Imprinting Disorders — Prader-Willi and Angelman

Key Mechanism 1: Paternal 15q11-q13 Loss-of-Function in Prader-Willi Syndrome

Key Mechanism 2: Maternal 15q11-q13 Loss-of-Function in Angelman Syndrome

Key Mechanism 3: Imprinting Control Region (ICR) Defects

Additional Mechanism: Epigenetic Modifications and Chromatin Architecture

Prader-Willi Syndrome Etiologies

• Paternal deletion of 15q11-q13 (~70% of cases): Type I deletions (BP1-BP3) represent ~45% and are recurrent due to low-copy repeats (segmental duplications). Type II deletions (BP2-BP3) account for ~25% and have better prognoses with milder obesity. Type III deletions (BP1-BP2) are rare (~5%). Deletions are typically de novo but can rarely result from unbalanced translocations inherited from a balanced carrier parent.

• Maternal uniparental disomy for chromosome 15 (~25% of cases): Both copies of chromosome 15 are maternally derived with no paternal contribution. This can result from nondisjunction events during meiosis I or II, or from post-zygotic mitotic errors. These patients have two functional copies of UBE3A and lack the paternal methylation defect but still manifest PWS due to lack of paternally expressed genes. Risk increases with advanced maternal age, particularly maternal age >35 years.

• Imprinting center defects/ICR mutations (~1-3% of cases): Deletions or point mutations in the ICR prevent proper establishment of paternal methylation marks. These may be inherited from asymptomatic fathers with ICR deletions that fail to undergo appropriate imprinting during spermatogenesis.

• Balanced translocation carriers (~1%): Parents carrying balanced translocations involving chromosome 15 have increased recurrence risk (10-15% if mother is carrier; up to 100% if father is carrier, though paternal carriers of t(15;15) are infertile).

Angelman Syndrome Etiologies

• Maternal deletion of 15q11-q13 (~70% of cases): Similar deletion patterns occur as in PWS (Type I, II, III), but AS results only when the maternal allele is deleted. Deletions are usually de novo. Parents with deletions can transmit either the deleted or normal chromosome depending on which was inherited.

• Paternal uniparental disomy for chromosome 15 (~2-3% of cases): Both chromosome 15s are paternally derived. Since the paternal allele is naturally imprinted and UBE3A is silenced, patients lack functional maternal UBE3A.

• UBE3A mutations (~10-15% of cases): Point mutations or small indels specifically affecting the UBE3A gene cause AS even in the absence of a deletion. These are often inherited from asymptomatic mothers who are heterozygous carriers; since males typically have only one active UBE3A on the maternal chromosome, affected males can inherit the mutation and remain unaffected. Affected females are usually de novo mutations or rarely inherited if mothers are mildly affected.

• Imprinting defects/ICR mutations (~1-3% of cases): As in PWS, defective imprinting of the maternal allele prevents UBE3A expression.

• Chromatin remodeling defects (rare): Mutations in genes encoding proteins that regulate chromatin structure (e.g., PCDH15 mutations) have been associated with atypical AS presentations.

Prader-Willi Syndrome

• Neonatal hypotonia and poor feeding: Infants present with severe hypotonia, weak cry, and poor suck reflex. Feeding difficulties and failure to thrive occur in the first weeks to months of life, requiring tube feeding in some cases. This reflects dysfunction of hypothalamic centers regulating motor tone and feeding drive. Hypotonia gradually improves but persistent weakness and poor motor development remain.

• Hypogonadism and cryptorchidism: Reduced testosterone and LH/FSH production lead to cryptorchidism (present in >90% of males) and micropenis. In females, the external genitalia may be hypoplastic. During puberty, gonadotropins fail to rise appropriately, and secondary sexual characteristics develop incompletely. Adults are almost universally infertile.

