genome instability

Genomic instability & DNA

repair I

Molecular Medicine

• Identifying the molecular basis of human disease

• Translating molecular defects into clinical symptoms

• Understanding molecular basis of human disease

Plan• DNA damage and genotoxic agents

• Effects on cells

• DNA repair

• Clinical implications of defective DNA repair - genomic instability

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

Sources of DNA damage

• Endogenous

• Free radicals

• Exogenous

• Genotoxic agents

Genotoxic agents• Radiation

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

– Non-ionizing• UV light

• Chemical agents– Alkylating agents, acridine etc.

• Biological agents– Viruses

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.)

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

DNA damage response (DDR)

How is DDR regulated?

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

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

11_21.jpg

Depurination and deamination

11_21_2.jpgA thymidine dimer

How chemical changecauses a mutation

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)

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

Proteins involved in NER

Global genome NER

Transcription coupled NER

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

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

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)

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

MMR

Clinical consequences of defective MMR

• HNPCC - Hereditary Nonpolyposis Colon Cancer