Fragile Sites - Lecture 9

Question 1

Fragile sites

Answer 1

Non-staining gaps in chromosomes

• seen in 5% of people
• Autosomal dominant
• inserted breakage caused by expansion of trinucleotide repeats
• Associated with chromosome breakage and late-replicating DNA

Question 2

Fragile X

Answer 2

FMR1 gene, X chromosome
-Moderate to severe intellectual disability
-Folate sensitive fragile site
-Second most important cause of ID after Down Syndrome
Symptoms:
-ID, enlarged testes, long ears, prominent forehead
-Milder effects in premutation carriers (Males - tremor/ataxia; Females - POF)
-50% of females who inherit full mutation will be affected
-Normal (<45 repeats)
-Grey zone (45-55 repeats)
-Premutation (56-200 repeats) - may be associated with increased stability of FMR1
-Mutation (200+ repeats) - full mutation associated with methylation and reduced expression of FMR1
-Can have interruption of CGG repeats by AGG

Question 3

Common Fragile Sites

Answer 3

• inducible by aphidicolin (DNA polymerase inhibitor that causes stress at replication sites)
• FRA3B most common fragile site in genome
• ATR-dependent DNA damage checkpoint is critical for maintaining stability of fragile sites
• Many micro-RNA loci lie within fragile sites

Question 4

Unstable repeates

Answer 4

• Large expansions in noncoding regions - interference with gene expression
• Small expansions in noncoding regions - intranuclear aggregation of protein

Question 5

Sherman Paradox

Answer 5

Anticipation

• disease more severe in successive generations
• due to expansion of trinucleotide repeat

Question 6

Approach to clinical analysis

Answer 6

Old - Southern blotting

New - PCR amplification - NGS

Question 7

Myotonic dystrophy

Answer 7

DM1 - CTG expansion
DM2 - CCTG expansion (lack interruption of repeat sequences)
-Intergenerational and somatic instability

Question 8

Huntington Disease

Answer 8

• Adult-onset neurogenerative disease
• CAG expansion on exon 1 - polyglutamines
• Preferential expansion of alleles in male germline
• Normal (10-35 copies)
• Affected (>35 copies)
• Reduced penetrance (36-39 copies, but can be affected)
• Intermediate (27-35 copies, but may expand to full mutation in next generation)

Question 9

Kennedy Disease

Answer 9

• CAG repeat expansion in exon 1 of androgen receptor gene (X chromosome)
• Female carriers do not have disease
• ONLY x-linked CAG repeat expansion disorder
• Normal (9-34 repeats)
• Reduced penetrance (35-37 repeats)
• Full penetrance (38+ repeats)
• Spinal and bulbar muscular atrophy
• Seems to require androgen with repeat number to induce disease (hence not present in females)