Chemical Carcinogenesis and Mutations

Learning objectives

1. Explain the difference between an initiator and a promoter and the role of each in carcinogenesis.

2. Describe the difference between an indirect and direct carcinogen and explain how cells convert indirect carcinogens into direct carcinogens.

3. List the types of DNA damage caused by carcinogens.

4. List common DNA repair mechanisms and the types of damage each one repairs.

5. List the different types of mutations and chromosomal alterations and describe how they contribute to cancer.

6. Explain the rationale for using carcinogenic agents as cancer therapies.

What is a carcinogen (provide examples)

• any agent or exposure with the capacity to cause cancer

• high energy radiation

  • X-ray, UV, Gamma Radiation

• Biologic

  • Viruses (HBV, HPV)

  • Bacteria (H. Pylori)

• Hormonal

  • estrogen, progesterone

• natural

  • tabacco,

• chemical

Initiators and promoters

Initiation and initiators

• Most carcinogen-mediated DNA damage does not result in Initiation.

• Initiation only occurs if the damaged DNA is not repaired leading to a mutation, AND the mutation alters a gene with a critical role in cellular growth, escape from senescence, survival, growth factor signaling (oncogenes mostly, but some tumor suppressor genes as well).

• Initiation is irreversible but Initiated cells must pass mutated genes to daughter cells for carcinogenesis to continue.

• Initiation does not equal transformation, it is just the first step and cells have many checkpoints to safeguard the genome and eliminate initiated cells, so most initiated cells will not progress to cancer.

  • Additional mutations or epigenetic events are needed to drive the transformation of an initiated cell

Promoters and promotion

• Promoters drive the clonal expansion of initiated cells resulting in a preneoplastic clone and eventually hyperplasia or the abnormal growth of cells.

• Additional Promotion allows for additional mutations to accumulate resulting in a malignant tumor.

• Promoters are not tumorigenic in absence of initiating mutations

• promoters can be

  • exogenous (chemicals)

  • endogenous such as

    • Unopposed estrogenic stimulation of the endometrium and breast

    • Chronic inflammatory (e.g., inflammatory bowel disease, chronic hepatitis, and Barrett esophagus)

    • Chronic wound or tissue repair

How do carcinogens influence cancer progression even after the tumor forms?

• they can promote the accumulation of additional mutations

What is a DNA adduct

• a segment of DNA covalently bound to a chemical moiety (foreign chemical or toxin)

How do carcinogens cause mutations to DNA + ex of DNA damage

• Carcinogens typically do not directly change the DNA sequence. They instead cause DNA damage (DNA adducts or strand breaks).

• If not repaired properly the adduct can result in errors during DNA replication resulting in a mutation.

• examples

  • Single base modifications- a base is chemically altered without dramatically altering the double-helix structure (oxidation, deamination, or alkylation)

  • Bulky” Adducts - single base adduct that disrupts helix structure

  • Dimers - 2 bases on the same strand (interstrand) or opposite strands (intrastrand) are covalently crosslinked

  • Strand breaks - single or double-strand breaks in DNA

List common carcinogens and how they damage DNA

• Ionizing Radiation (X-Ray, Gamma-Ray) causes chromosome breakage, translocations, and some point mutations

• UV Radiation – Causes cross-linking pyrimidine bases

• Chemical – Cause mostly single base DNA adducts and sometimes dimers

  • Aromatic Amines, Amides, Azo Dyes

• Natural –Aflatoxin B1 - metabolized to epoxides- forms DNA adducts, associated with p53 and Ras mutations and liver carcinogenesis

  • Betel Nut- forms DNA adducts

  • Benzopyrene and other Polyaromatic Hydrocarbons in cigarette smoke - metabolized to epoxides- forms DNA adducts

• Hormonal - tumor promoters

  • esrogen, progesterone

• Biological - Deliver oncogenes or inactivate TS genes. Can also act as promoters

  • Viruses (HBV, HBC, HPV, etc.), Bacteria (H. Pylori)

Define indirect and direct carcinogens and explain how indirect carcinogens are converted to direct carcinogens.

