L44. Genetics of Neurological Disorders

Question 1

What are the two types of neurodegenerative conditions that show motor signs changes

Answer 1

1. Acquired (cerebrovascular disease, tumours, alcoholism, vitamin deficiencies, etc)
2. Inherited: Subset are caused by a particular type of genetic mechanism: unstable repeat expansions

Question 2

A class of genetic conditions are caused by unstable repeat expansions.

What is meant by an “unstable repeat expansion”?

Answer 2

It is a kind of mutation where trinucleotide (or more) repeats exceed the normal threshold of normal.

They are characterised by the resultant expansion of a segment of DNA in this specific gene

Question 3

Is it normal to have repeat regions in the gene?

Answer 3

There are naturally occurring repeat regions present in ‘normal’ gene.

A specific number of repeat units make up this repeat region and each repeat is specific for each gene/condition

Question 4

What is a stable repeat segment?

Answer 4

When the repeat segment is below the theshold of normal range for that gene

Question 5

If there is repeat expansion in a sperm or egg cell that is below the threshold what is the implication…

• For the individual?
• For the offspring?

Answer 5

• The individual will not have the condition
• The offspring will most likely

Question 6

What is an unstable repeat segment?

Answer 6

When the repeating unit is above the threshold in the sperm/egg ± spomatic cells

Question 7

What is another term for an unstable repeat expansion?

Answer 7

Dynamic mutations

Question 8

What is meant by anticipation?

Answer 8

When the expansion size increases in following generations: the number of repeats increases with each generation

Question 9

What is the relationship between anticipation and onset and severity of the condition?

Answer 9

It is usually associated with earlier onset and/or greater severity of symptoms

The more number of repeats, generally the earlier and the worse

Question 10

What is the hypothesised mechanism of this repeat expansion?

Answer 10

Slipped mispairing is the current hypothesis:

1. The template strand of genomic DNA undergoes normal replication (meiosis)
2. During the replication processes the replicating strand dettaches inappropriately from the template
3. The replicating strand then slips from its proper alignment with the templant strand by one repeat length
4. This causes the process of replication to incorporate an extra repeat in the new strand

Question 11

Why is there a theshold of the repeating units and what sets it?

Answer 11

We don’t know

There is a different threshold that is unique to each gene so we don’t yet understand what defines this

Question 12

Complete the following statements…

• Unstable repeat expansion disorders are primarily __________
• _____ repeats are the most common types of repeats
• The have _________ inheritence patterns

Answer 12

• Unstable repeat expansion disorders are primarily neurological
• Triplet repeats are the most common types of repeats
• The have variable inheritence patterns

Question 13

What the four main trinucleotide repeat disorders?

Answer 13

1. Huntington Disease
2. Fragile X syndrome
3. Myotonic dystrophy
4. Friedreich ataxia

Question 14

What does the gene expression, protein formation and type of protein involved depend on?

Answer 14

Where in the gene a the unstable repeat expansion occurs

Question 15

What are the classes of unstable repeat expansions?

Answer 15

Class 1: Non-coding repeats causing a loss of function due to impaired transcription of the affected gene

Class 2: Non-coding repeats that confer novel properties on the RNA (a toxic gain of function)

Class 3: Repeats in a codon that confer novel properties on the affected protein leading to production of modified proteins that override normal functioning

Question 16

Give two examples of a class 1 (non-coding repeats leading to loss of function) unstable repeat expansion.

Answer 16

1. Fragile X syndrome
2. Friedreich ataxia

They occur in the same gene, but the number of different repeats causes different syndromes

Question 17

Give two examples of a class 2 (non-coding repeats leading to novel RNA properties) unstable repeat expansion.

Answer 17

1. Myotonic dystrophy
2. Fragile X associated tremor ataxia (FXTAS)

Question 18

Give two examples of a class 3 (coding repeats leading to impacted protein) unstable repeat expansion.

