Dynamic Mutation Diorders Flashcards
What are static mutations
stably transmitted to offspring and retained in somatic tissues through development
What are dynamic mutations
the mutations may continue to change during transmission to offspring and during tissue development (Creating mosaicssiM)
All diseases with dynamic mutations have what kind of symptoms
neurological
What sequences are dynamic
short tandem repeats
What specifically is the location of the dynamic mutation disorders
trinucleotide/triplet repeats
Where are trinucleotide repeats that are mutated found
eveywhere:exons, introns, 5’ UTR, 3’UTR
Trinucleotide repeats loci have what
normal polymorphic variation in repeat length with no clincal significance
backward slippage of trinucleotide repeat leads to what
expansion
forward slippage of trinucleotide repeat leads to what
contraction
what can happent o trinucleotide repeats during replciation that would lead to mutations
slippage
Is expansions or contractions more likely
expansion
why are expanions more likely
contractions do not cause phenotype (usually)
What is MOI of fragile X syndrome
X linked Dominant
What is genetic anticipation?
as a disease is transmitted from generation to generation it increases in severity
How is anticipation explained
increase in repeat sizes occur in each generation
for two alleles the top number and bottom number represent what in regards to dynamic mutations
top number is the number of repeats on normal allele
bottom number is number of repeats on disease allele
When is anticipation observed
increasein clinical severity or decrease in age of onset
The longer and more expansion, how does it affect severity
the more severe or earlier the age of onset
define normal alleles
not associated w/ disease, stable upon transmission
define mutable alleles
do not cause disease, show meiotic instability. their children at greater risk, person themself will not be affected
define reduced penetrance alleles
if they show phenotype its late onset and show meiotic instability
define disease alleles
associated w/ disease and full penetrance. show meitoic instability and for some other diseases also mitotic instability.
where is trinucleotide repeat
can be anywhere within structure of a gene
In exons what codon will mutation be in?
CAG - glutamine, always polyglutamine track and in diseaes it gets much longer which relates to pathology of disease.
Expansions in coding regions generally have shorter ____ ____ and sharper _____ b/w normal and pathogenic repeat sizes
pathogenic repeats
boundries
In Fragile X where is the repeat?
CGG
Where is repeat in Huntington?
CAG
Where is repeat in SCA?
CAG
Where is repeat in myotonic dystrophy
CTG
Where is repeat in Friedrich ataxia?
GAA
In huntington what is the bias
paternal - so it only expans when inherited from father
what is paternal expansion bias
genes only epand when in herited from father
Fragile X has what bias
CGG maternal expansion bias
Myotonic dystrophy has what bias?
CTG maternal expansino bias
cerebellar ataxias has what bias
CAG paternal expansion bias
How are repeat expansions measured?
PCR/gel electrophoresis
why would a pt only have one allele on PCR/gel?
if the repeat is too big PCR might not be able to detect
they could also be homozygous
what is the limit of PCR in measuring repeats?
limited to repeats that are less than 1000 repeats
what is used to measure very large repeat expansions
southern blotting
what leads tot he phenotype in FRAXA?
loss of protein (null mutation)
what leads to the phenotype in FRDA
loss of protein (null mutation)
what is MOI of FRAXA?
X linked dominant
what is MOI of FRDA
AR
What does FRAXA stand for
Fragile X syndrome
what does FRDA stand for
Friedreich Ataxia
What causes phenotype for myotonic dystrophy
altered RNA, gain of function
what causes pathology of huntington?
altered protein - gain of function
what causes pathology of SCA?
altered protein - gain of function
what does SCA stand for
Spinocerebellar Ataxias
What is the most commonly inherited ataxia
FRDA
What codons are expanded in FRDA
GAA
what is the gene affected in FRDA?
Frataxin (FXN) - intron
what does FXN stand for
frataxin
Neurodegeneration, Vision, hearing, speech problems, Muscle weakness, Spine Curvature, Diabetes, Progressive heart condition, Diagnosis: aged 5-15
Require a wheelchair by 20s is what disease
FRDA
What has GAA repeats in Intron 1?
FRDA
what are the two theories for how repeat in intron causes FRDA
Repeat forms an unusual DNA structure that interferes with RNA polymerase
Repeat favors the formation of heterochromatin
If FRDA is not caused by GAA expansion, what is the other way it is mutated
point mutation
Describe how FRDA doesn’t have anticipation
it’s AR, is only observed in one generation
Fe-S centers are found where
in complexes of ETC in mitochondria
what organelle is affected if ataxin isn’t working
mitochondria
below threshold how does diseae present
it will not present with phenotype
above threshold how does disease present
it depends on how many repeats
The size of which allele is important in FRDA
smaller allele
What is MOI in FRAXA
x-linked dominance
what codon has expansion in FRAXA
CGG
what gene has expansion in FRAA and by 5’ or 3’?
