Lecture 9: Repeat Expansion Disorders II Flashcards
(42 cards)
Give an example of a RED caused by expansions resulting in gain of RNA function
DM (myotonic dystrophy)
What is the inheritance pattern of myotonic dystrophy (DM)?
Dominant
Give 5 clinical features of DM
- myotonia (impaired muscle relaxation)
- Muscle wasting
- Insulin resistance
- Cardiac conduction defects
- Cataracts
- cognitive dysfunction
What is the typical age of onset of myotonic dystrophy?
Typically adult onset with age being determined by the number of repeats
Severe cases that are congenital (onset at birth) have most number of repeats (>1000)
How many forms of DM are there?
2 (DM1 and DM2) –> caused by repeat expansions in two different genes resulting in the same clinical features/disorder
What causes DM1?
expansion of trinucleotide CTG repeats in the 3’ UTR of DMPK gene
What is the pre-mutation and pathogenic repeat length for DM1?
Premutation = 37-50
Pathogenic = 50-1000
Does DM1 have a maternal or paternal expansion bias?
Maternal
Why does DM particularly affect muscle tissue?
Because it shows extensive somatic instability in proliferative cells and since DMPK (in DM1) and ZNF9/CNBP (in DM2) are highly expressed in muscle tissue, the repeats continue to expand and contract
What is the cause of DM2?
expansion of tetranucleotide CCTG repeats in intron 1 of ZNF9/CNBP gene
What is the pre-mutation and pathogenic repeat length for DM2?
Pre-mutation = 31-74
Pathogenic = 75-11000
Why, in the case of DM, can two repeat sequences located in non-coding regions of different genes cause the same disease?
They share a common pathogenic mechanism by RNA gain of function.
- The transcribed repeat containing RNA accumulates in discrete nuclear foci (can be detected in the cell nucleus)
–> as long as the gene in which the repeat is located is highly expressed in the muscle tissue, the effects will give a similar disease.
How does disease result from RNA gain of function?
The transcribed expanded repeat containing RNA can form imperfect hairpin structures due to G=C base pairing.
1. Sequester splicing proteins such as MBNL1 (normally involved in RNA splicing, sequestering of which results in aberrant alternative splicing of mRNAs)
2. Activation of PKC, which phosphorylates and stabilises CUGBP1 leading to increased availability and affects downstream mechanisms involved in processing of mRNAs (E.g. alternative splicing, mRNA translation and mRNA decay)
How can aberrant splicing resulting from RNA gain of function lead to the clinical features of DM?
Aberrant splicing of genes involved in production of:
1. insulin receptor = insulin resistance
2. chloride channel = myotonia
3. cardiac troponin _ = cardiac abnormalities
Genes that are affected in cataracts have not yet been identified.
How many disorders can result from expansion of the CGG trinucleotide repeat in the FMR1 gene?
Three:
- Fragile X syndrome (FXS) = 200-4000 repeats (full FRAXA mutation - gene silences, no FMRP)
- Fragile X-associated tremor/ataxia syndrome (FXTAS) = 55-200 repeats (pre-mutation stage)
- Fragile X-associated Premature ovarian insufficiency (FXPOI) = 55-200 repeats (pre-mutation stage)
How does the pathogenic mechanism resulting in FXS and the pre-mutation associated disorders FXTAS and FXPOI differ?
FXS = full FRAXA mutation - silencing of FMR1 due to CpG methylation and histone modification prevents FMRP expression (gene loss of function)
FXTAS/FXPOI = FRAXA pre-mutation - increased transcription leading to more mRNA containing expanded repeat, form unusual hairpin structures, sequesters RNA binding proteins resulting in dysregulation of proteins whose expression is usually regulated by these RNA binding proteins (RNA gain of function)
How can repeat associated non-ATG (RAN) translation of repeat containing RNA lead to the production of toxic peptides?
The repeat expansion can be transcribed in two directions to give sense and anti-sense repeat containing RNAs
The translation can initiate without the ATG initiation codon at non-canonical codons to give rise to multiple short peptides with amino acid repeats that are toxic and can aggregate
What is the cause of autosomal dominant amyotrophic lateral sclerosis (classical motor neuron disease)?
expanded GGGGCC hexanucleotide repeats in the first intron of C9orf72
What is the inheritance mode of RAN translation of repeat containing RNA?
Dominant (gain of function) - only need one allele to have expanded repeat to get symptoms of the disorder
What are the three different mechanisms that contribute to autosomal dominant amyotrophic lateral sclerosis (MND)?
- loss of function (In heterozygous individuals, haploinsufficiency means the mutated copy is not expressed but normal allele cannot compensate for loss of one allele)
- RNA toxicity
- Toxic RAN peptides
True or false: expanded repeats only show somatic instability in dividing/proliferative cells?
False: they may show some somatic instability in both dividing and non-dividing cells.
What happens when a repeat exceeds the threshold number of repeats for stable inheritance?
they show intergenerational instability (dynamic mutation) and may contract or expand through generations
What is special about the repeats that are prone to expansion?
They can all form unusual non-B DNA strucutres
What are the unusual non-B DNA structures that expansion prone repeat sequences can form?
- H-DNA
- G-quadruplex
- DNA unwinding element (A-T rich repeats, 2 hydrogen bonds = easily unwind)
- Hairpins
