L18 Dynamic mutations - non-coding Flashcards
Fragile-X syndrome
see onenote slides
Dynamic mutation in the 5’ UTR
Most common form of mental retardation
Quite common, which is unusual for dynamic mutations
On the X-chromosome (only 4% of the coding genome, carries 14% of genes involved in cognitive function)
- If a male gets a defective X-chromosome, would effect
- In a female, depends on how many mutant X’s are inactivated in => variable penetrance in females
Dynamic mutations
In genetics, a dynamic mutation is an unstable heritable element where the probability of expression of a mutant phenotype is a function of the number of copies of the mutation. That is, the replication product (progeny) of a dynamic mutation has a different likelihood of mutation than its predecessor.
Cells from severely impaired fragile-x people were cultured - what did they find?
see onenote
Sequenced the x-chromosomes, found that there was a bit missing from the chromosome at 27q3, in some cases the entire end of the chromsome was missing
Fragile site note at the base of the x-chromosome that was missing in an unaffected individual
Fragile-x syndrome differ from classical mendelian x-linked dominant disease pedigree
see onenote slides
Sherman paradox - in different generations, carrier females have different probabilities of producing affected chidren
Anticipation - later generations have an increasingly higher probability of affected offspring
How can there be carrier males for an x-linked disease?
see onenote
mutation identified as expansion in CGG repeat in 5’ TUR region of FMR1 gene
in unaffected population, the repeats are “impure” - there is an AGG rather than CGG every 9-10 repeats
Pre-mutations
see onenote slides
pre-mutations can amplify to full mutations when maternally inherited - dynamic mutations, change over time/different generations
pre-mutations explains the odd degrees
mutation not amplified in males but all daughters of these males will inherit a pre-mutation
CGG pre-mutation repeats expand when maternally transmitted
FMR1 mRNA levels increased in pre-mutation neurons but protein levels are reduced relative to normal range
increase in mRNA directly correlated to repeat length
How do the expansions affect protein and brain function?
see onenote
- expansions greater than 200 repeats result in epigenetic modifications
- silencing transcription, FMR1 mRNA greatly decrease/absent and no protein is produced
genetic defect classified as x-linked dominant loss of function
What does FMRP1 protein do?
see onenote slides
- enters nucleus, binds mRNA and transports them to dendrites of nerve cell
- regulates translation in dendrites
- important regulator of nerve cell protein synthesis and function
in FRX, expression of dendritic protein increased by approx 20% and neuron cannot respond/responds poorly, to incoming signals from interacting neurons
Pre-mutations and pathology
see onenote
pre-mutation indivduals not cognitively impaired but in males, autism is relatively common
in older pre-mutation males, parkinson-like tremor and memory problems may be present
about 20% of female pre-mature carriers cease menstruation at or before 40 years of age
Mechanisms that may contribute to fragile-x associated tremor/ataxia pathology
see onenote
Screening for full mutations and pre-mutations
see onenote
by PCR and southern blotting
Methylation dependent PCR
see onenote slides
if repeat number if close to 200, important to know if gene has been methylated
Methylation dependent PCR
- Change cytosine to uridine
- Won’t get a band if its methylated
- We’re looking for no bands but how do we know if our PCR has worked or there was some other error that resulted in no band
PCR with fluorescently labelled nucleotide
see onenote
gives quantitative measurement
- if two normal copies of very similar size are present, there will be greater fluorescence than if one normal copy is present
Treatment
see onenote
treatment available for behavioural symptoms, and co-morbidities e.g. epilepsy
testing allows for early intervention
