Everything You Need to Know About Trinucleotide Repeat Diseases for Step 1

Trinucleotide repeat diseases are high-yield USMLE genetics because they test core concepts like anticipation, parent-of-origin effects, and toxic gain-of-function vs loss-of-function mechanisms. Many of these disorders present with classic neuro findings and distinctive inheritance patterns that are frequently used in vignettes.

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What Are Trinucleotide Repeat Diseases?

Trinucleotide repeat diseases occur when a 3-nucleotide DNA sequence (e.g., CAG, CGG, CTG) becomes abnormally expanded in a gene. When repeat length crosses a threshold, it can disrupt gene expression or produce toxic proteins, causing disease.

Key Definitions (USMLE High-Yield)

• Repeat expansion: Increased number of repeated trinucleotides across generations.
• Anticipation: Earlier onset and increased severity in successive generations due to repeat expansion.
• Parent-of-origin effect: The tendency for expansion to worsen preferentially through maternal or paternal transmission (classic testable pairing).

First Aid cross-reference: Genetics → Trinucleotide repeat expansion (anticipation) and associated diseases.

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Pathophysiology: Why Repeat Expansions Cause Disease

The clinical phenotype depends on where the repeat is and what it does:

1) Coding Region Expansions → Toxic Protein (Gain-of-Function)

• CAG typically encodes glutamine, producing a polyglutamine (poly-Q) tract.
• Misfolded proteins aggregate → neuronal dysfunction/death.
• Classic diseases: Huntington disease, several spinocerebellar ataxias (often beyond Step 1 scope, but concept is testable).

2) Noncoding Region Expansions → Gene Silencing or RNA Toxicity

• 5' UTR expansions can reduce transcription (gene silencing).
• 3' UTR or intronic expansions can produce toxic RNA that sequesters RNA-binding proteins → abnormal splicing.

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The Big 4 Trinucleotide Repeat Disorders (Must-Know Table)

First Aid cross-reference:

• Huntington: Neurodegenerative disorders + genetics (AD, anticipation).
• Fragile X: X-linked disorders; dynamic mutation; autism.
• Myotonic dystrophy: muscular dystrophies; myotonia.
• Friedreich ataxia: AR disorders; neuro/cardiomyopathy association.

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Disease-by-Disease Deep Dive (Step 1 Focus)

1) Huntington Disease (HD)

Genetics & Pathogenesis

• Autosomal dominant
• CAG repeat expansion in HTT gene → toxic gain-of-function huntingtin protein
• Neuroanatomy: caudate nucleus atrophy → enlarged lateral ventricles

Clinical Presentation (Vignette Clues)

• Chorea (dance-like involuntary movements)
• Psychiatric changes: depression, irritability
• Cognitive decline → dementia
• Often mid-adulthood onset; earlier if repeats are larger

High-Yield Association

• Anticipation, classically worse with paternal transmission (spermatogenesis promotes expansion).

Diagnosis (Board Style)

• Clinical + family history; confirm via genetic testing for CAG repeat length
• Imaging may show caudate atrophy (supportive)

Treatment (Conceptual)

• Symptomatic: VMAT2 inhibitors (e.g., tetrabenazine) for chorea; supportive psych care
• No cure—test focuses more on inheritance + anticipation than management

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2) Fragile X Syndrome

Genetics & Pathogenesis

• X-linked dominant
• CGG repeat expansion in FMR1 (5' UTR)
• Leads to hypermethylation → decreased FMRP (important for synaptic function) → intellectual disability

Classic Clinical Presentation

• Intellectual disability (most common inherited cause)
• Autism spectrum behaviors, ADHD
• Long face, large ears
• Macroorchidism (post-pubertal)
• Mitral valve prolapse

High-Yield Associations

• Anticipation with maternal transmission (expansion to full mutation often occurs during oogenesis).
• Can show premutation effects (more Step 2/advanced): tremor/ataxia in older males (FXTAS), premature ovarian insufficiency—sometimes tested.

