Unstable DNA and Simple Repeat Expansions Flashcards
What causes variable symptoms in diseases caused by repeat expansions?
Variation in repeat length.
What are the features of spinocerebellar ataxia type 7?
Atxia of both gait and limbs - neuronal degeneration associated with: dysarthria, lower limb spasticity dysphagia, oculomotor abnormalities, myokymia, mental impairment.
Retinal degeneration:
- bilateral, symmetrical
- leads to blindness
What is the age of onset of spinocerebellar ataxia type 7?
0-60 mean is 29
Striking anticipation
Onset to death: few years for early onset, up to 30 years for late onset.
What is the inheritance pattern for spinocerebellar ataxia type 7?
Autosomal dominant
What is spinocerebellar ataxia type 7 caused by?
CAG repeat:
6-19 repeats in normal population
about 75% alleles have 10 repeats
Heterozygosity about 35%
Disease phenotype:
37-306 repeats
Average about 55 repeats
Larger alleles - increased severity, decreased age of onset.
Intermediate alleles (28-35): Not associated with disease, risk of expansion in succeeding generations.
Is there transmission bias in SCA7 families?
Yes, most transmissions are from affected mothers.
What is the mutation rate in the sperm of SCA7 male?
100%
Name 4 expanded polyglutamine repeat disorders.
Spinal and bulbar muscular dystrophy
Huntington disease
Spinocerebellar ataxia (SCA) types 1, 2, 3, 7 and 17
Dentatorubral pallidoluysian atrophy
What is happening in polyglutamine repeat disorders?
Gain of function of polyglutamine tract - protein aggregates.
Mitochondrial dysfunction
Proteosome dysfunction
Transcriptional mis-regulation
Induction of apoptotic pathway
What are the clinical features of myotonic dystrophy?
Cataracts Heart conduction defects Brain dysfunction Gastrointestinal tract dysfunction Insulin resistance Frontal balding in males Testicular atrophy Muscular atrophy Myotonia
What is the inheritance pattern of myotonic dystrophy?
Autosomal dominant - anticipation
What is the repeat expansion in myotonic dystrophy?
CTG
Are most congenital cases of myotonic dystrophy inherited from mother or father?
Mother
Excess of transmitting grandfathers.
What is SIX5?
A homeodomain transcription factor.
It is widely expressed at low levels, but highly expressed in the eye.
Reduction in expression is caused by mutant chromosome in DM - chromatin condensation in region of repeat.
Which gene is differentially spliced in myotonic dystrophy muscle?
CLC1
What are the therapeutic opportunities in DM?
Reversal of RNA dominance by displacement of protein sequestered on triplet repeat RNA (read this paper).
What is Friedreich ataxia?
Progressive neurodegeneration of both central and peripheral nervous system.
Ataxia of gait
Dysarthria
Hypertrophic cardiomyopathy
Diabetes mellitus
What is the inheritance pattern in Friedreich ataxia?
Autosomal recessive.
Which nucleotide sequence is repeated in FA?
GAA
In FA if 96% of patients are homozygous for GAA mutation what is happening in the other 4%?
Compound heterozygotes for point mutations:
Premature stops
Missense mutations
Loss of function
What kind of structures can polypurine sequences (such as GAA) form?
Triplex DNA
- no evidence for their existence in vivo
- do block transcription in vitro
- frataxin transcript levels reduced in patients.
Where is frataxin highly expressed?
Neurons
Cardiomyocytes
Pancreas
Skeletal muscle
What is frataxin?
A mitochondrial membrane protein.
What is frataxin involved in?
Intracellular iron homeostasis
Iron accumulates in mitochondria
Cells are susceptible to free radical change.
Have any animal models been made to study FXN?
Yes, mouse models using cre-lox.
Splice out fourth exon.
What is the phenotype of fxn mouse models?
Lose weight at 7 weeks Progressively develop signs of fatigue. Die at about 11 weeks Develop cardiomyopathy - degenerating mitochondria - iron deposits No skeletal muscle defects
What is the clinical presentation of fragile X syndrome?
Mental retardation Long faces Prominent ears Autistic like behaviour - no neurodegeneration - aberrant dendritic structures - excess dendrites (no pruning)
What can be seen on a karyotype of a patient with fragile X syndrome?
Observable fragile site on the long arm of the X chromosome.
Only visible in specific in vitro tissue culture conditions.
What is the incidence of fragile X?
1/1,500
Are males or females with fragile X more severely affected?
Males
What is the inheritance pattern of fragile X?
X-linked
What is unusual about the inheritance of fragile X?
Men with mutant chromosome don’t always have the phenotype.
Penetrance increases through generations.
Severity is always the same.
Due to triplet repeat.
Which triplet is repeated in fragile X?
CGG
How many CGG repeats are found in the general population and in fragile X?
General population 5-45
Affected patients 230-2000
Normal transmitting male 50-200
What does the CGG repeat cause?
Hypermethylation (CpG island) resulting in transcriptional repression.
Therefore no FRM1 mRNA produced.
no FMRP produced in males
Do normally transmitting males have any symptoms?
Can be prone to late onset tremor/ataxia.
How does repeat length change in fragile X if it’s repeated by males vs females?
Repeat increases down the generations if transmitted by females.
Not so much if transmitted by males.
In which gene is the CGG expansion in fragile X?
FMR1
Which region of FMR1 is the CGG expansion found?
5’ untranslated region.
What is FMRP?
Translational repressor
Associated with transcripts that are spatially translated.
In neurons translation often only occurs in end of synapses
Which mRNAs are affected by fragile X?
SEC7
SAPAP4, DAP-1, kinate receptor
GRMs - metabolic glutamate receptors involved in long term potentiation.
