Repair of DNA and XP
Errol C. Friedberg (2003) DNA damage and repair.
Nature 421: 436-440.
Xeroderma Pigmentosum (XP)
Symptoms include:
--- Extreme sensitivity
to sunlight
--- Early onset of skin
cancer
Why are XP patients sensitive to
sunlight?
8 gens
UV
Healthy
DNA
NER
Damaged
DNA
NO
REPAIR
Healthy
DNA
Xeroderma Pigmentosum (XP)
Hereditary Cancer Syndromes involving defects in DNA Repair
• Xeroderma pigmentosum..........Nucleotide Excision Repair
• Hereditary Nonpolyposis......... Colon Cancer Mismatch DNA Repair
• Ataxia Telangiectasia............... Double-Strand Break
• Fanconi Anemia DNA ..............Crosslink Repair
• Li-Fraumeni ..............................Nucleotide Excision Repair
• Breast-Ovarian Cancer.............. Double Strand Breaks
Nucleotide Excision Repair
Daño y reparación del DNA
Exogenous
Metabolism
Endogenous
Cell
Cycle
Arrest
DNA
Damage
DNA
Repair
DNA
Replication
Apoptosis
Permanent
Genetic
Alteration
Disease
Types of DNA damage
• Base Loss
• Base modification & Deamination
• Chemical Modification
• Photodamage
• Inter-strand crosslinks
• DNA-protein crosslinks
• Strand breakage
Base loss
Abasic site -loss of a nucleobase
(apurinic or apyrimidinic)
Deamination
Potential Sites of modification/damage
Chemical Damage
Alkylation
Oxidative damage
UV-induced damage
Pyrimidine dimers
Note: Cytosine residues
can also form dimers
Types of Damage Repair
• Photolyase
• De-alkylation proteins (not catalytic)
• Base Excision Repair
• Nucleotide Excision Repair (GG and TC)
• Recombination Repair
• Error-prone Repair
• Double strand Break Repair (if time permits)
UV-responsive photolyases
Direct reversal (de-alkylating proteins)
Base Excision Repair
Base Excision Repair
BER
NER
Nucleotide Excision Repair
(E.coli)
Nucleotide Excision Repair
(Global Genome Repair -Humans)
Nucleotide Excision Repair
(Transcription Coupled -Humans)
NER
Common features of GGR & TCR
Mismatch Repair
Recombination Repair
Other possible mechanisms of
recombination repair
Mecanismos de Reparación del DNA (DSB)
Recombinacion Homologa
RAD50
MRE11
NSB1
Error Prone Bypass
(E. coli)
Experimental evidence for Error prone repair
(E.coli)
Revertant in His- genes
(umuC mutated strain)
UV-responsive activation of
the umuC gene
DNA repair polymerases
DNA polymerase Eta (XP-V) - addition of two dA residues
across pyrimidine dimers
DNA polymerase Zeta - addition of random residues
across pyrimidine dimers
Non-Homologous End Joining (Double Strand Breaks)
Model for activation of DNA damage repair
Damage & Repair
• Multiple forms of DNA damage occur
• These are repaired constantly by several
mechanisms
• Failure to repair damage leads to mutations
• Often defects in damage sensing machinery
or DNA repair processes can be correlated
with increased incidence of diseases such as
cancer
Factors involved in Damage Sensing
Apoptosis and DNA Repair
• Objectives:
– 1) Understand that programmed cell death or
apoptosis is genetically controlled and is an
important factor in tumour growth
– 2) Describe the relationship of abnormal DNA
repair to genetic instability and cancer-prone
syndromes
– 3) Describe DNA-damage activated cell-cycle
checkpoints and how they prevent mutation and
abnormal cellular division
The
Importance
of
DNA-dsb
Repair
Van Gent et al, 2001
Molecular
DNA-dsb Repair
BRCA1/2
Van Gent et al, 2001
DNA Damage-Induced
Protein Interactions
Nbs1
Mre11
DAPI
H2AX
BP53 BRCA1
0
0.5
Merged
Rad50, Rad51
3
6
12
Time after IR (hrs)
Indicators or Biomarkers of DNA Repair ?
BRCA1/BRCA2
• Tumour suppressor genes associated with high risk of
breast cancer at young age with bilateral tumour risk
(BRCA1 also ovarian and prostate/colon; BRCA2 also
ovarian, male breast)
• These mutations account for 80% of familial breast
cancer; yet only 5-10% of breast cancers are familial,
the others are sporadic
• NOT associated with sporadic breast cancer (unlike
p53)
• Both proteins involved in the repair of DNA double
strand breaks and predispose to aberrant DNA
replication and lead to mutations and cancer
ATM
BRCA1/2 proteins may have a role in homologous DNAdsb repair due to ATM, rad50 and rad51 interactions and
mutant BRCA1/2 proteins can not interact appropriately
HNPPC and Mismatch Repair
• Hereditary non-polyposis colorectal cancer (HNPPC) is
a disorder of faulty mismatch repair and genetic
instability
• evolution of multiple tumours occurs much more
rapidly at young age (<50 yrs) and accounts for 2-4%
of all colonic cancers
• Have decreased ability to repair replication errors
(RER+ phenotype) due to a lack of the MLH1 or MSH2
genes which normally remove incorrect DNA base pairs
• Leads to DNA microsatellite instability (MSI)
detectable on gel electrophoresis; interestingly MSI+
colon cancers have a better prognosis following therapy
Microsatellite Instability (tandom repeats of DNA) is
a sign of MMR-deficiency and may be due to MSH2-MLH1
mutations (IHC) = Diagnostic Testing
Chromosomal Control and
Cancer: Telomeres
• Cells are capable of only a limited series of divisions before they
arrest or senesce and chromosomal fusions and cell death
• Telomeres caps of chromosome ends and function to prevent DNA
loss during DNA replication and provide a cellular clock for cell
proliferation
• They consist of a specific sequence TTAGGGG associated with
proteins (TEP, hTERT)
• As somatic cells normally age, telomeres reduce in length due to
decreased function of telomerase, an enzyme which is used by
germline cells to maintain telomere length = Cancer Prevention
or Tumour Suppression
• In human tumours, telomerase activity is abnormally high leading
to abnormal control of cell growth and proliferation
Normal and Abnormal Telomere Functioning
In Normal or Cancer Cells
Harrington, 2001
SUMMARY
-Apoptosis is triggered by a number of external stressors
including chemo- or radiotherapy and is controlled by the
p53, myc and bax-bcl-2 proteins
-Distinct morphologic features define apoptotic cells secondary to
caspase activity
-The relative level of apoptosis versus cell proliferation determines
selection of those mutant cells which may proliferate to form
a tumour
-Defects in DNA repair of UV-damage, DNA replication errors or
DNA-dsbs can lead to genetic instability and genetic mutation
-These defects lead to cancer-prone syndromes such as xeroderma
pigmentosum, HNPPC and BRCA1-associated tumours in which
patients are sensitive to specific DNA-damaging agents, develop
cancers at an early age and have chromosomal instability in tissues