DNA Repair: Lect 16 Flashcards
DNA damage:
- short term consequences;
- long term consequences;
- in thousands of cells per day
- due to basic chemistry of nucleic acids, UV light, chemicals, radiation, pH, smoking.
- reduced proliferation, altered gene expression and cell death.
- aging and diseases (cancer).
Molecular basis of mutation:
- Spontaneous or induced
- induced mutations increase rate of spont mutations.
Two classes of Spontaneous mutation:
- Errors of replication: mistakes made during replication.
- only occurs during S-phase
- wrong base incorporated by DNA poly.
- Tautomerism - Spontaneous lesions: chemical changes that occur spontaneously.
- occurs in resting cell/during replication.
Proof reading:
- a DNA repair mechanism
- slows down replication rate in order for the DNA to be checked.
- DNA poly has a 5’ to 3’ polymerase activity and a 3’ to 5’ exonuclease activity.
Bloom syndrome:
defect in BLM gene
- DNA helicase required for replication repair and recombination.
- Chrom instability resulting in many chrom breaks and sister chromatids exchanges.
- higher risk of cancer
Characteristics:
- smaller than average, narrow chin, prominent nose and ears, facial rash upon sun exposure.
- Diabetes, neurological, lung and immune syst deficiencies.
Fanconi anemia
- AR rare disorder
- multiple genes involved “locus heterogeneity”
- inc. spontaneous chrom breakage made worse by exposure to DNA cross linking agents.
- inc. risk of neoplasia.
Char:
-radial ray defects, pancytopenia, mental development problems, short stature.
Frameshift mutations:
- tends to occur at positions where there are base repeats (GTCGAAAAACTCA)
- DNA loops or kinks at these points and one or more bases are not copied or are copied twice.
Spontaneous lesions
- changes in resting cell
- extremely common
- increased by exposure to mutagens (sunlight).
- 3 main types
3 types of Spont. lesions:
- depurination:
- most common; breaking of glycosidic bond btwn base and sugar in purine nucleotides.
- sugar-phosphate remains but base is lost. - deamination:
- very common; loss of amine grp from base cytosine
- cytosine deaminates to form uracil but uracil is not in DNA; so easy to fix.
- if 5-Me-cytosine deaminates to thymidine = mutational hotspot. - oxidative damage:
- from production of reactive oxidative compounds due to oxidative metabolism. (superoxides, peroxides)
- addition of oxygen groups to nucleotide bases
- result in potential tranversion
Mutagens
- increase the frequency of nl mutations. (mismatches, depurinations)
- UV, ionizing radiation, aflatoxin, benzene, formaldehyde, mustard gases (serious ones)
Ionizing Radiation
- X-rays and radioactive particles.
- high energy particles or rays can cause many types of cellular damage and apoptosis.
- damage to DNA and heritable mutations.
UV light
- generates several deleterious photoproducts.
- interfere w/ normal pairing and block replication
- pyrimidine dimers or thymine dimers.
Spontaneous DNA lesions?
most often have apurinic sites.
Indirect DNA Repair
Nucleotide excision:
- removes more than a few base (~30) around damaged site.
- repairing of pyrimidine dimers from UV damage.
Base excision:
- repairs a single or few damaged bases by removing it
- methylation or oxidation damage
Mismatch Repair:
- post replication repair
- repairs mismatched bases from tautomerism.
Excision repair mechanism:
- recognition of damage
- removal of damaged base or region around damaged base via endonucleases.
- replacement of excised region; DNA poly fills in gap and Ligase seals it.
Xeroderma pigmentosum
- XP
- AR, mutations in 9 diff NER gene (locus heterogeneity)
- extreme sun sensitivity (blistering, freckles w/ hyperpigmented skin lesions)
- ocular involvement (conjunctivitis, ocular tumors)
- > 1000x incr in skin cancer.
- 20% have progressive neurologic degeneration.
- DNA damage is cumulative and irreversible once it happens.
Base excision repair:
- damaged nucleotides removed by DNA glycosylases which recognize specific damaged bases in DNA.
- ex: Uracil gylcosylase remove U from DNA, sugar phos removed by endonuclease then replace base and ligate.
Mismatch repair:
- post-replicative repair mechanism.
- form of excision repair
- mismatched bases recognized and excised.
- removal of small repeats that expand (3x expansion disorders).
- MMR proteins recognizes mismatch missed by proofreading.
- Repair occurs in S-phase(missed by PR) or G2 when genome is scanned for errors.
- Excise base around mismatch
- Repair by re-synthesis
Strand discrimination;
- how a cell knows which is the right strand.
- in prok; due to methylation
- in higher org; due to interaction w/ replication machinery or methylation.
Hereditary Nonpolyposis Colon Cancer
- mutations in genes encoding mismatch repair proteins MSH2, MLH1
- results in microsatelitte insability (diagnostic); simple repetitive DNA seq show size variability due to inaccurate replication.
Contraindicated treatment for Blooms syndrome?
-CT scan, UV tanning bed, X-ray, DNA damaging agents.
Double stranded breaks;
-two mechanisms that deals w/ this?
-difficult type of mutation to repair w/ high probability of loss of genetic material.
- Non-homologous end joining:
- more common and does not use hom chrom to repair the break. - Recombinational repair:
- uses hom. chrom
- less error prone
BRCA1 and BRCA2
Which is allelic heterogenous?
- in breast cells and other tissue
- mutations in 1 or 2 = 85% risk of breast cancer by age 70.
- ovarian cancer 55% for BRCA1 and 25% for BRCA2
-100s of mutations in BRCA1 gene = allelic heterogeneity
Somatic errors:
Germline errors:
- can result in cancer and aging
- can result in genetic diseases.
Increased error rate:
Genomic Instability:
- for proofreading and mismatch repair, produces mutator phenotype.
- mutated bases not repaired efficiently.
-mutations in genes involved in resting DNA repair and chrom break repair will de-stabilize the genome. e.g. Bloom syndrome.
Ataxia telangiectasia
- defects in ATM; a serine threonine kinae which detects DNA damage and activates cell cycle arrest and DNA repair proteins.
- AR inheritance.
- affects cerebellum and immune syst.
- incr incidence of cancer and ocular telangiectasia is common.
ATM/ATR signaling pathway:
Genotoxic stress > sensor proteins > apical kinases > signal relay proteins > effector proteins > responses.
