Flashcards in DNA Testing in Diagnosis of Neurological Disorders with Loss of Movement Control Deck (82):
How can you get a neurodegenerative disorder?
What are unstable repeat expansions?
Repeating units of 3+ nucleotides in tandem
What are more common: trinucleotide, or tetranucleotide repeats?
What is the more common sequence of unstable repeat expansions?
Are repeat expansions present in the normal gene?
Yes, with a specific range making up repeat region
What is repeat expansion?
When number of repeat units increases above certain threshold, associated with condition
Below the threshold, is the number of repeats stable?
Yes, in both gametes and somatic cells
Above the threshold, is the number of repeats stable?
No, unstable in gametes, and can also be unstable in somatic cells
- Passed onto subsequent generations
Why are repeat expansions also called dynamic mutations?
Size of expansion changes
What is anticipation?
Expansion size increases in following generations
Do all unstable repeat expansion diseases have anticipation?
What is anticipation associated with?
Greater severity of symptoms
Which cells are more likely to undergo repeat expansion?
Those undergoing DNA replication
- Some somatic cells
What is a possible mechanism of expansion?
1. Starting template strand of DNA
2. Replicating strand detaches inappropriately during replication
3. Replicating strand slips from proper alignment by one repeat length > mismatched repeat loops out
4. Newly synthesised strand contains extra repeat
How do unstable repeat expansion disorders present?
What is the inheritance pattern of unstable repeat expansion disorders?
Variable, but mostly autosomal dominant
What is the nature of a class 1 unstable repeat expansion?
Non-coding repeats > loss of protein expression/function
What is the consequence of a class 1 unstable repeat expansion?
Impaired transcription of affected gene
What is the nature of a class 2 unstable repeat expansion?
Non-coding repeats > confer novel properties on RNA
What is the consequence of a class 2 unstable repeat expansion?
RNA with toxic gain-of-function
What is the nature of a class 3 unstable repeat expansion?
Repeats in codon > confer novel properties to affected protein
What is the consequence of a class 3 unstable repeat expansion?
Production of modified protein > overrides function of normal protein
What are late-onset neurodegenerative disorders caused by repeat expansions characterised by?
Loss of movement control
Why is it important to diagnose the particular type of unstable repeat expansion disorder?
Risk for other family members
Can you have juvenile onset with unstable repeat expansion disorders?
Late onset is most common, but juvenile onset does occur
What is the symptom progression in late-onset neurodegenerative disorders caused by unstable repeat expansions?
Worsen over time > death
What is DNA testing useful for in unstable repeat expansion disorders?
- Pre-symptomatic at-risk relatives
- Prenatal testing
What is the inheritance pattern of Huntington's disease?
What is the prevalence of Huntington's disease?
1 in 10 000-20 000
- Varies from population to population
- Higher in places with founder effect
What is the age of onset of Huntington's disease?
Mean age = 40-50
Why can you get juvenile onset of Huntington's disease?
Number of repeats much higher
What are the main clinical features of Huntington's disease?
Do the clinical features change as Huntington's disease progresses?
- Early features
- Middle features
- Late features
Is current treatment for Huntington's disease disease-modifying?
No, mitigates some symptoms
What is the nucleotide sequence that is expanded in Huntington's disease?
In what region of DNA does repeat expansion happen in Huntington's disease?
In exon 1 of chromosome 4 of HTT gene
What protein does CAG code for?
What is the protein product of the HTT gene?
What are the physiological roles of huntingtin?
- Brain-derived neurotrophic factor
IC transport of other molecules
IC signalling and metabolism
What change to the protein structure does the expanded CAG huntingtin product have?
PolyQ tail at N-terminal
What is the effect of polyQ-huntingtin?
Toxic effect in basal ganglia, especially in medium spiny neurons (starts here)
What is the pathophysiology of Huntington's disease?
Progressive degeneration and loss of medim spiny neurons in striatum of basal ganglia
What do the brain scans of someone with Huntington's disease show?
Loss of brain tissue in basal ganglia, and other areas, including cerebral cortex
What is the normal number of repeats to have in the HTT gene?