Answer 18

1. Huntington disease
2. Some spinocerebellar ataxias (SCAs)

Question 19

Characteristics of neurogenerative disorders:

• Characterised by a _________________
• ________ and _______ clinical presentation
• Mainly ______ onset disorders
• Symptoms are __________ over time

Answer 19

• Characterised by a loss of movement control
• Variable and overlapping clinical presentations
• Mainly late onset disorders
• Symptoms are progressive (over time)

Question 20

What is Huntington disease?

Answer 20

A neurological condition caused by the death of brain cells in certain areas of the brain results in gradual loss of cognitive (thinking), physical and emotional function.

Question 21

Describe Huntington Disease in terms of …

• Inheritance Pattern
• Prevalance
• Age of Onset
• Main features [3]

Answer 21

• Autosomal dominant
• Around 1 in 10,000 - 20, 000
• Late onset (35-44 year olds; 5-10% are diagnosed <20 yrs)
• Movement/motor disorders (chorea), cognitive disorder, psychiatric/emotional disorder

Question 22

If one parent has Huntington disease, what is the chance of the offspring inheriting it?

Answer 22

Because it is autosomal dominant, there is a 1 in 2 chance (50%) of developing Huntingon disease

Question 23

What is the time from from diagnosis (clear onset of symptoms) to death for Hunginton Disease?

Answer 23

As a progressive disease, the time frame is roughly 10-15 years

Question 24

What are the early features of Huntington disease?

Answer 24

Clumsiness, agitation, irritability, apathy, anxiety, disinhibition, delusions/hallucinations, abnormal eye movements, depression

Question 25

What are the middle features of Huntington disease?

Answer 25

Involuntary movements, chorea, trouble with balance and walking, trouble with activities that require manual dexterity, slow voluntary movements, general weakness, weight loss, speech difficulties (dysarthria), stubbornness

Question 26

What are the late clinical signs of Huntington Disease?

Answer 26

Rigidity, bradykinesia (difficulty initiating and continuing movements), serious weight loss, inability to walk, inability to speak, danger of choking on food, inability to care for oneself Almost catatonic

Question 27

Is there a way of predicting the development of Huntington's disease before onset of symptoms?

Answer 27

There are very subtle prodromal signs such as cognitive and motor behaviour and brain imaging but they are not large enough to be used as a screening

Question 28

What is the genetic basis of Huntington Disease?

Answer 28

* **CAG** repeat expansion * In the **HTT** (HD) gene on **chromosome 4** * The gene spans 180kb with 67 exons * The repeat is in the **coding region** in exon 1 * Since CAG encodes glutamine = it is a polyclutamine problem

Question 29

What protein is affected in Huntington Disease? What is the normal role of it?

Answer 29

Huntingtin HTT protein * Involved in Transcription * Intracellular transport of other molecules * Intracellular signalling and metabolism * Reducing apoptosis

Question 30

The expanded CAG huntingtin product (polyQ-huntingtin) is toxic to the basal ganglia. What cell type is it particularly dangerous to?

Answer 30

Medium spiny neurons = progressive degeneration and loss of medium spiny neurons in the striatum of the basal ganglia But also widespread atrophy in the brain is caused eventually

Question 31

Describe the molecular pathology of Huntington Disease

Answer 31

1. PolyQ-huntingtin is cleaved by caspases and other enzymes genertaing N-terminal fragments with altered conformation (**these are toxic)** 2. They form aggregates and nuclear inclusions 3. The lost function of the normal HTT and the possible toxicity of the new mRNA might also contribute to pathogenicity

Question 32

What is the effect of the extended polyglutamine tract on the huntingtin gene in the cell? (the medium spiny neuron of the basal ganglia)

Answer 32

* Excitotoxicity * Oxidative stress * Impaired energy metabolism * Apoptosis * Altered interaction with proteins * Impaired protein clearance

Question 33

What is the spectrum of CAG repeat units and the severity and penetrance of HD? ie. the genotype/ phenotype correlation

Answer 33

**The number of repeats determines penetrance** ## Footnote Normal ≤ 26 rpts ‘Normal’, mutable (intermediate size) - paternal transmission 27-35 rpts Zone of reduced penetrance 36-39 rpts Affected (completely penetrant) ≥ 40 rpts (100% development of HD)

Question 34

CCG interruptions are often involved in the CAG repeat segments. What is the impact of this?