5’ UTR of FMR1
what does FMR1 stand for
fragile x mental retardation 1
describe how x chromosomes look in FRAXA
there’s little bits on the end that look like they could break off, considered fragile sites
what disease is the 2nd leading cause of mental retardation?
fraxa
prepubertal, neurological/behavir - mental retardation, hyperacitivyt, autism spectrum, tantrums, large testes post pubertiy, flat foot smooth skin is what disease?
FRAXA
can females be affected by FRAXS
yes
describe how females are affected by FRAXA phenotypically
less severey, same physical and behavior features as maales, lower frequency & milder involvement
what is observed as a discontinuity of staining
the fragile site of x chromosme in fragile x
what part of cgg can be methylated
c in cgg can be methylated causing hypermethylation
are chromosmes in fragile x broken
no (but they CAN break)
where is the fragile x site in the chromosoems in fragile x?
near the telomere of q arm
what is the sherman paradox?
risk of having affected offspring increases in each generation. clinical severity increases with each generation
what is the sherman paradox now referred to as?
anticipation
how is anticipation explained?
repeat expansions
What is the chance a daughter of a non transmitting male will be affected?
0% - the daughter will not be affected
what is the chance grandson of non-transmitting male will be affected
40%
what is the affect a grandaughter of non-transmitting male will be affected
16%
exansions are only in which germline
female
how big can repeats get in fragile x?
huge -over 2000 repeats
what would you use to detect FRAXA expansion
southern blotting
where would you design probe in southern blotting for FRAXA?
probe outside of repeat region in a stable part, but still where it is inside of region that will be cut
what are the symptoms of non-transmitting males (premutation alleles)?
many do have phenotype later in life - tremor/ataxia
if females have premutation alleles what will their phenotype be
premature ovarian failure
what will have tremor/ataxia?
male with premutation alleles of fragile x
what will have premature ovarian failure
female with premuatation alleles of fragile x
what can happen to each c site in CGG?
it can be mehtylated
if there a lot of methylations what happens in cgg?
hypermethylation will travel to promotoer and stop gene expression
What is MeCP2?
it binds to hypermethylated DNA and is involved in silencing
what does MeCP2 stand for
methyl-bnding domain protein
what does Sin3 do?
corepressor complex
what is histone deactylase ?
it’s recruited with Sin3
99% of FRAXA is caused by CGG expansion, if it’s cuased by something else what could it be
anything that causes loss of function - like nonsesnse, missense, frameshift, whole gene deletion
FXTAS stands for?
fragile x associated tremor/ataxia syndrome
describe FXTAS
late onset
inclusions in neurons and astrocytes throughout the brain
what causes FXTAS
gain of function @ RNA, increase in FMR1 mRNA
what is increased in FXTAS
FMR1 mRNA
What does FMR1 gene encode
FMRP (protein)
What does FMRP do
RNA-binding protein
FMRP binds mRNA tanscripts of multiple and genes and limits ____
translation
the translation of genes that FMRP that are limited b/c of it all have what in common
they are all involved in regulating the brain
In the absence of FMRP what happens to the proteins it limits?
increased synthesis of the proteins, disruption of cell signaling in mulctiple cell types
DM1 stands for what
myotonic dystophy
what is MOI of DM1?
AD
what codon has expansion in DM1?
CTG
What gene is affected in DM1?
3’ UTR region of DMPK gene
where is expansion in DM1?
CTG in female germline
Myotonia, muscle wasting, weakness Lower legs, hands, neck, face Abnormalities on electromyography Cataract Hypogonadism Frontal balding Insulin resistance what disease?
DM1
does DM1 display pleiotrophy?
yes
If CUGBP1 are upregulated what happens
things are spliced incorrectly
Does Friedrich Ataxia have anticipation?
none
98% of disease alleles with friedrich ataxia have what expansion?
