Trinucleotide Repeat Expansion

Question 1

Why do trinucleotide repeats (TNRs) expand?

Answer 1

Unknown mechanism. Hallmark is Instability

Question 2

What are 4 principals associated with TNR disorders?

Answer 2

1. Molecular characteristics and consequences of expansion differ.
2. Tendency for repeat expansions depends on transmitting parent
3. Those with abnormal number of TNRs who have no/fewer symptoms carry “pre-mutations”
4. TNR display genetic anticipation

Question 3

What are the 2 TNR polyglutamine diseases that occur in coding region of gene? What is disease presentation caused by?

Answer 3

1. Huntington’s
2. Spinocerebellar ataxia

(Caused by excess glutamine insertions in the protein)

Question 4

How is TNR expansion in the Non-coding region of the gene pathogenic?

Answer 4

Disease caused by diminished or absent target protein or ‘RNA toxicity’

Question 5

What is genetic anticipation?

Answer 5

With TNR expansion, the more repeats the more severe and early the onset of disease is.

This is different from typical mendelian genetics where expression is constant thru generations (apart from penetrance/phenotypic variability)

Question 6

What are some hallmark clinical progressions seen in pts w/ Huntingtons?

Answer 6

Saccadic extraocular eye movements: (these slow movements that are kind of like brief off center shifts of the eyes)

Ataxic Gait: poor stability and balance. Need a wide legged stance for balance and walking in tandem is difficult. Torso is also very unsteady

Choreatic movements: spontaneous non-patterned involuntary movements of muscles.

Question 7

What is the inheritence pattern of Huntington’s?

Answer 7

Autosomal dominant (progressive neurodegenerative disorder)

Question 8

Where does the increased repeats occur in HD? What is the pattern of repeats?

Answer 8

Glutamines(CAG)

1st exon in huntington protein.

Question 9

Where is huntington protein typically located? Where is it pathogenic?

Answer 9

Huntington protein expressed in brain and large number of body tissues

Pathology is restricted to CNS

is not a knock out of huntingtin, accumulation of these malshaped proteins

Aggregates -> 
Modify transcription
Induce proteolysis
Interfere with axonal transport
Disrupt synaptic transmission

Question 10

What is incidence of HD? Onset? Clinical features?

Answer 10

3-7/100,000

Onset is midlife with duration of 15-20years.

Features:
Involuntary choreiform movements
Cognitive impairment
Mood disorders and behavioral changes

Question 11

How many repeats are seen in HDwt? HDintermediate? HDpremut? HDmt?

What is the difference between them?

Answer 11

HDwt - 6-26 CAG repeats

HDint - 27-35 repeats => unstable, usually no disease, prone to repeating more

HDpremut - 36-39 repeats => may be incomplete penetrance (either +/- for disease, no alternative phenotype seen like in other TNRs)

HDmut - >39 repeats - pt is always affected.

Question 12

How does repeat expansion continue to expand in HD?

Answer 12

Spermatogenesis (thru the dad)

Mom can still have/transmit the diz, but if she is just a carrier (int/premut) then she won’t give the kids anymore repeats

Question 13

How can we diagnose HD?

Answer 13

Thru PCR.

TNR increases the size of the PCR product.

We would see heavier bands in pts with more repeats.

Question 14

What is the most common inherited cause of intellectual disability and autism spectrum disorders?

Answer 14

Fragile X syndrome.

(Remember trisomy 21s (Down syndrome) are the most common chromosomal cause of developmental delays. But those arent all caused by inheritance.)

Question 15

What is the inheritance pattern of Fragile X syndrome?

Answer 15

X-linked (hard to tell dominant vs recessive)

Question 16

What and where is the TNR in FXS?

Answer 16

CGG expansion at promoter of the fragile X mental retardation gene (FMR1)

5’UTR

Question 17

T/F FMR1 gene is only seen in mammals?

Answer 17

False, highly conserved across species

Question 18

What is FMR1wt repeat #? FRM1premut? FMR1mutation?

