Imprinting Disorders: Prader-Willi vs Angelman — MRCP Part 1
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Imprinting Disorders: Prader-Willi vs Angelman — MRCP Part 1

TL;DR

Prader-Willi syndrome and Angelman syndrome are classic genomic imprinting disorders involving chromosome 15q11-q13 and are highly testable in MRCP Part 1. The key distinction is parental origin: paternal deletion causes Prader-Willi syndrome, while maternal deletion causes Angelman syndrome. MRCP questions commonly test phenotype recognition, genomic imprinting, developmental features, obesity, seizures and examination traps.


Why this topic matters in MRCP Part 1

Exam questions commonly assess:

  • Genomic imprinting

  • Uniparental disomy

  • Chromosome 15q11-q13 abnormalities

  • Neonatal hypotonia

  • Hyperphagia and obesity

  • Developmental delay

  • Ataxia and seizures

  • Characteristic behavioural phenotypes

This topic is especially important because it combines:

  • Genetics

  • Neurology

  • Endocrinology

  • Paediatrics

  • Psychiatry

For broader revision, candidates should integrate this topic with structured resources such as the <a href="https://www.crackmedicine.com/mrcp-part-1" target="_blank">MRCP Part 1 overview</a>, <a href="https://www.crackmedicine.com/qbank" target="_blank">Free MRCP MCQs</a>, and <a href="https://www.crackmedicine.com/lectures" target="_blank">MRCP lectures</a>.


Understanding genomic imprinting

Genomic imprinting refers to selective silencing of genes according to parental origin.

In the chromosome 15q11-q13 region:

  • Some genes are expressed only from the paternal chromosome

  • Others are expressed only from the maternal chromosome

Loss of the active parental copy produces disease.

This explains why deletion of the same chromosomal region can produce two completely different syndromes.


High-yield comparison table

Feature

Prader-Willi Syndrome

Angelman Syndrome

Chromosomal defect

Loss of paternal 15q11-q13

Loss of maternal 15q11-q13

Main mechanism

Paternal deletion or maternal uniparental disomy

Maternal deletion or paternal uniparental disomy

Neonatal feature

Severe hypotonia

Developmental delay

Feeding pattern

Poor feeding initially, hyperphagia later

Feeding difficulties may occur

Behaviour

Food obsession, stubbornness

Happy demeanour, frequent laughter

Body habitus

Obesity

Usually lean

Speech

Mild-to-moderate impairment

Severe speech impairment

Seizures

Less common

Common

Movement disorder

Hypotonia

Ataxia, jerky movements

Puberty

Hypogonadism common

Usually preserved


The 5 most tested subtopics

1. Parent-specific chromosome deletion

This is the single most important fact.

Prader-Willi syndrome

  • Loss of paternal gene expression

  • Usually paternal deletion

Angelman syndrome

  • Loss of maternal gene expression

  • Usually maternal deletion

A common MRCP trap is reversing the parental origin.

2. Uniparental disomy

Questions may describe inheritance of both chromosome copies from one parent.

Examples

  • Maternal uniparental disomy → Prader-Willi syndrome

  • Paternal uniparental disomy → Angelman syndrome

The principle remains identical:

  • Loss of paternal expression → PWS

  • Loss of maternal expression → AS

3. Clinical presentation of Prader-Willi syndrome

The exam frequently describes the chronological progression.

Early infancy

  • Severe hypotonia

  • Weak cry

  • Poor feeding

  • Failure to thrive

Later childhood

  • Hyperphagia

  • Obesity

  • Learning difficulties

  • Behavioural disturbance

Additional features

  • Small hands and feet

  • Hypogonadism

  • Short stature

  • Sleep apnoea

The transition from poor feeding to excessive eating is highly characteristic.

4. Clinical presentation of Angelman syndrome

Angelman syndrome is primarily neurological.

Core features

  • Severe developmental delay

  • Minimal speech

  • Ataxic gait

  • Frequent smiling or laughter

  • Seizures

Additional clues

  • Tremulous limb movements

  • Hyperactivity

  • Sleep disturbance

Older literature used the term “happy puppet syndrome”, although this terminology is now outdated in clinical practice.

