyperphagia (insatiable appetite), rapid weight gain, central obesity, short stature, small hands and feet, and hypopigmentation.
Adolescence And Adulthood: Morbid obesity, type 2 diabetes mellitus, hypogonadism (cryptorchidism, delayed puberty, infertility), and mild to moderate intellectual disability (IQ 60-70).
Behavioral And Other Features: Obsessive-compulsive behavior, skin picking, temper tantrums, high pain threshold, sleep apnea, and scoliosis.

First-Line Testing: Methylation-specific PCR or MS-MLPA demonstrates an abnormal maternal-only methylation pattern.
Confirmatory Testing: Fluorescence In Situ Hybridization (FISH) or Chromosomal Microarray (CMA) confirms deletions. Single Nucleotide Polymorphism (SNP) microarray detects maternal uniparental disomy (UPD).
Endocrine Evaluation: Serum IGF-1, GH stimulation test, and bone age assessment.

Neonatal Care: Nasogastric feeding and physiotherapy for severe hypotonia.
Nutritional And Behavioral Control: Strict food supervision, locked kitchens, low-calorie diets, and targeted behavioral therapy for hyperphagia.
Endocrine Therapy: Growth hormone therapy is approved from infancy to improve linear growth, muscle tone, and cognition while reducing obesity (requires prior sleep study). Sex hormone replacement is initiated at puberty for hypogonadism.
Comorbidity Surveillance: Monitoring for scoliosis, type 2 diabetes, and continuous positive airway pressure (CPAP) for sleep apnea.

AS is a severe neurodevelopmental disorder caused by the loss of maternal UBE3A function at 15q11-13.
UBE3A encodes an E3 ubiquitin ligase expressed exclusively from the maternal allele in the brain.
Loss of UBE3A disrupts the ubiquitin-proteasome pathway in neurons, impairing synaptic function and leading to seizures, ataxia, and developmental arrest.

Infancy: Severe developmental delay, early hypotonia, feeding difficulties, and postnatal microcephaly.
Neurological Features: Profound intellectual disability, absent speech, early-onset seizures (occurring in 80% of cases, typically onset before 3 years), ataxia, gait apraxia, and tremulousness.
Behavioral Phenotype: Distinctive “happy puppet” behavior characterized by inappropriate and frequent laughter, a happy demeanor, hand flapping, hypermotoric behavior, and severe sleep disturbances.
Craniofacial Features: Wide mouth, protruding tongue, prominent jaw, deep-set eyes, and hypopigmentation (especially in deletion cases).

First-Line Testing: Methylation-specific PCR or MS-MLPA demonstrates an abnormal paternal-only pattern.
Confirmatory Testing: FISH or CMA confirms maternal deletion. SNP array detects paternal UPD.
Gene Sequencing: UBE3A sequencing is required if methylation is normal but clinical suspicion remains high (detects point mutations causing 10-15% of cases).
Neuroimaging And EEG: EEG reveals characteristic high-amplitude slow waves and notched delta patterns.

Neurological Care: Aggressive seizure control utilizing valproate, levetiracetam, or clobazam. Carbamazepine must be avoided. Ketogenic diets may be used for refractory epilepsy.
Developmental Support: Early intervention with physiotherapy for ataxia, and augmentative and alternative communication (AAC) devices to overcome absent speech.
Behavioral Therapy: Melatonin administration for sleep disturbances and targeted behavioral interventions.

Determining the exact genetic mechanism is mandatory for providing accurate recurrence risk counseling.

Etiologic Mechanism	PWS Frequency	AS Frequency	Recurrence Risk
Microdeletion (15q11-13)	65-75% (Paternal Deletion)	65-75% (Maternal Deletion)	Less than 1% (usually de novo). 50% if the parent carries a balanced translocation.
Uniparental Disomy (UPD)	20-30% (Maternal UPD 15)	5-10% (Paternal UPD 15)	Less than 1%.
Imprinting Center Defect	1-3%	3%	Up to 50% if an inherited genetic mutation disrupts the imprinting center.
Single Gene Mutation	Not Applicable	10-15% (UBE3A Mutation)	50% if the mother carries a germline UBE3A mutation.