Q.56 Which of the following disease in humans, is caused due to expanded microsatellite repeats?
1. Huntington’s disease.
2. Cystic fibrosis
3. Tuberculosis
4. Alkaptonuria
Trinucleotide Repeat Expansion Diseases: Huntington’s Confirmed
Huntington’s disease is caused by expanded CAG microsatellite repeats in the HTT gene, making option 1 correct.
Microsatellite Repeat Disorders
Microsatellites (short tandem repeats, 1-6 bp) expand pathologically (>normal length), causing >40 neurological disorders via protein gain-of-function (polyglutamine) or RNA toxicity.
Trinucleotide expansions (most common) show anticipation—worsening across generations.
Huntington’s exemplifies coding-region CAG repeats producing toxic polyglutamine tracts.
Correct Answer: Option 1
Huntington’s disease features >36 CAG repeats in exon 1 of HTT gene on chromosome 4, yielding mutant huntingtin protein aggregation in neurons.
| Disease | Genetic Cause | Microsatellite Type |
|---|---|---|
| Huntington’s (1) | CAG expansion (>36) in HTT | Yes—trinucleotide |
| Cystic fibrosis (2) | CFTR ΔF508 mutation | No—single nucleotide deletion |
| Tuberculosis (3) | Mycobacterium infection | No—infectious, not genetic |
| Alkaptonuria (4) | HGD point mutations | No—single gene defects |
Option Explanations
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Option 1 (Correct): Classic polyglutamine disease; repeat length correlates with onset/severity (juvenile <20 repeats).
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Option 2 (Incorrect): Autosomal recessive; CFTR chloride channel defect, not repeats.
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Option 3 (Incorrect): Bacterial infection (Mycobacterium tuberculosis), not heritable microsatellite disorder.
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Option 4 (Incorrect): Rare metabolic disorder; homogentisate accumulation from HGD enzyme deficiency via point mutations.
Other repeat diseases include fragile X (CGG), myotonic dystrophy (CTG), Friedreich ataxia (GAA)—all microsatellite-based, unlike these distractors. Key for NEET medical genetics syllabus.
