genome instability

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Genomic instability & DNA repair I

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genome Instability

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Page 1: genome Instability

Genomic instability & DNA

repair I

Page 2: genome Instability

Molecular Medicine

• Identifying the molecular basis of human disease

• Translating molecular defects into clinical symptoms

• Understanding molecular basis of human disease

Page 3: genome Instability

Plan• DNA damage and genotoxic agents

• Effects on cells

• DNA repair

• Clinical implications of defective DNA repair - genomic instability

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DNA damage• A large number of DNA lesions occur in our cells• Estimates: 1,000 - 1,000,000 DNA lesions / cell /

day• How do cells cope with these lesions?• DNA - the only molecule in our body that is

repaired• The rest are replaced

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Sources of DNA damage

• Endogenous

• Free radicals

• Exogenous

• Genotoxic agents

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Genotoxic agents• Radiation

– Ionizing• Particles (, ) and rays (X- and )

– Non-ionizing• UV light

• Chemical agents– Alkylating agents, acridine etc.

• Biological agents– Viruses

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Types of DNA damage caused by genotoxic agents

apurinic site (alkylating agents)

intercalation (acridine)

alkylation (alkylationg agents)

adducts (many chemicals)

pyrimidine dimers (UV radiation)

double-strand breaks (ionizing radiation)

interstrand cross-links (alkylating agents)

intrastrand cross-links (alkylating agents)

apyrimidinic site (alkylating agents)

single-strand break (ionizing & UV rad.)

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DNA damage response• Network of cellular mechanisms that

sense, signal and repair DNA damage• Linked with cell cycle checkpoint

mechanisms• Linked with mechanisms that regulate

apoptosis

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DNA damage response (DDR)

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How is DDR regulated?

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Clinical implications• Low fidelity repair - Genetic defects in cellular

mechanisms responsible for DNA damage response • Sensitivity to genotoxic stress

– When individual exposed to radiation or chemicals in cancer therapy

• Long term effects– Cancer predisposition– Premature ageing– Immunodeficiency– Infertility

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Clinical implications• Low fidelity repair - Genetic

defects in cellular mechanisms responsible for DNA damage response

• Sensitivity to genotoxic stress– When individual exposed to

radiation or chemicals in cancer therapy

• Long term effects– Cancer predisposition– Premature ageing– Immunodeficiency– Infertility

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11_21.jpg

Depurination and deamination

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11_21_2.jpgA thymidine dimer

How chemical changecauses a mutation

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DNA repair mechanisms• BER (base excision repair)• NER (nucleotide excision repair)

– Global NER

– Transcription coupled NER

• MMR (mismatch repair)• DSB repair (repair of DNA double strand breaks)

– Homologous recombination (HR)

– Non-homologous end-joining (NHEJ)

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NER• Two types

– Global genomic NER– Transcription coupled NER

• Steps – Damage recognition– Binding of a multi-protein complex at the damaged site– Double incision of the damaged strand– Removal of the damage-containing oligonucleotide from between

the two nicks– Filling in of the resulting gap by a DNA polymerase– Ligation

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Proteins involved in NER

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Global genome NER

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Transcription coupled NER

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Clinical consequences of dysfunctional NER

• Xeroderma pigmentosum (XP) (MIM 278700)

• Cockayne’s syndrome (CS) (MIM 216400)

• Trichothiodystrophy (TTD) (MIM 601675)

• Complementation groups: 7 XP, 2 CS & 2 TTD

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XP• Seven complementation

groups: XPA - XPG I.e. seven genes identified

• Symptoms: light sensitivity, pigmentation irregularities

• Frequent neurological defects

• High incidence of early onset skin cancer

• Elevated frequency of other forms of cancer

• Premature ageing

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CS• Abnormal and slow growth• Dwarfism• Skin photosensitivity, thin dry hair• Facial and limb abnormalities• Neurological abnormalities• Early death due to neurodegeneration • Premature ageing in some tissues• Molecular genetics

– Two complementation groups (CSA & CSB)– CSA - ERCC8 (5q11)– CSB - ERCC6 (10q11)

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TTD

• Sulfur deficient brittle hair and nails• Facial abnormalities and short stature• Ichthyosis (fish-like scales on the skin)• Light sensitivity in some cases• Premature ageing in some tissues• Molecular genetics

– XPD– TTD-A

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MMR

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Clinical consequences of defective MMR

• HNPCC - Hereditary Nonpolyposis Colon Cancer