Huntingtons Disease Flashcards
Define Huntingtons
progressive neuropyschiatric disorder caused by an autosomal dominant mutation on chromosome 4 charcaterized by motor cognitive and pyschiatric symptoms
What does the mutation in huntingtons do?
CAG nucleotide repeat expansion which encodes for a variably elongate polyglutamine chain to be present in the huntington HTT protein
Diagnosis?
Genetic testing
When was the huntingtons gene discovered
1993 by mcdonald et al
Describe the genetics of huntingtons?
chromosome 4 mutation autosomal dominant 100% penetrance usually 21 CAG repeats when the exon 1 of HTT gene > 36 repeats causes polyglutamine tracts
What did Brinkaman 1997 say?
greater length = more severe = anticipation
What is the function of Htt gene?
function unknown but interacts with many proteins an conserve across species such as drosophilia, mice, sea slug aplysia
Sah et al- rehulates apoptosis
Harges and wanker 2003- knockout mice did not survive embryonic development
Talk about mHtt aggregates
Glutamate = polar
Abunance causes links between proteins –> “sticky” molecules with H+ bonds
accumulation is toxic causing clathrin endocytosis dysfunction, mitochondrial toxicity an vesicular transport dysfunction
Choi et al 2014
Believe that wild type HTT is involved in long term memory as shown by experiments in aplysia where HTT was shown to be essential in pre and post synaptic for LTP
Talk about aggregates being neurotoxic and who says this?
cytoplasmic proteins are usually degraded by lysosomes
misfolded proteins- ubiquinated and degraded by proteosomes
dysregulation of this UPScauses cell death- bence 2011
what else does the mHtt aggregaates bind to?
sticky molecules that bind to other glutamate rich proteins including P53 and CREB binding protein
Describe evidence that the aggregates are neuroprotective
greenburg 1998 found that fragment size an folding didnt correlate with the rate of death so maybe aggregation is a beneficial mechanism to inactivate the toxic forms of huntington with expanded polyglutamine
how does tetrabenazine work?
basically blocks the VMAT which chucks dopamine into vessicles and also blocks the dopamine receptors so that there is LESS DOPAMINE because the damn dopamine acts on basal ganglia to INCREASE movement
Talk about gene silencing
ASO- anti-sense oligonucleotides is a form of treating genetic disorders where the ASO will bind to the complimentary mRNA and cause reduced traslation of HTT by trigerring RNase-mediated degradation
Talk about another form of gene silencing apart from ASO
RNAi approach- enogenous RNAi process where small inteferring RNAs(siRNA) bind to mRNA and cause formation of RISC that prevents the translation of HTT by cleaving the mRNA
what are some problems with ASO?
ASO currently cross the BBB but is soluble in CSF so is currently injected into the CSF whereas siRNAs are both insoluble in CSF and cannot easily cross the BBB and in-vivo research in mice has been used adeno-associated viruses to deliver the RNA strands to the brain
dont know long the levels can levels of HTT without causing side effects
immune response
side effects- off target effects and may bin to other mRNA
What is Ionis/Roche doing?
Will start a phase 3 clinical trial called generation HD-1 which will test non-selective ASO in humans which is crosses BBB-brain shuttle Because mouse models have demonstrated suppression of HTT by 70% for 4 months and was seen to be well tolerated
What does Stanek 2013 say?
improvement of motor phenotypes in HD mice correlted to the reduction in mHt levels in the ASO TRIALS
What are the ways in which HD studied?
cell lines - used to dissect molecular machinery involved
invertebrates- whole organism, simpler genetic manipulation
stem cells- induced pluripotency to generate neurons
How do N-terminal transgenics work?
carry small portion of the 5’ end of human Htt gene , exon 1 contains CAG repeat
Phenotypes= coordination, tremors, hypokinesia,
pronuclear injection leading to randomn integration of the transgene mouse genome
rapid onset
How do full length transgenics work?
Full length Htt gene is carried in either Yeast/bacterial artificial chromosome
the transgenes integrate at a single genomic locus
phenotypes show up graually over a few months
therapies to be targetted in full human Htt gene/protein which was USEFUL TO SEE IF THEY WORK
How do knock in mice work?
mouse embryonic stem cells are injected with specific number of CAG repeats introduced directly into mouse Htt gene
Grown on selective media an theninjecte into blastocyst of pseudo pregnant female
What is the Hdh-Q111
Transgenic mouse with exon-1 segment of Htt gene carrying 111 CAG repeats inserted into exon-1 of mouse Htt = effects on episodic memory
Aggregates focused in HC, Corpus Callosum, Lat ventricle = Monteiro 2010
Depression-like behaviour in forced swim test and splash test, also found qanxiety like behaviour - Orvoen et al. (2012)
Talk about anti-sense oligonucleotides in more detail
delivery of ASO- direct injection? microperfuion pump?
distribution- outer regions of brain are more accesible but how aboutthe deep brain?
choosing targets- target SNP in mHtt or long CAG repeats?
Side effects- off target effects may bind to other mRNA
Who says that ASO lasts about 4 months?
Kordasiewicz 2012
Relate memory deficits and Hdh-Q111
- Novel object recognition intact at 2 months
- Object-place recognition intact at 2/12, impaired at 13 in het/homo = Wilson 2013
- Object-context intact at 2/12, impaired at 13 = Langston 2010
Memory deficit due to impaired LTP
Can drugs to induce LTP benefit these mise models to reverse deficits?
= GABAa51A = Kind et al
