Repeat expansions Flashcards
What are dynamic mutations?
Nucleotide repeat tracts that are unstable and can expand/contract on generational transmission and causing disease when expansions are above a pathological threshold
What is a common feature of repeat expansions?
ANTICIPATION
Clinical features become more severe, more frequent and occur earlier in successive generations, due to further expansion of the repeat tract
Does not happen in all repeat expansion disorders e.g. PolyA (GCN) is more stable and therefore does not show anticipation
Example of sex specific anticipation
Paternal transmission of HD expansion = juvenile HD. The CAG is more unstable in spermatogenesis due to the increased no of meiotic divisions
What is the origin of a repeat expansion?
REPLICATION SLIPPAGE
Meiosis or mitosis
- DNA polymerase stutters while synthesising repetitive elements
- Formation of a loop in newly synthesised DNA = addition of extra repeat units
OR unequal crossover in meiosis = expansion on 1 allele and contraction of other allele
4 mechanisms of repeat expansion diseases
- LOF - reduced transcription of gene and decreased function
- GOF - PROTEIN toxicity from expansion of AUG-translated repeats
- GOF - RNA toxicity through gelation and sequestration of RNA-binding proteins
- GOF - PROTEIN toxicity from expansion of non-AUG-translated repeats
Function of FMRP
RNA-binding protein. It shuttles mRNA from nucleus to areas of cells for protein assembly. Important for synapse development in brain. Also expressed in testes and ovaries
Mechanism of Fragile X
CGG repeat expansion in FMR1 5’UTR resulting in hypermethylation of CpG island 250bp downstream near promoter. Results in transcriptional gene silencing and partial/complete loss of protein
Can have methylation and repeat expansion mosaicism (report of high functioning male with unmethylated full expansion)
FRAX clinical features
Males: moderate-severe ID, macrocephaly, long face, large ears, joint laxity, macroorchidism
Females: variable depending on skewed X-inactivation. Normal-moderate ID
PRE-MUTATIONS: FXTAS (late onset progressive ataxia) and FXPOI (early menopause), due to toxic RNA GOF not LOF
FRAX CGG ranges
NORMAL 6-50
INTERMEDIATE 46-58
PRE-MUTATION 55-200 (>90 = 90% chance of expansion into full mut range)
FULL MUTATION 200+
Also SNVs and del/dups report with no methylation defects
AR repeat expansion example
and mechanism
Friedreich ataxia
Homozygous GAA repeat expansion in intron 1 of FXN (not necessarily 2 the same size)
LOF. Formation of DNA-RNA hybrids (R loops) at GAA repeats block transcriptional machinery and trigger recruitment of heterochromatic factors by repressive chromatin marks = condensation of the locus
FXN protein function
FXN encodes frataxin, a mitochondrial protein that synthesises iron-sulphur clusters (ISCs) and enzymes in respiratory chain complexes 1-3
Less frataxin = less ISC = decreased iron-dependent enzyme activity = iron accumulation in mitochondria = iron mediated ROS = oxidative damage and apoptosis of tissues with high mitochondria content (heart, spinal cord, cerebral cortex)
Why does FRDA not show anticipation?
AR inheritance means disease is not seen in successive generations
Size of expansion does correlate with phenotype severity and age of onset
FRDA clinical features
Progressive ataxia and HCM, dysarthia, loss of position sense and paraplegia. Onset 10-15y. Death by 40y
FRDA GAA ranges
NORMAL 5-33
PRE-MUT 34-65
BORDERLINE 44-66
AFFECTED 66-1700
Shortest allele associated with FRDA = 44 (with large exp on other allele)
2% have SNV+expansion
Expansions arise from large normal (12-33), usually maternal
3x examples of:
GOF - PROTEIN toxicity from expansion of AUG-translated repeats
and mechanism
- Huntington (HTT)
- Spinocerebellar ataxia 7 (ATXN7) - extreme anticipation
- Spinal and Bulbar Muscular Atrophy (AR) - example of pleiotropy as missense muts cause AIS infertility(decreased sensitivity to testosterone)
Poly-Q (glutamine) CAG repeat expansions
Why are poly-Q expansions pathogenic?
CAG within ORF = expanded poly-Q tract in translated protein = conformational changes e.g. Beta sheets = insoluble aggregates e.g. intranuclear protein inclusions = cell proteotoxicity & degradation by UB pathway
Disease proteins are usually involved in multiple cell processes, so other dysfunctions in addition to proteotoxicity e.g. alter protein-protein interactions and localisation/stability, deregulated RNA metabolism
3 common features of Poly-Q disorders
Adult onset, complete penetrance and progressively fatal
Anticipation
No other mutation types and homozygotes same as heterozygotes
HD CAG ranges
NORMAL <27
INTERMEDIATE 27-35
AFFECTED (reduced penetrance) 36-39
AFFECTED (complete penetrance) >39
JUVENILE >60
HD clinical features
Neuronal dysfunction and cell death causes psychiatric disturbances, motor changes e.g. chorea and cognitive decline
Average age of onset = 40y
1 example of
GOF - RNA toxicity through gelation and sequestration of RNA-binding proteins
and mechanism
Myotonic dystrophy type 1. CTG expansion in DPMK 3’UTR
Long repeat RNA folds into complex structures that trap RBP (e.g. splicing regulators and TF) into RNA foci. This alters RNA processing/metabolism by decreasing RBP availability for it’s functions. Affect depends on the repeat length and expression levels of disease gene/RBP
Mechanism of DM1
DPMK encodes a Ser/Thr kinase expressed in cardiac & skeletal muscle. Expanded DMPK sequesters MBNL1, an alternative splicing regulator that is important for adult isoform splicing. This leads to increased CUGBP1 mediated splicing of an embryo isoform
DM1 CTG ranges
NORMAL 5-36
PRE-MUTATION 37-50
CLASSIC AFFECTED 51-150
JUVENILE >150
Presents with myotonia, distal muscle weakness, cataracts, testicular atrophy and cardiac conduction defects
What is RAN translation?
Repeat associated non-AUG translation is protein synthesis occuring in multiple reading frames within non-coding repeat regions, produces homopolymeric proteins.
RAN occurs due to IRES cap-independent ribosome binding, no PIC scanning for AUG, start codon fidelity decreased e.g. ACG or GUG
Example of
GOF - PROTEIN toxicity from RAN translation
ALS/FTD
GGGGCC expansion in C9orf72 accounts for 40% of familial ALS
NORMAL 5-24
AFFECTED 61-4000+
