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Xeroderma Pigmentosum (XP)
Symptoms include:
---Extreme sensitivity to sunlight
---Early onset of skincancer
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
DNADamageMetabolism
Exogenous
Endogenous
DNAReplication
Permanent Genetic
Alteration
Disease
CellCycleArrest
DNARepair
Apoptosis
Daño y reparación del DNA
• Base Loss• Base modification & Deamination
• Chemical Modification• Photodamage• Inter-strand crosslinks
• DNA-protein crosslinks• Strand breakage
Types of DNA damage
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)
Mecanismos de Reparación del DNA (DSB)
Recombinacion Homologa
RAD50
MRE11
NSB1
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
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
Apoptosis and DNA RepairApoptosis 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
DNA Damage-Induced DNA Damage-Induced Protein InteractionsProtein Interactions
Nbs1 Mre11 DAPI Merged
Time after IR (hrs)
0 0.5 3 6 12
H2AXBP53BRCA1 Rad50, Rad51
Indicators or Biomarkers of DNA Repair ?
BRCA1/BRCA2BRCA1/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 DNA-BRCA1/2 proteins may have a role in homologous DNA-dsb repair due to ATM, rad50 and rad51 interactions and dsb repair due to ATM, rad50 and rad51 interactions and mutant BRCA1/2 proteins can not interact appropriatelymutant BRCA1/2 proteins can not interact appropriately
HNPPC and Mismatch RepairHNPPC 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) isMicrosatellite Instability (tandom repeats of DNA) isa sign of MMR-deficiency and may be due to MSH2-MLH1a sign of MMR-deficiency and may be due to MSH2-MLH1
mutations (IHC) = Diagnostic Testingmutations (IHC) = Diagnostic Testing
Chromosomal Control and Chromosomal Control and Cancer: TelomeresCancer: 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 Preventionor Tumour Suppression
• In human tumours, telomerase activity is abnormally high leading to abnormal control of cell growth and proliferation
Normal and Abnormal Telomere FunctioningNormal and Abnormal Telomere FunctioningIn Normal or Cancer CellsIn Normal or Cancer Cells
GTTAGGGTTA
5'3'
TEP1hTERT
TEMPLATE
INACTIVE ACTIVE
shortened telomeres
cell senescence
stable telomeres
cell survival
AGING CANCER
TELOMERASE
Harrington, 2001
SUMMARYSUMMARY-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 orDNA-dsbs can lead to genetic instability and genetic mutation -These defects lead to cancer-prone syndromes such as xerodermapigmentosum, HNPPC and BRCA1-associated tumours in whichpatients are sensitive to specific DNA-damaging agents, develop cancers at an early age and have chromosomal instability in tissues