Unstable DNA and Simple Repeat Expansions

Question 1

What causes variable symptoms in diseases caused by repeat expansions?

Answer 1

Variation in repeat length.

Question 2

What are the features of spinocerebellar ataxia type 7?

Answer 2

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

Question 3

What is the age of onset of spinocerebellar ataxia type 7?

Answer 3

0-60 mean is 29

Striking anticipation

Onset to death: few years for early onset, up to 30 years for late onset.

Question 4

What is the inheritance pattern for spinocerebellar ataxia type 7?

Answer 4

Autosomal dominant

Question 5

What is spinocerebellar ataxia type 7 caused by?

Answer 5

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.

Question 6

Is there transmission bias in SCA7 families?

Answer 6

Yes, most transmissions are from affected mothers.

Question 7

What is the mutation rate in the sperm of SCA7 male?

Answer 7

100%

Question 8

Name 4 expanded polyglutamine repeat disorders.

Answer 8

Spinal and bulbar muscular dystrophy
Huntington disease
Spinocerebellar ataxia (SCA) types 1, 2, 3, 7 and 17
Dentatorubral pallidoluysian atrophy

Question 9

What is happening in polyglutamine repeat disorders?

Answer 9

Gain of function of polyglutamine tract - protein aggregates.

Mitochondrial dysfunction
Proteosome dysfunction
Transcriptional mis-regulation
Induction of apoptotic pathway

Question 10

What are the clinical features of myotonic dystrophy?

Answer 10

Cataracts
Heart conduction defects
Brain dysfunction
Gastrointestinal tract dysfunction
Insulin resistance
Frontal balding in males
Testicular atrophy
Muscular atrophy
Myotonia

Question 11

What is the inheritance pattern of myotonic dystrophy?

Answer 11

Autosomal dominant - anticipation

Question 12

What is the repeat expansion in myotonic dystrophy?

Answer 12

CTG

Question 13

Are most congenital cases of myotonic dystrophy inherited from mother or father?

Answer 13

Mother

Excess of transmitting grandfathers.

Question 14

What is SIX5?

Answer 14

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.

Question 15

Which gene is differentially spliced in myotonic dystrophy muscle?

Answer 15

CLC1

Question 16

What are the therapeutic opportunities in DM?

Answer 16

Reversal of RNA dominance by displacement of protein sequestered on triplet repeat RNA (read this paper).

Question 17

What is Friedreich ataxia?

Answer 17

Progressive neurodegeneration of both central and peripheral nervous system.

Ataxia of gait
Dysarthria
Hypertrophic cardiomyopathy
Diabetes mellitus

Question 18

What is the inheritance pattern in Friedreich ataxia?

Answer 18

Autosomal recessive.

Question 19

Which nucleotide sequence is repeated in FA?

Answer 19

GAA

Question 20

In FA if 96% of patients are homozygous for GAA mutation what is happening in the other 4%?

Answer 20

Compound heterozygotes for point mutations:
Premature stops
Missense mutations
Loss of function

Question 21

What kind of structures can polypurine sequences (such as GAA) form?

Answer 21

Triplex DNA

• no evidence for their existence in vivo
• do block transcription in vitro
• frataxin transcript levels reduced in patients.

Question 22

Where is frataxin highly expressed?

Answer 22

Neurons
Cardiomyocytes
Pancreas
Skeletal muscle

Question 23

What is frataxin?

Answer 23

A mitochondrial membrane protein.

Question 24

What is frataxin involved in?

Answer 24

Intracellular iron homeostasis
Iron accumulates in mitochondria
Cells are susceptible to free radical change.

Question 25

Have any animal models been made to study FXN?

Answer 25

Yes, mouse models using cre-lox. | Splice out fourth exon.

Question 26

What is the phenotype of fxn mouse models?

Answer 26

``` 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 ```

Question 27

What is the clinical presentation of fragile X syndrome?

Answer 27

``` Mental retardation Long faces Prominent ears Autistic like behaviour - no neurodegeneration - aberrant dendritic structures - excess dendrites (no pruning) ```

Question 28

What can be seen on a karyotype of a patient with fragile X syndrome?

Answer 28

Observable fragile site on the long arm of the X chromosome. Only visible in specific in vitro tissue culture conditions.

Question 29

What is the incidence of fragile X?

Answer 29

1/1,500

Question 30

Are males or females with fragile X more severely affected?

Answer 30

Males

Question 31

What is the inheritance pattern of fragile X?

Answer 31

X-linked

Question 32

What is unusual about the inheritance of fragile X?

Answer 32

Men with mutant chromosome don't always have the phenotype. Penetrance increases through generations. Severity is always the same. Due to triplet repeat.

Question 33

Which triplet is repeated in fragile X?

Answer 33

CGG

Question 34

How many CGG repeats are found in the general population and in fragile X?

Answer 34

General population 5-45 Affected patients 230-2000 Normal transmitting male 50-200

Question 35

What does the CGG repeat cause?

Answer 35

Hypermethylation (CpG island) resulting in transcriptional repression. Therefore no FRM1 mRNA produced. no FMRP produced in males

Question 36

Do normally transmitting males have any symptoms?

Answer 36

Can be prone to late onset tremor/ataxia.

Question 37

How does repeat length change in fragile X if it's repeated by males vs females?

Answer 37

Repeat increases down the generations if transmitted by females. Not so much if transmitted by males.

Question 38

In which gene is the CGG expansion in fragile X?

Answer 38

FMR1

Question 39

Which region of FMR1 is the CGG expansion found?

Answer 39

5' untranslated region.

Question 40

What is FMRP?

Answer 40

Translational repressor Associated with transcripts that are spatially translated. In neurons translation often only occurs in end of synapses

Question 41

Which mRNAs are affected by fragile X?

Answer 41

SEC7 SAPAP4, DAP-1, kinate receptor GRMs - metabolic glutamate receptors involved in long term potentiation.