DNA Testing in Diagnosis of Neurological Disorders with Loss of Movement Control Flashcards Preview

MD1 Neuroscience > DNA Testing in Diagnosis of Neurological Disorders with Loss of Movement Control > Flashcards

Flashcards in DNA Testing in Diagnosis of Neurological Disorders with Loss of Movement Control Deck (82):
1

How can you get a neurodegenerative disorder?

Acquired
Inherited

2

What are unstable repeat expansions?

Repeating units of 3+ nucleotides in tandem

3

What are more common: trinucleotide, or tetranucleotide repeats?

Trinucleotide

4

What is the more common sequence of unstable repeat expansions?

CAG

5

Are repeat expansions present in the normal gene?

Yes, with a specific range making up repeat region

6

What is repeat expansion?

When number of repeat units increases above certain threshold, associated with condition

7

Below the threshold, is the number of repeats stable?

Yes, in both gametes and somatic cells

8

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

9

Why are repeat expansions also called dynamic mutations?

Size of expansion changes

10

What is anticipation?

Expansion size increases in following generations

11

Do all unstable repeat expansion diseases have anticipation?

No

12

What is anticipation associated with?

Earlier onset
Greater severity of symptoms

13

Which cells are more likely to undergo repeat expansion?

Those undergoing DNA replication
- Gametes
- Some somatic cells

14

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

15

How do unstable repeat expansion disorders present?

Primarily neurological

16

What is the inheritance pattern of unstable repeat expansion disorders?

Variable, but mostly autosomal dominant

17

What is the nature of a class 1 unstable repeat expansion?

Non-coding repeats > loss of protein expression/function

18

What is the consequence of a class 1 unstable repeat expansion?

Impaired transcription of affected gene

19

What is the nature of a class 2 unstable repeat expansion?

Non-coding repeats > confer novel properties on RNA

20

What is the consequence of a class 2 unstable repeat expansion?

RNA with toxic gain-of-function

21

What is the nature of a class 3 unstable repeat expansion?

Repeats in codon > confer novel properties to affected protein

22

What is the consequence of a class 3 unstable repeat expansion?

Production of modified protein > overrides function of normal protein

23

What are late-onset neurodegenerative disorders caused by repeat expansions characterised by?

Loss of movement control

24

Why is it important to diagnose the particular type of unstable repeat expansion disorder?

Treatment
Prognosis
Risk for other family members

25

Can you have juvenile onset with unstable repeat expansion disorders?

Late onset is most common, but juvenile onset does occur

26

What is the symptom progression in late-onset neurodegenerative disorders caused by unstable repeat expansions?

Worsen over time > death

27

What is DNA testing useful for in unstable repeat expansion disorders?

Differential diagnosis
Predictive testing
- Pre-symptomatic at-risk relatives
- Prenatal testing

28

What is the inheritance pattern of Huntington's disease?

Autosomal dominant

29

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

30

What is the age of onset of Huntington's disease?

Mean age = 40-50
5-10% at

31

Why can you get juvenile onset of Huntington's disease?

Number of repeats much higher

32

What are the main clinical features of Huntington's disease?

Movement/motor disorder
Cognitive disorder
Psychiatric/emotional disorder
Progressive neurodegeneration

33

Do the clinical features change as Huntington's disease progresses?

Yes:
- Early features
- Middle features
- Late features

34

Is current treatment for Huntington's disease disease-modifying?

No, mitigates some symptoms

35

What is the nucleotide sequence that is expanded in Huntington's disease?

CAG

36

In what region of DNA does repeat expansion happen in Huntington's disease?

In exon 1 of chromosome 4 of HTT gene

37

What protein does CAG code for?

Glutamine

38

What is the protein product of the HTT gene?

Huntingtin

39

What are the physiological roles of huntingtin?

Transcription
- Brain-derived neurotrophic factor
IC transport of other molecules
IC signalling and metabolism
Reducing apoptosis

40

What change to the protein structure does the expanded CAG huntingtin product have?

PolyQ tail at N-terminal

41

What is the effect of polyQ-huntingtin?

Toxic effect in basal ganglia, especially in medium spiny neurons (starts here)

42

What is the pathophysiology of Huntington's disease?

Progressive degeneration and loss of medim spiny neurons in striatum of basal ganglia

43

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

44

What is the normal number of repeats to have in the HTT gene?

26 or less

45

What is the normal, mutable number of repeats to have in the HTT gene?

27-35

46

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

47

What is the number of repeats in the HTT if you're in the zone of reduced penetrance?

36-39

48

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

49

What is the number of repeats in the HTT to be completely penetrant?

40 or more

50

What do CCG interruptions in a sequence of CAG repeats in the HTT gene do?

Mitigate effects of CAG repeats

51

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

52

What are the outcomes of loss of normal huntingtin function?

Excitotoxicity
Oxidative stress
Impaired energy metabolism
Apoptosis

53

How is Huntington's disease classically tested for?

PCR using radioactive nucleotide tagging > gel electrophoresis > autoradiography

54

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

55

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

56

How do you chose the correct band in gel electrophoresis when there is a ladder pattern?

Focus on band that is largest and darkest

57

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

58

What is currently used to test for Huntington's disease?

PCR using fluorescent nucleotide tagging > fragment analysis on capillary electrophoresis > fluorescence detection

59

What is the inheritance pattern for spinocerebellar ataxias?

Autosomal dominant

60

What is the prevalence of spinocerebellar ataxias?

Frequency of different types varies in different populations

61

What is the age of onset of spinocerebellar ataxias?

Late onset

62

What are the main clinical features of spinocerebellar ataxias?

Progressive degeneration of
- Cerebellum
- Brainstem
- Spinocerebellar tracts
Affects
- Gait
- Hand coordination
- Speech
- Eye movements
Phenotypic variation

63

Which spinocerebellar ataxias are the most common?

SCA 1
SCA 2
SCA 3 = Machado-Joseph disease

64

In which populations is SCA 3/Machado-Joseph disease especially relevant?

Portuguese
Indigenous Australians, especially in North Arnhem land

65

Why does SCA 6 have no zone of reduced penetrance?

Unstable repeats don't undergo anticipation

66

What is the inheritance pattern for Friedreich ataxia?

Autosomal recessive

67

What is the prevalence of Friedreich ataxia?

2-4 in 100 000

68

What is the carrier frequency of Friedreich ataxia?

1/60 and 1/100 in Indo-Europeans

69

What is the age of onset of Friedreich ataxia?

Mean age 10-15
Usually

70

What are the main clinical features of Friedreich ataxia?

Progressive limb and gait ataxia
Cardiomyopathy in 65%
Diabetes mellitus in 30%

71

What is the nucleotide sequence that is repeated in Friedreich ataxia?

GAA

72

Where is the repeat expansion in Friedreich ataxia?

Intron 1 in FXN gene on chromosome 9

73

What is the normal function of the protein affected in Friedreich ataxia?

Binds Fe - important for mitochondrial function

74

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

75

What does the defect caused by Friedreich ataxia result in within the cell?

Accumulation of Fe in mitochondria > oxidative damage

76

What proportion of cases does the expansion mutation account for in Friedreich ataxia?

94-98%

77

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)

78

What is the protein that is affected in Friedreich's ataxia?

Frataxin

79

What is the normal range of repeats in the FTN gene?

5-33

80

What is the affected range of repeats in the FTN gene?

66-1700

81

What is the mutable range of repeats in the FTN gene?

34-65 uninterrupted

82

What often interrupts alleles longer than 27 GAA repeats in the FTN gene?

GAGGAA
Could stabilise allele

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