15. Triplet repeat disorders - gain of function Flashcards
What is the clinical phenotype of HD?
Motor (chorea, dystonia), progressive decline in cognitive ability, psychiatric disturbances,
What causes HD?
CAG expansion in HTT exon 1
When do expansions in HD usually occur? Why?
On paternal transmission
Due to increased number of meiotic divisions in spermatogenesis
Which STRs in coding regions show a GoF mechanism of disease?
HD
SCAs
DRPLA
SBMA
What proves a GoF mechanism of disease in STR disorders?
- No other mutations in the gene- deletions including HTT do not cause HD
- Expanded alleles are transcribed and transmitted
- HD homozygotes are phenotypically identical to heterozygotes
What theories could explain the molecular pathogenesis of GoF STRs?
- Build up of toxic aggregates produces by expansion in neurones. Protein aggregates (inclusion bodies) observed in HD & SCA3 patients
- Toxic fragments - abnormal HHT protein is substrate for proteolytic cleavage
- Expanded proteins accumulate & interact with TFs –> disrupt transcription
- Disruption of axonal transport
What is the molecular pathogenesis of DM1 and DM2?
RNA-mediated GoF
Expansion of non-translated repeats - altered RNA function instead
Describe the molecular pathogenesis of DM1
Coding region intact, CTG repeast transcribed into mRNA
mRNA folds into hairpins that accumulate - acquire toxic function by trapping MBNL1 protein
MBNL1 regulates splicing - reduction in its activity results in accumulation of embronic-specific transcripts/proteins in adult muscle
What causes DM1?
CTG repeat in 3’ UTR of DMPK - expressed in heart and skeletal muscle
When do DM1 alleles usually expand?
On maternal transmission
What is the clinical phenotype of DM1?
Multisystem disorder
Continuum including mild, classic & congenital forms depending on expansion size
Myotonia, progressive muscle weakness & wasting, ptosis, cataracts, testicular atrophy, cardiac conduction defects
What causes DM2?
Expansion of complex repeat unit in intron 1 of CNBP
No anticipation
What is the clinical phenotype of SBMA & how is it inherited?
Degeneration of lower motor neurones - proximal muscle weakness & atrophy, gynecomastia
XLR
What are the SCAs?
Neurodegenerative disease that affect the cerebellum
Incoordination of gait, speech, limb movements
Typically adult onset
What therapies are under investigation for HD?
Antisense oligos - inhibit expanded mRNA –> reduce conc of mutant HTT
Adeno-associated vectors expressing short hairpin RNAs - reduce expression levels