New analytical methods for genetic dissection of biological responses to DNA lesions

Two proteins that process damaged DNA may function in the same pathway, or in redundant (“synergistic”) pathways that respond to the same lesion(s), or in parallel pathways targeted to different lesions. Previously, extended plots of positive outcomes (such as cell survival) or negative outcomes (such as reduced tissue growth) vs. genotoxic dose yielded empirical estimates of wt and mutant resistances to DNA damage. Recently, wt and mutant outcomes have been compared at one or two doses. The criterion for parallel pathways was “additivity”: wt positive outcomes roughly equal to numerical sums of single-mutant positive outcomes or double-mutant negative outcomes equal to sums of single-mutant negative outcomes. For redundant pathways, wt positive outcomes or double-mutant negative outcomes were postulated to be “greater-than-additive” relative to single-mutant sums. Equations derived here to describe parallel and redundant pathways provide no rigorous theoretical justification for these criteria. Furthermore, simulations using these equations generate additive or greater-than-additive outcomes for both parallel and redundant pathways, depending on the values chosen for various parameters. Proposed new methods to compare wt vs. mutant plots of negative outcomes against doses of genotoxic agents yield three different but complementary estimates of resistances to DNA damage: respective plot slopes where mutant and wt outcomes are the same (inversely proportional to instantaneous damage-resistance strengths), respective total doses that cause the same outcomes (total resistance capacities), and respective dose thresholds where negative outcomes are first detected. Analyses of experimental examples suggest that greater-than additive threshold doses provide the most straightforward criteria for pathway redundancy.