Tri-nucleotide Repeat Disorder Flashcards
What are Tri-nucleotide repeat disorders?
These are caused by expansion of a triplet repeat. There are many examples of these all causing neurodegenerative/neuromuscular disorders. All diseases have threshold length that causes disease.
When the repeats are in coding regions 25-35 units will expand by about 10/generaiton. In non coding regions 55-200 units can expand 100/1000 units/generation.
What is anticipation?
Repeats increase during transmission from parents to children and this increased age of onset and disease severity making most autosomal dominant.
Expansion of the repeats can also occur in somatic cells and non dividing cells.
Where in the gene do these repeats often occur?
Can be seen anywhere in the gene. In the UTR pathology occurs via RNA whilst in the coding region it is due to protein.
How do repeat sequences expand and what influences this?
Formation of looped DNA intermediates which are then incorporated into RNA. However very long TNRs will be selected against due to dysfunction of the gene product, whether a cell is dividing - largest expansions seen in non dividing cells and finally parent of origin.
Explain how small expansions can occur?
Simple slippage of polymerase can lead to repeat or deletion but only accounts for 1-3 units
Larger (but still small) can occur if the polymerase is stalled by a TNR but the lagging strand continues. Fork then backs up and a large loop forms. (55-200 repeats)
Both of these can occur in dividing cells
Explain how large expansions can occur?
This occurs in non dividing cells due to repair (this includes meiotically arrested). Loops arise from repair of single strand break during excision of damaged DNA bases. Base excision repair and nucleotide excision repair both involved.
Repair by homologous recombination or non homologous end joining does not influence expansion.
Oxidised Gs recognised and removed by DNA Glycosylase OGG1. TNR displaced during gap filling. If MSH2 and MSH3 are involved then there will be expansion otherwise = deletion.
Do somatic expansions occur in vivo and do they contribute to disease pathology?
Yes in HT see much longer bands in somatic tissues, also replicated in mouse models. These expansion progress over time. Longer repeats show more instability.
KO of Msh2 in mice prevents expansion and also seems to reduce protein aggregation. Similar pattern seen with OGG1.
How does age of onset relate to repeat length and what can influence the onset of huntington’s disease?
There is a very strong negative relationship between number of repeats and age of onset. This relationship is stronger in longer repeats.
Biggest influences on disease are DNA repair genes. Most significant peak is FAN1 and you can block expansion of repeats by KO of this gene.
How do expansions leads to disease?
Protein toxicity e.g. HD soluble oligomers affecting transcription, cell transport and mitochondrial issues.
RNA toxicity - stem loop formation and accumulation in the nucleus. These can associate with splice proteins resulting in miss splicing of other proteins.
Also RAN translation.
What is RAN translation
Non ATG translation. First seen in SCA8 (spinocerebellar ataxia type 8). Previously thought disease was due to protein and RNA toxicity. But mutating start codon - still get protein translation.
Translation occurring because of the repeats and is frame independent giving 3 different proteins for each strand. All 6 proteins seen in the disease state.
Seen in patients
