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DNA Damage and Biodosimetry
• Radiation, through free radical formation,
causes damage to DNA
• Cells have very efficient DNA repair
mechanisms (argues against LNT hypothesis)
• Damage to DNA/chromosomes from radiation
can be measured to determine amount of
radiation exposure.
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Types of DNA repair:
Damage on/to single strands
• Incorrect base due to
– Spontaneous reactions
– Base analogs
– Copying errors
• Breaks in a DNA strand due to radiation
• Excision repair:
– Section of DNA with error cut out
– Enzyme puts in new DNA using complementary
strand
• Rejoining of broken strand
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Types of DNA repair:
Double strand breaks
• Homologous repair
– DNA has been copied, so sister chromatid is
available.
– Recombination event uses same (or similar DNA)
– May result in wrong section being used: mutation.
• Non-homologous end-rejoining
– Ends just stitched back together
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saturn.roswellpark.org/.../ DNA_Repair/SSBR.gif
Failure to repair
• Damaged DNA has consequences
– Senesence: cell stops dividing when it should
continue
– Apoptosis: damaged DNA stimulates programmed
cell death
– Carcinogenesis: damage DNA interferes with cell
regulation, cells divide out of control.
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6
http://en.wikipedia.org/wiki/Image:Dnarepair1.jpg
Deinococcus radiodurans
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• Gram positive bacterium
– Apparently fairly ancient
– High resistance to radiation damage
– Why? Earth never that radioactive.
• Highly efficient DNA repair
– Adapted to resist dessiccation, oxidation damage to
DNA
– Multiple copies of genes for DNA repair enzymes
Chromosomal aberrations
• Deletions, rings, dicentrics
Micronuclei: chromosome fragments
http://www.4p-supportgroup.org/images/figure12.jpg
http://www.infobiogen.fr/services/chromcancer/Tumors/Images/MeninKaryo.jpg
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Translocations
• Chromosomes swab pieces
– DNA breaks, attaches elsewhere
http://www.ucl.ac.uk/~ucbhjow/bmsi/lec7_images/rectrans.gif
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Biodosimetry: use of chromosomal change
to estimate radiation dose
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• Chromosomal aberrations
– Effective range: 35 -500 rem exposure
– Time from dosage must be considered due to repair
• 1000 hits occur and repaired before one seen
• Repair is continual, so #s decrease over time
– Dicentrics most commonly used
• Most dicentric chromosomes are radiation
induced
– Micronuclei
• From as low as 1 rem exposure; recent
exposures only.
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Translocations
http://www.jinr.dubna.su/drrr/
projects/gen/figb.gif
• Not readily repaired (caused by ds break repair)
– Good for estimating lifetime dose
– Background rate: 1-4 translocations per 1000 cells
Radiation Protection Dosimetry 88:77-86 (2000)
© 2000 Oxford University Press
Our Views on Issues in Radiation Biodosimetry Based on
Chromosome Translocations Measured by FISH
J.N. Lucas and W. Deng
A biomarker for retrospective biodosimetry for radiation
exposure and human health risk assessment would be most
useful if it employed an endpoint that is highly quantitative, is
stable over time, and is relevant to human health risk. The
frequency of chromosome translocations in individuals exposed
to low to medium doses of whole-body radiation serves these
goals. Recent advances in chromosome painting using
fluorescence in situ hybridisation (FISH) facilitate fast and
reliable measurement of chromosome translocations, a kind of
DNA damage linked to both prior clastogenic exposure and to
human health risk.
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Using FISH to spot translocations
www.rerf.or.jp/Gene/eng/fig/fishf.htm
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Current trends in biodosimetry
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• Molecular biomarkers for ionizing radiation exposure
(gene expression changes, blood proteins) can be
measured in real time using such diagnostic detection
technologies as miniaturized nucleic acid sequences
and antigen-based biosensors
– Certain genes are turned off or on by acute
radiation exposure; RT-PCR to detect, quantify.
• Examination of biological tissue (e.g. teeth) using EPR
(electron paramagnetic resonance)
– Ionization affects electron spin changes
Follow the money
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• After a large-scale radiological event, there
would be a major need to assess, within a few
days, the radiation doses received by tens of
thousands of individuals. By contrast, current
“high throughput” biodosimetry can, at best,
assess a few hundred individuals / day. The
Center for High-Throughput Minimally-Invasive
Radiation Biodosimetry is a research
consortium devoted to developing highthroughput radiation dose assessment
techniques.