Lecture 3 Flashcards
Repeat expansion Disorders:
- Diseases caused by expansion of repetitive DNA sequences
- Initially triplet repeat/trinucleotide repeat expansions (FXS, SBMA, DM, HD etc.)
- Penta- (SCA10 & SCA31), hexa- (SCA36 & ALS/FTD) & more complex (EPM1) pathological expansions exist
Repeat expansion disorders are…
Most autosomal dominant and GOF- \Look at diarma
What are the two main types of effect of mutations ( at what level) ?
Protein or RNA
Disease causing expansion
can occur throughout gene
Fragile X syndrome –
• X-linked condition
- fully penetrant males
- ~50% penetrance females
• Long face, protruding ears, low muscle tone, macroorchidism(large testicles)
• Learning disabilities – normal IQ to severe mental retardation
• Cytogenetic abnormality 1969
• FMR1 cloned 1991
FMRP-
- FXS caused by loss of FMRP function
- RNA-binding protein
- Associated with polyribosomes & mRNA granules transported to synapses and translation response to neural activity
- Functions as translational repressor, stalls protein synthesis during mRNP transport
- Important for LTP, learning & memory
Long expansion in FMRP causes what ??
Fragile X syndrome
Different lengths expansions
Give different diseases
FXTAS-
• 55-200 CGG repeats in FMR1
• Late onset, >50
• Intention tremor & ataxia
• White matter lesions middle cerebellar peduncle
• Ubiquitinated inclusions
• Normal FMRP levels → RNA toxicity
Protein still made inspite of long repeats GOF via RNA toxicity whereas fragile X is LOF
What is the effect of really long repeats ?
Interfere with the transcription and translation
- LOF no protein e.g Fragile X syndrome
Polyglutamine disorders-
- CAG triplet repeat expansion disorders
- 1991 - spinobulbar muscular atrophy (SBMA) or Kennedy’s disease
- 1993 - Huntington disease (HD) high prevalence in caucasions
- 1994 - Dentatorubral-pallidoluysian atrophy (DRPLA) Japanese equivalent of Huntingtons – low prevalence in the uk
- 1993-2001 - spinocerebellar ataxia (SCA) types 1,2,3,6,7 and 17 ( now over 50)
Common features of polyglutamine diseases-
• Autosomal dominant inheritance (except SBMA – X-linked recessive)
• Genetic anticipation (next slide)
• Inverse relationship between CAG repeat length and age of onset
• Toxic “gain of function” mechanism
• Protein accumulation and aggregation
Repeat gets longer age of onset gets earlier
Polyglutamine diseases show genetic anticipation-
• Age of onset becomes earlier in successive generations
• Mainly associated with paternal transmission
• Explained by inter-generational repeat expansions during spermatogenesis
As repeat get longer during generations the age of onset gets longer
Huntington disease (HD)-
- Huntington’s chorea described in 1872 by George Huntington
- Hereditary progressive neurodegenerative disorder
- Fatal - death usually 15-20 y after onset
- Frequency 4-10/100,000
- Autosomal dominant inheritance (before mendels work)
- Penetrance nearly 100%- very likely for disease to manifest if you have a long enough life span
HD Pathology-
- Severe atrophy of the striatum
- Cortical atrophy – overall reduction in brain size
- Loss of GABAergic medium-sized spiny striatal neurones (MSN)
- Juvenile cases more severe - widespread neuronal loss ( in cerebellar)
- Astrogliosis and microglial activation
Neuronal loss in the Striatum
Caudate nucleus doesn’t bulge into internal capsule/lateral ventricle
Huge neuronal loss in the caudate
are pathological signs of what disease ??
Huntingtons
HD geen and mutation
Tend to occur through paternal transmission
Toxic gain of function mechanism-
• Htt knockout embryonic lethal E7.5
• +/- mice normal – not haploinsufficiency
• Rescue embryonic lethality by breeding +/- mice with 72Q YAC transgenics
- 72Q/- mice develop normally, show HD-like symptoms as adults
evidence for GOF
Yac can rescue embryonic lethal phenotype –
HTT aggreagation
Htt protein aggregation in HD cortex-
Aggregates detected with anti-huntingtin or anti-ubiquitin antibodies
Range of nuclear and cytoplasmic pathology
No direct correlation between aggregation and neuronal
Protein sequestration-
- Many proteins (especially transcription factors) have long (15-42) non-pathological polyglutamine tracts
- Proteins with non-pathological polyglutamine tracts (e.g. TBP & CBP) can be sequestered into mutant polyglutamine aggregates
Transcriptional dysregulation in HD-
- CREB-binding protein (CBP) - transcriptional co-activator
- Present limiting amounts in cells
- Possesses histone acetyltransferase (HAT) activity
- Decreased histone acetylation & global dysregulation of transcription seen in HD
Histone deacetylase (HDAC) inhibitors as potential therapeutic agents in what disease ??
HD
-Silence genes using HDAC inhibitors
SBMA-
- Adult onset neuromuscular disease
- Muscle weakness and wasting especially in the extremities, face and throat
- Speech & swallowing difficulties, muscle cramps
- Degeneration of spinal and bulbar motor neurones
- Normal lifespan unlike ALS
- CAG repeat expansion in androgen receptor
- X-linked recessive – why? Whereas all others autosomal dominant ?!
Examples of neurodegenerative & neuromuscular disorders caused by non-coding repeat expansions
• Myotonic dystrophy
• Fragile X-associated tremor/ataxia syndrome – 60-200 CGG in 5’-UTR of FMR1
• Spinocerebellar ataxia 8, 10, 12, 31 & 36
• ALS/FTD – intronic GGGGCC in C9ORF72
Pathological non-coding repeat expansions tend to be longer than disease-causing coding repeat expansions
Myotonic dystrophy-
• Autosomal dominant neuromuscular disease
• Multisystemic – muscle, cardiac, cognitive, behavioural, eye, endocrine symptoms
• 1:20,000 births
• Congenital, childhood & adult forms – dependent on repeat length
Longer repeat length earlier onset
Myotonic dystrophy genetics-
- DM1 CTG expansion in 3’-UTR of DMPK
- DM2 CCTG expansion in 1st intron of CNBP/ZNF9
- DM1 more common
- Genetic anticipation seen in DM1, mainly maternal transmission
Mechanisms of non-coding repeat toxicity-
• RNA foci/protein sequestration - splicing (DM) - nucleocytoplasmic transport (ALS/FTD) • Repeat-associated non-AUG translation (RANT) • Haploinsufficiency
Myotonic dystrophy is an example of what opathy ?
Spliceopathy
Experiment to show why SBMA is X linked ?
Overexp 97 glutamine expressed phenotypes
Early on off spring maintained weight other did not
Severely affected males
Aggregated proteins – stuck in the well
Aggregated in nucleus- pathologically different in males then females
Castration – removal of testosterone phenotype is rescued
Dragging feet but better after castration
Sham operated protein accumulation
Castration – ameliorate phenotype
Testosterone induces symptoms and pathology in 97Q females
All phenotypes shown in males induced in females
SBMA
Testosterone reduction prevents phenotypic expression in a transgenic mouse model of spinal and bulbar muscular atrophy
