L19 Dynamic mutations in Protein Coding sequence Flashcards
Huntington’s Disease
see onenote
- Mendelian inheritance
- autosomal dominant
- complete pentrance
- if you have the mutation, 50% probability of passing mutant allele to both female and male offspring
Huntington’s - gene
see onenote slides
gene - htt (designated IT15)
- has variably sized CAG repeat in first exon of htt gene, resulting in protein from different alleles containing different numbers of glutamine, Q, repeats
gene is expressed in all body tissues throughout life but major impact of mutation is one the nervous system
PolyQ diseases
overall have expansions smaller than those disease where the repeat is in the untranslated region (like CCG in fragile-x) because polyQ becomes increasingly toxic to cells as the length increases
Huntington’s disease shows anticipation
see onenote
decreasing age of onset in succeeding generations indicates this disease shows anticipation
CAG repeat expansion
see onenote
number of repeats increasing in male line, not in female line
CAG repeat is expanded after passage through male
as length of repeat increases, age of onset decreases
- other genetic background components can also contribute to age of onset
huntington’s disease is due to a toxic gain of function resulting from expanded CAG repeat
The huntington gene and huntington protein
see onenote
htt gene - protein
see onenote slides
htt encodes a protein caleld Huntingtin (HTT)
specific function of HTT is unknown but null mutations in mice are embryonic lethal
HTT
- large protein
- has 36 HEAT (protein domain) repeats in the amino half of the protein
- repeats are binding sites for other proteins
- HTT thought to act as a scaffold protein
200 proteins known to interact with HTT, makes studying its function very difficult
RAN translation
see onenote
Translation int he repeat sequences starting at non-AUG codons
- occurs resulting in polyQ, poly-alanine and poly-serine non-functional proteins and peptides
Treatment prospects
see onenote
- screening
- CAG much easier to PCR accurately in comparison to CCG of FRX - IVF
Research focused on extending symptom free time into old age
Positive results in retarding progression obtained by
see onenote
- increase conc. of protective chaperone proteins
- using histone deacetylase inhibitors to improve overall transcription
- giving neuronal stimuli - “use it or lose it”
Reducing the levels of the toxin protein
see onenote slides
Toxic polyglutamine
- Can’t knock it out entirely as it is essential for development early on
Antisense-oligonucleotides
- Small sequences, complementary to the gene itself to reduce translation
- Needs to hit the CAG repeat, don’t want it to hit CAG of other proteins
- Don’t want it to affect the WT allele
IONIS ASO
see onenote
Used non-selective ASO against mutant - could bind either WT or mutant allele
- Outcome has been good so far
Need huntington’s protein for neurological development but in huntington’s disease, this protein is overexpressed => toxic
Amplification of tri-nucleotide repeats
see onenote
based on formation of DS nucleotide structures and aberrant DNA repair
What do the repeats expand?
- We only know part of it
- Formation of double stranded loop structure leads to expansion
- Mice without missense repair pathway do not expand if the protein is knocked out, missense repair pathway is involved somehow
How does it expand - requires:
- Double stranded loop structure
- Missense repair pathway
Mechanisms of expansion
see onenote
Pre-mutation
- Small expansion, could be due to slippage
- Unequal crossing over on X chromsome does not cause fragile-X, it is some sort of mismatch repair
How are non-coding repeats amplified in females and not in males, while coding repeats are amplified in males and not in females?
see onenote
- relates to germ cell production and maturation in different sexes
1. FRX expansions greater in older oocytes in pre-mutation women
2. men with FRX premutation do not pass on expansions to next generation
