Workshop 3

• Genetic toxicology is a branch of toxicology that focuses on the study of the genetic effects of chemicals, drugs, and other agents on living organisms
• Genetic toxicology

  Involves assessing the potential of substances to cause damage to the genetic material (DNA) within cells
• The primary aim of genetic toxicology is to identify substances that have the potential to induce mutations or other genetic alterations, which can lead to adverse health effects such as cancer, birth defects (congenital defects), and reproductive disorders
• Targets of DNA damage


  • Somatic cells (detrimental to the exposed individual)
  • Germ(minal) cells (potentially heritable effects)
  • Mitochondrial DNA (detrimental to the exposed individual & progeny via maternal inheritance)
• Somatic cells

  Any cells in the body that are not specialized for reproduction
• Germ cells

  Cells involved in reproduction and the formation of gametes (sperm and egg cells) in sexually reproducing organisms
• Mechanisms for Genetic Damage

  • DNA Damage
  • Induction of Mutations
  • Chromosome Aberrations
  • Interference with DNA Repair Mechanisms
  • Induction of Oxidative Stress
  • Impaired DNA Replication and Transcription
• DNA Damage


  Chemicals, radiation, and other agents can directly damage the structure of DNA molecules
• Induction of Mutations


  Changes in the DNA sequence that can result from errors during DNA replication or from the action of mutagenic agents
• Chromosome Aberrations

  Genotoxic agents can cause structural changes in chromosomes
• Interference with DNA Repair Mechanisms

  Cells have intricate DNA repair mechanisms that constantly monitor and repair damaged DNA
• Induction of Oxidative Stress

  Reactive oxygen species (ROS) generated during normal cellular metabolism or in response to exogenous agents can cause oxidative damage to DNA
• Impaired DNA Replication and Transcription

  Genotoxic damage to DNA can interfere with the processes of DNA replication and transcription, leading to errors in genome duplication and impaired gene expression
• Understanding the mechanisms of genetic damage is essential for evaluating the genotoxic potential of chemicals, drugs, and environmental agents, as well as for developing strategies to mitigate their adverse effects on human health and the environment
• Base substitution mutation

  A point mutation where one nucleotide base in the DNA sequence is replaced by another
• Transitions


  Involve the replacement of one purine base (adenine or guanine) with another purine base, or the replacement of one pyrimidine base (cytosine or thymine/uracil) with another pyrimidine base
• Transversions

  Involve the replacement of a purine base with a pyrimidine base, or vice versa
• Frameshift mutation

  The addition or deletion of one or a few base pairs (not in multiples of 3 [codon]) in protein-coding regions
• Types of Chromosome Aberrations

  • Structural chromosome aberrations
  • Numerical chromosome changes
• Structural chromosome aberrations

  Non-radiomimetic chemicals can arise from errors of DNA replication on a damaged template, while radiomimetic chemicals can directly induce strand breaks
• Numerical chromosome changes

  Involve non-diploid variations in chromosome number in the nucleus, such as monosomies, trisomies, and other ploidy changes
• Aneuploidy

  Gain or loss of individual chromosomes, resulting in an imbalance in the chromosome number
• Sister chromatid exchanges (SCE)

  Can be produced during S phase as a consequence of errors in the replication process and are apparently reciprocal exchanges
• DNA Repair Mechanisms

  • Base excision repair
  • Nucleotide excision repair
  • Mismatch repair
  • Homologous recombination
  • Non-homologous end joining
  • O6-methylguanine-DNA methyltransferase repair
• Base excision repair

  Removes damaged DNA bases and replaces them with the appropriate base
• Nucleotide excision repair

  Removes bulky lesions from DNA, involving a process with up to 30 proteins
• Homologous recombination

  Involves steps like exonuclease or helicase activity, strand invasion, Holliday junction formation, and cleavage and resolution
• Nonhomologous end-joining

  Involves DNA-dependent protein kinase, end processing, and ligation, and is considered more error-prone compared to homologous recombination
• Mismatch repair

  Recognizes and binds to mismatched bases, followed by excision, resynthesis, and ligation
• O6-methylguanine-DNA methyltransferase repair

  Protects against simple alkylating agents by transferring methyl groups from O6-methylguanine in affected DNA
• Cells in S phase (DNA synthesis) are most susceptible to genetic injury because they are actively replicating their DNA
• Homologous recombination (HR)

  One of the DNA repair mechanisms for DSBR (double-strand break repair)
• Nonhomologous end joining (NHEJ)

  One of the DNA repair mechanisms for DSBR (double-strand break repair)
• Mismatch repair

  1. Mismatched bases can be formed during DNA replication, genetic recombination, or chemically-induced DNA damage

  2. A specific protein recognizes & binds to the mismatch and additional proteins stabilize it

  3. This is followed by excision, resynthesis, and ligation
• O6-methylguanine-DNA methyltransferase repair

  By transferring methyl groups from O6-methylguanine in affected DNA, this repair mechanism protects against simple aklylating agents
• DNA enzymatic repair mechanisms developed to maintain fidelity and integrity of genetic information
• Enzymes are able to remove and replace damaged segments of DNA
• Enzymatic repair mechanisms are particularly useful during low-level exposure where excision repair enzymes are not fully saturated by excessive DNA damage
• Stimulation of repair activity following treatment at sublethal concentrations can indicate presence of DNA-directed toxicity
• Cells in S phase (DNA synthesis) are most susceptible to genetic injury because they have less time to repair the damage prior to mitosis