• Direct or Ultimate Carcinogens

  • Can directly react with DNA, RNA, or other molecules in cells causing damage

  • Typically electrophilic (electron-deficient) so react with nucleophilic (electron-rich) molecules (DNA, RNA, Proteins)

• Indirect or Procarcinogens

  • Require metabolic activation by cells to form a Direct carcinogen

  • Most carcinogens act Indirectly

Metabolic activation of carcinogens

• Indirect Carcinogens are metabolized by two sets of enzymes.

• The Phase I enzymes use oxidation, hydrolysis, and reduction to generate a reactive species

• which Phase II enzymes then prepare for excretion by detoxifying and conjugating to make it water-soluble.

Common DNA repair Mechanisms

• Mismatch Repair (MMR) replaces inserted, deleted, and mis-matched bases (usually due to replication errors) using the complementary strand as a template.

• Base Excision Repair (BER) corrects single base lesions that do not significantly distort the DNA helix structure or single strand breaks by excising/replacing the base

• Nucleotide Excision Repair (NER) removes bulky DNA lesions (bulky adducts and dimers from crosslinks by excising a small fragment of DNA from one strand. Opposite strand is used to replace excised bases.

• Homologous Recombination (HR) Repairs double-strand breaks and some intrastrand crosslinks by using the either sister chromatid or homologous chromosome as a template.

• Non-homologous End Joining (NHEJ) Repairs double-strand breaks without homologous DNA by joining the ends of the break.

List examples of familiar cancer syndromes that involve DNA repair and the genes that are mutated in each

• Li-Fraumeni syndrome: extremely high risk of developing cancers due to inherited mutations in p53

• Familial breast and ovarian cancer -is associated with inherited defects in homologous recombination repair genes (like BRCA1, BRCA2)

• Bloom syndrome: developmental anomalies and increased risk of developing many different types of cancer due to inherited loss-of-function mutations in a helicase that is required for homologous recombination repair

Image diagram of chemical carcinogenesis steps (includes initiation and promotion)

Chemotherapy as a carcinogen

• DNA reactive agents are often used as therapies for cancer

• these reactive agents are themselves carcinogens

• the goal is to create enough DNA damage to these rapidly dividing cells to cause cell death

• the possibility for new initiating events and secondary cancers exists

Some of the main causes of cancer

• Exposure to environmental carcinogens is thought to play a causal role in more than 90% of all cancers

• Exposure to avoidable carcinogens (cigarette smoke, alcohol, poor diet, and UV radiation) contributes to a high percentage of human cancers

• genes contribute to only about 5-10% of cancer

What are the 6 DNA mutation types

• point mutation

  • silent: do not change amino acid

  • missense: chnage amino acid to another amino acid which could be conservative or nonconservative

  • nonsense: premature stop codon

• substitution: one or more bases are replaces by the same number of bases

• insertion: one or more bases are added

• deletions: one or more bases are removed

• inversions: The sequence of bases is reversed

• frameshift

3 Different chromosomal alterations and how they lead to cancer

• Amplifications: increased copies of a gene

  • typically proto-oncogenes

• deletions: part of the chromosome is lost during DNA replication

  • deletion of tumor suppressor genes

  • deletion of the RB gene are common in retinoblastoma

• translocations: a chromosome abnormality caused by the exchange of parts of 2 non-homologous chromosomes

  • can increase protooncogene expression by promoter or enhancer substitution

  • generation of oncogenic fusion proteins

in what genes and what mutations would you predict would promote cancer

• carcinogenesis would involve nonsense mutations or non-conservative mutations or a frame shift in tumor supressors and gain of function missense mutations in proto-oncogenes

• loss of function

  • P53

  • PTEN

  • BRCA1/2

  • APC

• Gain of function

  • RAS

  • BRAF

  • RTKS