18.03.15 Triplet repeat disorders - GoF

Question 1

How are repeat expansions predicted to arise?

Answer 1

Strand slippage during DNA replication via DNA looping in actively dividing cells and/or transcription-mediated DNA repair pathways (where repeat sequences confuse transcription activating DNA repair mechanis, errors of which may result in expansion).

Question 2

What is suggested to cause the variation in triplet-repeat disorder severity during the course of disease in an individual or between generations?

Answer 2

Expansion in somatic cells.

Question 3

How is genetic anticipation defined?

Answer 3

Increasing severity and earlier onset of a disease as it is inherited through consecutive generations.

Question 4

What are the two groups of triplet repeat disorder/gain of function disorders?

Answer 4

Group I: Polyglutamine (polQ) disorder caused by GoF derived from the expansion of translated repeats (Altered protein function)

Group II: DIsorders caused by gain of function derived from the expansion or non-translated repeats (altered RA function)

Question 5

Give examples of group I TNR GoF disorders.

Answer 5

1. HD
2. SBMA
3. DRPLA
4. SCA (1,2,3,7,17)

Question 6

Give examples of group II TNR GoF disorders.

Answer 6

DM1 and 2
SCA8
SCA12

Question 7

What are the common properties of the polyQ diseases?

Answer 7

1. Mostly adult-onset. Some juvenile cases observed, especially when transmitted from the father.
2. The disease course is progressive, unremitting and usually fatal 10-30 years after onset
3. The clinical symptoms appear above a threshold of CAG repeats
4. Strong negative correlation between CAG repeat number and age of onset
5. Repeat sequence is unstable and its increase in size during transmission results in genetic anticipation (except SCA6)
6. Gene is ubiquitously expressed
7. Pathological protein accumulated in ubiquitinated neuronal intranuclear inclusions in several affected but also in non-affected brain structures.

Question 8

What is the incidence and the three main phenotypic components of Huntington disease?

Answer 8

3-10 in 100,000 in Western European populations.
1. Psychiatric disturbances (mood swings, personality changes, depression, paranoia)

2. Motor (chorea, dystonia, bradykinesia)
3. Cognitive (progressive cognitive decline).

Question 9

What are the different HD allele classes and their associated repeat ranges?

Answer 9

Normal <27
Mutable normal 27-35
Reduced penetrance 36-39
Full penetrance >40
Juvenile >60

Question 10

What is the average age of onset for HD?

Answer 10

40

5-10% have juvenile form (before 20)
25% present >50

Question 11

From which allele do large expansions predominantly occur?

Answer 11

Paternal; attributed to increased number of meiotic divisions in spermatogenesis.

Question 12

Describe exclusion testing

Answer 12

Used in cases where an individual with a 25% risk of developing HD requests predictive testing but the parent, who has a 50% risk, does not want to be informed about his or her risk.

Exclusion test can be offered using haplotype analysis. Almost exclusive in prenatal tesiting; pre-implantation test available.

Question 13

Describe the spinocerebellar ataxia.

Answer 13

AD group of neurodegenerative disease that affect the cerebellum and its main connections characterised by a generalised in-coordination of gait, speech and limb movements.

Usually adult-onset and shows clinical heterogeneity.

Question 14

What disease may show extreme anticipation?

Answer 14

SCA7; children with early-onset, severe disease die of disease complications long before the affected parent or grandparent is symptomatic.

Question 15

What is the prevalence of the AD cerebellar ataxias (ADCAs)?

Answer 15

1-3/100,000
SCA1 (6%)
SCA2 (13%)
SCA3 (23%)

Question 16

Describe Dentatorubral pallidoluysian atrophy (DRPLA)

Answer 16

Autosomal dominant progressive disorder

Ataxia, choreoathetosis, and dementia or character changes in adults >20 years

Ataxia, myoclonus, epilepsy, and progressive intellectual deterioration in children <20 years

Clinical presentation of myoclonus (spasmodic jerky contraction of groups of muscles) and epilepsy is strongly correlated with the size of expanded CAG repeats

The age of onset is from one to 72 years with a mean age of onset of 31.5 years.

Anticipation is prominent – affected offspring typically have symptoms 26 to 29 years earlier than fathers and 14 to 15 years earlier than affected mothers

A dosage effect is observed: homozygotes more severely affected than heterozygotes for the same repeat size

Question 17

In which populations is DRPLA more commonly observed?

Answer 17

DRPLA occurs predominantly in the Japanese, although Europeans, North Americans and African-Americans have all been reported with molecularly confirmed DRPLA. Prevalence: 1 in 200,000 in the Japanese.

Question 18

Describe the inheritance pattern of SBMA.

Answer 18

X-linked recessive disorder and occurs only in males. Females protected by having low levels of circulating androgens leading to lower levels of androgen receptor stimulation

Question 19

What are the clinical features of SBMA?

