DNA testing in diagnosis of neurological disorders

Question 1

Describe the two broad mechanisms by which neurodegenerative disorders develop?

Answer 1

Acquired (eg. cerebrovascular disease, alcholism)

Inherited: unstable repeat expansions

Question 2

What are unstable repeat expansions?

Answer 2

Expansion of a segment of DNA within a specific gene, which consists of repeating units of three or more nucleotides in tandem

Question 3

What is the difference between naturally occuring repeat regions and repeat expansions?

Answer 3

Repeat expansion occurs when the number of repeats increases beyond a certain threshold > associated with a condition

Question 4

Why are unstable repeat expansions referred to as dynamic?

Answer 4

Size of expansion changes

Question 5

Describe the phenomenon of anticipation?

Answer 5

Expansion size increases in following generations

Usually associated with ealrier onset and greater severity of symptoms

Question 6

Describe the mechanism by which expansion of repeats occurs?

Answer 6

Slipped mispairing

Question 7

Which body system do unstable repeat expansions usually affect?

Answer 7

Neurological

Question 8

Describe the different ways in which unstable repeat expansions can affect a gene?

Answer 8

Non-coding repeats > loss of protein function > impaired transcription

Non-coding repeats that confer novel properties on RNA > toxic ganin of function

Repeats in codon > novel properties on protein > novel gain of function

Question 9

Describe the general characteristics of neurodegenerative disorders caused by repeat expansions?

Answer 9

Loss of movement control

Late onset

Progressive

Question 10

Describe the genetics of Huntington disease?

Answer 10

Automsomal dominant

CAG repeat expansion in HTT gene of chrm 4

Repeat in exon 1 - CAG codes for glutamine

Question 11

Describe the prevalence of HD?

Answer 11

1 in 10,000 to 20,000

Question 12

Describe the onset of HD?

Answer 12

Late onset

Typically 40s-50s

Question 13

Describe the major features/symptoms of HD?

Answer 13

Movement/motor disorder

Cognitive disorder

Psychiatric/emotional disorder

Question 14

What is the normal role of the HTT gene?

Answer 14

Produces protein product - huntingtin

Seems to have roles in transcription

A lot we don’t know about it

Question 15

What is the expanded CAG huntingtin product referred to as?

Answer 15

polyQ-huntingtin

Question 16

Where does polyQ-huntingtin exert its toxic effects?

Answer 16

Medium spiny neurons in striatum of basal ganglia

Question 17

Describe the appearance of a HD brain on imaging?

Answer 17

Neurodegeneration

Initially starts in basal ganglia, but as disease progresses it spreads to whole brain

Question 18

Describe the basic molecular pathology of HD?

Answer 18

PolyQ-huntingtin is cleaved by caspases > generates N-terminal fragments with altered conformation > TOXIC

Form aggregates and nuclear inclusions - may be protective?

Question 19

Describe the genotype/phenotype corrleation in HD?

Answer 19

Normal: _<_26 repeats

Normal, mutable (paternal transmission): 27-35 repeats

Zone of reduced penetrance: 36-39 repeats

Affected: _>_39 repeats

Question 20

Describe the effects of CCG interruptions in HD?

Answer 20

Don’t code for glutamine > interrupt length of glutamine > can mitigate effects of CAG repeats and prevent onset of HD

Question 21

How is HD clascially tested for?

Answer 21

PCR, gel electrophoresis and autoradiography

Question 22

Describe the current method for HD testing?

Answer 22

PCR (fluorescent tagging), fragment analysis on capillary electrophoresis and fluoresence detection

Question 23

Describe the major features and symptoms of spinal cerebellar ataxias?

Answer 23

Autosomal dominant

Phenotypic variation

Progressive degeneration of cerebellum, brain stem and spinocerebellar tracts

Question 24

How are SCAs tested for?

Answer 24

Fragment analysis

Question 25

Describe the genetics of Freidreich ataxia?

Answer 25

Autosomal recessive

GAA repeat expansion in FXN gene on chrm 9

Repeat location within intron 1

Causes abnormal DNA secondary structure > reduced protein (frataxin) production

Question 26

Describe the main features/symptoms of Friedreich ataxia?

Answer 26

Progressive limb and gait ataxia

Cardiomyopathy

Diabetes mellitus

Question 27

What is the general age of onset for FA?

Answer 27

Puberty

Question 28

Describe the pathogenesis of FA?

Answer 28

Repeat expasnion causes decreased frataxin production

Causes mitochondrial iron accumulation > oxidative damage

Question 29

Describe the FA genotype/phenotype correlation?

Answer 29

Normal: 5-33 repeats

Affected: 66-1700

34-65: premutation range

Question 30

How is FA tested for?

Answer 30

Standard PCR - can be used because we are dealing with large numbers of repeats