Answer 18

FMR1wt - 6-44 repeats (does not expand)

FMR1premut - 55-200 CGG repeats

FMR1mutation - >200-4000 CGG repeats

Question 19

T/F: In fragile X syndrome there is more risk for expansion when transmitted by males?

Answer 19

False; females.

Question 20

What does the FMR protein do?

Answer 20

RNA-binding protein: Translational regulator for the target mRNAs.

Important in axonal transport/ generation of dendrites

Arrests development of neurons

Question 21

How does expansion >200 repeats in FXS manifest?

Answer 21

Full mutation: full transcriptional silencing of FMR1 gene. Promoter becomes methylated. Full cognitive disability

Affected males (w/ full mutation):
Cognitive disability (low IQ)
Post puberty changed: large narrow facew, large ears, hypotonia, macroochidism (large testes) - 90% of males

Question 22

How does an expansion of 55-200 repeats manifest in FXS?

Answer 22

Prevelence more common in females than males (for premutation)

Tremor-Ataxia Syndrome (FXS associated):
Risk males>females
Progressive neurodegenerative (cognitive decline in elderly)

Primary Ovarian Insufficiency:
Cessation of menses before 40yo
20% female with pre-mutation (1% in general population)

Question 23

What is hallmark premutation phenotype of FXS in females? How does it occur?

Answer 23

Primary Ovarian Insufficiency.

Enlarged promoter region in FMRgene leads to other binding proteins being sequestered (that were originally on other transcripts). Disrupt other factors function.

Question 24

What kind of atypical transmission patterns do we see in FXS?

Answer 24

Carrier males: 20% who carry FMR1premut (20-200 repeats) are clinically normal

Affected females:
30-50% carrier females with full mutation are affected
-due to selective S inactivation

Question 25

What is inheritance pattern of Friedreich’s ataxia?

Answer 25

Autosomal recessive disorder caused by GAA expansion at 1st intron of Frataxin gene (FRDA)

Question 26

How many cases of hereditary ataxia does FA account for?

Answer 26

50%

Question 27

What is repeat for FRDAwt? FRDAmut?

Answer 27

Normal : GAA 7-34 repeats

FA: 200-1200 GAA repeats

Question 28

What does TNR lead to in the Frataxin gene?

Answer 28

MRNA transcript loss

Question 29

How does FA influence mRNA transcript loss?

Answer 29

MRNA loses ability to splice correctly. Intron left in and mRNA broken down rapidly

Question 30

What kind of protein is Frataxin?

Answer 30

Mitochondrial

May have something to do with iron/oxidative stress

Question 31

Where is expression of frataxin the highest?

Answer 31

Heart and spinal cord (pancreas too)

Selective cell loss-> neurological dysfunction, cardiomyopathy, DM type 1

Question 32

What is inheritance pathway for Myotonic Dystrophy (type1)?

Answer 32

Autosomal dominant with expansion of CTG in 3’UTR of the dystrophia myotonia protein kinase (DMPK)

Question 33

What is the most common cause of adult onset nuscular dystrophy?

Answer 33

Type 1 MD

1/10,000

Question 34

What is repeat number for wt, premutation, mutation?

Answer 34

DMPKwt= 5-34 CTG alleles

DMPKpremut= 35-49 CTG repeats w/o clinical phenotype

DMPKmutation = >50 repeats, into the thousands

Question 35

In MD1, is there phenotypic expression of premutation gene?

Answer 35

No

Question 36

What are the phenotypic expression of MD1 in relation to repeat #?

Answer 36

MILD (late onset) = 50-150
Cataracts, mild myotonia

Classic MD1 (150-1000)
Muscle wasting weakness, myotonia, balding, cardiac conduction deficits

Severe/congenital (>1000): infantile hypotonia, resp dysfunction.

Question 37

How can males and females differ in their transmission of MD1?

Answer 37

Males - transmit repeats up to 1000

Females- may transmit thousands of repeats!

Question 38

How does MD1 manifest disease?

Answer 38

RNA-mediated toxicity

Alternative splicing that should occur may not occur due to recruitment of binding proteins attached to newly formed hairpin loop.