Huntington Disease

Question 1

Huntington Disease

Answer 1

hereditary chorea; neurological disorder characterized by spasmodic involuntary movements of the limbs or facial muscles

Question 2

3 features of Huntington’s chorea

Answer 2

1. Its hereditary nature; autosomal dominant
2. A tendency to insanity and suicide
3. A grave disease only in adult life

NOTE: chorea = neurological disorder

Question 3

Huntington’s: phenotype

Answer 3

1. Psychological impairment
   - Moody, depressed, unable to regulate emotions
2. Cognitive dysfunction
   - Forgetful, progressive dementia
3. Physical impairment
   - Tick, spasms, chorea, inability to walk, talk and eat

Question 4

T or F: Antidepressants and antipsychotics can successfully treat Huntington’s Disease

Answer 4

FALSE; Huntington’s Disease has no effective treatment or cure

Question 5

HTT gene

Answer 5

• “Huntingtin”
• p arm of chromosome 4
• exon 1 has a polymorphic CAG repeats

Question 6

Genetic cause of Huntington disease

Answer 6

• Huntingtin gene (HTT) Has excess CAG repeats on Chromosome 4, exon 1 = protein with poly-glutamine repeat
• Autosomal dominant

Question 7

Mechanism of Huntington Disease

Answer 7

• expansion of the CAG polymorphic repeats in exon 1
  = translates as a polyglutamine repeat in protein product

1. Long RNA molecule is toxic to neurons
2. Abnormal protein may have a new function that is detrimental to neurons

Question 8

Molecular Diagnosis of Huntington’s Disease

Answer 8

• TR-PCR = the enumeration of the number of CAG repeats in exon 1 is used for a molecular diagnosis
• Not everyone is tested if there is already a family history of the
  disease (autosomal dominant)

Question 9

<26 CAG repeats

Answer 9

Non-variant HTT gene

Question 10

26-35 CAG repeats

Answer 10

• will not develop disease
• However, the number of repeats can expand or contract in sperm, thus the next generation may have HD

Question 11

36-39 CAG repeats

Answer 11

• individuals will develop Huntington’s Disease in their lifetime
• reduced penetrance
• number of repeats can expand or contract in sperm, thus the next generation may have HD

Question 12

40+ CAG repeats

Answer 12

• individuals will develop Huntington’s Disease in their lifetime
• full penetrance
• number of repeats can expand in sperm, thus the next generation may have HD

Question 13

Technical challenge for PCR of CAG repeats

Answer 13

• Alleles with >100 CAG repeats are difficult to accurately replicate and enumerate using PCR = only normal alleles are detected and the variant allele is not amplified (allelic dropout)

Question 14

TR-PCR

Answer 14

**Triplet repeat-primed PCR:

• PCR method that generates different sized amplicons due to multiple annealing sites on the template
• Analysis of stutter patterns can be used to identify and enumerate alleles with expanded CAG repeats
• Failure to amplify a large expansion = differentiates homozygous alleles

Question 15

TR-PCR Primers

Answer 15

• uses 2-3 primers

Primer 1:
- labeled with a fluorophore
- determines specificity of reaction = anneals to a region flanking CAG repeats

Primer 2:
- anneals to the flanking DNA sequence at the opposite end of the CAG repeats

Primer 3: 3’- end binds randomly along the CAG, while its 5’ end is complementary to second primer