Causes of Mutations: DNA Damage

Question 1

Spontaneous DNA damage

Answer 1

Hydrolysis - Oxidative deamination results in:

• C –> U and Methyl-C –> T
• A –> Hypoxanthine (less than 10 cells a day)
• G –> Xantine (less than 10 cells a day)

Oxidising agents can increase rate

Question 2

Base Loss

Answer 2

• Due to hydrolysis of sugar-base bond
• Causes depurination and depyrimidation
• Results in abasic sites of DNA - AP (apurinic/apyrimidinic)

Question 3

Freq. of depurinations + depyrimidations

Answer 3

• 10,000 depurination per cell per day (purines are less stable)
• 500 depyrimidation per cell per day

If not repaired, during DNA replication an adenine nucleotide is inserted opposite the AP site

Question 4

Oxidative Damage

Answer 4

• Aerobic metabolism generates reactive oxygen species (ROS)
• e.g. Singlet oxygen, Hydrogen peroxide, Superoxide anion radicals, H radicals
• Cells actively scavenge reactive oxygen species

Question 5

What reactive oxygen species (ROS) do

Answer 5

• Attacks DNA causing strand breaks and base modifications
• Approx. 20,000 ‘hits’ occur per cell per day

Question 6

Methylation

Answer 6

• Methylation of adenine to form 3-methyladenine which blocks DNA replication
• 600 3-methyladenine residues are generated per cell per day

Question 7

Induced DNA damage - UV Light

Answer 7

• UV-C and UV-B induce pyrimidine dimers
• 2 adjacent pyrimidine bases (C or T) on the same strand are linked covalently
• These dimers block transcription + DNA replication
• If not repaired it can cause C to T mutations

Question 8

Induced DNA damage - Ionizing Radiation

Answer 8

• E.g. X-rays, radon, nuclear fallout
• Generates ROS species
• Can create localised damage with multiple hits including double strand breaks

Question 9

Induced DNA damage - Intercalating agents

Answer 9

• E.g. proflavin, acridine orange
• Cause base insertions or deletions resulting in frameshifts

Question 10

Induced DNA damage - Base adducts

Answer 10

• Many mutagens add chemical groups to the DNA bases
• Alkylating agents add alkyl groups to bases
• Polyaromatic hydrocarbons (PAHs) like benzopyrene + polychlorinated biphenyls are found in soot, exhaust gas, cigarette smoke + charred food

Question 11

Induced DNA damage - Base adducts - Aflatoxins

Answer 11

• Produced by moulds that grow on food
• Leads to base loss and formation of apurinic sites
• With hepatitis B/C exposure increases liver cancer risk

Question 12

Metabolism and toxicity

Answer 12

• Many chemicals are only toxic once metabolised like benzopyrene where a more toxic compound is created

Question 13

Mutational Signatures

Answer 13

• Exposure to different environmental agents can cause mutations
• Most agents produce complex signatures
• Whether a residue is mutated can depend on neighbouring sequences

Question 14

Skin cancer’s mutational signature

Answer 14

• If pyrimidine dimers are not repaired, they result in C to T and CC to TT transitions

Question 15

Lung cancer’s mutational signature

Answer 15

PAHs predominantly cause G to T transversions

Question 16

Mutational Signatures - 1,2-DMH

Answer 16

• 1,2-dimethylhydrazine is a DNA methylating agent
• Causes C to T mutations and C deletions