Lecture 8 Flashcards
Phenomenon of disorders with generational differences in phenotype
1905 - Nettleship - Children with degenerative genetic disorders showed symptoms earlier than parents
1918 - Fleischer - Myotonic dystrophy - Increased expressivity - showed worsening severity, earlier onset, with succeeding generations
Genetic Anticipation:
- Earlier onset, increasing severity in later generations
- Increased numbers of individuals with symptoms in later generations e.g. Sherman paradox
Expanded numbers of microsatellite repeats
1991 - Cause of Fragile X syndrome - Expanded number of CCG repeats in 5’ untranslated region of FMR1
- FMR1 - Fragile X Messenger Ribonucleoprotein 1 (FMRP)
1991 - Spinal and Bulbar Muscular Atrophy - Expanded number of CAG repeats in coding region of androgen receptor gene
Expanded repeats unstable - Expand on parental transmission in somatic tissue - dynamic mutation
Expanded repeats later explain genetic anticipation and Sherman’s paradox
Repeat Expansion Disease
- > 50 REDs identidied
- 13 different sequence repeats associated
- No repeats correspond to severity
- Expansion of one repeat does not promote expansion of other repeats
- REDs autosomal dominant, autosomal recessive, or X-linked
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Four partially overlapping mechanisms of disease
- Expansion of non-coding repeats - loss of function of gene containing repeat
Loss of function - Recessive inheritance for autosomal expansion e.g. Friedreich’s ataxia (GAA/TTC expansion in first intron of FXN) or Fragile X (CGG/CCG expansion in 5’ UTR of FMR1) (X-linked dominant)
- Expansion of CAG coding repeats - gain of function and production of polyglutamine tract containing abnormal protein e.g. Huntington’s, Kennedy’s/SBMA
- Expansions that lead to gain of function of RNA containing expanded repeat e.g. Myotonic dystrophy type 1/2, Fragile X associated tremor ataxia syndrome, Fragile X associated premature ovarian insufficiency
- Expansions resulting in Repeat Associated, Non-ATG, (RAN), translation of repeat containing RNA leading to toxic peptide production e.g. ALS/FTLD, DM1, FXTAS, HD
Fragile X symptoms
Fragile X syndrome - X-linked dominant
- Common cause of intellectual disability - Average IQ is 40
- Increased severity in males
- 1 in 4000-7000
- Common monogenetic cause of autism
- Mild abnormal facial features - sunken eyes, arched palate, large ears, macroorchidism
- Can cause otitis media, seizures, mitral valve prolapse, GI problems
FXS fragile site
Associated with FRAXA
Located at Xq27.3
FRA - FRAGILITY
X- on X chromosome
A - First fragile site described on X chromosome
Fragile site visualised as stained X chromosome gap in metaphase spread from cells grown under replicative stress
FRAXA site
CGG repeat
6-44 - stable - ave 30
45-54 - Grey zone for intermediate repeat
55-200 - Premutation
200 to >4000 - Full FRAX mutation
Genetic anticipation
Severity depends on X-inactivation bias
Carrier ALWAYS used in context of premutation females
FMR1 mutation has maternal expansion bias - some paternal contraction bias
Explain how FMR1 expanded mRNA silences FMR1 locus
- <40 CGG - FMRP produced in hESCs and differentiated cells - Active euchromatin characterised by H3K9Ac, H3K4Me - DNA unmethylated
- > 200 CGG - hESCs FMR1 transcribed/translated
- during differentiation expanded FMR1 mRNA causes silencing
- Differentiated neurones >200 CGG repressive heterochromatin
- MeCpG and H3K9Me2 - no FMRP expression
What causes fragile X
- Loss of Fragile X messenger ribonucleoprotein 1
- FMRP localizes to postsynaptic spaces of dendritic spines
- Shuttles in and out of nucleus transporting target mRNAs
- Phosphorylated FMRP binds/represses translation of ~400 dendritic mRNAs
- On receipt of synaptic signals, FMRP dephosphorylates - translation produces synaptic plasticity proteins
- In fragile X, no synaptic plasticity proteins form - leading to symptoms
What diseases are caused by CAG expansion
- 9 in total
- Repeat length, intrinsic protein function
Dominant, gain of function disorders e.g. HD OMIM143100
- Characterised by neuronal degeneration
- Clinical features - progressive, selective, neural cell death associated with choreic movements and dementia
Huntington’s disease
HTT gene affected
Normal allele 11-26 CAG repeats -> 11-26 Q residues
Mutable normal allele 27-35 CAG repeats
HD allele with reduced penetrance - 36-39 CAG
HD allele - >39 CAG repeats
- Longer repeats = earlier onset
- Paternal expansion bias >7 CAG
- Increased somatic instability associated with early onset
- Repeat numbers estimated using PCR
Huntingtin structure
- HTT protein consists of HEAT repeats (Hungtintin Elongation Factor 3 - protein phosphatase 2A and TOR1) in neurons
- Acts as scaffold to coordinate other proteins
- As transcriptional regulator
Huntington’s disease pathway
- HTT translation - Mutant huntingtin with PolyQ tract produced
- Proteolytic cleavage produces toxic N-terminal fragments
- Nuclear Translocation of fragments to nucleus
- Nuclear fragments oligomerize and aggregate -> dysregulated transcription
- Impaired proteostatis - Nuclear and Cytoplasmic aggregates impair cellular protein quality control
- Cytoplasmic inclusions marked by Ub
- Downstream effects: Synaptic dysfunction, Mitochondrial toxicity and metabolism, Axonal transport impairment
