More is Not Always More:
Unstable Repeat Expansion Disorders
SHP – Neurobiology of Development
and Disease
Two way trinucleotide repeats
can effect gene function
• occur in non-coding DNA (ie promoters)
transcriptional silencing, loss of function
• occur in coding sequence
lead to perturbation of gene function,
usually gain of function
Di Prospero and Fischbeck, Nature
Reviews Genetics 2005
Gatchel and Zoghbi, Nature Reviews
Genetics 2005
Gatchel and Zoghbi, Nature Reviews
Genetics 2005
Huntington Disease: the prototypical
unstable repeat disorder
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Effect about 5 in 100,000 men and women
Linked to a large protein called huntington
Highly penetrant autosomal dominant disorder
From diagnosis to mortality is ~10-25yrs.
Symptoms:
– Heritability
– Chorea
– Cognitive impairment, irritability, antisocial behavior
– Death in 15-20 years
– Degeneration of the caudate nucleus
Spinocerebellar Ataxia (SCA1-16)
• As name suggests, patients experience lack of motor
coordination.
• Multigenic disorder, each with their own distinctive loss of
neuronal subtypes and brain areas, durations, and
symptoms.
• Most seem to have a substantial degeneration of the
cerebellum.
• Autosomal dominant disorder.
Polyglutamine Tracts
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Normally contains 19-22
consecutive glutamines
Gene is manifest with 48 or more
repeats of the codon with more
repeats causing earlier onset of the
disease
PolyQ tract expands in successive
generations (particular in males)
PolyQ tracts are thought to be
toxic - fusing these proteins to
innocuous proteins makes those
proteins toxic (although the
normal protein is required for
normal pathology)
Cells expressing proteins with
expanded repeats contain
aggregates of protein within the
cell body and cytoplasm.
Consequences of Glutamine Expansion
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Expansion of polyQ tract can
enhance or alter protein-protein
interactions that normally occur
Protein accumulates into inclusion
bodies which may incorporate
chaperones (hsp70) and
ubiquitin/proteosome pathway
components
Huntingtin can be cleaved by
proteases including caspases and
caspases into more toxic peptide
fragments.
Mutant protein may interact with
or sequester transcriptional
regulators.
Interfere with other cellular
processes including RNA
metabolism, Ca2++ equilibria,
inhibiting mitochondrial function
and metabolism.
Gatchel and Zoghbi, Nature Reviews
Genetics 2005
Di Prospero and Fischbeck, Nature
Reviews Genetics 2005
Potential Therapeutic Strategies:
Diseases by Loss of Protein Function
• Include expanding
repeats: CGG, GCC,
GAA, CTG, and CAG
• Expansion of these C/G
rich repeats results in
methylation of CpG
stretches and
transcriptional silencing.
Fragile X Syndrome
• A leading cause of inherited mental
retardation
• X-linked
• Caused by expansion of CGG repeats in the
5’-UTR of the FMR1 (FMRP is protein
product) gene.
• 60-200 repeats are innocuous but over 200
are associated with the syndrome.
FMRP is Involved in RNA
Processing and Translation
• FMRP associates with many
components of the RNA
processing machinery.
• Mutations cause problems with
association of polyribosomes
• Loss of FMRP likely
destabilizes assembly of these
complexes
Gatchel and Zoghbi, Nature Reviews
Genetics 2005
Fragile X Neurons have Longer,
Immature Dendritic Spines
• FMRP seems to be involved in
dendritic development. Fragile
X patients contain
• Glutamate overstimulation
upregulates the expression of
FMRP, so mutant protein seems
to sensitize against excitoxicity
responses.
Gatchel and Zoghbi, Nature Reviews
Genetics 2005
Disease of Metabolism:
Friedreich Ataxia
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Autosomal recessive congenital disorder
The most commonly inherited ataxia
Caused by a mutation in the gene frataxin
Many parallels to muscular dystrophy although with degeneration of
the nervous system
• Symptoms:
– Muscle weakness in the limbs
– Ataxia
– Vision impairment
– Hearing loss
– Slurred speech
– Scoliosis
– Heart conditions
– Sugar intolerance
Frataxin (FXN)
• Frataxin associates with the inner
mitochondrial membrane
• Expansions of repeats in the FXN
prevent elongation of the transcript.
• Deletion of the gene results in
accumulation of iron in
mitochondria, which causes
accumulation of free radicals
 oxidative damage
• Decrease in production of proteins
with iron-sulfur clusters.
Gatchel and Zoghbi, Nature Reviews
Genetics 2005
Strategies for Treatment of
Friedreich Ataxia
• Enhancing Assembly/Production of
Fe/S cluster proteins
• Protection against oxidative damage
by free radicals in the cell.
• Free radicals may diminished by
potentiating the function of
superoxide dismutase (SOD).
• Chelation of Fe ions in the
mitochondria selectively
• Acetylation of histones to activate
gene transcription or prevent
silencing.
Di Prospero and Fischbeck, Nature
Reviews Genetics 2005
Prevention of
Silencing at the
Transcriptional Level
Might Diminish
Pathological
Symptoms
Di Prospero and Fischbeck, Nature
Reviews Genetics 2005
Tracking Microscope Used to
Visualize Huntingin Aggregation
and Cell Death
Many Neurons Die
without Forming
Inclusion Bodies
Levels of Diffuse
mutant Htt Protein
Protein Predict
Neuronal Death
Inclusion Body
Formation is
Protective Against
Htt-induced Death