Trinucleotide repeat diseases are classic “USMLE trap” territory: the stem gives you one or two clues, and the answer choices dare you to confuse anticipation with imprinting, or myotonia with dystonia. The good news is that once you learn to read these vignettes like patterns, every distractor becomes a memory hook instead of a time sink.
Tag: Genetics > Clinical Genetics
The Clinical Vignette (Q-bank style)
A 32-year-old woman comes to clinic because of progressive difficulty relaxing her grip and worsening fatigue. She says her “hands get stuck” after shaking hands. She also reports daytime sleepiness and has had trouble conceiving. On exam, she has bilateral ptosis, distal muscle weakness, and percussion myotonia in the thenar eminence. Slit-lamp exam shows early cataracts. Her father developed similar symptoms in his 40s, but her symptoms began in her late 20s and are more severe.
Which of the following is the most likely underlying genetic mechanism?
A. CGG trinucleotide repeat expansion with hypermethylation causing decreased gene expression
B. CAG trinucleotide repeat expansion causing a toxic gain-of-function protein
C. CTG trinucleotide repeat expansion in a gene with RNA-mediated toxicity and anticipation
D. Deletion of paternal 15q11-q13 due to imprinting error
E. Point mutation in dystrophin leading to absence of dystrophin protein
Step-by-Step: Identify the Diagnosis First
Key clues:
- Myotonia (“can’t relax grip”), distal weakness
- Early cataracts
- Infertility / hypogonadism and daytime sleepiness
- Anticipation (earlier onset + more severe in next generation)
That triad—myotonia + cataracts + endocrine issues—is a signature for myotonic dystrophy, especially type 1.
Correct answer: C. CTG trinucleotide repeat expansion in a gene with RNA-mediated toxicity and anticipation
The Correct Answer (C): Myotonic Dystrophy Type 1 (DM1)
What’s happening genetically?
- Repeat/sequence: CTG expansion
- Gene: DMPK (dystrophia myotonica protein kinase) on chromosome 19
- Mechanism: Toxic RNA gain-of-function
- The expanded CTG repeats are transcribed into RNA that sequesters RNA-binding proteins (notably MBNL), causing abnormal splicing of multiple transcripts.
Why the clinical picture fits
High-yield DM1 features:
- Myotonia (delayed relaxation after contraction; percussion myotonia)
- Muscle weakness (often distal > proximal in DM1)
- Early cataracts
- Cardiac conduction defects (think: AV block, arrhythmias)
- Endocrine: hypogonadism, infertility, insulin resistance
- CNS: daytime sleepiness, cognitive/behavior changes
- Anticipation: symptoms worsen and appear earlier in successive generations
Exam buzzwords
- “Handshake” (grip myotonia)
- “Hatchet facies,” ptosis (sometimes described)
- “Christmas tree cataracts” (classic but not always given)
Why Every Distractor Is Wrong (and What It Really Describes)
A. CGG repeat expansion with hypermethylation → Fragile X syndrome
Why it’s tempting: Another repeat-expansion disorder, often tested with anticipation-like language.
Why it’s wrong here:
- Fragile X is CGG repeat expansion in FMR1 (X chromosome)
- Leads to hypermethylation → silencing (loss of function)
- Typical findings:
- Intellectual disability, autism, long face, large ears
- Macroorchidism (postpubertal)
- Mitral valve prolapse
High-yield differentiator:
Fragile X = neurodevelopmental + physical features, not myotonia/cataracts.
B. CAG repeat expansion causing toxic gain-of-function protein → Huntington disease (and others)
Why it’s tempting: CAG expansions are heavily emphasized.
Why it’s wrong here:
- Huntington disease (HTT gene) is CAG expansion → polyglutamine → toxic protein gain of function
- Presents with:
- Chorea, dystonia
- Psychiatric symptoms (depression, irritability)
- Cognitive decline/dementia
- Often anticipation, especially with paternal transmission
High-yield differentiator:
Huntington = movement disorder + psychiatric + dementia, not myotonia/cataracts/infertility.
Other CAG/polyglutamine diseases exist (e.g., spinocerebellar ataxias), but the USMLE “core” association is Huntington.
D. Deletion of paternal 15q11-q13 due to imprinting error → Prader-Willi syndrome
Why it’s tempting: Family history + “genetics mechanism” makes people think imprinting.
Why it’s wrong here:
- Prader-Willi: loss of paternal expression at 15q11-q13
- Mechanisms: paternal deletion, maternal UPD, imprinting defect
- Classic features:
- Neonatal hypotonia
- Hyperphagia → obesity
- Hypogonadism
- Developmental delay
High-yield differentiator:
Prader-Willi has hypogonadism, yes—but the vignette’s myotonia + cataracts + distal weakness points strongly to DM1.
E. Point mutation in dystrophin leading to absence of dystrophin → Duchenne muscular dystrophy
Why it’s tempting: Muscle disease + genetics.
Why it’s wrong here:
- Duchenne is usually frameshift deletion (not typically a point mutation), X-linked recessive
- Presents in early childhood (age 2–5):
- Gowers sign
- Calf pseudohypertrophy
- Dilated cardiomyopathy
- Elevated CK
- No myotonia and no “can’t relax grip” story.
High-yield differentiator:
Duchenne/Becker = proximal weakness in boys, not adult-onset myotonia with cataracts.
High-Yield Trinucleotide Repeat Disease Table (Step 1 + Step 2)
| Disease | Repeat | Gene | Core clinical clues | Key concepts |
|---|---|---|---|---|
| Myotonic dystrophy type 1 | CTG | DMPK | Myotonia, distal weakness, cataracts, endocrine issues, conduction defects | Anticipation, RNA toxicity |
| Huntington disease | CAG | HTT | Chorea, psych changes, dementia | Gain-of-function protein, anticipation (often paternal) |
| Fragile X syndrome | CGG | FMR1 | ID/autism, long face, large ears, macroorchidism | Hypermethylation, loss of function, anticipation |
| Friedreich ataxia (often tested with repeats) | GAA | FXN | Ataxia, dysarthria, loss of vibration/position, scoliosis, hypertrophic cardiomyopathy, diabetes | Autosomal recessive, mitochondrial dysfunction |
How Q-Banks Like to Test This (Pattern Moves)
1) “Anticipation” isn’t enough—pair it with a signature symptom
- Anticipation + myotonia/cataracts → DM1 (CTG)
- Anticipation + chorea/dementia → Huntington (CAG)
- Anticipation + ID/autism + macroorchidism → Fragile X (CGG)
2) Mechanism matters
- DM1: RNA-mediated spliceopathy (toxic RNA)
- Huntington: toxic protein (polyglutamine)
- Fragile X: gene silencing (hypermethylation)
3) Don’t confuse imprinting with anticipation
- Imprinting: depends on parent of origin (Prader-Willi vs Angelman)
- Anticipation: repeat expansion across generations (worsening/earlier onset)
Quick Takeaways (What to remember on test day)
- Myotonia + cataracts + endocrine issues is a neon sign for myotonic dystrophy (CTG, DMPK).
- DM1 = toxic RNA (splicing defects), not protein toxicity.
- CAG = polyglutamine → classically Huntington.
- CGG = Fragile X → methylation and gene silencing.
- Imprinting syndromes are a separate category—don’t let them steal your repeat-expansion questions.