Answer 19

Main features are proximal muscle weakness, muscle atrophy and fasciculations

Additional features include gynecomastia, testicular atrophy, and reduced fertility as a result of mild androgen insensitivity

Symptoms of androgen insensitivity typically begin in adolescence

Neurologic symptoms typically begin between age 30 and 50 years

Question 20

What is the prevalence of SBMA?

Answer 20

Prevalence: ~1 in 300,000 males in Europeans & Asians.

Question 21

What evidence is there that the repeat expansion mutations produces a gain of function response? Give three.

Answer 21

1. No other mutations found in the gene.
2. The expanded allele is transcribed and translated.
3. A critical repeat threshold below which the repeat is non-pathogenic and above which causes disease.
4. Anticipation often seen - earlier onset with larger repeat sizes.
5. Late onset disorders normally dominantly inherited (an exception would be SBMA which is X-linked recessive).
6. HD homozygotes are clinically identical to heterozygotes
7. Patients with chromosomal deletions including the HD gene fail to manifest HD
8. CAG repeats introduced to housekeeping genes in mice result in neuropathology, but inactivation has no effect
9. Ataxin-1 null mice are viable, but CAG expanded ataxin-1 induces neurodegeneration

Question 22

Describe aggregation theory

Answer 22

1. Build-up of toxic products produced by expanded allele
2. effect of these products is particularly marked in neurons (cells which are long-lived and not replaced when lost).
3. Protein aggregates (inclusion bodies) containing expanded polyQ found in the nuclei of neurons from SCA3 and HD patients.
4. Aggregates of the mutant protein play a primary role in pathogenesis which fits with aggregation increasing with increased length, paralleling the genotype-phenotype relationship seen in HD (more inclusion bodies in brains of juvenile onset HD cases with large CAG repeat expansions).

Question 23

Which hypotheses have been proposed as a cause of the neurodegenerative phenotype of poly-Q disorders?

Answer 23

1. Aggregation theory
2. Toxic fragment hypothesis
3. Transcription dysregulation hypothesis

Question 24

Describe the toxic fragment hypothesis.

Answer 24

Abnormal htt with a toxic amino terminal fragment containing the polyQ stretch is a substrate for proteolytic cleavage by caspases and calpains

Amino terminal fragment expressed in mice is sufficient to produce HD like symptoms and a polyQ expanded HD protein with a caspase-6 cleavage site mutation (ie caspase resistant) is incapable of causing neurotoxicity.

Question 25

Describe the transcription dysregulation hypothesis

Answer 25

polyQ-expanded proteins accumulate in the nucleus inappropriately and interact with transcription factors/regulators to disrupt normal transcriptional functions, e.g. interference of CREB-binding protein (CBP) mediated transcription by huntingtin in a polyQ length-dependent fashion.

Question 26

What is some of the evidence against aggregation theory being a cause of neurodegeneration?

Answer 26

1, neurodegeneration has onset prior to aggregate formation in SCA1 mice and some mouse HD models and human brains exhibit neurodegeneration without aggregates.

2. In one study, neurons lacking visible inclusions had a higher risk of cell death, suggesting that visible aggregate formation can be protective.
3. Aggregates may not be toxic per se, but may signify the presence of misfolded proteins whose toxic action is actually occurring in the soluble phase and/or at the level of microaggregates.

Question 27

In HD, what are the two mechanisms for the axonal trafficking dysfunction?

Answer 27

Axons employ a microtubule dependent transport system powered by kinesin and dynein family members
Disruption of axonal transport leads to aggregation of accumulated cargo, resulting in neuronal dysfunction and degeneration.

1) PolyQ expansion of htt can disrupt binding of the HAP1/p150Glued complex to microtubules and motor complexes and thereby can depress BDNF axonal transport and release.
2) physical disruption of microtubule-dependent movement of cargo secondary to cytosolic aggregation of mutant htt

Question 28

Give a summary of the pathogenicity mechanisms in polyQ disorders.

Answer 28

1. The mutant protein is proteolytically processed by endogenous enzymes forming toxic fragments that form aggregates in the cytoplasm.
2. Toxic break down products may translocate into nucleus and generate nuclear aggregates by forming intermediate species and sequestering further vital proteins.
3. Accumulation of polyQ species can damage important cellular components and lead to mitochondrial dysfunction

Question 29

What are the therapeutic approaches for poly-Q disorders?

Answer 29

1. Decrease toxic fragments formation
2. Alter translocation
3. Modify aggregation
4. Improve correct protein folding
5. Increase proteosomal degradation
6. Enhance lysosomal protein degradation
7. Facilitate autophagic aggregate depletion
8. Rescue mitochondrial function