Alkylation, DNA leads to Increase, Mutations

Alkylated DNA may be ‘misread’ during DNA replication, leading to insertion of incorrect nucleotides. Upon replication, these changes become fixed as mutations. Subsequent replication propagates these mutations to daughter cells. Mutations in stem cells are of the greatest concern, as these will persist throughout the organism’s lifetime. Thus, increased mutations will be found in the cells of organisms that possess alkylated DNA.

Is it known how much change in the first event is needed to impact the second? Are there known modulators of the response-response relationships? Are there models or extrapolation approaches that help describe those relationships?

The shape of the dose-response curve for alkyl adduct formation versus mutation demonstrates that a threshold exists whereby alkyl adducts can be seen at low doses in the absence of increased mutations occurring. For example, following exposure of AHH-1 cells to increasing concentrations of MMS, a linear increase in alkylated DNA is measured. However, a hockey-stick shaped curve was found for mutations at HPRT in the same cells (Thomas et al. 2013). Thus, alkylation of DNA occurs at lower doses than mutation, and above a certain dose (where repair is saturated), mutation frequencies increase.

That DNA alkylation leads to mutation in spermatogonia in a similar hockey stick-shaped response (implying that a minimal dose must be exceeded) is supported by work using the LacZ Muta™Mouse assay. Exposure of male mice to the prototypical agent ENU was used to examine effects on spermatogonial stem cells, though the number of doses was limited (van Delft and Baan 1995). This analysis revealed that mutations did not occur at the lowest dose, where adducts are known to be measurable in other studies (van Zeeland et al., 1990). This data gap motivated a dose-response study using the Muta™Mouse model following both acute and sub-chronic ENU exposure by oral gavage at Health Canada (O’Brien et al., 2015). These data indicate a clear dose-response for single acute exposures, whereas a hockey stick-shaped dose-response occurs for lower dose sub-chronic (28 day) exposures. At the single acute high doses where the DNA repair machinery is expected to be overwhelmed (and thus higher levels of alkylation occur), significantly more mutations occur relative to the same dose spread out over 28 daily oral gavage exposures (O’Brien et al., 2015).

Additional contributors to the probability that an adduct will cause mutation include the site of alkylation, with agents that cause O-alkyl lesions being the primary mutagens, and the size of the alkyl group, with larger alkyl groups generally being more mutagenic.

A computational model to describe the mutational efficiency of different alkyl adducts has not yet been developed to our knowledge.

Alkylating agents are well-established to cause mutation in virtually any cell type in any organism.