1. with support from J.A. Swenberg & R. Budinsky
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

2. 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

3. Considerations
DNA is not pristine; ubiquitous burden of background & endogenous damage.
Background/endogenous damage can match DNA adducts from exogenous exposure to reactive chemicals: expect to be biologically equivalent.
Distinguish exogenous from background/endogenous with stable isotope-labelled or radiolabelled test material under controlled exposure experiments.
Background/endogenous DNA adduct levels vs exogenous levels: adduct-specific and varied.
Use of DNA adduct data can help better understand the shape of the dose-response 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

4. 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

5. 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

6. 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

7. Background/Endogenous Adducts vs Exogenous Adducts--EO
= 14C-N7-HEG
= 12C-N7-HEG
From Marsden et al., 2009
LHP 5/2011
ARA Workshop III

8. 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

9. 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

10. Vinyl Chloride
Angiosarcoma of the liver (ASL): recognized human cancer caused by high exposures to VC
Rare in general population (rare tumor)
Only 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
LHP 5/2011
ARA Workshop III

11. 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 dose-response for these chemicals,
to better inform the degree of conservatism currently associated with LNT risk assessment models.
LHP 5/2011
ARA Workshop III