Tissue Effects of Radiation at the Cellular Level by hcj


									 Tissue Effects of Radiation at the
           Cellular Level
Jeffrey Bryan, DVM, MS, PhD, DACVIM(Oncology)

         Cellular Radiation Effects

•   Cell membrane - Alteration in permeability
•   Cellular organelles - Functional Aberrations
•   Nuclear membrane - Altered permeability & Function
•   DNA - Chromosomes - Functional aberrations
               DNA (Chromosomes)
• The DNA makes up the chromosomes of the cell and carries all of the
  functional encoding information of the cell or organism

• All of the chromosomes together make up the genome

• The genome is composed of many genes (60,000 in humans)

• The individual genes are composed of sequences of nitrogenous bases
  attached to the molecular backbone. These sequences encode for protein
  functions etc. which control all cell functions

• Large areas of a DNA strand may not be expressed in individual cells
               DNA Structure
• Double stranded helix (twisted ladder millions of
  rungs long) with side rails of ladder composed of
  Sugar molecules bound together by a phosphate
• Rungs are composed of the nitrogenous bases
  Adenine, Thymine, Guanine and Cytosine.
• Adenine and Thymine combine to make up one type
  of rung and Guanine and Cytosine combine to make
  up another type.
• A given base may be on either side of the helix
                 DNA Structure
• DNA is a very large molecule. There are about 2 x
  109 base pairs in the mammalian genome distributed
  across 15-100 chromosomes.

• The stearic configuration (shape) of the molecule
  changes constantly and is important to function.

• DNA is replicated at cell division
DNA Structure
DNA Structure
DNA Size
DNA Radiation Injuries
   Mechanism of radiation Injury
• Direct ionization of a portion of the DNA

• Indirect injury by free radicals in the DNA
  – H+, 0H-, H202-, etc.
Mechanism of radiation Injury
          DNA Radiation Injuries
•   Base pair deletion
•   Cross-linking injuries
•   Single Strand Break
•   Double Strand Break
•   Multiple (complex) lesions
       Radiation Induced Chromosomal
•   Chromatid exchanges.
•   Sister Unions
•   Acentric Fragments
•   Rings
•   Dicentric Unions
Radiation Induced Chromosomal
Radiation Induced Chromosomal
Radiation Induced Chromosomal
Radiation Induced Chromosomal

Radiation Induced Chromosomal

       Chromosome 5 pair
Comet Assay
Radiation Induced Chromosomal
          DNA Replication
• DNA is replicated during S Phase prior to the
  onset of mitosis

• The original DNA is used as a template for the
  building of the new DNA.

• Quite rapid process, requires less than 15
DNA Replication
                   Cell Division
• Mitosis
  – Multistep process
     • DNA organizes into identifiable chromosomes
       (Prophase )
     • DNA aligns with centromeres on equatorial plate
     • DNA Separates and moves to opposite ends of cell
     • Cell cytoplasm divides at equatorial plate (Telophase)
Cell Division
• Cell resumes normal functional operations
• Only requires about one hour
• Through this process radiation induced
  aberrations in the DNA may result in
  significant loss of DNA to one or both of the
  daughter cells.
                 Cell Cycle
• Tissues grow and are maintained through cell
  replication (regeneration)
• Some cells never divide once adulthood is
• There are a specific set of steps involved
  – G1 (G0)     Gap Phase 1 Functional cell
  –S            Synthesis         DNA synthesis
  – G2          Gap phase 2 Rest
  –M            Mitosis           Cell Division
Cell Cycle
       Repair of Radiation Injury
• Cellular mechanisms are in place which can
  repair most if not all types of radiation injury
  to the DNA.
• Repair is a time sensitive process
• Repair is a cell cycle dependent process
• Repair is a dose rate dependent process
• Repair is dose dependent
• Repair is radiation type dependent
   Cellular Mechanisms of Repair
• Base Excision Repair
  – Damaged bases must be repaired
  – The complementary base on the opposite strand
    serves as a template.
  – This type of repair is quite efficient
  – Loss of this repair mechanism increases the
    incidence of mutations.
   Cellular Mechanisms of Repair
• Nucleotide Excision Repair (NER)
  Repairs DNA damage due to pyrimidine dimer
  adducts added to the DNA by injury.
  - Enzymatic removal of lesion and associated
  - Lesion is then sealed by DNA polyemerase and
  - Defective mechanism increases sensitivity to
       UV light
   Cellular Mechanisms of Repair
• Double Strand Break Repair
  – Non-homologous End Joining
     • Occurs primarily in G1 phase when no sister chromatid
       is present.
     • In some instances the base pair sequence is filled in by
       repair processes without a template.
     • Complex process with multiple pathways
     • Because it is an error prone process it tends to promote
       development of mutations.
Non-homologous End Joining
   Cellular Mechanisms of Repair
• Double Strand Break repair
  – Homologous Recombination repair
     • Uses sister chromatid as a template to faithfully
       recreate the damage section and join the ends together
     • Occurs in S phase when sister chromatids present
     • Error free process
     • Loss of ability increases radiation sensitivity and
       mutation rate.
Homologous Recombination repair
   Cellular Mechanisms of Repair
• Single strand break repair
• Occurs via similar pathway to Base Excision
• Efficiently done and vast majority of lesions
  are repaired.
• Predominately error free process
   Cellular Mechanisms of Repair
• Because of the efficiency of repair
  mechanisms for all but double strand breaks
  the majority of the cell killing occurring at low
  doses is due to double strand breaks which
  are not repaired.
• At high doses accumulated DNA injury due to
  many single strand breaks and base pair
  deletions becomes more important.
         Types of DNA Damage
• Lethal Damage
  – Irreversible and irreparable – fatal to cell
• Potentially Lethal Damage (PLD
  – Damage which is lethal unless modified by post
    irradiation events
• Sublethal Damage (SLD)
  – Repairable injury to the DNA
             Lethal Damage
• Non repairable injury associated with double
  strand breaks
• Increases with LET up to a point
• Increases with higher doses
      Potentially Lethal Damage
• Not repaired and is lethal under normal
• Repair increased by conditions which are
  suboptimal to the division of the cell
  – Reduced temperature
  – Hypoxia
  – Low pH
  – Others
• Increased capability = radioresistance
     Sublethal Damage Repair (SLD)
•   Refers to DNA damage that is repaired
•   Splitting radiation dose increases survival
•   Occurs in 1-6 hours after irradiation
•   Affected by phase of cell cycle
•   Affected by cell cycle time
    – Long cycle usually increases repair
• Indicated by shoulder on survival curve
    Repair is a time sensitive process
• Repair of DNA injury of all types is essentially
  complete by 6 hours post irradiation.

• External factors that affect cellular metabolic
  rate may delay or accelerate it

• Foundation of modern radiotherapy
  Repair is a cell cycle dependent process

• Different phases have different repair
  – Mitosis has the least repair capability
  – G2
  – G1/G0
  – S phase has the most repair capability

• Capability varies in G1 and S
              Next Time
• Cell Cycle and Differentiation Effects

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