Tandem repeat expansion disorders Flashcards
Outline the two main types of repetitive DNA sequences and their subtypes.
Tandem repeats
- Satellite DNA - 5-171bp, heterochromatin
- Minisatellites - 6-64bp, telomeres
- Microsatellites - 1-6bp, euchromatin
Interspersed repeats
- Alu family - 280bp, giemsa +ve, euchromatin
- Kpn (L1) family - 1300bp, giemsa -ve, euchromatin
Describe the 3 types of repeat expansions, and in each case state the effect on transcription and translation.
- Modest expansions within transcribed & translated regions - contribute to final protein
- Expansions in UTR - transcribed into RNA but not translated into protein
- Large scale expansions within untranscribed regions - do not appear in RNA/final protein
Describe the 4 types of DNA replication errors which could cause microsatellite expansion.
- Imperfect Okazaki fragment ligation
- Polymerase slipping
- Fork reversals
- Template switching
Outline the genetic features and mode of inheritance of Huntington’s Disease.
- Mutated HTT gene = IT15
- Chromosome 4p16.3
- Encodes Huntingtin
- Expansion of trinucleotide CAG repeat in exon 1
- Autosomal dominant
- Anticipation but CAG repeat size does not equal severity
- Longer repeat length associated with earlier onset but still variability
Describe the neuropathology and clinical features of Huntington’s Disease.
- Loss of GABAergic and cholinergic neurons in striatum (caudate nucleus and putamen) and globus pallidus
- Chorea - twitching, uncontrolled movements, spasms - earliest motor feature
- Bradykinesia and incoordination - disabling
- Attencion deficit
- Spatial disorder
- Memory disorder
- Disorganised speech
- Changes in personality and emotion
Describe the relationship between (CAG)n and Huntington’s phenotype.
- CAG repeats >39 = Huntington’s
- 36-39 repeats = may or may not have disease
- 27-35 repeats = normal but can expand
- 11-26 repeats = normal range
Explain the influence of de novo mutations on Hungtington’s.
- High de novo mutation rate
- 24% = no family history
- >10% of repeats in 27-35 range expand into disease range in each generation - late onset cases under ascertained or misdiagnosed
- Somatic expansions - unknown influence on disease
Outline the normal functions of huntingtin protein.
- Appears crucial for embryonic development and neurogenesis
- Loss of huntingtin function does not lead to HD
- Lethal in knockout mice embroys
- Expressed throughout brain
- Located in pre- and post-synaptic regions of axons and dendrites
- Mainly cytoplasmic but can be nuclear
- Associates with microtubules, vesicles and organelles
Explain the key features of Huntington’s Disease pathogenesis, with regard to HTT.
- Mutant HTT forms abnormal conformations
- Systems for handling abnormal proteins imapired in HD
- Cellular metabolic pathways impaired in HD
- HTT truncated > toxic N-terminal fragments
- Post-translational modifications of HTT influence toxicity
- Nuclear translocation of mutant HTT enhances toxicity
Describe the 3 main pathogenic pathways thought to be involved in Huntington’s Disease.
Polyglutamine toxic fragment
- Huntingtin cleaved by caspase-3 and polyQ fragment enters nucleus
- Fragment aggregates and interferes with other transcription factors
Axonal transport
- PolyQ expansion interferes with the interaction of Huntingtin associated protein HAP1 and p150 - a component of the dynein motor complex
- This results in reduced axonal transport
Mitochondrial malfunction
- PolyQ fragment may cause mitochondrial Ca2+ defects
- Expanded CAGs cause reduced mitochondrial ATP output
Outline the function of brain-derived neurotrophic factor (BDNF) and explain how this is thought to be affected in Huntington’s Disease.
- BDNF promotes growth, maturation and differentiation of neurons
- Active at synapses - helps to regulate synaptic plasticity
- Mutant HTT interferes with correct function of BDNF
- PolyQ fragment may interfere with BDNF transcription
Outline the clinical features of myotonic dystrophy (dystrophia myotonica, DM). What is the mode of inheritance?
