1.13 Imprinting Disorders (PWS, Angelman, Beckwith-Wiedemann, Russell-Silver)
Key Takeaways
- Prader-Willi syndrome results from lack of paternally expressed genes at 15q11-q13; Angelman syndrome results from lack of maternally expressed UBE3A in neurons—same region, opposite parental origin
- Mechanisms for PWS/Angelman include deletion, uniparental disomy (UPD), imprinting defects, and (for Angelman) UBE3A variants—each mechanism changes recurrence risk counseling
- Methylation analysis at 15q11-q13 is a common first-line screen that detects deletions, UPD, and imprinting defects but does not by itself distinguish among those mechanisms
- Beckwith-Wiedemann syndrome (11p15.5 overgrowth/tumor predisposition) and Russell-Silver syndrome (growth restriction) are imprinting disorders with overlapping loci but opposite growth phenotypes
- CGC exam traps hinge on parental origin: maternal UPD15 → PWS; paternal UPD15 → Angelman; do not invert them
1.13 Imprinting Disorders (PWS, Angelman, Beckwith-Wiedemann, Russell-Silver)
Quick Answer: Imprinted genes are expressed from only one parental allele. Prader-Willi (PWS) = missing paternal 15q11-q13 contribution; Angelman (AS) = missing maternal UBE3A function. Start many evaluations with methylation analysis, then determine mechanism (deletion vs UPD vs imprinting defect ± UBE3A sequencing) because recurrence risk depends on mechanism. Beckwith-Wiedemann (BWS) and Russell-Silver (RSS) are the classic 11p15 growth-opposite pair.
This is a perennial CGC trap topic: same cytogenetic band, opposite syndromes, opposite parental origins.
Genomic Imprinting in One Counseling Paragraph
Imprinting is an epigenetic parent-of-origin mark (often DNA methylation and related chromatin states) set in gametogenesis so that certain genes are expressed only from the maternal or only from the paternal allele. Disease occurs when the only active copy is lost—by deletion of that allele, UPD that duplicates the silent parental chromosome, an imprinting center defect, or (sometimes) a gene variant on the active allele.
| Term | Meaning for counseling |
|---|---|
| Imprinted locus | Parent-of-origin–dependent expression |
| Uniparental disomy (UPD) | Both chromosome copies from one parent |
| Imprinting center (IC) | Regulatory region controlling the imprint mark |
| Methylation test | Reads the epigenetic signature; abnormal pattern = disease-range result needing mechanism workup |
Prader-Willi vs Angelman (15q11-q13)
Phenotype Essentials
| Prader-Willi syndrome | Angelman syndrome | |
|---|---|---|
| Core cause | Lack of paternally expressed genes in 15q11-q13 | Lack of maternally expressed UBE3A in neurons |
| Infancy | Severe hypotonia, poor feeding, failure to thrive | Often normal birth growth; feeding may be poor; developmental delay emerges |
| Later childhood | Hyperphagia, obesity risk, intellectual disability, hypogonadism, behavioral rigidity | Severe developmental disability, minimal/absent speech, gait ataxia, happy demeanor/laughter, seizures, microcephaly |
| Inheritance cue | Missing paternal contribution | Missing maternal UBE3A contribution |
Mechanisms and Recurrence (Memorize the Logic)
| Mechanism | PWS result | Angelman result | Typical recurrence if parents unaffected* |
|---|---|---|---|
| Deletion of 15q11-q13 | Paternal deletion → PWS | Maternal deletion → AS | Usually low if parental studies normal; higher if parent carries rearrangements rare |
| UPD | Maternal UPD 15 → PWS | Paternal UPD 15 → AS | Usually low (sporadic nondisjunction-related), not 50% |
| Imprinting defect | Paternal imprint fails → PWS | Maternal imprint fails → AS | May be higher if a parental imprinting-center microdeletion is present |
| Gene variant | — | Maternal UBE3A pathogenic variant → AS | Up to 50% if mother carries the variant |
*Always individualize after laboratory mechanism assignment; the table is the board-level skeleton.
Highest-yield trap pair:
- Maternal UPD15 → PWS (two maternal chromosomes, no paternal imprint)
- Paternal UPD15 → Angelman (two paternal chromosomes, no maternal UBE3A expression)
If you reverse these on an exam item, you lose the question.
Testing Strategy at 15q
- Methylation analysis (e.g., SNRPN methylation) — abnormal in deletion, UPD, and imprinting defects for PWS/AS; efficient first-line when phenotype fits.
