TABLE 2.

Summary of adjacent key event relationships (KER)

Key event relationship (KER) title	Description	Examples of empirical evidence
Increases in Oxidative DNA damage leads to Inadequate DNA repair	At steady state, oxidative lesions generated by endogenous free radicals are readily repaired by basal repair mechanisms, mainly base excision repair (BER), to maintain baseline levels. However, if the level of oxidative DNA lesions (i.e., oxidized bases, abasic sites, strand breaks) increases above a cell's ability to detoxify, an exceeded repair capacity can lead to lack of, or faulty repair (i.e., increase in unrepaired lesions, repair intermediates, mispaired bases) – all indicators of inadequate DNA repair.	Concentration/Dose Concordance Threshold dose–response curve of 8‐oxo‐dG observed in the liver and urine of mice exposed to increasing doses of X‐rays; exceeded threshold indicates exceeded repair capacity leading to the observed increase in oxidative damage (Li et al., 2013) Temporal Concordance The 8‐oxo‐dG levels in Ogg1‐overexpressing Chinese hamster ovary cells returned to baseline within 4  hours following UV exposure, while wild type cells retained 70% of the lesions (Dahle et al., 2008) Other types of evidence Enrichment of 8‐oxo‐dG and γH2AX (DSB marker) in the same genomic regions was observed in human cells in culture by chromatin immunoprecipitation sequencing (ChIP‐seq) (Amente et al., 2019; Murata et al., 2012)
Inadequate DNA repair leads to Increases in DNA strand breaks	Exceeded BER capacity due to an increase in oxidative lesions can lead to an accumulation of repair intermediates, including AP sites and SSBs. Increase in the number of SSBs elevates the risk of two SSBs occurring in close proximity to each other; if two SSBs occur on opposite strands, it may be converted into a DSB, exacerbating the damage. Increase in unrepaired lesions and repair intermediates due to inadequate repair can further impede the repair of other damaged sites nearby. BER intermediates are known to be replication blocks that can cause a replication fork to stall and collapse. Collapsing of replication forks can cause DSBs, the most toxic type of DNA lesion.	Concentration/Dose and Incidence Concordance OGG1‐deficiency causing concentration‐dependent increase in strand breaks (Wu et al., 2008) BER‐proficiency leading to increase in strand breaks due to BER imbalance or interference with replication (Ensminger et al., 2014; Wang, Li, et al., 2018) Dose and Temporal Concordance Dose‐dependent increase in DSBs measured in primary human dermal cells after a 16‐h recovery following ionizing irradiation (Rydberg et al., 2005)
Increases in DNA strand breaks leads to Inadequate DNA repair	Increase in the number of strand breaks can exceed the repair capacity (DSB: NHEJ or HR; SSB: SSBR), resulting in prolonged presence of strand breaks (lack of repair). Increase in the occurrence of NHEJ may also increase the incidence of two incorrect ends being joined, altering the DNA sequence. DSBs may also lead to mutagenic salvage DNA repair pathways such as break‐induced replication (BIR) and microhomology‐mediated break‐induced replication (MMBIR).	Concentration and Incidence Concordance In the Rydberg et al. (2005) study above, dose‐dependent increase in misrejoined DSBs was also observed in primary human dermal cells after a 16‐h recovery following irradiation (Rydberg et al., 2005) In several studies, dose‐dependent increase in unrepaired DSBs was detected in irradiated mammalian cells after varying recovery periods, indicating exceeded repair capacity (Kuhne et al., 2000; Kuhne et al., 2005; Lobrich et al., 2000) Temporal Concordance In the studies above, the levels of DSBs were measured immediately after exposure and after recovery; inadequate repair was indicated by unrepaired or mis‐rejoined DSBs after the recovery period.
Inadequate DNA repair leads to Increases in Mutations	Higher incidences of NHEJ and mutagenic salvage repair can increase the chance of incorrect joining of two broken ends, altering the DNA sequence. Unrepaired base lesions can lead to point mutations, especially if they are able to form stable base pairs with incoming nucleotides during replication (e.g., 8‐oxo‐dG base pairing with adenine).	Incidence and Concentration Concordance Concentration‐dependent increase in DSB misrepair and mutation frequency in cells exposed to ionizing radiation (McMahon et al., 2016) A larger fold increase in KBrO3–induced mutations in Ogg1 −/− mice vs. wild type mice (Arai et al., 2002) Temporal Concordance A larger fold increase in spontaneous mutations in Ogg1 −/− mice at 14  weeks of age than at 9  weeks, compared to wild type (Minowa et al., 2000)
Inadequate DNA repair leads to Increases in Chromosomal Aberrations	If DSBs are not repaired in a timely manner, the ends can shift away from the original position, reducing the likelihood of error‐free repair. Unrepaired strand breaks and mis‐joined ends by incorrect repair can result in translocation, inversion, deletion of sections (unincorporated fragments), and other structural aberrations of the chromosome (e.g., ring and loop formation).	Concentration/Dose and Incidence Concordance Dose‐dependent increase in incorrect rejoining of DSBs in irradiated cells (McMahon et al., 2016) Clear, positive correlation between radiation dosage and clastogenic endpoints (CA, MN, copy number variants) reported by several studies (Arlt et al., 2012; Balajee, 2014; George et al.,  2009; Suto et al., 2015) Temporal Concordance DNA‐PK (DNA‐dependent protein kinase) was chemically inhibited and DSBs and MN were measured in gamma‐irradiated cells at 3  h and 24  h, respectively, post‐irradiation; irradiation dose‐dependent increase in DSBs and inhibitor concentration‐dependent increase in MN were observed (Chernikova et al., 1999)