- Subset of inherited muscular dystrophies
- Heterogeneous
- Affects children and adults
- Multisystem disease
- Core features = myotonia, muscular dystrophy, cardiac conduction defects, cataracts, endocrine disorders
- Autosomal dominant
Describe the main clinical manifestations of mytotonic dystrophy, including facial features.
- Myotonia and progressive myopathy
- Cardiac conduction defects, arrhythmias, cardiomyopathy
- Insulin resistance, glucose intolerance
- Testicular atrophy
- Cranial hyperostosis myotonia (irritability and prolonged contraction of muscles)
- Facial features - ‘hatchet face’, frontal balding, temporal wasting, drooping eyelids (ptosis), drooping mouth
Name the 4 clinical groups of DM1.
- Congenital - 100% from mother
- Childhood onset
- Adult onset “classic DM1”
- Late-onset/asymptomatic
Describe the tandem repeat expansion responsible for DM type 1. Explain the clinical significance of the number of repeats.
- CTG trinucleotide repeat in 3’ UTR of of DMPK (DM protein kinase) gene
- Normal (CTG)n = 5-37
- “Premutation” asymptomatic n = 38-49
- DM1 phenotype n = 50-4000
- Correlation between repeat number and severity/age of onset
- Highest number of repeats in congenital form
- Females with >500 copies preferentially transmit massive expansions
- Males with >500 copies preferentially transmit contractions
- Anticipation, somatic mosaicism
Outline the histopathological changes seen in DM type 1. What features are revealed by muscle biopsy?
- Early - myopathic changes, variable fibre size and internalised nuclei
- Late - numerous internalised nuclei, often in longitudinal chains
- Muscle biopsy reveals:
- Pyknotic nuclear clumps
- Abnormal muscle fibre types
- Predominantly type 1 fibre atrophy
Outline the proposed disease mechanisms in DM type 1 and the supporting evidence.
- Haploinsufficiency of DMPK - expansion at 3’ end may alter expression levels and protein amount. However, DMPK knockout mice do not have DM and no DMPK mutations are known to cause DM.
- Haploinsufficiency of nearby gene - CTG expansion may affect SIX5 promoter, altering chromatin structure and potentially affecting expression of multiple genes. However, studies have not found expansion to alter transcript levels of any genes.
- Nuclear RNA retention - ‘Gain of function toxic RNA’ mechanism acting in nucleus:
- CUG repeats form double-stranded hairpin structures, causing nuclear trapping.
- Expanded RNA accumulates in nucleus and sequesters splicing regulators, disrupting normal splicing of many genes > multisystem defects.
- Disruption of CLC-1 expression = myotonia.
- Disruption of IR expression = insulin resistance.
- Disruption of TNNT3 (cardiac troponin T) = cardiac problems.
Outline the clinical features of DM type 2 and the differences with DM1.
- Similar myotonia, weakness, cataracts, frontal balding, diabetes
- Pain and sleep disorders are common
- Age of onset 8-60 years
- No congenital form
- Prognosis more favourable than DM1 - cardiac involvement less common
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Describe the genetic mechanism and pathology of DM type 2.
- CNBP gene (zinc finger binding protein), 3q21.3
- Huge expansion of (CCTG)n in intron 1
- Healthy n < 24
- DM2 n = 75-11,000, avg ~ 5000
- Size does not correlate with severity/age of onset
Outline the pathological differences and similarities between DM1 and DM2.
- DMPK vs CNBP gene mutation
- CTG vs CCTG repeat expansion
- May be congenital vs no congenital form
- Rough correlation of repeat size and onset vs no correlation
- Distal>proximal weakness vs proximal>distal weakness
- Histopathology with type 1 fibre atrophy vs type 2 fibre atrophy
- Both toxic RNA diseases