- If methylation is abnormal → define mechanism: deletion analysis (FISH/CMA/MLPA), UPD studies, imprinting-center analysis as indicated.
- If methylation is normal but Angelman remains likely → UBE3A sequencing (methylation-negative Angelman is often UBE3A-related).
- Normal methylation makes classic PWS unlikely; rethink the differential.
Why methylation alone is not enough for recurrence counseling: An abnormal methylation result confirms an imprinting disorder pattern but does not tell you whether the cause was deletion, UPD, or IC defect—and those have different family risks.
Beckwith-Wiedemann Syndrome (BWS)
BWS is an overgrowth and tumor-predisposition imprinting disorder, primarily involving 11p15.5 (IGF2/H19 and KCNQ1OT1/CDKN1C regulation).
| Feature cluster | Examples |
|---|---|
| Growth | Macrosomia, macroglossia, hemihyperplasia |
| Abdominal wall | Omphalocele, umbilical hernia |
| Neonatal | Hypoglycemia (hyperinsulinism) |
| Lateralized overgrowth | Hemihyperplasia—tumor risk counseling still applies |
| Tumors | Increased risk of Wilms tumor, hepatoblastoma → protocolized surveillance (AFP, abdominal ultrasound per current guidelines) |
Mechanisms (conceptual): gain of paternal methylation marks / paternal UPD 11p15, loss of maternal IC2 methylation, CDKN1C variants, and others. Recurrence risk is usually low for sporadic epigenetic defects but can be higher with genomic alterations or maternally inherited CDKN1C variants—mechanism matters again.
Russell-Silver Syndrome (RSS / Silver-Russell)
RSS presents with prenatal and postnatal growth restriction, relative macrocephaly in many, body asymmetry, and feeding difficulties—clinically opposite to BWS growth excess.
| Molecular theme | Association |
|---|---|
| 11p15.5 | Hypomethylation at H19/IGF2 IC1 (paternal) — common identifiable cause |
| Chromosome 7 | Maternal UPD 7 in a subset |
| Other | Various rarer genetic causes; clinical diagnosis still used when molecularly negative |
Counsel growth supports, endocrine evaluation as indicated, and asymmetry monitoring. Do not treat RSS as “mild BWS”—the growth vectors and molecular signatures differ.
BWS vs RSS Comparison Table
| Beckwith-Wiedemann | Russell-Silver | |
|---|---|---|
| Growth | Overgrowth | Undergrowth |
| Key region | 11p15.5 imprinting | 11p15.5 and/or mat UPD7 |
| Classic adjuncts | Macroglossia, omphalocele, hypoglycemia, tumor risk | Relative macrocephaly, asymmetry, feeding issues |
| Counseling emphasis | Tumor surveillance + mechanism-specific recurrence | Growth/nutrition + mechanism-specific recurrence |
Prenatal and Postnatal Counseling Angles
Prenatal: BWS may be suggested by omphalocele, macroglossia, macrosomia, or placental mesenchymal dysplasia clues; RSS by severe IUGR with relative macrocephaly pattern. Molecular confirmation of imprinting disorders can be complex on prenatal samples—coordinate with labs experienced in methylation/UPD assays and discuss limitations.
Postnatal PWS: neonatal hypotonia/poor feeding → later hyperphagia planning, endocrine/obesity management, GH consideration per specialists. Postnatal Angelman: developmental disability, seizure management, communication supports. Avoid promising speech outcomes that the natural history rarely delivers; focus on augmentative communication.
Session Checklist for Imprinting Items
- Identify the syndrome phenotype (PWS vs AS vs BWS vs RSS).
- State the parent-of-origin rule in one sentence.
- Order thinking: methylation screen → mechanism → recurrence.
- Never quote 50% recurrence for UPD-related PWS/AS without a genomic reason to do so.
- For BWS, explicitly address tumor surveillance as part of natural-history counseling.
If Domain 1C chromosomal conditions are a three-legged stool, aneuploidy is leg one, microdeletions leg two, and imprinting—parent-of-origin epigenetics—is leg three. Boards love to kick that third leg.
Maternal uniparental disomy of chromosome 15 causes which condition, and why?
A methylation study at 15q11-q13 is abnormal in a child with classic Prader-Willi phenotype. What is the best next counseling concept regarding recurrence risk?
Which pairing correctly matches parental origin with Angelman syndrome?
In contrasting Beckwith-Wiedemann and Russell-Silver syndromes, which statement is most accurate?