10: Trinucleotide Expansion Disorders

Question 1

Trinucleotide Repeat Expansion Disease (TRED) Examples

Answer 1

Huntington Disease

Fragile X Syndrome

Friedreich Ataxia

OPMD

Question 2

TRED (overview)

Answer 2

Expansions in coding or non-coding
Common repeats: CGC, GAA, CAG, CGG, CTG, GCG
Expansion sites associated with hypermethylated regions and the ability to form stable hairpins (leading to faulty DNA replication, recombination and repair)

CPG is site for hyper methylation

DNA replication error

Question 3

TRED (locations)

Answer 3

Fragile X Syndrome- CGG- 5’ UTR

Friedreich ataxia- GAA- Intron

Huntington Disease- CAG- Exon

Question 4

TRED Causes

Answer 4

Mistakes during replication, d.s DNA break repair, BER of oxidative damage, single-stranded DNA gap repair

Gene conversion (unequal crossing over) during meiosis

Ex. Slippage of DNA polymerase during DNA replication: CG more stable and you get looping out of repeat regions, polymerase goes over this can get deletions or expansions (expand or contract)

Question 5

Huntington Disease (symptoms)

Answer 5

Chorea- movements
Ataxia- walking
Dysarthria- talking

Autosomal dominant

Question 6

Huntington Disease (gene)

Answer 6

HD gene= IT 1 5 located at 4p16.3
Encodes Huntington protein
67 exons
HD expressed in many tissues but really high in testes and brain (neocortex, cerebellar cortex, striatum, hippocampus)

Question 7

Huntington Protein

Answer 7

350kDa protein
Interacts with many other proteins
In both nucleus and cytoplasm
Regulate intracellular transport of specific proteins (transcription-TAF part of TFIID)
Shuttles TFs in and out of nucleus
May sequester TFs
Needed for normal embryonic development
Expansion of CAG region results in aggregation of mutant protein into inclusion bodies

Question 8

HD Mutation

Answer 8

CAG repeats located in exon 1 in coding region
Normally<26 repeats
CAG codes for glutamine (Q)

“PolyQ tract”

Meiotic instability in sperm thought to cause expansion (unequal crossing over)

Question 9

HD Repeat Length Meaning

Answer 9

<26 repeats (normal)
27-35 (does not cause HD)
36-39 (variable penetrance)
>39 (fully penetrant)

Question 10

HD Diagnostic Guidelines

Answer 10

PCR (put primers around expansion region and amplify product, determine sequence)

Need consent, use specific primers to distinguish CAGs from adjacent CCGs), size standards to analyze repeat lengths, for exact lengths use Southern blot, use CAG repeat numbers for both alleles.

Question 11

Huntington Inclusions in Hippocampal Cells

Answer 11

Glops

Could sequester important proteins
Could disrupt cellular structure/metabolism
Could be mechanism to trash bad proteins (ubiquitin)

Question 12

Fragile X Syndrome (symptoms)

Answer 12

X-linked Dominant (inheritance is complex, however, non-Mendelian
Developmental delay is fairly common in children (reason for many referrals to pediatric genetics)
Differential diagnosis: Down, Williams, Angelman, autism spectrum disorder…
More common in males
IQ less than 70
Large head, long face, prominent forehead and chin, protruding ears, connective tissue findings (joint laxity), large testes after puberty
Behavior abnormalities

Question 13

Fragile X Syndrome (gene)

Answer 13

CGG triplet expansion in FMR1 gene on X chromosome (5’ UTR)

X-linked dominant, but more complex

Promoter mutation in 5’UTR of FMR-1 (gene that encodes for FMRP)

Not mutation in protein, mutation in promoter

Question 14

FXS (mutation)

Answer 14

FXS caused by silencing promoter for FMR1 gene

Normal: 5-50 repeats
Premutation: 50-200 repeats
Mutation: >200 repeats

CpG region becomes long and get hypermethylation

Question 15

Fragile X (premutation)

Answer 15

55-200 CGG repeats

When premutated father passes to daughters it usually doesn’t expand, never passed to sons

When premutated mother has it, it can expand in oocytes and become full mutation
-coming from mother, more likely expansion

Question 16

FMRP (protein)

Answer 16

Protein for mRNA metabolism

Widespread expression, especially brain and testes

Implicated in: dendritic spine maturation, synapse formation, synaptic plasticity

Question 17

FMRP mRNA Metabolism in Neurons

Answer 17

Shuttling protein (transfer proteins in cell body)
Binds mRNAs (mRNA export out of nucleus, translational control, localization of specific mRNAs to dendrites and post-synaptic sites)
Involved in microRNA function

FMRP activates or represses translation depending on the mRNA
FMRP localizes in dendritic spines of neurons (pruning of dendritic spines needs specific proteins)

Question 18

FMR1 Gene (silencing)

Answer 18

Chromatin compaction and DNA methylation cause chromatin to turn off

Methylation attracts gene silencing complexes (HDACs-deacetylate histones, resulting in condensed chromatin and silencing of FMR1 promoter0

Question 19

Fragile X Syndrome Treatment

Answer 19

Management of symptoms

Question 20

Friedreich Ataxia (FRDA) (symptoms)

Answer 20

Neurological disorder (starting at 5-15 years of age):
Muscle weakness in arms and legs, loss of coordination, vision and hearing impairment, slurred speech, scoliosis, pes cavus (high plantar arches), diabetes, heart disorders (atrial fibrillation, tachycardia, hypertrophic cardiomyopathy)
GAA repeats
Autosomal recessive

Question 21

FRDA (treatment and genetic counseling)

Answer 21

Treatment is supportive

Genetic Counseling: autosomal recessive inheritance, 25% recurrence risk

Question 22

FRDA (molecular)

Answer 22

Progressive degeneration of nerve tissue in spine, leading to gait disturbance, speech problems and heart disease
Spine becomes thinner and nerves lose part of the myelin sheath
Need both alleles (autosomal recessive)

Question 23

FRDA Mutation in FXN Gene

Answer 23

FXN encodes protein frataxin (on chromosome 9)
Frataxin removes iron in cytoplasm around mitochondria
Iron build-up cause free radical damage to mitochondrial membrane (especially affecting nerve and muscle cells)

Question 24

FXN GAA Repeat Expansion

Answer 24

GAA in first intron (normal 6-36)
Expansion (70-1700) causes transcriptional repression
Most patients have repeats on both alleles

Number of repeats correlates to severity

4% are compound heterozygotes (expansion on one allele and other type of mutation in FXN on other allele)

Question 25

FRDA Future Treatment

Answer 25

Reverse chromatin condensing

Want to get rid of compaction

Manage symptoms