• Pathologic hyperphagia and obesity: Beginning typically in early childhood (6 months to 3-4 years), children develop insatiable appetite with obsessive food-seeking behavior. Satiety signals are absent; patients cannot sense fullness and will eat constantly if food is available. This leads to severe, progressive obesity by childhood, with associated metabolic complications (T2DM, hypertension, dyslipidemia). Hyperphagia is the hallmark feature that distinguishes PWS from other causes of neonatal hypotonia.

• Growth hormone deficiency: Reduced spontaneous GH secretion and blunted response to stimulation tests are nearly universal. This results in short stature with relative short limbs and trunk. Growth hormone deficiency also contributes to poor body composition (increased fat, decreased lean mass).

• Developmental delay and intellectual disability: Typically mild to moderate (IQ range 40-85). Global developmental delay affects motor, cognitive, and language domains. Speech is often delayed and dysarthric.

• Behavioral problems and psychiatric manifestations: Obsessive-compulsive behaviors, skin-picking (acanthosis nigricans-like lesions), compulsive behaviors around food, temper tantrums, and oppositional defiance are common. Psychiatric illness including depression, anxiety, and psychosis can emerge in adolescence and adulthood.

• Physical exam findings: Short stature, truncal obesity with relatively thin extremities, hypogenitalism, narrow forehead, downturned mouth, small hands and feet (acromicria), hypopigmented skin, light-colored hair and eyes.

• Metabolic complications: Insulin resistance, T2DM (40-50% by adulthood), fatty liver disease, dyslipidemia.

Angelman Syndrome

• Profound developmental delay and intellectual disability: Severe global delay typically evident by 6 months. Most patients have IQ <50. Speech development is severely delayed, and most remain nonverbal or use only a few words. Motor milestones are significantly delayed.

• Seizures: Present in ~80% by 3 years of age. Seizure types include absence seizures (most common), atonic seizures with drop attacks, and generalized tonic-clonic seizures. EEG shows characteristic 3 Hz spike-and-wave discharges. Seizures are typically resistant to standard antiepileptic drugs (refractory in ~30%) but often respond to specific agents like valproate and levetiracetam. Seizures may be less frequent and milder after age 20-30.

• Ataxia and movement disorder: Characteristic "puppet-like" gait with wide-based, stiff-legged, jerky movements. Tremor, ataxia, and hyperreflexia contribute. Patients have poor balance and coordination.

• Speech impairment: Severe expressive language delay with most patients remaining nonverbal or using <10 words. Speech is often dysarthric when present. Receptive language is usually better than expressive, suggesting the deficit is primarily expressive.

• Behavioral characteristics and "happy" demeanor: A distinctive "happy puppet" phenotype includes inappropriate laughter, frequent smiling, excitable affect, and apparent happiness despite severe disability. This is not true happiness but rather emotional dysregulation with inappropriate laughing and outbursts. Attention deficits, hyperactivity, and impulsivity are marked. Many have autism spectrum features.

• Sleep disturbances: Reduced need for sleep (often sleeping 5-6 hours per night), sleep apnea, and nocturnal seizures are common.

• Physical exam findings: Microcephaly (present in ~80%), flat back of head, protruding tongue, hypopigmented skin and light hair (variable), wide mouth with widely spaced teeth, characteristic EEG abnormalities.

• Other features: Feeding difficulties (though less severe than PWS), GI dysmotility, strabismus and nystagmus, hearing impairment in some.

Diagnostic Approach

Clinical Suspicion Triggers

• PWS: Neonate or young infant with hypotonia and poor feeding, followed by emergence of hyperphagia and obesity in early childhood; short stature; developmental delay; hypogonadism in males
• AS: Severe developmental delay and intellectual disability; seizures onset 6-12 months; ataxic gait; characteristic EEG; inappropriate laughing; autism spectrum features

Diagnostic Test 1: Methylation-Specific PCR/Southern Blot (Methylation Pattern Analysis)

• Prader-Willi Syndrome: Shows abnormal maternal methylation pattern (paternal allele should be unmethylated; if patient's paternal copy is absent or has maternal-like methylation, PWS is present). Sensitivity ~99%.