Answer 34

The CCG interuptions mitigate (dampen) effects of the CAG repeats (they are a good thing in this case)

Question 35

Describe the classical genetic testing of HD

Answer 35

PCR (radioactive nucelotide ‘tagging’), gel electrophoresis and autoradiography Because there is a use of enzyme in PCR there is a high risk of getting errors introduced when looking for small differences in repeat numbers. The ladder pattern where no single band is seen: they took the **darkest two bands as the positive**

Question 36

What is the current means of testing for HD?

Answer 36

PCR using *flourescent nucleotide 'tagging'* and **fragment analysis** on capillary electrophoresis and flourescence detection

Question 37

What is spinocerebellar ataxia?

Answer 37

A progressive, degenerative, genetic disease with multiple types (\>35 now described, 10 of which are due to unstable repeat expansions). Degeneration of the cerebellum is concerned with damaged coordination of movements and can bring on spasticity.

Question 38

Describe spinocerebellar ataxia in terms of... * Inheritance pattern * Prevalence * Age of onset * Main features

Answer 38

* Autosomal dominant * The frequency of the different types varies in different populations * Late onset (generally) * Progressive degeneration of cerebellum, brainstem and spinocerebellar tracts causing problems with gait, hand coordination, speech and eye movements

Question 39

What is the genetic basis of SCA?

Answer 39

CAG repeats occuring in a number of different genes (causing the different types)

Question 40

How is SCA 6 different to the others?

Answer 40

SCA6 is different because it codes a gene that encodes a subunit of a calcium channel. Abnormal repeats has a dominant negative effect and impacts on the normal channel subunits around it. SCA6 doesn’t have anticipation compared to the others

Question 41

What is Friedreich ataxia?

Answer 41

An nherited condition that causes progressive damage to the nervous system resulting in muscle weakness, loss of balance and coordination (ataxia), speech problems and heart disease.

Question 42

Describe Friedreich Ataxia in terms of: * Genetics * Prevalence * Carrier Frequency * Age of Onset * Main Features

Answer 42

* Autosomal recessive * 2-4 in 100,000 * 1/60 and 1/100 in Indo-europeans * Usually around puberty (10-15yrs) * Progressive limb and gait ataxia, cardiomyopathy in 65% (most common cause of death)

Question 43

Describe the genetic basis of Friedrich Ataxia

Answer 43

* **GAA** repeat expansion * in the **FXN** gene found on **chromosome 9** * The repeat is located in an intron (non-coding) leading to an abnormal secondary structure and reduced protein production

Question 44

What is the impact of the repeat GAA expansion in to the protein being coded by the affected intron? Protein is called **Frataxin**

Answer 44

It **doesn't change the protein** but it **swtiches off gene transcription** (changed secondary structure of RNA - hairpin loops that trigger epigenetic silencing) The defect in this protein causes mitochondrial iron accumulation leading to **oxidative damage**

Question 45

What is the spectrum of GAA repeat units and the severity and penetrance of Friedrich Ataxia? ie. the genotype/ phenotype correlation

Answer 45

Normal range: 5 to 33 repeats \* Affected range: 66 to 1700 repeats 34 - 65 (uninterrupted) repeats are regarded as premutation (mutable) range [Can expand more than 10-fold] \* Alleles longer than 27 GAA triplet repeats are often interrupted by a (GAGGAA)n sequence – stabilises alleles?

Question 46

Because the difference between normal and affected is relatively large in the affected range, what does this mean for diagnosing tools?

Answer 46

We can use normal PCR very easily

Question 47

What are some ethical issues in predictive/pre-symptomatic genetic testing [6]

Answer 47

* Different implications in laboratory diagnosis of genetic disease * Implications for not only patient, but also other family members * Formal consent, genetic counselling and confidentiality issues, especially with predictive testing (restricted access to results) * Insurance (life, disability) implications (NB family history) * Predictive testing in children not usually encouraged (NB ‘mature minors’) * Prenatal testing, including pre-implantation genetic diagnosis (PGD) possible – need to know mutation/haplotype in parent