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Radiobiological mechanisms
underlying sensitivity of nonhuman biota to ionising radiation
Outline
• Some facts about radiosensitivity of biota
• Some mechanisms
–
–
–
–
Extremophiles
avoidence
Adaptive responses
Selection
• Why study radiation response of organisms?
– Evolution of mechanisms
– Environmental protection concerns
– Cancer research/medical uses
Take home messages
• Radioresistance/ssensitivity must be considered in relation
to the endpoint e.g. reproductive fertility, death, enzyme
activity, individual organ sensitivity, ability to compete.
• Resistance to high doses of radiation is often the result of
evolutionary adaptation to a different environmental
extreme.
• Sensitivity can be due to genetic OR
epigenetic/environmental causes, complicating
determination of species sensitivity
• Determining the link between effect – harm – risk can be
challenging even at the level of the individual
Slide courtesy of Tom Hinton
Factors Influencing the Sensitivity of Plants to
Radiation
Increasing Sensitivity
Decreasing Sensitivity
Large nucleus
Large chromosomes
Acrocentric chromosomes
Small nucleus
Small chromosomes
Metacentric chromosomes
Low chromosome number
Diploid or haploid
Sexual reproduction
Long intermitotic time
High chromosome number
High polyploid
Asexual reproduction
Short intermitotic time
Long dormant period
Short or no dormant period
(Sparrow, 1961)
Taxonomic and Developmental Aspects of
Radiosensitivity
F.L. Harrison
S.L. Anderson
This paper was prepared for submittal to the
Symposia on Ionizing Radiation Stockholm, Sweden May 20-24, 1996
November 1996
Table 1. Summary showing ranges of LD50s obtained from acute
1
irradiation of organisms from different taxonomic groups.
Group
Dose, Gy
Protista
30 - 30,000
Invertebrates
2.1 - 1,100
Vertebrates
Fishes
Amphibians
Reptiles
Birds
Mammals
10 -> 600
7 - >22
3 - 40
5 - 20
2.5 - 150
Plants
1.5 - > 130
1
The radiation units in references were converted to grays for
comparative purposes and for some values are approximations.
LD50S FROM ACUTE IRRADIATION OF MAMMALS AND
1
FISHES.
Dose, Gy
Mammals
Humans
Monkey
Dog
Swine
Hamster
Mouse
Rabbit
Bat
Pisces
Goldfish
Mummichog
Tench
Guppy
Chinook salmon
Mosquitofish
Pinfish
3
6
2.5
2.5
6
6.4
7.5
150
3.75 - 100
10 -20
12 - 55
23.5
25
37
50
The dose rates known to cause sterility in different species have a
large range—0.23 to 1400 mGy/h. Differences occur because the
processes of gametogenesis are not the same from species to species,
and for a given species, the response of male and female reproductive
tissues may differ. In general, the testis is more radioresistant than
the ovary
Reproductive success for a given species
may be related not only to its sensitivity to
radiation during gametogenesis and early
development but also its reproductive
strategy. For example, in a highly fecund
species, the survival of early life stages
may be very low, and the loss of abnormal
embryos induced from radiation exposure
may be masked completely by those lost
from other ecological factors, such as food
limitation and predation.
CHANGES IN THE RADIOSENSITIVITY OF RAINBOW TROUT SALMO GAIRDNERII EXPOSED TO ACUTE
IRRADIATION DURING DEVELOPMENT
STAGE IN LIFE CYCLE
LD50 (GY)
GAMETE
0.5 - 1.0
CELL
0.58
2-32 CELL
3.1
GERM RING
4.5 - 4.6
EYED
4.1 - 9.0
ADULT
15
REF: HARRISON AND ANDERSON 1996
Table 5. S ensitivity of different endpoints in the polychaete worm Neanthes
arenaceodentata.
Dose, Gy
Endpoint
>0.3
>0.5
DNA-strand breakage
Reduced fertility
Increased sister chromatid exchanges
>2
Increased chromosomal aberrations
>50
Sterility
>100
Lifespan reduction
>500
Mortality
Figure 1. For the same LD50 value, the shape of species response curves from
radiation may differ significantly.
developing gametes or in the size of the gonad. They may be quantified also by
observing changes in the number of fertilized eggs produced and in the
morphology and physiology of the developing embryos. When early life stages are
irradiated, the effects quantified include the induction of abnormalities in the
embryos and increases in mortality. Although the database is far from complete,
sufficient information is available to permit some comparisons to be made.
