L9, Repeat Expansion Disorders II Flashcards
1
Q
DM Characteristics: (onset, inheritance, clinical features, types)
A
- Myotonic dystrophy -> Often adult onset; age of onset largely determined by number of repeats
- Second most common type of muscular dystrophy
- Dominant inheritance pattern
- Affects 1 in 8500 individuals worldwide
- Multisystemic disorder with varied clinical features (myotonia, muscle wasting, insulin resistance, cardiac conduction defects, cataracts, with cognitive dysfunction, ID in more sever form)
- Two forms: DM1, DM2
2
Q
Broad pathophysiology of DM1 and DM2:
A
- Different genes (non-coding regions)
- Disease caused by expansion resulting in gain of RNA function
- DM1: CTG repeats in 3’ UTR of DMPK gene
- DM2: CCTG repeats in intron 1 of ZNF9/CNBP gene
3
Q
Thresholds for repeats in DM1, additional features:
A
- DM1: CTG repeats in 3’ UTR of DMPK gene
- Normal: 5-37
- Premutation: 37-50
- Pathogenic: 50 -1000
- Congenital form: >1000
- Maternal expansion bias
- Extensive somatic instability in proliferative tissue particularly muscle
4
Q
Thresholds for repeats in DM2:
A
- DM2: CCTG repeats in intron 1 of ZNF9/CNBP gene
- Normal: <30
- Premutation: 31-74
- Pathogenic: 75-11000
5
Q
How does the RNA GOF mechanism work in DM?
A
- Transcribed RNA forms unusual structures -> imperfect hairpins in repeat containing sections
- DM1: CUG
- DM2: CCUG
- In both cases, GC pairing occurs -> partial base-pairing along hairpin
- These imperfect hairpins bind and sequester certain proteins
- e.g. DM1 hairpin sequesters MBNL1 -> loss of function (alternative splicing as a result), additionally results in PKC activation -> CUGBP1 stabilised and level thus increased -> alternative splicing, mRNA translation, mRNA decay
- See FC
6
Q
Give four examples of aberrant splicing as a result of DM:
A
- Insulin receptor -> insulin resistance
- Chloride channel -> Myotonia (Link to B302!)
- Cardiac troponin T -> Cardiac abnormalities
- Unidentified genes -> cataracts, testicular failure
7
Q
Discuss the different diseases arising from abnormal amounts of repeats in FMR1:
A
- Fragile X syndrome (200-4000 repeats)
- Fragile X-associated tremor/ataxia syndrome/FXTAS (40% males, 8% females with premutation repeats of 55-200) -> Late onset neuropsychiatric degenerative disorder
- Fragile X associated premature ovarian insufficiency/FXPOI (25% of females carrying premutation repeat 55-200)
8
Q
Describe the disease mechanisms in premutation repeat length potentially leading to FXTAS ad FXPOI:
A
- 5-55 repeats -> transcription, translation -> FMRP protein
- 55-200 repeats -> increased transcription, up to 8x normal RNA level (repeat stretch thought to sequester RNA binding proteins, resulting in dysregulation of protein expression)
9
Q
RAN: Effect
A
- Repeat-associated Non-ATG translation of repeat containing RNA
- Leads to production of toxic peptides
- Repeat expansion can be transcribed in two directions leading to sense and antisense transcripts -> protein toxicity in both cases (not fully understood; lack of ATG initiation codon)
10
Q
Amyotrophic lateral sclerosis: How does RAN translation cause this?
A
- Expanded GGGGCC repeats in first intron of C9orf72 cause autosomal dominant ALS
- 3 different contributory mechanisms…
- Loss of function -> haploinsufficiency
- RNA toxicity
- Toxic RAN peptides
- There is selective vulnerability of motor neurons in this condition, possibly as a result of differential expression of VEGF (higher expression may have a protective role)
11
Q
Describe the mechanism for repeat instability:
A
- Stable inheritance below threshold
- Above repeat threshold, intergenerational instability -> dynamic mutation (both expansion and contraction) -> somatic instability in both dividing and non-dividing cells
- Can depend on maternal vs paternal line
- Scale of expansion can vary hugely; from a few repeats to many hundreds of them
12
Q
Types of unusual non B-DNA structures formed by expanded repeats:
A
- H-DNA (triplex)
- G-quadruplex
- Hairpin (perfect or imperfect)
- DNA unwinding elements (must be AT rich region)
- Inverted repeats can form perfect cruciforms
- Direct repeats can result in imperfect cruciform structures (both stable and slipped strand from reannealing)
- These structures in DNA are hard to replicate and hard to transcribe
13
Q
How do stabilising interruptions affect RED alleles?
A
- Longer repetitive runs with stabilising interruptions (‘long-normal’ alleles) e.g. AGG insertions can sill have normal phenotype
14
Q
How may DNA replication lead to expanded/contracted repeats?
A
- Replication of dsDNA is asymmetric -> leading and lagging strands
- Structure prone sequence on lagging strand promotes instability
- Skipping hairpin on one -> contraction whereas stalling, reversal, restart -> expansion
15
Q
What processes are involved in transition from normal to unstable repeat sequence:
A
- Change of replication context of structure prone strand
- Two origins; first one would have unstable side on leading strand
- If the first origin becomes inactivated, the unstable region ends up on lagging strand -> much more likely to form hairpin structures -> instability