• Angelman Syndrome: Shows abnormal paternal methylation pattern (maternal allele should be unmethylated; if patient's maternal copy is absent or has paternal-like methylation, AS is present). Sensitivity ~99%.

• Methylation studies can detect deletions, uniparental disomy, and most imprinting defects, but cannot distinguish deletion from UPD in AS.

Diagnostic Test 2: Fluorescence In Situ Hybridization (FISH) or Chromosomal Microarray

• Sensitivity: ~98% for detecting deletions (both types I and II in PWS; maternal deletions in AS)
• Specificity: Excellent; however, FISH cannot detect UPD or ICR-only defects
• Result interpretation:
• One signal on metaphase chromosome 15 = deletion
• Two signals = no deletion (but does not rule out UPD or ICR defect)
• Chromosomal microarray is now preferred as first-line cytogenetic testing, offering high resolution and cost-efficiency.

Diagnostic Test 3: Uniparental Disomy (UPD) Testing

• Microsatellite analysis: Compares polymorphic markers on chromosome 15 between patient and parents. If patient has two identical alleles and both differ from each parent's contribution, UPD is present. Sensitivity ~99%.
• SNP-based microarray: Can detect UPD through loss of heterozygosity patterns;

Immediate/neonatal stabilisation (PWS)

• Feeding support: profound hypotonia with weak suck causes failure to thrive; use high-flow nipples, nasogastric or temporary gastrostomy feeding with calorie tracking. The AAP health-supervision clinical report for Prader-Willi syndrome frames this as the first phase of care, followed by anticipatory transition to calorie restriction once hyperphagia emerges.
• Airway assessment: obtain polysomnography and ENT evaluation for adenotonsillar hypertrophy before growth hormone is started.

First-line disease-directed therapy (PWS)

• Recombinant growth hormone (somatropin): FDA-approved for PWS and endorsed by Endocrine Society/Pediatric Endocrine Society consensus recommendations. It improves linear growth, lean-to-fat mass ratio, and motor development — it does not abolish hyperphagia. Started in infancy after sleep study; monitor IGF-1, glucose, scoliosis.
• Environmental food security: locked food storage, supervised meals, structured low-calorie diet and exercise. No drug reliably controls PWS hyperphagia; this is behavioural/environmental, not pharmacologic.
• Sex steroid replacement (testosterone in males, estrogen/progestin in females) for hypogonadism and bone health; orchiopexy for cryptorchidism, which the AUA recommends by roughly 18 months if descent has not occurred.
• Escalation: SSRIs for skin-picking/compulsivity; low-dose atypical antipsychotics (e.g., risperidone) for aggression or psychosis, accepting metabolic cost. Metformin or GLP-1 receptor agonists for coexisting type 2 diabetes per ADA Standards of Care. Bariatric surgery is not routine and remains controversial.

First-line therapy (Angelman)

• Antiseizure medication: broad-spectrum agents — valproate, levetiracetam, clobazam, or ethosuximide for atypical absence — consistent with general AAN/AES practice for generalized epilepsies. Convulsive or nonconvulsive status epilepticus is treated with IV benzodiazepine (lorazepam) then a second-line IV agent.
• Melatonin and sleep hygiene for the characteristic sleep fragmentation; intensive PT/OT/speech with augmentative communication devices.

Contraindicated/avoid

• Sodium-channel-blocking antiseizure drugs (carbamazepine, oxcarbazepine, phenytoin) and vigabatrin can aggravate myoclonic and absence seizures in Angelman syndrome.
• Growth hormone in PWS with severe obesity, untreated obstructive sleep apnea, or active respiratory infection.
• Valproate in females of reproductive potential without contraception (neural tube defects, neurodevelopmental harm).

