Background/Endogenous DNA Damage:
Considerations for Dose-Response & Risk Assessment
L. H. Pottenger & J. S. Bus,
with support from J.A. Swenberg & R. Budinsky
LHP 5/2011
ARA Workshop III
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Background/Endogenous DNA Damage
• DNA adduct = key event in MOA for mutation;
• Mutation = key event in mutagenic (direct DNA-reactivity)
MOA for cancer
• Background/Endogenous DNA damage: present in
untreated cells/tissues
Swenberg et al., 2011
LHP 5/2011
ARA Workshop III
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Considerations
1.
DNA is not pristine; ubiquitous burden of background & endogenous damage.
2.
Background/endogenous damage can match DNA adducts from exogenous
exposure to reactive chemicals: expect to be biologically equivalent.
3.
Distinguish exogenous from background/endogenous with stable isotopelabelled or radiolabelled test material under controlled exposure experiments.
4.
Background/endogenous DNA adduct levels vs exogenous levels: adductspecific and varied.
5.
Use of DNA adduct data can help better understand the shape of the doseresponse curve (linear vs. non-linear) at low doses for critical steps, in particular
doses for which exogenous adducts are either not observed or fall within the
range of background/endogenous DNA damage.
–
–
Recognized limitations & conservatism--DNA adduct is only beginning of first step
Use of specific adduct data to define a point-of-departure puts an upper bound on
potential risks, and serves to test the plausibility of risk values developed using
other approaches (Swenberg et al., 2011) .
LHP 5/2011
ARA Workshop III
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Background/Endogenous DNA Damage
•Swenberg estimates ~50,000 DNA lesions/cell at steady-state;
•Ubiquitous DNA damage; DNA is not pristine
LHP 5/2011
ARA Workshop III
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In some cases, same adducts!
• For certain cases, same adduct can be from
background/endogenous or from exogenous
source: formaldehyde (FA), vinyl chloride (VC),
ethylene oxide (EO)
– Biologically equivalent
• Only able to differentiate source with stable
isotope label (13C) or radiolabel (14C)
– Labelled test material; controlled exposures
LHP 5/2011
ARA Workshop III
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Background/Endogenous Adducts vs
Exogenous Adducts--FA
= 13CD2-N2-HMdG
--- = 12C-N2-HMdG
Range of background/endogenous N2-HMdG
From Swenberg et al., 2011
LHP 5/2011
ARA Workshop III
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Background/Endogenous Adducts vs
Exogenous Adducts--EO
= 14C-N7-HEG
=
12C-N7-HEG
From Marsden et al., 2009
LHP 5/2011
ARA Workshop III
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Adducts & Risk Assessment
• Recommendations: approach needs caution &
significant data (Jarabek et al., 2009):
– Structural identification
– Pro-mutagenic & persistent adduct
– Presence in target tissue
• Formation of DNA adduct = initial step
– Multistep process of MOA
• Mutation
• Mutagenic MOA for cancer
• Recognizing these caveats, where adequate data
available, develop potential risk values:
– Dose-response models for specific adduct as dose-metric
– Delineate upper bound of potential risk (very conservative)
– Possibly serve as point-of-departure (added safety factors)
LHP 5/2011
ARA Workshop III
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Formaldehyde
• Potential risk values calculated based on
N2-HMdG adducts:
– Nasal cancer: ~similar to EPA/IRIS risk value
(~1.5-28x lower risk than EPA)
– Systemic cancers: 45x ~ 19,000x difference
• Serve to test the plausibility of risk values
developed using other approaches
• Swenberg et al., 2011
LHP 5/2011
ARA Workshop III
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Vinyl Chloride
• Angiosarcoma of the liver (ASL): recognized human
cancer caused by high exposures to VC
• Rare in general population (rare tumor)
– Only 25-30 new cases/year in general US population
– Estimate background incidence of 2-3/10-6
• Concordance in human and animal model target
 Use VC data on εG background/endogenous levels &
background incidence ASL to estimate unit risk value for
ASL based on εG adducts as likely initial key event
(pro-mutagenic & persistent)
 Use VC data to compare expected ambient [VC] and
resulting predicted εG levels to estimated ASL incidence
 Use VC data to predict expected incidence of ASL from,
e.g., 1 ppm VC lifetime continuous exposure & compare
to risk value developed by EPA/IRIS
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ARA Workshop III
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Conclusions
• Consider background/endogenous adducts and/or
DNA lesions in risk assessment.
• Develop examples to establish conservative nature of
risk values based on DNA adducts.
• Use rich databases of existing chemicals to
understand what can be learned vis-à-vis linear &
non-linear/threshold dose-response models and upper
bounds on potential risk.
• Ability to quantify & differentiate endogenous adducts
(for at least some DNA-reactive materials)
– opportunity to better inform the shape of low-dose doseresponse for these chemicals,
– to better inform the degree of conservatism currently
associated with LNT risk assessment models.
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ARA Workshop III
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