26 or less
What is the normal, mutable number of repeats to have in the HTT gene?
What does it mean if the number of repeats in the HTT is mutable?
Won't develop Huntington's disease themselves
Have intermediate size allele
Higher risk of expansion in sperm
Paternal transmission > expanded number of repeats in offspring
What is the number of repeats in the HTT if you're in the zone of reduced penetrance?
What is your risk of developing Huntington's disease if you are in the zone of reduced penetrance?
Difficult to predict if you'll develop disease
What is the number of repeats in the HTT to be completely penetrant?
40 or more
What do CCG interruptions in a sequence of CAG repeats in the HTT gene do?
Mitigate effects of CAG repeats
What is the molecular pathology of Huntington's disease?
Aberrant mRNA splicing of exon 1 + polyQ-huntingtin cleaved by caspases > N-terminal fragments with altered conformation - toxic
Form aggregates and nuclear inclusions - may be partly protective cellular response via sequestration
Loss of function of normal HTT + possible toxicity of mRNA
What are the outcomes of loss of normal huntingtin function?
Impaired energy metabolism
How is Huntington's disease classically tested for?
PCR using radioactive nucleotide tagging > gel electrophoresis > autoradiography
Why is it difficult to get the exact repeat size when testing for Huntington's disease?
DNA polymerase in PCR makes mistakes > causes expansions, similar to those happening in cells
Why do you get a ladder pattern in gel electrophoresis when testing for Huntington's disease?
Stuttering effect of PCR > generates product of more than one size
How do you chose the correct band in gel electrophoresis when there is a ladder pattern?
Focus on band that is largest and darkest
What is the margin of error when testing DNA for repeat expansions? What does it mean for the patient?
Results are always plus or minus 1-2
Tricky if person on borderline
What is currently used to test for Huntington's disease?
PCR using fluorescent nucleotide tagging > fragment analysis on capillary electrophoresis > fluorescence detection
What is the inheritance pattern for spinocerebellar ataxias?
What is the prevalence of spinocerebellar ataxias?
Frequency of different types varies in different populations
What is the age of onset of spinocerebellar ataxias?
What are the main clinical features of spinocerebellar ataxias?
Progressive degeneration of
- Spinocerebellar tracts
- Hand coordination
- Eye movements
Which spinocerebellar ataxias are the most common?
SCA 3 = Machado-Joseph disease
In which populations is SCA 3/Machado-Joseph disease especially relevant?
Indigenous Australians, especially in North Arnhem land
Why does SCA 6 have no zone of reduced penetrance?
Unstable repeats don't undergo anticipation
What is the inheritance pattern for Friedreich ataxia?
What is the prevalence of Friedreich ataxia?
2-4 in 100 000
What is the carrier frequency of Friedreich ataxia?
1/60 and 1/100 in Indo-Europeans
What is the age of onset of Friedreich ataxia?
Mean age 10-15
What are the main clinical features of Friedreich ataxia?
Progressive limb and gait ataxia
Cardiomyopathy in 65%
Diabetes mellitus in 30%
What is the nucleotide sequence that is repeated in Friedreich ataxia?
Where is the repeat expansion in Friedreich ataxia?
Intron 1 in FXN gene on chromosome 9
What is the normal function of the protein affected in Friedreich ataxia?
Binds Fe - important for mitochondrial function
What does the repeat expansion cause in the FXN gene in Friedreich ataxia?
Abnormal secondary structure - maybe even triple helix, or induces heterochromatin > reduced protein production
What does the defect caused by Friedreich ataxia result in within the cell?
Accumulation of Fe in mitochondria > oxidative damage
What proportion of cases does the expansion mutation account for in Friedreich ataxia?
What accounts for the small proportion of cases that aren't due to an expansion mutation in Friedreich ataxia?
Compound heterozygotes with GAA repeat and other point inactivating mutation(s)
What is the protein that is affected in Friedreich's ataxia?
What is the normal range of repeats in the FTN gene?
What is the affected range of repeats in the FTN gene?
What is the mutable range of repeats in the FTN gene?