5. Endocrine and neurological complications

Prader-Willi syndrome complications

  • Morbid obesity

  • Type 2 diabetes mellitus

  • Obstructive sleep apnoea

  • Hypertension

  • Growth hormone deficiency

Angelman syndrome complications

  • Refractory epilepsy

  • Severe learning disability

  • Gait abnormalities

  • Communication impairment


High-yield revision framework

Prader-Willi syndrome

Think:

  • Paternal problem

  • Poor feeding first

  • Pudgy later

  • Puberty delayed

Angelman syndrome

Think:

  • Absent maternal expression

  • Ataxia

  • Always laughing

  • Absent speech


Mini-case MCQ

Question

A 6-year-old boy is reviewed because of obesity and behavioural problems. As an infant, he required nasogastric feeding because of profound hypotonia and poor suck reflex. He now demonstrates excessive eating behaviour and mild learning difficulties. Examination reveals hypogonadism and small hands.

What is the most likely underlying genetic abnormality?

A. Maternal deletion at chromosome 15q11-q13B. Paternal deletion at chromosome 15q11-q13C. Trisomy 18D. Mutation of MECP2E. CGG repeat expansion

Answer

B. Paternal deletion at chromosome 15q11-q13

Explanation

This is a classic presentation of Prader-Willi syndrome:

  • Neonatal hypotonia

  • Early feeding difficulty

  • Hyperphagia in childhood

  • Obesity

  • Hypogonadism

Prader-Willi syndrome results from loss of paternal gene expression at chromosome 15q11-q13.


Student preparing for MRCP Part 1 genetics and neurodevelopmental disorder questions.

10 facts to memorise before the exam

  1. Both disorders involve chromosome 15q11-q13

  2. Prader-Willi syndrome results from paternal deletion

  3. Angelman syndrome results from maternal deletion

  4. Genomic imprinting is parent-specific gene silencing

  5. Prader-Willi syndrome causes hyperphagia

  6. Angelman syndrome causes severe speech impairment

  7. Seizures are common in Angelman syndrome

  8. Hypogonadism is common in Prader-Willi syndrome

  9. Uniparental disomy may cause both conditions

  10. MRCP questions frequently test phenotype recognition rather than molecular detail alone


Common examination pitfalls

1. Confusing maternal and paternal deletion

This is the commonest error in genetics SBA questions.

2. Forgetting both disorders affect the same chromosome region

Candidates often incorrectly assume different chromosomes are involved.

3. Associating obesity with Angelman syndrome

Obesity is strongly associated with Prader-Willi syndrome.

4. Missing neurological clues

Ataxia and seizures strongly suggest Angelman syndrome.

5. Misunderstanding genomic imprinting

Imprinting is not mitochondrial inheritance. It refers to selective parental gene silencing.


Practical MRCP Part 1 study checklist

Before the examination, ensure you can:

  • Recall chromosome 15q11-q13 instantly

  • Differentiate paternal versus maternal deletion

  • Recognise neonatal hypotonia stems

  • Identify obesity-associated syndromes

  • Recognise seizure-heavy neurodevelopmental stems

  • Understand uniparental disomy

  • Recall endocrine complications of Prader-Willi syndrome

  • Differentiate Angelman syndrome from Rett syndrome

A useful companion topic is neurogenetics revision alongside developmental disorders. Candidates may also benefit from practising timed questions through the <a href="https://www.crackmedicine.com/mock-tests" target="_blank">MRCP mock test platform</a>.


How MRCP Part 1 commonly phrases these questions

Examiners often use subtle wording clues such as:

  • “Happy child with inappropriate laughter”

  • “Hyperphagia and obesity”

  • “Hypotonic infant”

  • “Genomic imprinting disorder”

  • “Minimal speech”

  • “Ataxic gait”

These descriptors are usually sufficient to identify the diagnosis without requiring detailed molecular genetics.


FAQs

What is genomic imprinting?

Genomic imprinting refers to parent-specific silencing of genes. Certain genes are expressed only from either the maternal or paternal chromosome.

Which parent deletion causes Prader-Willi syndrome?

Prader-Willi syndrome is caused by loss of paternal gene expression at chromosome 15q11-q13.

Which syndrome is associated with seizures and ataxia?

Angelman syndrome classically presents with seizures, ataxia and severe speech impairment.

Why does Prader-Willi syndrome cause obesity?

Hypothalamic dysfunction leads to hyperphagia and impaired satiety, resulting in progressive obesity.

Are imprinting disorders high yield for MRCP Part 1?

Yes. They are frequently tested because they combine genetics, neurology and endocrinology in recognisable clinical vignettes.


Ready to start?

Strengthen your preparation with structured revision via the MRCP Part 1 overview. Practise actively using the Free MRCP MCQs and simulate exam conditions with a Start a mock test.

For deeper understanding, combine this guide with lecture-based revision at:https://www.crackmedicine.com/lectures/

 
 
 
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