Diagnosis

• PCR/Southern blot for CGG repeats and methylation status

Treatment

• Supportive: early intervention, behavioral therapy, educational planning
• Treat comorbid ADHD/anxiety as needed

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3) Myotonic Dystrophy Type 1 (DM1)

Genetics & Pathogenesis

• Autosomal dominant
• CTG repeat expansion in DMPK gene (3' UTR)
• Causes toxic RNA that disrupts splicing (notably chloride channels) → myotonia

Clinical Presentation (Very Testable Cluster)

• Myotonia: delayed muscle relaxation (e.g., trouble releasing handshake)
• Distal muscle weakness
• Cataracts
• Cardiac conduction defects/arrhythmias
• Hypogonadism: testicular atrophy, infertility
• Frontal balding

High-Yield Associations

• Anticipation; congenital DM1 is classically linked to maternal transmission and can present with neonatal hypotonia/respiratory issues.

Diagnosis

• Clinical + genetic testing for CTG repeats
• EKG may show conduction defects (important complication)

Treatment

• Symptomatic: manage arrhythmias (may need pacemaker), supportive PT
• Myotonia may improve with membrane-stabilizing agents (more commonly emphasized on Step 2)

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4) Friedreich Ataxia (FRDA)

Genetics & Pathogenesis

• Autosomal recessive
• GAA repeat expansion in FXN (frataxin) intron → decreased frataxin
• Mitochondrial dysfunction → oxidative damage (esp in neurons and cardiac myocytes)

Clinical Presentation (Classic Board Vignette)

• Progressive gait ataxia
• Dysarthria
• Loss of vibration/proprioception (posterior columns)
• Pes cavus, scoliosis
• Hypertrophic cardiomyopathy
• Diabetes mellitus

Diagnosis

• Genetic testing for GAA repeats
• ECG/echo may show hypertrophic cardiomyopathy

Treatment

• Supportive: manage cardiomyopathy, diabetes, PT/OT
• Prognosis often driven by cardiac disease

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How USMLE Tests Trinucleotide Repeat Diseases

Pattern Recognition Triggers

• Anticipation + chorea + caudate atrophy → Huntington (CAG, AD, paternal)
• ID + autism + macroorchidism + long face → Fragile X (CGG, XLD, maternal)
• Myotonia + cataracts + conduction defects → Myotonic dystrophy (CTG, AD)
• Ataxia + hypertrophic cardiomyopathy + diabetes → Friedreich ataxia (GAA, AR)

High-Yield “One-Liners”

• CAG = polyglutamine = neurodegeneration (Huntington).
• CGG in FMR1 → methylation → silenced gene (Fragile X).
• CTG in DMPK → toxic RNA → splicing defects (myotonic dystrophy).
• GAA in frataxin → mitochondrial dysfunction → ataxia + cardiomyopathy (Friedreich).

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Diagnosis: Practical Board Approach

When you suspect a trinucleotide repeat disorder, the next best step is often:

• Targeted genetic testing for repeat size (PCR ± Southern blot depending on size/methylation).
• Consider family history + anticipation clues.

Tip: Southern blot is classically important for large expansions and methylation status (Fragile X).

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Treatment Principles (What Step 1 Expects)

Step 1 typically emphasizes mechanism and inheritance, but it’s still useful to know:

• Most therapies are supportive/symptomatic.
• Major morbidity often comes from:
  • Psych/neuro decline (Huntington)
  • Developmental/behavioral challenges (Fragile X)
  • Cardiac conduction disease (Myotonic dystrophy)
  • Cardiomyopathy (Friedreich)

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High-Yield Associations & Memory Hooks

Anticipation: Who Expands Through Which Parent?

• Huntington: paternal anticipation (sperm-driven expansion)
• Fragile X: maternal anticipation (oocyte expansion to full mutation)
• Myotonic dystrophy: anticipation; congenital form often maternal
• Friedreich ataxia: AR; anticipation not a classic emphasis

Inheritance Quick Sort

• AD: Huntington, Myotonic dystrophy
• X-linked dominant: Fragile X
• AR: Friedreich ataxia

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First Aid Cross-References (Where to Review)

Use these as quick anchors while you annotate:

• Genetics: Trinucleotide repeat expansion + anticipation
• Neuro: Huntington disease (caudate degeneration, chorea)
• Psych/Developmental: Fragile X (autism/ID, macroorchidism)
• MSK/Neuro: Myotonic dystrophy (myotonia, cataracts, conduction defects)
• Cardio/Neuro: Friedreich ataxia (hypertrophic cardiomyopathy + ataxia)