Prader-Willi — emergencies

• Acute gastric distention, necrosis, or rupture: binge eating plus impaired gastric emptying and a blunted vomiting reflex allows massive distention. Signalled by vomiting, abdominal distention/pain that seems out of proportion, or peritonitis — mortality is high and imaging should not be delayed. Surgical emergency.
• Choking/asphyxiation: hyperphagia with rapid ingestion and hypotonic oropharyngeal muscles. A leading cause of death.
• Central adrenal insufficiency: hypothalamic-pituitary dysfunction; presents as hypotension, hypoglycemia, and hyponatremia during febrile illness or surgery — treat with stress-dose glucocorticoids. Emergency.
• Blunted pain perception, absent fever response, and inability to vomit mask appendicitis, fractures, and sepsis — a normal-appearing PWS patient can be critically ill.

Prader-Willi — chronic

• Obesity cascade: obstructive sleep apnea, obesity hypoventilation, pulmonary hypertension/cor pulmonale, type 2 diabetes, NAFLD, dyslipidemia.
• Osteoporosis and fracture: combined hypogonadism and growth hormone deficiency reduce peak bone mass.
• Scoliosis and hip dysplasia from hypotonia; scoliosis may progress rapidly during GH-driven growth.
• Psychosis and affective illness, disproportionately in the maternal-UPD subtype.

Treatment-related (PWS)

• Growth hormone: sudden death reported in the first months of therapy, particularly with untreated OSA, obesity, or respiratory infection — hence mandatory pre-treatment polysomnography. Also worsening glucose tolerance, scoliosis progression, slipped capital femoral epiphysis (limp with hip/knee pain), and idiopathic intracranial hypertension (headache, papilledema).

Angelman

• Status epilepticus, including nonconvulsive/myoclonic status: presents as prolonged unresponsiveness, drooling, or myoclonus rather than convulsion — obtain EEG. Emergency.
• Aspiration pneumonia from dysphagia and GERD; constipation from dysmotility; scoliosis from truncal hypotonia and ataxia; injury from drop attacks.
• Antiseizure drug toxicity: valproate hepatotoxicity, hyperammonemic encephalopathy, pancreatitis, and thrombocytopenia; sodium-channel blockers paradoxically worsening seizures.

• Parent-of-origin is the whole question: paternal 15q11-q13 loss = Prader-Willi; maternal loss = Angelman. The uniparental disomy is the mirror image — maternal UPD causes PWS, paternal UPD causes Angelman. This swap is the single most common distractor on the exam.
• Two-phase PWS story: a floppy neonate with weak suck and poor feeding who is later an insatiable, obese toddler with short stature, small hands and feet, and cryptorchidism. Neonatal hypotonia alone should also raise congenital myotonic dystrophy (maternally transmitted) and spinal muscular atrophy — the emergence of hyperphagia is what pins PWS.
• Angelman buzzwords: happy puppet — inappropriate laughter, ataxic wide-based gait, microcephaly, seizures beginning around 6–12 months, near-absent speech with better receptive language.
• Best next step for either syndrome is DNA methylation analysis of 15q11-q13, not karyotype and not FISH. Methylation testing detects deletion, UPD, and imprinting-center defects with very high sensitivity.
• Methylation testing cannot tell you the mechanism — follow an abnormal result with microarray/FISH (deletion) and, if negative, UPD studies, then imprinting-center or UBE3A sequencing. This matters because recurrence risk is very low for de novo deletion and UPD but can approach 50% for imprinting-center defects and maternally inherited UBE3A mutations.
• UBE3A encodes an E3 ubiquitin ligase expressed only from the maternal allele in neurons — the classic tissue-specific imprinting example. Note that a normal methylation study does not exclude Angelman syndrome, since UBE3A point mutations leave methylation intact.
• Growth hormone in PWS improves height and body composition, not appetite, and requires a sleep study first; obtain hyperphagia control through environmental food restriction.
• In Angelman seizures, avoid carbamazepine, oxcarbazepine, phenytoin, and vigabatrin — they can worsen myoclonic and atypical absence seizures.
• Do not confuse with Beckwith-Wiedemann syndrome (11p15 imprinting: macrosomia, macroglossia, omphalocele, hypoglycemia, Wilms tumor risk).