L6 Repeat Expansion Disorders

Question 1

What are repeat expansion disorders?

Answer 1

REDs are diseases that are caused by the expansion of repetitive sequences of DNA in genes.

Question 2

What were the diseases that were originally discovered to be REDs?

Answer 2

These diseases are triplet repeat/trinucleotide repeat expansions.

• FXS (Fragile X Syndromes).
• SBMA (spinal and bulbar muscular atrophy).
• DM (myotonic dystrophy).
• Huntinton’s disease.

Question 3

How are most REDs thought to work?

Answer 3

Through a gain-of-function mechanism. The repeat causes the protein of RNA produced by that gene to obtain disease-causing properties. The toxicity can arise at either the RNA or the protein level.

There are, however, some loss-of-function diseases.

Question 4

What is FXS?

Answer 4

Fragile X Syndrome.

This is an X-linked condition that is fully penetrant (people that carry the mutation will show the phenotype) and only 50% penetrant in females.

FXS results in a long face, protruding ears, low muscle tone and macroochidism (abnormally large testicles). There are also a range of learning disabilities.

Question 5

What is FXTAS?

Answer 5

Fragile X-Associated Tremor/Ataxia Syndrome.

This is an adult-onset condition that is caused by shorted expansions in the same gene that causes FXS.

Question 6

What is FXS caused by?

Answer 6

A loss of FMRP (fragile X mental retardation protein).

FMRP is an RNA-binding protein associated with polyribosomes and mRNA granules. It functions as a translational repressor until the protein is required.

This process is important in LTP, learning and memory.

Question 7

What is FXTAS caused by?

Answer 7

55-200 CGG repeats in FMR1.

Question 8

What are the pathological symptoms of FXTAS?

Answer 8

• White matter lesions in the middle cerebellar peduncle.

- Ubiquitinated inclusions (protein aggregates).

Question 9

What are polyglutamine disorders? Give examples.

Answer 9

CAG triplet repeat expansion disorders.

• Spinobulbar muscular atrophy (SBMA).
• HD
• DRPLA (Dentatorubral-pallidoluysian atrophy; Japanese HD).
• Spinocerebellar ataxia (SCA) types 1, 2, 3, 6, 7, and 17.

Question 10

What are the common features of polyglutamine diseases?

Answer 10

• Autosomal dominant inheritance (with the exception of SBMA which is X-linked recessive).
• Genetic anticipation i.e. age of onset gets progressively earlier over successive generations. This is associated with paternal transmission.
• Inverse relationship between the CAG repeat lengths and age of onset.
• Works by a toxic ‘gain of function’ mechanism.
• Causes protein accumulation in nucleus and aggregation.

Question 11

Describe the pathology of HD.

Answer 11

Severe atrophy of the striatum, cortical atrophy, reduction in brain size and thickness of the cortex.

Areas that have lost neurons show astrogliosis (abnormal increase in the number of astrocytes due to the destruction of nearby neurons).

Question 12

Describe the different categories of HD with regards to repeat lengths.

Answer 12

• 27-35 repeats long means you yourself are not likely to develop the disease yourself, but your offspring are.
• 36-39 repeats means you have the allele but may never develop HD, there is not 100% penetrance.
• 40 or more repeats means that the person will develop the disease with all the symptoms.

Question 13

Describe protein aggregation in HD.

Answer 13

Protein aggregates are not restricted to areas that are affected by HD i.e. there is no direct correlation between aggregation and pathology.

Furthermore, only the N-terminus of the Huntington protein appears to be in these aggregates.

Question 14

What is protein sequestration?

Answer 14

The selective interaction of a protein with specific molecules in the cytoplasm. This interaction inhibits its transport into other areas of the cell.

Question 15

What proteins form polyglutamine aggregates and how?

Answer 15

Proteins that have long non-pathological polyglutamine tracts can be sequestered into mutant polyglutamine aggregates.

Question 16

What is CRB and how does it regulate gene transcription?

Answer 16

CREB-Binding Protein.