An example of changes in sensitivity among developmental stages is
Causes of sensitivity
•
•
•
•
UNSCEAR list but also
Heterozygous mutations – gene
dosage effects
Compromised defenses due to
other stressors
Lack of conditioning
exposure or induced
tolerance
Some examples of data from the
“experimental” field
• Field irradiators from Colorado
• Exp. aquatic mesocosms from Savannah River
plus work of Hingston et al using woodlice in a
mesocosm.
• Chernobyl and Fukushima accidents-, plants,
voles, swallows, butterflies and reindeer
• Hanford site and other uranium mines
• Chalk River, low dose facility and cooling ponds
Data on radiation effects for nonhuman species
Wildlife Group
Morbidity
Mortality
Amphibians
Aquatic invertebrates
Aquatic plants
Bacteria
Birds
Crustaceans
Fish
Fungi
Insects
Mammals
Molluscs
Moss/Lichens
Plants
Reptiles
Soil fauna
Zooplankton
No data
To few to draw conclusions
www.ceh.ac.uk/PROTECT
Some data
Reproductive
capacity
Mutation
www.ceh.ac.uk/PROTECT
Fixed assessment factor method
PNEV = minimal Effect Concentration / Safety Factor
Main underlying assumptions
In the frame of this approach,
extrapolations are made from:
•The ecosystem response depends on
the most sensitive species
•Acute to chronic
•One life stage to the whole life cycle
•Individual effects to effects at the
population level
•One species to many species
•One exposure route to another
•Direct to indirect effects
•One ecosystem to another
•Different time and spatial scales
•Protecting ecosystem structure
protects community function
www.ceh.ac.uk/PROTECT
Background radiation exposure for ICRP
RAPs (weighted dose rates)
Freshwater organisms –
0.4 – 0.5 μGy/h (Hosseini et al., 2010)
Marine organisms –
0.6 - 0.9 μGy/h (Hosseini et al., 2010)
Terrestrial animals and plants –
0.07-0.6 μGy/h (Beresford et al., 2008)
www.ceh.ac.uk/PROTECT
Background radiation exposure for ICRP
RAPs
Freshwater organisms –
0.4 – 0.5 μGy/h (Hosseini et al., 2010)
Derived screening dose rate
(10organisms
μGy/h)–is more than 10
Marine
times these
background
0.6 - 0.9
μGy/h (Hosseinivalues
et al., 2010)
Terrestrial animals and plants –
0.07-0.6 μGy/h (Beresford et al., 2008)
www.ceh.ac.uk/PROTECT
Zone and Classification
External
Gamma Dose
(Gy)
Conifer Death (4 km2)
Complete death of pines; partial damage to
deciduous trees
over 80 - 100
Sublethal (38 km2)
Death of most growth points, partial death of
coniferous trees, morphological changes to
deciduous trees
Medium Damage (120 km2)
Suppressed reproductive ability, dried needles,
morphological changes
Minor Damage
Disturbances in growth, reproduction and
morphology of coniferous trees
Post Chernobyl courtesy of Tom Hinton
10 – 20
4–5
0.5 – 1.2
(Kuzubov et al.1990)
Low dose data – a reminder!
– Reports of effects of less than 5 mGy on
• Tandem mutation frequency (microGy, Sykes in mice),
• Adaptive response using micronucleus endpoint (Stuart,
Redpath in frogs in vivo (microGy)and rat cell lines)
• Microsatellite instability (Dubrova in humans and mice)
• Oxidative stress response (Einset in plant root hairs )
• Calcium flux and bystander effect ( in rainbow trout, zebrafish,
prawns and various cell lines and communication of bystander
signals between fish- our group
Is an effect important even if not “harmful”?
Mechanisms and strategies
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•
•
•
•
Resistance due to DNA repair
Multiple copies of genome
Antioxidants/colour
Evasion of exposure
Up-regulation or down-regulation of antideath pathways (depending on whether
death is a beneficial outcome)
The tunicate story
Tunicate LD50 is about 3KGy BUT budding is inhibited
in the mGy region and rescue by normal tunicate grafts is
similarly affected. Allorecognition processes are very sensitive
to low radiation doses
Refs by Rinkevitch and Weissmann et al 1970-2008
Suggested mechanisms - extreme
radioresistance
• Multiple copies of the genome
– Cockroaches and many other insects
– Thermophilic bacteria
• Anti-oxidant colours/enzymes
– e.g. Rubrobacter radiotolerans
• DNA breaks protected
– D. radiodurans family
Nature Reviews – Microbiology 2007
Deinococcus radiodurans
• Part of a family including some of the
most radiation-resistant organisms
known
• Survives 5000 Gy of gamma radiation
• Genome is 4 circular molecules, 2
chromosomes, 1 megaplasmid, and 1
small plasmid
• Multiploid
• Genome lacks genes for RecB and
RecC (it has recD)
• Lots of interesting proteins
Courtesy of Michael Daly
Uniformed Services University
of the Health Sciences
DOUBLE STRAND BREAKS FORMED IN D RADIODURANS
COMPARED WITH E. COLI
Species
Genomes
per cell
E.coli K12
D. radiodurans
DNA DSB’s
At D37
Average distance
Between lesions
4-5
8-9
530,000 bp
8-10
>275
10,000 bp
RADICAL SCAVANGING AS AN ADDITIONAL
MECHANISM?
