10: Trinucleotide Expansion Disorders Flashcards
Trinucleotide Repeat Expansion Disease (TRED) Examples
Huntington Disease
Fragile X Syndrome
Friedreich Ataxia
OPMD
TRED (overview)
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
TRED (locations)
Fragile X Syndrome- CGG- 5’ UTR
Friedreich ataxia- GAA- Intron
Huntington Disease- CAG- Exon
TRED Causes
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)
Huntington Disease (symptoms)
Chorea- movements
Ataxia- walking
Dysarthria- talking
Autosomal dominant
Huntington Disease (gene)
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)
Huntington Protein
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
HD Mutation
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)
HD Repeat Length Meaning
<26 repeats (normal)
27-35 (does not cause HD)
36-39 (variable penetrance)
>39 (fully penetrant)
HD Diagnostic Guidelines
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.
Huntington Inclusions in Hippocampal Cells
Glops
Could sequester important proteins
Could disrupt cellular structure/metabolism
Could be mechanism to trash bad proteins (ubiquitin)
Fragile X Syndrome (symptoms)
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
Fragile X Syndrome (gene)
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
FXS (mutation)
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
Fragile X (premutation)
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
FMRP (protein)
Protein for mRNA metabolism
Widespread expression, especially brain and testes
Implicated in: dendritic spine maturation, synapse formation, synaptic plasticity
FMRP mRNA Metabolism in Neurons
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)
FMR1 Gene (silencing)
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
Fragile X Syndrome Treatment
Management of symptoms
Friedreich Ataxia (FRDA) (symptoms)
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
FRDA (treatment and genetic counseling)
Treatment is supportive
Genetic Counseling: autosomal recessive inheritance, 25% recurrence risk
FRDA (molecular)
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)
FRDA Mutation in FXN Gene
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)
FXN GAA Repeat Expansion
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)
FRDA Future Treatment
Reverse chromatin condensing
Want to get rid of compaction
Manage symptoms