GAA
2% of disease alleles with friedrich ataxia have what mutation?
point
What accumulates in mitochondria in friedrich ataxia?
iron
the repeat length of which allele is correlated with higher severity in friedrich ataxia
smaller allele
what is frataxin involved in?
heme synthesis & formation of Fe-S centers
In friedrich ataxia, why is the ETC and aconitase function reduced?
the frataxin gene which doesn’t function correctly in friedrich ataxia is involved in heme and Fe-S centers which are both important for ETC
In southern blot of FXN if the smaller allele is not that small, what does that mean?
pt is more severly affected, the smaller allele is correlated to the higher severity in friedrich ataxia
Expanded CGG repeat creates a large what in 5’ UTR
CpG island
What is significant beyond the CGG repeats in pts who have fragile x
the region is also methylated
What binds hypermethylated DNA?
methyl-binding domain protein (MeCP2)
What does MeCP2 stand for?
methyl-binding domain protein
What is the corepressor complex?
Sin3 complex w/ histone deacetylases (HDACs)
HDACs and other chromatin remodeling factors promote what
chromatin condensation & transcriptional repression
What is MBNL?
RNA-binding protein
MBNL & CUGBP1 have a role in what function
alternative splicing
The expanded RNA CUG repeats sequester what
MBNL (RNA-binding protein)
Does MBNL have loss or gain of function in Dm1?
loss
Does CUGBP1 have loss or gain of function in Dm1?
gain
In Dm1 RNA missplicing of multiple genes leads to what
the multisystematic presentation of the disease
Explain trans-dominant RNA in DM1
trans-dominant RNA
I believe it’s like a dominant affect b/c mutation in one RNA affects a bunch of different genes - pleiotropy
What is gene that is mutated in SCA1?
attaxin
WHat is MOI of huntington disease
AD
what does HD stand for
Huntington disesase
Where is repeat in HD
CAG
What gene is CAG repeating in Huntington disease
Huntingtin (HTT)
What does HTT stand for
Huntgintin
degeneration of neurons in striatum and cerebral cortex, depression, chorea, difficulty walking speaking, swallowing, late onset, death about 15 years after diagnosis, cognitive decline is what disease?
HD
What is treatment for HD?
supportive, pallative, tranquilizers, antidepressers
How do you test for HD?
PCR/gel electrophoresis
Why can you use PCR/gel electrophoresis to test for HD
repeats are fairly small
Any allele size above 36 for CAG repeats will have what?
HD
If it’s below 36 they will not be affected
What is MOI for SCA
AD
What is onset for SCA
late onset
what is degenerating in SCA
cerebellum
All SCA exhibit what
ataxia
wheelchair bound w/in 20 years of diagnosis, age dependent penetrance, ataxia, slowly progressive, is what disease?
SCA
How many loci are involved in SCA
27
SCA genes with exonic CAG expansions have what structural and functional similarity
none (at least it’s not obvious)
CAG expansion leads to what that causes pathology in SCA
gain of function
what is normal range of CAG repeats in the exon that would cause SCA
4-42
Does CAG repeats cause loss of function or gain of function of ataxin?
gain of function
CAG expands what tract
polyglutamine
when there are polyglutamine diseases the extra CAG repeats cause what?
insoluble aggregates and inclusions
B/w aggregates and inclusions which are the damaging ones that cause pathology?
inclusions are protective - the ones without aggregates are the neurons that have issues. soluble protein is what causes issues
How is Huntington protein modified
Sumo modification
what is sumolyation
sumo modification
neurons in striatum and cerebral cortex exhibit what
Rhes
What does Rhes promote
mHtt sumoylation preferentially over wt Htt
Sumo modifcation favors what
soluble form (which kills neurons)
Describe the potential novel therapy for HD
targets mRNA using antisense oligos ASOs which bind CAG expanded allele and trigger degradation by RNaseH
what disease is caused by altered mRNA function
myotonic dystrophy
which two have loss of function mutations?
FRDA and Fragile X
which two have loss of function mutations?
FRDA and Fragile X
In FRDA the expanded repeat interferes with what?
transcriptional elongation
What kind of expansions causes fragile site
CGG expansion
How many loci are involved in SCA?
27
Out of the 27 loci involved in SCA how many are involved in a CAG expansion in an exon?
7 loci
What is the normal range of CAG repeats in SCA?
4-42
What are the 6 mechanisms that might cause polyQ disease?
Proteolytic cleavage with toxic fragments Proteasome impairment Aggregate formation Mitochondrial Dysfunction Transcription dysregulation RNA toxicity?
What does mutant Htt do in cells?
aggregates
Is the aggregate that accumulates with mutant Htt harmful to the cells?
it’s not harmful in and of itself, it’s actually protective
Explain the pathology behind how Htt causes HD
mutant Htt promotes sumo modification which favors the soluble form - the soluble form is what is actually harmful