It regulates gene transcription by possessing histone acetyltransferase (HAT) activity.

HATs open up chromatin making DNA more accessible to transcription factors.

Question 17

What is happening to gene transcription in HD and how can this be treated?

Answer 17

There are very low levels of CRB in Huntington’s brains. This means that there is a decreased histone acetylation and a global dysregulation of transcription.

One way to treat this is by using HDAC (histone deacetylase) inhibitors. These drugs are in early stage clinical trials.

Question 18

What is SBMA?

Answer 18

Spinal and Bulbar Muscular Atrophy.

This is an adult-onset neuromuscular disease that is characterised by muscle weakness and wasting especially in the limbs, face and throat. This causes speech and swallowing difficulties and muscle cramps.

Question 19

What is the pathology of SBMA?

Answer 19

The degeneration of spinal and bulbar neurons.

Question 20

What is SBMA caused by?

Answer 20

CAG repeat expansions in the androgen receptor. Mutations are X-linked recessive.

Question 21

What did SBMA studies in transgenic mice show?

Answer 21

Male mice were getting sick a lot earlier than females. They also had a large amount of protein aggregates in their cell nuclei.

When the male mice were castrated (an attempt to decrease the amount of testosterone) they saw much less sickness and protein aggregation.

When exogenous testosterone was given to female mice, it caused the nuclear accumulation that was found in the males.

Question 22

What is the obligate step in the pathogenesis of SBMA?

Answer 22

Testosterone drives the nuclear translocation of the androgen receptor. Once the mutant androgen receptor is in the nucleus it starts to accumulate and aggregate.

Question 23

What are examples of neurodegenerative and neuromuscular disorders that are caused by non-coding repeat expansions?

Answer 23

• Myotonic dystrophy.
• FXTAS.
• Spinocerebellar ataxia 8, 10, 12, 31 &36.
• ALS (this is the most common genetic cause of motor neuron disease that we know of).

Expansions that are causing these diseases tend to be longer than coding repeat expansions.

Question 24

What is myotonic dystrophy?

Answer 24

This is an autosomal dominant neuromuscular disease. It is multisystemic affecting muscle, heart, cognitive function, behaviour, eyes and causes endocrine symptoms.

Question 25

What are the two different types of DM?

Answer 25

1) DM1 CTG expansion in 3’-UTR of DMPK (dystrophia myotonica protein kinase).
2) DM2 CTG expansion in 1st intron of CNBP/ZNF9 (zinc finger domain).

DM1 is more common than DM2 and shows genetic anticipation mainly in maternal transmission.

Question 26

What are the three methods of non-coding repeat toxicity?

Answer 26

• RNA foci/protein sequestration. These aggregates are not as stable and insoluble as protein aggregates, but there are suggestions that they can sequester various proteins and interfere with their functions.
• Repeat-associated non-AUG translation (RANT). This is where proteins are translated without the start codon AUG.
• Haploinsufficiency.

Question 27

What is one way in which MD is potentially working?

Answer 27

RNA foci sequester Muscleblind-like proteins (MBNL1-3) and CUG-binding protein/Elav-like family member 1 (CELF1).

When MBNL is sequestered it is unable to carry out its normal function. When CELF1 is sequestered, it becomes phosphorylated and stabilised which leads to upregulation.

Question 28

GGGGCC repeats are subjected to what? Where are they found?

Answer 28

RANT (they can be translated without a start codon AUG).

These repeats are found in cases of ALS/FTD that are caused by C9ORF72. d

Question 29

What is unusual about C9ORF72 cases of ALS?

Answer 29

One of the key pathological features of ALS is ubiquinitated inclusions that you can detect using anti-p62 antibodies. In ALS caused by C9ORF72, you see the inclusions in the cerebellum which is unique to this form of ALS.

Question 30

What happens if RAN translation occurs in a different reading frame? Where does this occur?

Answer 30

The proteins that are translated are a completely foreign string of amino acids.

This occurs in HD.