Prader-Willi and Angelman syndromes — branching viva

Opening framework

My framework has four layers. First, the shared mechanism — both are reciprocal disorders of genomic imprinting at chromosome 15q11-q13, where parent-of-origin-specific methylation controls which parental allele is expressed. Second, the reciprocal error — loss of paternally expressed genes (SNRPN, MAGEL2, NDN, SNORD116) produces PWS, and loss of maternal UBE3A expression in the brain produces AS. Third, the investigation — methylation analysis (MS-PCR or MS-MLPA) at the SNRPN locus is the first-tier test for both, because a single assay detects the imprinting abnormality and distinguishes PWS from AS. Fourth, the family — subtype determination (deletion, UPD, imprinting defect, UBE3A mutation) sets recurrence risk, and counselling must be imprinting-aware. [3] [4]

Recognising the two presentations

The neonate is Prader-Willi syndrome: profound hypotonia, poor feeding, weak cry, failure to thrive, narrow bifrontal diameter, almond-shaped palpebral fissures, thin upper lip, cryptorchidism, small hands and feet. This is a top differential of the floppy infant, alongside spinal muscular atrophy, congenital myotonic dystrophy, and metabolic disease — and methylation testing sorts it out. The three-year-old is Angelman syndrome: severe intellectual disability with absent speech, ataxic gait with uplifted arms, happy demeanour with frequent laughter, seizures, microcephaly, and fair skin and hair (in the deletion subtype). The happy demeanour and the seizure pattern are the two discriminators that should trigger methylation testing. [1] [2]

The molecular mechanism and the first-tier test

Genomic imprinting means that certain genes are expressed from only one parental allele, because the other is silenced by methylation. At 15q11-q13, the paternal allele expresses SNRPN, MAGEL2, NDN, and the SNORD116 snoRNA cluster; the maternal allele is silenced in this region but expresses UBE3A in neurons. The paternal UBE3A allele is itself silenced in neurons by the antisense transcript UBE3A-ATS, so the brain depends entirely on maternal UBE3A expression. Loss of paternal expression — by deletion, maternal UPD, or imprinting defect — eliminates the paternally expressed genes and produces PWS. Loss of maternal UBE3A — by deletion, UBE3A mutation, paternal UPD, or imprinting defect — eliminates functional UBE3A in the brain and produces AS. The first-tier test is methylation-specific PCR or MS-MLPA at SNRPN: absent paternal pattern confirms PWS, absent maternal pattern confirms AS (deletion, UPD, and imprinting-defect forms). [3] [4]

Subtype determination and recurrence risk

A positive methylation result must be followed by subtype determination, because subtype sets recurrence risk. FISH or chromosomal microarray distinguishes deletion from non-deletion. If a deletion is confirmed, recurrence risk is low (less than 1 per cent, de novo). If no deletion, UPD testing by microsatellite analysis of both parents determines whether the child has uniparental disomy — also low recurrence risk. If neither deletion nor UPD, an imprinting-centre defect is likely, and parental testing determines whether a parent carries the defect — this subtype carries up to 50 per cent recurrence risk. For AS, if methylation is normal but the phenotype is classic, UBE3A sequencing is indicated, because the mutation form has normal methylation and carries 50 per cent recurrence risk for carrier mothers. [4]

Syndrome-specific management

PWS management is anchored by growth hormone therapy (improves body composition, height, lean mass, cognitive outcomes, with surveillance for sleep apnoea and scoliosis), strict dietary and environmental control of hyperphagia (locked food, structured meals, behavioural strategies), endocrine management (hypogonadism, hypothyroidism, osteoporosis), and behavioural and developmental support. AS management is anchored by anticonvulsant optimisation (valproate, levetiracetam, clonazepam, ketogenic diet), augmentative and alternative communication (because expressive language is minimal but receptive is preserved), motor and physical therapy (gait, balance, tone), and sleep management. Neither syndrome has a cure, and both require lifelong multidisciplinary care with shared medical-home coordination. [1] [6] [7]

Branch: the imprinting-centre-defect family and the UBE3A trap

If a parent carries an imprinting-centre defect, the recurrence risk for subsequent children can approach 50 per cent, and prenatal or preimplantation genetic diagnosis must be offered. This is why subtype determination is not optional. The parallel trap is the methylation-normal child with classic Angelman features — the UBE3A mutation form. Methylation analysis is normal because the imprinting marks are intact, but the child still lacks functional UBE3A protein because of a point mutation. The reflex is UBE3A sequencing, and if a mutation is found, maternal carrier testing determines whether the mother carries it — with a 50 per cent recurrence risk if she does. The closing point is that cascade testing with imprinting-aware counselling is the obligation that completes every PWS and AS diagnosis, and the molecular subtype is the counselling answer. [4] [2]