Cellular Radiation Effects

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Cellular Radiation Effects

• Cell membrane - Alteration in permeability
• Cellular organelles - Functional Aberrations
• Nuclear membrane - Altered permeability
  Function
• DNA - Chromosomes - Functional aberrations

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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

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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

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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

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DNA Structure
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DNA Structure
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Mechanism of radiation Injury

• Direct ionization of a portion of the DNA
  molecule.
• Indirect injury by free radicals in the DNA
  environment.
• H, 0H-, H202-, etc.

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Mechanism of radiation Injury
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DNA Radiation Injuries

• Base pair deletion
• Cross-linking injuies
• Single Strand Break
• Double Strand Break
• Multiple (complex) lesions

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DNA Radiation Injuries
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DNA Radiation Injuries
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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 hours.

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DNA Replication
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Cell Division

• Mitosis
• Multistep process
• DNA organizes into identifiable chromosomes
  (Prophase )
• DNA aligns with centromeres on equatorial plate
  (Metaphase)
• DNA Separates and moves to opposite ends of cell
  (Anaphase)
• Cell cytoplasm divides at equatorial plate
  (Telophase)

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Cell Division
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Mitosis

• Cell resumes normal functional operations
  (interphase)
• Through this process radiation induced
  aberrations in the DNA may result in significant
  loss of DNA to one or both of the daughter cells.
  
• Only requires about one hour

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Radiation Induced Chromosomal Aberations

• Chromatid exchanges.
• Sister Unions
• Acentric Fragments
• Rings
• Dicentric Unions

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Radiation Induced Chromosomal Aberations
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Radiation Induced Chromosomal Aberations
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Cell Cycle

• Tissues grow and are maintained through cell
  replication (regeneration)
• Some cells never divide once adulthood is
  reached.
• 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

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Cell Cycle
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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

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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.

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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
  backbone.
• - Lesion is then sealed by DNA polyemerase and
  ligase.
• - Defective mechanism increases sensitivity to
  UV light

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Cellular Mechanisms of Repair

• Double Strand Break Repair
• Nonhomologous End Joining
• Occurs primarily in S 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.

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Cellular Mechanisms of Repair

• Double Strand Break repair
• Homologus Recombination repair
• Uses sister chromatid as a template to faithfully
  recreate the damage section and join the ends
  together properly
• Occurs in G1 phase when sister chromatids present
• Error free process
• Loss of ability increase radiation sensitivity
  and mutation rate.

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Cellular Mechanisms of Repair

• Single strand break repair
• Occurs via similar pathway to Base Excision
  Repair.
• Efficiently done and vast majority of lesions are
  repaired.

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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.

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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

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Lethal Damage

• Non repairable injury associated with double
  strand breaks
• Increases with LET up to a point
• Increases with higher doses

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Potentially Lethal Damage

• Not repaired and lethal under normal
  circumstances.
• Repair increased by conditions which are
  suboptimal to the division of the cell
• Reduced temperature
• Hypoxia
• Low pH
• Others
• Increased capability radioresistance

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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

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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

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Repair is a cell cycle dependent process

• Different phases have different repair
  capabilities
• Mitosis has the least repair capability
• G2
• G1/G0
• S phase has the most repair capability
• Capability varies in G1 and S

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S-phase Radiation Resistance

• Likely due to Homologous Recombination
• Can result in cell population synchrony
• S G2 blockade and increased survival in S
• More important in rapidly dividing cells
• May be important in some tumor lines

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Reassortment

• Cells in G2 M are preferentially killed
• Cells in S are preferentially spared.
• Alters proportion of cells in each phase
• Cell population tends to reestablish normal
  proportions within 2-3 cycles.
• Killed cells replaced by cells from G1
• Moves cells to more sensitive G2 S

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Repair - dose rate dependency

• Dose rate decreased by two mechanisms
• Splitting dose into smaller fractions w/ time
  between the fractions
• Smaller fractions increase time if spacing
  constant
• Reducing the actually rate at which dose is
  delivered
• Repair between ongoing during doses
• Repopulation may occur

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Repair is dose rate dependent

• At very low dose rates repair of SLD can keep up
  with radiation damage.
• SLD predominate type of injury.
• Repopulation can account for LD and SLD
• Dependent on cycling cell population
• Cell cycle time short relative to dose rate
• Affected by radiation quality
• Mutation rates may be increased

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Repair - dose rate dependency
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Repair - dose rate dependency
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Repair is dose dependent

• Lethal Damage increase with dose
• PLD increases with dose
• Accumulation of SLD increase with dose
• Survival curve is continuously bending
• Some repair always present
• Various forms of damage interact

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Repair is radiation type dependent

• Low LET radiation is repaired
• Little repair of High LET radiation injury
• Dense ionization track
• Double strand breaks more likely
• Energy deposition curve dependent
• Sublethal damage less important
• Single strand breaks, base pair deletion, etc.

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LET vrs. Survival