L22 DNA Repair and Associated Disorders Flashcards
what are the short term consequences of DNA damage?
- reduced proliferation
- altered gene expression
- cell death = apoptosis
what are the 2 major types of DNA mutations?
- induced
- spontaneous
what are the 2 classes of spontaneous mutation?
- errors of replication
- spontaneous lesions
what stage of cell division does Error of replication occur?
S phase
in Errors of replication, wrong base is incorporated by what? due to what?
DNA polymerase due to the chemistry of the nucleotides
define tautomerism
ability of certain chemicals to exist as a mix of 2 interconvertible isomers
what are the 2 functions of DNA polymerase?
- 5’ to 3’ polymerase activity
- 3’ to 5’ exonuclease activity
both for proofreading!
what is the defect in Bloom syndrome?
defect in BLM gene = RecQL3 DNA HELICASE!
what is the DNA helicase in Bloom syndrome normally required for when its not defective?
replication repair and recombination
what are sx/sx of Bloom syndrome?
- smaller
- narrow chin
- FACIAL RASH
- diabetes, neuro, lung, immune system deficiencies
- high incidence of cancer
what genetic manifestation is seen in Bloom syndrome?
chromosomal instability which results in increase sister chromatid exhanges
what is the defect in Fanconi Anemia?
multiple genes (N8)
Fanc A->H
related to DNA repair
Fanc A = 65 % of cases
what genetic manifestation is seen in Fanconi anemia?
increase spontaneous chromosome breakage which is increased by DNA cross linking agents
what are sx/sx of falcon anemia?
radial ray defects pancytopenia Mental retardation short increased risk of neoplasia
where do frameshift mutations tend to occur?
at positions where there are base repeats
what are frameshift mutations thought to result from?
‘slipping’ of DNA polymerases during replication of these repeats
what types of cells do spontaneous lesion occur in? due to what?
resting cells
due to the chemical nature of the DNA
what are the 3 main types of spontaneous lesions?
- depurination (most common)
- deamination
- oxidative damage
what happens in deprivation?
- what bond breaks?
- what remains and what is lost?
- what happens if it persists through replication?
glycosidic bond - between base and sugar in purine nt
remains - sugar-phosphate backbone
lost - base
persist - mutation can occur
what happens in deamination?
- loss of what?
- ___ deaminates to form ___
- is it easy to fix?
loss of amine group from base (particularly cytosine)
cytosine deaminates to form uracil (pairs with T)
easy to fix
in deamination, 5-methyl cytosine deaminates to form?
- what does it end up pairing with?
- what is this considered?
thymidine
T-G pair
both normal bases, could be repaired to TA or CG
mutational hotspot!
methylated cytosines produce a ______
mutational hotspot
*note - not all positions are mutable, hotspots include things like 5-methyl cytosine or repeated bases (AAAAAA)
what is the key to understand mutational hotspots?
you can see the same mutations occurring at a high frequency
what is oxidative damage a result of?
production of RO compounds due to oxidative metabolism (superoxides, peroxides etc.)
what does oxidative damage do to the cell?
What does this result in?
- causes oxidative damage to many parts of the cell including addition of oxygen groups to nt bases
- 8-oxo-7-hydroxyguanosine
results in misfiring with an A and potential TRANSVERSION
what is the harm in ionizing radiation?
high energy particles/rays can cause many types of cell damage up and including death
also causes extensive damage to DNA (base damaging type) including HERITABLE MUTATIONS
what does UV light generate?
deleterious photoproducts like cyclobutane pyrimidine dimers (CPD)
or
6-4 photoproducts (6-4 PP)
what type of linkages form as a result of UV light?
what does that interfere with?
covalent linkages between bases on the same strand!
interfere with normal pairing and block replication!
what are the main indirect repair groups?
- nucleotide excision
- base excision
- mismatch repair
what does nucleotide excision remove? Example?
more than a few (up to 30) base around a damaged site like seen with UV damage
what does base excision remove? Example?
a single (or a few) damaged bases like seen with methylation and oxidation
what is mismatch repair (post replication repair) remove? Example?
mismatched bases from tautomerism
what is the common mechanism for all excision repair
-recognize damage
-remove damaged base or region around it
-replace the excised region (using DNA polymerase)
+ligase seal
what steps of excision repair do you need specific repair proteins for?
to recognize damage
and to remove damaged base or region around it
what steps of excision repair do you need shared repair proteins for?
replacement of excised region
what is the defect in Xeroderma pigmentosum?
mutations in 9 different NER genes are capable of producing XP
9 possible mutations in the NER gene seen in XP is an example of..?
locus heterogeneity
what are the sx/sx of XP?
sun sensitivity
ocular involvement
increase risk of melanomas
maybe neurologic degeneration
DNA damage seen in XP is both ____ and ___-
cumulative and irreversible
what does nucleotide excision use to remove damage nt?
endonuclease
what does base excision repair use to remove damaged nt?
DNA glycosylases
what does the base excision repair use to remove sugar phosphate?
endonuclease
what repair mechanism is especially important with triplet expansion disorders?
mismatch repair
what does mismatch repair use to recognize mismatch?
MMR proteins (MSH 2/6, MLH1-3, PMS2
how does mismatch repair the excised new strand?
repaired by re-synthesis
how does the cell know what is the “right” strand?
-prokaryotes = methylation
humans = interactions with replication machinery + methylation
what is the defect in hereditary nonpolyposis colon cancer (HNCC)
mutations in mismatch repair genes MSH2*, MLH18, PMS1, 2 or MSH6
what is the result of the mutations in HNCC?
MICROSATELLITE INSTABILITY
what is micro satellite instability?
simple repetitive DNA sequences show size variability to inaccurate replication
what are difficult mutations to repair?
double stranded breaks
*dangerous in dividing cells = may lose genetic material
what are 2 mechanisms to deal with double stranded breaks?
- non-homologous end joining
2. recombinational repair
what is non-homologous end joining
more common
does not use homologous chromosome to repair the break
more prone to error
what is recombinational repair
DOES use homologous chromosome to repair the break
less error prone than NHEJ
what is BRCA 1 and 2 involved in?
DNA repair or apoptosis when DNA can’t be repaired
there are 100s of mutations that have been ID in the BRCA1 gene. What is this an example of?
allelic heterogeneity
what are the serious consequences associated with mutations in DNA repair genes
- increased error rate
2. genomic instability
what is the defect in ataxia telangiectasia?
defect is in ATM
what is the structure and function of ATM?
serene threonine kinase
detects DNA damage (sensor) and activated cell cycle arrest and DNA repair proteins
