Everything You Need to Know About Genomic Imprinting for Step 1

Genomic imprinting is one of those genetics topics that feels abstract—until you see how often it shows up in classic USMLE vignettes (Prader-Willi, Angelman, Beckwith-Wiedemann). If you can confidently answer “which parent’s allele is expressed?”, you’ll crush a common set of Step 1 questions.

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What Is Genomic Imprinting?

Genomic imprinting is an epigenetic phenomenon in which only one parental allele is expressed, while the other is silenced based on parent-of-origin.

• The silencing is typically due to DNA methylation (and histone modifications) laid down during gametogenesis.
• Imprinting marks are reset in the germline of the developing fetus and then re-established according to the fetus’s sex:
  • In males, new paternal imprints are set in sperm
  • In females, new maternal imprints are set in oocytes

High-yield takeaway: In imprinting disorders, the phenotype depends on which parent contributed the mutated/deleted/unexpressed allele.

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Pathophysiology (How Imprinting Causes Disease)

Because one allele is normally silenced, the body relies on the single active copy. Disease can occur if the active allele is lost or functionally absent. Major mechanisms include:

1) Deletion of the active allele (parent-specific)

• Example: Deletion on chromosome 15 can cause Prader-Willi or Angelman depending on whether the missing allele is paternal or maternal.

2) Uniparental disomy (UPD)

Uniparental disomy = receiving two copies of a chromosome from one parent and none from the other.

• Can occur via “trisomy rescue” or monosomy rescue.
• If the imprinted region requires one allele from each parent (normal situation), UPD can mimic a deletion.

Classic examples:

• Maternal UPD 15 → Prader-Willi
• Paternal UPD 15 → Angelman

3) Imprinting center (epigenetic) defects

• The DNA sequence may be normal, but methylation patterns are wrong → wrong allele is silenced/expressed.

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How to Recognize Imprinting in USMLE Vignettes

Look for:

• Same chromosomal region causing different syndromes based on parent of origin
• Mentions of UPD, “two copies from one parent,” or “methylation defect”
• Classic phenotype clusters (hypotonia + hyperphagia vs seizures + laughter)

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Core High-Yield Syndromes

Prader-Willi Syndrome (PWS)

Genetics (HY)

• Loss of paternal gene expression on chromosome 15 (15q11-q13)
• Usually due to:
  • Paternal deletion of 15q11-q13, or
  • Maternal UPD 15 (two maternal copies)

Clinical Presentation

Infancy:

• Neonatal hypotonia
• Poor feeding → later flips to hyperphagia

Childhood onward:

• Hyperphagia → obesity
• Intellectual disability
• Hypogonadism (often: cryptorchidism, delayed puberty)
• Behavioral issues

Diagnosis (Step-relevant)

• Methylation analysis can detect abnormal imprinting patterns.
• Microarray/FISH can identify deletions; UPD testing if deletion not found.

Treatment

• Multidisciplinary:
  • Strict dietary supervision
  • Growth hormone often used (supports growth/body composition)
  • Manage hypogonadism and developmental needs

USMLE clue phrase: “Hypotonia, hyperphagia, obesity” → think Prader-Willi.

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Angelman Syndrome (AS)

Genetics (HY)

• Loss of maternal gene expression on chromosome 15 (15q11-q13)
• Often due to:
  • Maternal deletion affecting UBE3A, or
  • Paternal UPD 15 (two paternal copies)
• Key gene: UBE3A (maternal allele expressed in parts of the brain)

Clinical Presentation (Classic “Happy Puppet”)

• Severe intellectual disability
• Seizures
• Ataxia
• Inappropriate laughter, smiling, excitability
• Often microcephaly

Diagnosis

• Methylation studies + UBE3A testing; microarray for deletions.

Treatment

• Seizure management, developmental therapies, supportive care

USMLE clue phrase: “Seizures + laughter + ataxia” → Angelman.

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Beckwith-Wiedemann Syndrome (BWS)

Genetics (HY)

• Imprinting defect on chromosome 11p15 affecting IGF2 (growth-promoting) and CDKN1C (growth-suppressing)
• Typically leads to increased IGF2 expression (overgrowth), often due to paternal allele dysregulation.

Clinical Presentation

• Macrosomia (large infant)
• Macroglossia
• Omphalocele/umbilical hernia
• Neonatal hypoglycemia
• Increased risk of embryonal tumors:
  • Wilms tumor
  • Hepatoblastoma

Diagnosis

• Clinical + methylation studies at 11p15; tumor surveillance protocols.

Treatment/Management

• Manage hypoglycemia, surgical issues (e.g., omphalocele), and tumor screening in childhood.

USMLE clue phrase: “Big baby + big tongue + omphalocele” → Beckwith-Wiedemann.

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High-Yield Associations and Testable Comparisons

PWS vs Angelman (Same Region, Different Parent)

Imprinting vs Anticipation vs Variable Expressivity (Don’t Mix These Up)

• Imprinting: parent-of-origin–dependent expression (epigenetic silencing)
• Anticipation: worsening/earlier onset in successive generations (often trinucleotide repeats)
• Variable expressivity: same mutation, different severity
• Incomplete penetrance: some with mutation show no phenotype

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Diagnostic Approach (What Step 1 Wants You to Know)

When a question hints at imprinting, expect:

• Methylation testing as a key diagnostic tool (detects imprinting pattern abnormalities)
• Microarray for deletions/duplications
• UPD studies if phenotype fits but no deletion is identified

Exam strategy: If the stem says “two copies from one parent,” jump to uniparental disomy and then decide which syndrome based on phenotype.

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Treatment Principles (Step 1/2 Level)

Most imprinting disorders are not “cured” genetically; treatment is supportive and preventive:

• PWS: diet control, endocrine management (e.g., GH), behavioral/developmental support
• Angelman: seizure control, developmental therapy
• BWS: manage neonatal issues + tumor surveillance

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First Aid Cross-References (Where This Lives in FA)

In First Aid for the USMLE Step 1, genomic imprinting is classically covered under:

• Genetics → Epigenetics / Genomic imprinting
• Autosomal dominant vs recessive vs X-linked patterns (contrast with imprinting)
• Syndromes:
  • Prader-Willi and Angelman (Chr 15 imprinting)
  • Beckwith-Wiedemann (Chr 11 imprinting, IGF2)

(Exact page numbers vary by edition; use the Genetics chapter index for “imprinting,” “Prader-Willi,” “Angelman,” and “Beckwith-Wiedemann.”)

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Ultra High-Yield “If You Only Memorize 6 Things”

1. Imprinting = parent-of-origin–specific gene silencing (DNA methylation).
2. PWS = loss of paternal expression (15q11-q13) → hypotonia, hyperphagia, obesity, hypogonadism.
3. Angelman = loss of maternal expression (15q11-q13, UBE3A) → seizures, ataxia, inappropriate laughter.
4. Maternal UPD 15 → Prader-Willi; Paternal UPD 15 → Angelman.
5. BWS (11p15, IGF2 dysregulation) → macrosomia, macroglossia, omphalocele, tumor risk.
6. Methylation testing is central for confirming imprinting defects.