Red pigment
anti oxidant activity??
Rubrobacter radiotolerans red pigmented highly radioresistant
Deinococcus radiodurans also red pigmented
1. E. Asgarani, H. Terato, K. Asagoshi, H.R. Shahmohammadi,
Y. Ohyama, T. Saito, O. Ymamoto and H. Ide (2000) J. Radiat. Res. 41, 19-34.
2. E. Asgarani, H. Funamizu, T. Saito, H. Terato,
Y. Ohyama, O. Yamamoto and H. Ide (1999) Microbiol. Res. 154,
185-190.
Other biochemical mechanisms
• Hypoxia
• Protective sugars (tetrahalose) in the exoskeleton
• Use of neurotransmitter antagonists such as L-DOPA
which can modulate stress responses
• Sensitization by plant polyphenols
• ROS mediates both pro-apoptotic and anti-apoptotic signaling, but the
precise mechanisms that lead to these polar outcomes are not yet clear.
• Growth factor pathways (e.g., EGFR, PDGFR) Bcl-2 Survivin Protein
kinase B/Akt MDR proteins ROI COX-2 NF-κB STAT3
Multiple copies of chromosomes?
•
Kira S. Makarova et al, Genome of the Extremely Radiation-Resistant
Bacterium Deinococcus radiodurans Viewed from the Perspective
of Comparative Genomics Microbiol Mol Biol Rev. 2001 March; 65(1): 44–79.
“copy number NOT correlated with radioresistance”
•
Alexander T. Smith et al HIP1 mediated excisions in Escherichia Coli.
“Data not consistent with a 'copy choice' mechanism of DNA rearrangement ---underlying extreme resistance to DNA damage”
•
The genome sequence of
the extreme thermophile Thermus thermophilus
Anke Henne,
Nature Biotechnology 22, 547 - 553 (2004)
Cross resistance?
• Anke Henne, Nature Biotechnology 22, 547 - 553
(2004) The genome sequence of the extreme
thermophile Thermus thermophilus
– enzymes of thermophilic and radioresistant
organisms are not only more thermostable, but also
more resistant to chemical agents than their
mesophilic homologs.
• In D. radiodurans, it is likely that the radioresistance
is due to mechanisms evolved to cope with
dessication
» Michael Cox p.comm
Several studies have shown that tardigrades can survive -irradiation well
above 1 kilogray, and desiccated and hydrated (active) tardigrades
respond similarly to irradiation. Thus, tolerance is not restricted to the
dry anhydrobiotic state suggesting possible involvement of an efficient,
but yet undocumented, mechanism for DNA repair. Other anhydrobiotic
animals (Artemia, Polypedium), when dessicated, show a higher
tolerance to irradiation than hydrated animals, possibly due to the
presence of high levels of the protective disaccharide trehalose in the dry
state.
But even though eggs were laid after 1kGy, they didn’t develop into
juvenilles. Adults appear resistant due to limited regeneration of adult
cells.
K. INGEMAR JÖNSSON Astrobiology 7, 757–766.
2007
Evolution of mechanisms
• Primitive mechanisms which evolved for other
purposes can be harnessed eg bystander
signalling, extremophiles
• Polymorphisms in enzymes can be selected for
leading to population drift
• Genomic instability can result in increased
diversity available for selection
VALUABLE MATERIAL FOR EVOLUTIONARY
BIOLOGISTS
Summary
• Theoretical basis for radiosensitivity not based on biology
• Very little data for most species/classes of organisms
• Actual studies from experimental and accident situations
show huge variation within and between species depending
on many factors
• Conservative generic screening values to try to take
account of variability
• Biological strategies to cope with radiation cross evolved
• Very low dose effects seen but relationship ti harm/risk not
established