Huntington Disease Genetics Quiz
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Questions and Answers

What is the consequence of having 36 or more glutamines in a row in the huntingtin protein?

  • Increased protein functionality
  • Enhanced DNA replication
  • Reduced neuronal cell death
  • Abnormal protein aggregation in neuronal cells (correct)

What is the primary mechanism by which the expanded CAG repeats affect the huntingtin protein?

  • Inhibition of DNA polymerase activity
  • Overexpression of the huntingtin protein
  • Instability of the HTT gene during DNA replication (correct)
  • Excessive signaling of neurons leading to high intracellular calcium

What is the likelihood that a child will inherit the affected gene if one parent has Huntington disease?

  • 25%
  • 100%
  • 50% (correct)
  • 75%

What is the region of the brain most affected by the abnormal protein in Huntington disease?

<p>Caudate and putamen of the basal ganglia (D)</p> Signup and view all the answers

What is the term used to describe the type of disease caused by the expanded CAG repeats in Huntington disease?

<p>Polyglutamine disease (D)</p> Signup and view all the answers

What is the primary mechanism by which Huntington disease symptoms appear earlier in each generation?

<p>Higher number of CAG repeats in the protein (C)</p> Signup and view all the answers

What is the term for the alleles with 27-35 CAG repeats that don't cause the disease but can expand to become full mutations?

<p>Pre-mutation alleles (A)</p> Signup and view all the answers

Why do new disease alleles and anticipation generally occur when the father is the affected parent?

<p>Because repeat expansion occurs more in the production of sperm than eggs (B)</p> Signup and view all the answers

What is the minimum number of CAG repeats required for a person to show 100% penetrance and develop Huntington disease?

<p>40 CAG repeats (D)</p> Signup and view all the answers

Why is the test for HD, which counts the number of CAG repeats, generally good at determining whether HD will develop in an at-risk individual?

<p>Because it can detect the number of CAG repeats, which correlates with penetrance (B)</p> Signup and view all the answers

Study Notes

Huntington Disease (HD)

  • A rare neurodegenerative disease caused by a repeated sequence of DNA that leads to an abnormal protein formation, resulting in abnormal movements and cognitive problems.
  • An autosomal dominant genetic disorder, meaning one affected copy of a gene is enough to cause disease.

Genetic Characteristics

  • Typically, the gene called huntingtin or HTT on chromosome 4 contains a triplet repeat of C, A, and G nucleotides 10-35 times in a row.
  • In people with HD, this repeat goes on for 36 or more times in a row.
  • The CAG repeat codes for the amino acid glutamine, resulting in 36 or more glutamines in a row in the huntingtin protein.

Effects of HD

  • The mutated protein aggregates within the neuronal cells of the caudate and putamen of the basal ganglia, causing neuronal cell death.
  • Cell death might be related to excitotoxicity, which is excessive signaling of these neurons, leading to high intracellular calcium.

DNA Replication and Repeat Expansion

  • The expanded CAG repeats affect DNA replication, causing DNA polymerase to lose track of which CAG it's on, resulting in extra CAGs being added.
  • This expansion occurs during cell divisions, leading to ample opportunities for repeat expansion, making the gene more unstable.

Anticipation and Inheritance

  • The phenomenon of anticipation means that HD families often show earlier symptom onset with each generation.
  • Children of a parent with HD can inherit even more CAG repeats than the parent, leading to an earlier age of symptom onset.
  • Repeat expansion occurs more in the production of sperm than eggs, resulting in anticipation and new disease alleles when the father is the affected parent.

Penetrance and Diagnosis

  • A person with 40+ repeats shows 100% penetrance and will have the disease.
  • People with 36-39 repeats can show reduced penetrance, with some having symptoms while others may not.
  • The test for HD, which counts the number of CAG repeats, is effective in determining whether HD will develop in an at-risk individual.

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Test your knowledge of Huntington disease, a rare neurodegenerative disorder caused by an autosomal dominant genetic mutation leading to abnormal movements and cognitive problems.

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