Lecture 8 | DNA Damage

Causative agent

Something that causes DNA damage

• a single agent may cause more than one type of damage

Lesion

Generic term for DNA damage

• mutations result when DNA damage is not properly repaired

Adduct

A type of lesion, and add-on to the DNA

Mutation

Change in DNA sequence

• not all mutations are carcinogenic, but a considerable amount can lead to cancer

Spontaneous DNA damage

Happens in the body in the absence of an external agent and can lead to mutations if not repaired

• can even be an initiating event in cancer

Sources:

• Natural instability of DNA

  • Deamination

  • Depurination

  • Oxidized bases

  • ss DNA breaks

• Oxidative damage

• DNA polymerase errors

Induced DNA Damage

• Alkylation

• Oxidative damage → ionizing radiation

• Bulky adducts, including UV dimers

When does spontaneous DNA damage occur

• Happened in the body in the absence of an external agent

• If not repaired, can lead to mutations

• Can be an initiating event in cancer

Deamination

Spontaneous DNA damage

Lose of NH2 is most common

C → U

DNA replication reads U as T, resulting in C to T point mutations → transition mutation

So… converts 5meC → T

• 5meC are regulatory marks in CpG islands and hotspots for mutagenesis and cancer

• 1/3 of all inherited human diseases are at 5meC in Cp6 islands

  

Depurination

Spontaneous DNA damage

Hydrolysis of glycosylic bond (5000/cell/day)

• Result in abasic site → single base substitution (point mutation)

Oxidative damage

Spontaneous and Induced DNA damage

Oxidized bases and single strand breaks

• caused by free radicals O2 -

• Result: oxidized bases → abasic sites

Most common: 8-oxo-deoxyguanosine (8-oxo-dG) & Thymine glycol

problems: 8-oxo-dG mispairs with deoxyadenosine

Where do are radicals come from for oxidative damage?

Both spontaneous and induced

Spontaneous causes: metabolism, inflammation

Induced: Smoking, radiation

Polymerase errors

Spontaneous DNA damage

Overall DNA replication error rate is 10^-10 mutations/bp/cell generation

results: mismatches (point mutations), misalignment (insertions, deletion)

• However, error rate is low in part because of our proof reading mechanism intrinsic to DNA polymerase

Main errors: mainly result from slippage and misalignments and are highest in regions of repetitive sequence

Alkylation

Induced source #1

Caused: Nitrosamines and other methylation

Results: point mutations

Methyldiazonium ion acts as an alkylating agent

Mechanism of radiation-induced cancers

IR penetrates a tissue and gives up energy through a series of random collisions with molecules in its path

• produces e- and free radicals (OH) that damage DNA by breaking chemical bonds

• radiation can damage DNA directly via free electrons

Type of DNA damage induced by radiation

1. Double stand breaks

2. Modified bases leading to basic sites due to chemical instability

3. Single strand breaks (easily repaired) and complex lesions (not easily repaired)

Law of Radiosensitivity

Radiosensitivity of a tissue is directly proportional to its mitotic activity and inversely proportional to the differentiation of the cell

Missense mutation

CHange in aa

Nonsense mutations

introduces a stop codon